Marshall - The Geomorphology and Physiographic Provinces of Central America

CENTRAL AMERICA: GEOLOGY, RESOURCES AND HAZARDS; BUNDSCHUH & ALVARADO (EDS) 1

Chapter 3

The Geomorphology and PhysiographicProvinces of Central America

JEFFREY S. MARSHALL

“We have been sailing placidly along thecoast…all day - a broad, low land, denselyclad in a green, tropical vegetation…. In fullview are three noble mountains - tall,symmetrical cones, with sides furrowed withwrinkle-like valleys veiled in a dreamy,purple mist that is charming to the eye, andsummits swathed in a grand turban ofrolling clouds. They say these are volcanoes,but we cannot see any smoke. No matter - itis a fairy landscape that is very pleasant tolook upon.” Mark Twain, December 1866,letter written aboard the steamer Columbiaoffshore of Guatemala, published in the SanFrancisco Alta California, March 15, 1867

3.1 INTRODUCTION

Central America encompasses an intricate mosaic of dynamic landscapes shaped by awide range of Earth surface processes. Few other regions worldwide exhibit acomparable magnitude of geomorphic diversity. Along the length of the CentralAmerican isthmus, highly variable tectonic, lithologic, and climatic domains (Figs. 3.1-3.3) are superimposed across a small land area (0.4 % of Earth’s total land surface).The resulting physiography is characterized by a heterogeneous array of geomorphicprovinces (Fig. 3.4), each featuring a distinctive landform assemblage that preserves aunique history of landscape evolution.

Over 1500 km in length, Central America reaches from the rugged Maya highlandsof Guatemala in the north, to the humid coast ranges of Panama’s Darién isthmus in thesouth. This narrow land bridge links the two American continents and forms a criticaldivide between the Pacific and Atlantic ocean basins, varying in width from less than100 km at the Panama Canal, to over 400 km across the interior highlands of Nicaraguaand Honduras. From towering volcanic peaks (>4000 m elevation) to jungle-shroudedalluvial lowlands, and from rugged tectonic shorelines to passive-margin lagoons,Central America embodies a geomorphic microcosm of remarkable diversity.

The physiographic architecture of Central America (Fig. 3.4) is defined primarilyby the northwest-trend of the Middle America Trench and Central American VolcanicFront (Fig. 3.1). These major morphotectonic features were formed by Cenozoicsubduction of the Cocos oceanic plate, and its predecessor, the Farallon plate, beneath

2 THE GEOMORPHOLOGY AND PHYSIOGRAPHIC PROVINCES OF CENTRAL AMERICA

the western margin of the Caribbean plate [1-10]. While the northern volcanic frontdeveloped over Paleozoic continental basement of North American origin (Maya andChortis blocks), the southern volcanic front formed on the Mesozoic oceanic basementof the Caribbean plate (Chorotega and Chocó blocks) (Fig. 3.1). Throughout theCenozoic, this first-order contrast in basement lithology has been overprinted with adiverse suite of rock formations (Fig. 3.2) generated in a variety of tectonic settings,including volcanic cordilleras, fore-arc and trench-slope basins, alluvial plains anddeltas, intra-arc rift valleys, fold-and-thrust belts, highland plateaus, and carbonateplatforms.

Figure 3.1. Tectonic map of Central America, showing the regional geometry of tectonic platesand basement blocks. Plate names are in white boxes (North American, Caribbean, Cocos,Nazca, and Panama). Large arrows show plate motions relative to Caribbean plate. Activeplate boundaries are shown as solid lines (with teeth on upper plate of convergent margins,and opposing arrows indicating transform motion). Shaded areas show basement blocks(Maya, Chortis, Chorotega, and Chocó). Dashed lines mark major bathymetric features.EPR: East Pacific Rise; GSC: Galapagos Spreading Center; PFZ: Panama Fracture Zone;CCRDB: Central Costa Rica deformed belt; NPDB: North Panama deformed belt; SPDB:South Panama deformed belt; EPDB: East Panama deformed belt.


Active tectonic deformation continues to shape the Central American landscapealong several complex plate-boundary zones (Fig. 3.1). In the north, the Motagua-Polochic fault system cuts across central Guatemala, accommodating sinistral shearingbetween the North American and Caribbean plates [11-13]. Along Central America’sPacific margin, rapid convergence (7-10 cm/yr) occurs between the Cocos andCaribbean plates at the Middle America subduction zone, generating large earthquakes[14-18], active volcanism [19-23], and pronounced upper-plate deformation [24-37].The oblique suduction of steeply dipping seafloor, produced at the East Pacific Rise,results in pervasive strike-slip faulting and rifting along the northern Central Americanmargin [24-27]. In contrast, the flat subduction of rough, hotspot-thickened seafloor,produced at the Galapagos Spreading Center, drives rapid uplift and crustal shorteningacross southern Central America [28-37]. Along the margins of Panama, rapiddeformation occurs in response to collision with South America to the east, and obliquesubduction of the Nazca plate to the south [38-42].

In addition to contrasting lithologic and tectonic domains, Central America alsohosts a wide array of climatic and ecological zones (Fig. 3.3), ranging from the humidtropical rainforests of the Caribbean and southern Pacific lowlands, with >4.0 m/yr ofrainfall, to the dry tropical savannahs of the northern Pacific coastal plains, with <1.0

Figure 3.2. Geologic map of Central America, showing the distribution of major rock units(based on map of R.Weyl [1]).


m/yr of highly seasonal precipitation [43-45]. Similarly, vegetation zones withinmountainous regions range from the humid cloud forests of volcanic highlands to thedwarf scrublands of the high-altitude páramo. Dramatic climatic and topographicgradients juxtapose 4000 m peaks that were glaciated in the Pleistocene [46] in closeproximity to humid lowland basins mantled by thick lateric oxisols [7]. Topographicextremes coupled with variations in slope aspect, wind direction, and orographicprecipitation result in extraordinarily diverse microclimates, vegetative cover, and soiltypes within single mountain ranges.

As a whole, the Central American isthmus forms a geologically tenuous landbridge that links the two American continents and creates a topographic divide betweenthe Atlantic and Pacific Ocean basins. This narrow landmass plays a vital role indirecting the ecological evolution of the Americas [47, 48], and may also profoundlyinfluence ocean circulation and global climate [49-51]. During the Cenozoic, a diversearray of landscapes has developed along the isthmus in response to dynamicinteractions between regionally variable rock types, complex plate-boundary tectonics,and an energetic tropical climate. Central America therefore serves as a uniquelaboratory for the study of a wide range of geomorphic processes and consequentpathways of landscape evolution.

Figure 3.3. Climate map of Central America, showing the regional distribution of KöppenClimatic Zones based on average temperature and precipitation (modified from [44]).


This chapter explores the regional geomorphology of Central America and definesa system of physiographic provinces (Fig. 3.4) that characterizes the overall landscapediversity of this dynamic region. The chapter is organized into sections that provide anoverview of the characteristic landforms and geomorphic processes that define each ofthe physiographic provinces. The first half of the chapter looks at the geomorphicprovinces of northern Central America, and the second half examines those of southernCentral America. Each section provides a brief review of current geomorphic researchwithin the physiographic provinces. Due to the wide range of possible topics, it isimpractical to cover every aspect of these diverse landscapes. The discussion istherefore limited to a subset of critical topics that provide a general flavor for theregional geomorphology of Central America. Several of these topics are explored ingreater detail in the four chapters that follow: Volcanism and volcanic landforms(Chapter 4), Karst landscapes (Chapter 5), Glacial geology and geomorphology(Chapter 6), and Coastal morphology and coral reefs (Chapter 7).

Figure 3.4. Map of the physiographic provinces of Central America as defined in this chapter.Solid lines indicate province boundaries. Numbers on map refer to the list of physiographicprovinces at right. Yucatán platform and Chortis highlands sub-regions: a, Northern pittedkarst plain; b, Southern hilly karst plateau; c, Petén karst plateau and lowlands; d, Easternblock-faulted coastal plain; e, Central Chortis plateau; f, Western rifted highlands; g, Easterndissected plateau; h, Honduran borderlands. Digital elevation model derived from NASAShuttle Radar Topography Mission (SRTM) image PIA03364.


3.2 GEOMORPHOLOGY OF NORTHERN CENTRAL AMERICA

Northern Central America straddles the boundary between two major crustal domains(Fig. 3.1), the Maya block of southern Mexico, Belize, and northern Guatemala, andthe Chortis block of southern Guatemala, Honduras, El Salvador, and Nicaragua [52-55]. These two basement terranes are juxtaposed across the Motagua-Polochic faultzone of central Guatemala, which defines the active North American-Caribbean plateboundary. The lithologic and structural contrast between the Maya and Chortis blocks(Fig. 3.2) exerts a first-order control on regional geomorphology in northern CentralAmerica. This geomorphic template is in turn affected by active plate-boundarydeformation along both the east-west trending Motagua-Polochic fault zone and thenorthwest-trending Middle America convergent margin along the Pacific coast. LateCenozoic volcanism, generated by subduction at the Middle America Trench impartsan additional influence on regional geomorphology.

Both the Maya and Chortis blocks (Fig. 3.1) encompass continental basementterranes of lower Paleozoic metamorphic and igneous rocks (Fig. 3.2) [52-58]. On theMaya block to the north, the basement is overlain by a thick sequence of upperPaleozoic clastic and carbonate sediments, upper Jurassic continental red beds, andCretaceous to Eocene carbonate and evaporitic rocks [59-61]. This sequence is exposedacross the Maya block within the Sierra Madre of southeastern Mexico, the northernhighlands of Guatemala, and the vast lowlands of the Yucatán platform. The basementof the Chortis block, to the south of the Motagua-Polochic fault zone, is overlain byMesozoic clastic and carbonate sediments, lower Teritiary red beds, and a thicksequence of Neogene ignimbrites [59, 62-64]. These rocks are exposed throughout theChortis highlands of southern Guatemala, Honduras, and Nicaragua.

While the Maya block has undergone only minor rotation relative to the NorthAmerican craton, the allocthonous Chortis block to the south, has experiencedsignificant southeastward displacement and rotation relative to its original positionnorthwest of the Maya block. The regional geomorphology of northern CentralAmerica is strongly influenced by the lithologic and structural contrasts between thesetwo adjacent tectonic blocks.

3.2.1 Maya Highlands Province

The Maya Highlands Province (Fig. 3.4) extends in a broad arc from México’s SierraMadre de Chiapas, eastward across the northern Guatemalan Altiplano, to the Mayamountains of southern Belize. These rugged highlands consist of a series ofmorphologically distinct mountain ranges separated by deep fault-controlled canyonsand occasional broad alluvial valleys.

The Maya highlands are developed across a Cretaceous-Paleogene age fold beltthat affects the underlying crystalline basement and its sedimentary cover [1-4]. Theintensity of deformation decreases toward the north, where a near-horizontal sectionforms the foundation of the Yucatán carbonate platform. The geomorphology of theMaya highlands is largely controlled by variations in the lithology and structural grainof deformed sedimentary rocks and metamorphic basement exposed within a series oferoding, high-altitude mountain belts [65].

In western Guatemala (Fig. 3.5), the Maya Highlands Province extends across theborder from Mexico’s Sierra Madre massif, forming the northwest-trendingCuchumatanes range. These rugged mountains encompass several distinct geomorphic


sectors, including an extensive high-altitude plateau (>3800 m) that was glaciatedduring the Pleistocene [46]. Toward the east, the Maya highlands decrease in elevation,extending into the Chamá and Santa Cruz ranges of east-central Guatemala (Fig. 3.5).Along their northern margin, the highlands descend toward the Yucatán platform,encompassing the Lacandón range (<800 m) of northern Guatemala and the Mayamountains of southern Belize (<1200 m).

3.2.1.1 Cuchumatanes Range

The Cuchumatanes range in northwestern Guatemala (Fig. 3.5) consists of a deeplydissected, northwest-trending, fault-bounded mountain block. This high-altitude range(>3800 m maximum elevation) encompasses a thick (>7500 m) section of deformedupper-Paleozoic to Mesozoic sedimentary rocks overlying a lower Paleozoic

Figure 3.5. Map of the physiographic provinces of northern Central America, showingsignificant geomorphic features of Guatemala, El Salvador, and portions of Mexico, Belize,Honduras, and Nicaragua. Digital elevation model derived from NASA Shuttle RadarTopography Mission (SRTM) image PIA03364.


metamorphic basement [61]. The Cuchumatanes range exhibits several distinctgeomorphic sub-regions, including a spectacular highland plateau (Altos de losCuchumatanes), a rugged mountainous zone of karst topography, and a lower-elevationarea of ridges and valleys developed on metamorphic basement [65].

The Altos de los Cuchumatanes plateau (3400-3800 m elevation), in the core ofthe Cuchumatanes range, is formed primarily on a 2500-m-thick section of Cretaceouslimestone and dolomite strata near the top of the Maya highlands sedimentary sequence[61]. This plateau was glaciated during the Pleistocene, leaving a relict landscape ofstriated bedrock, moraines, and outwash plains [46, 66, 67] (also see Chapter 6). Theabrupt margins of the Cuchumatanes plateau correspond with deep canyons developedalong a series of northwest trending faults and folds. The southern edge of the plateaufollows a prominent fault escarpment that separates the Maya highlands sedimentarysection from crystalline basement rocks exposed within the Motagua-Polochic faultzone to the south (Fig. 3.5).

3.2.1.2 Chamá and Santa Cruz Ranges

The Chamá range of north-central Guatemala (Fig. 3.5) exhibits a complexmountainous landscape formed over east-west trending folds in the Mesozoicsedimentary section [65]. Where carbonate rocks outcrop along the fold belt, theChamá mountains exhibit an undulating karst topography with scarce surface drainage.On intervening clastic rocks, the landscape consists of low-relief erosion surfacestraversed by meandering streams. To the east of the Chamá mountains, the Santa Cruzrange (Fig. 3.5) features a strikingly different mountain landscape of steep northwest-trending ridges cut by deeply-eroded drainages [65]. The drainage divides consist of aseries of orthogonal spurs that extend out from the primary ridges. This morphology isthe product of differential erosion along a system of parallel fractures within theunderlying ultramafic basement rocks [60].

3.2.1.3 Lacandón Range

The Lacandón range (Fig. 3.5) marks the northern edge of the Maya HighlandsProvince, extending in a broad arc across Guatemala’s southern Petén region to theMaya Mountains of Belize. This relatively subdued mountain chain (<800 m elevation)is formed along an arcuate belt of tightly folded Cretaceous limestone and dolomitestrata (La Libertad Arch) at the southern margin of the Yucatán platform [59]. Risingabove the humid Petén lowlands, the Lacandón range consists of a series of ridges andvalleys that exhibit a rugged karst landscape of abundant sinks, isolated knobs, andpoorly integrated surface drainages [65]. The central Lacandón range also encompassessome broader low-relief karst uplands with occasional limestone columns and towers.Along the southern margin of the Lacandón range (Fig. 3.5), an extensive alluvial-lowland has formed where several major rivers descend out of the higherCuchumatanes and Chamá ranges to the south [65]. This broad interior lowlandconsists of a network of major flood plains, developed where rapid sedimentationburied the underlying karst topography.

3.2.1.4 Maya Mountains

The Maya Mountains of Belize (Fig. 3.5) consist of a fault-bounded highland (<1200 melevation) that exposes granitic and meta-sedimentary basement rocks within an east-


northeast trending synclinorium [57, 58]. This isolated mountain block rises abruptlyabove the surrounding lowlands of the Petén region and the Belize coastal plain [65].River systems draining from the Maya Mountains experience sharp changes in channelmorphology as they cross the steep mountain front that separates deformed crystallinerocks within the massif from the generally flat lying carbonate platform of the adjacentlowlands [68]. The coastline east of the Maya Mountains (Fig. 3.5) is characterized bya series of small deltas constructed of coarse clastic sediments eroded from themetamorphic and igneous interior of the mountain block [69]. This coastal morphologydiffers considerably from the low-relief shorelines to the north and south, characterizedby broad estuaries and lagoons fed by stream networks draining carbonate terrains.

3.2.2 Yucatán Platform Province

Extending to the north of the Maya highlands, are the expansive carbonate lowlands ofPetén, northern Belize, and the Yucatán peninsula (Figs. 3.5 and 5.1). Together, theseregions encompass the most extensive karstlands of the North American continent,covering over 100,000 km2 [70-73] (also see Chapter 5). The Yucatán PlatformProvince features a wide array of karst landforms, including sinkholes, cenotes, dryvalleys, cockpits, towers, and elaborate cave networks. This vast carbonate platformcan be subdivided into several distinct physiographic regions (Fig. 3.4), each exhibitinga characteristic topography and a unique assemblage of karst landforms. Thegeomorphic character of each of these areas is closely linked to regional variations inlithology, structure, and depth to the groundwater table.

3.2.2.1 Northern Pitted Karst Plain

On México’s Yucatán peninsula (Fig. 3.4), a relatively subdued karst landscape isdeveloped across a flat-lying sequence of Cenozoic marine carbonate rocks [70, 71].The northern third of the peninsula consists of a low relief pitted karst plain (0-30 melevation), characterized by a dense network of sinkholes and cenotes (flooded collapsepits that access the groundwater table). Efficient subsurface drainage within the pittedkarst plain results in a complete absence of surface streams [74-77]. An extensiveinterconnected system of flooded caverns (up to 130 km long) lies beneath the northernYucatán lowlands. These caverns formed by aggressive karst dissolution during sealevel low stands of the late Pleistocene. Much of this cave network is now flooded bystratified groundwater consisting of a lower saline layer, capped by an overlying freshwater lens.

A prominent 180-km-diameter semicircular alignment of cenotes extends acrossthe northwestern Yucatán plain. This feature, known as the Ring of Cenotes, representsa concentric band of enhanced karst permeability that overlies the buried Cretaceous-age Chicxulub impact structure [78, 79]. Breaks within extensive coastal dune fieldsreveal where the buried impact structure intersects the northwestern coastline of theYucatán peninsula. These locations coincide with zones of concentrated groundwaterdischarge, as manifested by a high density of springs and flooded coastal wetlands.

3.2.2.2 Southern Hilly Karst Plain

The broad lowland of the northern pitted-karst plain (Fig. 3.4) is bordered along itssouthern edge by the prominent northwest-trending La Sierrita de Ticul fault scarp[70]. The Ticul escarpment consists of an abrupt line of hills that rise up to 50 m above


the northern lowlands. This chain of hills forms a distinct ridge that extends for nearly200 km across the northern Yucatán peninsula, dividing the northern pitted-karst plainfrom a hilly karst terrain to the south. The southern hilly-karst plain (Fig. 3.4) coversmuch of the Campeche region of the Yucatán peninsula, south of the Ticul escarpment.This extensive upland (60-80 m elevation) features an irregular karst terraincharacterized by abundant hills with intervening flat-floored depressions (poljes).Many of these depressions host ephemeral surface streams and contain alluvialsediments. These basins occasionally flood during the rainy season due to theinefficient karst drainage in this region.

3.2.2.3 Petén Karst Plateau and Lowlands

In the Petén region of Guatemala, and adjacent portions of northwestern Belize (Figs.3.4 and 5.1), a distinctly more rugged and heterogeneous karst terrain has formed on anolder sequence of Cretaceous-Paleogene carbonate rocks [65, 80]. The diversity ofkarst geomorphology across this region is controlled by local variations in the lithologyand structure of the carbonate bedrock. The karst plateau (<450 m elevation) ofnortheastern Petén exhibits a hilly landscape with local relief in excess of severalhundred meters. Surface drainage across the plateau is poorly developed and abundantsinkholes feed into an extensive network of solution fractures and caverns. Along theeastern margin of the Petén karst plateau, the landscape exhibits a rugged topographycontrolled by underlying normal faults [65, 81]. Local relief can exceed 100 m acrosssteep scarps formed along the fault-controlled margins of elongate depressions. Adiverse suite of karst landforms occurs in this area, including dry valleys, residuallimestone hills, isolated cockpits, sinkholes, solution corridors, open fissures, andelaborate cave systems. To the west of the Petén karst plateau, is a humid lowlandregion consisting of a vast low-relief alluvial plain covered by large swamps andnumerous lakes [65]. The river network draining this region exhibits an irregularpattern, interrupted locally by subsurface karst drainage. In some areas, an undulatingtopography has formed by karst dissolution and alluvial filling across a series of foldswithin the carbonate bedrock.

3.2.2.4 Eastern Block-faulted Coastal Plain

The low-relief coastlines of the Yucatán platform (Fig. 3.4) are characterized by broadlagoons, mangrove swamps, and seasonally-flooded marshlands. Along the platform’seast-facing Caribbean coast (including northern Belize), the elongate morphology ofcoastal lagoons and lowlands follows a series of north-northeast-trending, fault-bounded ridges and depressions [70]. Offshore, an extensive network of fringing reefsand coral cays has also developed along this structural grain (forming the world'ssecond longest barrier reef). This horst and graben structure is the result of broadlydistributed transtensional deformation along the North American-Caribbean transformboundary south of the Yucatán peninsula.

Groundwater discharge along the east coast of the Yucatán peninsula isconcentrated along the north-northeast-trending faults [71, 75, 76]. Mixing of meteoricgroundwater and saline water at the coast results in vigorous karst dissolution,producing a network of fracture-solution caverns that extend well inland [82-84].Alignments of cenotes and large lakes occur above these fractures on the coastal plain.Where these solution fractures intersect the coast, cave collapse results in progressiveenlargement of coves and lagoons. Gradual coalescence of lagoons by wave erosion


results in the formation of broad, crescent-shaped beaches along the eastern Yucatáncoast.

3.2.3 Motagua Fault Zone Province

A series of major northeast-trending river valleys have developed across centralGuatemala along sinistral strike-slip faults of the North American-Caribbean plateboundary (Fig. 3.4). This major fault zone separates the Maya and Chortis blocks alonga broad arc extending from the Guatemalan volcanic highlands along the Pacificmargin to the Gulf of Honduras on the Caribbean coast [1-6]. The two most prominentfault-controlled valleys are those of the Motagua and Polochic rivers (Fig. 3.5), whichdrain the interior highlands and flow eastward to the Caribbean Sea [65]. Offshore, theMotagua and Polochic fault systems merge with the Swan Islands fault, forming atransform boundary along the northern margin of the Caribbean plate [5, 6].

3.2.3.1 Motagua and Polochic valleys

The lower Motagua and Polochic valleys (Fig. 3.5) contain extensive alluvial plainsthat grade eastward into a broad delta on the Gulf of Honduras [65]. A major structuraldepression within the lower Polochic valley impounds Lake Izabal (590 km2), thelargest inland body of water in northern Central America. Along both valleys, offsetQuaternary river terraces, gravel fans, and tributary drainages attest to active sinistralslip along plate boundary faults [12, 13]. The damaging M 7.5 Guatemala earthquakeof 1976 produced left-lateral surface rupture along 230 km of the Motagua fault withan average of 1.1 m of horizontal displacement and 0.3 m of vertical displacement [11].Quaternary slip rates of 0.4-1.9 cm/yr have been determined from offset terrace treadsin the Motagua Valley [13].

While the Motagua fault presently accommodates a significant portion of plate-boundary slip, the presence of offset late-Neogene fluvial landforms along the Polochicvalley to the north (Fig. 3.5) indicates that the Polochic fault may have been theprimary plate-boundary between 10 and 3 Ma [85]. Prior to this (20-10 Ma), plate-boundary deformation may have been localized further to the south along the Jocotán-Chamelcón fault zone in northwestern Honduras. This now inactive fault system hassince been fragmented by east-west extension along a series of north-trending riftvalleys (Fig. 3.5) along the southern margin of the plate-boundary deformation zone[86-88].

3.2.3.2 Las Minas and Chuacús Ranges

From the broad alluvial valleys on the Gulf of Honduras, the Motagua and Polochicfaults extend westward into the Guatemalan highlands (Fig. 3.5), where they curve tothe northwest along a series of deeply incised valleys. In this region, the plate-boundary faults delimit a set of mountain-block slivers that expose lower Paleozoicmetamorphic and igneous basement rocks [56]. The faulted structure of thesecrystalline basement rocks largely defines the morphology of sub-parallel ridges andvalleys within the Las Minas and Chuacús ranges (Fig. 3.5) of central Guatemala [65].To the west, the plate boundary faults extend into Chiapas, México, where deformationbecomes more diffuse within the Sierra Madre massif [89].


3.2.4 Chortis Volcanic Front Province

The active Chortis volcanic front (Figs. 3.4 and 4.1) encompasses two major,northwest-trending morphotectonic segments, the Guatemalan Cordillera, formed alongthe western margin of the Chortis highlands, and the Salvadoran Cordillera, developedalong the southern boundary faults of the Median Trough graben [1-4, 19-23, 90-92](also see Chapter 4). Both of these cordilleras consist of aligned clusters ofstratovolcanoes and calderas localized along transverse faults that cut the Chortisvolcanic front.

3.2.4.1 Guatemalan Cordillera

Central America's highest volcanoes, Tacaná and Tajumulco (>4000 m elevation),mark the northwestern end of the Chortis volcanic front just south of the Motagua faultzone (Figs. 3.5 and 4.1). In general, the Guatemalan volcanoes are aligned in clusterswhere north to northeast-trending faults intersect the range front of the Teritiaryvolcanic highlands [93]. These frontal stratovolcanoes are often flanked by large siliciccalderas [94]. For example, a major volcanic complex in western Guatemala (Figs. 3.5and 4.1) that includes Santa María, Santiaguito, Santo Tomas, and Cerro Quemadovolcanoes has developed along a transverse, northeast-trending lineament that extendsinto the back arc [95-97]. The Quezaltenango Valley north of these stratovolcanoesmay represent an extinct caldera. Similar transverse clusters of frontal stratovolcanoesflanked by back-arc calderas occur along the entire Chortis volcanic front. Othervolcanic complexes along the Guatemalan cordillera (Figs. 3.5 and 4.1) include the SanPedro-Atitlán-Tolimán cluster (flanked by Atitlán caldera), the Yepocapa-Acatenango-Fuego lineament (flanked by Barahona caldera), and the Agua-Pacaya pair (flanked byAmatitlán caldera).

Near Guatemala City (Figs. 3.5 and 4.1), the twin stratovolcanoes of Acatenangoand Fuego form spectacular cones that tower 2000 m above the interior highlands andover 3500 m above the adjacent Pacific coastal plain [98]. Nearby Agua volcano adds athird hulking peak to the horizon of the modern Guatemalan capital. In 1541, a majordebris flow descended from Agua’s summit to obliterate the colonial capital of CiudadVieja [99]. Throughout the Quaternary, such debris avalanches and lahars haveconspired with occasional ash flows to construct a broad landscape of debris apronsthat extend outward from the Guatemalan volcanoes and on to the Pacific coastal plainbelow (Figs. 3.5 and 4.1). The potential for similar events in the future represents asignificant geologic hazard for the people of Guatemala [100].

3.2.4.2 Salvadoran Cordillera

In El Salvador, the active volcanic front is aligned along the southern boundary faultsof the Median Trough (Fig. 3.5), an elongate structural basin at the northwestern end ofthe Nicaraguan Depression. Similar to Guatemala, the Salvadoran volcanoes arelocalized where tranverse faults intersect the trough [20-23].

In western El Salvador (Figs. 3.5 and 4.1), the Izalco and Santa Anastratovolcanoes occur within a large, fault-controlled volcanic complex that alsoincludes the Coatepeque caldera [101]. This volcanic cluster is the source of a series ofQuaternary andesitic ignimbrites that extend northward into the Median Trough. LatePleistocene edifice collapse at Santa Ana volcano unleashed a massive debrisavalanche that traveled over 50 km to the south into the Pacific Ocean [102]. Like


Guatemala, the populated lowlands of El Salvador face a significant threat from debrisavalanches and lahars generated by gravitational failure along the volcanic front [103-106].

San Salvador volcano, adjacent to the country’s capital city (Figs. 3.5 and 4.1),consists of multiple remnants of Quaternary eruptive centers, including a large centralcrater (El Boquerón), and several surrounding peaks [105-107]. The nearby Ilopangocaldera forms a large lake-filled basin created by a series of major collapse events thatgenerated widespread tephra deposits found throughout central El Salvador [108, 109].Southeast of Ilopango caldera (Figs. 3.5 and 4.1), San Vicente volcano consists of abroad composite mass of two overlapping stratocones [103]. A late Quaternary debrisavalanche and lahar deposit forms a swath of hummocky terrain that extends 25 km tothe southeast of the volcano, reaching the Lempa river on the coastal plain.

Near the southern end of the Chortis volcanic front (Figs. 3.5 and 4.1), San Miguelvolcano forms an imposing composite cone that rises over 2000 m above the lowlandcoastal plain near the Gulf of Fonseca [104, 110]. As one of the most active volcanoesin El Salvador, this prominent peak is surrounded by a youthful landscape consisting ofmultiple, historic lava flows. Unlike most of the Chortis volcanoes, San Miguel iscomposed predominantly of basalt and shows little evidence of prior explosiveeruptions or debris avalanches.

3.2.5 Chortis Fore Arc Province

The Pacific coastal plain of Guatemala and El Salvador (Fig. 3.4) consists of a low-relief bajada of coalescing alluvial fans that extend up to 70 km seaward of thevolcanic front [111] (also see Chapter 7). This broad alluvial plain is constructed ofvolcanic ejecta and clastic sediments delivered by a network of debris-choked riversthat drain the interior volcanic highlands. In general, coastal topography along theChortis fore arc (Fig. 3.5) is relatively subdued, with only minor localized faultingaffecting the Quaternary strata. This low-relief coastal morphology strongly contrastswith the tectonically active coastlines of the Chorotega fore arc in southern CentralAmerica (Fig. 3.4). In that region, active faulting and rapid uplift have produced abruptcoastal topography along the rugged coastlines of Costa Rica and Panama.

3.2.5.1 Guatemalan Coastal Plain

The Guatemalan coastal plain (Fig. 3.5) forms a prominent bulge (>200 km long, andup to 70 km wide) along Central America’s northern Pacific coastline. This extensivealluvial lowland encompasses a series of overlapping debris fans, consisting of thickQuaternary sequences of volcaniclastic sands, gravels, pumaceous ash, and lahardeposits [111]. These materials have been deposited and reworked along a sub-parallelnetwork of river channels that descend from the adjacent volcanic highlands. Thespectacular stratovolcanoes that tower more than 3500 m above the coastal plain to theeast represent sources for pyroclastic flows, long-run-out lahars, and massive debrisavalanches [100]. Confined within deeply incised drainages along the volcanic front,such flows are capable of traveling great distances to inundate the coastal plain below.


3.2.5.2 Salvadoran Coastal Plain

Although narrower than the Guatemalan coastal plain, the Pacific lowlands of ElSalvador (Fig. 3.5) share a similar geomorphology of coalescing alluvial fans fed bycoarse detritus from the volcanic highlands. In western El Salvador, the coastlineexhibits a prominent headland referred to as the Acajutla peninsula (Fig. 3.5). Thissmall promontory extends 7 km offshore to its southern tip at Punta Remedios. TheAcajutla peninsula is the product of a massive late Pleistocene debris avalanchegenerated by edifice collapse at Santa Ana volcano, 50 km to the north [102]. Thesubaerial deposit covers 390 km2 with an additional estimated component of 150 km2

lying offshore. A 10-km-wide swath of hummocky terrain extends from the Acajutlapeninsula up to the source of the debris avalanche on Santa Ana volcano.

Southeast of the Acajutla peninsula (Fig. 3.5), the coastal plain narrows and givesway to rocky headlands and cliffs cut into resistant Pliocene volcanic rocks of theBálsamo range. This rugged volcanic range intersects the coast in a series ofsouthwest-trending ridges separated by deeply incised linear canyons. To the east, theSalvadoran coastal plain widens again where a series of rivers that drain the volcanicfront have deposited broad alluvial fans. The most prominent geomorphic feature alongthis coastal segment is the massive delta of the Lempa river (Fig. 3.5). This major riverdrains a significant portion of the interior Chortis highlands, transporting its massivesediment load across the Median Trough to the Pacific coast. The resulting, low relief,alluvial coastline is characterized by a series of elongate barrier islands and spits thatenclose extensive lagoons.

3.2.6 Chortis Highlands Province

The majority of the Chortis block (Fig. 3.4) consists of a broad, dissected, highlandplateau that extends from western Guatemala, across Honduras and El Salvador, tonorthern Nicaragua [27, 52-55, 112-114]. This mountainous topography reacheselevations >1 km and extends up to 400 km behind the active volcanic front. TheChortis highlands can be divided into four geomorphic sub-regions (Fig. 3.4) [27, 114]:a high-altitude central massif with concordant erosion surfaces; a western rifted plateausouth of the Motagua-Polochic fault zone; an eastern zone of heavily dissectedmountains facing the Caribbean lowlands; and an area of east-west trending, fault-bounded valleys and ridges along the northern coast. The geomorphic contrast betweenthese subregions reflects variations in lithology, proximity to plate boundaries, and astrong east-west climatic gradient across the Chortis highlands.

3.2.6.1 Central Chortis Plateau

The core of the Chortis Highlands Province (Figs. 3.4 and 3.5) consists of atectonically stable massif of Paleozoic metamorphic basement rocks and an overlyingsequence of folded Cretaceous sediments. Concordant high-altitude erosion surfaces(700-1000 m elevation) form a relatively level plateau across this region [27]. Whilemajor rivers have incised deep canyons into bedrock, the plateau remains largely intactwith only limited dissection by tributary networks (Fig. 3.5). The central Chortisplateau (Fig. 3.4) is isolated from the Caribbean Sea and Pacific Ocean, receiving lessrainfall (1.0-1.5 m/yr) than the rifted highlands to the west (>2 m/yr) and the heavilydissected Caribbean slope to the east (>3 m/yr) [27].


Tomographic imaging of the subducting Cocos plate reveals a slab gap extendingthe length of the Chortis highlands [27]. Slab break-off during the Miocene andassociated upwelling of buoyant mantle beneath the Caribbean plate may have inducedepeirogenic uplift of the Chortis highlands [27]. This regional-scale uplift, beginning inthe Middle to Late Miocene, led to deep entrenchment of meandering rivers throughoutthe region. Vertical meander incision with no lateral migration implies that upliftproceeded without local faulting or tilting, as a regional event that affected the entireChortis highlands [27].

3.2.6.2 Western Rifted Highlands

South of the Motagua-Polochic fault zone (Figs. 3.4 and 3.5), the Chortis highlands ofsouthern Guatemala and western Honduras are cut by a discontinuous series of small,north-trending, flat-floored rift valleys of late Miocene to Quaternary age [86]. Earlyworkers referred to these valleys under the collective term "Honduras Depression" [19,112]. However, instead of a single continuous feature as this term implies, theselocalized, independent rift basins are scattered across a broad area of the westernhighlands (Fig. 3.5) stretching from the Guatemala City graben in the west, to the Sulaand Comayagua valleys of Honduras in the east. Rifting across the western Chortisblock is attributed to regional extension in response to eastward movement of theCaribbean plate south of the arcuate Motagua-Polochic fault zone [27, 86-88, 113,114]. In a similar fashion, anticlinal folding has occurred north of the Motagua-Polochic fault zone in response to shortening within the southern Maya block [87].

The southwestern margin of the Chortis Highlands Province (Fig. 3.5) overlies a 2km thick sequence of pyroclastic rocks produced during the middle Miocene ignimbriteflare-up along the Central American volcanic front [63]. These materials weredeposited across a pre-existing low-relief terrain formed on underlying basement rocks.This extensive ignimbrite sheet buried earlier drainage networks and reset thelandscape for the development of a new system of low-gradient meandering rivers [27].The rift basins of the western Chortis highlands cut these deposits, providing a well-constrained maximum age for rifting (middle Miocene). In many cases, the rift-bounding faults also disrupt entrenched river meanders formed during the regionaluplift event that post-dates the ignimbrites.

3.2.6.3 Eastern Dissected Highlands

The eastern Chortis highlands of Honduras and Nicaragua (Fig. 3.4 and 3.6) encompassa rugged mountain landscape that faces the Caribbean lowlands of the Mosquito Coast[27]. This region is highly dissected by drainage networks and is traversed by severalmajor trunk rivers that descend from the interior highlands onto the coastal plain.These include the high-discharge Patuca, Coco, and Matagalpa rivers (Fig. 3.6), whichtransport large volumes of coarse sediment eroded from the uplifted interior. Althoughthe eastern portion of the highlands overlies the same basement rocks as the centralplateau, a sharp climate gradient toward the Caribbean coast leads to significantlygreater rainfall in this area (more than double the annual precipitation). Deepweathering and intense erosion have consumed the highland plateau in this region,leaving a heavily dissected, lower-elevation terrain (average of <500 m) characterizedby steep ridges and deep intervening valleys.


3.2.6.4 Honduran Borderlands

The northern margin of the Chortis highlands (Figs. 3.4 and 3.6), east of the riftedplateau, consists of a zone of east-west trending fault-bounded basins and ranges,including the Aguan Valley, and the Nombre de Dios and La Esperanza ranges [114].Five major east-northeast-trending faults traverse this region, forming the boundariesof elongate mountain blocks. Offset river channels and other geomorphic indicatorssuggest that these faults accommodate left-lateral transtension along the northern flankof the Chortis highlands. Transtensional fault blocks in this area may represent theonshore extension of normal fault-bounded basins within the offshore Honduranborderlands south of Swan Island fault zone. The difference in orientation andkinematics between the north-south oriented rift valleys to the west (Fig. 3.5), and theeast-west oriented basins and ranges to the east (Fig. 3.6), is attributed to variations inthe divergence angle between the Caribbean plate motion vector and the plate-boundary fault zone [114].

Figure 3.6. Map of the physiographic provinces of northern Central America, showingsignificant geomorphic features of Nicaragua, and portions of Honduras, El Salvador, andCosta Rica. Digital elevation model derived from NASA Shuttle Radar Topography Mission(SRTM) image PIA03364.


3.2.7 Mosquito Coast Lowlands Province

The Mosquito Coast Lowlands Province (Fig. 3.4) consists of a broad, thicklyvegetated alluvial plain, up to 150 km wide, along the east-facing portion of CentralAmerica's Caribbean coast. With annual precipitation rates of 4-6 m/yr, these humidlowlands represent one of the wettest regions on Earth. The alluvial plains of theMosquito Coast formed during the late Cenozoic atop a coalescing mass of deltaic sandand gravel deposits derived from the eroded interior highlands to the west [115]. Thesedeposits may reach a thickness of up to 4500 m in some areas. Neogene uplift of theChortis Highlands (Fig. 3.4) resulted in the deep incision of river drainages, producinga pulse of coarse, clastic sedimentation across the Caribbean lowlands [115].Progradation of these deltaic materials across the shallow Nicaragua bank offshore hasproduced a low-relief, lobate shoreline (Fig. 3.6) characterized by extensive mangroveswamps, broad tidal lagoons, elongate barrier islands, and scattered coral reefs.

3.2.7.1 Northern Mosquitia

The promontory of Cabo Gracias a Díos (Fig. 3.6) at the Honduras-Nicaragua bordermarks the apex of a massive Pliocene-Pleistocene age delta [115]. This extensivegravel complex is composed of sediments derived from the Coco and Patuca riverwatersheds within the Chortis highlands. Pleistocene stream piracy within thehighlands shifted deposition to the north of Cabo Gracias as the Patuca River capturedflow from the paleo-Coco drainage [115]. Rising sea level during the Holocene hastransferred deposition from the distal fan offshore, to a series of smaller nearshoredeltas along the modern coast. The constant interplay between shifting river courses,fluctuating sea level, and migrating deltas has been instrumental throughout the lateCenozoic in shaping the broad alluvial plains of the Mosquito Coast.

3.2.7.2 Southern Mosquitia

Along the southern Mosquito Coast (Fig. 3.6), in eastern Nicaragua, a dense network oflow-gradient rivers feed into a series of extensive coastal wetlands and lagoons. Theselagoons are protected behind large barrier spits that generally extend southward fromthe mouths of major rivers. Along the southern Nicaraguan coast, the monotonous,low-relief alluvial plain is disrupted by occasional hills and coastal cliffs formed byoutcrops of Paleogene to Quaternary volcanic rocks [116]. The Azul volcanic field, inthe jungle-covered lowlands west of Pearl lagoon, consists of three well-definedHolocene cinder cones. Along the coast between Perlas and Monkey points (Fig. 3.6), aseries of prominent cliffs expose resistant basalt flows interbedded with Tertiaryvolcaniclastic sediments. Tertiary lavas also form the basement of the offshore CornIslands, where coastal cliffs up to 100 m high expose massive basalt flows.

3.2.8 Nicaraguan Depression Province

The Nicaraguan Depression (Fig. 3.4) is a ~50 km wide structural trough that extendsfor over 600 km along the length of the active volcanic front from El Salvador, throughNicaragua, to northern Costa Rica [25, 116]. This elongate basin is generallyinterpreted as a half-graben, bounded along its southwestern margin by northwest-striking transtensional faults. The basin is most pronounced in Nicaragua where itcontains Central America’s two largest lakes, Lake Nicaragua (Cocibolca) and Lake


Managua (Xolotlán). Beginning in the Pliocene, the Nicaraguan volcanic frontmigrated toward the Middle America Trench and trench-perpendicular extensionwithin the upper plate opened the Nicaraguan Depression. Rifting along the Nicaraguanvolcanic front may reflect a late Cenozoic decrease in the plate convergence rate alongthe Pacific margin and hinge rollback of the subducting Cocos plate slab.

3.2.8.1 Central Nicaraguan Depression

In central Nicaragua (Fig. 3.6), the floor of the Nicaraguan Depression lies only 50 mabove sea level. Here, the Mateares fault forms a prominent 900-m-high scarp alongthe basin’s southwestern margin. Alluvial and volcanic fill within the basin may be upto 2000 m thick along the base of the Mateares scarp, indicating nearly 3 km of dip slipsince the Pliocene [25]. The northeastern boundary of the Nicaraguan Depression isgenerally defined by the 500-m-high mountain front of the interior Chortis highlandsformed on a thick sequence of Tertiary volcanic rocks [116, 117].

Damaging earthquakes in Managua (Fig. 3.6) in 1931 and 1972 resulted fromshallow rupture on northeast-trending, left-lateral, oblique-slip faults within a pull-apart basin along the southern shore of Lake Managua [118, 119]. These faults may bepart of a regional system of northeast-trending sinistral faults that accommodate dextralshear across the Nicaraguan depression between a forearc sliver at the coast and theinterior highlands to the east [26]. These cross-arc transtensional faults exert a first-order control on the shoreline morphology of Lakes Managua and Nicaragua. Theyalso may influence the spacing and orientation of volcanic centers along theNicaraguan volcanic front [22].

3.2.8.2 Median Trough, El Salvador

From northern Nicaragua, the Nicaraguan Depression extends across the Gulf ofFonseca (Fig. 3.6), where it continues along a more westerly trend as the MedianTrough of El Salvador (Fig. 3.5). In El Salvador, the topographic expression of thetrough is more subdued than Nicaragua, yet its general structure and stratigraphy arequite similar. As in Nicaragua, the volcanic front of El Salvador is localized along thesouthwestern margin of the trough where transverse faults intersect the basin.

Focal mechanisms for shallow upper-plate earthquakes along the southernboundary faults of the Median Trough indicate that these structures are predominantlyright-lateral strike-slip faults with minor components of extension [120-122]. Thesefaults have produced multiple damaging earthquakes throughout El Salvador’srecorded history [123]. The geomorphic expression of the Median Trough dies out nearthe Guatemalan border (Fig. 3.5) where transtensional deformation gives way to puredextral slip along the northwest striking Jalpatagua fault [124]. This fault forms amajor topographic scarp that cuts through the southern Guatemalan volcanic frontoffseting Neogene deposits [24].

3.2.8.3 Los Guatusos and San Carlos Lowlands, Costa Rica

South of Lake Nicaragua (Fig. 3.4), the Nicaraguan Depression extends into the LosGuatusos and San Carlos lowlands of the northern Costa Rican back arc (Fig. 3.7).Here, the depression is buried by a thick sequence of Quaternary alluvium and volcanicdebris shed from Costa Rica’s Guanacaste and Central volcanic cordilleras. Riversdescending from the volcanic front flow northward across the basin to join the San


Juan River (Fig. 3.7). This major river, which defines the border between Nicaraguaand Costa Rica, flows from Lake Nicaragua eastward to the Caribbean Sea. Thesouthern end of the Nicaraguan Depression in this area coincides with a rapid transitionfrom extensional to compressional tectonics within the back arc of Costa Rica’sCentral volcanic cordillera [34]. This transition is related to an abrupt change in thethickness and dip of the subducting Cocos plate beneath central Costa Rica.

3.2.9 Nicaraguan Volcanic Front Province

The Quaternary volcanic front of Nicaragua (Figs. 3.4 and 4.1) has developed along thefloor of the Nicaraguan depression, with most volcanic centers located along its fault-controlled southwestern margin [19-23, 116] (also see Chapter 4). Beginning at thesolitary Cosigüina volcano on the Gulf of Fonseca (Figs. 3.6 and 4.1), the Nicaraguanvolcanic front extends southward along the Los Marabios Cordillera to the spectacularstratocone of Momotombo on the shore of Lake Managua. The volcanic chain thencontinues southward, past the Apoyeque caldera, to the massive Las Sierras ignimbriteshield and the calderas of Masaya and Apoyo. The Cocibolca Cordillera (Figs. 3.6 and4.1), at the southern end of the volcanic front, extends from Mombacho volcano on thenorthern shore of Lake Nicaragua to the twin stratovolcanoes of Concepción andMadera on Ometepe island.

3.2.9.1 Cosigüina Peninsula

The Nicaraguan volcanic front begins at Cosigüina volcano (Figs. 3.6 and 4.1), on thesouthern shore of the Gulf of Fonseca [116]. This solitary volcano stands isolated on awide promontory that extends outward into the southern gulf from Nicaragua’snorthern coastal plain. Cosigüina consists of a broad, low-elevation, composite cone(872 m) with a pronounced ancient caldera rim along its northern slope, a young 300-m-high summit cone, and a 2-km-wide, prehistoric, lake-filled summit caldera. Thismassive composite volcano generated a powerful, explosive eruption in 1835 that sentpyroclastic flows into the Gulf of Fonseca (Fig. 3.6), and produced ash fall throughoutCentral America and as far away as Mexico and Jamaica [125]. This event is nowrecognized as one of the Western Hemisphere’s most powerful historic eruptions.

3.2.9.2 Los Marabios Cordillera

The Los Marabios range (Fig. 3.6), southeast of Cosigüina, consists of a denselyspaced series of composite volcanoes that form a prominent divide between thenorthern Pacific coastal plain and the Nicaraguan depression [116]. This alignment ofclustered volcanic vents includes the major stratovolcanoes, San Cristobal, Casita,Telica, Rota, Las Pilas, and Momotombo (Fig. 4.1). The rapidly growing cinder coneof Cerro Negro represents the newest addition to this range, having erupted asignificant volume of lava and pyroclastic material since the mid-1800s [126, 127].Momotombo volcano, at the southern end of the Los Marabios range, forms aspectacular 1300-m-high Holocene stratocone that towers above the northern shore ofLake Managua (Figs. 3.6 and 4.1).

During Hurricane Mitch in 1998, a rain-triggered landslide (1.6 million m3) fromthe summit of Casita volcano (Figs. 3.6 and 4.1) unleashed a devastating lahar (2-4million m3) that inundated several villages on the volcano’s Pacific flank, killing morethan 2500 people [128]. This volcano has been identified as a potential site for a future


sector collapse related to hydrothermal weakening of its summit edifice [129]. Steeptopography, energetic volcanism, and an aggressive tropical climate have conspiredthroughout the Quaternary to induce repeated gravitational failure along theNicaraguan cordillera. The resulting lahars in combination with pyroclastic flows haveconstructed a broad coastal debris apron that extends up to 30 km seaward of thevolcanic front.

3.2.9.3 Apoyeque and Las Sierras Shields

Between Lake Managua and Lake Nicaragua (Figs. 3.6 and 4.1), a series of calderasand explosion pits occur along the trace of the Mateares fault zone at the southwesternedge of the Nicaraguan depression [116]. The Apoyeque caldera complex, at thenorthern end of this chain, sits within a broad ignimbrite shield that forms the Chiltepepeninsula on the western shore of Lake Managua (Fig. 3.6). This low-lying (500 melevation) volcanic complex includes two large, lake-filled calderas that are the sourceof thick Quaternary pyroclastic deposits mantling the surrounding landscape. TheNejapa-Miraflores alignment, west of the city of Managua (Figs. 3.6 and 4.1), featuresa series of explosion pits, fissure vents, and cinder cones along a north-south trend thatmarks a right-step along the Nicaraguan volcanic front.

Southeast of Managua, the lake-filled calderas of Masaya and Apoyo occupy aprominent upland along the escarpment of the Mateares fault (Figs. 3.6 and 4.1). Thesetwo calderas formed during highly explosive pyroclastic eruptions in the lateQuaternary [130-133]. Subsequent activity along the southern margin of Masayacaldera, has constructed the twin volcanoes of Nindirí and Masaya, both sources ofhistoric lava flows.

The Masaya and Apoyo calderas (Figs. 3.6 and 4.1) both belong to the massiveNeogene-Quaternary Las Sierras volcanic shield (180 km3), located between LakesNicaragua and Managua [130-133]. The Las Sierras shield (Figs. 3.6 and 4.1) forms ananomalous, crescent-shaped highland (>900 m elevation) along the central Nicaraguanvolcanic front. This elongate massif consists of a broad, seaward-facing apron ofignimbrite deposits of Plio-Pleistocene age, produced primarily by unusually explosiveeruptions of mafic magma from the Masaya caldera.

3.2.9.4 Cocibolca Cordillera

The Cocibolca Cordillera (Fig. 3.6) of southern Nicaragua extends southeastward fromthe Las Sierras Shield to Ometepe island in Lake Nicaragua. This cordillera consists ofa widely spaced chain of four major volcanoes that form a series of promontories andislands within Lake Nicaragua. Standing on the northwestern shore of the lake,Mombacho volcano (Figs. 3.6 and 4.1) is a large stratocone with a jagged profile,scarred by multiple episodes of edifice collapse during the late Quaternary. Theseevents unleashed debris avalanches and lahars that form extensive hummocky depositsin the lowlands surrounding the mountain [134]. These deposits include an adjacentpromontory and group of small islands that extend out into the lake.

Zapatera volcano, southeast of Mombacho, consists of a low-lying volcanic shieldthat forms a broad island near the northwestern shore of Lake Nicaragua (Figs. 3.6 and4.1). This volcano features a 2-km-wide summit caldera with a 300-m-high central lavadome. To the southeast of Zapatera, the spectacular twin stratovolcanoes ofConcepción and Madera form Ometepe island, located in central Lake Nicaragua (Figs.3.6 and 4.1). Towering >1600 m above the lake, Concepción is one of Nicaragua’s


most active volcanoes, with 25 eruptions recorded in the last 120 years [135]. Recentlava flows and lahars continue to expand the shoreline of Ometepe island.

3.2.10 Sandino Fore Arc Province

The Sandino fore arc province (Fig. 3.4) extends along the entire Pacific coast ofNicaragua, from Punta Cosigüina on the Gulf of Fonseca, to Costa Rica's PuntaDescartes just north of the Santa Elena peninsula. This narrow coastal strip lies west ofthe Nicaraguan depression and volcanic front. It includes a northern low-relief coastalplain constructed of volcanic debris shed from the Los Marabios Cordillera (Fig. 3.6),and a southern cliff-lined coast where Cretaceous-Neogene marine sedimentary rocksof the Sandino fore-arc basin extend on land in a series of margin-parallel folds [25,136-139] (also see Chapter 7).

3.2.10.1 Northern Nicaraguan Coast

Nicaragua's northern Pacific coast, between the Gulf of Fonseca and Puerto Sandino(Fig. 3.6) consists of a low-relief alluvial plain developed on volcaniclastic debris shedfrom active volcanoes of the Los Marabios Cordillera. The 1998 Hurricane Mitchdebris flow from Casita volcano ran out on to the coastal plain for nearly 10 km,producing sediment laden flooding that extended even further along local streamchannels. Similar to the broad volcanic debris aprons of the Chortis fore arc to thenorth, the northern Nicaraguan coastal plain faces a significant hazard from highlymobile volcanic debris flows. The stream networks draining the Los Marabios range,deliver an abundant supply of volcaniclastic sediment to the coast. This material hasbeen reworked along this low-relief coastline, forming a system of large estuaries, deltaplains, and barrier beaches.

3.2.10.2 Southern Nicaraguan Coast

In sharp contrast to the northern coastline, Nicaragua's southern Pacific coast (south ofPuerto Sandino) is characterized by a rugged morphology of rocky headlands, seacliffs, and pocket bays (Fig. 3.6). The resistant grain of this coastline is controlled bystructures within Cretaceous-Paleogene marine sedimentary rocks of the Sandino Basin[25, 136-139]. These units outcrop within a series of margin-parallel folds, whichintersect the coastline at oblique angles, resulting in variable resistance to marineerosion. Along the central Nicaraguan coast, where these folds expose the Neogeneunits within the upper section of the Sandino Basin, a basin-and-range topography hasdeveloped along the coastal plain.

A notable geomorphic feature along this coastline is a 10-km-long MiddleMiocene basaltic dike [138] that armors the coastal bluff near the town of El Transito(Fig. 3.6). This feature extends offshore at El Transito Bay, forming a rock curtain thatconstricts the bay entrance. This resistant barrier contributed to unusually high waverun-up and a large death toll at this site during the 1992 Nicaraguan tsunami.

3.2.10.3 Las Sierras Shield

Between Puerto Sandino and Las Salinas (Fig. 3.6), Quaternary pyroclastic rocks of theLas Sierras volcanic complex extend seaward to the Pacific coast forming a prominentconvex bulge in the Nicaraguan coastline. A network of deeply incised barrancas is


developing on the Pacific slope of the range due to the rapid headward erosion of steepdrainages into these weak volcanic rocks. At the coast, a late Pleistocene marineterrace, referred to as the La Boquita surface [140], is cut across the Las Sierras ashflow tuffs and the steeply dipping Cretaceous-Neogene marine sediments of theSandino basin.

The Las Sierras tuffs have been correlated with a distinctive ash horizon inoffshore drill cores dated stratigraphically at 135 ka [133]. This age suggests that theLa Boquita marine terrace may have formed during the last interstadial sea level highstand at 125 ka [140]. This terrace occurs along the coast at elevations of 17-22 mattesting to active uplift at rates near 0.1 m/k.y. The La Boquita terrace occurs onlyalong the coastal segment adjacent to the Las Sierras massif (Fig. 3.6), suggesting thatcoastal uplift may be related to thermal expansion or erosion-driven isostatic reboundof the Las Sierras shield.

3.2.10.4 Punta Descartes, Costa Rica

At the southern end of the Sandino fore arc province (Fig. 3.6), offshore folds intersectthe coastline of northwestern Costa Rica forming the Punta Descartes (an anticline) andadjacent bays (synclines). On Punta Descartes, the west-northwest grain of the ridgesand valleys is controlled by a system of parasitic folds on a broader anticline. ElevatedHolocene shore platforms, beach ridges, and stream terraces record active coastalemergence on the Descartes headland [141]. Valley-fill terraces within coastalembayments extend several kilometers inland reaching 15-20 m elevation. Streamsdraining to the Bahía de Salinas are incised into these deposits exposing upliftedshallow bay to intertidal muds overlain by beach ridge sands and fluvial gravels.Radiocarbon ages for Holocene deposits indicate uplift rates of 2.0-3.5 m/ k.y. [141].

3.3 GEOMORPHOLOGY OF SOUTHERN CENTRAL AMERICA

In contrast to the continental Chortis block to the north (Fig. 3.1), most of southernCentral America consists of a Neogene-Quaternary volcanic belt that overliesMesozoic oceanic basement of Caribbean plate origin [1-10, 142]. This region, referredto as the Chorotega block (Fig. 3.1), includes all of Costa Rica and western Panama.The boundary between the Chorotega block and the Chortis block to the north isdefined by a major fault lineament extending from Costa Rica's Santa Elena peninsula(Fig. 3.7), eastward to the Hess Escarpment in the Caribbean Sea (Fig. 3.1). Thegeomorphology of the Chortega block reflects a dynamic history of Cenozoicvolcanism and upper-plate deformation influenced by complex tectonics along thesouthern Middle America Trench [7-10]. The eastern limit of the Chorotega block isdefined by a basement suture at the Panama Canal Zone that separates it from theChocó block beneath eastern Panama [4-6, 142]. The Chocó block (Fig. 3.1) consistsprimarily of Mesozoic igneous basement and overlying Tertiary sediments that extendalong Panama’s Darién isthmus and into the northwestern Colombian cordillera.

The Chorotega and Chocó blocks are situated within a region of complex tectonicsbetween four major converging plates: Caribbean, South America, Cocos, and Nazca(Fig. 3.1) [4-6]. Active collision between these plates has fragmented their margins intoa system of fault-bounded microplates that accommodate diffuse regional deformation[142-145] The Panama microplate (or Panama block) extends from the margin ofSouth America to central Costa Rica and includes portions of the Chocó and Chorotega


basement terranes. This rapidly deforming fragment of Caribbean crust is thusting overthe back arc in response to flat subduction of thickened seafloor (Cocos Ridge) in thewest, and collision with the South American craton to the east [34, 38-40].

Fold and thrust belts offshore of both northern and southern Panama (North andSouth Panama deformed belts) accommodate active convergence with the Caribbeanand Nazca plates (Fig. 3.1) [39-41, 146, 147]. Thrust faults in western Colombia(Atrato-Urubá suture zone) [148] and a broad zone of strike-slip faulting in easternPanama (East Panama deformed belt) [42] absorb the collision between the Panamablock and South America. The western boundary of the Panama block consists of adiffuse transpressional fault zone (Central Costa Rica deformed belt) that traversesCosta Rica from the Caribbean to the Pacific margin [34].

Central Costa Rica has long been recognized as a tectonic segment boundary alongthe Middle America convergent margin [20, 149]. An abrupt transition from the steepsubduction of smooth Cocos plate seafloor in the northwest (East Pacific Rise origin),to the flat subduction of rough, hotspot-thickened seafloor in the southeast (GalapagosSpreading Center origin) occurs along the Middle America Trench offshore of centralCosta Rica [150-154]. This transition coincides with pronounced changes in subductionzone seismicity [149, 150], arc volcanism [20-23, 36], and upper-platemorphotectonics [28-37].

The thickened seafloor offshore of southern Costa Rica includes the Cocos Ridgeand adjacent seamount domain (Fig. 3.1), products of hotspot volcanism along theGalapagos Spreading Center [151-158]. As this rough, sediment-poor seafloor entersthe subduction zone, the overriding plate margin experiences pronounced subductionerosion [159-161]. Flat subduction of this hotspot-thickened crust also drivestranspressional faulting along a broad deformation front (Central Costa Rica deformedbelt) that is propagating from the fore arc into the interior of the upper plate [34]. LateCenozoic slab flattening led to retreat of the magmatic front away from the MiddleAmerica Trench in central Costa Rica [36], and total extinction of volcanism directlyinboard of the Cocos Ridge in southern Costa Rica [162, 163]. Collision of the CocosRidge with the southern Costa Rican margin generates pronounced uplift and upper-plate shortening from the fore arc into the back-arc basin [28-37]. Along the centralCosta Rican margin, subducting seamounts on the northwest ridge flank erode the outertrench slope [151, 152] and produce corregated uplift of fore-arc fault blocks at thecoast [33-35].

3.3.1 Chorotega Volcanic Front Province

The volcanic front of southern Central America (Figs. 3.4 and 4.1) is segmented into aseries of distinct cordilleras that have evolved in response to variations in thegeometry, tectonics and geochemistry of subduction along the Middle America Trench[19-23, 36, 162-170] (also see Chapter 4). In Costa Rica, a dynamic history ofCenozoic tectonics generated a complex volcanic belt that includes the Guanacaste,Tilarán, Aguacate, Central, and Talamanca cordilleras (Figs. 3.7 and 4.1). In Panama,Quaternary volcanism has been limited to the Central Cordillera west of the CanalZone (Figs. 3.7 and 4.1). Spatial and temporal variations in the subduction system haveled to sharp contrasts in magma chemistry and eruption style along the length of theChorotega volcanic front [21-23]. Changes in slab thickness and dip have instigatedepisodes of volcanic front migration and rotation, resulting in a complex morphologyof overlapping cordilleras and intervening basins [36, 170]. Each of the volcanic


cordilleras of Costa Rica and Panama exhibits a unique geomorphology and geologichistory [171-177].

3.3.1.1 Guanacaste Cordillera

The northern segment of the Chorotega volcanic front (Figs. 3.7 and 4.1) consists of aQuaternary chain of shield-like stratovolcanos known as the Guanacaste Cordillera[169]. The four primary volcanoes of this chain, Orosí-Cacao, Rincón de la Vieja,Miravalles, and Tenorio, are constructed of coalescing lava flows and pyroclasticmaterial emitted from multiple vents. Unlike the Central Cordillera to the south, theGuanacaste volcanoes each form distinct mountains that rise sharply above asurrounding low-relief landscape. The gaps between these volcanoes allow for easypassage of the Trade Winds between the Caribbean and Pacific basins, resulting in anexceptionally dry climate along Guanacaste's Pacific coast.

Figure 3.7. Map of the physiographic provinces of southern Central America, showingsignificant geomorphic features of Costa Rica, and portions of Nicaragua and Panama.Digital elevation model derived from NASA Shuttle Radar Topography Mission (SRTM)image PIA03364.


Two remnant calderas occur along the Guanacaste Cordillera, and a broadignimbrite plateau (2000 km2) extends seaward from the base of the chain. Constructedof silicic tuffs emitted from pre-cordillera vents, the Guanacaste ignimbrites form agently undulating plain (Fig. 3.7) that ends in an abrupt 100-150 m high escarpmentnear the modern Pacific coast. Rivers draining the cordillera have incised deepbarrancas into the plateau.

The southern outpost of the Guanacaste cordillera is Arenal volcano, a relativelysmall (15 km3) and highly active Holocene strato-cone [178-180]. Located behind theeroded massif of the extinct Tilarán Cordillera (Figs. 3.7 and 4.1), this steep-sided cone(>1200 m high) rises abruptly above a structural trough in which it has formed. Adeadly pyroclastic blast in 1968 decimated 15-km2 of rainforest on the volcano’swestern flank. Since this event, Arenal has remained in constant activity, unleashingoccasional ash and lava flows that continue to alter the surrounding landscape.

3.3.1.2 Tilarán and Aguacate Cordilleras

The extinct Tilarán and Aguacate ranges (Fig. 3.7) consist of heavily dissectedremnants of stratovolcanoes and calderas composed of Neogene-Quaternary basaltic toandesitic lavas, breccias, tuffs, and lahar deposits [169, 181, 182]. Hydrothermalalteration and deep tropical weathering have destabilized the steep slopes of theseranges, resulting in pervasive landsliding.

Throughout the central Aguacate range (Fig. 3.7), deeply incised linear canyonshave developed along active, northwest-and-northeast-trending faults of the CentralCosta Rica deformed belt [34-36]. The Río Grande de Tárcoles cuts a deep gorgethrough the central Aguacate range, connecting rivers of the Central Valley basin (Fig.3.7) with the Pacific coastal plain to the southwest. Along the Tárcoles gorge and manyof its tributary canyons, resistant ignimbrite deposits form level benches and isolatedhilltops 50-100 m above the valley floor. Bedrock incision rates based on lateQuaternary isotopic ages for the ignimbrites [40], range from 0.1 to 0.5 mm/yr. Severalextinct, bowl-shaped calderas (e.g., Palmares and Atenas) centered along the northernflank of the Aguacate range may have generated silicic ignimbrites [36, 182].

3.3.1.3 Central Cordillera

The composite shield volcanoes of Costa Rica’s Central Cordillera, Platanar, Poás,Barva, Irazú, and Turrialba (Figs. 3.7 and 4.1), form an imposing northwest-trendingmountain range, located northeast of the extinct Aguacate Cordillera [169, 181]. Withpeak elevations ranging from 2000-3400 m, these massive, broad-shouldered volcanoestower above the adjacent low-relief landscape of Costa Rica’s densely populatedCentral Valley (Fig. 3.7). These are the largest volcanoes, in both area and volume, ofthe entire Central American volcanic front. Their summits exhibit wide calderas withmultiple craters and transverse alignments of parasitic cones [169]. A strong climaticgradient across the range results in greater weathering and erosion, deeper streamincision, and more frequent landsliding on the humid Caribbean slope.

Along both flanks of the Central Cordillera (Fig. 3.7), gravitational spreading ofthe volcanic massif generates prominent fault-propagation-fold scarps along the base ofthe mountains [183]. These structures offset a sequence of Quaternary lava flows, ashflow tuffs, lahar deposits, and tephra that drape across the volcanic slopes. Streams thatdrain the steep slopes of the Central Cordillera have cut a radial network of deeplyincised barrancas into the Quaternary volcanic sequence [36]. These canyons serve as


conduits that feed pyroclastic flows, lavas, and lahars toward the adjacent lowlands ofboth the Central Valley and Caribbean coastal plain.

3.3.1.4 Central Valley

The elongate Central Valley of Costa Rica (Fig. 3.7) consists of an east-west trendingbasin (600-1200 m elevation) situated between the active volcanoes of the CentralCordillera and the eroding volcanic remnants of the Aguacate range. Throughout theQuaternary, this highland basin filled with a thick accumulation (>1 km) of andesitic todacitic lavas, pyroclastic rocks, lahar deposits, and lacustrine sediments [36, 181, 182].

The floor of the Central Valley (Fig. 3.7) consists of a low-relief upland surfacewith deeply incised river canyons cut into the underlying Quaternary volcanic sequence[35, 36]. These abrupt canyons outline a pervasive network of seismically active,northeast and northwest trending transcurrent faults that offset Neogene-Quaternaryrock units throughout the Central Valley and adjacent volcanic ranges [34-36, 184-186]. In addition to controlling regional drainage patterns, these structures exhibitabundant geomorphic features associated with active faulting, including abrupt scarps,compression ridges, sag ponds, and perennial springs.

The active faults of the Central Valley mark the leading edge of the Central CostaRica deformed belt [34], a diffuse deformation front that is propagating into the upperplate in response to shallow subduction of hotspot-thickened seafloor beneath centralCosta Rica. This broad fault zone extends across the central Costa Rican volcanic front(Fig. 3.1), linking the North Panama deformed belt on the Caribbean coast, with theMiddle America Trench on the Pacific margin. Throughout Costa Rican history, thesefaults have produced damaging, shallow-focus earthquakes, including the 1910 eventthat destroyed the colonial capital Cartago [187, 188]. Periods of heightened seismicactivity along the Central Costa Rica deformed belt have been documented inassociation with large thrust earthquakes centered on both the Pacific and Caribbeanmargins [189-192]. This active deformation zone marks the western boundary of thePanama microplate [30, 34].

The fault-controlled drainage networks of the Central Valley feed into the Tárcolesgorge, a prominent canyon cut through the eroded highlands of the Aguacate Cordillera[35, 36]. This precipitous gorge provides a link between Central Valley rivers and thePacific coastal plain below. During the Middle-Late Quaternary, the Tárcoles riverbreached the Aguacate drainage divide, leading to progressive capture and rerouting ofCentral Valley drainage networks toward the Pacific slope [35, 36].

The geomorphic evolution of Costa Rica’s Central Valley and surroundingvolcanic cordilleras (Fig. 3.7) was profoundly affected by changes in Cocos platesubduction during the late Cenozoic. The propagation of irregular, hotspot-thickenedseafloor down the subduction zone led to a shallowing of the subducting slab and aprogressive retreat of the volcanic front from the Aguacate Cordillera to the CentralCordillera [36]. This northeastward expansion of the volcanic front resulted in theformation of the Central Valley basin and a shift in the location of the Pacific-Caribbean drainage divide. Linkage of the Central Valley drainage with the Pacificslope established a pathway for spillover of Quaternary pyroclastic flows and laharsonto the Orotina debris fan at the coast. Retreat of the volcanic front occurred onlyonshore of moderately thickened crust of the Cocos plate seamount domain, leading toformation of the Central Cordillera. Directly inboard of the subducting Cocos Ridge


(Fig. 3.1) to the south, volcanism shut off and rapid uplift maintains the drainage dividealong the crest of the Talamanca Cordillera [36].

3.3.1.5 Talamanca Cordillera

Southeast of the Central Valley, the extinct Talamanca Cordillera (Fig. 3.7)corresponds with a 175 km volcanic gap that extends into western Panama [20-23].These rugged mountains represent the only area of southern Central America above4000 m in elevation. Similar in age to the Aguacate Cordillera (Fig. 3.7), theTalamanca range is composed of a suite of Neogene-Quaternary intrusive (principallygranodiorites) and extrusive rocks (andesites) [162, 163, 166-170]. In contrast to theAguacate range however, rapid Quaternary uplift and unroofing caused by CocosRidge subduction has stripped off large volumes of extrusive rock from the Talamancarange exposing the intrusive core. The highest peaks of this range were glaciatedduring the Pleistocene, leaving striking examples of moraines, tarns, and striatedbedrock on mountaintops overlooking thickly vegetated tropical lowlands [46, 193-198] (also see Chapter 6).

3.3.1.6 Central Cordillera, Panama

The Talamanca volcanic gap of southern Costa Rica ends at the dormant Barú volcano(Fig. 3.7) at the western end of Panama's Central Cordillera [163-166]. This imposingstratovolcano towers above the Pacific coastal plain of western Panama. It features a 6-km-wide summit caldera that was breached on its western margin by a majorQuaternary debris-avalanche. Lava flows and lahar deposits on the southwestern flankof Barú volcano are affected by incipient faults of the Terraba thrust belt, which ispropagating southeastward along the margin in the wake of the migrating Panamatriple junction [199-200]. In addition to Barú, the Panamanian cordillera also featuresseveral other Quaternary volcanoes, including La Yeguada and El Valle [164-166].Isotopic ages on lava flows, in conjunction with youthful geomorphic features provideevidence for recent activity at these volcanoes.

3.3.2 Chorotega Fore Arc Province

The Chorotega fore arc (Fig. 3.4) extends along the Pacific coast from Costa Rica'sSanta Elena peninsula in the north (Fig. 3.7), to the Gulf of Panama in the south (Fig.3.8). This rapidly deforming convergent margin coast is characterized by abrupttopography [201] and a series of peninsulas and headlands that expose late Mesozoicoceanic basement rocks [202-205] and overlying pelagic to shallow marine sedimentsof Paleogene-Quaternary age [206-208]. These peninsulas and promontories, located inboth Costa Rica and Panama, include Santa Elena, Nicoya, Herradura, Quepos, Osa,Burica, Soná, and Azuero (Figs. 3.7 and 3.8). Like the adjacent volcanic front, theChorotega fore arc is highly segmented with sharp contrasts in structure and coastalmorphology linked to variations in the subducting Cocos and Nazca plates offshore[28-37] (also see Chapter 7).

Along the Chorotega fore arc (Fig. 3.4), the subducting Cocos and Nazca platesexhibit dramatic variations in thickness, roughness, dip, and convergence angle thatcoincide with sharp contrasts in the style of upper plate deformation. Overall, thesubducting seafloor has minimal sediment cover and, in many areas (e.g., offshoresouthern Costa Rica), is anomalously thick with substantial morphologic roughness


[151, 152]. As a result, subduction erosion produces scarring and subsidence alongmuch of the offshore fore-arc [159-161]. In contrast, however, the subaerial inner forearc along the coast has experienced uplift in many areas, consistent with out-of-sequence thrusting or underplating beneath the margin [33, 35, 37].

The overall Quaternary deformation pattern along the Costa Rican coast stronglyreflects the offshore bathymetry associated with the subducting Cocos Ridge (Fig. 3.1)and seamount domain [29, 33, 35]. In general, Quaternary uplift rates derived fromradiometrically-dated marine and fluvial terraces decrease parallel to the margin,moving northwestward away from the subducting Cocos Ridge. These rates range froma maximum of 6-7 m/k.y. astride the ridge in the south, to background rates of <1m/k.y. above smooth subducting crust in the north. This overall long-wavelength trendreflects a decrease in upward flexure of the overriding crust as the thickness of thesubducting plate diminishes away from the Cocos Ridge axis.

Shorter-wavelength roughness related to seamounts superimposes local variabilityon this background uplift pattern [33]. Along the central Costa Rican Pacific coast (Fig.3.7), uplift rates vary sharply across a series of margin-perpendicular faults thatsegment the coastal fore arc into discrete fault blocks [33-35]. While the central CostaRican fore arc exhibits strong segmentation across fault-bounded blocks, the fore arcregion south of Quepos (Fig. 3.7) deforms more uniformly by rapid shortening acrossthe Fila Costeña fold and thrust belt [37, 109, 200]

3.3.2.1 Santa Elena Peninsula and Papagayo Gulf

The northern Chorotega fore arc (Fig. 3.7) meets the Sandino fore arc of the Chortisblock along an abrupt boundary defined by the Murcielago fault zone on the SantaElena peninsula [141, 209]. This structure follows a linear E-W trending valley thatbisects the peninsula along its northern coast forming several prominent bays. TheMurcielago fault zone marks a sharp change in geology and geomorphology from thenorthwest-dipping hogback ridges formed on late Cretaceous Sandino Basin marinesediments in the north, to the peninsula's rugged interior highlands in the south formedon the Jurassic-Cretaceous ultra-mafic basement of the Santa Elena nappe [162, 202].

The morphology of the rugged shoreline extending southward from the SantaElena peninsula to the northen Nicoya peninsula (Fig. 3.7) is determined primarily byresistant fault-bounded basement outcrops and steep cliffs formed of the overlyingGuanacaste ignimbrites. At Punta Mala along the Bahía Culebra (Fig. 3.7), a survivingfragment of the ignimbrite mesa maintains steep columnar-jointed cliffs rising from afoundation of basement rocks. Coastal embayments here preserve a Quaternary fillwedge of shallow marine sediments that record active uplift [141].

3.3.2.2 Nicoya Peninsula

The Nicoya peninsula (Fig. 3.7) lies along an emergent segment of the Chorotega forearc south of the Santa Elena peninsula. Separated from the Costa Rican mainland bythe broad Gulf of Nicoya and Tempisque river basin, this large, rectangular peninsulacovers over 4800 km2 of the outer forearc [171-172]. The Nicoya peninsula’s ruggedPacific coastline features abundant pocket bays and sandy beaches, bounded by steep,rocky headlands. Uplifted marine terraces and paleo-beach deposits indicate activeemergence throughout the late Quaternary [210-215]. In contrast, the peninsula’s gulfcoast follows a low-relief alluvial plain with extensive mangrove estuaries. The coastal


piedmont along all sides of the Nicoya peninsula rises steeply into a mountainousinterior highland that reaches over 900 m in elevation.

Basement rocks exposed on the Nicoya peninsula consist of the Cretaceous-earlyTertiary Nicoya Complex, an intensely deformed oceanic sequence of pillow basalts,mafic intrusive rocks, and pelagic sediments [162, 202-205]. Along the margins of thepeninsula, a sequence of upper Cretaceous to Quaternary marine sediments drapesunconformably across the Nicoya Complex basement [206-208]. These sedimentsinclude Cretaceous-Paleocene turbidites, Eocene deep-water carbonates, Miocene shelfclastics and a shallowing upward sequence of Plio-Pleistocene shelf sandstones andconglomerates.

Located only 60 km inboard of the Middle America Trench (Fig. 3.1), the Nicoyapeninsula lies directly above the seismogenic zone [211, 213-215]. This uniquelocation results in pronounced seismic cycle deformation, which is readily observedalong the peninsula’s shorelines. Coseismic uplift of >1m affected the Nicoyapeninsula’s central Pacific coastline during the M 7.7 subduction earthquake of 1950[211]. Since that event, interseismic strain has led to notable subsidence along thissame shoreline. Seismicity and GPS data indicate that the peninsula occupies a high-potential seismic gap that is accumulating strain in advance of the next event [216,217].

A sequence of Quaternary marine and fluvial terraces on the Nicoya peninsula(Fig. 3.7) provide a record of continuing uplift along this segment of the Chorotegafore arc [35, 210-215]. High-elevation remnants of a Pliocene-Pleistocene marineerosion surface (Cerro Azul surface) are preserved within the peninsula's interiormountain block [210]. Deformation of this surface records differential uplift across aseries of mountain block faults. A lower elevation alluvial terrace (La Mansión surface)occupies interior river valleys at 4-10 m above local base level [210].

Near Cabo Blanco at the southern tip of the Nicoya peninsula (Fig. 3.7), aprominent late Pleistocene marine erosion surface (Cobano surface) is cut acrossPliocene-Pleistocene shallow-water sediments and late Cretaceous oceanic basalts [35,210-215]. This uplifted erosion surface forms a broad dissected mesa between theinterior mountains and abandoned sea cliffs near the coast. The Cobano surfaceencompasses at least four distinct Pleistocene marine terrace treads ranging in elevationfrom 15 m to 220 m above sea level. Age correlation with late Pleistocene sea levelhigh stands at 60-215 ka (marine oxygen isotope stages 3-7) indicate net uplift rates of1.0-2.0 m/k.y. [213-215]. Radiogenic ages (Optically Stimulated Luminescence) forterrace deposits on the lowest three treads are consistent with sea level correlations,indicating ages of 65-120 ka (marine oxygen isotope stages 3-5). The Cobano terracesexhibit a measurable decrease in tread elevation toward the northeast, away from theMiddle America Trench. Uplift and tilting, beginning in the middle to late Pleistocene,led to emergence of the Nicoya peninsula's southern tip and erosion of the Cobanoterraces during sea level highstands of the late Pleistocene.

An adjacent set of narrow (<1 km), low-lying (<20 m elevation) Holocene terraces(Cabuya surface) occur between the active shoreline and the abandoned sea cliffs alongthe seaward edge of the Pleistocene Cobano surface [211]. These wavecut platformshave emerged where uplift rates exceed the rate of late Holocene sea level rise.Radiocarbon dating of 35 samples from fossiliferous, intertidal sand and beach rockdeposits yielded late Holocene ages ranging between 0.3-7.4 ka [35, 211, 212]. Upliftrates decrease from a maximum of 6.0 m/ky near Cabo Blanco, to <1.0 m/ky along a20 km length of both the margin-perpendicular and margin-parallel coastlines of the


peninsula's southern tip. This trend indicates active rotation of Holocene paleo-shorelines toward the north, consistent with the tilt observed on the adjacentPleistocene Cobano terraces. Rapid uplift and northward tilting of the NicoyaPeninsula’s southern tip has been attributed to seamount subduction offshore of theCabo Blanco headland [35, 211, 212].

Along the northern coast of the Nicoya peninsula (Fig. 3.7), an additional latePleistocene marine erosion surface (Iguanazul surface) is cut across late Cretaceousseafloor basalts [213-215]. The Iguanazul surface consists of at least three separatewave-cut treads that preserve paleo-shorelines from 10-32 m in elevation. Agecorrelation with late Pleistocene sea level high stands at 80-215 ka (marine oxygenisotope stages 5-7) indicate net uplift rates of 0.1-0.3 m/k.y. Radiocarbon-dated beachrock horizons along the active beach yield Holocene ages consistent with recent upliftat <0.5 m/k.y. [213-215].

While the northern Nicoya peninsula lies onshore of the Cocos plate "smoothdomain", the southern peninsula sits inboard of subducting seamounts of the "roughdomain" (Fig. 3.1). The order-of-magnitude difference in Quaternary uplift ratesbetween the northern Nicoya peninsula (Iguanazul surface) and the southern peninsula(Cobano surface) may be linked to sharp contrasts in the roughness, thickness, and dipof the subducting Cocos plate offshore. Rapid uplift and block rotation of thepeninsula’s southern tip is consistent with seamount subduction along the projectedtrend of the Fisher seamount chain [35, 211, 212]. A large subduction earthquakecentered offshore of Cabo Blanco in 1990 (M 7.0) may have ruptured a seamountasperity, imaged both by aftershock locations and seismic tomography [190, 218].

3.3.2.3 Orotina-Esparza Coast

Along 150 km of coastline south of the Nicoya peninsula (Fig. 3.7), major trunk riversdraining the inner forearc flow along a system of active, coast-orthogonal faults. Thesesteep faults segment the inner forearc coastline into seven fault-bounded blocks withsharply differing Quaternary uplift rates as determined from elevated marine andfluvial terraces [30, 33-36]. These fault blocks are named (from north to south):Esparza, Orotina, Herradura, Esterillos, Parrita, and Quepos.

The coastline between the Nicoya Peninsula and the Herradura headland (Fig. 3.7)consists of a low-relief (<250 m) coastal piedmont (750 km2) along the base of theextinct Aguacate volcanic range [35, 36, 219]. This region includes the lower drainagebasins of the Barranca, Jesús María, and Tárcoles rivers, which flow southwestwardinto the Gulf of Nicoya. These rivers follow fault-controlled valleys incised withinNeogene-Quaternary nearshore sediments, volcaniclastic debris, and pyroclasticdeposits. The Barranca, Jesús María, and Tárcoles faults form the boundaries of theEsparza and Orotina fault blocks.

The low-lying Orotina fault block (Fig. 3.7) between the Jesús María and Tárcolesrivers is covered by a >100 m thick Quaternary sequence of lahar deposits, ash flows,volcaniclastic sands, and fluvial gravels [35, 36, 219]. During the early Quaternary, aseries of eruption-generated lahars descended from the volcanic front onto the coastalplain, forming the framework of a 25-km-wide debris fan (Orotina fan). Rapidheadward erosion across the Aguacate drainage divide led to stream capture within theCentral Valley (Fig. 3.7) and deep incision of the modern drainage system funnelingtoward the Tárcoles gorge. This shift in the location of the Pacific-Caribbean drainagedivide opened a pathway for pyroclastic flows to spill over onto the Orotina debris fan


at the Pacific coast. Meandering paleo-channels of the Tárcoles river are preservedacross the fan surface as inverted topographic ridges of welded tuff overlying rivergravels [35, 36].

The interior of the Orotina fault block is cut by a series of active northeast-strikingdip-slip faults resulting in horst and graben topography that exposes Miocenesediments and Plio-Pleistocene lahar deposits within isolated topographic highs [34-36]. These faults show up to 50 m of vertical displacement within a welded tuff datedat 350 ka. Earthquake focal mechanisms, historical ground ruptures, mappedQuaternary offsets, and mesoscale fault data from the Orotina debris fan are allconsistent with transtensional deformation across the northeast-striking margin-perpendicular faults.

Up to five late Quaternary alluvial fill terraces (10-260 m elevation) occur alongthe lower reaches of the fault-controlled Barranca and Tárcoles rivers [35]. Verticaloffsets of terrace treads and Holocene marine benches at the coast indicate active slipalong these faults. The upper river terrace (El Diablo surface) forms an extensiveupland along the foot of the Aguacate Cordillera [30, 35, 36, 219]. This surface caps athick accumulation (>50 m) of highly weathered alluvial gravel that exhibits adistinctive, bright-red, clay-rich soil. This deeply weathered fill terrace resemblesothers found along many of the major river systems draining the Chorotega forearc.

These pervasive valley-fill deposits are interpreted as uplifted alluvial prismsformed during sea level rise toward eustatic highstands of the late Quaternary [35].Lower, inset terraces may also reflect valley aggradation during periods of sea levelrise. A regional terrace correlation framework developed for the Costa Rican Pacificmargin [35] establishes constraints on the distribution and magnitude of fore arc upliftduring the Quaternary. These correlations are based on terrace elevations, radiometricages, soil and weathering rind characteristics, and stratigraphy of terrace deposits. Thetotal number of terraces, and the vertical spacing between them, varies along the coastwith respect to the magnitude of local tectonic uplift rates. This relationship suggeststhat terrace generation along this coastline is strongly controlled by the interaction ofrock uplift and eustatic sea level fluctuation.

3.3.2.4 Herradura Headland

The Herradura headland (Fig. 3.7) exhibits the highest topographic relief within theChorotega fore arc (>1700 m). This fault-bounded block exposes late Cretaceousoceanic basalts, which have been stripped of their sedimentary cover by rapidQuaternary uplift and erosion. The differential uplift between the Herradura block andadjacent lower-relief blocks is accommodated by dip slip along steep margin-perpendicular faults [33-35]. Holocene river terraces and wavecut benches attest torapid uplift along the Herradura headland. The absence of Pleistocene terraces heremay reflect high uplift rates and accelerated erosion along the steep mountain front.Rapid uplift of the Herradura block may be driven by seamount subduction beneath themargin, as suggested by extensive scarring of the margin wedge offshore [151, 152].

3.3.2.5 Quepos-Parrita Coast

The Quepos-Parrita coastal piedmont (Fig. 3.7) southeast of the Herradura promontoryconsists of a low-relief embayment in the coastal mountain front where the FilaCosteña thrust belt merges with the northern Talamanca Cordillera. This areaencompasses the Esterillos, Parrita, and Quepos coastal fault blocks inboard of the


rough domain on the subducting Cocos plate [33-35]. Several major rivers descendingfrom the interior highlands traverse the Quepos-Parrita piedmont on their way to thePacific coast. Active faulting in this area has disrupted drainage patterns, upliftingQuaternary river deposits to form flights of fluvial terraces [220-225]. High sedimentloads have formed a low-relief coastline with barrier beaches and mangrove estuaries,interrupted only by an isolated rocky headland at Quepos.

The rugged, high topography of the Herradura block (Fig. 3.7) descends abruptlyto the southeast where a major dip-slip fault separates it from the lower-relief upland ofthe Esterillos block [33-35]. The dissected upland surface of the Esterillos block isformed on a thick accumulation of alluvial gravels deposited by the paleo-Parrita river[35, 225]. Four separate late Pleistocene terrace treads occur on these deposits, rangingfrom 40-185 m in elevation above river level. These terraces extend up to 15 kmnorthwest of the Parrita river, indicating channel migration in response to lateQuaternary uplift and tilting of the Esterillos block. Like other fore-arc rivers along thecentral Costa Rican coast, the Parrita river flows along a steep dip-slip fault orientedperpendicular to the margin [33-35]. This structure accommodates differential upliftbetween the Esterillos fault block and the lower-elevation Parrita block to thesoutheast.

The smaller Quepos fault block (Fig. 3.7) has experienced pronounced upliftrelative to the surrounding Parrita lowlands. Similar to other peninsulas andpromontories along the Chorotega fore arc of Costa Rica and Panama, the Queposheadland exposes Cretaceous-Paleogene oceanic basement rocks [33]. Several drainagenetworks on the Parrita coastal plain are deflected around the Quepos highland andexhibit at least four late Quaternary terraces that attest to active uplift [220-223].

3.3.2.6 Fila Costeña

The Fila Costeña (Fig. 3.7) is a steep-fronted, linear mountain range that runs sub-parallel to Costa Rica's southern coastline from the Herradura headland to the Panamaborder [37, 220-224]. This abrupt coastal topography (>1 km of local relief) formedduring the late Cenozoic by rapid fore-arc shortening and crustal thickening inboard ofthe subducting Cocos Ridge [37]. Four major thrust faults imbricate Tertiary rocks ofthe Terraba basin, producing rapid uplift along the Fila Costeña range front. Thesefaults are exposed along the Terraba river gorge (Fig. 3.7), which cuts across the rangefront, linking the General and Coto Brus valleys with the Pacific coast [32, 37, 224].The Terraba river serves as the primary drainage for the Pacific slope of the TalamancaCordillera (Fig. 3.7) which rises to the northeast of the Fila Costeña. Four Quaternaryfluvial terraces occur along the Terraba gorge, with some surface elevations reaching>250 m above modern river level [224]. Irregular terrace profiles, offset graveldeposits, and sharp variations in mountain-front morphometry attest to active uplift andfaulting within the Fila Costeña thrust belt [220, 221, 224]. The frontal thrust of theFila Costeña separates a subsiding outer fore arc offshore of southern Costa Rica fromthe onland area of rapid uplift within the inner forearc [37]. This pattern is broken onlydirectly inboard of the axis of the subducting Cocos Ridge, where rapid uplift in theouter forearc has formed the Osa peninsula [29].

3.3.2.7 General and Coto Brus Valleys

The General and Coto Brus valleys (Fig. 3.7) occupy an elongate structural basin thatstretches for over 100 km along the Pacific slope of the Talamanca Cordillera. This


basin is separated from the Pacific coastal plain to the southwest by the Térraba thrustbelt within the Fila Costeña. A series of broad alluvial fans coalesce along the foot ofthe Talamanca Cordillera, forming an extensive piedmont surface covering over 400km2 of the valley bottom [226-229].

Tributaries of the General and Coto Brus rivers, which drain the Talamancahighlands (Fig. 3.7), have deeply incised within this fan complex leaving a sequence ofterrace remnants along canyon margins. These alluvial surfaces are distinguished fromone another based on geomorphic setting, sedimentary texture, and the morphologicand chemical characteristics of soils [226, 228]. The oldest geomorphic surfacescoincide with the extensive piedmont upland in the northwestern portion of the GeneralValley. These well-drained upland surfaces exhibit dark-red, deeply weathered lateriticoxisols. A series of lower fan surfaces with less-developed soils yield late Pleistoceneradiocarbon ages. The youngest alluvial surfaces consist of low elevation aggradationalterraces inset along river canyons and abandoned braided channel bars of the Generaland Coto Brus rivers.

3.3.2.8 Osa Peninsula

The Osa peninsula (Fig. 3.7) within the outer Chorotega fore arc of southern CostaRica has formed by rapid uplift and crustal shortening directly above the axis of thesubducting Cocos Ridge [29]. This rugged peninsula covers over 1200 km2 andexposes a highly deformed sequence of late Cretaceous-Paleogene oceanic basementrocks and acrreted marine sediments [204, 205]. The Osa peninsula segment of theMiddle America Trench is a known source for large (M ≥ 7.0) subduction earthquakesassociated with underthrusting of the buoyant Cocos Rdige [191, 192].

Quaternary marine sands, beach ridges, and alluvial gravels along the Osapeninsula's shorelines record high rates of tectonic uplift (6.5-2.1 m/k.y.) that descreasealong an arcward trend from the peninsula's interior, northeastward toward the DulceGulf [29, 224, 230]. Along the peninsula's abrupt seaward-facing coast, late Pleistoceneshallow marine sands dated at 27-50 ka are preserved in fill wedges overlyingbasement rocks at >75 m above sea level [230]. Along the northeastern coastalpiedmont, a sequence of uplifted beach ridges yield radiocarbon ages ranging from <1ka near the modern shoreline to >30 ka at an elevation of 25 m [29]. Rivers drainingthe coastal piedmont exhibit two extensive Pleistocene gravel terraces that form a thickalluvial apron across the fault-bounded mountain front [224]. These deposits overlienearshore marine sediments dated at >30 ka. Two lower terraces with late Holoceneradiocarbon ages occur adjacent to active channels attesting to continued uplift.

3.3.2.9 Burica Peninsula

The elongate Burica peninsula (Fig. 3.7) juts southward into the Pacific Ocean forminga 25-km-long promontory at the Costa Rica-Panama border. This emergent fragment ofthe outer Chorotega fore-arc exposes a basement of Cretaceous-Paleogene oceanicbasalts overlain unconformably by a Plio-Pleistocene sequence of marine sands,conglomerates, and turbidite beds [28, 231]. Facies relationships and faunalassemblages indicate that Pliocence subsidence was interrupted by rapid Pleistoceneuplift [28]. The Pio-Pleistocene sediments exhibit significant folding and verticaldisplacement along a prominent north-trending fault valley that bisects the peninsula.Uplifted wavecut platforms along the peninsula’s coast attest to ongoing deformation.


Faulting and uplift here are attributed to subduction of the Cocos Ridge and passage ofthe Panama Triple Junction [28].

3.3.2.10 Soná and Azuero Peninsulas

The Soná and Azuero peninsulas (Fig. 3.8) of western Panama together form a majorforearc promontory (>10,000 km2) that extends over 100 km southward into the PacificOcean from the Panamanian isthmus. This prominent coastal landmass forms theeastern edge of the Gulf of Chiriquí and the western shore of the Gulf of Panama (Fig.3.8). Both peninsulas feature central mountain ranges (>500 m elevation) that areseparated from the volcanic cordillera to the north by east-west trending lowlands. Thetwo peninsulas are separated from each other by the narrow north-trending Gulf ofMontijo (Fig. 3.8). Offshore to the southwest of the Soná peninsula, lies Coiba Islandwithin the Gulf of Chiriquí.

The mountains of the Soná and Azuero peninsulas are both cut by the northwest-trending Soná-Azuero fault [38]. This major left-lateral strike-slip fault forms aprominent lineament that cuts across the peninsulas along a series of aligned rivervalleys. Deformation along the Soná-Azuero fault affects a 40-km-wide zone markedby steep fault scarps and prominent linear valleys [38]. This fault separates two distinctsuites of basement rocks common to both peninsulas. South of the fault, the basementis comprised of homogenous Cretaceous seafloor basalts, while to the north it consistsof a heterogeneous late Cretaceous-Eocene volcanic arc complex of basalts andintrusive rocks overlain by intermediate lavas. Offshore to the southwest, the Coibafault zone runs parallel to the Soná-Azuero fault, resulting in uplift at Coiba Island[232].

3.3.3 Chorotega Back Arc Province

The Chorotega Back Arc Province (Fig. 3.4) extends from the vast Caribbean plains ofthe Tortuguero lowlands in northeastern Costa Rica (Fig. 3.7) to the abrupt emergentshorelines of the southern Limón and Bocas del Toro basins near the Costa Rica-Panama border. While the Tortuguero lowlands in the north feature an extensive low-relief alluvial plain with a relatively monotonous shoreline, the southern Limón andBocas del Toro region exhibits a narrow, higher-relief coastal plain with a ruggedshoreline of rocky headlands and intervening embayments [171, 172]. The sharpgeomorphic contrast between these regions reflects a sudden shift along the Chorotegaback arc from relatively stable tectonics in the north, to active crustal deformationwithin the North Panama deformed belt in the south.

3.3.3.1 Tortuguero Lowlands

The Tortuguero lowlands (Fig. 3.7) of the northern Chorotega back arc encompass anextensive alluvial plain that reaches 40-70 km seaward from the base of Costa Rica'sCentral volcanic cordillera. A series of major rivers draining the volcanic cordilleratraverse the alluvial lowlands, transporting a high sediment load for deposition acrossbroad inland flood plains and a coalescing delta complex at the coast. A sequence ofmassive alluvial fans has developed along the foot of the volcanic cordillera where themajor rivers exit the mountain front [229]. In many cases, modern rivers have incisedbelow extensive upland fan surfaces comprised of thick accumulations of Pleistocenefluvial gravels, capped by well-developed, deep-red, clay-rich soils [233]. These soils


and associated aggradational surfaces may be age correlative with the Pleistocenealluvial terraces observed along Costa Rica's Pacific fore arc [35].

Along the Tortuguero coast (Fig. 3.7), the low-relief, sediment-laden shorelinetraces a broad, continuous arc for over 120 km between the San Juan River in the northand the Limón headland in the south. This coastline consists of a 10-15 km wide bandof prograding, shore-parallel, beach ridges that stretch along the margin of the vastalluvial plain. The lower reaches of rivers approaching the coastline are often deflectedbetween the shore-parallel beach ridges, resulting in a coastal morphology of elongatelagoons and narrow barrier islands. At several locations along the Tortuguero coast, thelow-relief landscape is interrupted by abrupt hills generated by Neogene-Quaternaryback-arc volcanism. These volcanic hills, however, represent only a minor departurefrom the overall monotonous topography of the Tortuguero lowlands.

3.3.3.2 Southern Limón and Bocas del Toro Coast

The low-relief landscape of the Tortuguero region contrasts sharply with the ruggedemergent morphology of the southern Limón and Bocas del Toro coastlines to thesouth (Fig. 3.7). Along this southern segment of the Chorotega back arc, a narrowcoastal plain (<20 km) with undulating topography runs along the steep Caribbeanslope of the Talamanca mountains. This stretch of rugged coastline extends for >200km from Costa Rica's Limón headland in the northwest (Fig. 3.7), to Panama's Bocasdel Toro archipelago and Gulf of Mosquitoes in the southeast (Fig. 3.8).

The coastal morphology of the southern Limón region is characterized by a seriesof rocky promontories and coastal islands interspersed with pocket bays and widecrescent-shaped beaches. Coral reefs and prominent cliffs occur along some segmentsof coastline, while other areas feature broad estuaries, peat swamps, and barrierbeaches. At Bocas del Toro in northwestern Panama, an extensive low-relief coastalembayment (70 km wide) lies inboard of an emergent string of cliff-lined islands andpromontories. To the east of Bocas del Toro, the Chorotega back arc extends along theGulf of Mosquitoes where a series of river deltas form localized bulges in the coastline.

The rugged geomorphology of the Limón and Bocas del Toro region (Fig. 3.7) iscontrolled by active crustal shortening within the North Panama deformed belt alongthe Caribbean margin of the Panama block [234]. Rapid uplift above northeast-vergingthrust faults has led to the emergence of Quaternary coral terraces along the southernLimón coastline [235]. Active subsidence has also produced a low-relief troughinboard of the emergent islands of the Bocas del Toro archipelago. During the 1991M7.6 Valle de la Estrella earthquake, coseismic uplift of 0.5-1.5 m affected thesouthern Limón coast [236], while subsidence of 0.5-0.7 m resulted in inundation ofpeat swamps along the Bocas del Toro embayment [237]. Toward the northwest, thezone of active coastal deformation ends abruptly at the Limón headland, where thrustfaulting within the North Panama deformed belt gives way to oblique slip alongsteeply-dipping faults of the Central Costa Rica deformed belt [34, 235].

Uplift along the Limón-Bocas del Toro coast (Fig. 3.7) has exposed a Neogenesequence of marine to terrestrial sediments and volcanic rocks along coastal cliffs andislands [238-241]. These deposits are correlative with similar units mapped in theCanal Zone and Darién regions of Panama [242, 243]. As a whole, this rock sequenceprovides a detailed record of Neogene emergence along the Panama isthmus, resultingin closure of the oceanic strait between the Atlantic and Pacific Ocean basins [8, 9].


3.3.4 Canal Zone Lowlands Province

The Panama Canal Zone (Fig. 3.4) occupies a region of relatively low topographybetween the Central Volcanic Cordillera of western Panama and the mountainousDarien isthmus to the east (Fig. 3.8). This lowland encompasses a network of low-gradient river valleys that drain surrounding hills with peak elevations of less than1200 m [244]. The Pacific-Caribbean drainage divide descends to one of its lowestelevations in Central America (<200 m) in the low saddle of the Culebra Cut along thePanama Canal (Fig. 3.8). The most pronounced expression of low topography in thisregion is the once-extensive swamp within the broad valley of the Chagres river, nowcovered by the Panama Canal’s Gatún lake [245].

The stratigraphy of the Canal Zone Lowlands includes a Cretaceous volcanicbasement that is overlain unconformably by a thick sequence of Eocene-Mioceneshallow marine and terrestrial sediments [242, 244-248]. These rocks are moderatelyfolded, and were affected by late Neogene faulting and uplift that produced localizedbedrock highs within fault-bounded blocks [38, 245]. Within adjacent topographiclows, these sedimentary rocks are buried unconformably by a horizontal valley-fillsequence of Pleistocene-Holocene estuarine and swamp deposits referred to as theAtlantic Muck.

3.3.4.1 Gatún Fracture Zone

The low topography of the Canal Zone (Fig. 3.8) has been attributed to pervasivefaulting and fracturing that extends across the Panamanian isthmus within an 80-km-wide zone referred to as the Canal Discontinuity or the Gatún Fracture Zone [244, 245,248]. This major crustal discontinuity has been interpreted as a Neogene-age basementfault that divides the Chorotega block to the west from the Chocó block of easternPanama and western South America [4].

Figure 3.8. Map of the physiographic provinces of southern Central America, showingsignificant geomorphic features of portions of Panama and Colombia. Digital elevationmodel derived from NASA Shuttle Radar Topography Mission (SRTM) image PIA03364.


Seismic imaging along the Caribbean coast indicates that faulting in the CanalZone post-dates late Miocene strata and may in some cases displace overlyingQuaternary beds of the Atlantic Muck [245]. Prominent geomorphic lineaments,topographic breaks, and bends in river courses are all consistent with a young, fault-controlled landscape [244-246]. For example, a major northeast-trending faultlineament along the Río Gatún (Fig. 3.8) forms the southern boundary of a basementhighland at the northern end of the Canal Zone. This lineament extends eastward intothe northern coastal ranges of the Darién Isthmus and is interpreted as a major left-lateral strike-slip fault [38]. Abundant north-striking and generally east-facing scarpssouth of the Gatún fault suggest that the Canal Zone is undergoing active east-westextension across a series of normal faults [38]. These normal faults may represent thewestern termination of left-lateral shearing along major strike-slip faults within theDarién region of eastern Panama.

3.3.5 Darién Isthmus Province

The Darién Isthmus (Fig. 3.4) of eastern Panama is bounded by a northwest-trendingset of rugged mountains along both its Caribbean and Pacific coasts (Fig. 3.8). Theseinclude the San Blas and Darién ranges along the Caribbean coast, and the Majé,Bagre, and Sapo ranges along the Pacific side. These coastal highlands flank anelongate lowland basin centered along the broad valleys of the Bayano, Chucunaque,and Tuira rivers (Fig. 3.8). On the Pacific coast, the Gulf of San Miguel (Fig. 3.8)forms a deep bight into the coastal mountains where the Tuira river estuary drains fromthe central lowland into the ocean. Much of the humid and rugged landscape of theDarién Isthmus remains cloaked in dense tropical vegetation, contributing to itsreputation as one of Central America’s most remote regions.

The coastal massifs of the Darién Isthmus expose a Cretaceous-Eocene crystallinebasement complex comprised predominantly of highly deformed mafic igneous rocks[142, 243, 248, 249]. These rocks belong to the Chocó block (Fig. 3.1), an allocthonousoceanic basement terrane that lies beneath eastern Panama and western Colombia [4].The Chocó block is separated from the Chorotega block to the west, across Tertiarybasement faults of the Panama Canal Discontinuity.

The basement rocks of the Darién region are overlain along the flanks of thecoastal mountains by Eocene-Miocene pillow basalts and deep marine sedimentsdeposited prior to Panama’s collision with South America [243]. These rocks are in-turn buried by a thick, post-collisional sequence of Neogene siliciclastic sedimentswithin the core of a complex synclinorium centered along the Bayano-Chucunaquebasin.

Within Panama and southern Costa Rica, the Chocó and Chorotega basementterranes (Fig. 3.1) together form the modern Panama block, a semi-rigid microplatethat is caught between the Caribbean, Cocos, Nazca, and South American plates [5, 6].Fold and thrust belts offshore of both the northern and southern Darién isthmus (Northand South Panama deformed belts) accommodate active convergence with theCaribbean and Nazca plates [39]. Active collision between the Panama block and theSouth American craton to the east occurs across thrust faults of the Atrato-Urubá suturezone in western Colombia [148].


3.3.5.1 East Panama deformed belt

Ongoing collision between the Panama block and South America results in clockwiserotation and left-lateral slip along several major northwest-trending faults that cut theeastern Darién Isthmus [38]. Together, these structures constitute a diffuse deformationzone referred to as the East Panama deformed belt [42]. The Sansón Hills, Sambú,Majé, and Jaqué River faults (Fig. 3.8) each follow prominent northwest-trendinglineaments along the flanks of the Pacific coastal ranges.

The Sansón Hills fault forms the boundary between the Chucunaque basin (Fig.3.8) and the coastal highlands to the southwest [38, 42]. Hilly topography northeast ofthe fault exposes Paleogene sediments of the Chucunaque basin within a series ofbreached, en-echelon anticlines. These folded strata are juxtaposed against Cretaceousoceanic basement rocks exposed southwest of the fault within the estuarine lowlands ofthe Tuira river.

The Sambú fault, southwest of the Tuira river (Fig. 3.8), marks the boundarybetween the Bagre and Sapo massifs near the coast [38, 42, 250]. A left-step in thisfault bounds the Sambú pull-apart basin, which forms the entrance to the Gulf of SanMiguel and a broad alluvial valley between the Bagre and Sapo ranges (Fig. 3.8). TheMajé fault trends onshore to the northwest from the Gulf of San Miguel where it offsetsbasement rocks of the Majé massif [38, 42]. The Jaque River fault along the southwestrange front of the Sapo Massif follows the Pacific coast southeastward from the Dariénisthmus into western Colombia [38, 42, 250].

The northwest-trending left-lateral faults of the Darién Isthmus terminate at theirsoutheastern ends along a zone of north-trending thrust faults within the Panama-SouthAmerica suture zone [38, 42, 243]. Toward the northwest, the left-lateral faults die outas they approach north-trending extensional structures along the Canal ZoneDiscontinuity. The presence of undeformed Pliocene-Pleistocene sediments along faultvalleys in the eastern Darién Isthmus may indicate that the left-lateral faults are nolonger active [243]. If this is the case, the Panama block in this region may now beacting as a rigid plate as it collides with South America.

3.4 SUMMARY

The geomorphology of Central America is extraordinarily rich and varied (Fig. 3.4).This small fragment of land linking North and South America features a diversity oflandforms and geomorphic processes usually associated with an entire continent. Whileaccounting for only 0.4 % of the Earth's total land area, Central America encompassesa wide range of landscapes, from jungle-covered tropical lowlands to glaciatedhighland plateaus, from cloud-forested volcanic peaks to rolling dryland savannahs,and from rocky tectonic shorelines to low-relief barrier islands and mangrove estuaries.This chapter presents a broad overview of Central American geomorphology, anddefines a system of physiographic provinces (Fig. 3.4) that serves as a framework forcharacterizing regional landscapes and geomorphic processes.

The physiographic trend of the Central American isthmus is defined by the MiddleAmerica trench and volcanic front (Fig. 3.1), active since the initiation of subduction inthe late Cretaceous. Cenozoic magmatic rocks and adjacent sedimentary basins areoverprinted across four crystalline basement terranes, two of continental affinitybeneath northern Central America (Maya and Chortis blocks), and two of oceanicorigin (Chortis and Chocó blocks) beneath the southern isthmus. This geologic


template (Fig. 3.2) is in turn affected by active tectonics within three major plate-boundary deformation zones: the Motagua-Polochic transform faults of Guatemala, theMiddle America convergent margin along the Pacific coast, and the diffuse collisionalbelts along the margins of Panama and Costa Rica. The delicate interplay betweenCentral America’s highly variable climate and topography (Fig. 3.3) imparts anadditional influence over regional geomorphology.

The Maya Highlands Province (Fig. 3.4) of northern Central America includes aseries of high-altitude mountain ranges and plateaus that extend in a broad arc fromCentral Mexico, across northern Guatemala, to Belize. These highlands descend to thenorth onto the vast carbonate lowlands of the Yucatán Platform Province (Fig. 3.4), themost extensive karstlands of the North American continent. Along the southern marginof the Maya highlands, the Motagua Fault Zone Province (Fig. 3.4) extends acrosscentral Guatemala along a series of major fault-controlled valleys that define the activeNorth American-Caribbean plate boundary.

The Chortis Volcanic Front Province (Fig. 3.4) parallels the Pacific margin fromGuatemala to El Salvador, and consists of a series of clustered stratovolcanoes andcalderas arranged along transverse fault lineaments. Along the Pacific slope, theChortis Forearc Province (Fig. 3.4) encompasses a broad coastal plain made-up ofcoalescing alluvial debris fans that descend from the volcanic highlands. Behind thevolcanic front, and south of the Motagua fault zone, the Chortis Highlands Province(Fig. 3.4) extends across the mountainous interior of Guatemala, Honduras, andNicaragua. This extensive region of high topography includes a rifted plateau in thewest, a stable central massif, a northern coastal borderland of fault-bounded ridges, andan eastern zone of dissected mountains facing the Caribbean coast. The MosquitoCoast Lowlands Province (Fig. 3.4), east of the Chortis highlands, encompasses theextenisve alluvial plains along the Caribbean coast of Honduras and Nicaragua.

The Nicaraguan Depression Province (Fig. 3.4) consists of a fault-boundedstructural basin that extends for 600 km between the coastal fore arc and the Chortishighlands. This tectonic lowland includes the Median Trough of El Salvador, the Gulfof Fonseca, and Lakes Managua and Nicaragua. The Nicaraguan Volcanic FrontProvince (Fig. 3.4) encompasses a chain of stratovolcanoes and calderas centered alongthe southwestern boundary faults of the Nicaraguan depression. The Sandino Fore ArcProvince (Fig. 3.4), southwest of the Nicaraguan volcanic front, includes a low reliefcoastal plain composed of volcaniclastic debris in the north, and a rugged shoreline ofpocket bays and rocky headlands in the south.

The Chorotega Volcanic Front Province (Fig. 3.4) of Costa Rica and Panamaconsists of a segmented series of distinct cordilleras that have evolved in response tohighly variable tectonics along the southern Middle America margin. A major volcanicgap occurs along an uplifted segment above the subducted Cocos Ridge. The adjacentChorotega Fore Arc Province (Fig. 3.4) encompasses a rugged tectonic coastline,characterized by prominent uplifted headlands and peninsulas, as well as an active foldand thrust belt inboard of the Cocos Ridge. The Chorotega Back Arc Province (Fig.3.4) features a broad alluvial lowland in northern Costa Rica and a rugged coastlinewith uplifted coral reefs formed within an active back arc thrust belt. The Canal ZoneLowlands Province (Fig. 3.4) includes a swath of densely fractured, low relieflandscape that extends across central Panama. To the east, adjacent to South America,the Darién Isthmus Province (Fig. 3.4) consists of a fault-bounded series of humidcoastal mountain ranges flanking an elongate central valley.


As described throughout this chapter, the physiographic provinces of CentralAmerica (Fig. 3.4) each exhibit a characteristic assemblage of landforms that records aunique history of landscape evolution. The distinctive geomorphology of each provincereflects a dynamic interplay between regionally variable rock types, active plate-boundary tectonics, and an energetic tropical climate. As a whole, the physiographicprovinces of this dynamic region encompass an exceptionally diverse array oflandscapes and associated landforms. Central America, therefore, serves as anoutstanding geomorphic laboratory for the study of a broad range of surface processesand their imprint on the landscape.

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Marshall - The Geomorphology and Physiographic Provinces of Central America