Roeder 1988 2

TECTONICS, VOL. 7, NO. 1, PAGES 23-39, FEBRUARY 1988

ANDEAN-AGE STRUCTURE OF EASTERN CORDILLERA

(PROVINCE OF LA PAZ, BOLIVIA)

Dietricoh Roeder

The Anschutz Corporation, Denver, Colorado

Abstract. A Moho root beneath the

Bolivian Andes, 40 km deep, is consistent with 230 km of overlap of Neogene age on a single, trenchward dipping transcrustal thrust fault with 10 of finite ramp cutoff (Main Andean Thrust (MAT)). Only 10% of the Andean crustal volume is

ascribable to magmatic addition. The MAT is located within South American crust of full thickness. It intersects the basement top 450 km inland from the Neogene crustal margin. It is not a collision suture as shown by persistent pre-Neogene facies continuity. Thrusting is not accompanied by terrane accretion. The present bilaterally symmetrical thrust belt responds to elastic line loading and to Coulomb rheology. In the hanging wall of the MAT, a deep high-stress wedge base builds a steep critical slope. In the footwall, the foredeep response is fast subcritical growth by progradation and blind thrusting on a low-stress decollement. Interaction is maintained by out-of-sequence renewal of movement on the MAT.

Copyright 1988 by the American Geophysical Union.

Paper number 7T0615. 0278-74 07 / 88 / 007T-0615510. 00

INTRODUCTION

In orthern Bolivia, the Andean orogen contains on its cratonic east flank a

marginal fold-thrust belt of Oligocene to Recent age [Jordan et al., 1983]. In this segment of the Andes, bedrock is not covered by volcanics and in part has potential for commercial hydrocarbons. Therefore, some information is available. Surface mapping and photogeology by the Bolivian state oil company Yacimientos Petroliferos Fiscales Bolivia (YPFB), reflection seismic data and sparse drilling by private industry, potential- field data, and refraction seismic data add quantitative aspects to the puzzle of compressional subduction systems termed Andean by Dewey and Bird [1970].

Data available for the present study have been condensed into three semibalanced structure sections across the external parts of the eastern Cordillera between 14S and 16S latitude. These sections and their support data show that the retroarc system is a nested, conjugate, bilaterally symmetrical thrust complex, with 230 km of overlap on a single intracrustal detachment. Modeled as a Coulomb plastic wedge, the thrust system supports the view that the Andean topography is upheld largely by crustal compression rather than by underpinning and buoyancy alone. The cross sections are

24 Roeder: Andean-Age Structure of Eastern Cordillera

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consistent with an elastically loaded and flexed craton; they constrain the wide range of rigidities allowed by the Bouguer gravity data.

I cannot relate quantitatively recent Andean structure to the bimodally dipping Peru-Chile Benioff zone. Intracrustal architecture is unknown except for a very large Moho root. However, 70 to 90% of the crustal volume including root and inclined slab can be accounted for by the crustal overlap. About 20% may be Mesozoic to Paleogene newly accreted crust, and 10% may be additions by magmatism.

The present paper is an update of a speculative synthesis prior to the present crop of data [Roeder, 1982a]o It results from an early phase of hydrocarbon exploration. It is an example of what can be done in overthrust belts with modern techniques and a moderate amount of data.

SETTING

Figure 1 shows a zonation of the Bolivian Andes into topographically and geologically defined

parallel belts. Reviews of Bolivian geology are by Ahlfeld and Branisa [1960], Martinez and Tomasi [1978], and Martinez [1980].

The western Cordillera, largely on Chilean territory, is a plateau below 5000 m with widely spaced cones above 6000 m. It is a Neogene to Recent magmatic arc intruding and covering a poorly known crustal slab which may be a southern extension of the Arequipa basement massif. It is the foreland of west vergent thrusting of Triassic and Tertiary age in the Altiplano and the eastern Cordillera.

The Altiplano is a Tertiary and Quaternary fluviatile complex of internal drainage at 4000 m elevation. Interbasin ridges and a few oil wells expose a passive-margin series of Paleozoic to late Cretaceous age with minor magmatic constituents, all involved in a west vergent fold-thrust belt of mixed late Paleozoic and Andean kinematic ages. In some areas, enough pre-Tertiary is exposed to demonstrate the structural style. Ages of thrusting, however, are in part based on angular unconformities only and may, in

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The eastern Cordillera is a belt of

folded Paleozoic sediments intruded by groups of late Triassic and Neogene granitoid plutons. It forms a continuous ridge above 5000 m peaking in Illampu (7010 m) and Illimani (6882 m). It contains a 20- to 50-km-wide west slope underlain by a major post-Miocene thrust, and a 100-km-wide east slope down to 1000 m of elevation. Easterly drainage dissects this slope and, in the La Paz river, has retrograded into the Neogene lake beds of the Altiplano.

The sub-Andean zone is 50 to 100 km

wide and continues the east-Andean slope eastward and below 1000 m, Bedrock

consists of Paleozoic sediments largely conformable with overlying Mesozoics and Tertiary. The post-Paleozoic series are preserved in flat-bottomed synclines and on the back limbs of ridge-forming east- vergent thrust fronts.

The foreland of the sub-Andean zone is

the Amazon River drainage area at less than 500 m. This area is vaguely specified as Madre de Dios basin and Beni basin; it contains fluviatile Neogene above a gently truncated and SW-dipping pre-Eocene series. A zero edge of pre-Neogene sediments forms the southwest limit of the

outcropping or subcropping Brazilian shield somewhere 100 to 200 km NE of the sub-Andean east front. There is no

topographic rise associated with the shield.

STRATIGRAPHY

The material involved in Andean thrusting consists of South Anerican crust, a Paleozoic passive-margin prism, thin late Paleozoic to Eocene interarc or cratonic deposits, and an Oligocene to Recent foredeep fill. In the following, the stratigraphic sequence is summarized as a narrative legend to the cross sections (Figures 2 to 5).

PC, basement with Andean and late Proterozoic ages, is documented on the Brazilian Shield and in the Pampas foreland massifs of Argentina as a thin sliver in the frontal thrust of the study area, in a basement core (1750 m) from an Altiplano well, and in exposures in the western Cordillera.

COS D, an Ordovician through Devonian turbiditic to deltaic passive-margin series, may reach 15 km in thickness

[McBride et al., 1983]. Now located in the eastern Cordillera, this depocenter may reflect a Paleozoic continental edge and may suggest that thin crust in the western Cordillera is a pre-Andean carry-over and not exclusively a product of subductive erosion.

P C, disconformable, 1-km-thick Permo- Carboniferous series, grades upward from Gondwana deposits into tropical-marine carbonates [Helwig, 1972]. The top of the series is gradually truncated eastward to a zero edge within the sub-Andean.

K, TE, volcanogenic redbeds, shallow marine sandstones and shales, locally intercalated with South American pre- Atlantic basalt flows, spans late Cretaceous to Eocene and thins eastward from 2.5 to less than 1 km between the Altiplano and the sub-Andean.

TM-QD and TM-C, is a series conformable with an Oligocene to mid-Pliocene fluviatile foredeep fill. The correlatable formations Quendeque and Charqui, preserved in synclines and strike belts of the sub-Andean, are 4 km thick at the frontal thrust and 6 km in the westernmost outcrops. The youngest preserved layers are estimated to be early to mid-Pliocene in age. There is no net foredeep sedimentation at present.

OROGENY, METAMORPHISM, AND MAGMATISM

If the Andean orogeny is defined in time to the age of its foredeep fill, it becomes critical to distinguish Andean deformation and pre-Andean deformation.

In the high core of the eastern Cordillera, the Paleozoic series is folded into 0.01- to 1-km-size kink folds at or past locking position, with axial plane spaced cleavage and with upright axial planes. With few local exceptions, the strike of these folds is consistent with clearly Andean strike directions.

Folding in the eastern Cordillera is dated by infolded unconformable septa and synclines as pre-late Cretaceous. In several modern syntheses but not in the present report, the folding is ascribed to early and late Paleozoic "Hercynian" phases [Martinez and Tomasi, 1978; Martinez, 1980; Dalmayrac et al., 1980].

In the sub-Andean, strike belts involve Neogene and are clearly Andean in age. Their Paleozoic portions show folds of the same style as in the high core. Stratigraphic persistence of disconformable contacts suggests that the

Roeder: Andean-Age Structure of Eastern Cordillera 27

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Fig. 4. Semi balanced structure cross section of sub-Andean belt in the Madidi area of northern Bolivia, located on Figure 1. Solid areas are Eocene to Westfalian sandy units. Dotted areas are foredeep fill. Letters are stratigraphic units explained in the text.

folds affecting the Paleozoics are Andean in age.

In the high core of the eastern Cordillera, metamorphism ranging from undetectable to lower greenschist facies [Martinez and Tomasi, 1978] can be ascribed to burial within the passive- margin series at a Barrovian gradient.

Tin-tungsten bearing granitoids occur in two nearly coincident trends of plutons. Synkinematic monzogranites with K-Ar dates of 225-202 m.y. [McBride et al., 1983] and with contact aureoles in the andalusite-sillimanite fields of the

A12 SiO 5 polymorph point [Bard et al., 1974] date the pre-Cretaceous folding as late Triassic and probably subduction related.

Post-kinematic discordant subvulcanic plutons without contact aureoles and with K-Ar dates of 28 to 19 m.y. [McBride et al., 1983] suggest arc magmatism during the main event of Andean intracrustal or retroarc thrusting.

SEMIBALANCED CROSS SECTIONS

Three sections, 67 km, 85 km, and 116 km long, cross the Subandean zone, between the Main Andean Thrust and the orogenic front, at strike spacings of 57 and 130 km. Their locations are shown in Figure 1. The sections are shown in Figures 2, 3, and 4. Their cross-strike extent is about

one half of the eastern Cordillera; they are confined to the area where data are

available and where Cretaceous and younger rocks are preserved from erosion and clearly define Andean structure.

The sections are constructed by hand and by honoring stratigraphic thicknesses, dip attitudes, and normal ranges of ramp cutoff angles. No systematic attempt at line-length balancing other than restoration to the depositional state (Figure 5) has been made. No attempt has been made to adapt ramp cutoff angles to modification by folding.

The series of sections shows a

northwesterly decrease in width of, and number of structural units within, the sub-Andean belt.

As the amount of transport on the Main Andean Thrust is not constrained, this may or may not reflect a decrease in Andean eastward thrusting. However, the geological map of Peru shows a western termination of this sub-Andean segment near the Urubamba river at 7330vW longitude.

EXTERNAL PORTION OF SUBANDEAN

An external segment contains two major thrusts: en echelon, the Eva-Eva and the Caquiahuaca. They rise from a regional decollement at estimated ramp angles of 20 to 23 . Slivers of Precambrian gneiss at the base of the Caquiahuaca thrust sheet [Davila et al., 1965] show that the basal decollement extends clear to the

orogenic front without any step-up or shallower toe additions.

The bodies of the external thrust

sheets rise with steep to vertical dips above flat or synclinally folded frontal segments in thrust contact.

This may suggest that the external


thrusts evolved from blind thrusts soling out upward at an estimated depth of 2 km below the synkinematic surface. Footwall imbrications below the external thrusts, as assumed in Figure 3, are likely but not documented in available data.

Backthrusting affects the bodies of the external thrust sheets. Seismic data (industry, unpublished) suggest that backthrusts tend to occur with preference at the upper fault bend of a ramp.

Section 2 shows that over the 50-km- wide external zone, the basal decollement ramps up in two mappable steps about 3.5 km. Part of this step-up, however, is achieved by stratigraphic thinning in the Paleozoic series.

A third imbricate unit, Quiquibeycito, appears only in Figure 2. Farther northwest, its space is occupied by the footwall imbrications of folded-thrust structures attributed to the internal portion of the sub-Andean. The Quiquibeycito thrust front is a tight anticline with an overturned faulted east limb, overlying a west-dipping sole thrust. This and other, similar, thrust fronts cannot be geometrically restored with ease. They may represent either Suppean second-mode folds [Suppe, 1983] or polyphase tightly folded-thrust structures (Do Roeder, in preparation). Both interpretations suggest an origin from earlier blind thrusts at 2 or 3 km below a former land surface.

INTERNAL PORTION OF SUBANDEAN

In section 1, the footwall of the Main Andean Thrust contains the poorly controlled Caranavi anticline, the well- controlled Alto Beni syncline, and a complex frontal anticlinorium of the Marimonos range. Seismic control at Marimonos (industry, unpublished) suggests, but not clearly outlines, footwall imbrication beneath the Marimonos

frontal thrust which juxtaposes Devonian onto Miocene. The footwall imbrications, therefore, are composed of the late Paleozoic to Miocene series.

The steep east limb of the Caranavi anticline contains a backthrust near the

base of the Devonian. This suggests a ramp top in the Marimonos frontal thrust. West dips in the west flank of the Caranavi anticline constrain the depth extent of this ramp and leave room for one or two duplex-type footwall imbrications.

The ramp geometry of the Marimonos

frontal thrust, as constructed in Figure 2, requires that the footwall imbrications beneath the Caranavi anticline are composed of the late Paleozoic to Tertiary section.

In Figure 3, the internal position of the sub-Andean contains the north extension of the Marimonos thrust sheet

with two open faulted anticlines with a regional southeasterly axial plunge. Mapping by YPFB [Davila et al., 1965] shows a half-window with Miocene and older rocks beneath the Devonian of the eastern one of the two anticlines. By section construction, both anticlines are folded- thrust structures with complex and poorly known footwall imbrications in Paleozoic to Miocene rocks.

The north extension of the Main Andean Thrust (MAT) is traceable on detailed mapping into an imbrication of Paleozoic series with seemingly small displacement. As mapped, the trace of the MAT shows progressive down-section truncation through several thrust sheets, suggesting major out-of-sequence thrusting in the sense of Boyer and Elliot [1982] or less likely, considerable mapping errors. The northwesterly continuity of the MAT thrust mass is supported by a 20- to 30-km-wide belt of east dips with east-dipping backthrusts.

This MAT dip panel suggests a major ramp through much more sedimentary thickness of early Paleozoics than is documented in the sub-Andean zone. Only its east half is included in section 2. The section does not show the dip extent of the MAT nor the structure in its foreland. The depth extent of the backthrusts in the MAT dip panel is also unknown but most likely is confined to the zone above the MAT.

The ramped footwall unit below the Marimonos thrust in section 2 rises

northwestward by axial plunge to form the Uchupiamonas anticline of Figure 4, with Devonian exposed in the core. Seismic data, depicted as depth-converted line tracings, clearly outline, by ramp cutoffs and bed-parallel tread seqments, two thrust faults and geometrically imply a third thrust. Ramping in the surface thrust sheet is supported by two backthrusts, labeled Madidi, in the northeast limb of the Uchupiamonas anticline. The sole fault of the surface unit can be traced across the adjacent Tuichi syncline and tied into the Caquiahuaca thrust complex. This thrust

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has the sigmoidal shape of a duplex structure, with a cross-strike succession of an anticline and a syncline.

At depth, the Uchupiamonas duplex structure shows major repetition at footwall imbrications. However, these imbrications affect the older Paleozoics not present in the external zone. The Tuichi syncline is too deep to allow the southwesterly extension of late Paleozoic to Eocene rocks into the footwall imbrications of the Uchupiamonas structure.

Internal to the Uchupiamonas structure, a tightly folded and backthrusted belt represents the combined Marimonos unit and the front of the out-of-sequence thrust identified with the MAT. Some of the tight folds expose half-windows and suggest in- place folding after the out-of-sequence emplacement of the MAT mass.

RESTORATION OF CROSS SECTIONS

Figure 5a, 5b, and 5c show simplified and small-scale restorations of the three cross sections to the depositional state.

Figure 5d shows a restored structure section from the Appalachian fold-thrust belt in the United States.

In spite of the palinspastic uncertainty at the footwall of the out-of- sequence MAT, the restoration suggests a constant bulk thrust overlap of 137 km. Expressed as negative elongation, the bulk strains E vary between -0.60 and -0.67 for the three cross sections.

The restoration also suggests that the poorly understood Main Andean Thrust includes a major ramp in its hanging wall through 10 km or more of rock thickness. This is suggested in the field by the sudden appearance of Cambro-Ordovician quartzites in a thickness of 5 km, and also by the extensive panels of east- dipping Paleozoic strata in the hanging wall of the MAT. The ramp angle is not documented but has been assumed at about 16 for the MAT and about 10 for the internal part of the Marimonos thrust.

The restoration (Figure 5d) pertains to a structure cross section by Roeder [1982b] which is not reproduced in the present paper but which is located near, and is structurally similar to, cross section 1 by Mitra [1986] and cross section 3 by Kulander and Dean [1986].

The restoration (Figure 5d) shows a similar amount of thrust overlap, a similar structure, and, possibly, a

similar dynamic setting to the sub-Andean belt of northern Bolivia. In front of a basement-involving thrust (Blue Ridge Front or Stanley thrust explored by the southern Appalachian COCORP line), a large thrust sheet covers 2/3 or more of the width of the external belt. The sole of

the central decollement is bed-parallel and located 3 to 6 km below the

synkinematic surface. The frontal belt contains two simple thrust sheets with a blind thrust at the front. The Blue Ridge Front thrust has not yet been described as out of sequence. However, Witherspoon [1981] and Woodward [1985] describe its discordant attitude.

MEDIAN PANEL OF EASTERN CORDILLERA

An 80-km-wide panel west of the Main Andean Thrust contains the orographic backbone of the eastern Cordillera. It consists of flat lying and/or mildly folded early Paleozoics with basement at shallow depth, and with greenschist-facies metamorphism in its deepest parts. It contains the depocenter of the Paleozoic passive-margin prism and most of the Andean plutonic bodies known in the eastern Cordillera. Part or most of the open, locally tight-folding in this panel is dated by late Triassic cooling ages in synkinematic plutons; it is clearly older than all of the sub-Andean structure described in the foregoing.

The vaguely known fold pattern, mappable along the highway from La Paz to Caranavi, suggests some degree of conformity of folding between the basement top and the bedding within the Paleozoic series.

ALTIPLANO THRUST BELT

In the footwall of an east-dipping west-vergent thrust in the west slope of the high (eastern) Cordillera, a poorly known, mostly covered, partly graben- offset, partly plutonized thrust belt extends to the west edge of the Altiplano. It is west-vergent and perhaps 60 km wide. Precambrian basement cored in the San Andres well at 2744 m of depth (+1184 m elevation) below an undisturbed Tertiary and Cretaceous series [Lehmann, 1978] marks the western foreland.

Thrusting is polyphase and dated as early Oligocene to Pliocene [Martinez, 1980]. Hills emerging from the Pleistocene lake beds show style elements of a common


supracrustal, detached fold- thrust belt. One of the best exposed and described areas of the Altiplano thrust belt extends along the east shore of Lake Titicaca [Rivas, 1968].

Basement may be involved in some of the Altiplano thrusts. At depth they may intersect in a conjugate fashion with the east- vetgent Andean thrusts.

NESTED ANDEAN FOLD-THRUST BELT

Seen as a unit in cross section, the sub-Andean belt, the median panel, and the Altiplano fold-thrust belt together form a conjugate, nested, bilaterally symmetrical complex with 137 km shortening in the sub- Andean part, an estimated 30 to 50 km transport on the Main Andean Thrust, perhaps 10 km of shortening in the median panel, and, as a vague guess by style analogy, perhaps 40 km in the Altiplano fold-thrust belt. The total estimated

Andean shortening is 217 to 237 km; the bulk strain is about 50% derived from a

present width of the orogen of 240 km. Available data pertaining to the strain

rate of this orogeny suggest a low finite bulk strain rate and at least one major spike; constraints on both data sets are vague.

If all Andean deformation is attributed to the time interval of the Inca and

Quechua phases from early Oligocene to mid-Pliocene. one obtains a transport rate T of 2.1x10- cm s-1 or 0.66 cm/yr, and a bulk rate of _ngative elongation of -4.48x10 -16 s .

Seismic stratigraphy of the foredeep fill (industry, unpublished) suggests that all of the Quendeque and Charqui is prekinematic to all thrusting in the sub- Andean, which therefore must have achieved 135 km of overlap in 5 million years or less. This conclusion requires, in the sub-Andean only, a transport rate T of 8.56x10 -8 cm s -1 or 2.7 cm/yr, and a bulk elongation rate of-4.1x10 -15 s -1.

As in the classical geosynclinal theory [e.g. Press and Siever 1978], the nested thrust belt pair of the eastern Cordillera is laterally confined by foredeeps with coreward dipping basement. It coincides with a major Phanerozoic depocenter, and it has synkinematic calc-alkali intrusions in its core. It does not mark the site of a suture. The entire Phanerozoic sediment

series is continuous across the orogen, disregarding thickness changes and Neogene to Recent erosion. All known sediments

were deposited on Precambrian continental crust, and much of these on crust of cratonic thickness. The only facies change of significance is that the paleobathymetry of the Paleozoics changes from intertidal or shallow subtidal in the east to bathyal or deeper in the west. This remarkable structure must have formed

by intracrustal compression essentially during the latest, shortest span of its geological history.

It is the task of the second part of this paper to attempt a geodynamic definition of this compression.

ELASTIC FORELAND FLEXURE

Structure contours on pre-Tertiary horizons in the sub-Andean foreland and

seismic data (industry, unpublished) suggest a foredeep with a bottom that slopes mountainward at 2 to 3 Following a common practice (reviews by Turcotte [1979] and by Karner and Watts [1983]), the sub-Andean foredeep is interpreted as an elastic response to the Andan load (Figure 6). A quantitative approach [Lyon- Caen et al., 1985] defines the load and the shape of the deflected foreland by their Bouquer expression and searches for appropriate flexural rigidities. A geometric approach [Turcotte and Schubert, 1982; Roeder, 1980], applied in the present study, iteratively varies rigidities, amounts of deflection, and locations of line loads to fit deflection shapes to structural data.

The purpose of this modeling is first, to define those crustal conditions that produce the available, incomplete, set of supracrustal data. Secondly, a definition of the foreland slope and the base of deformation is needed in constructing structure cross sections. Thirdly, the slope at the base of deformation is a constituent of the thrust mechanics.

A downwarp produced by Andean thrusting is depicted by the foredeep sediments deposited during Andean thrusting, that is, by the base of the Quendeque formation in the undisturbed foreland and in the parautochthonous external sub-Andean zone. Mountainward beyond the point where thrust dislocations displace the foredeep base vertically, the downwarp continues only as a theoretical "regional" horizon. The regional is defined as the locus of a stratigraphic pick influenced by its subsidence history but not by thrust dislocations. The regional is a concept

32 Roeder' Andean-Age Structure of Eastern Cordillera

well known to thrust-belt explorationists [Shaw, 1963; Keating, 1966; Dahlstrom, 1970], and it is an essential part in constructing balanced cross sections.

Elastic load modeling of fold-thrust belts can be seen as an effort to quantify the regional.

The elastic downwarp of a foredeep is an effect of crustal mechanics, but the basement top does not follow the downwarp, because its position is the sum of all downwarps, many of them elastic. Nonforedeep events of downwarp include the development of the Paleozoic passive margin, the late Triassic folding episode, and the telescoping of the preforedeep section during Andean thrusting. Anelastic and viscous elements of foredeep downwarp, essential in the modeling of basin stratigraphy, are comparatively small and hence neglected in the present analysis.

The deflection of the Moho from its

(poorly controlled) regional depth beneath the South American craton to the Andean root [James, 1971] also contains a percentage of elastic foredeep downwarp and Andean thrust overlap. In addition, it is expected to contain the adiabatic downwarp and sediment loading of the Paleozoic passive margin, the westward rise due to prePaleozoic or early Paleozoic extension and margin formation, the effect of Triassic compression, and an unknown but controversial amount of

magmatic addition to crustal thickness. In constructing a flexural-load model

for the sub-Andean of northwest Bolivia, three variables have been considered.

1. The rigidity D has a geometric expression X o as the distance between the zero edge of foredeep sedimentation and the center of the load depression. The zero edge of foredeep sedimentation is poorly defined as the edge of Brazilian shield rocks buried beneath a thin coat of Pleistocene beds. There are two choices

for locating the load depression. In a minimalist or fixist model, the load center would be located at the divide

between eastward Andean thrusting and westward Andean thrusting, about 135 km SW of the frontal thrust of Figure 2. In a mobilistic model, the load center would be located at the trough of the Andean Moho root below the Altiplano, about 300 km SW of .the frontal thrust.

2. The load as a force has a geometric expression as a maximum deflection Yo in the trough of the line-loaded downwarp. There are no independent constraints for

Roeder' Andean-Age Structure of Eastern Cordillera 33

the maximum deflection in the minimalist-

fixist model. The mobilist model explains the Andean Moho root as a load-deflected

crustal overlap. The deflection, therefore, should be in the same order of magnitude as the Moho root, about 30 km.

3. The location of the flexure relative to structural data is variable

horizontally along the line of cross section. There is no morphologically or geologically expressed flexural bulge beyond the line of zero deflection, possibly because of interference by undefined subsidence above cratonic extensional features.

For the present study, the horizontal location of the flexure has been fixed by prosecting linearly the base of the Quendeque beyond a foreland seismic line (industry, unpublished) to the northeast. This projection yields a zero edge of foredeep subsidence 100 km NE of the sub- Andean thrust front.

A continuous section of the southwest

flank of the Eva-Eva thrust sheet [Davila et alo, 1965] furnishes a located depth to the regional base of the Quendeque (for method, see Roeder et al. [1978]). Successful geometric deflection models are simply those that meet the zero edge of foredeep depression, the regional base foredeep near the Eva-Eva thrust and one of the two arbitrarily placed locations of maximum deflection.

There are large differences in the overall geometry of the successful models, but they coincide reasonably within the area of control, that is, in the foreland and in the external part of the sub- Andean.

The best fitting minimalist flexure model has a rigidity D of 1.9x1023 Nm, corresponding to an effective elastic slab thickness of 33 km, and corresponding to a flexural length X o of 230 km, or a flexural parameter of 9.7615x106 or A of 1.0244x10 -7. It meets the control depth at the Eva-Eva thrust sheet if its maximum deflection beneath the structural divide of the eastern Cordillera is 11.4 km.

The downdip extent of the deflected foreland beneath the sub-Andean fold- thrust belt is 130 km in this model.

The best fitting model of crustal overlap has a rigidity D of 1.8x1024 Nm, corresponding to an effective elastic slab thickness of 70 km, and corresponding to a flexural length X o of 400 km, or a

flexur_l parameter of 1.7x107 and A of 5.85xl 8. The flexed surface meets the

Eva-Eva control depth of 4 km if its maximum deflection, located beneath the Altiplano, is 27.5 km.

The downdip extent of the deflected foreland beneath the eastern Cordillera is 300 km in this model.

No attempt has been made to quantify the deflecting load. Therefore, it is interesting to compare the adopted models to the gravity-derived models [Lyon-Caen et al., 1985] applied to an Andean segment 700 km further south. The range of rigidities considered reasonable is about the same in both approaches. The minimum downdip extent of the foreland plate required to transmit the load as a deflection, of 150 to 200 km [Lyon-Caen et al., 1985] would favor the mobilistic model of the present study.

MECHANICS OF THRUSTING

Fold-thrust wedges based on a variety of flow laws describe and quantify the geological mechanism known as gravity spreading [Bucher, 1956; Elliott, 1976; Chappie, 1978], with tectonic transport up an inclined decollement in response to a wedge shape, to a forward topographic slope, and to a compressive push from the rear.

A rigid-plastic flow law applied to a moving fold-thrust wedge assumes a state of impending Coulomb-Navier failure and a geometry of critical taper [Davis et al., 1983], that is, a critical sum of topographic slope and decollement dip. The interplay of internal deformation, frontal growth by added thrusts, foreland sedimentation, and erosion can be described as causes and effects accompanying a wedge that must maintain its critical taper in order to move and to transmit crustal convergence to the surface.

In the theory of critical Coulomb fold- thrust wedges, shortening within emplaced thrust sheets indicates adjustment to a subcritical state. Frontal growth and, possibly, some forms of normal faulting indicate adjustment to a supercritical state. The taper is reduced by erosion, foreland sedimentation, and lithospheric deflection under the orogenic load. The taper is increased by tectonic thickening, by thrust imbrication below the axial panel, and by transport onto outer, less deflected foreland areas.

Material properties that determine the actual angles of the critical taper


include the shear strength of the wedge material, the shear strength of the base of deformation, and the ratio of pore fluid pressure to lithostatic pressure. In the sub-Andean belt these properties are unknown, but open to analysis and speculation.

STATE OF TAPER IN CORDILLERAN EAST SLOPE

A numerical look at the thrust-belt

data presented here follows the concepts and procedures by Davis et al. [1983]. Somewhat blurry results suggest that the main range of the eastern Cordillera is being tilted eastward above a "sticky," probably basement-involving western thrust belt. The results also suggest that the external portion of the sub-Andean belt is a new and still subcritical toe addition to the thrust sheet above the Main Andean Thrust. This segment is now undercritical but may have been higher, steeper, and overcritical in the recent past.

The topographic profile can be divided into an internal segment sloping eastward at m = 3.5 and into an external segment with m--0.6 . The sole of the thrust wedge, depicted on the cross sections (Figures 2, 3, and 4) as a decollement near the base of the sediments, dips at = 4.8 under the internal segment and at = 3.8 under the external segment. The wedge segments are 75 and 100 km long, respectively, and sufficient to satisfy the size requirement. The MAT may serve as the boundary between a weakly deformed, basement-involving, strongly eroded, but steeply sloping segment and a typical fold-thrust belt segment with style indicators of active thickening, subcritical state, and very low topographic slope.

Following the consensus for fractured rock complexes, Byerlee's law of an internal friction = 0.85 is assumed throughout the thrust wedge segments. For the basal friction b, a range has been considered between 0.7 for an attached and possibly basement-involving sole and 0.2 for a detached sole, floating on a through-going glide plane.

The ratio of fluid pressure to geostatic pressure A has also been considered at a range of between 0.3 and 0.8. This range spans the conditions of a body of granite and quartzite with strong erosion and bleed-off of any overpressure to conditions of a sedimentary series

involved in an average foredeep and foothills setting.

Table 1 shows observed and assumed input into the critical taper equation and the topographic slope predicted for the state of critical taper. Although the rock properties of the wedge segments are unknown, I believe that the assumed values bracket the actual values, and that the difference between observed and predicted slope does permit a dynamic interpretation of Andean thrusting.

Based on the data on Table 1, the internides are undercritical if there is no effective detachment at the wedge base. The model of crustal overlap does require an active low-angle thrust below the internides, but its assumed depth of 14 to 24 km may be below the validity of Coulomb mechanics and in the viscous realm [Platt, 1987]. A weak base in one of the early Paleozoic slates would make the east slope of the axial zone overcritical, but probably its basement-involved fold style keeps it in place.

The externides are undercritical even with a weak base. This is consistent with the structural style in part of the external sub-Andean belt, where folding and backthrusting appears to be thickening emplaced thrust sheets.

Other parts of the externides suggest quiet toe addition, presumably a reaction to an overcritical state. The introduction of cohesion into the wedge modeling may help to explain this contradiction [Dahlen et al., 1984]. The flat and possibly blind-thrusted toe ends of cohesive wedges are described to move without Coulomb failure while the more internal wedge part is building up to critical taper.

Based on measured data and the same assumptions made to describe the state of taper, the maximum length of an undeformed but transported toe slab is about 70 km [Dahlen et al. 1984, Equation 25]. This is in the same order of magnitude as the restored length of the thrust units shown in Figure 2: Eva- Eva 50 km Caquiahuaca, 35 km' Quiquibeycito, 70 km; and Marimonos 70 km.

These numbers suggest that cohesive prefailure transport and blind thrusting may have been the persistent thrust style in the sub-Andean belt since deposition of the Neogene clastics, perhaps similar to the mechanism suggested by Seely [1977].

The poorly known western thrust wedge involves sedimentary rocks up to the Neogene as well as probably the basement.


Table 1. Showing Data and Results of Coulomb-Wedge Models for Eas tenon Cordillera

a b c d e f g h Model Setting a i po/p I b A a crit

i State

1 Internides, dry wedge, attached

2 Internides, dry wedge, weak base

3 Internides

we t wedge, weak base

4 Internides, we t wedge, decollement

5 Externides, we t wedge, weak base

6 Externides, we t wedge, decollement

3.5 4.8 0.37 0.85 0.7 0.5 3.19 7.23

2.4 4.7 0.37 0.85 0.45 0.5 3.51 4.29

3.5 4.8 0.37 0.85 0.45 0.8 3.51 2.09

3.5 4.8 0.37 0.85 0.3 0.8 3.60 0.77

0.6 3.8 0.46 0.85 0.45 0.8 3.51 2.40

0.6 3.8 0.46 0.85 0.3 0.8 3.60 0.97

Very undercritical

Near critical, undercritical

Overcri tical

Very overcritical

Very undercritical

Near critical, undercritical

a Topographic maps are 1:250,000. b Semibalanced section. c Assumed [Turcotte and Schubert, 1982]. d Assumed (Byerlee v s Law) . e Assumed [Jaeger and Cook, 1969]. f Assumed [Davis et al., 1983, Table 1]. g Calculated [Davis et al., 1983, Equation (28)]. h Calculated [Davis et al., 1983, Equation (22)]. i Conclusion (preferred settings are underscored).

An attached state would allow slope steepening of up to 5 , but above a weak base, critical taper and transport below failure conditions would be reached already at a slope of 1.5 . PRESENT THRUST WEDGE

In its Pleistocene configuration, the eastern Cordillera shows a major east- vergent thin-skinned fold-thrust belt and, back to back with it, a west-vergent critical or subcritical thrust belt tilting the hinterland common to both belts clockwise or eastward. Most of about 15 km of uplift and part of the eastward tilt are a carryover of east dip in the

hanging wall of a major thrust ramp on the MAT. However, the present rotation is not caused by thrust movement over a ramp top in the MAT, but by the stacking of thrust sheets at the Altiplano foot of the axial mountain chain. This is suggested by the palinspastics of the sub-Andean thrust units in the foreland of the MAT. There is

no ramp top suggested in the stratigraphic record.

The steepening of the axial panel is still undercritical, pinned by basement- involved folds. The Cordillera is building a large volume of critical slope material to push the accumulated low-dip, weak-base material of the sub-Andean. It is being counteracted by erosion in the high


TRENCH COAST MISTI TITICACA MURURATO M.A.T. EVA-EVA

,11, I 100 KM

... ;,.' . . '"'"" ".::."9"'. =.

:Fig. 7. Crustal sketch cross section of Andes in northern Bolivia, located on :Figure l, showing a possible distribution of crustal domains, based on structural geology of eastern Cordillera, and on Moho root after data by Tames [1971]. Solid areas are ?hanerozoic sedLments in eastern Cordillera, Neogene voltanits, and Neogene accretionary wedge of ?eru-Chile trench. Dotted area is area of possible magmatic crustal addition during Oligocene and Neogene. Dashed lines are faults. Dashed-dotted [ines are boundaries between crustal domains with letter labels. Solid circles are earthquake foci gathered by Grange et al. [1984] into their section 2. A; Phanerozoic sediments of eastern Cordillera and Altiplano; B; crust of Andean foreland, footwall of Main Andean Thrust; C; crust of hanging wall of MAT, Precambrian continental crust. D; Triassic- to Paleogene-age crust accreted and deformed at innerwall of Peru- Chile trench; E; Neogene accretionary wedge; F volumetrically void space, possibly occupied by Neogene to Recent magmatic addition and M, Moho.

Cordillera, but not, at present, by foredeep deposition

The need for an out-of-sequence thrust as a late surface branch of the MAT is not

clear. It may be blocking and overriding the sub-Andean thrusts, thereby terminating the era of thrust-belt growth by cohesive slab push. Alternatively, it may be part of the thickening by conjugate thrusting and more efficient than a low- angle thrust.

The need for a west-facing thrust belt is also not clear. However, development of the western belt is essential for building critical taper and enough high mass in the median panel. There are many examples of backthrusting on the rear ends of thrust wedges [Roeder, 1973; Karig et al., 1979; Davis et al., 1983 Figure 10; Platt,

CRUSTAL BALANCING

The Andean Moho root in Bolivia [James, 1971] area-balances against a crustal overlap of 230 km with an excess of 10% (Figure 7). This excess is located beneath the Recent magmatic arc and may represent magmatic addition of material with crustal densities and velocities. This conclusion is poorly supported by the history of the continental margin as it is understood at present, and it is not supported by intracrustal data.

Crustal data are limited to the Moho

root [James, 1971] and to the outlines of an area of low-strength rocks or magmatic melts beneath the western Cordillera [Wigget, 1986].

A velocity inversion within the rooted part of the crust [Ocola and Meyer, 1972J had been interpreted as crustal overlap [Roeder, 1982] but has been rejected in newer refraction-seismic work [Wigget, 1986]. There are no Moho data on the cratonic crustal thickness and, hence, no crustal support of the foredeep slope. Any attempt to combine thrust-belt geometry with crustal geometry, therefore, can only be made based on the thrust-belt data. This is done in the following way.

The east Cordilleran fold-thrust belt suggests crustal thrust overlap between a hanging wall at surface and a footwall downwarped to 25 or 30 km.

The leading edge of the hanging-wall Moho may be located, by coincidence, near the footwall ramp top, with a Moho extending toward the Peru-Chile trench. However, no such surface is known there.

To maintain thrust contact with crust over crust, the dip length of the transcrustal ramp must be at least 190 km at a ramp angle of 10 or 11 . This ramp geometry is consistent with the Moho root and with the elastic flexure data of the foreland. A shorter ramp and a steeper cutoff angle do not produce a reasonably balanced cross section. A void space between the hanging-wall crust and the

Roeder: Andean-Age Structure of Eastern Cordill era 37

beveled footwall crust is assumed to be filled with material of crustal velocity and composition. The size of this space can be estimated volumetrically or by cross-sectional area.

Assuming crust-crust overlap over the full dip extent along the MAT, the hanging-wall crust thins from 35 km beneath the Altiplano to 15 km near the coast and the Neogene accretionary wedge. In this model, thrust-overlapping crust between the coast and the east edge of the sub-Andean foredeep covers about 90% of the cross-sectional area included by the Moho root.

Paleozoic lithofacies suggests that the pre-Andean edge of the continental crust was located perhaps 200 km further inboard than today, with the pre-Paleozoic crust thinning to 15 km or less at a distance of 180 km west of the Paleozoic depocenter and the Triassic magmatic arc. In this model, an additional 17 to 20% of the Moho root area is not original South American cratonic crust. This area is the present western Cordillera. It has reported outcrops of Precambrian basement, but it may consist either of magmatic arc or of deformed and plutonized accretionary wedge of Triassic to Cretaceous age, with minor lumps of Precambrian basement.

The Andean model suggested by crustal balancing, with a single large crustal overlap, differs from the Peruvian model of crustal imbrication [Suarez et al., 1983]. However, it is similar to a configuration that the Appalachians may have had before Atlantic peripheral bulging uplifted and flattened the crustal root and the crustal overlap.

Acknowledgments. I have received help from colleagues and friends at Yacimientos Petroliferos Fiscales Bolivia (YPFB) in La Paz and Santa Cruz, at Shell in The Hague and La Paz, and at Anschutz in Denver. The paper is published by permission of Shell in The Hague and of Anschutz in Denver. It has benefited much from reviews by David James and Albert Bally, and from editorial work by Annamarie Argandona.

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(Received August 4, 1986' revised July 16, 1987; accepted July 17, 1987.)

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