Various types of sedimentary basins that contain a greatvariety of sedimentary successions and structural attrib-utes form along strike-slip faults. These basins have com-monly been called “pull-apart basins.” However, webelieve that the term “strike-slip basin” is more suitablebecause “pull-apart basins” are only one of many varietiesthat may develop along strike-slip faults.

Strike-slip basins range from small sag ponds (length= tens of meters) to larger areas of subsidence (width of50 km). The basins are commonly small but very deep. Theyare typically elongate, with a typical length-to-width ratioof 4:1 and a maximum ratio of about 10:1. The surfacedimensions can be measured structurally from the bound-ing faults and flexures or physiographically from the areaof subsidence. The definition of a basin’s size and shapecan be difficult, however, because of (1) basin-margin defor-mation by younger folds and faults, (2) basin marginsdefined by sag rather than faulting, (3) bounding faultswith variable dip directions, (4) merging of surface faultsinto master faults at depth, and (5) displacement of basinmargins from basin depocenters by continued strike-slipmovement. Some types of strike-slip basins may also havesubhorizontal faults at their bases.

Classification of faults and basins. Strike-slip faults canform in various plate-tectonic settings along continentalmargins, within continental plates, within oceanic plates,and within zones of plate collision and extension (Figure1). Sylvester (1988) divided the faults into transform andtranscurrent types. Transform faults, which form plateboundaries and penetrate the entire crust, include ridgetransform faults that separate oceanic spreading ridges,boundary transform faults that separate crustal plates, andtrench-linked transform faults that are subparallel totrenches in convergent settings. Transcurrent faults, whichlie within crustal plates and penetrate only the upper crust,include indent-linked faults in areas of plate collision andtectonic escape or extrusion, intracontinental faults thatseparate different tectonic domains, tear faults that accom-modate displacement within or along the margins ofallochthonous blocks; and transfer faults that connect over-stepping fault segments.

In 1995, we suggested that strike-slip basins can be clas-sified into six different types on the basis of their geome-try and kinematic setting (Figure 2). The classificationprovides relatively easy recognition of basins in their ini-tial stages of formation. The basins generally form intranstensional settings, local areas of extension along strike-slip faults, and commonly are subsequently inverted intranspressional settings, local areas of shortening alongstrike-slip faults. As strike-slip basins evolve through timeand space, however, they may be strongly modified as aresult of significant translation along the principal strike-slip faults. For example, where basins encounter releasingbends (changes in strike of the principal fault that gener-ates local transtension) they undergo subsidence, and

where basins encounter restraining bends (changes in strikeof the principal fault that generates local transpression) theyundergo uplift and erosion. The basins or parts of thebasins are thus commonly subjected to multiple cycles ofsubsidence and uplift while undergoing translation, muchlike the motion of a porpoise as it moves through water.As a result, recognition of the initial nature of long-livedstrike-slip basins is generally difficult.

• Fault-bend basins typically develop at releasing bendsalong strike-slip faults (Figure 2a). Extension results asone fault block slides past and away from the other, caus-ing the sliding block to sag into the extending zone, thusforming an elongating area of subsidence along the faultbend. The basins are strongly asymmetric, have prominentcoarse-grained aprons along their principal displacementzones, and are commonly lens-shaped in map view.

• Stepover basins generally develop from transtension thatdevelops between the unconnected ends of two parallel-to-subparallel strike-slip faults or strands of the same fault.The en-echelon arrangement of the faults or strands canresult in local extension of the block between the faults andgeneration of an elongating basin through time (Figure 2b).These basins are typically more symmetric than fault-bendbasins and have coarse-grained aprons along both basinmargins. They may also contain abundant transverse struc-tures that can segment the basin into separate subbasinsand multiple depocenters. Stepover basins typically formbetween left-stepping faults in left slip and between right-stepping faults in right slip.

• Transrotational basins develop between strike-slip faultsas a result of the rotation of blocks about a subvertical axisin the same direction as the principal shear strain, clock-wise in right simple shear and counterclockwise in left sim-ple shear (Figure 2c). Triangular gaps or basins form alongthe margins of the rotated blocks, with the rate and mag-nitude of rotation depending upon the rate of shear strain.Subhorizontal detachment faults generally underlie thebasins and rotated blocks, separating them from unrotatedunderlying crust.

• Transpressional basins are generally long, narrow struc-tural depressions that lie parallel to but outboard ofrestraining bends in strike-slip faults (Figure 2d). They arecommonly bounded by regional oblique-slip or reverse

1146 THE LEADING EDGE OCTOBER 1999 OCTOBER 1999 THE LEADING EDGE 0000

Strike-slip basins: Part 1TOR H. NILSEN, Consulting Geologist, San Carlos, California, U.S.ARTHUR G. SYLVESTER, University of California of Santa Barbara, Santa Barbara, California, U.S.

Figure 1. Plate-tectonic settings of strike-slip faults(from Woodcock, 1986). The diagram is a northeast-ward-looking schematic representation of the collisionof India and Asia from the southern Indian Ocean.

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Editor’s Note: The Geologic Column, which appears monthly in TLE,is (1) produced cooperatively by the SEG Interpretation Committee andthe AAPG Geophysical Integration Committee and (2) coordinated byM. Ray Thomasson and Lee Lawyer.

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faults that dip back into the principal strike-slip displacement zone and generate positive“flower or palm-tree” structures adjacent toareas of transpressional uplift. Subsidence ofthese basins results from flexural loading of themarginal areas, leading to the development ofminiforeland-type basins that are commonlycomplicated by folding and lie adjacent to theuplifted blocks. Although coarse-grainedaprons derived from erosion of the upliftedblocks are generally well developed along theproximal margin, the basins may be dominatedby axial systems of sediment transport paral-lel to the uplifted blocks.

• Polygenetic basins develop as a result of localstrike-slip in larger regions of generally diver-gent or convergent tectonics (Figure 2e). Inextensional settings, transfer faults and accom-modation zones, which commonly link at var-ious scales the principal bordering normalfaults or segments of these faults, may containand bound small strike-slip basins. In conver-gent settings, strike-slip faults and basins maybe confined to the upper plate of allochthonousthrust sheets along which they accommodatedifferential rates or amounts of slip.

• Polyhistory basins develop where episodes ofstrike slip alternate with or are replaced byepisodes of extensional rifting, contractilethrusting, or other styles of deformation (Figure2f). The resulting basins are complex, can be ofalmost any size and shape, and incorporatemultiple episodes of subsidence, sedimenta-tion, and deformation under shifting tectonicsettings. Although these basins may also incor-porate the characteristics of many other typesof strike-slip basins, they tend to be even morecomplex because of their variable tectonicstyles.

Structural framework. Four principal factorscontrol the structural patterns that developalong strike-slip faults:

• the kinematic framework (transtensional,transpressional, or parallel)

• the magnitude of the displacement• the material properties of the rocks and sed-

iments in the deforming zone• the configuration of pre-existing structures

The strike of the fault relative to the block- or

1148 THE LEADING EDGE OCTOBER 1999 OCTOBER 1999 THE LEADING EDGE 0000

Figure 2. Diagrammatic sketches and mapsof six types of strike-slip basins (fromNilsen and Sylvester, 1995). (a) Fault-bendbasin (left) with map of La Gonzalez basin(right). (b) Stepover basin (left) with mapof part of Dead Sea rift (right). (c)Transrotational basins (black areas). (d)Transpressional basins (dotted areas) inmap view (left) and cross section (right). (e)Polygenetic basins (dotted areas) inregional extension (left) and regional com-pression (right). (f) Polyhistory basins (dot-ted areas) in map view (right) initiallyformed as an asymmetric rift basin (left).

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plate-motion vectors determines whether the fault or faultsegment has components of transtension or transpression.The basins that develop along and adjacent to the faultsmay change either gradually or abruptly through timeand space from transtensional to transpressional. Theupward-branching “flower” or “palm-tree” structures char-acteristic of both settings range in cross-section from sim-ple, single-strand features with uniform dips to complex,multistrand features with variable dip amounts and direc-tions (Figure 3). In transpressional zones, most faults thatmake up “positive flower structures” in cross-section willhave apparent reverse senses of slip, whereas in transten-sional zones, most faults that make up “negative flowerstructures” will have apparent normal senses of slip.

In cross-section, positive flower structures will gener-ally have an overall antiformal character with abundantfolds that result from the net shortening, whereas the neg-ative flower structures will generally have an overall syn-formal character with minor folds that result from netextension. Because the overall strike-slip deformation mayinclude local oblique as well as dip-slip displacements,rotated blocks, and both structurally higher rocks charac-terized by brittle failure and structurally lower rocks char-acterized by more ductile failure, all faults must be studiedcarefully in four dimensions in order to reconstruct thedeformation pattern.

The geometry of the resulting strike-slip basins is con-trolled by numerous factors including:

• basin-forming process and type of strike-slip basin

• amount of lateral and vertical curvature of the princi-pal bounding faults

• depth to the brittle-ductile transition in the underlyingcrust

• nature of and structural fabric of the basement rocks• amount of displacement along the principal fault and

all other faults• age of the basin relative to the age of the principal

bounding faults• length of overlap of overstepping fault segments

Strike-slip basins can be divided into “hot” and “cold”types based on whether the mantle has been involved intheir formation. In hot basins, uniform-extension modelswith modifications for lateral heat loss have been appliedwith some success. In cold basins, which are generallythin-skinned, postdeformational thermal subsidence isgenerally insignificant. As a result of greater lateral heatloss, narrower basins usually subside at faster rates thanwider basins. Although rates of subsidence and sedimen-tation during basin formation may be extraordinarily highin strike-slip basins, rates of uplift and erosion duringbasin uplift can be equally high. Because many strike-slipbasins pass through multiple phases of subsidence anduplift during their complex evolution, different parts of thesame basin may undergo rapid uplift and rapid subsi-dence at the same time.

Depositional framework. The sedimentary fill of strike-slip basins is commonly extraordinarily variable and com-plex, depending in part upon the extent to which the basinsare submarine, sublacustrine, and/or subaerial. The fol-lowing characteristics appear to be common among the

0000 THE LEADING EDGE OCTOBER 1999 OCTOBER 1999 THE LEADING EDGE 1149

Figure 3. Conceptual diagrams of flower or palm-treestructures in right simple shear (from Sylvester, 1988).(a) From Lowell (1972). (b) From Sylvester and Smith(1976). (c) From Woodcock and Fisher (1986). (d) FromBartlett and others (1981). (e) Adapted with modifica-tion from Ramsey and Huber (1987). (f) From Steel andothers (1985).

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basin fills of continental or continental-margin strike-slipbasins (Figure 4):

• diverse depositional facies, commonly including fault-scarp breccia, talus, landslide, alluvial-fan, fluvial, delta,fan-delta, shoreline, shallow- and deep-lacustrine, shallow-and deep-marine, evaporite, chemical precipitate, and car-bonate

• an asymmetric distribution, with the finer grained anddeeper water deposits accumulating subparallel to andadjacent to the most active strike-slip margin

• distinctive coarse-grained basin-margin facies, typicallysmall and steep debris-flow-dominated alluvial fans alongthe principal basin-bounding fault margins vs. large andgently sloping streamflow-dominated alluvial fans alongthe inactive or less active basin margins

• dominated volumetrically by axial depositional systemsoriented parallel to the principal bounding fault system

• abrupt facies changes• very diverse petrography• derived from multiple provenance areas as a result of lat-

eral displacement• texturally and compositionally immature because of the

generally short transport distances from provenance tobasin

• very thick stratigraphic succession compared to basin size

• basinwide to basin-margin upward-coarsening parase-quences

• high sedimentation rates of ~3 mm/y• abundant seismically triggered synsedimentary defor-

mation and slumping• depocenters that migrate laterally along the principal

bounding strike-slip fault in the same direction as themigration of the provenance areas

Oceanic strike-slip basins, typically developed adjacentto or within transform faults offsetting spreading ridges,will contain wholly different features, because they nor-mally develop far from major continental sources of sed-iment. These basins commonly contain only brecciasderived from uplifted oceanic crust, hemipelagic andpelagic clays, and associated biogenic sediments. Thepreservation potential of these basins is very low, however,because of eventual subduction of the oceanic crust in theplate-tectonic cycle.

Recognition of ancient strike-slip basins. Reliable crite-ria for the recognition of strike-slip basins may be devel-oped from both structural and depositional characteristics.Useful criteria include the following relationships:

• lateral offsets of matched provenance areas and deposits,

1150 THE LEADING EDGE OCTOBER 1999 OCTOBER 1999 THE LEADING EDGE 0000

Figure 4. Tectonic and sedimentary comparison of Hornelen Basin (Norway), Ridge Basin (California), and theLittle Sulphur Creek basins (northern California) from Nilsen and McLaughlin (1985). Orientations and scales ofbasins vary considerably but are shown here at approximately the same orientation and size for comparison. Thelength of the basins is approximately 50 km for Hornelen, 25 km for Ridge, and 10 km for the Little Sulphur Creek.MF = Maacama Fault. SAF = San Andreas Fault. SGF = San Gabriel fault.

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especially coarse-grained facies• lateral offsets of the same or related depocenters• lateral offsets of locally unique rock types or well-dated

geomorphic features such as shorelines and incised valleys

• presence of known strike-slip faults on one or morebasin flanks

• presence of en echelon folds on the flanks of or betweenmapped faults that systematically step in the same manner

• lateral migration of the basin depocenter parallel to theprincipal flanking strike-slip fault

• presence of coarse-grained landslide, alluvial-fan, fan-delta, submarine/and sublacustrine-apron, and relateddeposits along the principal flanking basin-marginstrike-slip fault that cannot be compositionally matchedwith the presently juxtaposed provenance

• presence of very thick but laterally restricted sedimen-tary successions characterized by high sedimentationrates, variable facies, abrupt facies variations, variablepetrography, and extensive upward-coarsening parase-quences

• localized uplift and erosion associated with abundantunconformities within and along the basin margins thatare coeval with adjacent thick sedimentary successions

• inability in seismic or structural cross-sections to cor-rectly two-dimensionally balance stratigraphic volumesor lengths because of strike-slip perpendicular to theplane of the section

The recognition of strike-slip basins is commonly difficult,however, because of numerous factors, including:

• their depositional and structural histories are complex• lateral movements along the principal zones of strike-

slip displacement may be difficult to prove conclusively• lateral movements commonly detach, rotate, and trans-

late basins or parts of basins from their places of origin,making paleogeographic and palinspastic restorationsdifficult

• polycyclic episodes of subsidence and uplift commonlyremove major parts of the stratigraphic record and struc-tural history of the basins

• the sedimentary fill of many basins in intracontinentaland continental margin settings is entirely nonmarine,resulting in poor stratigraphic correlation and age control

• two-dimensional cross-sectional views of the basins mayresemble rift and foreland basins

• strike-slip faults may be reactivated as other types offaults as tectonic settings change

• oceanic strike-slip basins are commonly destroyed bylater subduction.

Conclusions. At least six main types of strike-slip basinscan be defined on the basis of their fault patterns andmechanisms of formation. The basins form in diverse tec-tonic settings and are commonly deformed and reformedas fault blocks rise, fall, converge, diverge, and are later-ally translated in space and time. The form, structure,

depositional settings, and history of these basins dependupon the complex interplay of structural, depositional, cli-matic, and paleogeographic factors. Most long-lived strike-slip basins are polycyclic and undergo repeated episodesof generally transtensional subsidence and transpressiveuplift within complex flower structures. Laterally dis-placed provenance areas, basin-margins, sedimentaryfacies, and depocenters provide general criteria for therecognition of areas of strike-slip faulting.

An understanding of strike-slip basins may beenhanced by a thorough understanding of how other typesof basins form and develop. Because strike-slip faulting isa significant component of many plate-bounding andintraplate settings, better understanding of strike-slipbasins will also aid in the understanding of other tectonicsettings. Continued fault displacement and polycyclicepisodes of sedimentation and uplift commonly place mostlong-lived strike-slip basins in the general category of“complex basins.” Their original paleotectonic frameworkmay be difficult to recognize and restore without detailed4-D reconstructions.

Suggestions for further reading. Basin Analysis: Principles andapplications by Allen and Allen (Blackwell, 1990). “Evolutionof pull-apart basins and their scale independence” by Aydinand Nur (Tectonics, 1982). “The types and roles of stepoversin strike-slip tectonics” by Aydin and Nur (in Strike-slipDeformation, Basin Formation, and Sedimentation, SEPM, 1985).“Pull-apart” origin of the central segment of Death Valley,California” by Burchfiel and Stewart (GSA Bulletin, 1966).“Deformation and basin formation along strike-slip faults” byChristie-Blick and Biddle (in Strike-slip Deformation, BasinFormation, and Sedimentation). “Seismic characteristics andidentification of negative flower structures, positive flowerstructures, and positive structural inversion” by Harding(AAPG Bulletin, 1985). “Identification of wrench faults usingsubsurface structural data: Criteria and pitfalls” by Harding(AAPG Bulletin, 1990). “Convergent wrench fault and posi-tive flower structure, Ardmore Basin, Oklahoma” by Hardinget al. (AAPG Studies in Geology Series 15, 1983). “Tectonics ofsedimentary basins” by Ingersoll (GSA Bulletin, 1988).“Development of pull-apart basins” by Mann et al. (Journal ofGeology, 1983). “Comparison of tectonic framework and depo-sitional patterns of the Hornelen strike-slip basins of Norwayand the Ridge and Little Sulphur Creek strike-slip basins ofCalifornia” by Nilsen and McLaughlin (in Strike-slipDeformation, Basin Formation, and Sedimentation). “Strike-slipbasins” by Nilsen and Sylvester (in Tectonics of SedimentaryBasins, Blackwell, 1995). “Characteristics of strike-slip fault sys-tems” by Reading (in International Association of SedimentologistsSpecial Publication 4, 1980). “The Marinduque intra-arc basin,Philippines: Basin genesis and in situ ophiolite developmentin a strike-slip setting” by Sarewitz and Lewis (GSA Bulletin,1991). “Origin of the Yaracuy Basin, Bocono-Moron fault sys-tem, Venezuela” by Schubert (Neotectonics, 1986). “Strike-slipfaults: by Sylvester, (GSA Bulletin, 1988). The San Andreas FaultSystem by Wallace (USGS Professional Paper 1515, 1990). “Therole of strike-slip fault systems at plate boundaries” byWoodcock (Philosophical Transactions of the Royal Society ofLondon, 1986). LE

Acknowledgments: We thank anonymous reviewers for helpful critiquesand Ray Thomasson for encouraging us to complete this first part of abrief but hopefully useful review of a most complex subject.

Corresponding author: T. Nilsen, nilsen@pacbell.net

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