This document is downloaded from DR-NTU, Nanyang Technological University Library, Singapore. Architecture of transpressional thrust faulting in the San Title Bernardino Mountains, southern California, from deformation of a deeply weathered surface. Author(s) Spotila, James A.; Sieh, Kerry. Spotila, J. A., & Sieh, K. (2000). Architecture of transpressional thrust faulting in the San Bernardino Citation Mountains, southern California, from deformation of a deeply weathered surface. Tectonics, 19(4), 589–615. Date 2000 URL http://hdl.handle.net/10220/8429 © 2000 AGU. This paper was published in Tectonics and is made available as an electronic reprint (preprint) with permission of American Geophysical Union. The paper can be found at DOI: [http://dx.doi.org/10.1029/1999TC001150]. One print or electronic copy may be made for personal use only. Rights Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law. TECTONICS, VOL. 19, NO. 4, PAGES 589-615 AUGUST 2000 Architecture of transpressional thrust faulting in the San Bernardino Mountains, southern California, from deformation of a deeply weathered surface JamesA. Spotila Departmentof GeologicalSciences, Virginia Polytechnic Institute and State University, Blacksburg Kerry Sieh Divisionof Geologicaland Planetary Sciences, California Institute of Technology,Pasadena Abstract. To investigatethe architectureof transpressional boundariesexists, strike-slipdisplacement alone cannot deformationand its long-termrelationship to platemotion adequately accommodate relative movements of the in southern California, we have studied the deformation lithosphere [Dewey et al., 1998]. Instead, far-field patternand structuralgeometry of orogenywithin the San horizontal plate motions are manifest as complex Andreas fault system. The San Bernardino Mountains deformationalong the plateboundary, which can involve haveformed recently at the hub of severalactive structures significantvertical strain [Teyssieret al., 1995]. The that intersectthe SanAndreas fault eastof Los Angeles. accommodationof horizontal plate motion by convergent This mountainrange consistsof a groupof crystalline or divergentstructures is not a simpleprocess, and its blocksthat have risen in associationwith transpressive characteristicsarenot predicted by simpletectonic theory. plate motionalong both high- and low-anglefaults of a As a result, there are many modelsof transpressive complex structural array. We have used a deeply deformationthat are debatedin termsof geometry[e.g., weathered erosion surface as a structural datum to constrain Sylvester,1988; Lemiski and Brown, 1988] and mechanics the patternof vertical deformationacross fault blocksin and [e.g., Wilcoxet al., 1973; Mount and Suppe,1987] of adjacentto this mountainrange. By subtractingthe structures. hangingwall and footwall positionsof this preuplift In southernCalifornia the obliquitybetween plate horizonwe havedetermined vertical displacement along motiondirection and the San Andreasfault system,the two major thrust faults. We concludethat one fault, the primarytransform, is 27ø [DeMets,1995] (i.e., the "Big North Frontalthrust, has played a moresignificant role in Bend";Figure 1). The resultingoblique convergence is raisingthe largefault blocksand can explain the uplift of accommodatedin the far field by a wide fold andthrust belt all but a few crustalslivers. On the basisof thepattern of (the western TransverseRanges [Namson and Davis, displacementassociated with this thrust fault we have also 1988]). In the near field the San Andreas fault exhibits inferredfault zone geometry beneath the range. Rather than nearlypure strike-slipdisplacement, although localized simplysteepening into a high-anglefault zone or flattening rockuplift has occurred within the faultzone itself (e.g., into a decollement,the thrustfault may have a complex, Yucaipa Ridge [Spotila et al., 1998]) and acrossnarrow curviplanargeometry. The patternof rock uplift also beltsass6ciated with local geometricperturbations of the enablesusto calculate the total motion accommodated by fault zone (e.g., the restraining bend at SanGorgonio Pass thisorogeny. We estimatethat >6 km of convergence.(5% [Allen,1957; Jones et al., 1986]). Obliqueplate motion of the total plate motionin the last 2 Myr) has occurred. is thusaccommodated by two architecturesof deformation: This horizontalshortening is associatedspatially with the wide, internally deforming thrust sheets (i.e., 15-km-widerestraining bend in the SanAndreas fault zone decollements)and narrow, high-angle zones (i.e., flower near SanGorgonio Pass. The entirerange may thushave structures). These general cases mimic the two broad risenbecause of a smallgeometric complexity in the San architectures that have been observed elsewhere in Andreasfault ratherthan the obliquityof far-field plate transpressiveenvironments [Sylvester, 1988; Lemiskiand motion. Brown, 1988; Lettis and Hanson, 1991; Vauchez and Nicolas, 1991; BeauJoin, 1994]. However, the 1. Introducti6n relationshipbetween these deformation styles is not clear. By definition,continental transform plate boundaries are First, it is not obviouswhy two stylesexist. Does far- dominatedby horizontal plate motions. However, along fielddeformation represent distributed slip partitioning due transpressiveor transtensionalmargins, where significant to obliqueconvergence, while near-fielddeformation results obliquity between plate motion vectors and plate from localgeometric complexities along the strike-slip fault itself?.Second, it is not clearhow low-angleand Copyright2000 by the AmericanGeophysical Union. high-angle deformation zones interact at intermediate distancesfrom strike-slip fault zones. Do thrust faults Paper number 1999TC001150. merge with the strike-slip zone itself, or are they 0278-7407/00/1999TC001150512.00 completelydecoupled in the subsurface? 589 59O SPOTILAAND SIEH: ARCHITECTUREOF TRANSPRESSIONALTHRUST FAULTING 19 ø Sierra Nevada broad zone of convergence,sinistral shear, and clockwise rotation Mojave Desert diffuse zone of dextral shear Pacific Ocean California [ 100km .:?,• .•......... sinistralslip faults and clockwise rotation Arizona Mexico Figure 1. Activetectonic context of the SanBernardino Mountains (SBMs). The "Big Bend"of the SanAndreas fault occurs south of the Garlockfault (GF) andnorth of the smallrestraining bend at San GorgonioPass (circle). Relativeplate motion vector is from DeMets [ 1995]. ECsz,eastern California shearzone (gray); eTR, easternTransverse Ranges (ruled); N-IF, Newport-Inglewoodfault; Pen. Ranges,Peninsular Ranges; PMF, PintoMountain fault; SAF, SanAndreas fault (m, Mojave;sb, San Bernardino;cv, CoachellaValley segments);SGMs, San Gabriel Mountains;SJF, San Jacintofault; SJMs,San Jacinto Mountains; SM-CF, SierraMadre-Cucamonga fault; W-EF, Whittier-Elsinorefault; wTR, westernTransverse Ranges. To better understandhow horizontal plate motion is San Andreasfault zone or the low-angledeformation that manifestas crustaldeformation along transpressive strike- occursat greaterdistance from its trace. Although both slip fault systems,the architectureand kinematichistory of have been proposed as models for the SBMs [Sadler, structuralarrays at intermediatedistances to strike-slipfault 1982; Li et al., 1992], the subsurfacestructural geometry zonesmust be quantified. The San BernardinoMountains and displacements of the faults have not been well (SBMs) extend from near-field to intermediate distance constrained. Previous efforts have been stymied because from the San Andreasfault zone (Figure 1) and have been the SBMs consistmainly of crystallinebedrock that cannot elevatedover the pastfew million yearsin associationwith be palinspasticallyrestored in balancedcross sections. transpressivedeformation [Dibblee, 1975;Meisling and We have used a geomorphicfeature as a structural Weldon, 1989; Spotila et al., 1998]. This young datum to quantify the patternof rock uplift in the SBMs mountainsystem thus representsan ideal opportunityto and thereby place constraintson the displacementand study long-term transpressiveorogenesis associated with geometryof the major structures. Atop the SBMs and the San Andreas fault' system. The bulk of the range acrossthe surroundinglowlands is a low-relief, deeply consistof the broad Big Bear plateau, which has been weathered,granitic erosion surface (Figure 3). Geologic raised along two east-west trending thrust faults with and geomorphicrelationships argue that this surfaceis opposingdips (North Frontal thrust system(NFTS) and relict, in that it formed prior to recentmountain building Santa Ana thrust (SAT)) (Figure 2). Critical to and was subsequentlyseparated by the major thrustfaults understandingthe structuralkinematics of the SBMs is of the range [Oberlander, 1972; Meisling, 1984; Spotila, knowingwhether the subsurfacegeometry of thesethrusts 1999]. We have used the distribution of this easily mimics the high-angledeformation observed within the identified geologic marker to reconstructthe vertical SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 591 592 SPOTILAAND SIEH:ARCHITECTURE OF TRANSPRESSIONALTHRUST FAULTING displacement field with some certainty. Our results Mio-Pliocene Santa Ana Sandstone [Sadler, 1993]. demonstrate that the NFTS has been the dominant Basalts in this unit indicate