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North America Transform Deformation JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B10, PAGES 19,975-20,010, OCTOBER 10, 1994 Deformation across the western United States: A local estimate of Pacific-North America transform deformation EugeneD. Humphreysand Ray J. Weldon II Deparmaentof GeologicalSciences, University of Oregon,Eugene Abstract. We obtain a locally basedestimate of Pacific-NorthAmerica relativemotion andan uncertaintyin this estimateby integratingdeformation rate alongthree different pathsleading west acrosssouthwestern North America from east of the Rio Grande Rift to nearthe continentalescarpment. Data areprimarily Quaternary geologic slip rate estimates,and resultingdeformation determinations therefore are "instantaneous"in a geologicsense but "longterm" with respectto earthquakecycles. We deducea rate of motionof the Pacificplate relative to North Americathat is 48 :E2mm/yr, a rate indistinguishablefrom that predictedby the globalkinematic models RM2 and NUVEL-1; however, we obtain an orientation that is 5-9 ø counterclockwise of these models. A morewesterly motion of the Pacificplate, with respectto North America, is calculatedfrom all threepaths. The relativelywesterly motion of the Pacificplate is accommodatedby deformationin the NorthAmerican continent that includes slip on relativelycounterclockwise-oriented strike-slip faults (including the SanAndreas fault),which is especiallyrelevant in andsouth of theTransverse Ranges, and a margin-normalcomponent of net extensionacross the continent,which is especially relevantnorth of the TransverseRanges. Deformationof the SW United Statesoccurs in regionallycoherent domains within which the styleof deformationis approximately uniform.In thevicinity of theTransverse Ranges, two importantshear systems splay fromthe SanAndreas fault: the easternCalifornia shear zone trending NNW fromthe eastemTransverse Ranges and the trans-Peninsularfaults trending SSE from the westernand central Transverse Ranges. Within the TransverseRanges the right-lateral SanAndreas fault steps left, seeminglyrequiring large amounts of convergencethere. However,most of thisconvergence is avoidedthrough a "funnelingflow" of the crust towardthe westernTransverse Ranges and into the relativelynarrow central California CoastRanges and the northernmotion of theMojave. The formerprocess involves an alternationof rotationdirection from counterclockwise(in and southof the central TransverseRanges) to clockwise(in the westernTransverse Ranges). Introduction regional plate margin kinematics consistent with the globally derived Pacific-NorthAmerican plate velocity, The occurrenceof broadlydistributed young faulting in the westernUnited States(Figure la) affordsan oppor- these models included unrealisticallygreat deformation tunity to study the process of continental transform rates somewherein the southernCalifornia region. The deformationin a well-studiedregion. Essentialto sucha models of Hill [1982] and Bird and Rosenstock[1984] study is an understandingof the regionalkinematics of include excessiveconvergence rates acrossthe length of this broadlydistributed and diversedeformation field. An the TransverseRanges. Weldon and Humphreys[1986] importantaspect is knowledgeof the far-field plate introduceda model that has relatively low convergence motionsthat fundamentallydrive transformdeformation. rates across the central and eastern TransverseRanges Previous kinematic models of the southwestern United and high convergencerates acrossthe westernTransverse States have been constructed so that the total deformation Ranges by including relatively high rates of strike-slip across the region is consistentwith global kinematic faulting and a E-W orientedshortening in the continental models such as RM2 [Minster and Jordan, 1978] or borderland. They also included no deformation east of NUVEL-1 [DeMets et al., 1990]. In order to keep the San Andreas system south of the Garlock fault. Saucier and Humphreys [1993] and Humphreys and Weldon [1991] included deformation east of the San Copyright1994 by theAmerican Geophysical Union. Andreasfault and still found that NUVEL-1 boundary conditionsresulted in margin-normalcontraction. Papernumber 94JB00899. In this paper we considerthe deformationencountered 0148-0227/94/94JB-00899505.00 along three paths leading from the North American 19,975 19,976 HUMPHREYS AND WELDON: WESTERNU.S. DEFORMATION a / GreatNorthernBasin -/- BasinandRange// / ColoradoPlateau 0 -,,--- • NorthAmerica Iio 0 250 500 750 1000 I00 Figure1. Faukmap of axeascrossed by thethree paths of integrationacross the Pacific-North America boundary.(a) The deforming SW United States and adjacent regions. Heavy lines represent the most important faultsthat accommodate relative plate motion, and lighter lines represent other faults. Paired parallel lines are spreadingcenters. Bars are on downthrown sides of normal faults. Three paths are shown leading from stable NorthAmerica to thePacific plate. Arrows indicate velocity with respect to stableNorth America. Map symbolsare HF, Hurricane fault; LMF, Lake Mead fault; WFF, Wasatch Front fauk; and LVSZ, Las Vegas shearzone. (b) Faukmap of thesouthern California region. Projection is oblique Mercator projection about the RM2 Pacific-NorthAmerica pole [Minster and Jordan, 1978] (which is indistinguishablefrom the projection aboutthe NUVœL-1 Pacific-North America pole [DeMets et al., i990]). Map abbreviationsare BPF, Big Pine fault;CBF, Coronado Bank fault; ECSZ, Eastern Califomia shear zone; FCF, Furnace Creek fault; HMF, Hunter Mountainfault; IF, Imperialfault; LSF, Laguna Salada fault; N-IF, Newport-Inglewoodfault; NDVF, Northern DeathValley fault; OVF, Owens Valley fault; PVF, Panamint Valley fauk; RCF, Rose Canyon fault; SBM, San BernardinoMountains; SDTF, San Diego Trough fault; SDVF, Southern Death Valley fauk; SGF, San Gabriel fault;SGM, San Gabriel Mountains; SGP, San Gorgonio Pass; SIF, SanIsidro fault; SMB, Santa Maria Basin; SMF,San Miguel-Vallicitos fault; SSF, San Simeon fault; SYF, Santa Ynez fault; and VB, Ventura Basin. HUMPHREYS AND WELDON: WESTERN U.S. DEFORMATION 19,977 Sierra Nevada Range SMB--' MojaveDesert ] Outer ¸ Borderland Ran .. .. .. ß ß . ß ß ß . ... ß . ß .. ß ß . .. .. ß ß ß ... ß ß : . \ ß . .' \ . .. ß . ß ß \ ß ß . - \ ß . Figure 1. (continued) interiorto thePacific plate (Figures la and2). In doing global kinematicmodels, and the precisionof our so, we obtainboth a descriptionof the transform-estimate is comparableto thatoffered by the presently accommodatingdeformation and a locallybased estimate availableglobal kinematic models RM2 and NUVEL-1. of Pacific-NorthAmerica plate motion. The estimate of We obtaina velocityfield that avoids high (--1 cm/yr) relativeplate motionthat we obtainis independentof convergencerates both in the central to eastern 19,978 HUMPHREYS AND WELDON: WESTERN U.S. DEFORMATION Figure9 • Sierra Nevada Path ]% Transverse RangesPath Peninsular RangesPath s \ \ \ ... ß .. Figure2. SouthernCalifornia index map. Shownare the three paths (and their subpaths) used in integrating deformationto obtainour locally based Pacific-North America relative velocity estimate. Also shown are the areasof Figures9-13. HUMPttREYS AND WELDON: WESTERN U.S. DEFORMATION 19,979 TransverseRanges and in the offshoreregion by having of years). In the absenceof suchdata we use longer- thePacific plate move more westerly, relative to North termslip rate estimates, and in lieuof reliablegeologic America,than is derivedfrom the global models, and by data,we consider geodetic data. Where geodetic data are includingregional rotations and a greaterrole for faulting used,care is takento avoidthe elasticstrain field near eastof theSan Andreas fault. majorfaults. This priority in dataselection is motivated by a desirefor maximumconsistency. Descriptionof Method An importantaspect of thisanalysis is a formal inclusion of uncertainty. To describethe uncertainty Therelative motion between two points can be found associatedwith eachsn'ucmre encountered along the by integratingvelocity changes along the lengthof an lengthof a path,we ascribeprobability functions to the arbitrarypath connecting the two points[Minster and rateand orientation chosen for thatsu'ucture. If velocity Jordan,1984]. In practice,this calculationusually datafor activestructures encountered along a pathare involvesa summationof deformationrates for known independentof one another,the probabilityfunction activefaults encountered along the chosenpath [e.g., describingtotal motion encountered along the path can be Weldonand Humphreys, 1986]. Integrationalso should determinedby convolvingthe probabilityfunctions for includevelocity gradients across rotating rigid blocks and eachof the featuresconsidered. Because independent acrossregions of continuouslydistributed deformation. If velocityestimates can be determinedfor severalpaths a completeaccounting is madeof all velocitychanges joining two points,we cancombine the individual-path encounteredalong any chosenpath, then the velocity estimatesof relativevelocity by takingthe productof calculatedat theend of thepath gives the correct relative theirend-of-path probability functions to providea better motionbetween the two points.This is trueregardless of estimateof the relativevelocity between two points. If the natureof the deformationfield awayfrom the path, datafrom each path are not completely independent from eitheron the surface of theEarth or beneathits surface. one another,the estimatedprobability function for the Our path choices(Figure 2) are guided by the
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