Subsurface Imaging of the Garlock Fault, Cantil Valley, California

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B9, PAGES 14,461-14,479, AUGUST 10, 1991

SubsurfaceImaging of the Garlock Fault, Cantil Valley, California

JOHN N. LOUIE AND JIZENG QIN

Dept. of Geosciences,Penn State University, University Park, Pennsylvania

Imaging of the Garlock fault from seismic reflection data tests tectonic models for the Mojave Desert region of southernCalifornia. Models developed from geologic and geodetic evidence of fault movement rates disagreeon whetherthe Garlock fault shoulddip north or south. Such models are further at odds with focal mechanismand dislocationanalyses consistent with a vertical strike-slip fault. Our analysisdemon- stratesa nonvertical,southward dip for the Garlock fault. Consortium for Continental Reflection Profiling Mojave line 5 collectedseismic reflection data acrossthe Garlock in Cantil Valley, where the 3-kin-deepCan- til pull-apart basin has developedbetween the southwestand east branchesof the fault. We analyze the reflectiondata for evidenceof the fault's structureto 5 km depths. Garlock fault plane reflectionson unpro- cessedshot gathersmerge with the first arrival as the sourceprogresses to the surface trace of the east branch,explicitly tracingthe Garlock fault as a seismicreflector from depthto the mappedcontemporary fault trace. The apparentvelocity of thesereflections gives a 37ø+10ø southdip for the eastbranch of the fault. A prestackKirchhoff sum migrationimages the east branchreflector with a 45ø southdip. Our migrationproc- ess fully accountsfor the bending of seismic rays through the strong lateral velocity variations in Cantil basin. The south dip of the Garlock's east branch togetherwith basementsteps that decreaseto the south suggestthat Cantil basin developed from a 0.4-1 mm/yr component of extension normal to the fault. Developmentof the basinby detachmentprovides a mechanismto widen it as left-lateralmotion lengthensit, allowing the pull-apartto maintainthe globally constantlength-to-width ratio of 3. Detachmentby the Gar- lock at Cantil Valley similarly requires0.4-1 mm/yr of dextral strike slip on a northweststriking fault suchas the Helendale that cuts the Mojave between the Garlock and Pinto Mountain faults. Such motion is consistentwith regional tectonicmodels.

INTRODUCTION 1975; Carter, 1980, 1982]. About 60 km of offset have accu- Conflictsbetween tectonic models previously suggestedfor mulated since the early Cenozoic [Smith, 1962; Smith and the Mojave Desert region of southernCalifornia could be Ketner, 1970]. As one of the major active elements of the resolvedby the subsurfacemapping of active faults bounding regionalSan Andreasfault system,the Garlock fault plays a key the region. The questionof whetherthe Garlockfault dipsto role in the tectonics of southern California. the north or to the southrepresents the essenceof the conflict The fault can be broken into two major segments,which we betweenseveral of thesemodels. We analyzeseismic reflection call the southwestbranch and the east branch (Figure 2). The data recorded over the Garlock fault in order to trace the fault southwest branch, striking northeasterly, exhibits a complex, from its mappedsurface location to shallow crustaldepths. anastomosing surface trace, aseismic creep, and continuous New seismicdata analysisand migrationmethods can be used minor seismicity. The east branch strikeseast-northeast to east- to obtain subsurfaceimages of the fault. These methodsare erly and in contrasthas a simple, locked trace and only sporadic basedon the recognitionand modelingof strongfault plane seismicity [Astiz and Allen, 1983; Louie et al., 1985]. The reflectionsand on prestackmigration through a stronglylaterally characteristicsof the Garlock fault's segmentationare similar to heterogeneousvelocity model. The lateralheterogeneities, con- the segmentationof the San Andreas fault systeminto creeping strainedby geologicaland geophysical data, require a migration and locked sectionshaving different seismicpotentials proposed usingcurved ray paths. The subsurfaceimages of the Garlock, by Allen [1968]. Much of the recorded seismicity associated togetherwith the velocitymodel and its constraints,suggest a with the Garlock occursnear the junction of the southwestand southeastdipping, active listric fault. Sucha pictureexplains east branches,in Cantil Valley [Louie et al., 1985; Dokka and the continuityof midcrustalreflectors beneath the Garlockand Travis, 1990, Figure 6]. Left-lateral strike-slip focal mechan- indicatesa mechanismfor the growthof Cantil basin. isms predominate[Astiz and Allen, 1983]. Many workers [Bird and Rosenstock, 1984; Carter et al., Tectonic Setting 1987; Dokka and Travis, 1990; Golombeck and Brown, 1988; The Garlock fault separatesthe Basin and Range from the Luyendyket al., 1985; Ross et al., 1989] believe this segmenta- Mojave Desert(Figure 1), two of southernCalifomia's major tion of the Garlock, particularly its bend from northeasterlyto tectonicprovinces. Major late Tertiaryand Quaternarytectonic easterlystrike, to be related to tectonicblock (or flake) displace- featuresend abruptlyat the fault, which promptedDavis and ments and rotations within the Mojave and the Sierra Nevada Burchfiel[1973] to call it an intracontinentaltransform structure. (Figure 1). Paleomagnetic measurements suggest clockwise Left-lateralslip on the Gafiock may have begun in the late rotationof the southernmostSierra Nevada, or TehachapiMoun- Miocene [Loomisand Burbank, 1988] and has probablycontin- tains by 45 ø [Kanter and McWilliams, 1982], the western ued at rates close to 10 mm/yr throughthe last 5 m.y. [Smith, Mojave by 25 ø [Golombeckand Brown, 1988], and the central Mojave by 50ø-100ø [Ross et al., 1989]. Much of this rotation is constrainedto the Miocene before 16 Ma, perhapsdue to a broad simple shear that resulted in oroclinal bending across Copyright1991 by the AmericanGeophysical Union. severalareas of southernCalifornia [Luyendyket al., 1985]. Paper number 91JB01273. To bend the active Garlock fault, however, some rotation 0148-0227/91/91JB-01273505.00 must be occurringat present. Carter et al. [1987] dated the 40 ø

14,461 14,462 LOUIEAND QIN: IMAGINGOF THE GARLOCK FAULT

BASIN

% BAR-•. STOW'• MOJAVE

DESERT

TRANSVERSE Pinto Mtn.

Fig. 1. Tectonicmap of the Mojave Desert and surroundingregions in southernCalifornia includingmajor faults from Jennings[1975] and geographicfeatures. Shaded areas indicate the blocksof a schematictectonic model for the Mojave based on modelsby Carter et al. [1987] with somemodifications according to the model of Dokka and Travis [1990]. The block boundaryfollowing the Helendalefault has an interpreted0.4-1 mm/yr rate of dextral slip. The long arrows suggestthe directionof relativeplate motion. Half arrowsindicate the senseof motionsacross faults and betweenblocks. The shortsolid arrowslocate areas of compressionfrom block rotation,while the openarrows locate areas of extension.

of clockwise rotation of the eastern TransverseRanges (Figure produce thrusting at the White Wolf fault [Stein et al., 1) as post-late Miocene. They suggestedthat 100 km of right- 1981](Figure1) and explainthe tilting of Quaternarysediments lateral shear, distributed over a broad north-northwesttrending in the E1 Paso Mountainsnorth of Cantil Valley (Figure 2). zone between the central and eastern Mojave, are causing the Bird and Rosenstock's[1984] prediction of a north dipping rotation. Northwest trending, right-lateral faults such as the Garlock fault (their Figure 5) suggeststhat it could flatten to Helendale, Lockhart, and Blackwater [Dibblee, 1961] that cut form the master decollementcontrolling Basin-and-Rangeexten- throughthe entire Mojave Desert betweenthe Garlock and Pinto sion between the Sierra Nevada and Death Valley (Figure 1). Mountain faults would carry the shear(Figures 1). The Garlock'sappearance as a boundarybetween regions having Dokka and Travis [1990] developed a tectonic block model very different Cenozoic deformational styles [Davis and for the Mojave region from mappingof thesenorthwest trending Burchfiel,1973] would be an obviousconsequence of a decolle- faults and geodeticmeasurements [Sauber et al., 1986]. Their ment coming to the surface. However, Bird and Rosenstock model (Figure 1) places9-14% of the total Pacific-NorthAmer- [1984] find smaller deformation rates for active Basin-and- ica transform shear, or-65 km, within the south central Mojave Rangeextension north of the Garlock (2-5 mm/yr) than $auber over the last 11 m.y. However, they do not find evidence for et al. [1986] measure for the dextral fragmentation of the the northwest continuationof any of the right-lateral faults to Mojave Desert region (5-10 mm/yr). One might expect the the Garlock. The Blackwater fault may be an exception. West detached, unrooted blocks above a decollement to be more of it, Dokka and Travis [1990] do not model any dextral shear active than an adjacentrooted region. affectingthe southside of the Garlock in the region of Figure 2. Aside from the questionof the directionof dip of the Garlock They attributethe bending of the Garlock trace to the east to fault, a more fundamentalissue is whether a strike-slip fault can oroclinal folding. have a significantlynonvertical dip on a large scale. Brady and Bird and Rosenstock [1984] assembled available geologic Troxel [1981] have mapped south dips at the surfacenear the information on fault offsets and rates of motion into a easternend of the Garlock. Serpa and Dokka [1988] proposeda comprehensivetectonic block model for the present-daydefor- southdip for the fault to severalkilometers depth from seismic mation of the Mojave as well as the rest of southernCalifornia. reflectiondata in the Rand Mountains (Figure 2). Such interpre- Their model assumesrigid blocks, with a few scatteredareas tations are at odds, however, with the vertically dipping fault exhibiting continuumdeformation. Central to the model is the planes interpretedby seismologistssuch as Astiz and Allen clockwise rotational motion of two blocks comprising the [1983] from focal mechanismsor by Lippincott et al. [1985] Mojave Desert, separatedby the Blackwater-Calicofault (Figure from dislocation models of groundwater level fluctuations. 1). With this geometry,Bird and Rosenstock[1984] predict These conflictssuggest close investigationof the Garlock fault oblique thrustingalong a north dipping southwestbranch of the to resolve whether it has a nonverticaldip and, if so, whether it Garlock fault. They opt for convergenceacross the southwest may detachthe Basin and Rangeto the north or the Mojave to branch rather than extension across the east branch in order to the south. LOUIEAND QIN: IMAGING OF THE GARLOCK FAULT 14,463

Deep Structure high 8.3 km/s sub-Moho velocities. This anomaly is not offset Several geophysical studies have attempted to directly by the San Andreas fault, prompting them to suggestthat the observe the geometry of tectonic structuresbelow the Mojave plate boundary in the mantle "... is displaced from the San Desert. An extensive Consortium for Continental Reflection Andreas fault at the surface" [Hadley and Kanamori, 1977, p. Profiling (COCORP) deep crustal reflection survey probed the 1469]. central Garlock fault and the western Mojave in 1982 [Cheadle This displacementof the plate boundary is supportedby the et al., 1985, 1986]. Lines 3 and 5 of this survey (Figure 2) tomographicinversions of Pg and Pn phase delays performed crossed the Garlock. Louie et al. [1986], Louie [1990], and over southern California by Hearn and Clayton [1986a, b]. Serpa and Dokka [1988] have examined shallow and midcrustal Their results outline a -5% velocity variation at the Moho reflectionsfrom line 3. This paper analyzesthe data from line below the central Mojave, while mid-crustalvelocities vary most 5. Cheadle et al. [1986] describethe surveyparameters. Essen- prominently to the west at the intersectionof the San Andreas tially, four vibrators sweepingfrom 8 to 32 Hz at 0.1-km station and Garlock faults. The Moho-depth variation follows Hadley intervals led an off-end spreadof 96 linear geophonearrays of and Kanamori's [1977] deep plate boundary from the central 24 elements over 0.2 km. The receiver spacing was also 0.1 Mojave south to the Salton trough (Figure 1). Such evidence km, with nominal source-to-receiver offsets between 0.5 and 10 suggestedto Lemiszki and Brown [1988] the presenceof active km. Line 3 was run progressingfrom northeastto southwest, subhorizontal detachment faults below the central and western while line 5 progressedfrom southto north (Figure 2). Mojave Desert, consistentwith the high rates of block rotation Common midpoint (CMP) stacked sections of the COCORP and fragmentationin the Mojave [Bird and Rosenstock,1984; survey data revealed several subhorizontalreflectors in the mid- Sauber et al., 1986]. Geometric links between the Garlock fault dle and deep crust, extending across most of the western and any such detachmentsmay be found in the COCORP line 5 Mojave. Cheadle et al. [1985] made three alternativeinterpreta- reflection data. tions of these structures on the basis of their overall southwest Cantil Basin dip and ramp-and-flatgeometry. All the interpretationspropose that the reflectorsoriginated as regional thrust or decollement Cantil Valley, a closed topographic depressionbeginning 15 surfaces between the late Paleozoic and the Miocene. km northeast of the town of Mojave, separatesthe southwest Cheadle et al. [1985, 1986] found two gently dipping and east branchesof the Garlock fault with a -4 km left step reflectors below the Garlock fault on line 3 in the middle and (Figures 2 and 3). COCORP Mojave line 5 crossesthe valley deep crust. Both structurespass from north to south without on the southwestend at an oblique angle of 600-70ø . Aydin and interruptionbelow the fault. Assumingthe structuresto be older Nut [1982] showed that the valley, long recognizedas a trans- than the Garlock, Cheadle et al. showedthat each has enough tensional, or pull-apart basin, is one of the largest such struc- dip along the Garlock's strike direction to produce a detectable tures in the San Andreas system. The total width of the depres- interruption if they had been shifted by the 60 km total offset sion, at-10 km compared to its -40 km length, supportedtheir [Smith, 1962; Smith and Ketner, 1970] of the Garlock. Cheadle observation that pull-apart basins widen as lengthening by et al. and Lemiszki and Brown [1988] therefore interpreted strike-slip movement continues. We call the basin formed by active strike slip on the Garlock to be limited to the upper 9 km this processat Cantil Valley "Cantil basin." of the crust, the depth of the midcrustalreflector. The topography of Cantil Valley is somewhat asymmetric, Cheadle et al. [1985, 1986] did not think that a broadly dis- with gentler slopes on the southeastside, near the projection of tributed shear zone below 9 km depth could provide the con- the southwest branch of the Garlock, than on the northwest side, tinuity of the midcrustal reflector. Without finding direct evi- along the east branch (Figures 2 and 3). Mabey [1960] found dence for the subsurfacegeometry of the Garlock fault, Cheadle the elongatednegative Bouguer anomaly centeredon Cantil Val- et al. [1985] extended the fault down to the 9-km structure with ley to be the largest in the western Mojave, exceeding40 mGal a north or vertical dip. They suggestedthat the midcrustal of variation with gradients as high as 9 mGal/km. Dibblee structure forms the active decollement for Basin-and-Range [1967] assessedthe thickness of Neogene sedimentsaccumu- extension north of the Garlock. Lemiszki and Brown [1988] lated within the basin from the gravity togetherwith well infor- related detachment below the Garlock to the rotation of crustal mation. Wildcat petroleum prospectsbottomed as deep as 1.45 blocks in the Mojave region southof the fault as well. km in Cenozoic sedimentsnear the center of the basin (Figures Serpa and Dokka [1988] examined apparent midcrustal 2 and 3). Just outside the main part of Cantil basin, south of reflections recorded by COCORP line 3 throughout the Rand the southwestbranch of the Garlock, drill holes hit "quartz mon- Mountains area. They found evidence that these events were zonite" bedrock no deeper than 0.18 km. Dibblee [1967] inter- actually deep sideswipe from a south dipping Garlock fault. prets almost 3 km of vertical displacementat each branch of the Other evidence on line 3 for the geometryof the Garlock is a Garlock, putting a deep graben structure between them. The possible termination of a Rand thrust reflector [Louie et al., centering of the gravity anomaly at about one third of the dis- 1986]. Here we look closely at the north end of the other line, tance between the east and the southwest branches of the Gar- line 5, where it crossesthe Garlock fault zone in Cantil Valley lock led Mabey [1960] to conclude that the sediment to base- (Figure 2) for additional evidence of the fault's subsurface ment interface dips 23 ø northwest at the bottom of the graben, geometry. reaching 3 km depth againstthe east branch. Imaging the subsurfacegeometry of the Garlock requires Small, active branches and strands of the Garlock fault seismicvelocity informationboth in the shallow and the deep abound in Cantil Valley. Many have accumulated vertical crust. Basic informationon deep crustalvelocities in the central scarpsin recentalluvium more than 1 m high [Clark, 1973](Fig- Mojave is available from the analysesof refracted phasesgen- ure 3). Most strike northeasterly. COCORP line 5 crossesinto erated by quarry blasts of Kanamori and Hadley [1975] and the center of Cantil basin between two of Clark's strands of the Hadley and Kanamori [1977]. Below the Transverse ranges southwestbranch near vibrator point (VP) number 335. This Hadley and Kanamori [1977] observed a strip of anomalously stationis within 0.1 km of the mapped strands. The end of line Jennings,1975 ...... ------Gravity contoursfrom QuaternaryHannaet al.,faults 1975;from Nilsen and Chapman,1974 .... • (10 milligal intervals) index contours ...... • (at 100 and 140 milligals)

Towns ...... 35 ø 30' Minimum boreholedepth to bedrock, m ...... Actual boreholedepth to bedrock, m ......

COCORPLine with MojaveVP no ...... l_0?/ Quaternaryfault noted byDibblee, 1958 ...... O

35 ø 15'

Km N 200• California ß

©Mojave

35 ø O'

Mojave Desert

0 200 I I I 118 ø 30' 118 ø 15' 118 ø O'

Fig. 2. Locationmap of thenorthwest Mojave Desert region of California.Shading indicates elevation contours from USGS 15-minquadrangles, contour interval 1000 feet (305 m). Lowestcontour at 2000 feet (609 m).

I I I

Scale, km i

8OO

/I oo

Fig. 3. Detailof Figure2 showingthe location of faultstrands within Cantil Valley. Crosssections in Figures4, 6, 7, 8, 9, 11, 12, and 13 followthe line of sectionindicated, which is the northernpart of COCORPline 5. VP 382 is at the northend of the line. The crosseswith solid circles locate boreholes labeled with depthsdrilled without hitting bedrock The crossesin opencircles are boreholeswith depthsto drilledbedrock. LOUIEAND QIN: IMAGING OF THE GARLOCK FAULT 14,465

5 is 4.7 km north at VP 382, which is at the end of Neuralia prestack Kirchhoff migration techniquesthat fully account for Road at Randsburg-RedRock Road near its intersectionwith lateral velocity variations by using curved rays. These efforts State Highway 14. Strandsof the east branch of the Garlock include statisticaltests to determine which parts of the images mappedby Clark [1973] end 1 km eastof VP 382. They would result from coherentdata phasesrather than uncorrelatednoise, projectto line 5 at 0.3 km southof VP 382, near VP 379. Con- as well as considerationof how changesin the assumedvelocity sideringthe low 8-32 Hz frequenciesand 0.2 km overlapping model affect fault imaging and positioning. The final step is to receiver arrays employedby the COCORP survey, the surface verify the validity of all the proceduresby applying them to a expressionof the east branchis for all practicalpurposes at VP full wave syntheticdata set. A valid velocity model should at 382, and the southwestbranch is at VP 335 (Figure 3). least produce syntheticssimilar to the data. Images constructed The east branch of the Garlock has another strand, the E1 from the syntheticspoint out the strengthsand weaknessesof Paso fault [Dibblee, 1952], which is not crossedby COCORP the migration methods. From this processing,the subsurface line 5. Its southwest end is 1.5 km north of VP 382, from geometry of the Garlock fault can be traced. The proceduresare which it extends to meet the main branch at the northeast end of constrainedby additional geophysicaland geological data and Cantil Valley (Figure 3). While Jennings[1975] shows it as the presenceof clear fault plane reflections. Quaternaryin age, neither Clark [1973] nor Pampeyan et al. Initial Velocity Model [1988] map it as recently active. Mabey [1960] models the wedge-shapedregion betweenthe E1 Paso fault and the main We first seek to develop an initial velocity model below strand of the east branch as a bedrock shelf 1.2 km deep, pro- COCORP line 5 that can be constrainedby data other than the jecting northwestof the centralpart of Cantil basin. He does reflection arrivals. Each vibrator point of the COCORP lines not, however, place any of the youngest,lowest-density sedi- was recorded by receivers to offsets near 10 km. The survey ments, suchas thosefound in the centralbasin, atop the shelf. thus comprises,in one way, a large number of unreversedshal- Dibblee [1958] noted a northweststriking scarp -2 km long low crustal refraction profiles. First arrival times can be picked on aerial photographs,cutting an actively alluviated plain. to an accuracyof 0.05 s or better over nearly the entire data set. COCORP line 5 crosses it 12.5 km south of VP 382, at VP 258. The times provide, in this area, refraction velocitiesto depthsof The apparent scarp roughly follows the 2275-foot (693 m) 2-3 km. We use reciprocal sections to control refractor dip. elevation contour,which is within the closeddepression of Can- Assumingreciprocity for seismicwaves, travel times from many til Valley. Pleistocenelake shorelinesmapped by Clark [1973] sourcesto one receiver point provide a reversed travel time to the northeastat the other end of Cantil Valley only reach, curve to contrast with times picked from the standardcommon however, a maximum elevation of 1985 feet (605 m). We will sourcegather. regard this feature, like Jennings[1975] and Pampeyanet al. The reciprocal sectionsshow that no refractors below line 5 [1988] do, as a Quaternaryfault trace. north of VP 192 (Figure 2) dip more than 8ø over any distance greater than 1 km. Refractor velocities, corrected with simple METHODS AND RESULTS dipping-layer relations for the 20-8ø dips found, range from 0.9 Imagingthe subsurfacegeometry of the Garlockfault in Can- km/s at the surface in recent, unsaturated alluvium to 5 km/s at til Valley requiresthe applicationof a combinationof seismic 1.3 km depth in granitic rocks just south of the southwest reflectionanalysis techniques. The processis made much more branchof the Garlock. The strongnear-surface velocity gradient difficult by the presenceof the steep-walled,3-km-deep Cantil limits the depth penetration of the refraction analysis. It also basin, filled with Cenozoic sedimentshaving much lower veloci- affects other aspectsof the recorded data by trapping seismic ties than the older metasedimentaryand igneousrocks bounding energy near the surfacewith critical angles as small as 10ø, gen- the basin. The basin thus presentslarge lateral velocity varia- erating multiples, and by bending rays having very large angles tions juxtaposedagainst the structuresthat we seek to image. of incidenceat deep interfacesto nearly vertical incidenceat the While the large velocity contrastsdo providevisible fault plane surface. reflectionsin the reflection data, they make the imaging and The first-arrival times also show abundant evidence for lateral correctplacement of the reflectorsmuch more difficult. The velocity heterogeneity. Throughoutline 5 north of VP 192 the most common seismic reflection processingand interpretation apparentrefraction velocity changesabruptly at pointsremaining techniqueswere not designedto work well in the presenceof fixed to the ground relative to the passageof the source and large lateral velocity variations. Correct imaging within Cantil receiver spread. These points are each within 0.3 km of VP basin requires accountingfor the bending of seismic rays 240, VP 280, and VP 335. All are at the center of a diffraction through these lateral variations. We employ innovative but and a time offset of the first arrivals,suggesting vertical faulting computationallyexpensive curved-ray methods after verifying of the refractors. The times show that each fault has a throw their applicabilitywith synthetictests. down to the north. From south to north the vertical throws The first step of our analysisis to developa velocity model increasefrom 0.2 km at VP 240, through0.5 km at VP 280, to from available geophysicaland geologicalinformation. This >1 km at VP 335, which is at the southwestbranch of the Gar- model acts as a constrainton the reflection imaging process;our lock. Dibblee's [1958] Quaternary fault trace is midway results should be consistentwith gravity, refraction, and geo- between the two southern subsurfacefaults, at VP 258 (Figure logic structuredata. The secondstep is to identify reflections 3). from the fault structures of interest within the unprocessed Figure4 showsthe velocitymodel constructed from this sim- seismic data. The presenceof obvious, coherent reflections ple analysisof refraction times by Qin [1989]. The deepest givesus someconfidence that our imagingresults are relatedto refractionsfrom within Cantil basin turn at -2 km depths in 4 earth structureand not just noise. The nonverticallypropagating km/s material. The reciprocalsections suggest that this interface nature of these events alone suggeststhat conventionalCMP dips 8ø south. However,no more than a 1-km samplingof the stackingmethods will not work well [Phinneyand Jurdy, 1979]. refractorgenerates first arrivals;so this dip is questionable.The The third and major stepis to imagethe fault reflectorswith data do not exhibit reflections from this refractor, so the inter- 14,466 LOUIEAND QIN: IMAGINGOF THEGARLOCK FAULT

..v/p335 V 382 N S Q k'm/s 3.5 km/s o 2.2 km/S O ½antil : B:asin 4 km/s

Fig. 4. Refraction velocity model cross sectionfor COCORP Mojave line 5 from VP 192 to VP 382 (Figure 2). The Q indicateswhere line 5 crossesDibblee's [1958] Quaternaryfault strand. Lighter gray indicatessediments; darker gray indicates bedrock.

face is probably gradational. Deeper refractions from higher- the unprocessedseismic data. However, the extreme lateral velocity bedrock cannot be seen within the basin, apparently variationsdistort the arrivalssufficiently to confusetheir origins. because the basin is relatively deep compared to its narrow Visualizationtechniques must be used to recognizethe charac- width of 4 km. Any refractionspropagating south from a source teristicsof the reflections,followed by full wave modelingto at VP 382 at the basin bottom would hit the southwest branch confirm their origins. before reaching the crossover distance at depth. The first Figure 5 presents constant source gathers from three arrivals do not give any information on basement depth or COCORP line 5 vibrator points within Cantil basin. The data velocity below Cantil basin. have been enhanced only by the original vibrator correlation The lack of crossovertogether with Mabey's [1960] gravity [Cheadleet al., 1986] and trace equalization. The gatheron the modeling and drill hole depthsdo indicatea basin depth greater left is from a source at VP 362, near the center of Cantil basin. than 1.5 km. Figure 4 gives the basin a 3-km depth and verti- The gather on the right is from VP 382 at the north end of line cally dipping sides to agree with the gravity analysis and 5, 2 km away, and atop the surfacetrace of the east branchof Dibblee's [1967] crosssections. Cantil basin clearly representsa the Garlock fault (Figure 3). The centergather is from an inter- considerable lateral velocity heterogeneity, having contrasts mediate sourceposition. perhapsas large as 100% at 2 km depth. The most interestingarrival in these gathersis the hyperbolic The line 5 refractions,however, do not define the geometry diffraction extending from -2 to 3.5 s travel time (Figure 5, of the basin. Neither does the CMP-stacked section published "diffraction" arrival). At VP 362 its left side is asymptoticto by Cheadle et al. [1986]. For the moment we assume90 ø dip- the the first arrival, arriving about 0.5 s later. Animation of the ping boundingfaults, subjectto the analysisof line 5 reflection progressivesequence of shot gathers (not shown) has shown us arrivals. No velocity information is available north of VP 382, that the left side of this diffraction becomes coincident with the so we simply bring the 5 km/s bedrock to the surface. Below 4 first arrival as the sourcepoint moves north, to the surface trace km depth, we assumetypical regional velocitiesfrom Kanamori of the east branch at VP 382. The apex of the hyperbola and Hadley's [1975] profiles. The bedrock fault throws southof remains at VP 335, which moves left to increasing offset in Cantil basin, on the other hand, will allow this initial velocity these displays. The animated display makes it easier to recog- model to put neededconstraints on the reflectionimaging proc- nize the hyperbola against coherentmultiples of the first arrival ess. trapped in the shallow low-velocity layer. Figure 5 shows as well the progressionof this diffraction toward the first arrival. Fault Plane Reflections What is striking about the left, or south, side of this The degree of lateral velocity variation presentwithin Cantil diffraction hyperbolais its great amplitude. In every trace it is basin is strikingly manifestedby the COCORP line 5 reflection as least as strong as the first arrival. Louie et al. [1988] demon- arrivals. The faulted sidesof the basin generatereflections often strated the occurrence of such high-amplitude fault plane rivaling the first arrivals in amplitude,making them obviousin reflections in a COCORP survey that crossedthe San Andreas LOUIEAND QIN.' IMAGING OF THE GARLOCK FAULT 14,467

262 Receiver VP 357 275 Receiver VP 370 282 Receiver VP 377 -2

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.... o .--

Fig. 5. (Top row) Trace-equalizedprestack shot recordsfrom COCORP Mojave line 5 and (bottom row) the corresponding syntheticrecords. The left and centergathers are from sourcespositioned within Cantil basin,between the southwestand east branchesof the Garlock fault at VP 362 and VP 375. The gatherson the right are from a source at the north edge of the basin, atop the surfaceexposure of the east branchat VP 382. On each gather,north is to the right at the 0.5-km minimum offset. Maximum offset is 10 km. The hyperbolicdiffraction is a reflectionfrom the east branchof the Garlock fault (left side of hyperbola)and its multipleoff the southwestbranch (right side). The fine dashedline locatesthe times abovewhich, for stackingand migration,the traceswere muted to zero.

fault in central California. In that case,horizontally propagating An important result of this observationis that it explicitly refractionsbounced off large lateral heterogeneitiesat the fault, traces the east branch of the Garlock fault, as a seismic reflector, producingarrivals in the shot recordsthat were traveling back from depth to its mapped surface trace. This positively estab- toward the source. The reverse moveout of those arrivals made lishes continuity between the reflector at depth and a known, them easy to recognize, but on line 5, the high-amplitude contemporarytectonic feature. arrivalshave a more conventionalpositive moveout. We can estimatethe dip of both branchesof the Garlock fault Figure 6 proposes the geometry of a reflection ray that directly from the diffractionsin Figure 5. The dip of a plane accountsboth for the movement of the hyperbola toward the reflectorcan be found from the asymptoticapparent velocity of first arrival and for its high amplitude. A fault plane reflection its diffraction hyperbola in a common midpoint record section from the southdipping east branch,perhaps at a point north of with a simple formula modified from Claerbout [1985, p. 162] the source,propagates south to refract throughthe steeplydip- 2 2 t2¾2 = 4•5m sin c• + X2c0s2•, (1) ping southwestbranch. At the southwestbranch a multiple is reflected north to the receivers within Cantil basin. As the the where t is diffraction arrival time at source-to-receiver offset X sourcemoves north to the surfaceexposure of the east branch, from a plane having dip c• surfacinga distance•Sm from the the reflection point below on the east branch moves up and midpointbetween the sourceand receiver. A constantvelocity becomescoincident with the source(Figure 6). In this way the v above the reflector must be assumed. separationbetween the reflectionand the first arrival decreases It is easy to see that the travel time includesa zero-offset until they becomeone arrival. time at the apex of the hyperbolaplus a moveoutthat increases 14,468 LOUIEAND QIN: IMAGINGOF THEGARLOCK FAULT

ß Receivers SourceCantil

mmmmmmmm!

Fig. 6. Schematiccross section showing ray pathswithin Cantil basinthat could generatethe diffractionsin Figure 5. The multiplereflection ray from the southwestbranch is representedby the shorterdashes.

with offset X. At large offset, the apparent velocity of the reflection. It also suggestsa smallervelocity contrastacross the diffraction tail is about southwest branch of the Garlock than across the east branch at

X ¾ 1-3 km depths. Despitethe use by COCORP of long sourceand -- = (2) t cos o• receiver arrays, the bending of all arrivals by the low-velocity surfacelayer to within 15ø of vertical ensuresthat the array In a common shot record, the apex of the diffraction hyperbola attenuationof these mostly horizontallypropagating phases is is displacedto a finite offset in the up-dip direction, but the negligible. hyperbola is asymptoticto the same slope. Equivalenceof the Each gather (Figure 5) showsa bend in first-arrivalapparent slopesis clear from the cosine correctionsderived by Claerbout velocities at VP 335. The bend locates the lateral velocity [1985, p. 190], which reduce to giving the apparent velocity changefrom 2.2 km/s Cantil basin sedimentsto 5 km/s granitic with respectto receiver location g as basement(as in Figure 4) at the southwestbranch of the Gar- dg_ v lock. The record at VP 382 (Figure 5, right) showsan earlier - , (3) dt cos ot arrival that can only be distinguishedfrom correlationsidelobes by animatinga sequence(not shown)of 50 shotrecord displays. for large offsets. We use this equationto obtain dip estimates This arrival rises out of the 5 km/s refraction as the source from the apparentvelocities of diffraction tails. progressesnorth to the end of the line, indicatingthat it follows In Figure 5 the left sides of the labeled diffractions are from a faster bedrock path entirely below the slow basin sediments the east branch of the Garlock fault. They have an asymptotic (Figure 5, "subbasin"arrival). If so, it suggestsa steeplydip- apparentvelocity dg/dt close to 6 km/s. The basementrefrac- ping southwestbranch carrying low velocities below 2 km tion above the diffraction, from south of Cantil basin where the depth. A refractionalong the bottom of the basin could then cut basementis shallow (Figure 4), gives us a 4.8 km/s velocity to below the comer of the 1.5-km-deep bedrock shelf at the assume for v. Using these values and likely accuraciesfor southwestbranch (Figure 4), emergingearlier than a wave pro- dg/dt and v of 20%, equation(3) yields a 37ø+10ø southdip for pagatinghorizontally through the basin to the shelf. the east branch. For the right sidesof the diffractionhyperbolas in Figure 5, the southwestbranch multiples, we assumethe 2.2 Variable Velocity Imaging km/s basin refraction velocity (Figure 4) for v. These arrivals Keepingthe establishedsurface-to-depth continuity of the east are asymptoticto a dg/dt of 4.48 km/s, giving the southwest branchof the Garlock fault in mind, we now attemptto image branch a noah dip of 600+20ø. and locate it within the subsurface.Our initial trials employed The large amplitude of the left, south side of the reflection methods assuming some type of velocity homogeneity. The hyperbolas(Figure 5) may be a consequenceof the juxtaposition unsatisfactoryresults demonstrated the need to use not only a of low-velocity Cenozoic Cantil basin sedimentsagainst Meso- prestack migration, as could be predicted from the work of zoic granitic basement [Jennings, 1977] at the east branch. A Phinney and Jurdy [1979] but also to accountfor ray bending lateral velocity variation from 2.2 to 5 km/s would reflect throughthe stronglateral velocity heterogeneities. strongly. The relative weaknessof the right, noah side of the Preprocessing. In initial processingof the vibrator-correlated hyperbola results from its origin as a second-order,multiple records,we applied a sphericaldivergence amplitude correction LOUIEAND QIN: IMAGINGOF THE GARLOCK FAULT 14,469

and a quantile-basedtrace equalization(in the manner of Wig- We easily calculate these travel times with Vidale's [1988] gins et al. [1985]) to the data before imaging. The divergence finite difference solution of the eikonal equation in two dimen- correction was based on velocities we picked from constant sions. Any laterally variable velocity model, such as our initial velocity CMP stacksat 0.4 km/s intervals between 2 and 6 km/s refraction velocity model in Figure 4, can rapidly yield first- of the line 5 segmentnorth of VP 193 (Figure 2). We simply arrival travel times between any two points. To migrate the choosevelocities giving the highestreflection amplitudeon the prestackline 5 data, we calculatetravel times from each vibrator final stack,poor as it was. First-arrivalmutes picked by hand point north of VP 300, to each of 40,000 possible reflection from each shot gatherlimit the partsof the gatherswe use. Fig- points spacedat 50 m within a 200x200 grid 10 km wide and ure 5 shows the mutes used on three of the shot records. 10 km deep. Both sourcesand receiversoccupy the same vibra- The stack(not shown)is quite similar to a line drawingpub- tor points in the COCORP surveys. About 20 min are neededto lished by Cheadle et al. [1986] of their line 5 stack. The Gar- calculate the -4 million times altogether,on a 4-Mflop work sta- lock fault-plane reflectionsidentified above appear as short, tion. gently south dipping arrivals near 2.5 s two-way travel time Vidale's [1988] method may yield time errors of the order of below VP 350. One could not on the basis of these stacks con- 1%. It can also producetimes that are too late by a larger per- nect the reflection to the surface location of the east branch at centagewhere a wave front encountersa sharpvelocity contrast VP 382. of a factor of 2 or more. Incorrect times can blur the migrated Kirchhoffmigration. The pre-stackKirchhoff migrationproc- images and leave artifacts. These limitations are minor, how- ess we use to properly image thesereflections is similar to those ever, comparedto the likely inaccuracyof the laterally hetero- used by McMechan and Fuis [1987] and by Louie et al. [1988]. geneousvelocity models we are able to develop. We avoid The migrationis a back projectionof assumedprimary reflection some of the error by partially smoothingthe large lateral varia- amplitudesinto a depth section. It has been identified by Le tions in our velocity models. Bras and Clayton [1988] as the tomographicinverse of the Figure 7, top right, showsfirst-arrival times to every point in acousticwave equationunder the Born approximationin the far a 10x10 km section north of VP 300 from a source at VP 382, field, utilizing WKBJ rays for downwardcontinuation and two- calculatedthrough a velocity model including a laterally hetero- way reflectiontravel time for the imagingcondition. geneousCantil basin. The times to every point from a sourceat This inverse requires knowledge of the source wavelet for VP 300 are shown to the left. Assuming reciprocity of seismic crosscorrelation with each seismictrace. We crudely approxi- waves, this sectioncould also representtimes from every depth mate this aspect by cross correlation with a boxcar function point to a receiver at VP 300. Thus, by adding the times in 0.032 s long, close to the central period of the reflection both sectionswe derive the two-way travel time from VP 382, arrivals. This operationhas the effect of smoothingas well as diffracting at any depth point, to a receiver at VP 300 (Figure 7, centeringthe reflectionpulses. We do not apply any otherproc- bottom). These times allow the back projection of reflected essingto the tracesbefore migration. arrivals recordedby that source-receiverpair. The constanttime In another initial test we migrated the prestack line 5 data contoursshown in Figure 7 are the prestackmigration impulse north of VP 300 at 11 constantvelocities spaced at 0.2 km/s responseof this velocity model to reflectedarrivals in that one from 2 to 4 km/s. As with the CMP stacks,these images (not seismogram.Deriving a similar two-way time sectionfor each shown) suggestthat no constantvelocity migration can image of the 4800 traces used allows their migration through the the east branchreflector in a positionthat would allow it to rea- heterogeneousvelocity model. sonably be connected to the surface trace of the fault. Both Statistical enhancementand artifacts. We use the method of CMP stackingand constantvelocity prestackKirchhoff migra- Harlan et al. [1984] to emphasize portions of the migrated tion assumethat the reflectionstravel along straightrays. The images most different from a migration of uncorrelatednoise. strong lateral velocity variationsin Cantil basin must produce Specifically, "noise data" are estimatedby randomly reversing stronglybent rays. The use of laterallyvariable stacking veloci- the sign of half of the data traces. Separatemigrations of the ties doesnot mitigate this problem,as bent rays distortreflection data and the noise data produce two images that we scan to hyperbolasin the CMP gathers. Standard normal moveout obtain two histogramsof amplitudedistribution. The data histo- correctionscannot match distortedhyperbolas, thus decreasing gram divided by the noise histogram,with a "water level" of the focusingof the reflectionin the outputstacked section. 0.1% of the maximum of the noise histogram, estimates an PrestackKirchhoff migration does not require matching of amplitude histogram of coherent signal focused by the migra- hyperbolasbut does rely on the accuratelocation of a back- tion. Bayesian estimationusing the three histogramsgives the projectedarrival within the migratedsection. Assumingstraight signal expectationfor each point of the data migration [Harlan rays in Cantil basin resultsin the mislocationof back-projected et al., 1984, equation 2]. A smoothedimage of the signal reflections. The mislocationssum to a less coherentimage of a expectationweights the data migration to emphasize structures reflector once its reflections on different source-receiver combi- focused from coherent data. This process serves as well to nations have been added. The inability of either techniqueto attenuate"smile" artifactsin areasof the image having little data properly reconstructthe surface-to-depthcontinuity of the east coverageor incorrect travel times. branchof the Garlock fault suggestswe use a nonlinearmigra- Another sourceof artifactsin the migrationsis the aliasing of tion that doesnot assumestraight ray paths. nonverticallypropagating reflections by the relatively wide 0.1 Migration with curved rays. The ability to calculate travel km receiver spacing. Inspection of Figure 5 shows that most times in laterally variable media can avoid the limitations of reflected energy appearsbetween the frequenciesof 20 and 32 straight ray approximations. A diffraction tomographicback Hz. At 32 Hz, reflectionshaving an apparentvelocity dg/dt of projectionprocedure like Kirchhoff migration does not require less than 6.4 km/s will alias, while 20 Hz energy will alias at definition of ray paths. Only the travel time from each source velocities below 4 km/s. Figure 5 also indicates that the east and receiver point to each reflectionpoint at depth needsto be branch fault plane reflectionshave apparent velocities near 6 found. km/s, so they will not be aliased on average. The southwest 14,470 LOUIE^•D QI•: IM^GI•G OFTH• GARLOCKF^ULT

Receiver at VP 300 Source at VP 382 VP 300 400 0300 ...... VP 400

Two-Way ReflectionTravel Time ...... 400 '3 S

E 3s

Fig. 7. Travel times for a laterally heterogeneousCantil basin velocity model, calculatedwith Vidale's [1988] finite difference method. Contour interval 0.5 s.

branch events,however, arrive at 4.5 km/s, so they will alias to branchmay passthrough only the low-velocity basin or through a large extent. An aliasedevent blurs into smile artifactsupon the basin and the bedrock shelf to the southwest. Even with the migration. These smilesare the incompletelycanceled impulse large velocity heterogeneityat the walls of the basin, accounting responseof migration.The statisticalenhancement process for the travel time of curvedrays properly locatesthe reflector. above will attenuate these aliased reflections as well. Yet it is clear that this a priori velocity model deleteriously A more seriousproblem for the migrationof COCORP line 5 affectsthe image. The reflectoris strongjust below the bottom may be its oblique crossingof Cantil basin. Reflectionpoints of the basin in the velocity model, which appearsto cut it off. on oblique, steeplydipping structuressuch as the branchesof Certainly, where seriouserrors exist in the velocity model, the the Garlock fault are very likely not in the vertical plane of the reflectorimages will not focusbut will presenta group of smile ,surveyline. This conditionviolates our assumptionthat a two- artifactsinstead. A logical solutionwould be to give the north dimensional model can be used to calculate times. We recog- wall of Cantil valley, at the right of Figure 8, a south dip to nize the improvementpossible from three-dimensionalvelocities extend from the surfacetrace of the east branch to the imaged and time calculations (such as Vidale [1990b]) but do not reflector. attemptthem here becauseof their extreme cost. We computeseveral more migrationsby changingthe veloc- Results and velocity models. Figure 8 shows the result of ity model, regeneratingthe travel times, and migratingthe data migratingCOCORP line 5 data throughthe refractionvelocity again. Other geological and geophysicalconstraints, such as model (Figure 4) in the manner describedabove. This image gravity modeling [Mabey, 1960] and the line 5 refraction times, plots the migration on top of the model to show their corre- supportthe trial velocity modelswe use. Changingthe dips of spondence.The east branchreflector now appearswith a posi- the walls of Cantil basin appearsto have the most effect on the tion and dip that could conceivablyallow it to be connectedto resultingimage. Figure 9 showsone such image that contains the surface fault trace. Proper positioning of the structure the strongest, most continuous east branch reflector we could clearly requiresmigration througha laterally variable velocity obtain. The eastbranch reflector extends to 4 km depth,hinting model. Dependingon receiverposition, reflections from the east that the low-velocity basin may extend deeper than 3 km. LOUIEAND QIN: IMAGING OF THE GARLOCK FAULT 14,471

300 leastBranchv ace 400

East Branch

Fig. 8. PrestackKirchhoff migration of the northern portion of COCORP line 5 using travel times calculatedthrough the initial velocity model developedfrom refractedarrival times (Figure 4). The migration, the wavy part of the image, is plotted on top of the velocity model, which showsthrough as polygonalregions.

Unfortunately, the gravity, borehole, and refraction data do not Southwestbranch. The image of Cantil basin in Figure 9 constrain the shape of the bottom half of Cantil basin. This does not show any evidence for the geometry of the southwest trial-and-error procedure has no quantitative basis at present, branch of the Garlock fault, on the south side of Cantil basin since it is difficult to predict how travel times will changefor near VP 335 at the surface. Figure 5 shows the relatively low the nonlinear curved rays we employ or what the positioning amplitudeof the southwestbranch reflection,which is on the errors may be. We cannot claim that Figure 9 is the result of a north (right) side of the hyperbolascentered at VP 335. Where "best"velocity model. the east branchreflection is separatefrom the first arrival, both a The coincidencein Figure 9 of the east branch reflector and primary and a multiple reflectionfrom the southwestbranch are the wall of Cantil basin in the velocity model does suggestthat present. Both have inversemoveouts in thesegathers, showing the model has some basis in reality. Since we use the model decreasesin travel time with increasingoffset. only to generatetravel times for back projectingthe phasetimes, These reflections have all been recorded by receivers, and it just controlsthe placementof reflectors. The amplitudesfor sources, within Cantil basin between VP 335 and VP 382. back projectioncome directly from the data and have no relation Receivers south of VP 335, on the other hand, recorded the to the velocity model. Coincidencetherefore suggeststhat the high-amplitudeeast branch reflection that appearsas the south model boundary is properly located. As a result, we interpret (left) side of the hyperbolasin Figure 5. A migration of only Figure 9 to show that the east branch dips 45 ø south to 4 km those traces having sourcesand receivers within Cantil basin depths, confirming our results of examining the unprocessed producesan image (not shown) of a steeplydipping structure1 data. km south of the the southwestbranch at VP 335. This image is In Figure 9 the imaged reflector does not extend shallower the incorrectback projectionof multiple reflectionswith primary than 1.5 km. Our muting of the first arrivals from the data reflection times but does confirm the steep dip of the southwest removes the reflections from the shallowest part of the east branch. Aliasing of the reflection togetherwith its low ampli- branch. Figure 5 shows how the south side of the hyperbola tude make the dip directionand locationof the southwestbranch falls within the mute as it moves with source proximity to the unclear. surfaceexposure of the east branch. Unmuted data violate the Born approximationinherent in the migration, which can only SyntheticTests handle primary reflections. Only the original data can demon- In the proceduresabove we developed a laterally variable stratethe depth-to-surfacecontinuity of the reflector. velocity model for COCORP Mojave line 5 in Cantil basin. By 14,472 LOUIEAND QIN: IMAGINGOF THEGARLOCK FAULT

]East Branch Tracel ISWBranchTrace•lOantilBasi/nl/ ..,300 vP/ /4o U •'•'••-••••,•.•d • ..-...... •••...... -, / ...... •-• :-.?•;?...... "•'•;½':•:.:: •?:•......

-!-

EastBranch Garloc

6.3 10

Fig. 9. PrestackKirchhoff migrationof the northernportion of COCORP line 5 using travel times calculatedthrough an adjustedvelocity model. As in Figure 8, the wavy migrationis plottedon top of the polygonalvelocity model. The east branchreflector is now more continuousand coincideswith the northernboundary of Cantil basin.

generatingsynthetic seismograms from this model, we can test stationgenerated the 4800 synthetictraces in under27 hours. its fit to the seismicdata. Further, by subjectingthe synthetic Figure 10 shows a visualizationof the volume that results data to the the same analysis methods that we use for the field from stackingup the syntheticshot records like a deck of cards. data, we can evaluatethe limitationsof our imaging techniques. Figure 5 shows three of the syntheticrecords individually. The While thesecomparisons cannot show that our velocity model is near right face of the volume in Figure 10 showsthe record for unique, they do suggestthat it is consistentwith the data and VP 382 from the lower right of Figure 5. The solid surfaces how the data show the geometry of the Garlock fault. representthe higher-amplitudearrivals within the volume. All We generate synthetic seismogramswith a finite difference of the arrivals that we used above to locate the branches of the solution of the two-dimensionalacoustic wave equation. This Garlock fault are clear in the syntheticsand similar in form to explicit algorithm implements a second-orderapproximation of the COCORP line 5 field data records. In particular, Figures 5 the time derivative and a fourth-orderapproximation for spatial and 10 shows how the east branch reflection generates a and material property derivatives [Vidale, 1990a]. The synthet- diffraction at the southwestbranch and merges with the first ics include all ray parameters, refractions, head waves, arrival when the source reaches VP 382. The figures also diffractions, and multiple reflections describedby the acoustic confirm the large amplitude of the east branchreflection. How- wave equation, but not P to S mode conversionsor intrinsic ever, the syntheticsgive its multiple off the southwestbranch attenuation [Vidale et al., 1985]. and the basin bottom reflection too large an amplitude. Since The syntheticseismograms have sourceand receiver positions the model gives the southwestbranch and the basin bottom with the same geometry as in the line 5 survey from source equal reflection coefficientsto the east branch, producingequal point VP 333 north to VP 382. The velocity model is modified syntheticamplitudes in Figures5 and 10, true reflectivitiesmust from Figure 4 to give Cantil basin the shapeshown in Figure 9. differ greatly to producethe amplitudedifferences we observe. In addition, we further simplify the model by (1) eliminatingthe We subjectthis syntheticdata set to the sameprocedures that 1 km/s low-velocity surface layer, to avoid near-surfacemulti- we apply to the COCORP line 5 data. These include spherical ples; and (2) giving the interior of the basin a 2.2 km/s constant divergencecorrection before stackingand trace equalizationand velocity, to test our ability to image the sidesof the basin rather muting before stackingor migration. Figure 11 summarizesthe than the detailed velocity structurewithin it. A 4-Mflop work- results. The constantvelocity Stolt migration (Figure 1l a)[Stolt, LOUIEAND QIN.' IMAGING OF THE GARLOCK FAULT 14,473

Fig. 10. Visualizationof the acousticsynthetic data volume generatedfor 50 sourcepoints from VP 333 to VP 382. Large positiveamplitudes are renderedas solid objects;smaller or negativeamplitudes are renderedtransparent. The top face of the volume would representa stackingdiagram. The left face is a zero-offsetsection from south(back) to north (front). The right face of the volume would be the common sourcerecord of VP 382, from 0.5 km (left) to 10 km (right) of offset. The syntheticcommon source records in Figure 5 are essentiallythree parallel slicesfrom this volume.

1978] of the near-offset time section (near left side of Figure offsets. Such arrivals do not contributeto the image in the con- 10) obviously images the geometry of the model Cantil basin stant velocity migration of Figure 1l c. For these reasons the very well. The COCORP data, unlike these noise-freesynthet- constantvelocity method works for the syntheticsbut not for the ics, are heavily contaminatedwith source-generatednoise at the COCORP line 5 data. Migration of the strong east branch near offset (Figure 5). So, like most field reflection data, the reflectionsmust accountfor the lateral velocity variation at Can- longer-offset traces must be incorporated into the imaging. til basin. CMP stacking, however, does not properly image the junction We properly image the junction of the east branchof the Gar- between two structuresof different dip, as the Stolt-migrated lock with the basin bottom by prestack migration through the stack shows(Figure 1lb). Stackingalso acts as a dip filter that correct velocity model. The image in Figure 1ld matches the has emphasizedthe amplitude of the flat basin bottom reflection model basin boundarieswith the back-projectedreflections, giv- over the dipping east branchreflection. ing both structuresthe same reflectivity. However, large smile The constant velocity prestack migration (Figure 1l c) does artifacts are also present. The artifacts result from poor cover- not suffer this limitation. Given the constant model velocity age of reflection points beneath the basin as well as the within the basin, it is able to form an accurate image from the improper migration of high-amplitude,aliased southwestbranch reflectionson the near-offset portions of the synthetic gathers. multiples of the east branch reflection. Figure 5 shows far more source-generatednoise on the line 5 Since we wish to distinguish the dip direction of the east field data at near offsetsthan on the synthetics. Any imaging of branch of the Garlock, we generatean additional syntheticdata the north edge of Cantil basin must incorporatethe strong east set from a model that has been altered to give it a dip to the branch reflectionsrecorded by receiversoutside the basin at far north. Figure 1l e shows the result of migrating these synthetics 14,474 LOUIEAND QIN.' IMAGING OF THE GARLOCK FAULT

A: 2.2 km/s Migrated Near-Offset B' 2.4 km/s MigratedStack 333 VP 383 -1 -3 0333.=•...... ,,,, VP 383

ß ....

...... ß

ß.. '-:'.....•>.

.,• •.•½ . "i.' -.•' %•.;':%•: ..•.

C' 2.4 km/s CV Kirchhoff D' Kirchhoff w/Vel. Model 0300 VP 400 3OO VP 4OO

::'•:'• '•"•i' '!i...... • • ..: E •'" ,::7'*'7%Z'"•'-:"':•"....•.4:.. :• ...."';""':!? ...... '" :•&Z..;......

10 lO -4 Reflectivity 4 ....-.:•;•: :::::..:;:;!i!:'i•i:?.:.i:.:::.::•i:.{;"; ..: . E'North Di(•v/Vel. Model F' N Dip w/S-Dip Model o 03.0¸ ...... 400 300...... • VP 400 ..... t-'-:-:-:• ...... :.•;!•!i'. 'ii:;•:•i!•...... :::•,'..-::•:!:::: ...... ::, ...... :::

...... '-'::. :.•..;.!:i:;½!,..:•:::'"•

..'..... ' ':....c.:':"":E-.-'".•:•

6.3 10 lO

Fig. 11. Imagescomputed from the synthetic data set of Figure10. Thetop row shows constant-velocity Stoltmigrations of (a) thenear-offset section and (b) a stackedsection. The middle row showsprestack Kirchhoff migrations (c) at constant velocityand (d) withthe laterally variable velocity model used to generatethe synthetics. The variable velocity prestack migrationsare plottedatop their migration velocity models in the mannerof Figures8 and9. The bottomrow showsthe variablevelocity prestack migrations of a modelgiving a northdip to theeast branch of theGarlock, (e) properlymigrated withthe north dip model and (f) improperlywith the south dip model. Figures 1la and1 lb are5 kmwide, while Figures 1lc, 1l d, 1l e, and 1If are all 10 km wide.

throughtheir velocity model. Sincethe fault is now completely DISCUSSION off the end of the sourceand receiver line anddips away from Tracing the east branch of the Garlock fault to the subsurface them,it doesnot image. Figure1 If givesthe impropermigra- with a southdip with COCORP Mojave line 5 data'confirmsa tion of the northdip syntheticsthrough our originalmodel hav- previousinterpretation of nonverticaldip from line 3 by Serpa ing a southdipping east branch. If datafrom a northdipping and Dokka [1988]. Thesetwo piecesof seismicevidence apply east branchwere migratedthrough a velocitymodel having a to the centralportion of the eastbranch of the Garlock, where it south dipping east branch, the basin bottom reflector would be skirts the E1 Paso Mountains(Figure 2). Our evidencefor a bent downwardat the falsevelocity interface. This effectmay southdip calls into questionsome aspectsof the model block not be detectablein the field data in the absenceof a strong geometryused by Bird and Rosenstock[1984] to predicta north basin bottom reflector. These synthetictests assure us that our dip on thissegment, west of the Blackwaterfault (Figure2). curved-raymigrations are meaningfulfor the Cantil basinveloc- At greaterissue is how an activestrike-slip fault like the Gar- ity models. The observedreflection (Figure 5) togetherwith its lock can have a nonverticaldip on a large scale. The focal migration(Figure 9) are proofof a southdip to the eastbranch mechanismsof Astiz and Allen [1983] and the dislocat.ion of the Garlock fault. modelsof Lippincottet al. [1985]appear to beconsistent with a LOUIEAND QIN: IMAGING OF THE GARLOCK FAULT 14,475 vertically dipping fault to depths near 10 km. On the other Developmentof Cantil basin hand, both small-scale geological investigationssuch as by The geometry in cross sectionof the basement-to-sediment Brady and Troxel [1981] and the regionalgeologic syntheses of interface in Cantil Valley and along COCORP line 5 to the Bird and Rosenstock [1984], Carter et al. [1987], and Dokka south (Figures2 and 3) suggeststwo alternatehypotheses for [1989] suggestlong-term movements on a nonverticalGarlock Quaternaryfault geometrywithin the Garlockfault zone. Figure fault. Serpa and Dokka [1988] found direct seismicevidence of 12 shows these models. In either case, the thickness of sedi- nonverticaldip to several kilometersdepth. We confirm this mentaryfill in Cantil basin,the down-to-the-northfaulting south evidencein Cantil Valley for a 45ø southdip to 4 km depth. In of the basin, and the south dip of the east branch require an addition,we can link the dippingreflector to the surfacetrace of extensionalcomponent of motion acrossthe Garlock fault. The the currentlyactive fault with certainty. opening of Cantil basin requires2-4 km of extension. This Our effort to resolve this issue of a nonvertical Garlock fault componentis minor compared to the 60 km of left-lateral involves two phasesof interpretation. First, we reconcile our motion but has nonethelessproduced major vertical deformation. migratedimages with geologicand geophysicalobservations in Both the negativeflower structure(as definedby Harding and and near Cantil Valley. This processleads to a model for the Lowell [1979]) and the detachment model incorporate the development of Cantil basin. Second, we discuss how this suggestionby Cheadle et al. [1985, 1986] and Lemiszki and model fits into the tectonic framework of the Mojave region. Brown [1988] of a continuous horizontal structure underlying Our model is consistent with many important observations. the Garlock near 9 km depth. Although the significanceof this However, we cannotat this point resolvethe conflictsbetween a structurehas not been confirmedby other observations,neither nonverticalGarlock and seismicityand sourcemechanism data, its existence or location are crucial features of our models. In so we will leave them for further investigation. the flower structuremodel (Figure 12, top) we arbitrarily bring

Q 20 km cantil

20 km

Fig. 12. (Top) Diagrammaticnegative flower structureand (bottom)detachment interpretations for Cantil basin. Q locates Dibblee's[1958] Quaternaryfault. The Garlockshould terminate above the continuousreflector noted by Cheadleet al. [1985, 1986] 9 km below line 3. 14,476 LOUIEAND QIN.' IMAGINGOF THEGARLOCK FAULT all branchesof the flower down to one point at the horizontal flower structuremodel suggestsmore equal settlingof each side structure. The structuremust be currently active and able to of the basin. The asymmetryof the surfacetopography (Figure carry the motion of the Garlock as horizontal detachment. The 3) suggestsdifferential subsidence, as doesthe asymmetryin the flower model does not, however, determinewhether the deep Bouguer gravity anomaly. Mabey [1960] models the gravity detachmentmay be active to the south,north, or on both sides anomaly asymmetryby giving the basementbelow Cantil basin of the Garlock. Also, this model does not specify the relative a 23ø dip to the north, from 1.2 km depth at the southwest ratio of horizontalversus vertical deformationon any particular branchto 3 km depth at the east branch. branchof the fault. It is ad hoc in that it matchesmany of the The larger subsidencerate that we suggestfor the east branch observationsfrom Cantil Valley, such as the south dip on the over the southwestbranch is consistentwith the much larger east branchof the Garlock, yet allows detachmentof the Inyo reflection amplitudesobserved from the east branch (Figures 5 block and rootingof the Mojave. and 9). Rapid subsidenceof 3 km within 5-10 m.y. againstthe The detachmentmodel (Figure 12, bottom) alternatively east branch would juxtapose recent, relatively unconsolidated transfers strike-slip motion on the Garlock fault southward to basinfill againstthe pre-Tertiarybasement rocks of the E1 Paso low-angle motion on a horizontaldetachment fault. This detach- Mountains, forming a large velocity discontinuityand a strong ment may merge with the 9-km-deep structure,detaching the reflector. At the southwestbranch, the young sediment would Mojave block to the southof the Garlock. Any of the regional, not be carried down as deep or as fast, forming a weaker southwestdipping reflectorsimaged by Cheadle e! al. [1985] reflector. Placing much more strike-slip motion on the east could be candidatesfor this detachment.This interpretationof branch as well, as suggestedby the detachmentmodel, is also thosereflectors would be radicallydifferent from the int.erpreta- consistentwith it being a strongreflector. tion of Cheadlee! al. [1985, 1986], who proposedfault agesno Differential subsidencewithin Cantil basin would be easy to youngerthan Miocene. Some of thesestructures, seen on test if reflections from interfaces within the basin sediments COCORP line 3, include contrasts in Poisson's ratio of more could be seenin the image of Figure 9. Unfortunately,we have than 10% [Louie, 1990], which may be consistentwith the phys- removed most of the shallow basin reflections with the first- ical properties expected for an active detachmentin southern arrival mutes (Figure 5). Future work may be able to target California [Louie, 1988]. thesereflections and tie them to the logs of wildcat oil prospects Under the detachment model the southwest branch of the drilled within Cantil basin. Garlock as well as the faults to the south (Figure 3) would be We prefer the detachmentmodel over a negativeflower struc- complementaryfaults to the detachmentthat surfacesat the east ture becauseit proposesa simplemechanism for the broadening branch. As such they should have a much lower ratio of hor- of Cantil basin. Aydin and Nut [1982] use this basin as one of izontal to vertical motion than the east branch,possibly being the larger examplesof a transtensionalgraben in their global only vertical. The detachmentmodel also demands differential compilation. This compilation establishesthat such grabens subsidencewithin Cantil basin,with much more rapid slumping maintain a constantratio of length to width of between3 and 4. on the northwestside of Cantil Valley (Figure 12, bottom). The The grabens must constantlygrow wider as lateral slip makes

krn

Helendale?

Fig. 13. Schematicview of detachmentstructure for Cantilbasin, looking southwest along Cantil Valley at a 10x20 km block of the crust10 km deepcentered on COCORPline 5. Tertiaryand Quaternary sediments are rendered as the uppertransparent layer. The middletransparent layer correspondsto the upperdetached basement. The faultssouth of Cantilbasin are given the northwest strike of Dibblee's [1958] fault. LOUIEAND QIN: IMAGINGOF THEGARLOCK FAULT 14,477

them longer. Aydin and Nut [1982] suggestCantil basin formed the Garlock (Figure 9) and its extensioninto the midcrust as an by coalescenceof severaladjacent left stepsin the Garlock fault. active obliquedetachment (Figure 12, bottom)has broad impli- Their model, however, implies that the eventual width of Cantil cations for the tectonic configurationof the Mojave Desert basin was set when the Garlock first developed and that the region. Bringing the Garlock below the Mojave province, as basin could not broadenany further. The structuresthat they suggestedby Serpa and Dokka [1988], rather than below the summarizein their global compilationhave a wide range of Basin and Range as suggestedby Davis and Burchfiel [1973] ages; one could not expect preset widths to lead to such a con- and Bird and Rosenstock[1984], makes the Mojave region a stantratio betweenlength and breadth. rootlesstectonic "flake." If the midcrustalreflectors identified by Aydin and Nut [1982] also suggestthat• the developmentof Cheadle et al. [1985, 1986] are in some way related to this new fault strands away from the establishedfault zone could regional detachment, then the relative shifts and rotations serve to broadenthe basinsas lateral slip continues. It is hard betweensubregions of the Mojave couldbe takingplace among to imagine,as they point out, how new faults could developout- flakes possibly less than 10 km thick. This picture could side a preexistingand actively slippingzone of weakness. The account for the lower rates of deformation north in the Basin advantageof our detachmentmodel (Figure 12, bottom) is that it and Range [Bird and Rosenstock,1984] versusthe higher rates gives an obvious mechanismfor the broadeningof the fault south in the Mojave [Sauber et al., 1986]. zone. As the dip slip on the oblique detachment continues, Carter et al. [1987] suggestthat Cantil basindeveloped from lesseningcurvature along the deep listric geometry results in the terminationof right-lateralslip on a northweststriking fault slumpingback towardthe headwallof the detachment,develop- such as the Lockhart against the Garlock (Figure 1). They ing new fault strands. Maintaininga constantratio of strikeslip envisionednorthwest striking, dextral faults cutting the entire to dip slip then producesa constantratio of grabenlength to Mojave between the Garlock and Pinto Mountain faults. width. Against the Garlock, each of the "slivers" between these faults The key to this mechanismfor broadeningthe grabenis the would develop extensional features at their northwest comers nonverticaldip of the fault. However,one would not expecta and compressionalfeatures at their northeastcomers. Alterna- major strike-slip fault like the Garlock to be able to maintain a tions between relatively high topography along the Garlock shallow dip, becausethe vertical orientationof the intermediate southwestof Cantil Valley, low topographyin Cantil Valley, principalstress for suchfaults predicts fracturing along a verti- and highs northeastof the valley (Figure 2) suggestwhere these cal plane. We can envisionthe northernmostpart of the Mojave slivers could intersect the Garlock. block on Figure 12 (bottom) "breakingoff" and being left Our model, which would floor each of the slivers with a behind by continuedstrike slip on a fault trace that has now detachmentsurfacing at the Garlock, is consistentwith that of shiftedto the south. If, however,we assumethat motionbegins Carter et al. [1987]. In particular,it requires2-4 km of dextral along a shallow dipping, preexistingzone of weakness,then strike slip on some combinationof the Spring, Helendale, or establishinga vertical fault trace for strike slip could take some Lockhartfaults (Figures 1 and 2), havingthe sametiming as slip time. on the Garlock. The faults we observed below COCORP line 5 In fact, Cantil basin offers a possibleoriginal but now inac- southof Cantil basin (Figures4 and 12), if they have the same tive strike-slip trace in the E1 Paso fault, 1.5 km north of the trend as the Quaternaryfault mappedby Dibblee [1958](Figure east branch (Figure 3). Shifting the active trace south to the 3), could be the northwestextension of this systemnear the Gar- presenteast branch early in the developmentof the basin could lock. Figure 13 shows a renderingof how these faults would explain the 1.2-km-thick accumulation of older sediment contributeto the geometryof Cantil basin. betweenthe east branch and the E1 Paso fault modeledby The divisionof the Mojave into two rotatingblocks by Bird Mabey [1960] from gravity data. At present,the active strike and Rosenstock[1984] is alsoconsistent with our model. They slip should be shifting south from the east branch to the divide the blocksnear the Harper or Blackwaterfaults (Figures southwestbranch of the Garlock. A test of this hypothesis 1 and 2) and assigncompression across the southwestbranch of would be to establish an earlier date for the initiation of strike- the Garlock at the westernMojave block insteadof putting slip motion on the east branch than on the southwestbranch of extension across the east branch at the eastern block. Thrust the Garlock within Cantil Valley (Figure 3). This test would faulting during the 1952 Kern County earthquakeon the White only apply to the timing of contemporarymotions on small Wolf fault (Figures1 and 2) requiresthe compressionacross the strandsof the Garlock within the valley. It is not relatedto the southwestbranch. If we divide the blocksinstead at the Spring, Mioceneinitiation of the southwestbranch as a regionaltransfer Helendale,or Lockhartfaults and allow both compressionto the structurein advanceof the east branch [Dokka, 1989]. southwest and some extension to the east, then our model of The comparisonof the detachmentmodel against the negative extensionat Cantil Valley could be compatiblewith the fault flower structuremodel for Cantil Valley helps establishtests of displacementdata synthesizedby Bird and Rosenstock[1984]. the preferreddetachment model. Given the observedsouth dip Dokka and Travis [1990] challengethis model by asserting of the east branch of the Garlock fault, the flower structure that faults such as the Helendale or Lockhart do not show evi- model is set up to allow detachmentto the north instead of the dence of significantstrike slip northwestof the Barstow area. south. The fact that the detachmentmodel predictsmore of the The geodetic deformationin the south central Mojave that geologicaland geophysicalfeatures of Cantil Valley than the Sauber et al. [1986] observe on these faults Dokka and Travis flower structuremodel suggeststhat detachmentof the Mojave truncatenear Barstow by an east trendingzone of compression to the south is more realistic than forcing detachmentof the and extension(Figure 1). The 6-10 mm/yr of dextral deforma- Basinand Rangeto the north (Figure 1). tion previouslythought to be on northweststriking faults in the westernMojave they transferto high rates of defr.•rmationin the Fragmentationof the Mojave Bristol Mountainsin the easternMojave. Such large faults so Our imagingof the basinboundary reflectivity developed by far to the east have not yet been substantiated. differentialsubsidence against the southdipping east branchof Dokka and Travis [1990, Figure 6] refer to patternsin earth- 14,478 LOUIEAND QIN: IMAGINGOF THEGARLOCK FAULT quake epicentrallocations in the Mojave to supporttheir model. 10-km-thick "tectonic flake" is consistentwith fault offset rate, On their figure, we observelittle in the seismicitydistribution to seismicity, and deep seismic velocity data assembledfor the substantiate an active structure between Barstow and a northwest region. termination of the Helendale fault. Instead, there is some minor seismicity that could connect the Helendale trace north to the Acknowledgments.The COCORP Mojave data were graciouslyprovided by Sid Kaufman of Cornell University, with assistancefrom the Lockhart (Figures 1 and 2). They also use King's [1985] geo- SeismologicalLaboratory at the California Institute of Technology. detic results from a network north of Barstow to require J.Q.'s and much of J.N.L.'s researchwere generouslysupported by the minimal dextral slip in the north central Mojave. King's meas- ResearchInitiation Grant Programat Penn StateUniversity and by the urementscertainly restrain possible movement on the Harper or National Science Foundation under grant EAR-8904993. Chevron Blackwater faults, but his stations did not extend farther west U.S.A. Inc. has given crucialsupport to the Departmentof Geosciences at Penn State in providingthe computingresources used herein. John than 115ø15'W longitude and so would not cross any slip car- Vidale of U.C. Santa Cruz graciouslyprovided his travel time code as ried betweenthe Helendale and Lockhart faults (Figure 2). well as valuableadvice and discussionson imagingtechniques. Tom Data publishedby Dokka and Travis [1990] thus do not rule Heam of Cornell University gave instructivecriticism. The authors benefitedfrom input by many at Penn State'sDepartment of Geosciences out the 0.4-1 mm/yr of right-lateral slip on a northweststriking andfrom commentsby RobertNowack and two anonymousreviewers. fault in the western Mojave required by our model to open Can- til basin. This extensionmay in fact help to keep the required REFERENCES offset on the Harper and and Blackwater faults to a minimum Allen, C. R., The tectonicenvironments of seismicallyactive and inac- and allow them to terminate south of the Garlock, as shown by tive areasalong the San Andreasfault system, StanfordUniv. Publ. Geol. Sci., 11, 70-82, 1968. Dokka and Travis [ 1990]. Westward extensionof King's [1985] Astiz, L., and C. R. Allen, Seismicityof the Garlock fault, California, geodeticstudy to crossour proposedHelendale-to-Lockhart con- Bull. Seismol. Soc. Am., 73, 1721-1734, 1983. nection could test whether extension at Cantil basin is causedby Aydin, A., and A. Nur, Evolution of pull-apartbasins and their scale the intersectionof a regionally extensiveright-lateral fault with independence,Tectonics, 1, 91-105, 1982. the Garlock. Bird, P., and R. W. Rosenstock,Kinematics of presentcrust and mantle flow in southemCalifomia, Geol. Soc. Am. Bull., 95, 946-957, 1984. These considerationsshow that our model giving the Garlock Brady, R. H., and B. W. Troxel, Easterntermination of the Garlock fault fault a south dip to actively detach the Mojave region is con- in the Avawatz Mountains, San Bernardino County, California sistentwith other geologicaland geophysicaldata. The deep (abstract), Geol. Soc.Am. Abstr. Programs,13, 46-47, 1981. seismic velocity distributions found by Hadley and Kanamori Carter, B. A., Quaternarydisplacement on the Garlockfault, California, [1977] and Hearn and Clayton [1986a, b] further suggestthat a in Geologyand Mineral Wealthof the CaliforniaDesert, edited by D. L. Fife and A. R. Brown, SouthCoast Geological Society, Santa Ana, detachmentsurfacing at the Garlock may be connectedin some Calif., pp. 457-466, 1980. way to a lateral shift in the plate boundary from the eastern Carter, B., Neogene displacementon the Garlock fault, California Mojave at the Moho and the western Mojave at the surfacetrace (abstract), Eos Trans. AGU, 63, 1124, 1982. of the San Andreasfault. The lateral motionsalong sucha shal- Carter,J. N., B. P. Luyendyk,and R. R. Terres,Neogene clockwise tec- tonicrotation of the easternTransverse Ranges, California, suggested lowly dipping detachmentcould be driving the fragmentationof by paleomagneticvectors, Geol. Soc. Am. Bull., 98, 199-206, 1987. the Mojave at deformationrates above 10 mm/yr. Further work Cheadle,M. J., B. L. Czuchra,C. J. Ando, T. Byrne, L. D. Brown, J. E. could attempt to define the geometry of this detachmentin the Oliver, and S. Kaufman,Geometries of deepcrustal faults: Evidence eastemMojave and near the San Andreasfault. from the COCORP Mojave survey,in ReflectionSeismology: The ContinentalCrust, Geodynam.Set., vol. 14, editedby M. Barazangi CONCLUSIONS and L. Brown,pp. 305-312, AGU, Washington,D.C., 1985. Cheadle,M. J., B. L. Czuchra,T. Byrne, C. J. Ando, J. E. Oliver, L. D. We have analyzed and interpretedseismic reflection data from Brown, S. Kaufman,P. E. Malin, andR. A. Phinney,The deepcrustal COCORP Mojave line 5 acrossthe Garlock fault in the Mojave structureof the Mojave Desert, California, from COCORP seismic Desert of southern California. Strong fault plane reflections reflection data, Tectonics,5, 293-320, 1986. from this line (Figures 5 and 6) establishthe continuityof the Claerbout, J. F., Imaging the Earth's interior, 398 pp., Blackwell Scientific, Palo Alto, Calif., 1985. east branch of the Garlock fault as a seismic reflector from its Clark, M. M., Map showingrecently active breaks along the Garlock mapped surfacetrace to 4 km depth in Cantil Valley. Imaging and associatedfaults, California, U.S. Geol. Surv.Map, 1-741, 1973. techniquesassuming straight ray paths cannot properly locate Davis, G. A., and B. C. Burchfiel, Garlock fault: An intracontinental this fault reflectorin the subsurface.Strong lateral velocity con- transform structure, southern California, Geol. Soc. Am. Bull., 84, trastsat the sidesof 3 km deep Cantil basin (Figure 4) bend the 1407-1422, 1973. Dibblee, T. W., Jr., Geology of the Saltdale quadrangle,Califomia, reflection rays. A novel prestack Kirchhoff migration fully Bull. Calif. Div. of Mines and Geol., 160, 66 pp., 1952. accountingfor curvedrays throughthe strongvariations locates Dibblee,T. W., Jr., Geologicmap of the CastleButte quadrangle, Kern the east branchwith a southdip. This image (Figure 9) is sta- County, California, U.S. Geol. Surv. Miner. Invest.Field Stud.Map, tistically better than migrationsof noise and is constrainedby an MF-170, 1958. Dibblee, T. W., Jr., Evidenceof strike-slipfaulting along northwest- initial velocity model developedfrom complementaryrefraction, trendingfaults in the Mojave Desert, U.S. Geol. Surv. Prof. Pap., gravity, and borehole data. The imaged 45ø dip of the east 424-B, B197-B199, 1961. branch agrees with 37ø+10ø dip estimatesmade by simple Dibblee, T. W., Jr., Areal geologyof the westernMojave Desert,Cali- analysisof the apparentvelocities of diffraction tails. Basement fomia, U.S. Geol. Surv.Prof. Pap., 522, 153 pp., 1967. topographyand the strongreflectivity of the eastbranch suggest Dokka, R. K., The Mojave extensionalbelt of southernCalifornia, Tec- tonics, 8, 363-390, 1989. a model where the Garlock fault dips south to detach the Dokka, R. K., and C. J. Travis, Late Cenozoicstrike-slip faulting in the Mojave block (Figures 12 and 13). This model explainshow Mojave Desert, Califomia, Tectonics,9, 311-340, 1990. Cantil basin can grow wider as it lengthenswith slip on the Golombeck, M.P., and L. L. Brown, Clockwise rotation of the western Garlock. Extensionacross Cantil basin requires0.4-1 mm/yr of Mojave Desert, Geology,16, 126-130, 1988. Hadley, D., and H. Kanamori, Seismic structure of the Transverse dextral strike slip on a northwest-strikingfault such as the Ranges,Califomia, Geol. Soc. Am. Bull., 88, 1469-1478, 1977. Helendalebetween the Garlock and Pinto Mountain faults (Fig- Hanna, W. F., H. W. Oliver, R. F. Sikora, and S. L. Robbins, ure 1). Detachmentof the Mojave region by the Garlock as a Bakersfield sheet, Bouguer gravity map of California, scale LOUIEAND QIN: IMAGING OF THE GARLOC•: FAULT 14,479

1:250,000, Calif. Div. of Mines and Geol., Sacramento, 1975. McMechan, G. A., and G. S. Fuis, Ray equation migration of wide- Harding, T. P., and J. D. Lowell, Structuralstyles, their plate-tectonic angle reflections from southern Alaska, J. Geophys. Res., 92, 407- habitats,and hydrocarbontraps in petroleumprovinces, AAPG Bull., 420, 1987. 63, 1016-1058, 1979. Nilsen, T. H., and R. H. Chapman,Trona sheet,Bouguer gravity map of Harlan, W. S., J. F. Claerbout,and F. Rocca, Signal/noiseseparation and California, scale 1:250,000, Calif. Div. of Mines and Geol., velocity estimation,Geophysics, 49, 1869-1880, 1984. Sacramento, 1974. Hearn, T. M., and R. W. Clayton, Lateral velocity variationsin southern Pampeyan,E. H., T. L. Holzer, and M. M. Clark, Modern groundfailure California, I, Results for the upper crust from Pg waves, Bull. in the Garlock fault zone, Fremont Valley, California, Geol. $oc. Am. $eismol. $oc. Am., 76, 495-509, 1986a. Bull., 100, 677-691, 1988. Hearn, T. M., and R. W. Clayton, Lateral velocity variations in southern Phinney, R. A., and D. M. Jurdy, Seismicimaging of deep crust, Geo- California, II, Results for the lower crust from Pn waves, Bull. physics,44, 1637-1660, 1979. $eismol. $oc. Am., 76, 511-520, 1986b. Qin, J., Upper crustal velocities and structuralimaging of the Garlock Jennings,C. W., Fault map of California, Geol. Data Map Set. Map 1, fault zone, Mojave Desert, California, M.S. paper, 126 pp., Penn scale 1:750,000, Calif. Div. of Mines and Geol., Sacramento, 1975. State Univ., University Park, 1989. Jennings,C. W., Geologic map of California, Geol. Data Map Set. Ross, T. M., B. P. Luyendyk, and R. B. Haston,Paleomagnetic evidence Map 2, scale 1:750,000, Calif. Div. of Mines and Geol., Sacramento, for Neogeneclockwise tectonic rotations in the centralMojave Desert, 1977. California, Geology, 17, 470-473, 1989. Kanamori, H., and D. Hadley, Crustal structure and temporal velocity Sauber, J., W. Thatcher, and S. 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Paso basin, southernCalifornia: Implicationsfor the Miocene develop- Stolt, R., Migrationby Fouriertransform, Geophysics,43, 23-48, 1978. ment of the Garlock fault and uplift of the Sierra Nevada, Geol. $oc. Vidale, J. E., Finite-difference calculation of travel times, Bull. Seismol. Am. Bull., 100, 12-28, 1988. Soc. Am., 78, 2062-2076, 1988. Louie, J. N., The earthquakepotential of seismic reflectors near the Vidale, J. E., Comment on "A comparisonof finite-difference and southern San Andreas fault (abstract), Eos Trans. AGU, 69, 1419, Fourier method calculations of synthetic seismograms" by C. R. 1988. Daudt et al., Bull. Seismol. Soc. Am., 80, 493-495, 1990a. Louie, J. N., Physical propertiesof deep crustal reflectorsin southern Vidale, J. E., Finite-difference calculation of traveltimes in three dimen- California from multioffset amplitudeanalysis, Geophysics,55, 670- sions, Geophysics,55, 521-526, 1990b. 681, 1990. Vidale, J. E., D. V. Helmberger,and R. W. Clayton, Finite-difference Louie, J. N., C. R. Allen, D. C. Johnson, P. C. Haase, and S. N. Cohn, seismogramsfor SH waves, Bull. Seismol.Soc. Am., 75, 1765-1782, Fault slip in southernCalifornia, Bull. Seismol.Soc. Am., 75, 811-833, 1985. 1985. Wiggins, R., G. S. Kenny, and C. D. McClure, Common-depthpoint Louie, J. N., S. H. Fagerson,and R. W. Clayton, Imaging and properties methodfor determiningand displayingthe shear-velocityreflectivities of structures below the Rand Mts. (abstract), Eos Trans. AGU, 67, of a geologicformation, U.S. Patent 4,534,019, 1985. 1189, 1986. Louie, J. N., R. W. Clayton, and R. J. Le Bras, 3-d imaging of steeply J. N. Louie and J. Qin, Department of Geosciences,Penn State dipping structurenear the San Andreas fault, Parkfield, California, University, 442 Deike Building, University Park, PA 16802 Geophysics,53, 176-185, 1988. ([email protected]). Luyendyk, B. P., M. J. Kamerling, R. R. Terres, and J. S. Hornafius, Simple shear of southernCalifornia during Neogene time suggested by paleomagneticdeclinations, J. Geophys.Res., 90, 12,454-12,466, 1985. (Received August 10, 1990; Mabey, D. R., Gravity survey of the westernMojave Desert, California, revised January22, 1991; U.S. Geol. Surv. Prof. Pap., 316-D, 72 pp., 1960. acceptedApril 24, 1991.)