Garlock Fault: an Intracontinental Transform Structure, Southern California

Home , Basin and Range Province, Death Valley, Death Valley Fault Zone, Fault block, Garlock Fault, Salt Spring Hills

GREGORY A. DAVIS Department of Geological Sciences, University of Southern California, Los Angeles, California 90007 B. C. BURCHFIEL Department of Geology, Rice University, Houston, Texas 77001

Garlock Fault: An Intracontinental Transform Structure, Southern California

ABSTRACT Sierra Nevada. Westward shifting of the northern block of the Garlock has probably contrib- The northeast- to east-striking Garlock fault uted to the westward bending or deflection of of southern California is a major strike-slip the San Andreas fault where the two faults fault with a left-lateral displacement of at least meet. 48 to 64 km. It is also an important physio- Many earlier workers have considered that graphic boundary since it separates along its the left-lateral Garlock fault is conjugate to length the Tehachapi-Sierra Nevada and Basin the right-lateral San Andreas fault in a regional and Range provinces of pronounced topogra- strain pattern of north-south shortening and phy to the north from the Mojave Desert east-west extension, the latter expressed in part block of more subdued topography to the as an eastward displacement of the Mojave south. Previous authors have considered the block away from the junction of the San 260-km-long fault to be terminated at its Andreas and Garlock faults. In contrast, we western and eastern ends by the northwest- regard the origin of the Garlock fault as being striking San Andreas and Death Valley fault directly related to the extensional origin of the zones, respectively. Basin and Range province in areas north of the We interpret the Garlock fault as an intra- Garlock. Recent models for development of continental transform structure which sepa- that province related to intracontinental rates a northern crustal block distended by spreading east of an east-dipping subduction late Cenozoic basin and range faulting from a zone along the Cenozoic margin of western southern, Mojave block much less aifected by North America may best account for the dilational tectonics. Earlier ideas that the Gar- differential east-west extension which has oc- lock fault terminates eastward at the Death curred in the crustal blocks to the north and Valley fault zone appear to us to be in error, south of the Garlock fault. although right-lateral offsetting of the Garlock Other possible examples of intracontinental along that zone by about 8 km is necessary. transform faults in the southwestern Cordillera Displacement along the Garlock fault must with geometries similar to that of the Garlock increase westward from its eastern terminus, a fault include the left-lateral Santa Cruz-Sierra point of zero offset now buried beneath alluvial Madre fault zone along the southern margin of deposits in Kingston Wash to the east of the the western Transverse Ranges, and the right- Death Valley fault zone. Much of the dis- lateral Las Vegas shear zone and Agua Blanca placement on the Garlock fault due to east- fault of Baja California. west components of basin and range faulting appears to have been derived from block fault- INTRODUCTION ing in the area between Death Valley and the The Garlock fault zone of southern Cali- Nopah Range. Westward displacement of the fornia strikes northeastward to eastward and crustal block north of the Garlock by exten- has been recognized as a major structural ele- sional tectonics within it totals 48 to 60 km ment of this region for nearly 50 years. It is in the Spangler Hills-Slate Range area and equally important as a physiographic boundary, probably continues to increase westward at for it separates along its length the Tehachapi- least as far as the eastern frontal fault of the Sierra Nevada and Basin and Range provinces

Geological Society of America Bulletin, v. 84, p. 1407-1422, 5 figs., April 1973 1407

Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1407/3418372/i0016-7606-84-4-1407.pdf?casa_token=jv0gNjCIMW0AAAAA:UsGY5Ien0hbR0j-xzOfYLKmZBj1Ffs3jvP6Wu4-2aW17Ro16_h1drYrjalKmW7u8Gv5rMlNs by California Geological Survey, 19774 on 20 July 2019 Figure 1. Displacement criteria, Garlock fault, southern California. Displaced features, for example AA', BB', are explained and referenced in text. Base map from Geologic Map of California, scale, 1/250,000 :Los Angeles sheet (Jennings and Strand, 1969), Bakersfield sheet (Smith, 1964), and Trona sheet (Jennings and others, 1962). Inset: Location map of Garlock, Big Pine, and San Andreas faults in southern California.

of pronounced topography to the north from ing major regional significance, although the the Mojave Desert block of more subdued important paper by Hill and Dibblee (1953) topography to the south. All previous authors first gave the Garlock its present level of recog- have considered the 260-km-long left-lateral nition. Hill and Dibblee suggested that the fault to be terminated at its western and eastern left-lateral Garlock and Big Pine faults had ends by the northwest-striking San Andreas once been continuous before being disrupted and Death Valley fault zones, respectively along the San Andreas, and that displacements (Fig. 1). This fault geometry, although some- along these two faults have produced the west- what unusual, posed no serious problems until ward bending of the San Andreas near its inter- Smith (1962) offered evidence for a 64-km section with them (Fig. 1). They interpreted left-lateral displacement along the Garlock the Garlock and Big Pine faults as left-lateral fault based on an offset late Mesozoic dike shears conjugate in a regional strain pattern to swarm. The absence of an equivalent total off- right-lateral faults of the San Andreas system. set of the Garlock's bounding fault zones—the Others, for example, Kupfer (1968), have also San Andreas and Death Valley zones—posed a considered the Garlock and San Andreas faults serious geometric problem, especially in light as conjugate shears and have attempted re- of Smith's estimate of displacement along the gional strain and stress analyses for southern Garlock as being equal to one-quarter of its California on this basis. entire length. The idea that the Garlock and San Andreas Although the San Andreas fault zone swings faults are regionally related has led some geolo- westward near its junction with the Garlock gists (for example, Hill and Dibblee, 1953; fault, it is the Garlock-Death Valley fault zone Bucher, 1955; Hewett, 1955, PI. 1; D. L. junction that presents the most serious geo- Anderson, 1971) to conclude that the wedge- metric problems. Here, offset features suggest- shaped Mojave Desert structural block, bounding up to 64 km of left-lateral displacement ed on the west by the northwest-striking, along the Garlock fault are found on the south right-lateral San Andreas fault and by. the side of the Garlock within 16 km of its junction northeast-striking, left-lateral Garlock fault with the right-lateral Death Valley fault zone, (Fig. 1), must be undergoing eastward dis- thus producing radically different breadths of placement away from the intersection of the terrane on the two sides of the fault between two faults. This conclusion raises serious struc- these offset features and the Death Valley fault tural problems at the previously inferred zone. Smith recognized the geometric prob- eastern end of the Garlock fault where the lems posed by the inferred geometry of the presumably "active" Mojave block is bounded Garlock and its bounding faults, but he con- by the Garlock fault to the north and the cluded that "the evidence that the two dike Death Valley fault zone to the east. These swarms document 40 mi of lateral displacement problems are discussed in detail below. is stronger than the evidence that the tectonic pattern at the ends of the fault precludes it" DISPLACEMENTS ACROSS THE (Smith, 1962, p. 103). We agree, and offer in GARLOCK FAULT this paper an explanation for the geometry and Smith's study (1962) of the northwest- origin of the Garlock fault which is compatible trending dike swarms in Mesozoic granitic with regional geologic relations in the southern terranes north and south of the Garlock fault Death Valley-eastern Mojave Desert region. zone led him to conclude that approximately 64 km of offset has occurred along the fault EARLY STUDIES since the presumed late Mesozoic emplacement The Garlock fault was discovered and named of the dikes (Fig. 1, AA'). This displacement, by Hess (1910) during studies in the Randsburg although equivalent to one-quarter of the area of the north-central Mojave Desert. Hulin length of the known fault trace, has been (1925) mapped the fault in his study of the strongly supported by subsequent field studies. Randsburg quadrangle; he recognized dis- Smith and others (1968) described a previ- placement of Quaternary and older rock units ously unrecognized thrust fault in the southern along it, and concluded that total displace- Slate Range to the north of the Garlock fault ments of left-lateral nature amounted to ap- (Fig. 1, B). Here, cataclasized Mesozoic granitic proximately 8 km. Hulin (1925) and Noble rocks and gneisses of Precambrian age overlie (1926) both regarded the Garlock fault as hav- Mesozoic metavolcanic and granitic rocks along

the west-dipping (30° to 40°) Layton Well be offset some 65 krr.. from each other (Jahns thrust. In reconnaissance studies south ol the and others, 1971; 196ii, oral commun.). Garlock fault in the eastern Granite Moun tains In summary, a left-lateral offset of about 56 (Fort Irwin Military Reservation) and only km of pre-Cenozoic structures and rocks across 13 km west of its junction with the Deat:i the central part of th:: Garlock fault zone ap- Valley fault zone, we have found a major pears well established. Offset elements on the thrust fault which we regard as being part of south side of the fault can be traced to within the same thrust (Fig. 1, B'). Here, as in the 13 km of the presumed termination of the Gar- Slate Range, a section of sheared and phylloni- lock fault at its junction with the northwest- tized crystalline rocks several thousands of feet striking Death Valley fault zone. Near this thick overlies Mesozoic metavolcanic rocks 1 junction the Avawatz and easternmost Granite along a west-dipping (40°) thrust contact. This Mountains correlate geologically with displaced thrust lies some 56 to 64 km east of the Layton terranes in the westernmost Panamint Range Well thrust in the Slate Range, but this separa- and the Slate Range. But where to the south tion distance may not represent true strike- of the Garlock fault, if the Garlock terminates slip offset. Because of the moderate dip of the at the Death Valley fault zone, is the offset thrust, dip-slip components of displacement equivalent of the 32- to 40-km-wide Owlshead along the Garlock might significantly affect Mountains plutonic tei rane (shaded area, Fig. the horizontal separation distance observed be- 1) which lies between the southern Panamint tween the two thrust segments. Range and the Death Valley fault zone? This Smith and Ketner (1970) present evidence geologic and geometric enigma is the heart of for a 48- to 64-km left-lateral offset of the the Garlock problem. lithologically distinctive, and at least in part Permian, Garlock Formation from north of the RELATIONS BETWEEN THE fault in the El Paso Mountains (Fig. 1, C) to GARLOCK AND DEATH VALLEY the south of the fault in Pilot Knob Valley FAULT ZONES (Fig. I, C')- The correlative strata in both Smith (1962) reviewed various attempts to areas strike north to northwest and because resolve the geometric problems at the junction they dip steeply eastward they are well ori- of the Garlock and Dea th Valley fault zones. ented for definition of strike-slip separation Hewett once suggested (1 955) that at its eastern along this portion of the Garlock fault. end the Garlock fault turns southward and Other possible but less convincing large off- becomes a thrust fault dipping westward under sets of stratigraphic units or sequences across the Avawatz Mountains. The Mojave block to the Garlock fault have been inferred. Dibblee the south of the Garlock was thus viewed by (1967a, p. 115) suggested that low-grade meta- Hewett as being a thrus.: plate uplifted with morphic rocks of the Rand Schist near Rands- respect to northern provinces and displaced burg (Fig. 1, D') and of the "Pelona Schist" eastward intermittently through Tertiary and within the Garlock fault zone to the west (Fig. into Quaternary time. T rie presence of a Gar- 1, D) may once have been adjacent; they are lock "thrust" fault beneath the Avawatz now 72 km apart. Finally, Jahns and others Mountains has since been discounted (fahns believe that sequences ol: late Precambrian and and Wright, 1960). early Paleozoic miogeosynclinal strata in the Three general possibilities remain for resolv- Avawatz Mountains south of the Garlock ing the problems at the G'arlock-Death Valley fault (Fig. 1, E') and in the southern Panamint fault zone junction: Range to the north of the fault (Fig. 1, E) may 1. The Garlock fault terminates at the right- lateral Death Valley fault zone and the 48- to 64-km displacement along the Garlock is ac- 1 The thrust lies to the northeast of Drinkwater Lake commodated in the junction area. approximately along 116° 30' W. long. It is not the thrust 2. The Garlock fault once continued east of fault depicted on the Trona Sheet of the Geologic Ma-> the junction, but its eastward extension has of California (Jennings and others, 1962) to the north cl been offset in a right-lateral sense along the Drinkwater Lake. Rocks designated as Mesozoic metavolcanic rocks immediately to the east of this mapped younger, northwest-strikin;:; Death Valley fault thrust are sheared and phyllonitized granitic rocks in zone and displaced many miles to the south. the basal portion of the thrust plate we have described 3. The Garlock fault crosses the Death Val- briefly above. ley fault zone in the junction area and con-

tinues eastward past it, despite conclusions to out the Great Basin province to the east the contrary by other geologists. (Eaton, 1932). Eaton also believed that the Hypotheses pertinent to the first possibility westward shifting (sliding) of the Sierra have been raised by several authors. Grose Nevada crustal block was responsible for the (1959) and Jahns and Wright (1960) suggested westward bending of a once straighter San that great vertical movements coupled with Andreas fault and for the compressive, post- strike-slip displacements in the junction area Miocene deformation in the southern Coast may have accommodated the merger of the Ranges to the west of the San Andreas (1932, Garlock and Death Valley fault zones. These Fig. 4). suggestions were made, however, prior to More recently, Hamilton and Myers (1966) Smith's recognition of tens of kilometers of attempted to explain some of the displacement displacement along the Garlock and are no along the Garlock fault by noting the presence longer compatible with local geology in light of extensional (basin and range) structure north of such large displacements. The junction area of the fault and west of Death Valley and its of the Garlock and Death Valley fault zones is absence in the Mojave structural block to the characterized by many fault splays and an ap- south: parent southward turning of some east-west faults in the Garlock zone, for example, the The Garlock and Death Valley faults merge and Arrastre Spring fault which bounds the Ava- mutually deflect each other (Jahns and Wright, watz Mountain block on its western side (Fig. 1960), but the Garlock does not continue east 1, F'). Jahns and others (1971) suggest that the beyond the Death Valley zone, nor does it appear Arrastre Spring fault "may well be the major farther southeast in a position offset along the Death Valley fault. South of the Garlock is the Mojave easterly expression of the Garlock zone." We Desert block of generally old, low mountain blocks, believe that local structural relations do not largely buried by basin deposits in the west. North support this contention. The Garlock fault of the Garlock fault is a region of high and exceed- would have to curve through an angle of 50° to ingly active fault-block ranges, the Sierra Nevada, become the Arrastre Spring iault. Large left- Argus, Panamint, and Black Mountains. The Gar- lateral displacement along a curvilinear Garlock lock thus has the position and relative offset re- fault would necessitate rotations of 50° of struc- quired by the explanation that the block faulting tural elements. Smith (1962) suggested that to the north is due to crustal extension, because the dike swarm south of the Garlock fault in the fault marks the south end of the currently stretching mass, although the total amount of the Granite Mountains has been rotated in a lateral displacement is too large to be accounted clockwise direction by 20° to 25° from its offset for by this mechanism alone. The aggregate topo- extension in the Spangler Hills north of the graphic height of the major range-front scarps Garlock. However, several kilometers to the north of the Garlock fault is 7 or 8 km; if the bed- east of the dike swarms the northwest strike of rock relief is twice the topographic relief, at most the Layton Well thrust in the Slate Range and 15 km of extension could have accompanied the its offset extension in the eastern Granite Mountains show no discernible relative rota- 2 The Arrastre Spring fault zone might be an offset tion, even though the latter locality is within equivalent of the Panamint Valley fault zone (Fig. 1, G) 8 km of the junction of the Garlock and Ar- which separates the Slate and Panamint Ranges and ap- 2 rastre Spring faults (Fig. 1, B' and F'). pears to cross the Quail Mountains just north of the Garlock fault. In both areas, the faults separate a Meso- The nature of the Garlock fault as a bound- zoic metavolcanic and plutonic terrane on the west from ary between diverse structural and physio- a late Precambrian-Paleozoic miogeosynclinal terrane graphic provinces has prompted past attempts intruded by Mesozoic plutonic rocks on the east. The to explain some or all of its displacement by eastward bend of the southern part of the Panamint calling upon differential extensional behavior Valley fault zone complements the westward bend of the in crustal blocks to the north and south of it. northern part of the Arrastre Spring zone, and both Eaton (1932) was probably the first to develop bends could be attributed to left-lateral drag along the this line of reasoning. He concluded that the Garlock fault. If a correlation exists, dip-slip displacements along the Panamint-Arrastre Spring fault zone Sierra Nevada had shifted westward in post- must have occurred both prior to inception of the Gar- Miocene time, with a maximum displacement lock fault and subsequently, since the present Panamint of up to 96 km at its southern end. Shifting of Valley and Arrastre Spring faults involve Pliocene and the Sierran block was thought to have been Quaternary sediments, and at least one of the two faults, accompanied by extensional faulting through- the Panamint Valley, is still active.

Figure 2. Reconstruction of possible pre-Garlock lock fault and 56 km of subsequent left-lateral displace- relations between Panamint, Owlshead, and Avawatz ment along it. The reconstruction does not attempt to terranes assuming original continuity of the Mesozoic undo the distensional effects of Cenozoic normal dike swarm and the Layton Well thrust across the Gar- faulting in the terrane north of the Garlock (see text).

formation of the present ranges. Strike-slip dis- Valley to account for 48 to 64 km of crustal placement due to this process would increase west- extension in a terrane now only 64 to 80 km ward. wide. Although Hamilton and Myers thus concluded To support this contention we have pre- that block faulting to the north of the Garlock pared a reconstruction of pre-Garlock relations could account for 15 km or one-third to one- between the Avawatz, Panamint, and Owls- quarter of the total displacement along it, head areas by reversing 56 km of left-lateral they were at a loss to account for the remainder. slip along the Garlock and thus aligning the B. W. Troxel and L. A. Wright (1969, oral two portions cf Smith's offset dike swarm and commun.) modified the Hamilton-Myers con- ;he Layton Well thrust and it:; eastern Granite cept by inferring a different geometry for the Mountains equivalent (Fig. 2). Despite the range-front faults north of the Garlock. Troxel alignment of these features it can be seen that and Wright assume that the range-front faults the traces of the Death Valley fault zone east become progressively flatter at depth before of the Avawatz and Owlshead Mountains are merging into a horizontal and deep-seated si ill separated by about 56 km. To bring these basal detachment surface beneath the block traces into alignment by undoing the exten- bounded on the south by the Garlock and on sional effects of basin and range faulting in the the east by Death Valley. This inferred fault- terrane north of the Garlock and west of the block geometry could, they believe, allow a Death Valley fault zone requires that the crustal distension north of the Garlock equiva- breadth of the northern terrane be reduced by lent to the total 48- or 64-km displacement 3C0 percent. Normal faults do exist within this along it. They, too, suggest that the Garlock terrane (Fig. 3), but they are far too few and terminates at its intersection with the Death too limited in dip-slip displacement to have Valley fault zone on the northeastern side of produced such an enormous delation of the the Avawatz Mountains. The principal draw- Slate Range to Death Valley region. back to their alternative, in our opinion, is that We therefore conclude that most of the there are not enough normal faults in the ter- Owishead Mountains must origina lly have been rane between the Slate Range and Death situated east of the longitude of the Avawatz

Mountains (as in Fig. 2). The westward shifting clusions from geologic reconnaissance along by extensional tectonics of the Owlshead ter- projections of the Death Valley fault zone rane to its present position northwest of the south of its intersection with the Garlock are Avawatz Mountains requires that the Garlock in accord with the estimates of Wright and fault also extends east of the eastern edge of Troxel. There seems to be no basis, for example, the Avawatz Mountains as defined by the for postulating large strike-slip displacements present trace of the Death Valley fault zone between areas of outcrop along the southward (Fig. 2). The problem of finding the offset projection of the Death Valley fault zone in counterpart of the Owlshead Mountains south the Silver Lake area (Fig. 1, Avawatz Moun- of the Garlock fault is complicated by the tains, the hills west of Silver Lake, and the nature of the Owlshead area. The Owlshead Halloran Hills to the east). Mountains are underlain largely by Mesozoic Even if tens of kilometers of displacement granitic rocks (Gastil and others, 1967) that have occurred along the Death Valley fault comprise an apparently concordant plutonic zone, no problems pertinent to the Garlock complex at least 32 km wide in an east-west fault are resolved since we can find no offset direction (Fig. 2). This complex lies entirely Garlock fault tens of kilometers south of the north of the Garlock fault and accordingly does junction. Mapping by Hewett (1956), our- not have a direct offset counterpart to the selves (unpub. data), and others in the area south, although metamorphic country rocks southeast of the junction of the Garlock and south of the plutonic complex have been cut Death Valley fault zones reveals a continuous by the Garlock. Slices of this country rock terrane, represented by marbles and other pre-Cenozoic geologic terrane extending from metamorphic rocks, may be present within the the Silurian Hills south to Kelso Valley, north Garlock fault zone 13 to 19 km east of the of the Providence Mountains (Fig. 3). A left- southeastern Owlshead Mountains (Noble and lateral fault probably does lie beneath Kelso Wright, 1954), although offset counterparts and Ivanpah Valleys, east of the south-pro- south of the zone and still farther east have not jected trace of the Death Valley fault zone, been found. The likely present location for but this fault is too far to the south (96 km), most of the country rock terrane which once its trend too northerly (N. 55° E.), and its lay south of the Owlshead Mountains plutonic likely displacement (11 to 13 km) too small to complex is east of the present Avawatz Moun- be compatible with its being an offset eastern tains and Death Valley fault zone in the area segment of the Garlock. now covered by Quaternary alluvial deposits We, therefore, regard theories postulating a of Silurian lake basin. major right-lateral offset of the Garlock fault along the Death Valley fault zone as lacking We thus regard the first general possibility both evidence for large lateral displacements on for explaining geometric relations at the eastern the latter, and an offset equivalent of the Gar- end of the Garlock fault, that is, that the Gar- lock fault in areas to the south and east. lock fault terminates at the Death Valley fault zone and never extended farther east, as EASTERN EXTENSION OF geologically untenable. This then brings us to THE GARLOCK FAULT the second possibility—that the Garlock fault As an alternative to the geologically unsatis- once extended east of the Death Valley fault fying hypotheses that the Garlock fault either zone, but that its eastern extension has been ends at the Death Valley fault zone or that it offset many miles by right-lateral strike-slip has been displaced many miles along it, we faulting along the younger Death Valley fault propose that the Garlock fault must cross the zone. Recent right-lateral displacements along Death Valley fault zone in the junction area this zone have been documented by Hill and and continue eastward past it. Minor right- Troxel (1966), but the amount of total dis- lateral displacement of the Garlock fault by placement has been the subject of controversy. faults in the Death Valley fault zone is required Hamilton and Myers (1966) hypothesized 48 by the geometry of bedrock exposures in the km of right-lateral displacement on the Death junction area, but certainly no more than 8 km. Valley fault zone, although Wright and Troxel An offset of the Garlock fault by this amount (1967) have presented geologic evidence favor- would place its eastern continuation beneath ing less than 8 km of offset along it. Our con- the Quaternary deposits of Kingston Wash

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Va Key Figure 3. Geologic relations, eastern portion of Garlock fault and vicinity, California. Base map from (Jennings, 1958), and Kingman sheet (Jennings, 1961). Geologic Map of California, scale, 1:250,000: Trona Geologic data from above maps and studies of B. C. sheet (Jennings and others, 1962), Death Valley sheet Burchfiel and G. A. Davis (1971; unpub. data).

(Fig. 3). Continuity of the pre-Cenozoic geo- Wash of stream channels and the inferred posi- logic terrane in areas east of Kingston Wash, tion of the fault. including the Nopah and Kingston Ranges, Mesquite Mountains, Shadow Mountain, Clark GARLOCK FAULT AS A Mountain and Mescal Ranges, and other areas TRANSFORM STRUCTURE to the south (Burchfiel and Davis, 1971), re- Our proposed extension of the Garlock fault quire the termination of the Garlock fault beneath Kingston Wash coincides with an east- beneath the wash. Cambrian strata in the Nopah ward continuation across the Death Valley fault block continue southeastward into the fault zone of the boundary between basin and uppermost (Winters Pass) thrust plate of the range structure to the north and a Mojave-style Clark Mountain thrust complex. Thus, the area to the south where major range-front Nopah fault block is part of the continuous faults are lacking. In areas to the north of our geologic terrane east of Kingston Wash. We inferred Garlock trace and east of Death Valley conclude that the Garlock fault either termi- are numerous large, east-tilted fault blocks, nates beneath the alluvial deposits of Kingston including the southern Black Mountains, Sara- Wash along the south-projected trace of the toga-Ibex Hills, the Saddle Peak, Salt Spring, Nopah Range frontal fault or that it extends and Dublin Hills, and the Resting Spring and several miles farther east to where other, less Nopah Ranges (Fig. 3). important, high-angle faults can be inferred to In postulating an eastward extension of the exist beneath Quaternary alluvium (Fig. 3). Garlock fault and by designating an inferred The location of the Garlock fault beneath terminus for it, we are in a position to re- the western part of Kingston Wash is restricted analyze the geometry of terranes north and by the areas of exposed bedrock present in this south of the eastern third of the Garlock fault. area. It must pass north of the Silurian Hills We have drawn a northwest-trending reference and south of the Salt Spring Hills.3 Both areas line parallel to regional structural trends and are characterized by the distinctive southern through the continuous geologic terrane east of facies of the Kingston Peak Formation of the Kingston Wash (Fig. 3). Measurements west- Pahrump Group (Troxel, 1967, p. 35) which ward from this line to northwest-trending fea- contrasts with other local occurrences of this tures which were also formerly continuous, but formation. This similarity and the anomalous have since been offset by the Garlock, enable geosynclinal sections of the two areas—both us to compare the present breadths of once- sections are relatively thin and lack the late equivalent terranes now north and south of the Precambrian Noonday Dolomite (Wright and fault, for example, the western limit of the Troxel, 1966)—suggest that the two areas may vertical dike swarms mapped by Smith (1962) be offset from each other along the buried north and south of the Garlock fault. To the Garlock fault. north of the Garlock fault, the east-west No trace of the Garlock fault can be seen on breadth of terrane between the western edge aerial photographs of the western part of of the dike swarm and our eastern reference line Kingston Wash or to the south of the Salt of continuous terrane is approximately 160 km. Spring Hills, in contrast to its prominence on The breadth of what we interpret to be the photographs in the Leach Lake playa area 32 equivalent terrane south of the Garlock fault km to the west. It is likely that the Garlock (since it has the same western and eastern fault is inactive in the former areas, although boundaries) is approximately 96 km, or 64 km the possibility exists that the absence of sur- less. We believe that the increased breadth of ficial faulting can be attributed to the high the northern terrane and the left-lateral strike- rates of Quaternary sedimentation in these slip displacement along the Garlock fault can areas and to a general parallelism in Kingston be explained by adopting and expanding upon the basic ideas of Hamilton and Myers (1966) and Troxel and Wright (B. W. Troxel, 1969, 3 Strata in the northern Salt Spring Hills strike nearly oral commun.) that the northern terrane has north, whereas strata in the southern Salt Spring Hills been distended by basin and range faulting, strike about N. 80° W. The progressive southward whereas the southern (Mojave) terrane has not. change in strike between the two areas may indicate By relating the Garlock fault to basin and drag effects along the left-lateral Garlock fault; it is not range structure to the east of Death Valley (as compatible with right-lateral displacements within the well as to the west), in contrast to earlier nearby Death Valley fault zone.

hypotheses, many more normal faults are avail- the necessary structural and stratigraphic data able to produce the extra breadth of terrane to make a fault-by-fs ult summation of east-west north of the Garlock. Much of the displace- extension across the terrane in question, but ment on the Garlock fault, in fact, appears to we believe that our geometric analysis of rela- have been derived from block faulting in the tions at the eastern end of the Garlock fault area between Death Valley and the Nopah and our estimate of total extension within the Range. northern terrane are both sound and geometri- The stratigraphic throw on some cf the cally inescapable. The area between the Nopah range-front faults to the east of Death Valley Range and the Spangler Hills-Argus Range is considerable. For example, the late Precam- may, therefore, be one. of extreme distension in brian to Paleozoic miogeosynclinal sect on in comparison with the Great Basin as a whole. the east-tilted Nopah Range fault block has a The Garlock fault thus appears to be an thickness in excess of 6,500 m (Hazzard, 1937). intraccntinental transform structure since it It is largely repeated by normal faulting i.i the is a boundary or narrow zone of accommodation adjacent Resting Spring Range to the west. between one crustal alock which has grown Assuming, therefore, a stratigraphic throw of wider because of late Cenozoic normal faulting six and one-half km and a westward dip of 55° and another block, the Mojave, which has not on the Nopah frontal fault, an east-west exten- (or which has not been dilated as much). The sion (the horizontal component of down-dip eastern end of the Garlock fault is thought to displacement) due to this fault alone is 3.7 to lie east of the Death V alley fault zone and to 4.5 km, depending upon the inferred eastward be a fixed point of zero strike-slip displacement. dip of the stratigraphic section prior to faulting Extensional strain within the terrane north of (40° to 0°). Other fault configurations, for the Garlock and west of this fixed point has example, a flatter dip or flattening of this and been accommodated regionally by a westward other frontal faults at depth (Moore, 1960; deflection of the San Andreas fault and the Wright and Troxel, 1969; R. E. Anderson, crustal block to the west of it. These relations 1971), can produce even greater amounts of are shown diagrammatically in Figure 4. Like extension. certain other types of transform faults (Wilson, According to our analysis, the terrane sojth 1965) the length of the Garlock is not constant, of the Garlock between its eastern termi ius but should increase with :ime as distension con- and the western edge of the Granite Mountains tinues in areas to the north of it, unless, as Ross dike swarm is 72 km wide, whereas the width (1970) has suggested, portions of the western of the equivalent terrane to the north of the end of the northern block (the San Emigdio Garlock is 138 km or nearly 100 percent wider. Mountains) are sliced off along the San Andreas There is a precedent in other recent studies of fault and transported northward along it. basin and range structure for postulating such Evidence for an increase in strike-slip dis- a large percentage of distension. Profett (197. ) placement westward along the Garlock fault has concluded that east-west extension due to as required by the transform hypothesis pro- normal faulting in the Yerington district of posed here is, on the basis of available data, Nevada exceeds 100 percent. Even greater dis- meager and inconclusive.' There is, however, tension is proposed by R. E. Anderson'(1971, no evidence against this postulate. All geologic unit Nil; Fig. 2, AA') within terranes of criteria whxh have been used for displacement closely spaced low-angle normal faults in south- estimates (Fig. 1, A-E) lie considerably west ern Nevada. Hamilton (1969, p. 2421) sug- of the inferred eastern terminus of the Garlock, gested that extension within the entire Basir and all by their geologic nature are somewhat and Range province might be several times the inconclusive when individually considered. A total amount of dip-slip displacement; his '•8- to 64-km Cenozoic offset of the steeply Figure 3 indicates total Cenozoic extensions o.: dipping Garlock Formation (CC') and the up to 50 percent. Nevertheless, our postulatec vertical Mesozoic dike sws.rm (AA') does ap- 48- to 64-km extension within the terrane north of the Garlock fault between the Nopah pear well established, but apparently similar Range and the Spangler Hills (Fig. 1) is com- displacement values of features farther east are parable in magnitude to that inferred for the 4 Unfortunately, access to the Sarlock fault zone be- entire Great Basin province by other authors, tween the Slate Range and the A/awatz Mountains and for example, Thompson (1959, 48 km), and to :he geologic terranes adjacen : to it is severely re- Stewart (1971, 72 km). Unfortunately we lack stricted because this critical 80-kra-long area lies within the closed boundaries of military /eservations.

Figure 4. Northward diagrammatic view of Garlock generalized and are shown only north of the Garlock fault, southern California, as a boundary between a fault. Geographic localities: SJV = San Joaquin Valley, northern, distended crustal block (Basin and Range SN = Sierra Nevada, PV = Panamint Valley, DV = province) and a southern, nondistended crustal block Death Valley, NR = Nopah Range, KR = Kingston (Mojave Desert). Topographic relations are highly Range.

in question. As mentioned previously, the (Prodehl, 1970) indicates that the northern strike-slip offset of the Layton Well thrust and central Mojave block lacks the anomalous from its southern continuation in the eastern low-velocity upper mantle so characteristic of Granite Mountains may be less than the 56 km the Basin and Range province (see, for exam- measureable from Figure 1 (BB') because of ple, Scholz and others, 1971). Prodehl inter- the low dip of the thrusts and the possibility of prets the Basin and Range province north of dip-slip displacement along the Garlock. The the Garlock fault as having a crust 31 to 36 km 56- to 64-km sinistral separation of late Pre- thick, a thickness defined by the depth to the cambrian-Cambrian miogeosynclinal strata in strongest velocity gradient beneath this region, the Avawatz Mountains and Panamint Range and as having an anomalous upper mantle with may be due in part to forceful shouldering P„ velocities less than 8.0 kmps. In contrast, aside of country rocks during emplacement of the crust beneath the central and northern the extensive and apparently concordant Owls- Mojave Desert is interpreted as being 29 to head Mountains plutonic complcx (see Fig. 2); 31 km thick (Scholz and others, 1971, Fig. 11) under any circumstances, displacement esti- and as being underlain by mantle with veloci- mates based on the present distribution of ties equal to or greater than 8.0 kmps (Scholz originally widespread stratigraphic units must and others, 1971, Table 2); these "normal" be viewed with extreme caution. upper mantle velocities are found elsewhere in the western United States only beneath the GEOPHYSICAL EVIDENCE central Rocky Mountains and the California Our interpretation of the Garlock fault as a Coast Ranges. major structural boundary between crustal Our transform model for the Garlock fault blocks with radically different late Cenozoic accords well with conclusions regarding the tectonic histories is supported by available, Garlock based on regional seismicity. Allen and but limited, geophysical data. Although many others (1965, p. 577) concluded from a study geologists assign the Mojave Desert to the of the relations between recent seismicity and Basin and Range (Great Basin) province, a structure in southern California that "the recent reinterpretation of seismic refraction entire Garlock fault zone . . . seems to have profiles across the southwestern United States served more as a boundary between seismic

provinces than as a locus of seismic activity." East-west transcurrent (strike-slip) faults which They regard the western and northern Mojave bound terranes of north-striking, closely Desert block between the San Andreas, and spaced, low-angle normal faults have recently Garlock faults as seismically and, therefore, been described by R, E. Anderson (1971) in tectonically, stable with respect to ad' .ice it the Basin and Raiv:e province of southern blocks, although, as they recognized, the time Nevada. His studies p rovide, on a much smaller span of their investigation (1934 to 1963) may scale of development than the Garlock fault, be inadequate to support such a conclusion. interesting analogs to the relations between Our tentative conclusion, expressed earlier, basin and range structure and the Garlock that that part of the Garlock fault to the east fault postulated here. of Death Valley is inactive, whereas the Gar- A possible example of a Garlock-type intra- lock to the west shows physiographic indica- continental transform fault elsewhere in south- tions of Holocene (if not historic) activity, re- ern California is the Santa Cruz-Malibu Coast- quires that Death Valley should also be a Santa Monica-Raymond-Sierra Madre fault boundary between an area of active basin and zone which separates the offshore Continental range faulting to the west, and an area of in- Borderland and the onshore Peninsular Ranges activity between Death Valley and the Nopah from the east-trending Transverse Ranges to Range to the east. This relation appears to be the north (Fig. 5). This fault zone bears a confirmed by the freshness of fault scarps cut- number of geometric similarities to the Gar- ting alluvial fans along the western fronts of lock. Northwest-trending basin and range the Panamint and Slate Ranges and the Black structure is highly developed in Borderland Mountains (Fig. 3), coupled with the absence areas south of it and is absent in the Transverse of such scarps along mountain fronts farther Ranges to the north (Clements and Emery, east. A westward shift with time in late 1947; Allen and others, 1965, PI. 1; Moore, Cenozoic basin and range faulting has been 1969, PI. 13, Fig. 19). .Left-lateral displacement documented by geochronologic studies of asso- along the portion of this zone which extends ciated Great Basin volcanism (Armstrong, from the Pacific Ocean to the San Andreas 1970; Fleck, 1970a; McKee, 1971; Scholz and fault is estimated by Teats (1968) and Yerkes others, 1971). Most workers agree that basin and Campbell (1971) as totalling 88 km be- and range structure in this portion of the tween the Santa Monica Mountains to the province began to develop in the Miocene, north and the Los Angeles Basin and Santa Ana perhaps as long ago as 16 to 17 m.y. (Stewart, Mountains to the south.5 If initial strike-slip 1971; McKee, 1971). displacement on this ftult zone was related to extensional tectonics within the Continental The low level of current seismic activity Borderland, then the left-lateral nature of along the Garlock fault to the west of Death faulting suggests that ftult displacement would Valley (Allen and others, 1965) may reflect (1) increase eastward from an offshore point of a recurrence interval for basin and range fault- zero displacement. This hypothesis is supported ing in areas to the north of the Garlock w.iich by the apparent lack of left-lateral offset of the is longer than the short period of historic time continental margin west of Santa Cruz (see within southern California, or (2) a changeover Moore, 1969, PI. 13; compare the 5,000-ft and in the tectonic regimen of this portion of Cali- 10,000-ft isobaths; Fig. 5), and by the pro- fornia from one of east-west extension to one nounced eastward deflection of strands of the of northwest-aligned right-lateral strain. The San Andreas fault (Nor :h and South Branches, Death Valley area itself shows, for example, a Mill Creek, Mission Creek, and Banning present preponderance of right-lateral strike- faults) near the intersec :ion of the Santa Cruz- slip motion along active or recently active Sierra Madre fault zon: with the San Andreas faults (Hill and Troxel, 1966; Burchfiel and (Dibblee, 1968). It is interesting that the two Stewart, 1966; Stewart, 1967). major deflections in trend of the San Andreas FAULTS ANALOGOUS TO fault in California occu r in close proximity to THE GARLOCK FAULT

It is likely that the Garlock fault is not a 5 Present fault displacements along the eastern half unique transform structure within the south- of the Santa Cruz-Sierra Madre fault zone, where it western United States, but perhaps only one defines the southern margin of the uplifted San Gabriel of many strike-slip faults which separate crustal Mountains crystalline block, are of reverse type (north blocks with differential extensional behavior. side up).

Figure 5. Areas of pronounced basin and range Sierra Madre fault zone comprises the California structure in the southern California region and the Borderland province. Bathymetry in this offshore area is positions of possible left-lateral transform faults of Gar- from Moore (1969, Pi. 13). lock-type. The offshore area south of the Santa Cruz-

junctions of that fault with the east-striking Still other possible examples of intraconti- Garlock and Santa Cruz-Sierra Madre fault nental transform faults of Garlock type within zones—both of which bound extensional geo- the southern California area are the east-west logic provinces of basin and range type (Fig. Pinto Mountain, Blue Cut, and Santo Tomas 5)-6 faults (Fig. 5), all of left-lateral type and with displacements of 9 to 16 km (Dibblee, 1967b), 6 The San Jacinto fault does not exhibit the deflection 5 to 6 km (Hope, 1970), and 14 km (Krause, shown by adjacent portions of the San Andreas near the 1965), respectively. Each has geologic and eastern end of the Santa Cruz-Sierra Madre fault zone geometric characteristics suggestive of an origin (Fig. 5). It can be argued that the San Jacinto fault, by at least in part related to differential amounts of far the most active member of the San Andreas fault extension by normal faulting in terranes to the system in southern California (Allen and others, 1965), is younger than the San Andreas of this region and that north and south of the faults. it represents a tectonic bypassing of the bent and pos- The recent literature also provides several sibly locked portion of the San Andreas fault in the probable examples of right-lateral, intraconti- southern and eastern Transverse Range area. nental transform faults in the southwestern

Cordillera. Burchfiel and Stewart (1966) pro- ponents of east-west extension and right-lateral posed that extensional "pull-apart" c;~ tie shear. According to our interpretation, how- central segment of Death Valley is related to ever, basin and range structure originally devel- right-lateral strike-slip movement aloiv; the oped in southern California during east-west bounding Furnace Creek and Death Valley to east-northeast-west-southwest extension fault zones. Fleck (1970b) has suggested that (with respect to present geographic coordi- the right-lateral Las Vegas shear zone is a :rans- nates), that is, in a direction parallel to the form fault separating two centers of late Ceno- Garlock and Santa Cruz-Sierra Madre trans- zoic volcanism (and extension)—one to the form faults, and without significant right-lateral north of the shear zone in the Nevada Test components of faulting. Site and the other to the east and south cf the Thompson (1971) and Scholz and others shear zone along the Colorado River. Similar (1971) have recently proposed that develop- ideas are expressed by Anderson and others ment of the Basin and Range province is due (1972) on the basis of geologic and geochrono- to intracontinental spreading east of an east- logic studies at the southeastern end of the Las dipping subduction zone along the Cenozoic Vegas shear zone in the vicinity of Lake Mead. margin of western North America. Both have Finally, Hamilton (1971) has proposed a trans- suggested that more active east-west distension form model for the right-lateral strike-slip within the Basin and Range province occurred Agua Blanca fault of northern Baja, California, with the changeover documented by Atwater that is strikingly similar to the Garlock model (1970) from a compressive stress regimen of proposed here. Hamilton believes that proven subduction type to the present strike-slip tec- displacements along the fault of 5 to 10 km tonic regimen exemplif.ed by the San Andreas can be accounted for by extensional (normal) fault system. Scholz and others (1971) and faulting south of the Agua Blanca fault and Garfunkel (1966) have also concluded that the near and at its eastern end. Garlock fault may delineate the southern boundary of a more rapidly spreading portion CONCLUSION of the Basin and Range province than is found Many earlier workers have considered that to the south, an idea that agrees well with the the left-lateral Garlock fault is conjugate to geometry and genesis of the Garlock fault as the right-lateral San Andreas fault in a regional developed in this paper. strain pattern of north-south shortening and east-west extension, the latter expressed in part ACKNOWLEDGMENTS as an eastward displacement of the Mojave Discussions with Dcnald F. Palmer have block away from the junction of the San contributed to the development of ideas ex- Andreas and Garlock faults. In contrast, ,ve pressed in this paper. Earlier versions of this regard the origin of the Garlock fault as be ng manuscript were critically reviewed and sig- directly related to the extensional origin of the nificantly improved by Allen M. Bassett, John Basin and Range province in areas north of the C. Crowell, Warren Hamilton, Thomas L. Garlock. Atwater (1970) has proposed a moael Henyey, and Bennie W. Troxel, to whom our for the structural development of the Basin sincere appreciation is ex tended. This study is and Range and Continental Borderland prov- the outgrowth of regional tectonic studies in inces which requires (1) the existence of a broad the southern Cordillera supported by grants transform "zone" between the North America n "rom the National Science Foundation—GA- and Pacific plates and (2) right-lateral strik;- 21401 and GA-1562 (Davis), and GA-21375 slip motion across this zone, possibly for as and GA-1079 (Burchfiel). long as the past 30 m.y. According to this mocel the fault blocks of the two provinces owe the r REFERENCES CITED origin to oblique rifting within the transform Allen, C. R„ St. Amand, ?., Richter, C. F., and "zone." The trend of this zone is northwest Nordquist, J. M., 1965, Relationship between and the direction of oblique rifting (or seismicity and geologic structure in the south- "stretching") is considered by Atwater (1970, ern California region: Seismol. Soc. America Fig. 14) to be northwest-southeast, that is, Bull., v. 55, p. 753-79/ parallel to the San Andreas transform. Hamil- A.nderson, D. L., 1971, Tire San Andreas fault: ton and Myers (1966, Fig. 6) have similarly Sci. American, November, p. 52-67. favored a model of oblique rifting for the Basin Anderson, R. E., 1971, Thin-skin distension in and Range province which combines com- Tertiary rocks of southeastern Nevada: Geol. Soc. America Bull., v. {12, p. 43-58.

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