Garlock Fault Research Articles

Late Cenozoic structure and tectonics of the northern Mojave Desert

In the Fort Irwin region of the northern Mojave desert, late Cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age. Kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral‐thrust slip on northwest striking, moderately dipping segments at the east ends of the blocks. Crustal blocks ∼7–10 km wide by ∼50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block. Faulting initiated after ∼11 Ma, and Quaternary deposits are faulted and folded. We document a minimum of 13 km cumulative sinistral offset in a north‐south transect from south of the Bicycle Lake fault to north of the Drinkwater Lake fault. Paleomagnetic results from 50 sites reveal two direction groups in early and middle Miocene rocks. The north‐to‐northwest declinations of the first group are close to the middle Miocene reference pole. However, rock magnetic studies suggest that both primary and remagnetized directions are present in this group. The northeast declinations of the second group are interpreted as primary and 63.5° ± 7.6° clockwise from the reference pole and suggest net post middle Miocene clockwise rotation of several of the east trending blocks in the northeast Mojave domain. The Jurassic Independence Dike Swarm in Fort Irwin may be rotated 25–80° clockwise relative to the swarm north of the Garlock fault, thus supporting the inference of clockwise rotation. Using a simple‐shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks, we predict ∼23° clockwise rotation using the observed fault slip, or one‐third that inferred from the paleomagnetic results. The discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks, where most of our paleomagnetic sites are located. However, at least 25°–40° of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible “rigid‐body” rotation of the region evidenced by clockwise bending of northwest striking domain‐bounding faults. Our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the Eastern California Shear Zone over the last 10 m.y. occurred within the northeast Mojave Domain. The remainder must be accommodated in adjacent structural domains, e.g., east of the Avawatz Mountains and west of the Goldstone Lake fault. Supporting Appendices 1 and 2 are available on diskette or via Anonymous FTP from kosmos.agu.org, directory APEND (Username ‐‐ anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W, Washington, DC 20009 or by phone at 800‐966‐2481; $15.00. Payment must accompany order.

Simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern California

We determined a detailed three-dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern California using local and teleseismic data simultaneously. We used 131 372 P-wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the Caltech-US Geological Survey Southern California Seismic Network. Compared with previous local and teleseismic studies, the station coverage is considerably improved, and the number of data used is greatly increased in the present study. This, together with the local and teleseismic joint inversion approach, results in a unified three-dimensional (3-D) P-wave velocity model of the crust and mantle in southern California with a higher resolution than the previous models. The result obtained shows very heterogeneous structures in the crust and upper mantle in southern California. Shallow crustal structures correlate well with surface geological features. Sedimentary basins such as the Los Angeles Basin, Ventura Basin and Santa Maria Basin are imaged well as low velocities, and batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. The velocity in the crust is generally low in the Mojave Desert. The crustal velocities change abruptly across major faults such as the San Andreas, San Jacinto, Elsinore and Garlock faults. In the upper mantle, three major anomalies are found, beneath the Transverse Ranges, Salton Trough and Southern Sierra Nevada. The Transverse Ranges feature appears as a curtain-like, east-trending and north-dipping high-velocity anomaly with a thickness of approximately 50 km, and extends most deeply at its eastern end to about 250 km. The Salton Trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth. The Southern Sierra Nevada feature is a slab-like high-velocity anomaly which extends to about 250 km. The mantle velocity is generally low in the upper 100–200 km beneath the volcanic areas in southern California. The seismic velocity anomalies in the mantle beneath southern California are considered to result from small-scale mantle convection.

Kinematics of the Eastern California shear zone: Evidence for slip transfer from Owens and Saline Valley fault zones to Fish Lake Valley fault zone

Late Quaternary slip rates and satellite-based geodetic data for the western Great Basin constrain regional fault-slip distribution and evolution. The geologic slip rate on the Fish Lake Valley fault zone (the northwest extension of the Furnace Creek fault zone) increases northward from about 3 to 5 mm/yr, in agreement with modeled geodetic data. The increase coincides with the intersections of the Deep Springs fault, connected to the Owens Valley fault zone, and of other faults connected to the Saline Valley fault. The combined geologic and geodetic data suggest that (1) the northwest-striking faults of the Eastern California shear zone north of the Garlock fault are connected by north- to northeast-striking normal faults that transfer slip in a series of right steps, and (2) the amount and distribution of slip among the many faults of this broad, complex plate boundary have changed through time.

Significance of seismic reflections beneath a tilted exposure of deep continental crust, Tehachapi Mountains, California

The focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface, with the Tehachapi Mountains in California being the case in point. As a prime example of the lower crust, these mountains expose Cretaceous gneisses that formed 25–30 km down in the Sierra Nevada batholith and appear to be underlain by the ensimatic Rand schist. Integrated geophysical and geological studies by the CALCRUST program have produced a cross section through this post‐Mid‐Cretaceous structure and suggest a general model for its development. Seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the San Joaquin Basin's Tejon embayment. Two south dipping reverse faults on the rim of the Tejon embayment were discovered in the reflection data and verified in the field. The faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the Rand schist. The upper and lower crusts are separated by a zone of laterally discontinuous reflectors. Reflections from the lower crust form a wedge, the base of which is a nearly flat Moho at 33 km. Regional geological relations and gravity models both suggest that the reflective zone corresponds to the Rand schist and the newly recognized faults account for its Neogene exposure. Alternatively, the reflective zone maybe part of the gneiss complex, suggesting that the schist either lies deeper or is not present under the gneisses. If the Rand schist underlies the Tehachapi Mountains and Mojave region to their south, a model for their evolution can be constructed from regional geological relations. It seems that during Late Cretaceous Laramide subduction the protolith of the schist was thrust eastward beneath the Mojave. Along this portion of the Cordilleran batholithic belt the subduction was evidently at very low angles. The bottom of the batholith was removed and replaced by a thick section of schist, fluids from which weakened the overlying batholith. This thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat‐lying slabs. When emplacement of the schist ended in latest Cretaceous/earliest Paleocene, the underlying mantle rose, compensating for the extension and providing material for magmatic underplating. In the Neogene, transpression and rotation of the upper crust along the San Andreas and Garlock faults resulted in the exposure of the schist.

Jurassic plutonism and crustal evolution in the central Mojave Desert, California

Middle to Late Jurassic plutonic rocks in the central Mojave Desert represent the continuation of the Sierran arc south of the Garlock fault. Rock types range from calc-alkaline gabbro to quartz monzonite. Chemical and isotopic data indicate that petrologic diversity is attributable to mixing of crustal components with mantle melts. Evidence for magma mixing is scarce in most plutons, but emplacement and injection of plutons into preexisting wallrocks (e.g. pendants of metasedimentary rocks) suggests that assimilation may be locally important. Field and petrographic evidence and major and trace element data indicate that the gabbros do not represent pure liquids but are, at least partly, cumulates. The cumulate nature of the gabbros coupled with field evidence for open-system contamination means that trace element contents of gabbros cannot be used to fingerprint the Jurassic mantle source, nor can isotopic data be unequivocally interpreted to reflect the isotopic composition of the mantle. Correlation of Sr and Nd isotropic composition with bulk composition allows some constraints to be placed on the mantle isotopic signature. Gabbros and mafic inclusions from localities north of Barstow, CA have the most depleted mantle-like isotopic signatures (Sr(i)≈0.705 and ɛNd(t)=≈0 to +1). However, these rocks have likely seen some contamination as well, so the mantle source probably has an even more depleted character. Gabbros with the lowest Sr((i) and highest ɛNd(t) are also characterized by the highest 207Pb/204Pb and 206Pb/204Pb in the entire data set. This may be a feature of the mantle component in the Jurassic arc indicative of minor source contamination with subducted sediment as has been observed in modcrn continental arcs. Locally exposed Precambrian basement and metasedimentary rocks have appropriate Sr, Nd and Pb isotopic signatures for the crustal end members and are possible contaminants. Incorporation of these components through combined anatexis and assimilation can explain the observed spread in isotopic composition. Evidence for a depleted mantle component in these gabbros contrasts with the enriched subcontinental mantle component in Jurassic arc plutons further to the east and suggests there may have been a major mantle lithosphere boundary between the two areas as far back as the Late Jurassic. Crustal boundaries and isotopic provinces defined on the basis of initial isotopic composition (Sr(i)=0.706 isopleth) are difficult to delineate because of the correlation of bulk composition with Sr and Nd isotopic composition and because values may differ depending on the age of the rocks sampled within a given area. Data from plutons intruded into rocks known or inferred to be Precambrian are, however, shifted dramatically (highest Sr(i) and lowest ɛNd(t) toward Precambrian values. The least isotopically evolved rocks (lowest Sr(i) and highest ɛNd(t)) occur within the eugcoclinal belt of the Mojave Desert. This zone has been previously identified as a Precambrian rift zone but more likely represents a zone where mantle magmas have been intruded into isotopically similar crustal rocks of the eugeocline with minor input from old Precambrian crust.

Water table characterization for environmental hydrogeologic monitoring in fault zones: examples from California, potential applications in Chile

Potentially contaminating activities such as mining and waste disposal, which require environmental hydrogeologic monitoring, may be located in or above fault zones. Two case studies of ground water monitoring at waste disposal sites in California illustrate the effect such fault zones may have on water tables. A fault gouge area in the Garlock fault zone has a potentiometric surface (a conceptual surface plotted using water elevations in monitoring wells and springs) which intersects the ground surface. The planar Kern Bluff fault produces a stepped water tabel. It is suggested that due to geologic and other environmental similarities, studies of environmental impacts of projects along the Ataca fault zone and Pocuro fault zone in Chile may encounter similar hydrogeologic phenomena.

Aftershocks from the 28 June 1992 Landers earthquake: Northern Mojave desert to the Coso Volcanic Field, California

Abstract Following the 28 June 1992 Landers earthquake, a well-developed linear pattern of primary and triggered aftershocks occurred within the Eastern California Shear Zone from near the eastern end of the big bend in the San Andreas to northern Owens Valley. In this article we investigate primary aftershocks on the Calico-Blackwater and Harper fault zones, and triggered aftershocks in Indian Wells Valley (Little Lake fault zone) and the Coso Volcanic Field. These areas are directly northwest of the Landers rupture terminus and fall within the Southern Owens Valley Seismic Gap described by Hill et al . (1985). Focal mechanisms from selected events show dextral slip on northwest-trending faults similar to those that ruptured in the main Landers event. From the cumulative seismicity, we estimated the triggered seismicity rates from the Landers earthquake on the Little Lake fault zone (230 events in the first 10 days to 2.5 events/day during the following 5 months) and the Coso Volcanic Field (250 events in the first 10 days to 3.6 events/day during the following 5 months). Triggered seismicity in the Coso Volcanic Field may have been caused by advective gas overpressure. The continuity of aftershock data agree with a significant shear zone passing through the northern Mojave desert into the southern Owens Valley by way of the Harper fault, Little Lake fault zone, and Coso Volcanic Field. Geologic data (surface faults) also support structural continuity within the zone, with the exception of the Garlock fault.

Space geodetic measurement of crustal deformation in central and southern California, 1984–1992

We estimate the velocity field in central and southern California using Global Positioning System (GPS) observations from 1986 to 1992 and very long baseline interferometry (VLBI) observations from 1984 to 1991. Our core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north‐south, and a semimajor axis of approximately 3 mm/yr oriented east‐west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position (“repeatability”), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of change in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 ± 1, 5 ± 1, 5 ± 1, and 2 ± 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnetic declinations. Our estimates indicate that the “San Andreas discrepancy” can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1–2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands and the mainland.

Inversion of seismic reflection traveltimes using a nonlinear optimization scheme

We present the use of a nonlinear optimization scheme called generalized simulated annealing to invert seismic reflection times for velocities, reflector depths, and lengths. A finite‐difference solution of the eikonal equation computes reflection traveltimes through the velocity model and avoids ray tracing. We test the optimization scheme on synthetic models and compare it with results from a linearized inversion. The synthetic tests illustrate that, unlike linear inversion schemes, the results obtained by the optimization scheme are independent of the initial model. The annealing method has the ability to produce a suite of models that satisfy the data equally well. We make use of this property to determine the uncertainties associated with the model parameters obtained. Synthetic examples demonstrate that allowing the reflector length to vary, along with its position, helps the optimization process obtain a better solution. We put this to use in imaging the Garlock fault, whose geometry at depth is poorly known. We use reflection times picked from shot gathers recorded along COCORP Mojave Line 5 to invert for the Garlock fault and velocities within the Cantil Basin below Fremont Valley, California. The velocities within the basin obtained by our optimization scheme are consistent with earlier studies, though our results suggest that the basin might extend 1–2 km further south. The reconstructed reflector seems to suggest shallowing of the dip of the Garlock fault at depth.

Holocene slip rate of the Central Garlock Fault in southeastern Searles Valley, California

A Late Pleistocene shoreline at the overflow level of Searles Lake has been displaced 82 to 106 m (preferred value is 90 m) in a left‐lateral sense and 2.5 m (net) north side up along the Garlock fault, at the southeastern corner of Searles Valley. Previously published radiocarbon dates from both surface and subsurface strata indicate that the most recent highstand of Searles Lake ended sometime between 10,000 and 13,800 14C years ago. The maximum slip rate of the Garlock fault in southeastern Searles Valley is thus 11 mm/14C yr. If part of the offset of the shoreline occurred during older lakestands, then the slip rate may be somewhat less. A channel incised after the most recent highstand, however, is offset about 68 m, indicating that the minimum slip rate is 5 mm/14C yr. Subjective evaluation of the constraints on the offset and on the age of the shoreline yields a preferred rate of 6–8 mm/14C yr at this site. Assuming Bard et al.'s (1990) recent calibration of the radiocarbon time scale, the calibrated slip rate of the Garlock fault is between 4 and 9 mm/yr with a preferred value of 5–7 mm/yr. This estimate is similar to a previous estimate of the Holocene slip rate and is slightly less than an estimate derived from modelling of geodetic data. Extension north of the Garlock fault in Indian Wells and Searles valleys contributes no more than 3 mm/yr left slip to the Garlock fault.

Geometry of the Garlock fault zone based on seismic relection data

Detailed examination of shot gathers from line 3 of the Consortium for Continental Reflection Profiling Mojave experiment indicates a low‐velocity, high‐amplitude seismic event that may originate from a south dipping portion of the Garlock fault zone. Part of this event was previously interpreted as a subhorizontal surface located in the midcrust. Our analysis indicates, however, that the distance to this event varies consistently with the distance of the shot and receivers from the fault zone and that the velocities are too low for seismic waves traveling through predominantly crystalline rocks. These characteristics are more consistent with the presence of a subvertical reflector located near the Garlock fault zone than with the previously inferred subhorizontal reflector beneath the seismic line.

Aspects of the crustal structure of the western Mojave Desert, California, from seismic reflection and gravity data

Seismic and gravity data taken along line 1 of the 1982 Consortium for Continental Reflection Profiling (COCORP) Mojave Desert Survey (N‐S profile, ∼30 km long) have been used to characterize the upper crust north of the San Andreas fault in the western Mojave block of southern California. Consortium for Continental Reflection Profiling seismic reflection data were reprocessed to emphasize the upper 5 seconds (two‐way travel time). The resultant common depth point (CDP) sections provided starting models for generating a refined geologic cross‐section using a combination of ray tracing (forward modeling) and gravity interpretation. The forward modeling was used to validate the existence of faults and constrain their dips. The gravity data were used to refine the overall model, particularly in poor data areas on the CDP sections. Gravity data, taken along three nearby profiles parallel to primary line of section, were also used to determine the structural trend. Results from the first two seconds indicate the presence of a series of ENE striking reverse faults beneath the late Tertiary and Quaternary sedimentary cover of the western Mojave. The faults dip northward and offset the sediment‐basement interface. The largest such feature has an apparent throw of ∼1.8 km and exhibits a subtle scarp at the Earth's surface suggesting Holocene displacement. The orientation of these faults, although not an instantaneous representation of the present‐day stress field, is consistent with NNW compression across the western Mojave block and WNW striking San Andreas fault, as determined from nearby focal mechanisms and in situ stress measurements. The faults also appear to be closing small sedimentary basins in the Mojave block, which may have formed during an earlier extensional phase, similar to what is happening on a much larger scale in the Los Angeles basin to the south of the San Andreas fault. Reflections between 2 and 5 s, coupled with the local geology and gravity modeling, are consistent with the presence of the Pelona/Rand schist in the subsurface beneath the western Mojave. The upper surface of the schist (i.e., Vincent/Rand thrust equivalent) rises southward toward the San Andreas fault where it is displaced vertically (up to the south) at least 5 km along the E‐W trending Hitchbrook fault, such that the schist crops out between the Hitchbrook and subparallel San Andreas to the south. The same structure may exist beneath the Tehachapi mountains, with the roles of the Hitchbrook and San Andreas faults played by the north and south branches of the Garlock fault, respectively. The rising or arching of the basement toward the San Andreas fault (and toward the Garlock) is not only reflected in the geology and topography local to these faults in many places but is also generally observed on seismic reflection profiles in the vicinity of these faults in the western Mojave. Furthermore, the arching is also consistent with a strong component of fault normal compression.

Nearfield interseismic vertical strain on the Central Garlock Fault, southern California, 1984–1991

Repeated precise leveling of fault‐crossing arrays at two sites on the central and western parts of the Garlock fault discloses minor height changes of a few centimeters across the fault in seven years. During that time, the seismicity of the fault has been very low relative to other parts of California. Although other investigators have measured a millimeter or two of horizontal displacement per year near each of the leveling arrays, the vertical changes measured in this study probably result from nontectonic differential subsidence across the fault due to the effects of the 1985–1991 drought in southern California. The vertical displacement at Cameron could turn out to be tectonic, but at the Koehn Lake site the subsidence may be related to nearby pumping of groundwater. The changes and rates of heights reported here should not be extrapolated in time or space, because of the relatively short period of observations, and because these sites measure only minuscule parts of the 250 km‐long fault.

Surficial offsets on the Central and Eastern Garlock Fault associated with prehistoric earthquakes

Geomorphic features offset along the central and eastern Garlock fault record the amount of surface slip associated with prehistoric earthquakes. Along the easternmost 90 km of the fault, the smallest offsets cluster around 2–3 m of left‐lateral slip, apparently associated with the most recent earthquake on this portion of the fault. Larger offsets along this part of the fault, especially in Pilot Knob Valley, cluster around values consistent with 2–4 m of slip in each of the past several events. Farther west, south of El Paso Mountains, offset geomorphic features suggest that each of the past two earthquakes on this stretch of the Garlock fault was produced by about 7 m of slip, whereas the third event back was produced by about 4 m of slip. Vertical displacements of geomorphic features range from 0% to 30% of the left‐lateral offsets. Within Pilot Knob Valley (along the southern side of the Slate Range), vertical displacements are consistently up on the northern side, whereas within the Avawatz Mountains both north‐ and south‐side‐up vertical displacements are present. On the basis of the geomorphic offsets, the geometry of the Garlock fault, and the precedents set by historical strike‐slip earthquakes elsewhere, a number of different rupture patterns are plausible. These range from rupture of the entire Garlock fault in a single event with a maximum magnitude of about Mw=7.8, to separate rupture of the western segment and of the central and eastern segments combined, with approximate magnitudes Mw≤1.1 and Mw=7.5, respectively, to separate rupture of even shorter segments, producing earthquakes of magnitudes Mw=6.6 to Mw=7.5. In conjunction with available slip rates for the Garlock fault, the geomorphic offsets suggest that average recurrence intervals are probably within the range of 600–1200 years south of El Paso Mountains, about 200–750 years in Searles Valley, about 200–1300 years in Pilot Knob Valley, and about 200–3000 years near Leach Lake and in the Avawatz Mountains.

The Shumake volcanic dome-hosted epithermal, precious metal deposit, western Mojave Desert, California

Several Tertiary volcanic-hosted precious metal deposits occur in the Mojave block in southern California which is bounded on the north by the Garlock fault and on the southwest by the San Andreas fault. The Middle Buttes volcanic dome complex hosts several low-grade gold and silver orebodies and is comprised of Miocene-age coalescing lava domes, flows, vent breccias, and pyroclastic deposits lying unconformably on Cretaceous basement of the Sierra Nevada batholith. The volcanic rocks range in composition from andesite through rhyolite and are cut by numerous faults.The Shumake deposit, over 5 million tons of ore with an average grade of 0.043 oz per ton (1.5 ppm) Au and 0.4 oz per ton (14 ppm) Ag, is centered around two large quartz veins and the intervening altered rhyolites. The large veins are characterized by pyrite, arsenopyrite, and silver sulfosalts, with minor base metal sulfides and gold. Gold in the altered rhyolite occurs with stockwork quartz veinlets and an oxidation assemblage of iron oxides, scorodite, and red kaolinite. Alteration of the rhyolites is best characterized by sericite, composed of fine-grained illite and muscovite, and stockwork quartz veinlets with associated adularia. Fluid inclusions indicate that the stockwork quartz formed around 210 degrees C. The quartz, sericite, and minor adularia assemblage is underlain by propylitic alteration and overlain in one part of the deposit by alunite and white kaolinite alteration. These main-stage alterations were followed by an oxidation assemblage which is interpreted to be related to collapse of the hydrothermal system. The deposit is dominated by anomalous abundances of Au, Ag, Hg, As, and Sb. Spatial geochemical variations suggest that the two massive quartz veins were dominant components of the hydrothermal plumbing system.The paleohydrologic system at the Shumake deposit is analogous to the adularia-sericite type of volcanic-hosted epithermal precious metal deposits. Faults and fractures provided the pathways for hydrothermal fluid movement. Vein textures suggest episodic mineralization that can be explained by seismic pumping of hydrothermal fluids. Since a close temporal relationship exists between volcanism and ore deposition, magmas probably provided heat to the hydrothermal system. Thus, for the Shumake deposit, faulting provided both the pathways and driving mechanism for the hydrothermal system while heat was provided by magmas.

Subsurface imaging of the Garlock Fault, Cantil Valley, California

Imaging of the Garlock fault from seismic reflection data tests tectonic models for the Mojave Desert region of southern California. Models developed from geologic and geodetic evidence of fault movement rates disagree on whether the Garlock fault should dip north or south. Such models are further at odds with focal mechanism and dislocation analyses consistent with a vertical strike‐slip fault. Our analysis demonstrates a nonvertical, southward dip for the Garlock fault. Consortium for Continental Reflection Profiling Mojave line 5 collected seismic reflection data across the Garlock in Cantil Valley, where the 3‐km‐deep Cantil pull‐apart basin has developed between the southwest and east branches of the fault. We analyze the reflection data for evidence of the fault's structure to 5 km depths. Garlock fault plane reflections on unprocessed shot gathers merge with the first arrival as the source progresses to the surface trace of the east branch, explicitly tracing the Garlock fault as a seismic reflector from depth to the mapped contemporary fault trace. The apparent velocity of these reflections gives a 37°±10° south dip for the east branch of the fault. A prestack Kirchhoff sum migration images the east branch reflector with a 45° south dip. Our migration process fully accounts for the bending of seismic rays through the strong lateral velocity variations in Cantil basin. The south dip of the Garlock's east branch together with basement steps that decrease to the south suggest that Cantil basin developed from a 0.4–1 mm/yr component of extension normal to the fault. Development of the basin by detachment provides a mechanism to widen it as left‐lateral motion lengthens it, allowing the pull‐apart to maintain the globally constant length‐to‐width ratio of 3. Detachment by the Garlock at Cantil Valley similarly requires 0.4–1 mm/yr of dextral strike slip on a northwest striking fault such as the Helendale that cuts the Mojave between the Garlock and Pinto Mountain faults. Such motion is consistent with regional tectonic models.

Anomalous folds associated with the east-central part of the Garlock fault, southeast California

Research Article| May 01, 1991 Anomalous folds associated with the east-central part of the Garlock fault, southeast California GEORGE I. SMITH GEORGE I. SMITH 1U.S. Geological Survey, M.S. 902, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information GEORGE I. SMITH 1U.S. Geological Survey, M.S. 902, 345 Middlefield Road, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (5): 615–624. https://doi.org/10.1130/0016-7606(1991)103<0615:AFAWTE>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GEORGE I. SMITH; Anomalous folds associated with the east-central part of the Garlock fault, southeast California. GSA Bulletin 1991;; 103 (5): 615–624. doi: https://doi.org/10.1130/0016-7606(1991)103<0615:AFAWTE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The east-central part of the left-lateral Garlock fault, in southeast California, is associated with three broad folds that trend and plunge northeast. The folds, which lie north, south, and astride the fault, postdate deposition of alluvial gravels that at one time formed a continuous northwest-sloping fan; the gravels rest conformably on lake beds that contain the Lava Creek B ash bed (0.62 Ma). The anticline, named after Christmas Canyon, has vertical relief of as much as 100 m. Topographic profiles suggest that the gravels exposed along the synclinal troughs to its southeast (Pilot Knob Valley syncline) and northwest (Teagle Wash syncline) lie at about the same elevations as they did when part of the originally active fan, implying that the synclines are passive results of the anticline development. If so, the original slope of the middle Pleistocene alluvial sediment sheet was about 0.6° northwest. The age and dimensional controls allow calculation of the rates of tilting and horizontal shortening. Corrected tilting rates vary from about 2.5° to 4.7°/m.y.; the total shortening of about 7.5 m developed at a minimum rate of 12.5 m/m.y.The orientations of these folds imply northwest-southeast compression, which is anomalous when viewed in the context of regional tectonics. The left-lateral Garlock fault and the north-northwest-trending Searles Valley syncline and Slate Range anticline imply northeast-southwest compression, which geophysical criteria confirm. Geologic evidence of northwest-southeast compressional deformation, dating to pre-Miocene (Paleocene?) time, however, is known from several other localities along the Garlock fault. These anomalous features are interpreted as evidence that episodes of north-west-southeast compression along the Garlock fault do occur, and although oriented about 90° from the average of conventional—and presently observed—compression directions, they have occurred during much (most?) of the Cenozoic Era, including the past 0.6 m.y. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Cenozoic strike‐slip faulting in the Mojave Desert, California

Recent tectonic models for southern California treat the entire Mojave Desert Block as the site of distributed simple shear during late Cenozoic time. These models consider that much of the region is composed of a series of narrow blocks, bounded by active NW striking, right‐slip faults that have facilitated the distortion and rotation of the region about vertical axes during translations. As much as 100 km of cumulative right slip is predicted for these faults by some of these models. These kinematic models require that the faults of the Mojave Desert Block merge with the Garlock fault, which is viewed as the intact northern boundary that served to accommodate the distortion of the Mojave Desert Block by simple shear. Map‐scale structural relations are used to test explicit and implicit features of kinematic models proposed for the region. These relationships indicate that late Cenozoic NW striking, right‐slip faults of the Mojave Desert Block possess the following characteristics: (1) the faults are discontinuous, with only the Calico‐Blackwater fault spanning the entire Mojave Desert; (2) the faults terminate before reaching the Garlock fault; (3) faults south of an irregular line extending from near Barstow eastward to Ludlow and to Soda Lake are continuous and well developed and have a cumulative net slip of &gt;40 km, whereas faults to the north are discontinuous and display &lt;12 km of right slip; and (4) there is a northwestward decrease in net slip along most of the faults. A new kinematic model is proposed to reconcile these new observations with existing data. We assert that integrated strain within the province since middle Miocene time is not regionally homogeneous as predicted by simple shear models but is instead partitioned into six major domains. The domains probably have deformed and rotated about vertical axes independently of each other and are separated by zones of shortening or extension or by strike‐slip faults. Strike‐slip faults and folding have likely accommodated internal deformation and rotation of some of the domains. The model predicts that the Mojave Desert has been the site of ∼65 km of right shear since middle Miocene time. The broad network of faults of the Mojave Desert Block along with similar strike‐slip faults of the Death Valley region constitute a regional zone of right shear, named here, the Eastern California shear zone. Because of its probable physical connection to the San Andreas fault system, the Eastern California shear zone may have accommodated a significant portion of Pacific‐North American transform motion. The Eastern California shear zone accounts for 9–14% of the total shear, predicted from plate tectonic reconstructions, along the Pacific‐North American transform boundary since ∼10.6 Ma. The kinematic connection of the normal faults of the Death Valley region, with the San Andreas fault system via the faults of the Mojave Desert accords with the deduction of Atwater (1970) that late Cenozoic extension in portions of the Basin and Range province is related to Pacific‐North American transform shear. Finally, the present arcuate trace of the Garlock fault is ascribed to oroclinal folding within the broad zone of distributed shear of the Eastern California shear zone.

Crustal strain near the Big Bend of the San Andreas Fault: Analysis of the Los Padres‐Tehachapi Trilateration Networks, California

In the region of the Los Padres‐Tehachapi geodetic network, the San Andreas fault (SAF) changes its orientation by over 30° from N40°W, close to that predicted by plate motion for a transform boundary, to N73°W. The strain orientation near the SAF is consistent with right‐lateral shear along the fault, with maximum shear rate of 0.38±0.01 μrad/yr at N63°W. In contrast, away from the SAF the strain orientations on both sides of the fault are consistent with the plate motion direction, with maximum shear rate of 0.19±0.01 μrad/yr at N44°W. The strain rate does not drop off rapidly away from the fault, and thus the area is fit by either a broad shear zone below the SAF or a single fault with a relatively deep locking depth. The fit to the line length data is poor for locking depth d less than 25 km. For d of 25 km a buried slip rate of 30 ± 6 mm/yr is estimated. We also estimated buried slip for models that included the Garlock and Big Pine faults, in addition to the SAF. Slip rates on other faults are poorly constrained by the Los Padres‐Tehachapi network. The best fitting Garlock fault model had computed left‐lateral slip rate of 11±2 mm/yr below 10 km. Buried left‐lateral slip of 15±6 mm/yr on the Big Pine fault, within the Western Transverse Ranges, provides significant reduction in line length residuals; however, deformation there may be more complicated than a single vertical fault. A subhorizontal detachment on the southern side of the SAF cannot be well constrained by these data. We investigated the location of the SAF and found that a vertical fault below the surface trace fits the data much better than either a dipping fault or a fault zone located south of the surface trace.

Thematic mapper and field investigations at the intersection of the Death Valley and Garlock fault zones, California

Analysis of processed images and detailed field investigations have provided significant information concerning the late-Pliocene and Quaternary evolution of the intersection of the Garlock and Death Valley fault zones. The imagery was used to determine patterns of sedimentation and age relationships on alluvial fans and to determine the geometry, styles of deformation, and relative ages of movements on major and minor faults in the study area. The field investigation often confirmed the inferences drawn from the images and provided additional tectonic and geomorphologic data about the Quaternary deformation of the region. All the data gathered in the course of this project support the contention that the Garlock fault zone terminates in the Avawatz Mountains and that the Death Valley fault zone continues south of the intersection for at least 50 km, forming the eastern boundary of the Mojave province.