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Geology topics

John W. Hillhouse

Publications and source records attributed to John W. Hillhouse.

At least 19 recordsLinked to original sources
USGS2017

Fission-track age (400,000 yr) of the Rockland tephra, based on inclusion of zirco grains lacking fossil fission tracks

A zircon fission-track age of about 400,000 yr B.P. has been determined for the Rockland tephra, a widespread pyroclastic layer in northern California and western Nevada. New ages of zircon separates from both proximal and distal exposures of this layer range from 370,000 to 460,000 yr; ages of the best material provide a narrower range, from 370,000 yr for unwelded ash-flow tuff to 420,000 yr for distal air-fall ash that appears to be uncontaminated by clastic detritus or xenocrysts. Detrital or xenocrystic grains in the ash-flow tuff may have been annealed during emplacement and cooling of the tuff. Detrital and xenocrystic zircons are identified on the basis of their physical characteristics and distinctly older ages. Independent stratigraphic and magnetostratigraphic data constrain the age of the Rockland tephra between 300,000 and 600,000 yr, a range that is compatible with the fission-track age. Zircon grains containing no spontaneous (fossil) tracks are regarded as part of the normal population of comagmatic grains because maximum ages calculated for these grains form a population that mimics the distribution of ages of individual zircon grains that contain fossil tracks; modal ages of both groups fall between 250,000 and 500,000 yr. Induced fission tracks from grains that lack fossil tracks are included in the age calculations, resulting in significantly younger and more coherent dates than would result if these tracks had been omitted, especially those of the finer-grained distal samples. ?? 1991.

Quaternary Research↗
USGS2013

Analysis of Neogene deformation between Beaver, Utah and Barstow, California: Suggestions for altering the extensional paradigm

For more than two decades, the paradigm of large-magnitude (~250 km), northwest-directed (~N70°W) Neogene extensional lengthening between the Colorado Plateau and Sierra Nevada at the approximate latitude of Las Vegas has remained largely unchallenged, as has the notion that the strain integrates with coeval strains in adjacent regions and with plate-boundary strain. The paradigm depends on poorly constrained interconnectedness of extreme-case lengthening estimated at scattered localities within the region. Here we evaluate the soundness of the inferred strain interconnectedness over an area reaching 600 km southwest from Beaver, Utah, to Barstow, California, and conclude that lengthening is overestimated in most areas and, even if the estimates are valid, lengthening is not interconnected in a way that allows for published versions of province-wide summations. We summarize Neogene strike slip in 13 areas distributed from central Utah to Lake Mead. In general, left-sense shear and associated structures define a broad zone of translation approximately parallel to the eastern boundary of the Basin and Range against the Colorado Plateau, a zone we refer to as the Hingeline shear zone. Areas of steep-axis rotation (ranging to 2500 km 2 ) record N-S shortening rather than unevenly distributed lengthening. In most cases, the rotational shortening and extension-parallel folds and thrusts are coupled to, or absorb, strike slip, thus providing valuable insight into how the discontinuous strike-slip faults are simply parts of a broad zone of continuous strain. The discontinuous nature of strike slip and the complex mixture of extensional, contractional, and steep-axis rotational structures in the Hingeline shear zone are similar to those in the Walker Lane belt in the west part of the Basin and Range, and, together, the two record southward displacement of the central and northern Basin and Range relative to the adjacent Colorado Plateau. Understanding this province-scale coupling is critical to understanding major NS shortening and westerly tectonic escape in the Lake Mead area. One north-elongate uplift in the Hingeline shear zone is a positive flower structure along a strike-slip fault, and we postulate that most other large uplifts are diapiric, resulting from extension-normal inflow of ductile substrate, rather than second-order isostatic responses to tectonic unloading. We also postulate that large steep-axis rotations, and some small ones as well, result from basal tractions imparted by gradients in southerly directed subjacent ductile flow rather than by shear coupling imparted by laterally variable elongation strains. The shortening strain recorded in the rotations and related structures probably matches or exceeds the magnitude of lengthening, even for the Lake Mead area where we do not question local large (~65 km) west-directed lengthening. We assess the results of extensive recent earth-science research in the Lake Mead area and conclude that previously published models of N-S convergence, westerly tectonic rafting, and N-S occlusion are valid and record unique tectonic escape accommodation for south-directed displacement of the Great Basin sector of the Basin and Range. Genetic ties between the south-directed displacement and plate-interaction forces are elusive, and we suggest the displacement results from body forces inherent in the Basin and Range.

Arizona, California, Nevada, Utah↗
USGS2012

Magnetic fabric, flow directions, and source area of the Lower Miocene Peach Springs Tuff in Arizona, California, and Nevada

We have used anisotropy of magnetic susceptibility (AMS) to define the flow fabric and possible source area of the Peach Springs Tuff, a widespread rhyolitic ash flow tuff in the Mojave Desert and Great Basin of California, Arizona, and Nevada. The tuff is an important stratigraphic marker from the Colorado Plateau to Barstow, California, a distance of 350 km; however, the location of its source caldera is unknown. Dated at 18.5 Ma by 40 Ar/ 39 Ar, the tuff erupted during the early stages of Miocene extension along the lower Colorado River. The thicker accumulations (>100 m) occur at Kingman, Arizona, and in the Piute Mountains, California, on opposite sides of the Colorado River extensional corridor. Our AMS studies produced well-defined magnetic lineations in 30 of 42 sites distributed throughout the tuff. Typical ratios of the principal AMS axes are 1.01 for the magnetic lineation ( k max / k int ) and 1.02 for the foliation ( k int / k min ); the bulk magnetic susceptibility of the Peach Springs Tuff averages 2.0×10 −3 in the SI unit system. The subhorizontal lineations, which presumably parallel the flow directions, form a pattern radiating outward from the approximate center of the outcrop area. Magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff. The lineation and imbrication indicate a source region near the southern tip of Nevada. Defining the best intersection of the AMS lineations required restoration of major extension, strike-slip faulting, and associated tectonic rotation in the disrupted tuff. The optimum intersection of magnetic lineations lies in the southern Black Mountains of Arizona on the eastern side of the Colorado River extensional corridor. No caldera structures are known from that area, but the area contains thick sections of the Peach Springs Tuff above a silicic volcanic center. The caldera may be buried under younger deposits in the Mohave Valley of Arizona. Tertiary granite in the Newberry Mountains may represent a deeper level of the Peach Springs Tuff vent that has been exhumed by detachment faulting.

Journal of Geophysical Research↗
USGS2012

Early Cretaceous paleolatitude of the Yukon-Koyukuk province, Alaska

We report results from a paleomagnetic study of the Yukon-Koyukuk province, a key region for reconstructing the Cretaceous paleogeography of northern Alaska and northeastern Asia. The province lies between the displaced continental fragment of Arctic Alaska and the accreted terranes of southern Alaska. Although Lower Cretaceous volcanogenic rocks of the Yukon-Koyukuk province preserve primary remanent magnetizations, the primary component is masked by a pervasive overprint. The over-printing magnetization, which has dual polarity, was acquired after a Late Cretaceous orogeny left the region highly deformed. By fitting planes to tilt-corrected thermal demagnetization data and solving for the best intersection of the corresponding remagnetization circles, we separated the primary component, presumably a vestige of the original thermoremanent magnetization, from the overprinting component of the natural remanence. The primary magnetization corresponds to a mean paleopole of Early Cretaceous age at 60.2°N, 159°E. The mean paleolatitude is 69°N with the 95% confidence ellipse ranging from 60° to 75°N. This analysis was successful because the tilt corrections of the sampled strata had sufficient variation to ensure a strong intersection of the remagnetization circles. We infer that Lower Cretaceous rocks of the Brooks Range and the Yukon-Koyukuk province were once separated by a wide ocean basin that was closed by subduction, probably during the Cretaceous. The northern limb of the Angayucham terrane, which lies between the Brooks Range and the Yukon-Koyukuk province, may contain remnants of the now-closed ocean basin.

Journal of Geophysical Research Solid Earth↗
USGS2011

Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40 Ar/ 39 Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies. Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek–Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ∼4.83 Ma. We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

California, Nevada↗
USGS2010

Clockwise rotation and implications for northward drift of the western Transverse Ranges from paleomagnetism of the Piuma Member, Sespe Formation, near Malibu, California

New paleomagnetic results from mid-Tertiary sedimentary beds in the Santa Monica Mountains reinforce the evidence for large-scale rotation of the western Transverse Ranges, and anisotropy measurements indicate that compaction-induced inclination flattening may resolve a long-standing controversy regarding the original paleolatitude of the rotated block. Previously published paleomagnetic data indicate that post-Oligocene rotation amounts to 70°–110° clockwise, affecting the Channel Islands, Santa Monica Mountains, and Santa Ynez Mountains. The Sespe Formation near Malibu consists of a lower member dominated by nonmarine sandstone and conglomerate and an upper section, the Piuma Member, which consists of gray-red sandstone and mudstone interbedded with minor tuff and limestone beds. The Piuma Member has a paleomagnetic pole at 36.6°N, 326.7°E (A 95min = 5.0°, A 95max = 9.6°), obtained by thermal demagnetization of 34 oriented cores from Oligocene and early Miocene beds. After correcting for plunge of the geologic structure, the data are consistent with significant clockwise rotation (77° ± 7°) of the region relative to stable North America. Rotation of the western Transverse Ranges is generally viewed as a consequence of Pacific–North American plate interactions after 28 Ma, when east–west subduction gave way to northwest transform motion in southern California. Inclinations from the Piuma study indicate a paleolatitude anomaly of 11° ± 7° and are consistent with a mean northward drift that exceeds generally accepted San Andreas fault displacement by a factor of 3. However, sedimentary inclination error may accentuate the anomaly. Anisotropy of isothermal remanent magnetization indicates inclination flattening of approximately 8°, and correction for the effect reduces the paleolatitude anomaly to 5.3° ± 5.8°. Compaction may explain the inclination flattening in these sedimentary rocks, but the process does not adequately explain lower-than-expected inclinations found in previous studies of Miocene volcanic rocks of the western Transverse Ranges.

Geochemistry, Geophysics, Geosystems↗
USGS2010

Reconnaissance geochronology of tuffs in the Miocene Barstow Formation: implications for basin evolution and tectonics in the central Mojave Desert

Early to middle Miocene lacustrine strata of the Barstow Formation are well dated in just a few places, limiting our ability to infer basin evolution and regional tectonics. At the type section in the Mud Hills, previous studies have shown that the lacustrine interval of the Barstow Formation is between ~16.3 Ma and ~13.4 Ma. Elsewhere, lake beds of the Barstow Formation have yielded vertebrate fossils showing the Hemingfordian/Bartovian transition at ~16 Ma but are otherwise poorly dated. In an attempt to clarify the age and depositional environments of the lake deposits, we are mapping the Barstow Formation and dating zircons from interbedded tuffs, as well as testing ash-flow tuffs for the distinctive remanent magnetization direction of the widespread Peach Spring Tuff. Thus far, our new U-Pb zircon ages inficate that the Barstow lake beds contain tuff beds as old as 19.1 Ma and as young as 15.3 Ma. At Harvard Hill, Barstow lake beds contain a thick tuff dated at 18.7 Ma. On the basis of zircon ages, mineralogy, zircon chemistry, and paleomagnetic results, we consider the thick tuff to be a lacustrine facies of the Peach Spring Tuff. We have identified the Peach Spring Tuff by similar methods at eight localities over a broad area, providing a timeline for several fluvial and lacustrine sections. The new dates indicate that long-lived lacustrine systems originated before 19 Ma and persisted to at least 15 Ma. The onset of lacustrine conditions predates the Peach Spring Tuff in most Barstow Formation sections and may be older than 19.5 Ma in some places. The new data indicate that the central Mojave Desert contained narrow to broad lake basins during and after extension, and that Barstow lacustrine deposits did not exclusively postdate extensional tectonics. At present, it is unclear whether several separate, small lake basins coexisted during the early to middle Miocene, or if instead several small early Miocene basins gradually coalesced over about 6 millions years to form one or two large middle Miocene lake basins.

Mojave Desert↗
USGS2010

Correlation of the Miocene Peach Spring Tuff with the geomagnetic polarity time scale and new constraints on tectonic rotations in the Mojave Desert, California

We report new paleomagnetic results and 40 Ar/ 39 Ar ages from the Peach Spring Tuff (PST), a key marker bed that occurs in the desert region between Barstow, California, and Peach Springs, Arizona. The 40 Ar/ 39 Ar ages were determined using individual hand-picked sanidine crystals from ash-flow specimens used in previous paleomagnetic studies at eight sites correlated by mineralogy, stratigraphic position, and magnetic inclination. Site-mean ages, which range from 18.43 Ma to 18.78 Ma with analytical precision (1 s.d.) typically 0.04 Ma, were obtained from areas near Fort Rock, AZ; McCullough Mts, NV; Cima Dome, Parker Dam, Danby, Ludlow, Kane Walsh, and Stoddard Wash, CA. The regional mean age determination is 18.71 ± 0.13 Ma, after the data were selected for sanidine crystals that yielded greater than 90% radiogenic argon (N=40). This age determination is compatible with previous 40 Ar/ 39 Ar dating of the PST after taking various neutron-flux monitor calibrations into account. We report paleomagnetic results from eight new sites that bear on reconstructions of the Miocene basins associated with the Hector Formation, Barstow Formation, and similar fine-grained sedimentary deposits in the Barstow region. Key findings of the new paleomagnetic study pertain to age control of the Hector Formation and clockwise rotation of the Northeast Mojave Domain. Our study of a rhyolitic ash flow at Baxter Wash, northern Cady Mountains, confirms the correlation of the PST within the Hector Formation and prompts reinterpretation of the previously determined magnetostratigraphy. Our model correlates the PST to the normal-polarity zone just below the C6-C5E boundary (18.748 Ma) of the astronomically tuned Geomagnetic Polarity Time Scale. After emplacement of the Peach Spring Tuff at Alvord Mountain and the Cady Mountains, the southern part of the Northeast Mojave Domain (between Cady and Coyote Lake faults) underwent clockwise rotation of 30°–55°. Clockwise rotations increase with distance northward from the Cady fault and may reflect Late Miocene and younger accommodation of right-lateral motion across the Eastern California Shear Zone. The new results also expand the area known to be affected by the Peach Springs eruption, and confirm that a pink ash-flow tuff surrounding Daggett Ridge near Barstow is part of the PST.

Arizona, California, Nevada↗
USGS2010

Reconnaissance geochronology of tuffs in the Miocene Barstow Formation: Implications for basin evolution and tectonics in the central Mojave Desert

Early to middle Miocene lacustrine strata of the Barstow Formation are well dated in just a few places, limiting our ability to infer basin evolution and regional tectonics. At the type section in the Mud Hills, previous studies have shown that the lacustrine interval of the Barstow Formation is between ~16.3 Ma and ~13.4 Ma. Elsewhere, lake beds of the Barstow Formation have yielded vertebrate fossils showing the Hemingfordian/Barstovian transition at ~16 Ma but are otherwise poorly dated. In an attempt to clarify the age and depositional environments of the lake deposits, we are mapping the Barstow Formation and dating zircons from interbedded tuffs, as well as testing ash-flow tuffs for the distinctive remanent magnetization direction of the widespread Peach Spring Tuff. Thus far, our new U-Pb zircon ages indicate that the Barstow lake beds contain tuff beds as old as 19.1 Ma and as young as 15.3 Ma. At Harvard Hill, Barstow lake beds contain a thick tuff dated at 18.7 Ma. On the basis of zircon ages, mineralogy, zircon chemistry, and paleomagnetic results, we consider the thick tuff to be a lacustrine facies of the Peach Spring Tuff. We have identified the Peach Spring Tuff by similar methods at eight localities over a broad area, providing a timeline for several fluvial and lacustrine sections. The new dates indicate that long-lived lacustrine systems originated before 19 Ma and persisted to at least 15 Ma. The onset of lacustrine conditions predates the Peach Spring Tuff in most Barstow Formation sections and may be older than 19.5 Ma in some places. The new data indicate that the central Mojave Desert contained narrow to broad lake basins during and after extension, and that Barstow lacustrine deposits did not exclusively postdate extensional tectonics. At present, it is unclear whether several separate, small lake basins coexisted during the early to middle Miocene, or if instead several small early Miocene basins gradually coalesced over about 6 million years to form one or two large middle Miocene lake basins.

California↗
USGS2008

Dike orientations in the late jurassic independence dike swarm and implications for vertical-axis tectonic rotations in eastern California

Analysis of the strikes of 3841 dikes in 47 domains in the 500-km-long Late Jurassic Independence dike swarm indicates a distribution that is skewed clockwise from the dominant northwest strike. Independence dike swarm azimuths tend to cluster near 325?? ?? 30??, consistent with initial subparallel intrusion along much of the swarm. Dike azimuths in a quarter of the domains vary widely from the dominant trend. In domains in the essentially unrotated Sierra Nevada block, mean dike azimuths range mostly between 300?? and 320??, with the exception of Mount Goddard (247??). Mean dike azimuths in domains in the Basin and Range Province in the Argus, Inyo, and White Mountains areas range from 291?? to 354?? the mean is 004?? in the El Paso Mountains. In the Mojave Desert, mean dike azimuths range from 318?? to 023??, and in the eastern Transverse Ranges, they range from 316?? to 051??. Restoration for late Cenozoic vertical-axis rotations, suggested by paleodeclinations determined from published studies from nearby Miocene and younger rocks, shifts dike azimuths into better agreement with azimuths measured in the tectonically stable Sierra Nevada. This confirms that vertical-axis tectonic rotations explain some of the dispersion in orientation, especially in the Mojave Desert and eastern Transverse Ranges, and that the dike orientations can be a useful if imperfect guide to tectonic rotations where paleomagnetic data do not exist. Large deviations from the main trend of the swarm may reflect (1) clockwise rotations for which there is no paleomagnetic evidence available, (2) dike intrusions of other ages, (3) crack filling at angles oblique or perpendicular to the main swarm, (4) pre-Miocene rotations, or (5) unrecognized domain boundaries between dike localities and sites with paleomagnetic determinations. ?? 2008 The Geological Society of America.

Special Paper of the Geological Society of America↗
USGS2007

Stratigraphy, paleomagnetism, and anisotropy of magnetic susceptibility of the Miocene Stanislaus Group, central Sierra Nevada and Sweetwater Mountains, California and Nevada

Paleomagnetism and anisotropy of magnetic susceptibility (AMS) reveal pyroclastic flow patterns, stratigraphic correlations, and tectonic rotations in the Miocene Stanislaus Group, an extensive volcanic sequence in the central Sierra Nevada, California, and in the Walker Lane of California and Nevada. The Stanislaus Group (Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation) is a useful stratigraphic marker for understanding the post–9-Ma major faulting of the easternmost Sierra Nevada, uplift of the mountain range, and transtensional tectonics within the central Walker Lane. The Table Mountain Latite has a distinctively shallow reversed-polarity direction (I = −26.1°, D = 163.1°, and α 95 = 2.7°) at sampling sites in the foothills and western slope of the Sierra Nevada. In ascending order, the Eureka Valley Tuff comprises the Tollhouse Flat Member (I = −62.8°, D = 159.9°, α 95 = 2.6°), By-Day Member (I = 52.4°, D = 8.6°, α 95 = 7.2°), and Upper Member (I = 27.9°, D = 358.0°, α 95 = 10.4°). The Dardanelles Formation has normal polarity. From the magnetization directions of the Eureka Valley Tuff in the central Walker Lane north of Mono Lake and in the Anchorite Hills, we infer clockwise, vertical-axis rotations of ∼10° to 26° to be a consequence of dextral shear. The AMS results from 19 sites generally show that the Eureka Valley Tuff flowed outward from its proposed source area, the Little Walker Caldera, although several indicators are transverse to radial flow. AMS-derived flow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane.

California, Nevada↗
USGS2007

A 3-Dimensional Model of Water-Bearing Sequences in the Dominguez Gap Region, Long Beach, California

A 3-dimensional computer model of the Quaternary sequence stratigraphy in the Dominguez gap region of Long Beach, California has been developed to provide a robust chronostratigraphic framework for hydrologic and tectonic studies. The model consists of 13 layers within a 16.5 by 16.1 km (10.25 by 10 mile) square area and extends downward to an altitude of -900 meters (-2952.76 feet). Ten sequences of late Pliocene to Holocene age are identified and correlated within the model. Primary data to build the model comes from five reference core holes, extensive high-resolution seismic data obtained in San Pedro Bay, and logs from several hundred water and oil wells drilled in the region. The model is best constrained in the vicinity of the Dominguez gap seawater intrusion barrier where a dense network of subsurface data exist. The resultant stratigraphic framework and geologic structure differs significantly from what has been proposed in earlier studies. An important new discovery from this approach is the recognition of ongoing tectonic deformation throughout nearly all of Quaternary time that has impacted the geometry and character of the sequences. Anticlinal folding along a NW-SE trend, probably associated with Quaternary reactivation of the Wilmington anticline, has uplifted and thinned deposits along the fold crest, which intersects the Dominguez gap seawater barrier near Pacific Coast Highway. A W-NW trending fault system that approximately parallels the fold crest has also been identified. This fault progressively displaces all but the youngest sequences down to the north and serves as the southern termination of the classic Silverado aquifer. Uplift and erosion of fining-upward paralic sequences along the crest of the young fold has removed or thinned many of the fine-grained beds that serve to protect the underlying Silverado aquifer from seawater contaminated shallow groundwater. As a result of this process, the potential exists for vertical migration of seawater into the producing aquifers and subsequent landward migration of intruded waters beneath the existing Dominguez barrier. Incipient invasion of the Silverado aquifer by chloride-enriched waters is observed in a recently drilled well located along the crest of the fold seaward of the barrier and at a depth of 440 feet (134 meters). These new observations and interpretations indicate that the new sequence-based approach to defining the stratigraphy of the Dominguez Gap area may have important implications for seawater intrusion management. To test this, it will be useful to overlay existing water-quality and water level data onto the framework model and to incorporate the new stratigraphy into a transport model.

Open-File Report↗
USGS2005

Highly magnetic Upper Miocene sandstones of the San Francisco Bay area, California

[1] A high-resolution aeromagnetic survey of the San Francisco Bay area shows prominent positive anomalies over distinctive blue sandstones of Late Miocene age. The total-field survey was measured at a nominal height of 300 m above the land surface along flight lines spaced 0.5 km apart. Anomalies with amplitudes up to 200 nT correlate with sandstones of the San Pablo Group, and these anomalies are similar in strength to the magnetic signatures of serpentinites and basalts in the surveyed region. Andesitic sandstone of the Neroly Formation, the upper part of the San Pablo Group, has high magnetic susceptibility (0.013 SI units, volume) and relatively strong natural remanent magnetization (0.29 A/m). Total magnetization of the sandstone is two thirds induced and one third remanent magnetization. The presence of coarse-grained magnetite detritus, low coercivity of remanence, low thermal stability of remanence, and multidomain properties is consistent with the NRM being a viscous remanent magnetization that grew during the Brunhes normal-polarity chron. The strong magnetic signature of the Upper Miocene sandstones allows their delineation over distances as great as 100 km, through areas where they are concealed by landslides and younger deposits. The sandstones are important structural markers for understanding the complex folding and faulting associated with active fault systems in the San Francisco Bay area.

California↗