Eastern Transverse Ranges Research Articles

Titanite Petrochronology Records Secular Temperature and Fluid Evolution During Ductile Deformation: An Example From Late Cretaceous Shear Zones in the Eastern Transverse Ranges

AbstractQuantifying the timing and conditions of ductile deformation is essential for quantitative models of lithospheric deformation. Yet, directly constraining these variables and documenting how they change during a single deformation event remain difficult. We present titanite microstructural, zoning, trace‐element, and U‐Pb data from <m‐scale mylonitic shear zones in the Eastern Transverse Ranges, CA (Joshua Tree National Park) that record (a) the timing and conditions of ductile deformation and (b) the evolution of deformation conditions within a single, punctuated ductile deformation event. Titanite grains in sheared plutonic rocks yield a bimodal distribution of dates (∼151 and ∼75 Ma) interpreted as the independently constrained pluton crystallization and shear zone deformation ages, respectively. Synkinematic titanite neocrystallization at ∼75 Ma documents early deformation at ∼640–710°C under hydrous fluid conditions. Recrystallized titanite rims with ∼75 Ma dates indistinguishable from the neocrystallized titanite record Zr‐in‐titanite temperatures of 600–610°C and higher F contents, suggesting that the fluid became a more halogen‐rich brine as deformation progressed and the system cooled. Chemical zoning related to fluid‐driven recrystallization occurs in tandem with lattice misorientations and partially to fully reset U‐Pb dates in bent titanite, implying that reactive fluid flow is the ultimate mechanism by which dates are reset in titanite deformed by dislocation creep. These data document a punctuated (e.g., ∼1–3 Myr) phase of high‐temperature deformation that accompanied local pluton emplacement and demonstrate that titanite petrochronology can capture multiple, discrete reactions associated with crustal weakening and variables that affect the rheological evolution of ductile shear zones.

Steady Long‐Term Slip Rate on the Blue Cut Fault: Implications for Strain Transfer Between the San Andreas Fault and Eastern California Shear Zone

AbstractThe Eastern Transverse Ranges (ETR) province of California contains a system of E‐W‐trending left‐lateral faults accommodating clockwise block rotation between the San Andreas Fault (SAF) system in the Coachella Valley and the Eastern California Shear Zone (ECSZ) in the Mojave Desert. Building upon established geometric relationships, we estimate that this rotation across the ETR may transfer right‐lateral strain from the SAF to the ECSZ at a time‐averaged rate of ∼4.3–7.7 mm/yr. Through geomorphic mapping and the analysis of 38 10Be surface exposure samples, we derive a long‐term slip rate of 1.26 ± 0.50 (2σ) mm/yr over the late Pleistocene, yet analysis of the displacement record indicates a rate decrease ∼71 kya. While a rate change on this fault could have implications for possible plate boundary reorganization in this area, we argue that this slip rate variability more likely reflects routine fluctuation about a steady lifetime slip rate.

Moho Complexity in Southern California Revealed by Local PmP and Teleseismic Ps Waves

AbstractThe Moho discontinuity plays an important role in crustal growth and evolution. In this study, we delineate the Moho geometry in southern California by jointly using local Moho‐reflected waves PmP and teleseismic Moho‐converted waves Ps. To well constrain the Moho geometry, we have developed a two‐stage process to pick PmP waves and have created a reliable PmP travel time data set with a total of 10,192 picks. We have also extracted 38,648 high‐quality P‐wave receiver functions (RFs). The Moho depth is initially estimated via the common conversion point (CCP) stacking of RFs and then refined by inverting the PmP travel time data in a community velocity model (CVM‐H, version 15.1.1). The newly built Moho geometry is generally consistent with the California Moho Model version 1.0 (CMM‐1.0), that is, a shallow Moho beneath the Salton Trough (23 km), a uniformly shallow Moho beneath the Mojave Desert and the Basin and Range (<29 km), and a sliver of deep Moho under the western Peninsular Ranges, the eastern Transverse Ranges, and the western Sierra Nevada (>34 km). However, our Moho model reveals some new features different from the CMM–1.0, such as a deep Moho (∼34 km) beneath the northern end of the central and western Transverse Ranges, consistent with the observation of deep seismicities due to a thick brittle crust there. We also find a gradual transition from the lower crust to the uppermost mantle beneath the western Peninsular Ranges, leading to the rareness of pickable PmP waves as well as weak Moho‐converted signals there.

Beyond Ethnography and Beneath the Bedrock Mortars: The Eastern Transverse Ranges of Southern California as Early Holocene Refugia

ABSTRACT Rock Camp (CA-SBR-342) in the San Bernardino Mountains has been regarded as a seasonal Late Prehistoric Complex acorn processing base camp since it was excavated in the 1960s. Reanalysis of its collections and new investigations at the nearby Willow Creek Crossing sites permit reevaluation of archaeological models for the Transverse Ranges of southern California and the adjacent Mojave Desert. Diagnostic artifacts, site structure, and obsidian hydration dating indicate montane Greven Knoll I (9,400-4,000 BP) occupations in inland southern California several millennia earlier than has been previously posited. Moreover, the presence of stemmed points suggests that the Greven Knoll Pattern of the Encinitas Tradition was introduced into the region directly from the Mojave Desert during the onset of the Altithermal. This analysis also serves as a reminder for archaeologists to use ethnohistoric models with caution and to look beyond the obvious.

Testing models of Laramide orogenic initiation by investigation of Late Cretaceous magmatic-tectonic evolution of the central Mojave sector of the California arc

Abstract The Mojave Desert region is in a critical position for assessing models of Laramide orogenesis, which is hypothesized to have initiated as one or more seamounts subducted beneath the Cretaceous continental margin. Geochronological and geochemical characteristics of Late Cretaceous magmatic products provide the opportunity to test the validity of Laramide orogenic models. Laramide-aged plutons are exposed along a transect across the Cordilleran Mesozoic magmatic system from Joshua Tree National Park in the Eastern Transverse Ranges eastward into the central Mojave Desert. A transect at latitude ∼33.5°N to 34.5°N includes: (1) the large upper-crustal Late Cretaceous Cadiz Valley batholith, (2) a thick section of Proterozoic to Jurassic host rocks, (3) Late Cretaceous stock to pluton-sized bodies at mesozonal depths, and (4) a Jurassic to Late Cretaceous midcrustal sheeted complex emplaced at ∼20 km depth that transitions into a migmatite complex truncated along the San Andreas fault. This magmatic section is structurally correlative with the Big Bear Lake intrusive suite in the San Bernardino Mountains and similar sheeted rocks recovered in the Cajon Pass Deep Scientific Drillhole. Zircon U-Pb geochronology of 12 samples via secondary ionization mass spectrometry (SIMS) (six from the Cadiz Valley batholith and six from the Cajon Pass Deep Scientific Drillhole) indicates that all Cretaceous igneous units investigated were intruded between 83 and 74 Ma, and Cajon Pass samples include a Jurassic age component. A compilation of new and published SIMS geochronological data demonstrates that voluminous magmatism in the Eastern Transverse Ranges and central Mojave Desert was continuous throughout the period suggested for the intersection and flat-slab subduction of the Shatsky Rise conjugate deep into the interior of western North America. Whole-rock major-element, trace-element, and isotope geochemistry data from samples from a suite of 106 igneous rocks represent the breadth of Late Cretaceous units in the transect. Geochemistry indicates an origin in a subduction environment and intrusion into a crust thick enough to generate residual garnet. The lack of significant deflections of compositional characteristics and isotopic ratios in igneous products through space and time argues against a delamination event prior to 74 Ma. We argue that Late Cretaceous plutonism from the Eastern Transverse Ranges to the central Mojave Desert represents subduction zone arc magmatism that persisted until ca. 74 Ma. This interpretation is inconsistent with the proposed timing of the docking of the Shatsky Rise conjugate with the margin of western North America, particularly models in which the leading edge of the Shatsky Rise was beneath Wyoming at 74 Ma. Alternatively, the timing of cessation of plutonism precedes the timing of the passage of the Hess Rise conjugate beneath western North America at ca. 70–65 Ma. The presence, geochemical composition, and age of arc products in the Eastern Transverse Ranges and central Mojave Desert region must be accounted for in any tectonic model of the transition from Sevier to Laramide orogenesis.

New geodetic constraints on southern San Andreas fault-slip rates, San Gorgonio Pass, California

AbstractAssessing fault-slip rates in diffuse plate boundary systems such as the San Andreas fault in southern California is critical both to characterize seismic hazards and to understand how different fault strands work together to accommodate plate boundary motion. In places such as San Gorgonio Pass, the geometric complexity of numerous fault strands interacting in a small area adds an extra obstacle to understanding the rupture potential and behavior of each individual fault. To better understand partitioning of fault-slip rates in this region, we build a new set of elastic fault-block models that test 16 different model fault geometries for the area. These models build on previous studies by incorporating updated campaign GPS measurements from the San Bernardino Mountains and Eastern Transverse Ranges into a newly calculated GPS velocity field that has been removed of long- and short-term postseismic displacements from 12 past large-magnitude earthquakes to estimate model fault-slip rates. Using this postseismic-reduced GPS velocity field produces a best-fitting model geometry that resolves the long-standing geologic-geodetic slip-rate discrepancy in the Eastern California shear zone when off-fault deformation is taken into account, yielding a summed slip rate of 7.2 ± 2.8 mm/yr. Our models indicate that two active strands of the San Andreas system in San Gorgonio Pass are needed to produce sufficiently low geodetic dextral slip rates to match geologic observations. Lastly, results suggest that postseismic deformation may have more of a role to play in affecting the loading of faults in southern California than previously thought.

Constraints on rock uplift in the eastern Transverse Ranges and northern Peninsular Ranges and implications for kinematics of the San Andreas fault in the Coachella Valley, California, USA

AbstractThe nexus of plate-boundary deformation at the northern end of the Coachella Valley in southern California (USA) is complex on multiple levels, including rupture dynamics, slip transfer, and three-dimensional strain partitioning on nonvertical faults (including the San Andreas fault). We quantify uplift of mountain blocks in this region using geomorphology and low-temperature thermochronometry to constrain the role of long-term vertical deformation in this tectonic system. New apatite (U-Th)/He (AHe) ages confirm that the rugged San Jacinto Mountains (SJM) do not exhibit a record of rapid Neogene exhumation. In contrast, in the Little San Bernardino Mountains (LSBM), rapid exhumation over the past 5 m.y. is apparent beneath a tilted AHe partial retention zone, based on new and previously published data. Both ranges tilt away from the Coachella Valley and have experienced minimal denudation from their upper surface, based on preservation of weathered granitic erosion surfaces. We interpret rapid exhumation at 5 Ma and the gentle tilt of the erosion surface and AHe isochrons in the LSBM to have resulted from rift shoulder uplift associated with extension prior to onset of transpression in the Coachella Valley. We hypothesize that the SJM have experienced similar rift shoulder uplift, but an additional mechanism must be called upon to explain the pinnacle-like form, rugged escarpment, and topographic disequilibrium of the northernmost SJM massif. We propose that this form stems from erosional resistance of the Peninsular Ranges batholith relative to more-erodible foliated metamorphic rocks that wrap around it. Our interpretations suggest that neither the LSBM nor SJM have been significantly uplifted under the present transpressive configuration of the San Andreas fault system, but instead represent relict highs due to previous tectonic and erosional forcing.

Transrotation Induced by Crustal Blocks Moving through Restraining Double Bends, with Southern California Examples

AbstractWithin the San Andreas system of southern California, vertical-axis clockwise rotations have been attributed to (1) capture of the Monterey microplate, during which oceanic lithosphere beneath North America was transferred to the Pacific plate imparting basal traction and inducing northwestward motion of overlying North American crust, and (2) torsional force couples imposed at the ends of rotating panels. In contrast, some crustal blocks that are smaller and/or weaker than crust on opposite sides of faults have experienced clockwise transrotation as they have moved through restraining double bends: (1) rapid 40° clockwise rotation of the eastern Transverse Ranges (6–0 Ma), starting with the southernmost parts and progressing northward; (2) rapid 23° clockwise rotation of the Morales Formation in the Cuyama Valley (3–0 Ma); and (3) continuing clockwise rotation of the westernmost Transverse Ranges. The magnitude of rotation matches the angle between the strikes of fault segments on either side of ...

Reconciling disparate estimates of total offset on the southern San Andreas fault

Late Cenozoic tectonic reconstructions of southern California (United States) have been hindered for decades by disagreement over the total displacement on the southern San Andreas fault; disparate estimates of dextral offset range from 160 km to 240 km. Prior estimates were based on purely translational models that neglect the effects of transrotational strain on the orientation and distribution of key geologic markers. We present a new reconstruction that integrates published lithologic, structural, and paleocurrent data with the effects of clockwise rotation in the eastern Transverse Ranges (ETR). This model yields 200 ± 14 km of cumulative offset on the southern San Andreas fault system in the Salton Trough. The disparity between our estimate and a lower estimate of 160 km for the Mojave segment can be explained by a combination of transtensional elongation along the southwest boundary of the ETR, transfer of strain to the eastern California shear zone, and diffuse crustal shortening in the Big Bend region. This reconstruction reconciles previously incompatible and long-debated models, and places an important new constraint on the tectonic evolution of the Pacific–North America plate boundary in southern California.

Phylogeography of a stream-dwelling frog ( Pseudacris cadaverina) in southern California

Recent phylogeographic studies of animal taxa in California have revealed common geographic patterns of evolutionary divergence and genetic diversity that are generally attributable to landscape influences. However, there remains a paucity of knowledge on the evolution of freshwater taxa in southern California. Here, we investigate phylogeographic patterns in a stream-dwelling frog ( Pseudacris cadaverina). Two hundred and twenty-one individuals were collected from 46 populations across the species’ range in southern California. Using 1100 bp of sequence data from cytochrome b and tRNA-Glu, we conducted phylogenetic analyses, analysis of molecular variance, and nested clade phylogeographic analysis to gain insight into the factors contributing to the distribution of genetic diversity in P. cadaverina. We tested for evidence of two putative phylogeographic breaks and tested hypotheses that genetic diversity in this species is partitioned into (1) major watersheds, (2) mountain ranges, and (3) coastal and desert regions. Our results suggest that the eastern Transverse Ranges are the center of origin for extant P. cadaverina lineages and that the observed genetic structure in this species was established during the Pleistocene Epoch. There is strong support for three major haplotype groups and a Transverse Range break in P. cadaverina that is concordant with breaks found in numerous other taxa. The distribution of genetic diversity in P. cadaverina is due in large part to the separation of populations into different major watersheds and mountain ranges. Gene flow appears to be generally limited among disjunct populations throughout the region and some desert populations have been isolated by historical habitat fragmentation.

Basin geometry and cumulative offsets in the Eastern Transverse Ranges, southern California: Implications for transrotational deformation along the San Andreas fault system

The Eastern Transverse Ranges, adjacent to and southeast of the big left bend of the San Andreas fault, southern California, form a crustal block that has rotated clockwise in response to dextral shear within the San Andreas system. Previous studies have indicated a discrepancy between the measured magnitudes of left slip on through-going east-striking fault zones of the Eastern Transverse Ranges and those predicted by simple geometric models using paleomagnetically determined clockwise rotations of basalts distributed along the faults. To assess the magnitude and source of this discrepancy, we apply new gravity and magnetic data in combination with geologic data to better constrain cumulative fault offsets and to define basin structure for the block between the Pinto Mountain and Chiriaco fault zones. Estimates of offset from using the length of pull-apart basins developed within left-stepping strands of the sinistral faults are consistent with those derived by matching offset magnetic anomalies and bedrock patterns, indicating a cumulative offset of at most ~40 km. The upper limit of displacements constrained by the geophysical and geologic data overlaps with the lower limit of those predicted at the 95% confidence level by models of conservative slip located on margins of rigid rotating blocks and the clockwise rotation of the paleomagnetic vectors. Any discrepancy is likely resolved by internal deformation within the blocks, such as intense deformation adjacent to the San Andreas fault (that can account for the absence of basins there as predicted by rigid-block models) and linkage via subsidiary faults between the main faults.

The Basin and Range Province as a Composite Extensional Domain

The Basin and Range province is a composite Cenozoic taphrogen with a present configuration and internal geometry derived from superimposed multiple phases of extensional deformation driven by varied geodynamic impulses. Different components of the composite taphrogen derive from: (a) post-orogenic Late Paleocene to Middle Miocene slab rollback beneath overthickened crust of the Sevier-Laramide orogen, to induce migratory arc magmatism, tectonic denudation of Cordilleran metamorphic core complexes within associated domains of crustal extension, and belts of backarc rifting including pre-Middle Miocene phases of the Rio Grande rift; (b) Eocene transrotational tectonism that pivoted and displaced crustal elements of the Paleogene forearc in the Pacific Northwest; (c) late Early Miocene and younger crustal extension in the northwestern Basin and Range province (Numic subtraphrogen including segments in the Great Basin, Southeast Oregon Plateau, and Idaho-Montana border regions) as structural accompaniment to northward migration of the Mendocino triple junction; and (d) late Middle Miocene and younger crustal extension in the southeastern Basin and Range province (Piman subtaphrogen including the Gulf of California extensional province, interior Meseta Central province, and post-mid-Miocene Rio Grande rift) succeeding the rapid southward migration of the Rivera triple junction. A transtaphrogen bridge spanning the Mojave block, Eastern Transverse Ranges, and a broad region in southeastern California and southwestern Arizona experienced only limited post-mid-Miocene extensional deformation during evolution of the active Numic and Piman subtaphrogens lying to the northwest and southeast. The compound evolutionary history of the Basin and Range province renders moot any unitary theories for its geodynamic behavior.

Estimating crustal thickness in southern California by stacking PmP arrivals

We use observations of the Moho‐reflected phase PmP to constrain crustal thicknesses and upper mantle velocities in southern California. We stack normalized absolute values of seismograms from local events in time and range bins after aligning on the initial P arrival and applying a range correction to adjust the various source depths to the surface. Although most individual seismograms do not allow accurate determination of a PmP arrival time, imaging the whole data set in this way shows clear PmP arrivals at ranges from about 90 km to over 250 km. PmP‐ Pg and PmP‐ Pn differential times can be measured from the image and used to estimate the Moho depth and upper mantle velocity. For southern California, we obtain an average crustal thickness of 28 km and an upper mantle velocity of 7.8 km/s. Next, we map lateral variations by repeating this procedure for stacks of subsets of the data in which the traces are grouped in caps by Moho reflection point. Estimates of Moho depth range from 18 km in the Salton Trough to 33 km beneath the eastern Transverse Ranges and 36 km beneath the southernmost Sierra Nevada. The upper mantle velocities generally increase from southwest to northeast across the region. We also map postcritical PmP/Pg amplitudes which vary by a factor of 4 with the highest amplitudes in the northwest Mojave Desert. Preliminary experiments with stacking SmS arrivals indicate strongly correlated SmS and PmP amplitude variations. These results provide a guide to source‐receiver paths that may produce anomalously strong Moho‐reflected phases during future earthquakes.

Seismicity and stress‐drop in the Eastern Transverse Ranges, southern California

Stress‐drops for small to moderately sized earthquakes in Southern California are found to vary systematically with source‐depth and location (tectonic environment). We determine high‐quality fault‐plane solutions, plus depth and source duration, for 17 significant (M >3.9) aftershocks associated with the June 28, 1992 Big Bear sequence, including the more recent April 4, 1994 19:04 GMT Mw 4.6 Lake Arrowhead aftershock, and a Mw 4.2 Banning Pass event which occurred on May 31, 1993 at 08:55 GMT. Given source durations and moments obtained from long‐period source estimations, and assuming a circular fault model, we estimate stress‐drop for each event. Big Bear aftershocks are moderate to high (>100 bars) stress‐drop. Events deeper than 12 km are generally high stress‐drop (>100 bars), while shallower events exhibit moderate to high stress‐drops. These results are compared with a similar analysis of Landers aftershocks in the Mojave block. For the Big Bear region, stress drops appear to correlate with depth, with the deepest events yielding the highest stress‐drops. In general, events in this region yield higher stress‐drops than events occuring in the Mojave block and those associated with the Landers and Joshua Tree sequences. Comparisons of ML to MO are consistent with the stress‐drop results: deep, high stress‐drop events show elevated ML to MO ratios.

Thermal regime of the southern Basin and Range Province: 1. Heat flow data from Arizona and the Mojave Desert of California and Nevada

With about 150 new heat flow values, more than 200 values of heat flow are now available from the crystalline terranes of southern California, the Basin and Range Province of Arizona, and Paleozoic sedimentary rocks of the southwestern Colorado Plateau (CP). Heat flow ranges from about 5 mW m−2 on the CP near Flagstaff, Arizona, to more than 150 mW m−2 in the crystalline rocks bordering the Salton Trough in SE California. The heat flow pattern within this region is complex, although it correlates with regional physiographic and tectonic features. Unlike the adjacent Sierra Nevada Batholith where heat flow is a linear function of near‐surface radiogenic heat production, no statistically significant correlation exists between heat flow and heat production in the study area, possibly because of its complex tectonic history, involving lateral movement of basement terranes, and relatively young heat sources and sinks of different strengths, ages, and durations. Contemporary and Neogene extensional tectonism appears to be responsible for the very high heat flow (>100 mW m−2) associated with the Salton Trough and its neighboring ranges, the Death Valley fault zone and its southward extension along the eastern boundary of the Mojave block, and zones of shallow depth (<10 km) to the Curie isotherm (as inferred from aerornagnetic data) in west central Arizona. Low (<60 mW m−2) heat flow in the Peninsular Ranges and eastern Transverse Ranges of California may be caused by downward advection associated with subduction and compressional tectonics. Relatively low heat flow (67±4 mW m−2) is also associated with the main trend of metamorphic core complexes in Arizona, and the outcropping rocks in the core complexes have a low radioactive heat production (1.3±0.3 μW m−3) compared to the other crystalline rocks in the region (2.1±0.2 μW m−3).

Deformation across the western United States: A local estimate of Pacific‐North America transform deformation

We obtain a locally based estimate of Pacific‐North America relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern North America from east of the Rio Grande Rift to near the continental escarpment. Data are primarily Quaternary geologic slip rate estimates, and resulting deformation determinations therefore are “instantaneous” in a geologic sense but “long term” with respect to earthquake cycles. We deduce a rate of motion of the Pacific plate relative to North America that is 48±2 mm/yr, a rate indistinguishable from that predicted by the global kinematic models RM2 and NUVEL‐1; however, we obtain an orientation that is 5–9° counterclockwise of these models. A more westerly motion of the Pacific plate, with respect to North America, is calculated from all three paths. The relatively westerly motion of the Pacific plate is accommodated by deformation in the North American continent that includes slip on relatively counterclockwise‐oriented strike‐slip faults (including the San Andreas fault), which is especially relevant in and south of the Transverse Ranges, and a margin‐normal component of net extension across the continent, which is especially relevant north of the Transverse Ranges. Deformation of the SW United States occurs in regionally coherent domains within which the style of deformation is approximately uniform. In the vicinity of the Transverse Ranges, two important shear systems splay from the San Andreas fault: the eastern California shear zone trending NNW from the eastern Transverse Ranges and the trans‐Peninsular faults trending SSE from the western and central Transverse Ranges. Within the Transverse Ranges the right‐lateral San Andreas fault steps left, seemingly requiring large amounts of convergence there. However, most of this convergence is avoided through a “funneling flow” of the crust toward the western Transverse Ranges and into the relatively narrow central California Coast Ranges and the northern motion of the Mojave. The former process involves an alternation of rotation direction from counterclockwise (in and south of the central Transverse Ranges) to clockwise (in the western Transverse Ranges).

Conflicting tectonics? Contraction and extension at middle and upper crustal levels along the Cordilleran Late Jurassic arc, southeastern California

Effects of mid-Mesozoic contraction followed closely in time by extension are present in mid- to upper-crustal plutonic rocks in the Chuckwalla Mountains of the eastern Transverse Ranges, California. Late Jurassic movement along a steeply dipping, right-lateral mylonitic shear zone is bracketed between 159 and 147 Ma via U-Pb dated plutons. Depth of emplacement vs. time data based on hornblende geobarometry and U-Pb geochronology of Mesozoic plutons indicate that a period of dramatic uplift affected the Chuckwalla Mountains during the Late Jurassic, contrasting sharply with data from the (then) nearby San Gabriel Mountains. Subsequent latest Jurassic extensional tectonics is suggested by alkalic plutonism, nearly synchronous intrusion of the Late Jurassic Independence dike swarm, and possibly by deposition of the McCoy Mountains Formation. We conclude that both contraction and extension were significant at upper- and mid-crustal depths in the Chuckwalla Mountains region during the Late Jurassic, and speculate that the combined influence of oblique convergence and the opening of the Gulf of Mexico may have caused the contrasting and nearly contemporaneous tectonic modes.

Palinspastic reconstruction of southeastern California and southwestern Arizona for the Middle Miocene

A paleogeographic reconstruction of southeastern California and southwestern Arizona at 10 Ma has been made based on available geologic and geophysical data. Clockwise rotation of 39° has been reconstructed in the eastern Transverse Ranges, consistent with paleomagnetic data from late Miocene volcanic rocks, and with slip estimates for left‐lateral faults within the eastern Transverse Ranges and NW‐trending right‐lateral faults in the Mojave Desert. This domain of rotated rocks is bounded by the Pinto Mountain fault on the north. The model requires that the western part of the late Miocene Pinto Mountain fault was a thrust fault, which gained displacement to the west, because of the absence of evidence for rotation of the San Bernardino Mountains or for significant right slip faults within the San Bernardino Mountains. The Squaw Peak thrust system of Meisling and Weldon (1989) may be a western continuation of this fault system. The Sheep Hole fault bounds the rotating domain on the east. East of this fault an array of NW‐trending right slip faults and south‐trending extensional transfer zones has produced a basin and range physiography while accumulating up to 16 km of right slip. This maximum is significantly less than the 37.5 km of right slip required in this region by a recent reconstruction of the central Mojave Desert (Dokka and Travis, 1990a). Geologic relations along the southern boundary of the rotating domain are poorly known, but this boundary is interpreted to involve a series of curved strike slip faults and linking normal faults that accommodated noncoaxial extension. Quaternary movement on the Pinto Mountain and nearby faults is unrelated to the rotation of the eastern Transverse Ranges, and a hiatus during part of Pliocene time followed the deformation which produced the rotation. The reconstructed Clemens Well fault in the Orocopia Mountains, proposed as a major early Miocene strand of the San Andreas fault, projects eastward towards Arizona, where early Miocene rocks and structures are continuous across its trace, making large displacements on this structure unlikely. The model predicts a 14° clockwise rotation and 32% extension during late Miocene and early Pliocene time along a NW‐trending line parallel to the present trace of the San Andreas fault. Palinspastic reconstructions of the San Andreas system based on this proposed reconstruction may be significantly modified from current models.

Discriminating quarry blasts from earthquakes using coda waves

AbstractA new quarry blast-earthquake discrimination method is presented based on the analysis of seismic coda waves. Quarry blasts and local earthquakes in the area encompassing the south-central Mojave Desert and Eastern Transverse Ranges were used to test this method. We found that the coda decay rate Qc−1 is significantly higher for quarry blasts than earthquakes for lower frequencies (1.5 and 3 Hz) for lapse time up to about 30 sec. This result is attributed to the greater contribution of surface waves to quarry blasts due to the shallowness of their source depth. The difference in Qc−1, however, disappears for lapse time greater than 30 sec for the same frequency range as well as for higher frequencies (6 and 12 Hz) for lapse time greater than 20 sec. This suggests that the coda waves, at the lapse time greater than these critical values, are dominated by the same type of body waves, probably S waves, for both quarry blasts and earthquakes. The power spectrum P0(ω) obtained after the correction for attenuation was compared between earthquakes and quarry blasts at the same stations, and a significant difference was found in the spectral shape between these two data sets. The curves of power spectrum P0(ω) versus frequency for quarry blasts decrease more sharply than for earthquakes at high frequencies, indicating a lack of energy in high frequencies for quarry blasts as compared to earthquakes. The different frequency dependence of power spectrum P0(ω) between quarry blasts and earthquakes is attributed to their different source properties and can be used for seismic discrimination of quarry blasts from earthquakes.

Paleomagnetism of Middle Miocene volcanic rocks in the Mojave‐Sonora Desert region of western Arizona and southeastern California

Paleomagnetic directions have been obtained from 190 early to middle Miocene (12–20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave‐Sonora desert region of western Arizona and southeastern California. These flows generally postdate early Miocene tectonic deformation accommodated by low‐angle normal faults but predate high‐angle normal faulting in the region. After detailed demagnetization experiments, 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations (TRM). Because of the episodic nature of basaltic volcanism in this region, the 179 flows yielded only 65 time‐distinct virtual geomagnetic poles (VGPs). The angular dispersion of the 65 VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9° within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from rocks of similar age in stable North America and from a paleomagnetic pole calculated from rocks of similar age in Baja California. The coincidence of paleomagnetic poles from the Mojave‐Sonora desert region with reference poles from the stable continental interior indicates that (1) significant vertical axis net tectonic rotations have not accompanied post‐middle Miocene high‐angle normal faulting in this region; (2) there has been no detectable post‐middle Miocene latitudinal transport of the region; and (3) long‐term nondipole components of the middle Miocene geomagnetic field probably were no larger than those of the recent (0–5 Ma) geomagnetic field. In contrast, paleomagnetic data indicate vertical axis rotations of similar age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major structural discontinuity in the vicinity of the southeastward projection of the Death Valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations.