Mapping of the surface breaks that resulted from the San Fernando earthquake of 9 February 1971 reveals that the pattern of faulting was highly complex; it consisted of a number of segments that produced ground displacements and acceleration throughout the entire northern end of the San Fernando Valley. Instead of occurring on the frontal fault zone, as might have been expected, the faulting occurred on the valley side of the frontal fault system, which separates the crystalline rocks of the San Gabriel Mountains from the Tertiary sediments of the San Fernando Valley. However, the new fault system does, in many cases, follow breaks in slope and subtle escarpments that suggest faulting along these zones in the recent geologic past.
Research Article| August 01, 1976 Heat flow in Lake Tahoe, California-Nevada, and the Sierra Nevada–Basin and Range transition T. L. HENYEY; T. L. HENYEY 1Department of Geology Sciences, University of Southern California, Los Angeles, California 90007 Search for other works by this author on: GSW Google Scholar T. C. LEE T. C. LEE 1Department of Geology Sciences, University of Southern California, Los Angeles, California 90007 Search for other works by this author on: GSW Google Scholar Author and Article Information T. L. HENYEY 1Department of Geology Sciences, University of Southern California, Los Angeles, California 90007 T. C. LEE 1Department of Geology Sciences, University of Southern California, Los Angeles, California 90007 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1976) 87 (8): 1179–1187. https://doi.org/10.1130/0016-7606(1976)87<1179:HFILTC>2.0.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 T. L. HENYEY, T. C. LEE; Heat flow in Lake Tahoe, California-Nevada, and the Sierra Nevada–Basin and Range transition. GSA Bulletin 1976;; 87 (8): 1179–1187. doi: https://doi.org/10.1130/0016-7606(1976)87<1179:HFILTC>2.0.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 Heat-flow measurements made in Lake Tahoe, California-Nevada, demonstrate that the transition from subnormal heat flow in the Sierra Nevada to above-normal heat flow in the Basin and Range province occurs west of the assumed physiographic boundary between these two areas, contrary to earlier belief. In addition, these data, together with data of other workers, clearly reveal the sharpness of this transition, which suggests that the causative thermal sources and (or) sinks must be restricted to depths not greater than the uppermost mantle. The way in which heat-flow data constrain the current hypotheses of crustal structure and evolution of the Sierra Nevada–Basin and Range provinces is illustrated with a tectonic model that employs a post-Cretaceous shallow-dipping subduction zone beneath the Sierra Nevada and an active upper-mantle diapir under the Basin and Range province during late Cenozoic time. 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.
Water levels in the Los Angeles Aqueduct in southern California fluctuate in a manner that are not easily attributable to normal aqueduct operations. Simple hydraulics suggests that large scale earth tilt can register as water level anomalies with a sensitivity of about .01 ft/microradian. Two aqueduct anomalies which coincide spatially and temporally with independently observed deformational phenomena are used to explore this suggestion.
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.
Research Article| January 01, 1973 Tectonic Elements of the Northern Part of the Gulf of California THOMAS L. HENYEY; THOMAS L. HENYEY 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90007 Search for other works by this author on: GSW Google Scholar JAMES L. BISCHOFF JAMES L. BISCHOFF 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90007 Search for other works by this author on: GSW Google Scholar Author and Article Information THOMAS L. HENYEY 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90007 JAMES L. BISCHOFF 1Department of Geological Sciences, University of Southern California, Los Angeles, California 90007 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (1): 315–330. https://doi.org/10.1130/0016-7606(1973)84<315:TEOTNP>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation THOMAS L. HENYEY, JAMES L. BISCHOFF; Tectonic Elements of the Northern Part of the Gulf of California. GSA Bulletin 1973;; 84 (1): 315–330. doi: https://doi.org/10.1130/0016-7606(1973)84<315:TEOTNP>2.0.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 Results from a continuous seismic survey along closely spaced ship tracks in the northern Gulf of California are presented in terms of the tectonics of this region. Apparent vertical offsets of the most recent sediments, ranging in height from several to a few hundred meters, are associated with the central basins (Delfin and Wagner basins), indicating they are the loci of active tectonism. Structural relations inferred from mapping these features are consistent with plate tectonic concepts of the Gulf. Delfin basin represents a single, complex, northeast-southwest–trending, spreading center. Two parallel transform faults, which flank Angel de la Guarda Island and strike northward into Delfin basin from the south, and a complementary transform fault to the north represented by the Wagner basins, end at this spreading center. With the possible exception of the San Jacinto fault, no correlation of active faults was found between the northern Gulf and contiguous land areas. Interpretations of other geophysical and geological data are complicated by the high sedimentation rate in the northern Gulf, yet are generally consistent with our conclusions. Spatial and temporal characteristics of plate boundaries in the northern Gulf are probably influenced by the proximity of continental structures. 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.
Paleomagnetic secular variation (PSV) records have been recovered from three marine sediment cores from Santa Catalina basin, California continental borderland, in order to more accurately date these late Quaternary sediments. The PSV records are not high in resolution due to low sedimentation rates coupled with a 3‐cm sampling interval and some inherent smoothing of the PSV signal during remanence lock‐in. However, the PSV waveforms are sufficiently clear to permit their correlation among the three separate cores. These PSV records can also be correlated with four independent PSV calibration curves from western North America that have detailed radiocarbon age control. The four calibration curves are developed in this paper to improve the dating and regional comparison of PSV records from western North America. The PSV correlations establish time‐depth curves for the three cores which indicate that the sediments are all younger than about 11,000 years B.P. The relative accuracy of the time‐depth curves is approximately 200 years, which represents an order‐of‐magnitude improvement in the chronology of these sediments. Sedimentation rates derived from the three time‐depth curves indicate a constant rate of 20–25 cm/kyr for the last 6700 years throughout Santa Catalina basin, and more variable rates (but constant within each core) of 13–86 cm/kyr prior to 6700 years B.P. In all three cores, the change in sedimentation rate corresponds to a subtle but distinct change in lithology. These changes probably indicate a major shift in paleoceanographic processes within Santa Catalina basin 6700 years B.P.
A 30 km‐long N‐S seismic reflection line was shot by California Consortium for Crust Studies (CALCRUST) across the southern Mojave Desert and onto the northern flank of the San Bernardino Mountains in southern California. On the northern end of the seismic section, the reflectivity increases markedly in the midcrust at a depth corresponding to a two‐way travel time of 4 to 5 s (12–15 km), suggesting a transition between nonreflecting brittle upper crust and reflecting ductile lower crust. The high reflectivity disappears at about 8 s (24 km) and may be correlated with a change in seismic velocity in the lower crust from 6.3 km/s to 6.8 km/s. A band of reflectivity between 9.5 and 10 s (27–30 km) is believed to represent the Moho. The midcrustal relectivity transition and Moho both deflect downward toward the San Bernardino Mountains uplift over the entire length of the profile. The deflection of the midcrustal transition (12°) appears greater than that of the Moho (6°), resulting in a thinning of the lower crust to the south beneath the uplift. In addition, the midcrustal transition coincides with the base of the seismogenic zone (brittle‐ductile transition?) which is also dipping southward beneath the San Bernardino Mountains, while the Moho deflection is consistent with elastic flexure resulting from edge loading by the San Bernardino Mountains which have been thrust over the Mojave block. It is suggested that the thinning of the lower crust beneath the San Bernardino Mountains is a result of north directed ductile flow in response to loading by the over thickened upper crust. Since a portion of the load is transmitted through the lower crust to the Moho, the time constant for flow equilibrium must be of the order of or greater than that for the time of uplift (≥2 m.y.).
High-frequency, wideband (20 Hz to 16 kHz) recording instruments for the detection of minute seismic emissions in borehole environments have been designed, developed, and deployed in three deep wells within seismically active regions. Two of these well sites are within a few km of the San Andreas fault near Palmdale, California; the third site is at the Monticello Reservoir, South Carolina. Seismicity near the Monticello Reservoir is induced by the recent impounding of water. The sensor is a highly-sensitive hydrophone emplaced at the bottom of the fluid-filled well. The surface recording package is an analog event recorder complete with event-detecting logic and digital delay circuit. At all three sites, numerous minute seismic emissions were detected with dominant spectral energy in the band 0.5 to 5 kHz and a peak at about 2 kHz. These events have durations of the order of 10 to 100 milliseconds and waveforms similar to a near-field earthquake greatly scaled down in size. Risking downward extrapolation of the duration time vs. magnitude relationship, these events may be assigned magnitudes in the range of -1 to -5; as such we call them nanoearthquakes. With their high-frequency content and in view of the strong attenuation in the upper crust, these nanoearthquakes are probably occurring at distances less than one km from the sensor. If laboratory results are applicable to field situations, the frequency of occurrence of nanoearthquakes may reflect the state of ambient stress, and their rate of occurrence may be useful in identifying the approximate time when a large earthquake is imminent.
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