Northern Peninsular Ranges Research Articles

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.

Rediscovery of the southern California endemic American Keeled Slug Anadenulus cockerelli (Hemphill, 1890) after a 68-year hiatus

ABSTRACTWe report the first records of the southern California endemic American Keeled Slug Anadenulus cockerelli (Hemphill, 1890) in 68 years. Due to the restricted range of this species and lack of recent detection, it has received a NatureServe conservation status of Critically Imperilled or Imperilled. This species has always been difficult to detect, and will likely continue to be. This may be the result of genuine rarity, or may be the result of being cryptic and occurring in a microhabitat only ephemerally accessible to humans. We created a Species Distribution Model for A. cockerelli, which strongly predicts its occurrence where it is known, as well as some locations where it is currently unknown, such as in the northern Peninsular Ranges and in the San Bernardino Mountains in California. Threats to the continued existence of this species in southern California include human encroachment and resulting habitat loss, the climate changing to be hotter and drier, increased fire frequency, oak death due to beetle infestation, and other introduced species. Anadenulus cockerelli should continue to be surveyed and monitored into the future in order to better understand its distribution and level of conservation concern.

Delayed Dynamic Triggered Seismicity in Northern Baja California, México Caused by Large and Remote Earthquakes

We analyze the seismicity in northern Baja California, Mexico, that occurred one month before and one month after the 11 March 2011 ( M w 9.1) Tohoku‐Oki, Japan, earthquake and for two other large and remote earthquakes, the 27 February 2010 central Chile ( M w 8.8) earthquake and the 11 April 2012 northern Sumatra earthquake ( M w 8.6). The northern region of Baja California exhibits high microseismic activity and moderate‐size earthquakes. The seismicity in this region is monitored by the seismic network Red Sismica del Noroeste de Mexico (RESNOM) operated by the Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE). We use the RESNOM seismic catalog to search for changes in local seismic rates that occurred after the passing of surface waves generated by the three large earthquakes studied. When we compare one month of seismicity before and after the M w 9.1 Japan earthquake, in the biggest of the three events analyzed, we observe the absence of triggered seismicity in the northern Peninsular Ranges and an increase of seismicity south of the Mexicali Valley, where the Imperial fault jumps southwest and the Cerro Prieto fault continues south. We also observe an increase of seismicity rate in the Cucapah fault and in the San Pedro Martir fault regions. The Cucapah fault region, the most active of the three regions during the studied period, shows an increase of seismicity several days after the 2010 Chile ( M w 8.8) and the 2011 Japan ( M w 9.1) earthquakes. This observation can be interpreted as evidence of delayed seismicity and confirms the observation of Rubinstein et al. (2009) that earthquakes tend to be triggered in regions with high ambient seismicity rates. We observed delayed triggering of earthquakes generated by two of the three remote earthquakes analyzed. These observations can be explained by dynamic triggering of prolonged fault creep as proposed by Shelly et al. (2011). Online Material: Maps of seismicity and M c.

The Role of Soil Chemistry in the Geographic Distribution ofCeanothus otayensis(Rhamnaceae)

Ceanothus otayensis McMinn (Rhamnaceae) was previously known only from metavolcanic-derived soils of the northern Peninsular Ranges—predominantly the San Ysidro Mountains—in San Diego County, California, and adjacent Baja California, Mexico. Recently, a new population of C. otayensis was discovered on sedimentary soils at Marine Corps Air Station Miramar, 25 km northwest of the next nearest known population. Sedimentary deposits at the new locality are thought to produce unusual soils. It is possible that the disjunct distribution of C. otayensis is a response to soil conditions, a phenomenon frequently seen in other members of Ceanothus, for instance on serpentine. The present study uses soil chemistry data for seven populations and subpopulations of C. otayensis (metavolcanic: n = 5; sedimentary: n = 2), as well as 22 populations of closely related Ceanothus, to determine whether soils of C. otayensis are chemically distinct from those of closely related Ceanothus, and answer the following question: are sedimentary-derived soils at the new locality chemically similar to metavolcanic-derived soils that support all other known populations of the species? Soils of C. otayensis proved to be chemically distinct from soils of closely related Ceanothus, with significantly lower levels of nitrate, sulfur, and conductivity. Sedimentary and metavolcanic soils of C. otayensis proved to be chemically indistinguishable from one another (P < 0.05), with low levels of all assayed nutrients other than Ca, suggesting a chemical similarity among the soils of C. otayensis that may help explain its disjunct distribution. Population size estimates indicate that the new disjunct locality at Marine Corps Air Station Miramar supports about 75 adult individuals.

Seismic heterogeneity and small‐scale convection in the southern California upper mantle

We invert traveltime residuals of teleseismic P and S phases for 3‐D perturbations in VP, VS, and VP/VS structures in the southern California upper mantle. The tomographic inversion uses frequency‐dependent 3‐D sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals. In addition to separate VP and VS tomography, we jointly invert the P and S data sets for VP/VS perturbations by imposing a smoothness constraint on the δlnVS/δlnVP field. The regional upper mantle is very heterogeneous with the greatest amplitude of velocity and VP/VS perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern California. Our imaging improves the resolution of the geometry and amplitude of these features. In addition to the major structures imaged and discussed in previous papers, we find a large high‐velocity anomaly at depths between ∼340 and 500 km beneath the Borderland, Transverse Ranges, and northern Peninsular Ranges. This anomaly is separate from the well‐known uppermost mantle Transverse Ranges anomaly, which extends no deeper than ∼175–200 km. The strongest low‐velocity, high VP/VS anomaly is found beneath and west of the Salton Trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning. The magnitudes of VP, VS, and VP/VS perturbations and knowledge of regional 1‐D velocity and attenuation lead us to conclude that the asthenosphere contains up to ∼1% partial melt extending to depths as great as 150–200 km.

Teleseismic surface wave study forS-wave velocity structure under an array: Southern California

SUMMARY Teleseismic surface wave data from the dense seismographic network in Southern California (CISN, California Integrated Seismic Network) are used to construct S-wave velocity maps for the region. Surface wave phase velocity measurements are achieved by way of a waveform-matching spectral-domain technique that was developed for use within the CISN array. A two-station phase velocity measurement technique was developed, which takes advantage of the fact that, for surface wave arrivals from any azimuth, there are multiple pairs of CISN stations, which are almost exactly on the same great-circle paths. To ensure the validity of this assumption, particle motions were analysed for measured pairs within the network, showing that the deviation between expected backazimuth from ray theory and observed backazimuths are small enough to be analysed by a simple distance correction. This method was applied to 114 large earthquakes for time periods between 1999 and 2002 and led to the retrieval of phase velocity maps for Rayleigh waves between 20 and 55 mHz and for Love waves between 25 and 45 mHz. Finite frequency effect for propagating surface waves was approximately taken into consideration using a geometrical technique. An ellipse, determined by wavelength and great-circle distance between the source and receiver can describe an influence zone for the finite frequency effect. Phase velocity map reconstructions, using a method which incorporates this effect, show minimal changes in phase velocity maps for our region, but do extend solvable velocity regions. Inversion for S-wave velocity distribution with depth shows that deep structural contrast across the San Andreas extends into the upper mantle. A slow velocity anomaly is evident under the Southern Sierras and the Salton Sea, the latter region indicating the northward extension of tectonic activity in the Gulf of California. Fast velocity roots that reach 60 km are found under the western part of Transverse Ranges and the Northern Peninsular Ranges. Reconstruction of past location of the Western Transverse Range microplate implies that these two ranges may have been adjacent to one another historically. Both the Western Transverse range fast velocity root and the Peninsular range fast velocity root may be a remnant of old oceanic plate.

Chapter 11

Studies of fossil remains of age-diagnostic land mammals have allowed recognition of assemblages of late Uintan, early and late Arikareean, and early Hemingfordian (late middle Eocene, early Oligocene to early Miocene) age in the Sespe Formation and equivalent marine formations of the northern Peninsular Ranges Provinces and western Transverse Ranges in Los Angeles, Orange, Santa Barbara, and Ventura counties, southern California. Recent fossil recovery efforts have resulted in the recognition of new land mammal assemblages in the Santa Ana Mountains and San Joaquin Hills of Orange County, the Santa Monica Mountains of Los Angeles County, and Simi Valley, Ventura County. The late Arikareean fauna appears to represent a new assemblage that has not been recognized previously in the fossil land mammal record of southern California. The presence of a late Uintan assemblage near the base of the undifferentiated Sespe and Vaqueros formations (S/V) in the northern Santa Ana Mountains of Orange County suggests that the base of the unit is similar in age to the base of the Sespe Formation in the Simi Valley area. The top of the S/V in the northern Santa Ana Mountains and San Joaquin Hills of Orange County and in the Santa Monica Mountains of Los Angeles County is approximately 10.4 million years younger than the top the Sespe Formation in the Simi Valley area, at South Mountain, and along Oak Ridge in Ventura County. In the northern Santa Ana Mountains and the Santa Monica Mountains, early Hemingfordian land mammal assemblages occur stratigraphically below late Hemingfordian land mammal assemblages in the overlying marine and continental Topanga Formation. 1 Curator Emeritus, Vertebrate Paleontology Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007. 2 Paleo Environmental Associates, Inc., 2248 Winrock Avenue, Altadena, CA 91001-3205 and Vertebrate Paleontology Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA

Development and Distribution of Lateritic Paleosols in the Paleogene Silverado Formation, Northern Peninsular Ranges, CA

Development and Distribution of Lateritic Paleosols in the Paleogene Silverado Formation, Northern Peninsular Ranges, CA

Regional crustal thickness variations of the Peninsular Ranges, southern California

We used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern Peninsular Ranges, California. Depth to the Moho varies from ∼37 km beneath the western Peninsular Ranges batholith to ∼27 km at the western edge of the Salton trough, an average apparent dip of ∼10° to the west over a lateral distance of 60 km. We previously obtained a similar result for a profile ∼100 km to the south (a Moho dip of ∼20° over 30 km lateral distance). In both cases, the Moho depth variations do not correlate with topography of the eastern batholith, but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes. These observations suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges, and that compensation is through lateral variations in crustal or upper mantle density rather than through an Airy root.

Late Quaternary tectonism along the Pacific coast of the Californias: a contrast in style

Abstract Marine terraces and associated features are examined along 1500 km of coast at the seaward margin of that part of the North American plate that has been captured by the Pacific plate. This coast embraces parts of three morphotectonic provinces, each with its own distinctive tectonic style, namely the southern Coast Ranges, western Transverse Ranges, and northern Peninsular Ranges. Based primarily on the 125 ka shoreline, four magnitudes of late Quaternary deformation are recognized: (1) high uplift rates exceeding 1.0 m per 1000 years; (2) moderate uplift rates between 1.0 and 0.1 m per 1000 years; (3) low uplift rates less than 0.1 m per 1000 years; and (4) variable net subsidence. The highest rates of uplift and subsidence are associated with the intense fold-thrust tectonics of the western Transverse Ranges nearest the Big Bend in the San Andreas fault, and diminish in magnitude to east and west. Moderate uplift rates occur in both the southern Coast Ranges and the northern Peninsular Ranges adjacent to the western Transverse Ranges, commonly associated with block uplift and subsidence between high-angle reverse faults within a broader strike-slip scenario. This suggests that compression and transpression are significant features of these areas and that the continued rotation and relative westward motion of the western Transverse Ranges, within the broader context of shear distributed across the San Andreas fault system, continue to have an impact on regions immediately to the north and south. Uplift diminishes with distance from the Transverse Ranges and is low for Baja California south of Punta Banda. The style of late Quaternary deformation is similar to that defined by earlier Quaternary marine limits, but intensity varies, most notably in the western Transverse Ranges.

SKS splitting beneath southern California

Measurements of SKS phase splitting were obtained from nineteen seismic stations in southern California. The fast polarization directions are 53° at the southern end of the Great Valley, 82±8° in the western Transverse Ranges and northern Peninsular Ranges, 95±4° in Mojave Desert, and 70° on San Clemente Island. The splitting time ranges from 0.8 to 1.8 seconds, which is consistent with an anisotropic layer of 100 to 200 km thick for 4% anisotropy.

A cross section of the Los Angeles Area: Seismically active fold and thrust belt, The 1987 Whittier Narrows earthquake, and earthquake hazard

Retrodeformable cross sections across the Los Angeles area interpret the Pliocene to Quaternary deformation to be a developing basement‐involved fold and thrust belt. The fold and thrust belt is seismically active as evidenced by the 1987 Whittier Narrows earthquake (ML = 5.9) and the 1971 San Fernando earthquake (MW = 6.6). The structural geology of the Los Angeles area is dominated by three major compressional uplift trends: (1) the Palos Verdes anticlinorium and western shelf, (2) the Santa Monica Mountains anticlinorium, and (3) the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains. These trends result from major thrust ramps off a detachment(s) at 10–15 km depth. Thrusts of the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains reach the surface; the other two uplifts are associated with blind thrusts. Compressional seismicity is concentrated along these thrust ramps. The 1987 Whittier Narrows earthquake probably occurred on the Elysian Park thrust which underlies the Santa Monica Mountains anticlinorium. The thrust interpretation accounts for the geometry of the anticlinorium, the seismological characteristics of the earthquake, and the geometry of coseismic uplift. The earthquake and aftershocks occurred within a structurally complex, narrow zone of Miocene and Pliocene northwest trending faults that cross the anticlinorium at a high angle. These northwest trending faults are interpreted to be reactivated faults now behaving as tears in the Elysian Park thrust and not the result of active right‐lateral deformation extending into the Whittier Narrows area. Our analysis suggests the Whittier Narrows earthquake sequence occurred within a structurally weakened zone along the Elysian Park thrust. We also suggest that the Whittier fault is not an important Quaternary structure and may not be seismogenic. The regional cross section is a nonunique solution, and other possible solutions are considered. Multiple solutions arise from the presence of two intersecting compressional belts in the Los Angeles area: the Transverse Ranges and the northern Peninsular Ranges. The belts may be due to one or more regional detachments and the northern Peninsular Ranges may be northeast or southwest vergent. The deformed top of the crystalline basement along the regional cross section requires a minimum of 15.0 km of north‐south convergence between the Palos Verdes Hills and the San Andreas fault regardless of the structural solution. Restoration of our cross section solution requires 21.4 km of north‐south convergence on top of the crystalline basement (including 6.4 km of slip continuing offshore to make structures of the continental borderland) and 29.7 km of convergence on the basal detachment. Geologic relationships suggest major shortening started between early and late Pliocene time (2.2–4.0 Ma) which yields a minimum convergence rate of 3.8–6.8 mm/yr between the Palos Verdes Hills and the San Andreas fault. Convergence rates for our solution range from 5.4 to 13.5 mm/yr between the edge of the continental borderland and the San Andreas fault. Slip rate estimates for the Elysian Park thrust along the eastern portion of the Santa Monica Mountains anticlinorium are 2.5–5.2 mm/yr. If the 1987 Whittier Narrows earthquake is the characteristic earthquake along the 170‐km‐long anticlinorium, then recurrence intervals are estimated to be 5.6–12.1 years. This recurrence is higher than the historic seismic record along the Santa Monica Mountains anticlinorium and suggests that (1) our slip rates are too high, (2) some crustal shortening is taken up aseismically, and/or (3) earthquakes larger than the 1987 Whittier Narrows earthquake will occur along the anticlinorium, but less frequently.

Analysis of the seismicity detected in 1982-1984 in the Northern Peninsular Ranges of Baja California

The San Miguel fault system strikes in the northwest direction within the Peninsular Ranges of Baja California. The study concentrates primarily on the southern sector of the San Miguel fault (SSM) which is about 20 km long and had a large rupture in 1956 . There is a high rate of seismicity along the SSM. In a time period of 23 days, 109 events were located, with duration magnitudes between 1.4 and 2.8. Focal depths are between 5 and 15 km. Epicentral locations show three clusters of seismicity. Two clusters are on the SSM. Another cluster is closer to a normal fault that strikes parallel 8 km to the southwest of SSM. The seismicity is stationary, with an average of 20 events per day. A “ b ” value of 0.92 ± 0.02 is characteristic for the background seismicity. A larger “ b ” value of 1.4 ± 0.07 is associated with the cluster at the northwest end of SSM. Those inconsistent “ b ” values suggest a heterogeneous state of stress . Most fault plane solutions in the Northern Peninsular Ranges of Baja California indicate strike-slip motions (one normal mechanism is also observed). The greatest principal stress is oriented approximately north-south .

Evidence for Late Cretaceous crustal thinning in the Santa Rosa mylonite zone, southern California

The Santa Rosa mylonite zone is part of a 100-km-long belt of deformation along the eastern margin of the northern Peninsular Ranges batholith, southern California. In the northern Santa Rosa Mountains, complexly deformed mylonitic plutonic and metasedimentary tectonites are cut by a stacked sequence of five low-angle faults. Evidence from field relationships and the analysis of minor structures, microstructures, and textures (preferred orientation) suggest that the coaxial component of deformation (“pure shear”) dominated over a noncoaxial (“simple shear”) component. It is proposed that mylonitic deformation and low-angle faulting, which appear to be kinematically associated, are the result of Late Cretaceous extension and thinning of the continental margin.

Provenance and Depositional Mode of Upper Cretaceous Chatsworth Formation Conglomerates, Simi Hills, California: ABSTRACT

Conglomerates of the Upper Cretaceous Chatsworth Formation occur as lenses of concentrated clasts in channels and as clasts dispersed at the base of thick, coarse-grained, graded sandstone beds. The matrix of the conglomerates consists of grains ranging from silt to granule size (4 mm) and comprises between 20 and 78% of any one conglomerate unit. The matrix composition ranges from 30 to 80% quartz, 25 to 60% feldspar, and 5 to 20% lithic fragments, with accessory biotite up to 5%. The conglomerate clast population is composed principally of clasts in the pebble size range. Five distinct rock types are recognized within the conglomerate clast population: blue-black argillite, 4 to 20%; felsic volcanites, 4 to 28%; felsic plutonites, 16 to 24%; arkosic sandstones and silts one, 0 to 12%; and a group of genetically related quartz-rich clasts, 17 to 46%. The quartz-rich clasts include sandstones and siltstones with continuous textural gradation from well-preserved sandstone through partially recrystallized sandstone with sutured grains, into metamorphosed, foliated quartz sandstone and quartz schist types. In addition, a conglomerate unit may contain between 0 and 14% authigenic rip-up clasts. The Chatsworth Formation, as a whole, is recognized to be a deep-sea fan complex upon which the primary depositional agent for sand was turbidity currents. Lenses of concentrated pebble conglomerates originated as debris flow, whereas beds of dispersed pebble clasts are of turbidity current origin. Paleocurrent data and the conglomerate clast composition for the Chatsworth Formation indicate that its detritus was derived from a source terrane to the south of the Simi Hills. The Santa Monica Mountains basement complex contains a large mass of argillite and felsic plutonite, but contains no felsic volcanite or quartz-rich suite of rocks. The basement in the northern Peninsular Ranges includes representatives of the principal rock types recognized in the clasts of the Chatsworth Formation conglomerates and, therefore, it is the best possible choice for the provenance. Extensive Franciscan terrane also lies south of the Chatsworth conglomerates, but no Franciscan detritus is recognized in the Chatsworth Formation. End_of_Article - Last_Page 1686------------

Early 20th-century uplift of the northern Peninsular ranges province of southern California

Repeated leveling in the northern Peninsular Ranges province identifies an early 20thcentury episode of crustal upwarping in southern California. The episodic vertical movement is broadly bracketed between 1897 and 1934, and the main deformation is bracketed within 1906–1914 and involved regional up-to-the-northeast tilting of the Santa Ana block of as much as 4 · 10 −6 rad and elevation changes exceeding 0.4 m in the Perris block and parts of the San Jacinto block, Transverse Ranges, and the Mohave block. Primary tide station records containing occasional entries since 1853 at San Pedro and San Diego show no evidence of episodic crustal movement, suggesting that the uplifted area hinged along coastal fault zones forming the west boundary of the Santa Ana block. Physiographic features and recent studies of Quaternary marine terraces by others show that this episode of regional tilting and uplift is a part of the continuing tectonic process in southern California. A crude, questionable coincidence exists between the uplift episode and a period of increased seismicity (1890–1923) in the northern Peninsular Ranges characterized by a number of moderate-size ( M > 6) earthquakes on NW-trending strike-slip faults. However, releveling data are too sparse to associate the uplift development clearly with any one event.

Seismic structure of the Transverse Ranges, California

Research Article| October 01, 1977 Seismic structure of the Transverse Ranges, California DAVID HADLEY; DAVID HADLEY 1Seismological Laboratory, California Institute of Technology, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar HIROO KANAMORI HIROO KANAMORI 1Seismological Laboratory, California Institute of Technology, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1977) 88 (10): 1469–1478. https://doi.org/10.1130/0016-7606(1977)88<1469:SSOTTR>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 DAVID HADLEY, HIROO KANAMORI; Seismic structure of the Transverse Ranges, California. GSA Bulletin 1977;; 88 (10): 1469–1478. doi: https://doi.org/10.1130/0016-7606(1977)88<1469:SSOTTR>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 Travel-time data obtained from both natural and artificial events occurring in southern California indicate a major, lateral crustal transition within the Transverse Range Province. The eastern crust is very similar to the adjacent Mojave region, where a crustal velocity of 6.2 km/sec is typically observed. The western ranges are dominated by an extensive 6.7 km/sec layer. Pn velocity beneath the western Mojave, Transverse Ranges, and northern Peninsular Ranges is 7.8 km/sec. The crustal thickness of these provinces is 30 to 35 km. The Transverse Ranges do not have a distinct crustal root. Unlike other provinces within southern California, the Transverse Ranges are underlain at a depth of 40 km by a refractor with a P-velocity of 8.3 km/sec. P-delays from a vertically incident, well-recorded teleseism suggest that this velocity anomaly extends to a depth of 100 km. These data indicate that this high-velocity, ridge-like structure is coincident with much of the areal extent of the geomorphic Transverse Ranges and is not offset by the San Andreas fault. Four hypotheses are advanced to explain the continuity of this feature across the plate boundary: (1) dynamic phase change; (2) a coincidental alignment of crust or mantle anomalies; (3) the litho-sphere is restricted to the crust; (4) the plate boundary at depth is displaced from the San Andreas fault at the surface. Within the context of the last model, we suggest the plate boundary at depth is at the eastern end of the velocity anomaly, in the vicinity of the active Helendale-Lenwood-Camprock faults. The regionally observed 7.8 km/sec layer is suggested as a zone of decoupling necessary to accommodate the horizontal shear that must result from the divergence of the crust and upper mantle plate boundaries. The geomorphic Transverse Ranges are viewed as crustal buckling caused by the enhanced coupling between the crust and upper mantle which is suggested by the locally thin, 7.8 km/sec layer. 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.