Salinia revisited: a crystalline nappe sequence lying above the Nacimiento fault and dispersed along the San Andreas fault system,central California

Salinia, as originally defined, is a fault-bounded terrane in westcentral California. As defined, Salinia lies between the Nacimiento fault on the west, and the Northern San Andreas fault (NSAF) and the main trace of the dextral SAF system on the east. This allochthonous terrane was translated from the southern part of the Sierra Nevada batholith and adjacent western Mojave Desert region by Neogene-Quaternary displacement along the SAF system. The Salina crystalline basement formed a westward promontory in the SW Cordilleran Cretaceous batholithic belt, relative to the Sierra Nevada batholith to the north and the Peninsular Ranges batholith to the south, making Salinia batholithic rocks susceptible to capture by the Pacific plate when the San Andreas transform system developed. Proper restoration of offsets on all branches of the San Andreas system is a critical factor in understanding the Salinia problem. When cumulative dextral slip of 171 km (106 mi) along the Hosgri–San Simeon–San Gregorio–Pilarcitos fault zone (S–N), or dextral slip of 200 km (124 mi) along the Hosgri–San Simeon–San Gregorio–Pilarcitos–northern San Andreas fault system, is added to the cumulative dextral slip of 315–322 km (196–200 mi) along the main trace of the SAF north of the San Emigdio–Tehachapi mountains, central California, there is a minimum amount of cumulative dextral slip of 486 km (302 mi) or a maximum amount of cumulative dextral slip of 522 km (324 mi) along the entire SAF system north of the Tehachapi Mountains. When these sums are compared with the offset distance (610–675 km or 379–420 mi) between the batholithic rocks associated with the Navarro structural discontinuity (NSD) in northern California, and those in the ‘tail’ of the southern Sierra Nevada granitic rocks in the San Emigdio–Tehachapi mountains, central California, a minimum deficit of from ～100 km (～62 mi) to a maximum deficit of ～189 km (～118 mi) is needed to restore the crystalline rocks associated with the NSD with the crystalline terranes within the San Emigdio and Tehachapi mountains – the enigma of Salinia. Two principal geologic models compete to explain the enigma (i.e. the discrepancy between measured dextral slip along traces of the SAF system and the amount of separation between the Sierra Nevada batholithic rocks near Point Arena in northern California and the Mesozoic and older crystalline rocks in the San Emigdio and Tehachapi mountains in southern California). (i) One model proposes pre-Neogene (>23 Ma), Late Cretaceous or Maastrichtian (<ca. 71 Ma) to early Palaeocene or Danian (ca. 66 Ma) sinistral slip of 500–600 km (311–373 mi) along the Nacimiento fault and of the western flank of Salinia from the eastern flank of the Peninsular Ranges (sinistral slip but in the opposite sense to later Neogene (<23 Ma) dextral slip along and within the SAF system. (ii) A second model proposes that the crystalline rocks of Salinia comprise a series of 100 km- (60 mi-) scale allochthonous (extensional) nappes that rode southwestward above the Rand schist–Sierra de Salinas (SdS) shear zone subduction extrusion channels. The allochthonous nappes are from NW–SE: (i) Farallon Islands–Santa Cruz Mountains–Montara Mountain, and adjacent batholithic fragments that appear to have been derived from the top of the deep-level Sierra Nevada batholith of the western San Emigdio–Tehachapi mountains; (ii) the Logan Quarry–Loma Prieta Peak fragments that appear to have been derived from the top of a buried detachment fault that forms the basement surface beneath the Maricopa sub-basin of the southernmost Great Valley; (iii) The Pastoria plate–Gabilan Range massif that appears to have been derived from the top of the deep-level SE Sierra Nevada batholith; and (iv) the Santa Lucia–SdS massif, which appears to be lower batholithic crust and underlying extruded schist that were breached westwards from the central to western Mojave Desert region. In this model, lower crustal batholithic blocks underwent ductile stretching above the extrusion channel schists, while mid- to upper-crustal level rocks rode southwestwards and westwards along trenchward dipping detachment faults. Salinian basement rocks of the Santa Lucia Range and the Big Sur area record the most complete geologic history of the displaced terrane. The oldest rocks consist of screens of Palaeozoic marine metasedimentary rocks (the Sur Series), including biotite gneiss and schist, quartzite, granulite gneiss, granofels, and marble. The Sur Series was intruded during Cretaceous high-flux batholithic magmatism by granodiorite, diorite, quartz diorite, and at deepest levels, charnockitic tonalite. Local nonconformable remnants of Campanian–Maastrichtian marine strata lie on the deep-level Salinia basement, and record deposition in an extensional setting. These Cretaceous strata are correlated with the middle to upper Campanian Pigeon Point (PiP) Formation south of San Francisco. The Upper Cretaceous strata, belonging to the Great Valley Sequence, include clasts of the basement rocks and felsic volcanic clasts that in Late Cretaceous time were brought to a coastal region by streams and rivers from Mesozoic felsic volcanic rocks in the Mojave Desert. The Rand and SdS schists of southern California were underplated beneath the southern Sierra Nevada batholith and the adjacent Salinia-Mojave region along a shallow segment of the subducting Farallon plate during Late Cretaceous time. The subduction trajectory of these schists concluded with an abrupt extrusion phase. During extrusion, the schists were transported to the SW from deep- to shallow-crustal levels as the low-angle subduction megathrust surface was transformed into a mylonitic low-angle normal fault system (i.e. Rand fault and Salinas shear zone). The upper batholithic plate(s) was(ere) partially coupled to the extrusion flow pattern, which resulted in 100 km-scale westward displacements of the upper plate(s). Structural stacking, temporal and metamorphic facies relations suggest that the Nacimiento (subduction megathrust) fault formed beneath the Rand-SdS extrusion channel. Metamorphic and structural relations in lower plate Franciscan rocks beneath the Nacimiento fault suggest a terminal phase of extrusion as well, during which the overlying Salinia underwent extension and subsidence to marine conditions. Westward extrusion of the subduction-underplated rocks and their upper batholithic plates rendered these Salinia rocks susceptible to subsequent capture by the SAF system. Evidence supporting the conclusion that the Nacimiento fault is principally a megathrust includes: (i) shear planes of the Nacimiento fault zone in the westcentral Coast Ranges locally dip NE at low angles. (ii) Klippen and/or faulted klippen are locally present along the trace of the Nacimiento fault zone from the Big Creek–Vicente Creek region south of Point Sur near Monterey, to east of San Simeon near San Luis Obispo in central California. Allochthonous detachment sheets and windows into their underplated schists comprise a composite Salinia terrane. The nappe complex forming the allochthon of Salinia was translated westward and northwestward ～100 km (～62 mi) above the Nacimiento megathrust or Franciscan subduction megathrust from SE California between ca. 66 and ca. 61 Ma (i.e. latest Cretaceous–earliest Palaeocene time). Much, or all, of the westward breaching of the Salinia batholithic rocks likely occurred above the extrusion channels of the Rand-SdS schists; following this event, the Franciscan Sur-Obispo terrane was thrust beneath the schists, perhaps during the final stages of extrusion in the upper channel. Later, the Sur-Obispo terrane was partially extruded from beneath the Salinia nappe terrane, during which time the upper plate(s) underwent extension and subsidence to marine conditions. Attenuation of the Salinia nappe sequence during the extrusion of the Franciscan Complex thinned the upper crust, making the upper plates susceptible to erosion from the top of the Franciscan Complex near San Simeon, where it is now exposed. In the San Emigdio Mountains, the relatively thin structural thickness of the upper batholithic plates made them susceptible to late Cenozoic flexural folding and disruption by high-angle dip–slip faults. The ～100 km (～62 mi) of westward and northwestward breaching of the Salinia batholithic rocks above the Rand-SdS channels, and the underlying Nacimiento fault followed by ～510 km (～320 mi) of dextral slip from ～23 Ma to Holocene time along the SAF system, allow for the palinspastic restoration of Salinia with the crystalline rocks of the San Emigdio–Tehachapi mountains and the Mojave terrane, resolving the enigma of Salinia.     

Anthropogenic subsidence in the Mexicali Valley,Baja California,Mexico, and slip on the Saltillo fault

Deep fluid extraction in the Cerro Prieto geothermal field (CPGF) has caused subsidence and induced slip on tectonic faults in the Mexicali Valley (Baja California, Mexico). The Mexicali Valley is located in the southern part of the Salton Trough, at the boundary between the Pacific and North American plates. The Valley is characterized by being a zone of continuous tectonic deformation, geothermal activity, and seismicity. Within the Cerro Prieto pull-apart basin, seismicity is concentrated mainly in swarms, while strong earthquakes have occurred in the Imperial and Cerro Prieto transform faults, that are the eastern and western bound of the basin. Since 1973, fluid extraction at the CPGF has influenced deformation in the area, accelerating the subsidence and causing rupture (frequently as vertical slip or creep) on the surface traces of tectonic faults. Both subsidence and fault slip are causing damage to infrastructure like roads, railroad tracks, irrigation channels, and agricultural fields. Currently, accelerated extraction in the eastern part of CPGF has shifted eastwards the area of most pronounced subsidence rate; this accelerated subsidence can be observed at the Saltillo fault, a southern branch of the Imperial fault in the Mexicali Valley. Published leveling data, together with field data from geological surveys, geotechnical instruments, and new InSAR images were used to model the observed deformation in the area in terms of fluid extraction. Since the electricity production in the CPGF is an indispensable part of Baja California economy, extraction is sure to continue and may probably increase, so that the problem of damages caused by subsidence will likely increase in the future.     

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.     

Four-dimensional transform fault processes: progressive evolution of step-overs and bends

Many bends or step-overs along strike–slip faults may evolve by propagation of the strike–slip fault on one side of the structure and progressive shut-off of the strike–slip fault on the other side. In such a process, new transverse structures form, and the bend or step-over region migrates with respect to materials that were once affected by it. This process is the progressive asymmetric development of a strike–slip duplex. Consequences of this type of step-over evolution include: (1) the amount of structural relief in the restraining step-over or bend region is less than expected; (2) pull-apart basin deposits are left outside of the active basin; and (3) local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes. This type of evolution of step-overs and bends may be common along the dextral San Andreas fault system of California; we present evidence at different scales for the evolution of bends and step-overs along this fault system. Examples of pull-apart basin deposits related to migrating releasing (right) bends or step-overs are the Plio-Pleistocene Merced Formation (tens of km along strike), the Pleistocene Olema Creek Formation (several km along strike) along the San Andreas fault in the San Francisco Bay area, and an inverted colluvial graben exposed in a paleoseismic trench across the Miller Creek fault (meters to tens of meters along strike) in the eastern San Francisco Bay area. Examples of migrating restraining bends or step-overs include the transfer of slip from the Calaveras to Hayward fault, and the Greenville to the Concord fault (ten km or more along strike), the offshore San Gregorio fold and thrust belt (40 km along strike), and the progressive transfer of slip from the eastern faults of the San Andreas system to the migrating Mendocino triple junction (over 150 km along strike). Similar 4D evolution may characterize the evolution of other regions in the world, including the Dead Sea pull-apart, the Gulf of Paria pull-apart basin of northern Venezuela, and the Hanmer and Dagg basins of New Zealand.     

The kinematic paradox of the San Andreas Fault

The Pacific-North America plate boundary along the San Andreas fault system is notoriously a right-lateral transpressive margin where both almost pure thrust and strike-slip tectonics take place. The Pacific plate travels WNW, forming an angle of about 25° with the boundary. Since the Pacific is moving WNW faster than North America, right lateral transtension should result along the San Andreas system. North America, in turn, travels westward obliquely to the boundary and a left-lateral transpressive component would be expected along the same margin. Therefore, the right-lateral transpression of the San Andreas system can be partitioned into (i) a sinistral transpression along the southwestern margin of the North America plate obliquely overriding (ii) a faster right lateral transtension occurring along the transfer margin of the Pacific plate between the East Pacific rise in the California Gulf and the Gorda ridge to the north-west. This is due to the oblique trend of the Pacific and North America plate margins with respect to their motion in a absolute reference frame.
The geodynamics of California is marked by a unique setting in which there is a special subduction where, in contrast with classic subduction zones, the footwall of the subduction plane is obliquely diverging from the hanging wall in an E-W section, while it is converging at slower rates in a NE-SW direction. The extensional E-W component is absorbed into the Basin and Range rifting, whereas the compressive NE-SW component is mainly expressed in the Coast Ranges and California offshore. The compression perpendicular to the San Andreas is then not intrinsic in the strike-slip movement, but it is rather an independent tectonic factor. Therefore, the San Andreas system cannot be considered as an archetype of a pure strike slip fault.     

Changes in rate of fault creep

Aseismic slip or fault creep is occurring on many faults in California. Although the creep rates are generally less than 10 mm/yr in most regions, the maximum observed rate along the San Andreas fault between San Juan Bautista and Gold Hill in central California exceeds 30 mm/yr. Changes in slip rates along a 162 km segment of the San Andreas fault in this region have occurred at approximately the same time at up to nine alinement array sites. Rates of creep on the fault near the epicenters of moderate earthquakes (M_L 4–6) vary for periods of several years, decreasing before the main shocks and increasing thereafter, in agreement with prior observations based on creepmeter results. The change of surface slip rate is most pronounced within the epicentral region defined by aftershocks, but records from sites at distances up to 100 km show similar variations. Additionally, some variations in rate, also apparently consistent among many sites, have a less obvious relation with seismic activity and have usually taken place over shorter periods. Not all sites exhibit a significant variation in rate at the time of a regional change, and the amplitudes of the change at nearby sites are not consistently related. The time intervals between measurements at the nine array sites during a given period have not always been short with respect to the intervals between surveys at one site; hence, uneven sampling intervals may bias the results slightly. Anomalies in creep rates thus far observed, therefore, have not been demonstrably consistent precursors to moderate earthquakes; and in the cases when an earthquake has followed a long period change of rate, the anomaly has not specified time, place, or magnitude with a high degree of certainty. The consistency of rate changes may represent a large scale phenomenon that occurs along much of the San Andreas transform plate boundary.     

Palaeoseismology in steep terrain: The Big Bend of the San Andreas fault, Transverse Ranges, California

Transpressional tectonics are typically associated with restraining bends on major active strike-slip faults, resulting in uplift and steep terrain. This produces dynamic erosional and depositional conditions and difficulties for established lines of palaeoseismological investigation. Consequently, in these areas data are lacking to determine tectonic behaviour and future hazard potential along these important fault segments. The Big Bend of the San Andreas fault in the Transverse Ranges of southern California exemplifies these problems. However, landslides, probably seismically triggered, are widespread in the rugged terrain of the Big Bend. Fluvial reworking of these deposits rapidly produces geomorphic planes and lines that are markers for subsequent fault slip. The most useful are offset and abandoned stream channels, for these are relatively high precision markers for identifying individual faulting events. Palaeoseismological studies from the central Big Bend, involving ¹⁴C ages of charcoal fragments from trench exposures, illustrate these points and indicate that the past three faulting events, including the great 1857 earthquake, were relatively similar in scale, each producing offsets of about 7–7.5 m. The mean recurrence interval is 140–220 years. The pre-1857 event here may be the 1812 event documented south of the Big Bend or an event which took place probably between 1630 and 1690. Ground breakage in both events extended south of the Big Bend, unlike the 1857 event where rupture was skewed to the north. The preceding faulting event ruptured both to the north and south of the Big Bend and probably occurred between 1465 and 1495. All these events centred on the Big Bend and may be typical for this fault segment, suggesting that models of uniform long-term slip rates may not be applicable to the south-central San Andreas. A slip-rate estimate of 34–51 mm a^{− 1} for the central Big Bend, although uncertain, may also imply higher slip in the Big Bend and highlights difficulties in correlating slip-rates between sites with different tectonic settings. Slip rates on the San Andreas may increase within the broad compressional tectonics zone of the Big Bend, compared to the north and south where the plate boundary is a relatively linear and sub-parallel series of dominantly strike-slip faults. Partitioning slip within the Big Bend is inherently uncertain and insufficient suitably comparable data are available to sustain a uniform slip model, although such models are a common working assumption.     

Structural characteristics of the Altar Basin,Northwest Sonora,Mexico

Eight two-dimensional, multichannel seismic reflection lines were acquired, processed, and interpreted to study the structure of the Altar Basin, which is part of the Salton Trough tectonic province. We identified two basin-bounding zones characterized by different degrees of strain: the Cerro Prieto–Altar deformation zone (CPADZ) and the Altar–Caborca deformation zone (ACDZ). The CPADZ is bounded on the west by the Cerro Prieto fault and on the east by the Altar fault. To the north, the strike of both faults changes slightly from a NW to more NNW direction. In the CPADZ, the thickness of the crust decreases southward towards the Gulf of California, and is associated with a deformation-developing fault. The CPADZ has a rotation component orientating these faults in an oblique direction to the Cerro Prieto fault, whereas within the ACDZ, a geometric coherence of synthetic and antithetic faults exists, creating horsts and graben striking N37° W. The Altar fault is recognized by basement interruption, with a vertical component of ～1 km, striking at N37° W and dipping 83° SW. On the northeastern side of the Altar Basin, the basement configuration shows that the minimum time of basement record (～0.4 s of two-way travel time) and the time curve gradient decrease in the NE–SW direction. The depocentre is ～6 km deep in the central-west portion of the basin. We identified a graben between the Rosario and Tinajas Altas mountains (Rosario Basin). The extension–connection of the Altar and Rosario basins to the south is not well defined; nevertheless, these basins could represent the link between the Colorado River and the Gulf of California during the late Miocene, whereas this link was abandoned in the Pliocene as subsidence migrated towards the northwest into the Cerro Prieto and Laguna Salada basins.     

Fault location and fault activity assessment by analysis of historic level line data, oil-well data and groundwater data, Hollywood area, California

Transecting the Los Angeles metropolitan area in a general E-W direction are major north-dipping reverse faults comprising the Santa Monica—Raymond Hill fault zone, a segment of the frontal fault system separating the Transverse Ranges from the Peninsular Ranges geomorphic provinces of southern California. Pleistocene or Holocene movement is evident along some segments of these faults, but urban development precludes accurate location and assessment of Quaternary movement by conventional mapping techniques. At present no conclusive evidence of Holocene surface rupture has been found onshore west of the Raymond Hill segment of the fault zone, but the geologic conditions and urban development in the area are such that the possibility of Holocene movement cannot be excluded at this time. Groundwater barriers in Pleistocene sediments are indicative of Quaternary faulting on the Santa Monica fault segment west of the Newport—Inglewood fault zone. Most literature indicates that movement along the Beverly Hills—Hollywood segment east of the Newport—Inglewood fault zone terminated in Late Miocene or Pliocene time, and there is no general agreement on the location of faults in this segment. However, recent work by the Division of Mines and Geology, by Geotechnical Consultants, Inc., and others suggests that the Santa Monica fault transecting the Hollywood area is associated with a zone of differential subsidence that varies from 100 to 400 m wide, depending on the resolution of repeated leveling survey data and with a groundwater barrier determined from analysis of oil-well and water-well data. Additional exploration is essential to test our present geologic model and to evaluate the earthquake hazard and seismic risk of faults in the area.     

GPS-derived strain in northwestern California: Termination of the San Andreas fault system and convergence of the Sierra Nevada–Great Valley block contribute to southern Cascadia forearc contraction

GPS-derived velocities (1993–2002) in northwestern California show that processes other than subduction are in part accountable for observed upper-plate contraction north of the Mendocino triple junction (MTJ) region. After removing the component of elastic strain accumulation due to the Cascadia subduction zone from the station velocities, two additional processes account for accumulated strain in northern California. The first is the westward convergence of the Sierra Nevada–Great Valley (SNGV) block toward the coast and the second is the north–northwest impingement of the San Andreas fault system from the south on the northern California coastal region in the vicinity of Humboldt Bay. Sierra Nevada–Great Valley block motion is northwest toward the coast, convergent with the more northerly, north–northwest San Andreas transform fault-parallel motion. In addition to the westward-converging Sierra Nevada–Great Valley block, San Andreas transform-parallel shortening also occurs in the Humboldt Bay region. Approximately 22 mm/yr of distributed Pacific–SNGV motion is observed inland of Cape Mendocino across the northern projections of the Maacama and Bartlett Springs fault zones but station velocities decrease rapidly north of Cape Mendocino. The resultant 6–10 mm/yr of San Andreas fault-parallel shortening occurs above the southern edge of the subducted Gorda plate and at the latitude of Humboldt Bay. Part of the San Andreas fault-parallel shortening may be due to the viscous coupling of the southern edge of the Gorda plate to overlying North American plate. We conclude that significant portions of the upper-plate contraction observed north of the MTJ region are not solely a result of subduction of the Gorda plate but also a consequence of impingement of the western edge of the Sierra Nevada–Great Valley block and growth of the northernmost segments of the San Andreas fault system.     

An archaeoseismic approach and method for the study of active strike-slip faults

The use of archaeology to study earthquake hazards provides a human dimension to an issue of modern societal concern. We developed an archaeoseismic approach to the study of prehistoric earthquakes on active strike-slip faults. This approach employs a combination of standard archaeological and paleoseismic techniques. We have successfully applied this approach and its attendant methods to an archaeological site that straddles and has been offset by the San Andreas fault in northern coastal California. Resultant fault parameters, including cumulative rate of slip and timing of the penultimate event, are comparable to results of strictly paleoseismic investigations at other sites on this fault. The archaeoseismic approach furnishes a number of advantages over geologic studies in terms of the availability and number of potential study sites, the abundance of datable materials, and the array of potential piercing features with which to constrain fault history. © 1997 John Wiley & Sons, Inc.     

Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico)

The El Arco porphyry copper deposit is located in central Baja California and is a resource containing >600 Mt of ore with ∼0.6% copper. It was emplaced within a relatively primitive Jurassic island arc and it was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith. The porphyritic stock intrusion and ore formation at El Arco has recently been dated at ∼165 Ma [Valencia et al. GEOS 24:189 (2004)]. This age is much older than Aptian-Albian K–Ar ages previously reported from El Arco [Barthelmy, Geology of El Arco-Calmallí area, Baja California, México. MSc Thesis, San Diego State University, CA (1975); Baja California Geology. San Diego State University, CA, pp 127–138 (1979)]. The copper mineralization at El Arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas. Mafic dikes that intruded the deposit are not mineralized, but they are affected by post-ore low-grade metamorphism. The dikes are compositionally the most primitive rocks, while host rock andesites and the porphyry stock display typical volcanic arc characteristics. The Pb isotope data from sulfides, feldspars, quartz, and whole-rock samples indicate that: (1) the copper-bearing porphyry stock and the surrounding andesites evolved from a similar source with an average μ-value of 9.43; (2) no external Pb was added during mineralization; (3) some Pb isotope compositions were slightly disturbed by a later metamorphic event. Strontium and Nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust. Our data favor a model for the formation of the El Arco deposit linked to a Triassic to Jurassic intra-oceanic arc system, cropping out at the western margin of central Baja California in the Cedros-Vizcaíno region. The intra-oceanic arc together with the El Arco deposit was accreted to the active continental margin of North America and metamorphosed during the Early Cretaceous. This model is in disagreement to earlier models that favor the El Arco deposit formation being linked to the Cretaceous continental margin.     

Cretaceous to Cenozoic sequential kinematics in the forearc–arc transition: effects of changing oblique plate convergence and the San Andreas system with implications for the La Paz fault (southern Baja California, Mexico)

We studied metasediments and mylonitic arc granitoids from the forearc–arc transition of southern Baja California, Mexico. Thin section analyses and field evidence show that metamorphism of the forearc–arc transition is of the high T/P active margin type. The heat was provided by Cretaceous arc intrusions. Field observations and thin section analyses, including the time/temperature deformation path, demonstrate that the study area was first affected by dextral, ductile shearing followed by ductile, sinistral, possibly transpressive strike-slip parallel to the magmatic arc during the Cretaceous. Both intervals are related to changing oblique plate convergence and, thus, identified as trench-linked strike-slip effects. The geometric relationship between arc-dipping foliation, stretching lineation and shear sense indicates that the arc may have been pressed onto the rocks of the study area during sinistral shearing. The sinistral interval lasted up until regional cooling (Early Cenozoic?). Because the La Paz fault is closely associated with the forearc–arc transition, it must have the same Cretaceous to Early Cenozoic kinematic history. The northern segment of the La Paz fault is a modern, brittle, strike-slip fault interpreted as a dextral synthetic fault of the San Andreas system which opened the Gulf of California (Mar de Cortés/Golfo de California). We found no evidence for Miocene Basin and Range extension.     

Spatial radon anomalies on active faults in California

Radon emanation has been observed to be anomalously high along active faults in many parts of the world. We tested this relationship by conducting and repeating soil-air radon surveys with a portable radon meter across several faults in California. The results confirm the existence of fault-associated radon anomalies, which show characteristic features that may be related to fault structures but vary in time due to other environmental changes, such as rainfall. Across two creeping faults in San Juan Bautista and Hollister, the radon anomalies showed prominent double peaks straddling the fault-gouge zone during dry summers, but the peak-to-background ratios diminished after significant rain fall during winter. Across a locked segment of the San Andreas fault near Olema, the anomaly has a single peak located several meters southwest of the slip zone associated with the 1906 San Francisco earthquake. Across two fault segments that ruptured during the magnitude 7.5 Landers earthquake in 1992, anomalously high radon concentration was found in the fractures three weeks after the earthquake. We attribute the fault-related anomalies to a slow vertical gas flow in or near the fault zones. Radon generated locally in subsurface soil has a concentration profile that increases three orders of magnitude from the surface to a depth of several meters; thus an upward flow that brings up deeper and radon-richer soil air to the detection level can cause a significantly higher concentration reading. This explanation is consistent with concentrations of carbon dioxide and oxygen, measured in soil-air samples collected during one of the surveys.     

Particle-size distributions of low-angle normal fault breccias: Implications for slip mechanisms on weak faults

Slip on low-angle normal faults is not well understood because they slip at high angles to the maximum principal stress directions. These faults are considered weak and their motion cannot be explained using standard Byerlee friction and Andersonian fault mechanics. One proposed mechanism for weak fault slip is reduction of effective normal stress induced by high pore-fluid pressure. This mechanism is likely to allow dilation of the fault zone and, therefore, affect the particle-size distribution of fault breccia, which has been shown to differ for unconstrained versus constrained comminution. High pore-fluid pressure can cause dilation which leads to unconstrained comminution. We analyze samples from the footwalls of two low-angle normal faults in southern California (West Salton and Whipple detachment faults) to determine the fault-rock textures and grain-size distributions (GSDs). The GSDs are fractal with fractal dimensions ranging from ∼2.6 to 3.4. The lower end of this range is thought to reflect constrained comminution and only occurs in samples from the footwall of a small-offset “minidetachment” fault about 100 m below the Whipple detachment. The higher fractal dimensions are common in cataclasites related to the main faults and also reflect constrained comminution but are overprinted by shear localization. Our GSDs are similar to those from natural and laboratory-deformed fault rocks from strong faults. We conclude that if high pore-fluid pressure aided slip on these faults, it did not strongly affect mechanisms by which brecciation occurs, implying that fluid pressure generally was sublithostatic. Independent evidence exists for lithostatic fluid pressure that having dropped or cycled to hydrostatic levelsin the minidetachment, but our GSD results suggest that periods of high fluid pressure were too short or infrequent for unconstrained comminution to have been the dominant cataclastic mechanism. Fractal dimensions of ∼2.6 for these samples suggest that little subsequent abrasion occurred due to shear localization, consistent with minor offset on the minidetachment. Main detachment footwall samples with fractal dimensions ≥3 reflect constrained comminution followed by shear-related abrasion, and suggest that seismic cycling was important in formation of main detachment cataclasites.     

Detrital zircon ages in Palaeozoic and Mesozoic basement assemblages of the Peninsular Ranges batholith,Baja California,Mexico: constraints for depositional ages and provenance

The origin and continuity of Phanerozoic lithostratigraphic terranes in southern and Baja California remain an unsolved issue in Cordilleran tectonics. We present data from eight detrital zircon samples collected across the southern extent of the Peninsular Ranges that help constrain the provenance of detritus and the depositional ages of these basement units. Detrital zircon signatures from units in the eastern Peninsular Ranges correlate with Palaeozoic passive margin assemblages in the southwestern North American Cordillera. Units in the central belt, which consists of Triassic–Jurassic metasedimentary turbidite assemblages that probably deformed in an accretionary prism setting, and Cretaceous metasedimentary and metavolcanic units that represent the remnants of a continental margin arc, were derived from both proximal and more distal sources. The westernmost units, which are locally structurally interleaved with the Triassic through Cretaceous units of the central belt, are Cretaceous deposits that represent a series of collapsed basin complexes located within and flanking the Cretaceous Alisitos volcanic island arc. Cretaceous intra-arc units show little influx of cratonal material until approximately 110 Ma, whereas coeval sediments on the northern and eastern flanks of the Alisitos arc contain abundant cratonal detritus. Intra-arc strata younger than approximately 110 Ma contain large amounts of Proterozoic and older detrital zircons. These data suggest that basins associated with the Alisitos arc were either too distant or somehow shielded from North American detritus before 110 Ma. In the case of the former, increased influx of continental detritus after 110 Ma would support a tectonic model in which the arc was separated from North America by an ocean basin and, as the arc approached the continent, associated depositional centres were close enough to receive input from continental sources.     

What can strike‐slip fault spacing tell us about the plate boundary of western North America?

The spacing of parallel continental strike‐slip faults can constrain the mechanical properties of the faults and fault‐bounded crust. In the western US, evenly spaced strike‐slip fault domains are observed in the San Andreas (SA) and Walker Lane (WL) fault systems. Comparison of fault spacing (S) vs. seismogenic zone thickness (L) relationships of the SA and WL systems indicates that the SA has a higher S/L ratio (~8 vs. 1, respectively). If a stress‐shadow mechanism guides parallel fault formation, the S/L ratio should be controlled by fault strength, crustal strength, and/or regional stress. This suggests that the SA‐related strike‐slip faults are relatively weaker, with lower fault friction: 0.13–0.19 for the SA vs. 0.20 for WL. The observed mechanical differences between the San Andreas and Walker Lane fault systems may be attributed to variations in the local geology of the fault‐hosting crust and/or the regional boundary conditions (e.g. geothermal gradient or strain rate).     

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone,Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.     

A microstructural study of fault rocks from the SAFOD: Implications for the deformation mechanisms and strength of the creeping segment of the San Andreas Fault

The San Andreas Fault zone in central California accommodates tectonic strain by stable slip and microseismic activity. We study microstructural controls of strength and deformation in the fault using core samples provided by the San Andreas Fault Observatory at Depth (SAFOD) including gouge corresponding to presently active shearing intervals in the main borehole. The methods of study include high-resolution optical and electron microscopy, X-ray fluorescence mapping, X-ray powder diffraction, energy dispersive X-ray spectroscopy, white light interferometry, and image processing.The fault zone at the SAFOD site consists of a strongly deformed and foliated core zone that includes 2–3 m thick active shear zones, surrounded by less deformed rocks. Results suggest deformation and foliation of the core zone outside the active shear zones by alternating cataclasis and pressure solution mechanisms. The active shear zones, considered zones of large-scale shear localization, appear to be associated with an abundance of weak phases including smectite clays, serpentinite alteration products, and amorphous material. We suggest that deformation along the active shear zones is by a granular-type flow mechanism that involves frictional sliding of microlithons along phyllosilicate-rich Riedel shear surfaces as well as stress-driven diffusive mass transfer. The microstructural data may be interpreted to suggest that deformation in the active shear zones is strongly displacement-weakening. The fault creeps because the velocity strengthening weak gouge in the active shear zones is being sheared without strong restrengthening mechanisms such as cementation or fracture sealing. Possible mechanisms for the observed microseismicity in the creeping segment of the SAF include local high fluid pressure build-ups, hard asperity development by fracture-and-seal cycles, and stress build-up due to slip zone undulations.     

Holocene displacement along branch and secondary faults in the San Andreas fault zone, southern California

Detailed geologic mapping of the San Andreas fault zone in Los Angeles County since 1972 has revealed evidence for diverse histories of displacement on branch and secondary faults near Palmdale. The main trace of the San Andreas fault is well defined by a variety of physiographic features. The geologic record supports the concept of many kilometers of lateral displacement on the main trace and on some secondary faults, especially when dealing with pre-Quaternary rocks. However, the distribution of upper Pleistocene rocks along branch and secondary faults suggests a strong vertical component of displacement and, in many locations, Holocene displacement appears to be primarily vertical. The most recent movement on many secondary and some branch faults has been either high-angle (reverse and normal) or thrust. This is in contrast to the abundant evidence for lateral movement seen along the main San Andreas fault. We suggest that this change in the sense of displacement is more common than has been previously recognized.The branch and secondary faults described here have geomorphic features along them that are as fresh as similar features visible along the most recent trace of the San Andreas fault. From this we infer that surface rupture occurred on these faults in 1857, as it did on the main San Andreas fault. Branch faults commonly form “Riedel” and “thrust” shear configurations adjacent to the main San Andreas fault and affect a zone less than a few hundred meters wide. Holocene and upper Pleistocene deposits have been repeatedly offset along faults that also separate contrasting older rocks. Secondary faults are located up to 1500 m on either side of the San Andreas fault and trend subparallel to it. Moreover, our mapping indicates that some portions of these secondary faults appear to have been “inactive” throughout much of Quaternary time, even though Holocene and upper Pleistocene deposits have been repeatedly offset along other parts of these same faults. For example, near 37th Street E. and Barrel Springs Road, a limited stretch of the Nadeau fault has a very fresh normal scarp, in one place as much as 3 m high, which breaks upper Pleistocene or Holocene deposits. This scarp has two bevelled surfaces, the upper surface sloping significantly less than the lower, suggesting at least two periods of recent movement. Other exposures along this fault show undisturbed Quaternary deposits overlying the fault. The Cemetery and Little Rock faults also exhibit selected reactivation of isolated segments separated by “inactive” stretches.Activity on branch and secondary faults, as outlined above, is presumed to be the result of sympathetic movement on limited segments of older faults in response to major movement on the San Andreas fault. The recognition that Holocene activity is possible on faults where much of the evidence suggests prolonged inactivity emphasizes the need for regional, as well as detailed site studies to evaluate adequately the hazard of any fault trace in a major fault zone. Similar problems may be encountered when geodetic or other studies, Which depend on stable sites, are conducted in the vicinity of major faults.