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Introduction to the Special Issue on Paleoseismology of the San Andreas Fault System
Liquefaction
Introduction
Geologic and Tectonic Setting
Late holocene slip rate and earthquake history for the northern Calaveras fault at Welch Creek, eastern San Francisco Bay area, California
Kinematics of transpressional deformation in the eastern San Francisco Bay region, California
The northern San Gregorio fault zone: Evidence for the timing of late Holocene earthquakes near Seal Cove, California
Empirical observations regarding reverse earthquakes, blind thrust faults, and quaternary deformation: Are blind thrust faults truly blind?
Correlation, ages, and uplift rates of Quaternary marine terraces: South-central coastal California
Emergent Quaternary marine terraces are present along most of the south-central California coastline from San Simeon on the north to the Santa Maria Valley on the south. Detailed mapping of these terraces provides new data for assessing the locations, style, and rates of Quaternary deformation in the region. The distribution, correlation, and ages of the terraces have been studied near San Simeon and on the flanks of the San Luis Range between Morro Bay and the Santa Maria Valley. In the San Simeon study area, sequences of four and five marine terraces have been mapped to the northeast and southwest, respectively, of the southern onshore reach of the San Simeon fault zone. From youngest to oldest, they are the Point (Q p ), San Simeon (Q s ), Tripod (Q t ), Oso (Q o ), and La Cruz (Q lc ) terraces. They are interpreted to correlate with marine oxygen isotope stages 3 or 5a (60 or 80 ka), 5a or 5c (80 or 105 ka), 5e (120 ka), 7 (210 ka), and 9 (330 ka). A uranium-series age of 46 ± 2 ka and a weighted mean average thermoluminescence age of 95 ± 13 ka have been obtained for samples collected from the lowest two emergent terraces, respectively, on the southwestern side of the fault zone. Estimated ages and correlation of terraces across the San Simeon fault zone are based on lateral correlation of the Tripod (Q t ) terrace to the well-dated ∼120-ka Cayucos terrace, comparison of relative soil profile development, and comparison of geomorphic expression and terrace altitudinal spacing. Comparison of the relative altitudinal spacing of terraces with paleosea-level curves developed from worldwide data indicates uplift rates of approximately 0.17 ± 0.02 m/kyr southwest, and 0.16 ± 0.01 m/kyr northeast of the fault, and approximately 0.24 m/kyr for the uplifted and warped areas within the fault. Terrace altitudinal spacing for the lowest three terraces on San Simeon Point, however, indicates that uplift during the past 120,000 yr in this area has not been uniform adjacent to the active traces of the San Simeon fault zone. In the San Luis Range study area, a flight of at least 12 elevated marine terraces is present between Morro Bay and the northwestern margin of the Santa Maria Valley. The lower two terraces (Q 1 and Q 2 ) in this sequence are interpreted to correlate to marine oxygen isotope substages 5a (80 ka), and 5e (120 ka), respectively. These correlations are well constrained by 12 uranium-series ages of coral and vertebrate bone samples, 12 amino acid racemization analyses, and 14 paleoclimatic analyses of invertebrate faunal assemblages. The ages of the lower two terraces provide local calibration of the terrace sequence for correlation with paleosea-level curves developed from worldwide data. For terraces equal to or younger than about 330 ka, we have estimated terrace ages and uplift rates by correlating shoreline angle altitude and terrace altitudinal spacing to these curves. Uplift rates based on the present altitude of the 120-ka terrace in this region range from approximately 0.06 to 0.23 m/kyr. Late Pleistocene uplift rates throughout the entire coastal region between San Simeon to the Santa Maria Valley are comparable to rates observed elsewhere in California, which are approximately 0.1 to 0.3 m/kyr in tectonic regimes characterized by predominantly strike-slip faulting. The rates are considerably less than maximum rates of 3 to 5 m/kyr for the region directly south of the Mendocino Triple Junction, 5 to 7 m/kyr for areas characterized by significant crustal shortening, such as the Ventura anticline in the Transverse Ranges, and 0.8 m/kyr for the Santa Cruz Mountains region adjacent to the restraining bend in the San Andreas fault. Estimates of the position of sea level (with respect to the present) during the ∼80-ka sea-level highstand range from about −19 m to near the present level. Estimated paleosea level during the ∼80-ka high stand in the San Luis Range study area, assuming a +6 m paleosea-level estimate for the ∼120-ka terrace and uniform uplift since formation of the ∼120-ka terrace, is −4 ± 1 m (relative to present sea level). This value is in general agreement with other recent estimates from coastal California, Mexico, and Japan, but is significantly higher than previous estimates from New Guinea and Barbados.
The Los Osos fault zone is a west-northwest-trending reverse fault in the Pacific coastal region of San Luis Obispo County, California. The fault zone extends as a discontinuous en echelon zone from the Hosgri fault zone in Estero Bay southeast to an intersection with the West Huasna fault zone near Twitchell Reservoir, a distance of up to 57 km. The fault zone is divided into four segments based on distinct changes in recency of activity and slip rate along the fault: abrupt changes in elevation of the bordering San Luis Range, en echelon separation of fault traces, intersection with known or inferred branching or crossing structures (e.g., faults, subsiding basins), and changes in geomorphic expression from a range-front fault to an intrarange fault. From northwest to southeast, we propose naming the segments the Estero Bay, Irish Hills, Lopez Reservoir, and Newsom Ridge segments. The Estero Bay segment, 11 to 15 km long, lies primarily offshore in Estero Bay. Its recency of activity is unknown. The segment is poorly imaged on seismic reflection data and is weakly expressed in sea-floor bathymetry, suggesting a low rate of late Quaternary activity. The Irish Hills segment, which is 17 to 21 km long, exhibits the strongest expression of Holocene activity and is a well-defined range-front fault. Detailed mapping of marine terraces and trenching of fluvial deposits show that this segment has had recurrent late Pleistocene and Holocene movement at a long-term slip rate of 0.2 to 0.4 mm/yr. The adjacent Lopez Reservoir segment is a 15- to 19-km-long, poorly defined range-front fault that displaces older Quaternary alluvium. Detailed mapping and trenching indicate no Holocene activity. The Newsom Ridge segment is an 8-km-long, intrarange fault that has poor geomorphic expression and appears not to displace late Pleistocene deposits.
The San Luis Range, a prominent west-northwest-trending topographic and structural high along the coast of south-central California, is one of a series of elongated structural blocks in the Los Osos/Santa Maria (LOSM) domain. The range is uplifting as a relatively rigid crustal block along bordering northwest-striking reverse faults. Altitudes and ages of marine terraces show that the range is uplifting at rates of between 0.12 and 0.23 m/kyr, with little or no internal deformation. Major geologic structures within the range, including the Pismo syncline and the San Miguelito, Edna, and Pismo faults, do not deform Quaternary deposits or landforms and are not active structures in the contemporary tectonic setting. The northeastern margin of the range is bordered by the Los Osos fault zone, a southwest-dipping reverse fault that separates the uplifting San Luis Range from the subsiding or southwest-tilting Cambria block to the northeast. The fault zone has had recurrent late Pleistocene and Holocene displacement at a long-term slip rate of 0.2 to 0.7 mm/yr. Uplift of the range is accommodated, entirely or in part, by displacement along this fault zone. The southwest margin of the San Luis Range is bordered by a complex system of late Quaternary reverse faults that separates the range from the subsiding Santa Maria Basin to the southwest. The fault system includes the Wilmar Avenue, San Luis Bay, Olson, Pecho, and Oceano faults, all of which dip moderately to steeply to the northeast. The cumulative net dip-slip rate of displacement for this system of faults ranges from about 0.16 to about 0.30 mm/yr. Slip rates on individual faults generally range from 0.04 to 0.11 mm/yr. We infer that the style and rates of deformation occurring within and bordering the San Luis Range are representative of the style and rates of deformation occurring elsewhere in the LOSM domain. Crustal shortening in the domain is accommodated primarily by reverse faulting along the margins of structural blocks and by uplift, subsidence, or tilting of the blocks. In the southern and southeastern parts of the domain, crustal shortening also may be accommodated by active folding and thrust faulting. The west-northwest structural grain and tectonic style within the LOSM domain is unique in the south-central coastal California region, and is transitional between the west-trending structural grain of the western Transverse Ranges and the north-northwest-trending grain of the Santa Lucia and San Rafael Ranges. We interpret that Quaternary deformation within the domain is related to transpression along the North America/Pacific plate margin, renewed late Cenozoic clockwise rotation of the western Transverse Ranges, and convergence of the domain against the relatively stable Salinian crust that underlies much of the Santa Lucia and San Rafael Ranges to the northeast.
Estimated Pleistocene slip rate for the San Simeon fault zone, south-central coastal California
The San Simeon fault zone disrupts a flight of emergent marine terraces and offsets a series of drainages near San Simeon Point along the coast of south-central California. Detailed studies of the offset marine terraces and drainages have provided data that we have used to estimate the late Pleistocene slip rate for this fault zone. In this study, we mapped four and five marine terraces to the northeast and southwest, respectively, of the southern onshore reach of the San Simeon fault zone. These terraces correlate with sea-level highstands at ∼60 or 80, ∼80 or 105, ∼120, ∼210, and ∼330 ka. The marine terrace strandlines are displaced by the San Simeon fault zone along two or possibly three primary fault traces within a zone of shearing and warping up to 500 m wide. Ratios of horizontal to vertical slip are 8:1 to greater than 50:1, demonstrating that the fault is predominantly a right-lateral strike-slip fault. Estimated slip rates based on the present locations of strandlines for the San Simeon (80 or 105 ka), Tripod (120 ka), and Oso (210 ka) terraces, and paleogeographic reconstructions of the shoreline configurations during their development, range from about 0.4 to 11 mm/yr, with the best constrained values ranging from 1 to 3 mm/yr. Slip rates based on deflections and apparent offset of drainages across the primary active traces of the San Simeon fault zone are in agreement with the 1-t o 3-mm/yr values estimated from the marine terrace study. The San Simeon fault zone, therefore, accommodates a significant amount of transpressional strain along the North America-Pacific plate margin. The fault zone is part of the larger San Gregorio-San Simeon-Hosgri system of near-coastal faults. The geologically determined slip rate of 1 to 3 mm/yr is comparable to geodetically modeled estimates of fault-parallel shear west of the San Andreas fault.
Assessment of the Style and Timing of Surficial Deformation Along the Central Reelfoot Scarp, Lake County, Tennessee
FIELD GUIDE TO THE TECTONIC BOUNDARY BETWEEN THE NORTHERN SAN JOAQUIN VALLEY AND THE CENTRAL DIABLO RANGE
CENOZOIC STRATIGRAPHY OF THE NORTHERN SAN JOAQUIN VALLEY, CENTRAL CALIFORNIA
ABSTRACT Cenozoic deposits in the northern San Joaquin Valley are thin and incomplete, but they are of potential interest because they are the record of Cenozoic tectonic, volcanic, and climatic events in the surrounding regions. The lower Paleogene section of the northern San Joaquin Valley is represented by two marine sequences. The section above the Eocene is virtually all nonmarine and is separated from the older Paleogene section by an unconformity that marks a widespread late Eocene and Oligocene regression. Lithologic similarities between upper Eocene to upper Miocene sequences exposed on the east and west sides of the northern San Joaquin Valley strongly suggest a one-to-one correlation of units. Lithologic equivalents of the east-side units— the lone, Valley Springs, and Mehrten Formations—are recognized on the west side of the valley, mostly as part of what had been mapped in the past as the San Pablo Formation. The earliest significant uplift of the Diablo Range is evidenced by a radiolarian chert-bearing conglomerate of Eocene age. The post-Eocene unconformity indicates a period of significant regional deformation. A change from Sierran to Coast Range provenance in the late Miocene indicates major uplift of the Diablo Range. Finally, an angular unconformity in the latest Pliocene or Pleistocene marks the beginning of the last major uplift of the range.
ABSTRACT Stratigraphic relationships and patterns of Quaternary tectonic-geomorphic development together record a complex history of late Cenozoic deformation and crustal shortening at the northeastern Diablo Range-northwestern San Joaquin Valley mountain front. Neogene uplift of the northern Diablo Range is marked by a regional reversal in sediment transport direction in the ancestral northwestern San Joaquin Valley and deposition of a late Miocene-Pliocene fanglomerate. The fanglomerate and older Neogene strata were subsequently uplifted and tilted homoclinally eastward as mountain-front deformation migrated eastward into the ancestral San Joaquin Valley. Drill-hole data indicate that Neogene and underlying units are essentially flat-lying in the central San Joaquin Valley, so the east-tilted strata must flatten across a synformal flexure located at or directly east of the present mountain front. Pediments were beveled across the east-tilted strata in middle Pleistocene time. Mapping and reconstruction of the pediments reveal that the surfaces have been uplifted, tilted eastward, gently folded and locally faulted. East-draining streams have nested, inset sequences of middle to late Pleistocene terraces and alluvial fans that also have been variously uplifted, folded, tilted or offset. Similar patterns of late Cenozoic tectonism are observed in the northern Coast Ranges and western Sacramento Valley, and are consistent with surface deformation related to blind thrust faulting within an eastward-tapering tectonic wedge or triangle zone. Propagation of an underthrust wedge can account for uplift and homoclinal tilting along the northeastern Diablo Range piedmont. The regional synformal flexure at the base of the homocline lies approximately above the wedge tip and is analogous to similar structures at the leading edges of many fold and thrust belts. Structural and stratigraphic relationships indicate that tectonic wedges were emplaced beneath the western Great Valley in early Tertiary time or earlier. Pleistocene uplift and tilting west of the wedge tip may be related to movement on reactivated west-vergent thrust faults splaying upward from the roof thrust of the wedge, or from the development of second-order wedge-style pop-up blocks. Reverse faults displace Pleistocene deposits above the wedge tip and locally accentuate the steepness of the mountain front. Instrumental microseismicity and historic large magnitude earthquakes elsewhere along the western margin of the Great Valley suggest that the underlying blind thrust faults may be seismically active and thus may pose a heretofore unrecognized seismic hazard to communities in the northwestern San Joaquin Valley.
INTERPRETIVE STRUCTURAL CROSS SECTION FROM THE CENTRAL DIABLO RANGE TO THE SAN JOAQUIN VALLEY, CALIFORNIA
ABSTRACT An interpretation of the structure of the Central Diablo Range and its relation to the northern San Joaquin Valley is presented. The interpretation recognizes the need to separate in time and space at least three tectonic events: (1) formation of the basin of deposition of the Great Valley Sequence (GVS), (2) Paleogene convergence to produce the Diablo Range antiform, and (3) Late Cenozoic convergence to produce the present topography of the Range. The basin of deposition for the GVS was formed on two separate portions of the Foothills Terrane Arc. The Jurassic and Lower Cretaceous portion of the GVS, which has only been recognized in the Range, was deposited on oceanic crust on the west side of the Foothills Terrane Arc in a distal setting. The proximal Upper Cretaceous portion of the GVS was deposited on Foothills Terrane Arc after ductile deformation of the Arc substrate and tectonic stripping of the Arc superstructure. The ductile deformation within the Arc occurred synchronously with deposition of the distal portion of the GVS. It is suggested that the Arc superstructure detached in the Lower Cretaceous, slid slowly southwestward during the late Lower Cretaceous and Upper Cretaceous, and now forms the tectonic wedge beneath the Diablo Range, which has been interpreted by Wentworth and others (1984) from reflection seismic data. The Central Diablo Range is an anticline expressed by GVS and younger sedimentary rocks dipping “radially” away from a core of Franciscan Formation. Everywhere the GVS is separated from the Franciscan core by a complex fault zone ( Jayko and others, 1987 ) called the “Coast Range Thrust.” This “thrust” was initiated by gravity driven slumping of the GVS into the Franciscan trench, perhaps during the Late Cretaceous as a part of the detachment of the arc superstructure. The anticline was created in essentially its present form by Paleogene thrusting of the distal GVS and Franciscan package northeastward over the proximal GVS in the northern San Joaquin Valley on a gentle southwestward dipping thrust plane. Thrusting continued, perhaps intermittently, on the same thrust structures through the middle Miocene. The present elevation of the range is due to a reactivation of these same structures after deposition of the Plio-Pleistocene Tulare Formation. More than 2,600 feet of structural relief developed on the base of the Tulare Formation between the San Joaquin Valley and the Diablo Range during the latest Cenozoic at the latitude of the Panoche Hills. The interpretation is presented on a conceptually balanced cross section constructed along Garzas Creek. Potential hydrocarbon traps are indicated.