Late Cenozoic uplift and shortening in the central California Coast Ranges and development of the San Joaquin Basin foreland
Published:December 28, 2018
- PDF LinkChapter PDF
Donald D. Miller, Stephan A. Graham, 2018. "Late Cenozoic uplift and shortening in the central California Coast Ranges and development of the San Joaquin Basin foreland", Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson, Raymond V. Ingersoll, Timothy F. Lawton, Stephan A. Graham
Download citation file:
The Coast Ranges Province of central California provides an important record for the timing of convergence of the North American–Pacific plate boundary along the San Andreas fault system. The sedimentary record, in conjunction with seismic interpretation and backstripping methods, constrains the age of onset of Diablo and Temblor Range uplift and concurrent subsidence of the San Joaquin Basin along the San Andreas fault to 6.2–5.4 Ma. This age of convergence and uplift of the Coast Ranges is compatible with plate-tectonic circuit models, where clockwise rotation of Pacific–North American plate motion produced plate convergence at this latitude. However, changes in plate motion do not explain a widespread structural and sedimentary event at ca. 3.5 Ma that is evident in the western San Joaquin Basin and other basins in California. Possible drivers for the 3.5 Ma event include eustatic sea-level change, geomorphic forcing, epeirogeny related to mantle lithosphere removal, and flexural tilt of the Sierra Nevada–Great Valley microplate.
The Coast Ranges Province of central California is located along the San Andreas fault system at the western margin of the Sierra Nevada–Great Valley microplate (Argus and Gordon, 1991, 2001) and the western edge of the North American continental plate (Fig. 1). The Coast Ranges Province consists of many small mountain ranges along both sides of the San Andreas fault (Page et al., 1998), which has separated the Pacific plate from the North American plate since early Neogene time (Wilson, 1965; Atwater, 1970, 1989). The objective of this study was to understand the timing and causes of uplift of the Coast Ranges Province based on the Upper Cenozoic stratigraphic record, along with reflection seismic and geohistory-subsidence data. We focused on the southern Diablo and Temblor Ranges, which are temporally and spatially linked to subsidence of the adjacent San Joaquin Basin (Fig. 2) in a spatial pairing akin to a foreland system (Fig. 3). In doing so, we sought to improve understanding of the sequence of local tectonic events in the evolution of the Sierra Nevada–Great Valley microplate and contribute to regional tectonic models for North American–Pacific plate interactions (e.g., Saleeby et al., 2012, 2013, and references therein).
Late Cenozoic uplift and shortening along the full length of the California Coast Ranges have long been recognized (e.g., Reed, 1933). Compressional structuring (Mount and Suppe, 1987; Zoback et al., 1987) and shortening of the crust perpendicular to the San Andreas fault (Namson and Davis, 1988; Wentworth and Zoback, 1989; Bloch et al., 1993) are widely viewed as reflecting oblique convergence of the Pacific and North American plates (Harbert and Cox, 1989; Harbert, 1991) during the Neogene. Balanced structural cross sections along the length of the eastern boundary of the Coast Ranges and the adjacent Central Valley of California demonstrate that late Cenozoic shortening (Fig. 4) along the plate boundary was much more extensive than the San Joaquin Basin. Indeed, late Cenozoic shortening affected the full width of the Coast Ranges: Namson and Davis (1988) estimated 11 km shortening east of the San Andreas fault and 22 km west of the fault, for a total of 33 km of shortening across the entire Coast Ranges at the latitude of their study.
A small clockwise rotation of the Pacific plate rotation is hypothesized to have caused a switch from a dominantly strike-slip system with slightly oblique extension along the San Andreas fault system to one of strike slip with oblique convergence (Minster and Jordan, 1984). Estimates of the angle of plate convergence range from 8° to 23° (Cox and Engebretson, 1985, and erratum; Harbert, 1991; Cande et al., 1995; Atwater and Stock, 1998), with high-resolution estimates from DeMets and Merkouriev (2016) of 12°. Geodetic satellite data indicate that motion perpendicular to the strike-slip San Andreas fault is mainly convergent (Argus and Gordon, 2001). Alternatively, Saleeby et al. (2013) suggested that late Neogene shortening may have resulted from flexural upwarping linked to mantle lithosphere removal beneath the southern Sierra Nevada and San Joaquin Basin.
Age estimates for uplift of the Coast Ranges derived from plate-circuit models are highly variable. Previous estimates for the initial convergence of plates based on marine studies range from younger than than 9.8 Ma (Stock and Molnar, 1982), to ca. 8 Ma (Atwater and Stock, 1998), 6–5 Ma (Cande et al., 1992), ca. 5.9 Ma (Cande et al., 1995), 5.5 Ma (Cox and Engebretson, 1985, and erratum), 5–3.2 Ma (Pollitz, 1986), 3.9–3.4 Ma (Harbert, 1991), and 3.86–3.4 Ma (Harbert and Cox, 1989). High-resolution reconstructions of DeMets and Merkouriev (2016) estimated that convergence began after 5.2 Ma.
The geologic record of coastal California contains numerous unconformities and associated nonmarine deposits in Neogene coastal basins (Fig. 1) related to uplift at ca. 5.0–3.5 Ma (Christensen, 1965; Page et al., 1998). The mainly nonmarine deposits include the Santa Clara Formation of the Santa Clara Basin (with 3.6 Ma basalts near the base; Nakata et al., 1993), the Tulare Formation of the San Joaquin Basin (beginning deposition ca. 4 Ma), the Paso Robles Formation of the Salinas Basin (ca. 4.0 Ma, with “late late Miocene” fauna as the basis for a possible 5–6 Ma age; Repenning and Tedford, 1977), and the Livermore and San Benito Gravels in central California (as reviewed by Page et al., 1998, and references therein). To the north, in the Sacramento Basin, the Tehama Formation (3.4 Ma) overlies a major unconformity (Unruh and Moores, 1992) correlative with the Tuscan Formation on the east side of the Sacramento Valley (Helley and Harwood, 1985), both including the 3.3 Ma Nomlaki Tuff (Sarna-Wojcicki et al., 2011) near their bases. Deformed strata occur below a Pliocene disconformity in the Point Arena Basin (Bachman and Crouch, 1987). To the south in the Santa Maria Basin, the Foxen Mudstone (3.8–3.4 Ma) records an abrupt influx of siliciclastic marine sediment above local unconformities (Behl and Ingle, 1995). The Morales Formation in nearby Cuyama Basin indicates a transition from strike-slip to compressional tectonics, possibly beginning prior to 3.57 Ma (Ellis et al., 1993). Unconformities occur in the Santa Cruz (3.0–2.8 Ma) and Cuyama Basins (2.8 Ma; Graham, 1987), with an estimate for uplift in the Los Angeles Basin at 3 Ma (Mayer, 1987), similar to the Diablo Range (Crowell, 1987). The Santa Maria Basin preserves the sedimentary signature of at least two regional tectonic events, during the latest Miocene to Pliocene at 5.5 Ma (McCrory et al., 1995) and at 3.8–3.4 Ma (Behl and Ingle, 1995) or ca. 3 Ma (McCrory et al., 1995), with an angular unconformity within the Purisima Formation in the Santa Cruz area spanning ca. 4.5–3.6 Ma (Powell et al., 2007).
In summary, the marine record of Pacific plate rotation relative to the North American plate indicates initial plate convergence in the central Coast Ranges after 5.2 Ma (DeMets and Merkouriev, 2016), with estimates ranging from 9.8 to 3.5 Ma. Geologic studies document uplift and shortening within the Coast Ranges beginning ca. 5 Ma, with a widespread overprint of nonmarine deposition in multiple basins beginning ca. 3.5 Ma.
STRATIGRAPHIC RECORD OF COAST RANGE UPLIFT IN THE WESTERN SAN JOAQUIN BASIN
Upper Cenozoic deposits of the San Joaquin Basin preserve the history of basin subsidence, which in turn provides age control for Coast Ranges uplift. The Miocene Monterey Formation is the oldest unit of interest here (Fig. 5). It consists of biosiliceous deposits and turbidites that were deposited in bathyal conditions (Graham and Williams, 1985). Biostratigraphy dates the unconformity at the top of the Monterey at 6.5 Ma (Dumont, 1988), which is slightly younger at Chico-Martinez Creek (Fig. 2) than in offshore California basins farther west (Barron, 1986), indicating the time-transgressive character of deposition.
The overlying Miocene Reef Ridge Formation is predominantly biogenic in the Kettleman Hills–Lost Hills area. Terrigenous detrital content increases upward overall, similar to deposits in the Santa Maria Basin to the west (e.g., Dumont and Barron, 1995), indicating contributions from a new sediment source. Some authors include the Reef Ridge Formation within the Monterey Formation (e.g., Graham and Williams, 1985), because distinguishing the two units on the basis of lithology alone can be difficult. Other authors distinguish the Reef Ridge Formation on the basis of diatom biostratigraphy, where ages of 6.0 and 5.4 Ma have been established for lower and upper zones at Lost Hills (Dumont, 1988, 1989; Butler and Dumont, 1991). The Reef Ridge Formation is partially age equivalent to the Etchegoin Formation at the Lost Hills oil field, based on subsurface relationships (Dumont, 1988; Schwartz, 1990). We place the base of the Reef Ridge Formation slightly below the biostratigraphic designation of Dumont (1988), to include the informally named Reef Ridge marker and Williamson sandstone horizons, which are extensive in the subsurface of our study area. These markers represent the onset of siliciclastic deposition, supplanting fine-grained biosiliceous sedimentation, ca. 6.2 Ma.
Clastic facies of the overlying Miocene and Pliocene Etchegoin Formation (Anderson, 1905; Woodring et al., 1940) were deposited as thick (>1500 m) time-transgressive progradational strata under persistent shelf, tidal, and nonmarine conditions (Graham and Williams, 1985; Loomis, 1990). Steward (1997) recognized eight sea-level cycles of transgression and regression in this prograding unit. Loomis (1990) noted the partial time-equivalence of the Etchegoin Formation with Reef Ridge deposits and estimated an age of 6.5 Ma for the base of the Etchegoin unit, whereas Dumont (1988) assigned 5.4 Ma for the underlying top of Reef Ridge sampled at the more basinward location in Lost Hills. The range of dates is consistent with the interpreted progradation that Loomis (1990) observed from the northwestern basin margin. The age of the top of the Etchegoin Formation is estimated to be 4.8 Ma in our study area (Table 1), slightly younger than the 4.83 Ma Lawlor tuff deposits that occur near the top of the formation at Kettleman Hills (Fig. 1; Sarna-Wojcicki and Davis, 1991; Sarna-Wojcicki et al., 2011). The detrital composition of the western facies of the Etchegoin Formation indicates derivation from the newly uplifted Diablo Range (Loomis, 1990).
The overlying Pliocene San Joaquin Formation consists of brackish and nonmarine strata of variable thickness, up to nearly 700 m (Loomis, 1990). In Coast Ranges outcrops, the Cascajo conglomerate at the base of the San Joaquin Formation overlies an angular unconformity, where the angle of discordance with underlying beds exceeds 30° (Woodring et al., 1940; Loomis, 1990). Basinward in Kettleman Hills outcrops, the angular bedding relationship is no greater than 5° and described as “exceptionally deep local scouring at the base of the conglomerate assigned to the Cascajo” (Woodring et al., 1940, p. 50). This correlates to the basinwide Cascajo-Mya marker zone (Fig. 5), interpreted as a sequence boundary in the San Joaquin Basin (Miller, 1999). A tuff bed in the overlying Tulare Formation near the contact with the San Joaquin Formation (Fig. 5) is exposed in an unnamed arroyo at Kettleman Hills (sec. 6, T. 23 S., R. 19 E.; State of California, 2010) that correlates with the Putah tuff dated at 3.3 Ma (Sarna-Wojcicki et al., 2011). Although the tuff occurs below the original Tulare–San Joaquin contact mapped by Woodring et al. (1940), abundant freshwater fossils are diagnostic of the Tulare Formation (Woodring et al., 1940), including Carnifex sanctaerlarae marshalli, Valvata humeralis californica, abundant Amnicola longinqua, and ostracods and fish remains (A. Almgren, 1998, written commun.). As with the underlying Etchegoin Formation, the detrital composition of the western facies of the San Joaquin Formation indicates derivation from the uplifted Diablo Range (Loomis, 1990).
The Pliocene–Pleistocene Tulare Formation is a widespread nonmarine deposit (Watts, 1894; Anderson, 1905) that is locally unconformable along the western basin margin, while it conformably overlies the San Joaquin Formation toward the basin depocenter. The base is characterized by the oldest fully nonmarine strata that commonly contain lacustrine mollusks, including the gastropod Amnicola. The base can be transitional with the San Joaquin Formation (Woodring et al., 1940). North-prograding seismic clinoforms (Fig. 6) in the basin axis illustrate sediment derivation from the fold belt and the time-transgressive nature of sedimentation (Miller, 1999), where the 3.3 Ma Nomlaki Tuff (Poletski, 2010) occurs in Tulare strata at Kettleman Middle Dome in the La Salida area (sec. 17, T. 23 S., R. 19 E.; State of California, 2010) and in the San Joaquin Formation at North Dome in Arroyo Doblegado (sec. 16, T. 22 S., R. 18 E.; State of California, 2010), located 12 km away (Sarna-Wojcicki, 1998, personal commun.). This study used the tephra date to constrain the age of lithologic transition from brackish to nonmarine strata, consistent with global cooling 3.5–2.5 Ma and eustatic sea-level drop of 30 m ca. 3 Ma (Fig. 5; Miller et al., 2011).
In summary, the western San Joaquin Basin near Lost Hills preserves a thick Upper Neogene stratigraphic section with robust age control: (1) 3.4 Ma or slightly older at the base of the Tulare Formation; (2) 4.8 Ma or slightly younger at the top of the Etchegoin Formation; and (3) 5.4 Ma at the top and 6.2 Ma at the base of the Reef Ridge Formation. Age assignments for prograding strata are specific to our study area and compatible with the regional stratigraphic synthesis of Scheirer and Magoon (2007). The Miocene ages in Figure 5 and Table 1 are based on correlations of benthic foraminiferal stages to the radiometric time scale (Finger, 1995).
SEISMIC ARCHITECTURE OF WESTERN SAN JOAQUIN BASIN DEPOSITS
The seismic profile in Figure 7 illustrates several key structural and stratigraphic relationships that constrain the timing of uplift of the Temblor Range. Shortening and progressive uplift developed folds over blind thrust faults of the Lost Hills–Kettleman Hills fold belt (Namson and Davis, 1988; Medwedeff, 1989; Bloch et al., 1993; Wickham, 1995), subparallel to the basin margin, where the interplay between sedimentation and deformation provides age control for deformational events. The well-log cross section in Figure 8 confirms our interpretation of the stratigraphic continuity of all formation units across the anticlinal fold in areas of poor seismic resolution over the crest of the Lost Hills anticline.
The seismically imaged angular unconformity of 20° to 30° in Figure 7A is further documented by dipmeter well logs in the Atlantic Richfield, Getty Fee No. 1 well (sec. 15, T. 27 S., R. 20 E.; State of California, 2010). The age of 3.5 Ma for the unconformity at the western end of the seismic line is based on stratigraphic age control of conformable units at the eastern end of the seismic line in Figure 7.Figure 7D illustrates numerous seismic-reflector terminations that are distributed throughout the onlap wedge of San Joaquin strata against the fold. The seismically imaged angular unconformity at the basin margin (Fig. 7C) is composed of small basinward-stepping discontinuities and thinning sections toward the Coast Ranges uplift that are not readily apparent at outcrop scale. This occurs where the scale of the seismic resolution combines multiple depositional surfaces, each with a small angularity to underlying beds, to form a single seismic reflector. Thus, the transition progresses from the basin margin to conformable deposits of a depositional sequence boundary of basinwide significance.
In summary, the seismic profile in Figure 7 documents at least two significant basin events. First, faulting and folding at the Coast Ranges basin margin began before Etchegoin deposition, in agreement with Medwedeff (1989) and Bloch et al. (1993). We date this at 5.4 Ma, as described above. Second, folding and uplift of Coast Ranges strata continued throughout deposition of the San Joaquin Formation. Beginning ca. 3.5 Ma, wedge-top deposits (terminology of DeCelles and Giles, 1996) overlie the angular unconformity, with deposition continuing to the present.
The seismic stratigraphic relationships and downward-concave shape of the subsidence curve, discussed below, fit those of a foreland-basin setting, as described by DeCelles and Giles (1996). The deepest depozone consists of “foredeep deposits” of the Etchegoin and Reef Ridge Formations that generally exhibit continuous seismic reflectors. Rapid basin subsidence at the northwestern basin margin accommodated more than 1600 m of Etchegoin strata that show an upward increase in sandstone content toward the northwestern basin margin (Loomis, 1990). The overlying San Joaquin Formation laps onto the basinward flank of the Lost Hills fold (Fig. 7D) to define the proximal foredeep of DeCelles and Giles (1996). Where seismic reflectors show an apparent lack of stratigraphic continuity over the fold, our work (Fig. 8) and numerous oil-field studies support the stratigraphic studies of Woodring et al. (1940), which indicate that the sedimentary unit was deposited and preserved continuously during folding.
The “wedge-top” deposits of the uppermost Tulare Formation strata overlie the angular unconformity with the Cascajo-Mya and the overlying Tulare Formation (Fig. 7C). The wedge-top stratigraphy tapers toward the sediment source, where the Cascajo conglomerate overlies the angular unconformity. Time-equivalent deposits in the subsurface basin depocenters, informally named the Mya sand, consist of sand, silt, and mud that are conformable with underlying units.
BACKSTRIPPING, SUBSIDENCE ANALYSIS, AND LITHOLOGY OF SEDIMENTARY DEPOSITS
Backstripping and subsidence analysis provide analytical techniques for calculating the timing of tectonic history from the sedimentary record. A chronology of subsidence rates can be calculated from ages and decompacted thicknesses of beds using the methods of Steckler and Watts (1978) and van Hinte (1978). These systematically reverse the events of progressive sediment loading and compaction from the stratigraphic record to yield basin-subsidence rates (e.g., Dickinson et al., 1987). Total subsidence can be determined from the thickness of a sequence, lithology, depositional bathymetry, and eustatic sea-level data (Table 1).
Total basin subsidence can be divided into components of sediment loading and tectonics. For this study, we calculated the sediment-load component and isostatic adjustment of the lithosphere using an assumption of one-dimensional Airy isostasy. The residual was ascribed to tectonic drivers. The tectonic-subsidence history plots a curve with information about timing that is independent of the sediment loading, thus providing information about tectonic stresses.
Subsidence analyses of the two stratigraphic sections shown in Figure 9 complement the seismic analysis described for Lost Hills. These sections are located at either end of the seismic transect in Figure 2, one at a key outcrop at the basin margin, and the other from a key well near the basin depocenter. Backstripping parameters for the basin-margin location were taken from outcrop studies along the Chico Martinez Creek, in secs. 2 and 11, T. 27 S., R. 20 E. (State of California, 2010) (Fig. 2). The parameters in Table 1 include biostratigraphy, paleobathymetry, and present stratigraphic thickness from Graham and Williams (1985), without modeling diagenetic silica effects as described by Mosher (2013), who measured a reduced part of the section. Parameters for the depocenter position of the Great Basins #31X-10 well in sec. 10, T. 27 S., R. 22 E. (State of California, 2010). provide a complete stratigraphic section of Miocene–Pliocene–Pleistocene strata (Schwartz, 1990). Paleobathymetry data are from Woodring et al. (1940), Bandy and Arnal (1969), and Schwartz (1988); lithostratigraphy was determined from well logs using gamma-ray, spontaneous-potential, and resistivity data correlated to a regional stratigraphic framework (Graham and Williams, 1985).
The resulting subsidence history at the basin margin (Fig. 9A) displays two unconformities, one at 17 Ma and the other between 6.5 and 2.2 Ma. The large range of subsidence and uplift that occurred at the section near the basin margin reflects proximity to the San Andreas fault system, which is the presumptive tectonic driver. Nearly vertical beds are exposed along Chico Martinez Creek that document deformation related to uplift prior to development of the Pliocene unconformity.
In contrast, the full stratigraphic sequence penetrated by the Great Basins #31X-10 well near the basin depocenter constrains the age of uplift that produced the angular unconformity revealed on the seismic transect of Figure 7. Total and tectonic subsidence accelerated at 5.4 Ma (Fig. 9B), exhibiting concave-down profiles at the onset of Etchegoin deposition. Subsidence rates were high during Etchegoin deposition (exceeding 2 km/m.y.; Table 1). Closer inspection shows the onset of subsidence during deposition of the Reef Ridge Formation and before 5.4 Ma. The concave-down total subsidence profile, beginning ca. 6.2 Ma and accelerating ca. 5.4 Ma, is characteristic of flexural subsidence in foreland basins (e.g., Dickinson et al., 1987; DeCelles and Giles, 1996).
Lithologic transitions preserved in the stratigraphic record signal uplift of the Coast Ranges between ca. 6.2 and 5.4 Ma, with an overall increase in siliciclastic input. The upper contact of the Reef Ridge Formation with the Etchegoin Formation at Lost Hills constrains the youngest age for the onset of subsidence to ca. 5.4 Ma. However, the transitional nature of the Reef Ridge–Etchegoin contact suggests a change in basin subsidence that began during Reef Ridge deposition. The Williamson sandstone horizon at the base of the Reef Ridge Formation (discussed above) records the earliest change in the sedimentary record in our area of study, slightly below the 6.0 Ma horizon of Dumont (1988) at Lost Hills.
In summary, our subsidence analysis (Fig. 9) is consistent with flexural subsidence related to thrust loading during Coast Ranges uplift that began no later than 5.4 Ma and possibly as early as 6.2 Ma. These ages correspond to the onset of clastic deposits of the Etchegoin Formation and earlier deposition of the Reef Ridge Formation. Neither our subsidence data from the western San Joaquin Basin nor changes in lithology indicate a separate tectonic event at 3.5–3.4 Ma. The development of the angular unconformity (Fig. 7) and largely nonmarine wedge-top deposits at ca. 3.4 Ma suggests the need for an alternative explanation for a regional event.
Although strike slip on San Andreas system faults has accommodated most motion between the Pacific and North American plates in central California in the late Cenozoic (e.g., Dickinson et al., 2005; Sharman et al., 2013), our study points to a component of lithospheric convergence as the primary driver behind Coast Ranges shortening and uplift along the California margin, which was coupled with subsidence in the western San Joaquin Basin. Clockwise rotation of the Pacific plate to the west of the Coast Ranges appears to have been the direct cause for shortening, uplift, and flexural subsidence. The horizontal compressive stress field perpendicular to the San Andreas fault (Mount and Suppe, 1987) appears to have propagated the fold belt onto the buttress formed by the edge of Sierran basement of the Great Valley (observed on reflection-seismic data; e.g., Wentworth and Zoback, 1989; Bloch et al., 1993). The resulting cross-sectional configuration is that of a foreland fold-and-thrust/basin architecture (Figs. 3 and 4), best characterized as a transpressional foreland (e.g., Ingersoll, 2012). Tectonic and sedimentary loading from the Coast Ranges further contributed to the westward tilt of the Sierra Nevada and Great Valley basement, as modeled by Rentschler and Bloch (1988). Geomorphic forcing caused by erosional unloading of the Sierra Nevada and redistribution of the resulting sediment load into the Great Valley (Small and Anderson, 1995) also contributed to the westward tilt of the microplate (Saleeby et al., 2013).
East of the Coast Ranges, Ducea and Saleeby (1998) interpreted delamination and removal of mantle lithosphere beneath the southern Sierra Nevada and Great Valley based on geophysical and geological evidence. Saleeby et al. (2013) modeled the role of regional upper-mantle buoyancy to drive vertical tectonics of uplift and subsidence of the Sierra Nevada and Great Valley (Le Pourhiet et al., 2006), emphasizing rapid delamination of the Sierran arc root after 5 Ma. Their modeling predicted a flexural upwarp of several hundred meters in the area occupied by the Coast Ranges in response to removal of mantle lithosphere beneath the central and eastern San Joaquin Basin. However, their model did not incorporate kinematics of the San Andreas fault system, and it neglected the estimated 12° of clockwise rotation of the Pacific plate (i.e., in the direction of shortening of the California margin) after 5.2 Ma (e.g., DeMets and Merkouriev, 2016), crustal shortening evident in cross sections (Fig. 4), and geodetic data showing mainly convergence perpendicular to the predominantly strike-slip San Andreas fault (Argus and Gordon, 2001). Alternatively, we hypothesize that late Cenozoic removal of mantle lithosphere and the location of the delamination hinge in the southern Sierra foothills described in the model of Saleeby et al. (2012) might have impinged on the lithosphere of the Pacific plate and caused the small clockwise rotation to produce oblique convergence of plate boundaries and the shortening within the Coast Ranges observed regionally northward along the plate boundary (Fig. 4).
A shift in the triple junction and opening of the Gulf of California at 5.5 Ma (Sedlock and Hamilton, 1991) indicate a regional adjustment to the Pacific–North American plate boundary that was concurrent with Coast Ranges uplift. More robust data indicate this occurred as a transition between 6.3 and 4.7 Ma (Oskin et al., 2001). In summary, while recognizing a component of subsidence of the southern San Joaquin Basin related to arc root delamination, we conclude that the direct cause for Coast Ranges uplift was a manifestation of Pacific–North American plate adjustments.
Convergence across the San Andreas fault and uplift of the Coast Ranges in central California began ca. 6.2 Ma and not later than 5.4 Ma, as deduced from three data sets from the stratigraphic record: (1) onset of fold deformation, (2) subsidence history, and (3) changing lithologic composition concurrent with uplift deformation and coupled basin subsidence. Seismic data and sedimentary stratal architecture are diagnostic of shortening in the Coast Ranges in a compressive stress field, with related flexural subsidence of the western San Joaquin Basin.
In contrast, a second regional event expressed in the sedimentary record at ca. 3.5 Ma appears to have been unrelated to a change in Pacific–North American plate motions. Our study shows that the western San Joaquin Basin subsided during that time primarily due to increased sediment load, without evidence for a significant change in an imposed tectonic force. Our study indicates an angular unconformity and overlying nonmarine wedge-top sediments of this age, with no evidence of a clear tectonic signal in our subsidence analysis (Table 1; Fig. 5). Nonmarine wedge-top deposition in the San Joaquin Basin coincided with a sea-level drop at 3.3 Ma (Miller et al., 2011). Alternative mechanisms for this might be related to delamination of mantle lithosphere and crustal buoyancy (e.g., Saleeby et al., 2013) and climate-related geomorphic forcing (Small and Anderson, 1995). The likelihood of multiple forcing factors at play warrants further synthesis of geological and geophysical data across multiple basins in coastal California.
We acknowledge the contribution of Seismic Exchange, Inc., for allowing publication of the shallow seismic profiles shown in this publication. We also acknowledge the editorial efforts of J.R. Schwalbach, R.G. Stanley, R.V. Ingersoll, and the editors of this volume, whose contributions materially improved the clarity of this publication.