Steven Kidder

Net dextral slip, Neogene San Gregorio-Hosgri fault zone, coastal California : geologic evidence and tectonic implications

Reinterpretation of onshore and offshore geologic mapping, examination of a key offshore well core, and revision of cross-fault ties indicate Neogene dextral strike slip of 156 ± 4 km along the San Gregorio–Hosgri fault zone, a major strand of the San Andreas transform system in coastal California. Delineating the full course of the fault, defi ning net slip across it, and showing its relationship to other major tectonic features of central California helps clarify the evolution of the San Andreas system. 2 W.R. Dickinson et al.

Late Cretaceous gravitational collapse of the southern Sierra Nevada batholith, California

The Sierra Nevada batholith is an ∼600-km-long, NNW-trending composite arc assemblage consisting of a myriad of plutons exhibiting a distinct transverse zonation in structural, petrologic, geochronologic, and isotopic patterns. This zonation is most clearly expressed by a west-to-east variation from mafic to felsic plutonic assemblages. South of 35.5°N, the depth of exposure increases markedly, and fragments of shallow-level eastern Sierra Nevada batholith affinity rocks overlie deeper-level western zone rocks and subjacent subduction accretion assemblages along a major Late Cretaceous detachment system. The magnitude of displacement along this detachment system is assessed here by palinspastic reconstruction of vertical piercing points provided by batholithic and metamorphic pendant structure and stratigraphy. Integration of new and published U-Pb zircon geochronologic, thermobarometric, (U-Th)/He thermochronometric, and geochemical data from plutonic and metamorphic framework assemblages in the southern Sierra Nevada batholith reveal seven potential correlations between dispersed crustal fragments and the Sierra Nevada batholith autochthon. Each correlation suggests at least 50 km of south- to southwest-directed transport and tectonic excision of ∼5–10 km of crust along the Late Cretaceous detachment system. The timing and pattern of regional dispersion of crustal fragments in the southern Sierra Nevada batholith is most consistent with Late Cretaceous collapse above the underplated accretionary complex. We infer, from data presented herein (1) a high degree of coupling between the shallow and deep crust during extension, and (2) that the development of modern landscape in southern California was greatly preconditioned by Late Cretaceous tectonics.

U-Pb detrital-zircon geochronology of northern Salinian basement and cover rocks

Salinia is an out-of-place granitic terrane in central coastal California whose debated origin is critical to understanding the tectonic history of southwestern North America. Salinian metasedimentary and sedimentary rocks that respectively host and cover its predominant arc rocks should contribute important data about its origin and kinematic history, but pervasive intrusion, high-grade metamorphism and Cenozoic erosion of the Salinian block have inhibited their widespread characterization and correlation. To further address these problems, we report 605 U-Pb detrital-zircon geochronologic ages collected by laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) from seven Salinian metasedimentary framework (Sur Series) and sedimentary cover samples. Samples collected from the Sur Series contain Late Archean (2.5–2.9 Ga), late Paleoproterozoic (1.6–1.9 Ga), Mesoproterozoic (0.9–1.5 Ga), Neoproterozoic (0.65–0.8 Ga), Paleozoic (250–450 Ma), and possibly Mesozoic U-Pb detrital-zircon ages. Samples collected from Upper Cretaceous cover units have various age-peak distributions, which collectively include late Paleoproterozoic (1.6–1.8 Ga), early Mesoproterozoic (1.35– 1.55 Ga), Permo-Triassic (220–290 Ma), and Jurassic-Cretaceous (80–190 Ma) peaks. From these data, several interpretations are made. (1) Maximum depositional ages of the Sur Series and cover intervals are 280– 360 Ma and 78–90 Ma, respectively. (2) The presence of Late Archean, early Paleoproterozoic, and Neoproterozoic zircons in Salinian metasedimentary rocks suggest that uplift and erosion of adjacent basins recycled sediment onto Salinia. (3) The abundant pre-Mesoproterozoic detrital-zircon ages in Sur Series and cover units preclude the possibility that Salinia originated in southern Mexico, as has been previously suggested. (4) Five of six key detrital-zircon age peaks identifi ed in Salinian basement and cover units are nowhere more closely arranged than in the Mojave Desert–Peninsular Ranges region of Baja and southern Alta California. (5) Paleozoic and early Mesozoic detrital zircons in Sur Series and cover units match the ages of several plutonic events that occurred along the western margin of North America—however, Permian ages favor a Mojave Desert origin over other candidates. Collectively, these and other data suggest that Salinia resided in the Mojave Desert–Peninsular Ranges region from the late Paleozoic until the Late Cretaceous, after which it was rapidly exhumed, deposited upon, and then translated outboard and northward to its current position.

Late Cenozoic denudation and uplift rates in the Santa Lucia Mountains, California

Apatite (U-Th)/He ages from a vertical transect through the Santa Lucia Mountains, central California Coast Ranges, are used to reconstruct the history of exhumation and of bedrock and surface uplift in this region since ca. 6 Ma. We find a direct correlation between (U-Th)/He ages and elevation, which we interpret to correspond to denudation rates of ;0.35 mm/yr between 6 and 2 Ma. The onset of bedrock uplift and exhumation ca. 6 Ma followed a change in plate motion ca. 8 Ma. After 2 Ma, denudation rates increased substantially (;0.9 mm/yr). This is a rare instance in which long-term average bedrock (;0.85 mm/yr) and surface (;0.20 mm/yr) uplift can be calculated from denudation rates and stratigraphic data. The post‐2 Ma denudation rate is about one order of magnitude higher than independently determined river erosion rates in the area. We suggest that this discrepancy indicates that exhumation of the steep western slopes of this segment of the Coast Ranges has been dominated by mass wasting via landslides, rather than fluvial erosion, at least since ca. 2 Ma. We also show that the bedrock uplift is predominantly tectonic, not isostatic.

Tectonic and magmatic development of the Salinian Coast Ridge Belt, California

[1] We present new field, structural, petrographic, and geochronologic data on a rare midcrustal (� 25 km) exposure of a Cordilleran arc, the Coast Ridge Belt, located in the Santa Lucia Mountains of central California. The study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks (the ‘‘Sur Series’’), which is dominated by metaigneous tonalites, diorites, and gabbros with subordinate metasedimentary quartzite and marble. Inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late Paleozoic or later. Minor magmatism in the Coast Ridge Belt began in the Early or Middle Cretaceous, but magmatic activity was most intense during a short period time from 93 to 81 Ma, based on U-Pb zircon ages of a felsic gneiss and two less-deformed diorites. The time period 93– 81 Ma also brackets a period of extensive thickening and high-temperature ductile deformation. While a thrusting cause for ductile deformation cannot be ruled out, we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust. Magmatism ended throughout Salinia between81and76Ma,coincidentwiththeattainmentof peak pressure and temperature conditions of 0.75 GPa and 800� C. Exhumation followed immediately, bringing the Coast Ridge Belt to the surface within 8 My at a rate of at least 2–3 mm/yr. Exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern California arc during the early Laramide orogeny and that may be related to underthrusting of the forearc at that time. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8035 Structural Geology: Pluton emplacement; 3640 Mineralogy and Petrology: Igneous petrology; 8030 Structural Geology: Microstructures; 8124 Tectonophysics: Earth’s interior—composition and state (1212);

Constraints from rocks in the Taiwan orogen on crustal stress levels and rheology

interval. We estimate a maximum strain rate of 7.0 � 10 � 14 s � 1 by distributing the geodetic convergence rate throughout a region homogeneously deformed under horizontal compression. These stress, strain rate and temperature estimates are consistent with the predictions of widely applied dislocation creep flow laws for quartzite. The samples record stress levels at the brittle-plastic transition, indicating a coefficient of friction (m) of 0.37 in the upper crust consistent with results based on critical taper. Integrated crustal strength of the Hsuehshan range amounts to 1.7 � 10 12 N/m based on our analysis, consistent with potential energy constraints based on topography. Other strength profiles are considered, however high crustal stresses (>300 MPa) conflict with our analysis. The study supports the use of the recrystallized grain size piezometer in quartz as a quick and inexpensive method for resolving stress histories in greenschist facies rocks. For consistency with the independent constraints presented here, we find it accurate to within +20%/� 40%, significantly better than previously recognized.

Role of extrusion of the Rand and Sierra de Salinas schists in Late Cretaceous extension and rotation of the southern Sierra Nevada and vicinity

The Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent Mojave-Salinia region along a shallow segment of the subducting Farallon plate in Late Cretaceous time. Various mechanisms, including return flow, isostatically driven uplift, upper plate normal faulting, erosion, or some combination thereof, have been proposed for the exhumation of the schist. We supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the Rand and Sierra de Salinas schists. These data indicate that the schist was transported to the SSW from deep to shallow crustal levels along a mylonitic contact (the Rand fault and Salinas shear zone) with upper plate assemblages. Crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth, suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary. The resulting flow type within the channel is that of general shear extrusion. Integration of these observations with published geochronologic, thermochronometric, thermobarometric, and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation. We present a model whereby trench-directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate.

Shear heating not a cause of inverted metamorphism

An archetypal example of inverted metamorphism purportedly resulting from shear heating is found in the Pelona Schist of southern California (United States). Recent studies demonstrate that the Pelona Schist was subducted and accreted at the onset of Laramide fl at subduction under thermal and kinematic conditions not considered in earlier numerical models. To test the shear heating hypothesis under these conditions, we constructed a thermokinematic model of fl subduction initiation involving continuous accretion of the schist. A neighborhood algorithm inversion demonstrates that available metamorphic and thermochronologic constraints in the Sierra Pelona mountains are satisfi ed only if accretion rates were 0.2‐3.6 km/m.y and shear heating was minimal (shear stress 0‐19 MPa). Minimal shear heating is also consistent with an inversion of models constrained by thermochronology of the East Fork (of the San Gabriel River) exposure of the schist. Shear heating inhibits the formation of modeled inverted gradients during accretion and should not be considered an important factor in their generation.

The Recrystallized Grain Size Piezometer for Quartz: An EBSD-based calibration

We have re-analyzed samples previously used for a quartz recrystallized grain size paleopiezometer, using electron backscatter diffraction (EBSD). Recrystallized and relict grains are separated using their grain orientation spread (GOS), which acts as a measure of intragranular lattice distortion and a proxy for dislocation density. For EBSD maps made with a 1 μm step-size, the piezometer relationship is D = 103.91 ± 0.41 ∙ σ−1.41 ± 0.21 (for RMS mean diameter values). We also present a ‘sliding resolution’ piezometer relationship, D = 104.22 ± 0.51 ∙ σ−1.59 ± 0.26, that combines 1 μm step-size data at coarser grain sizes with 200 nm step-size data at finer grain sizes. The sliding resolution piezometer more accurately estimates stress in fine-grained (< 10 μm) samples. The two calibrations give results within 10% of each other for recrystallized grain sizes between 10 μm and 100 μm. Both piezometers match the original light-optical microscopy quartz piezometer within error.

A geologic window into a subduction megathrust

trench (the schist of Sierra de Salinas). The two units were originally located some 150 kilometers apart across the plate margin. In the earliest stage of the Laramide orogeny, the sediments became attached to the lower plate, in this case the subducting Farallon plate, and were transported downward before reattaching to the upper, North American plate by tectonic underplating. During that process, the top of the sedimentary section became the subduction megathrust, which is currently exposed as the Salinas shear zone. Subsequently, the megathrust migrated downward, reuniting the underplated sediments with the upper plate and preserving the fossil shear zone. Extensional collapse and erosion of the upper plate resulted in more than 30 kilometers of exhumation in the latest Cretaceous, eventually bringing the deep arc rocks, the shear zone, and the schist to the surface. During the late Cenozoic, the entire package of rocks was transported to its present location in central California by more than 300 kilometers of right-lateral slip along the Cenozoic San Andreas fault system (Figure 1). However, none of the right-lateral slip faults within the Coast Ranges significantly affects the pre-Cenozoic tectonic assembly