Sierra Nevada Microplate Research Articles

Mid-Miocene to Present Upper-Plate Deformation of the Southern Cascadia Forearc: Effects of the Superposition of Subduction and Transform Tectonics

The southern Cascadia forearc undergoes a three-stage tectonic evolution, each stage involving different combinations of tectonic drivers, that produce differences in the upper-plate deformation style. These drivers include subduction, the northward migration of the Mendocino triple junction and associated thickening and thinning related to the Mendocino Crustal Conveyor (MCC) effect, and the NNW translation of the Sierra Nevada-Great Valley (SNGV) block. We combine geodetic data, plate reconstructions, seismic tomography and topographic observations to determine how the southern Cascadia upper plate is deforming in response to the combined effects of subduction and NNW-directed (MCC- and SNGV-related) tectonic processes. The location of the terrane boundaries between the relatively weak Franciscan complex and the stronger Klamath Mountain province (KMP) and SNGV block has been a key control on the style of upper-plate deformation in the southern Cascadia forearc since the mid-Miocene. At ∼15 Ma, present-day southern Cascadia was in central Cascadia and deformation there was principally controlled by subduction processes. Since ∼5 Ma, this region of the Cascadia upper plate, where the KMP lies inboard of the Franciscan complex, has been deforming in response to both subduction and MCC- and SNGV-related effects. GPS data show that the KMP is currently moving to the NNW at ∼8–12 mm/yr with little internal deformation, largely in response to the northward push of the SNGV block at its southern boundary. In contrast, the Franciscan complex is accommodating high NNW-directed and NE-directed shortening strain produced by MCC-related shortening and subduction coupling respectively. This composite tectonic regime can explain the style of faulting within and west of the KMP. Associated with this Mendocino Crustal Conveyor crustal thickening, seismic tomography imagery shows a region of low velocity material that we interpret to represent crustal flow and injection of Franciscan crust into the KMP at intracrustal levels. We suggest that this MCC-related crustal flow and injection of material into the KMP is a relatively young feature (post ∼5 Ma) and is driving a rejuvenated period of rock uplift within the KMP. This scenario provides a potential explanation for steep channels and high relief, suggestive of rapid erosion rates within the interior of the KMP.

Late Cenozoic structure and tectonics of the southern Sierra Nevada–San Joaquin Basin transition, California

AbstractThis paper presents a new synthesis for the late Cenozoic tectonic, paleogeographic, and geomorphologic evolution of the southern Sierra Nevada and adjacent eastern San Joaquin Basin. The southern Sierra Nevada and San Joaquin Basin contrast sharply, with the former constituting high-relief basement exposures and the latter constituting a Neogene marine basin with superposed low-relief uplifts actively forming along its margins. Nevertheless, we show that Neogene basinal conditions extended continuously eastward across much of the southern Sierra Nevada, and that during late Neogene–Quaternary time, the intra-Sierran basinal deposits were uplifted and fluvially reworked into the San Joaquin Basin. Early Neogene normal-sense growth faulting was widespread and instrumental in forming sediment accommodation spaces across the entire basinal system. Upon erosion of the intra-Sierran basinal deposits, structural relief that formed on the basement surface by the growth faults emerged as topographic relief. Such “weathered out” fossil fault scarps control much of the modern southern Sierra landscape. This Neogene high-angle fault system followed major Late Cretaceous basement structures that penetrated the crust and that formed in conjunction with partial loss of the region’s underlying mantle lithosphere. This left the region highly prone to surface faulting, volcanism, and surface uplift and/or subsidence transients during subsequent tectonic regimes. The effects of the early Neogene passage of the Mendocino Triple Junction were amplified as a result of the disrupted state of the region’s basement. This entailed widespread high-angle normal faulting, convecting mantle-sourced volcanism, and epeirogenic transients that were instrumental in sediment dispersal, deposition, and reworking patterns. Subsequent phases of epeirogenic deformation forced additional sediment reworking episodes across the southern Sierra Nevada–eastern San Joaquin Basin region during the late Miocene break-off and west tilt of the Sierra Nevada microplate and the Pliocene–Quaternary loss of the region’s residual mantle lithosphere that was left intact from the Late Cretaceous tectonic regime. These late Cenozoic events have left the high local-relief southern Sierra basement denuded of its Neogene basinal cover and emergent immediately adjacent to the eastern San Joaquin Basin and its eastern marginal uplift zone.

Dynamic Strains for Earthquake Source Characterization

ABSTRACT Strainmeters measure elastodynamic deformation associated with earthquakes over a broad frequency band, with detection characteristics that complement traditional instrumentation, but they are commonly used to study slow transient deformation along active faults and at subduction zones, for example. Here, we analyze dynamic strains at Plate Boundary Observatory (PBO) borehole strainmeters (BSM) associated with 146 local and regional earthquakes from 2004–2014, with magnitudes from M 4.5 to 7.2. We find that peak values in seismic strain can be predicted from a general regression against distance and magnitude, with improvements in accuracy gained by accounting for biases associated with site–station effects and source–path effects, the latter exhibiting the strongest influence on the regression coefficients. To account for the influence of these biases in a general way, we include crustal‐type classifications from the CRUST1.0 global velocity model, which demonstrates that high‐frequency strain data from the PBO BSM network carry information on crustal structure and fault mechanics: earthquakes nucleating offshore on the Blanco fracture zone, for example, generate consistently lower dynamic strains than earthquakes around the Sierra Nevada microplate and in the Salton trough. Finally, we test our dynamic strain prediction equations on the 2011 M 9 Tohoku‐Oki earthquake, specifically continuous strain records derived from triangulation of 137 high‐rate Global Navigation Satellite System Earth Observation Network stations in Japan. Moment magnitudes inferred from these data and the strain model are in agreement when Global Positioning System subnetworks are unaffected by spatial aliasing.

Evidence for Moho‐lower crustal transition depth diking and rifting of the Sierra Nevada microplate

AbstractLithospheric rifting most often initiates in continental extensional settings where “breaking of a plate” may or may not progress to sea floor spreading. Generally, the strength of the lithosphere is greater than the tectonic forces required for rupture (i.e., the “tectonic force paradox”), and it has been proposed that rifting requires basaltic magmatism (e.g., dike emplacement) to reduce the strength and cause failure, except for the case of a thin lithosphere (<30 km thick). Here we isolate two very similar and unprecedented observations of Moho‐lower crustal transition dike or fluid injection earthquake swarms under southern Sierra Valley (SV: 2011–2012) and North Lake Tahoe (LT: 2003–2004), California. These planar distributions of seismicity can be interpreted to define the end points, and cover ~25% of the length, of an implied ~56 km long structure, each striking N45°W and dipping ~50°NE. A single event at ~30 km depth that locates on the implied dipping feature between the two swarms is further evidence for a single Moho‐transition depth structure. We propose that basaltic or fluid emplacement at or near Moho depths weakens the upper mantle lid, facilitating lithospheric rupture of the Sierra Microplate. Similar to the LT sequence, the SV event is also associated with increased upper crustal seismicity. An 27 October 2011, Mw 4.7 earthquake occurred directly above the deep SV sequence at the base of the upper crustal seismogenic zone (~15 km depth).

Internal deformation of the southern Sierra Nevada microplate associated with foundering lower lithosphere, California

Quaternary faulting and background seismicity in the southern Sierra Nevada microplate are concentrated east and south of the Isabella anomaly, a high-velocity body in the upper mantle interpreted to be lower Sierra lithosphere that is foundering into the asthenosphere. We analyzed seismicity in this region to evaluate patterns of upper crustal deformation above and adjacent to the Isabella anomaly. Earthquakes in the southern Sierra and San Joaquin Valley were relocated using joint hypocentral inversion and double-difference techniques, and groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor. The deformation field derived from this analysis reveals two distinct departures from horizontal plane strain associated with distributed northwest-directed dextral shear east of the Pacific plate: (1) heterogeneous extension and crustal thinning in the high Sierra and western foothills east of the Isabella anomaly; and (2) pronounced counterclockwise rotation of the principal strains from regional trends in the southwestern Sierra Nevada and across the Kern Arch. Based on comparison with a three-dimensional tomographic model, the extension in the southern Sierra is spatially associated with relatively thinner crust and anomalous low P-wave speeds in the upper mantle (40–90 km depth range) directly east of the Isabella anomaly. These relations suggest that seismogenic crustal thinning is localized above upwelling asthenosphere that is replacing foundering lithosphere. Counterclockwise rotation of strain trajectories in the southwest Sierra occurs southeast of the Isabella anomaly, and is associated with seismogenic west-northwest–striking dextral faults. We suggest that the deformation here represents westward encroachment of dextral shear into the microplate from the eastern California shear zone and southern Walker Lane belt. The strain rotation may reflect the presence of local stresses associated with relaxation of subsidence in the vicinity of the Isabella anomaly. Westward propagation of foundering lithosphere, with spatially associated patterns of upper crustal deformation similar to those documented herein, can account for observed late Cenozoic time- and space-transgressive deformation in the southern Walker Lane belt east of the Isabella anomaly, and is a potentially observable consequence of the foundering process in other orogens.

Kinematics of the west-central Walker Lane: Spatially and temporally variable rotations evident in the Late Miocene Stanislaus Group

The Walker Lane currently accommodates ∼20% of the dextral motion between the Pacific and North American plates. This accommodation occurs on regional-scale systems of strike-slip and normal faults located between the northwestward-translating Sierra Nevada microplate and the east-west–extending Basin and Range. At the western edge of the central Walker Lane (lat ∼38°–39°N) is a region of crustal blocks bounded by asymmetric basins and normal faults, here defined as the west-central Walker Lane. Although this region is apparently devoid of major active strike-slip faults, the presence of Neogene clockwise vertical-axis tectonic block rotations indicates the accommodation of dextral shear. We measured vertical-axis rotation by comparing remanence directions of widespread members of the Eureka Valley Tuff of the Late Miocene Stanislaus Group within the west-central Walker Lane to the same units on the Sierra Nevada microplate. Results show that the study area is organized into discrete domains with heterogeneous regional distribution of clockwise vertical-axis rotation, ranging from ∼10° to 60°, since ca. 9.5 Ma. The highest measured magnitudes of vertical-axis rotation (∼50°–60° clockwise) are interpreted as a region of high deformation that includes the asymmetric Bridgeport Valley. Previous work underestimated vertical-axis rotation magnitudes in the region because published reference directions for two of the three members of the Eureka Valley Tuff (By-Day Member and Upper member) derive from the rotated region. We recalculate a reference direction for the By-Day Member of declination 353.2°, inclination 43.7°; α 95 = 10.8°. This corroborates a reference direction for the By-Day Member from the Stanislaus Group type section, situated on the relatively stable Sierra Nevada microplate, providing a robust reference direction for paleomagnetic studies. We present a kinematic model in which dextral shear in the west-central Walker Lane is accommodated by ∼30° of clockwise rotation in the Sweetwater Mountains and Bodie Hills since the Late Miocene. This model incorporates rotation magnitudes, paleostress orientations, edge effects, and bounding faults of rotating tectonic blocks to reveal timing, patterns, and mechanisms of crustal deformation. The results and models presented here elucidate the complex and evolving nature of the west-central Walker Lane. The rotational history of dextral shear accommodation demonstrates that the west-central Walker Lane should be included as an important part of the Walker Lane transtensional zone. The results presented in this study not only improve understanding of deformation in the Walker Lane, but illuminate the potentially significant contribution of crustal block vertical-axis rotations in other transtensional regions of the world.

Birth of a plate boundary at ca. 12 Ma in the Ancestral Cascades arc, Walker Lane belt of California and Nevada

The Walker Lane belt of eastern California and western Nevada is the northernmost extension of the Gulf of California transtensional rift, where the process of continental rupture has not yet been completed, and rift initiation can be studied on land. GPS and earthquake focal mechanism studies demonstrate that the Walker Lane belt currently accommodates NW-SE–directed movement between the Sierra Nevada microplate and the North American plate, but the timing and nature of rift initiation remains unclear. I present a model for plate-margin-scale initiation of the Gulf of California and Walker Lane transtensional rifts at ca. 12 Ma; localization of rifting in both was initiated by thermal weakening in the axis of a subduction-related arc undergoing extension due to slab rollback, and thermal weakening in the arc was enhanced by stalling of the trenchward-migrating precursor arc against a thick Cretaceous batholithic lithospheric profile on its western margin. Rifting succeeded very quickly in the Gulf of California, due to stalling of Farallon slabs, but the Walker Lane transtensional rift has been unzipping northward along the axis of the Cascades arc, following the Mendocino triple junction. I infer that plate-margin-scale Walker Lane transtension was signaled by the development of an unusually large and voluminous transtensional arc volcanic center, the ca. 11.5–9 Ma Sierra Crest–Little Walker arc volcanic center. I show that the style of faulting in this large Miocene arc volcanic center closely matches that of Quaternary transtensional structures in the central Walker Lane, where it lies, and that it differs from southern and northern Walker Lane structures. This indicates that the temporal transition from E-W Basin and Range extension to NW-SE Walker Lane transtension occurred earlier than most workers have inferred. I also summarize new data which show that the central Sierra Nevada range front (from Long Valley to the Tahoe Basin) lies squarely within the Walker Lane belt, not to the west of it as previous workers have inferred.

Lithospheric convective instability could induce creep along part of the San Andreas fault

Research Article| September 01, 2013 Lithospheric convective instability could induce creep along part of the San Andreas fault Laetitia Le Pourhiet; Laetitia Le Pourhiet 1Université Pierre-et-Marie-Curie-Paris 06, Institut des Sciences de la Terre, Paris F-75005, France2CNRS, Unité Mixte de Recherche 7193, Paris F-75005, France Search for other works by this author on: GSW Google Scholar Jason Saleeby Jason Saleeby 3Division of Earth and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Laetitia Le Pourhiet 1Université Pierre-et-Marie-Curie-Paris 06, Institut des Sciences de la Terre, Paris F-75005, France2CNRS, Unité Mixte de Recherche 7193, Paris F-75005, France Jason Saleeby 3Division of Earth and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Publisher: Geological Society of America Received: 18 Nov 2012 Revision Received: 16 Apr 2013 Accepted: 05 May 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (9): 999–1002. https://doi.org/10.1130/G34244.1 Article history Received: 18 Nov 2012 Revision Received: 16 Apr 2013 Accepted: 05 May 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Laetitia Le Pourhiet, Jason Saleeby; Lithospheric convective instability could induce creep along part of the San Andreas fault. Geology 2013;; 41 (9): 999–1002. doi: https://doi.org/10.1130/G34244.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Along the western border of the Sierra Nevada microplate, the San Andreas fault (California, United States) is comprised of three segments. Two (north and south segments) are locked and support large earthquakes (e.g., the M 7.7 1906 San Francisco and the M 7.8 1857 Fort Tejon earthquakes), while the central segment, from Parkfield to San Juan Bautista, is creeping. Based on mechanical models, we show that the late Pliocene–Quaternary convective removal (delamination) of the southern Sierra Nevada mantle lithosphere and associated uplift of the Sierra Nevada Mountains causes the Great Valley upper crust to deform by flexure and buckling. Additional three-dimensional flexural models imply that the local flexural bulge overlaps with the creeping segment of the fault system, while geological observations indicate that the local weakening of the San Andreas fault started at the same time that the Sierra Nevada started its recent phase of uplift. We argue that bending stresses promote lithostatic pore pressure to occur in the depth range of 7–15 km, causing the effective strength of the fault to vanish, and locally favoring creep. Our results suggest for the first time that earthquake cycles along a major plate boundary may be influenced by convective instabilities in the adjacent upper mantle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sierra Crest graben-vent system: A Walker Lane pull apart within the ancestral Cascades arc

We show here that transtensional rifting along the eastern boundary of the Sierra Nevada microplate (Walker Lane rift) began by ca. 12 Ma in the central Sierra Nevada (USA), within the ancestral Cascades arc, triggering voluminous high-K intermediate volcanism (Stanislaus Group). Flood andesite (i.e., unusually large-volume effusive eruptions of intermediate composition) lavas erupted from fault-controlled fissures within a series of grabens that we refer to as the Sierra Crest graben-vent system. This graben-vent system includes the following. 1. The north-northwest–south-southeast Sierra Crest graben proper consists of a single 28-km-long, 8–10-km-wide full graben that is along the modern Sierra Nevada crest between Sonora Pass and Ebbetts Pass (largely in the Carson-Iceberg Wilderness). This contains fissure vents for the high-K intermediate lavas. 2. A series of north-northwest-south-southeast half-grabens on the western margin of the full graben, which progressively disrupted an ancient Nevadaplano paleochannel that contains the type section of Stanislaus Group (Red Peak–Bald Peak area). These Miocene half-grabens are as much as 15 km west of the modern Sierra Nevada crest, and vented high-K lavas from point sources. 3. Series of northeast-southwest grabens define a major transfer zone along the northeast side of the Sierra Crest graben. These extend as much as ∼30 km from the modern range crest down the modern Sierra Nevada range front, in a zone ∼30 km wide, and vented high-K lavas and tuffs of the Stanislaus Group from point sources. Range-front north-south and northeast-southwest faults to the south of that, along the southeast side of the Sierra Crest graben, did not vent volcanic rocks (although they ponded them); those will be described elsewhere. We present evidence for a dextral component of slip on the north-northwest–south-southeast normal faults, and a sinistral component of slip on the northeast-southwest normal faults. The onset of transtension immediately preceded the high-K volcanism (within the analytical error of 40 Ar/ 39 Ar dates), and triggered the deposition of a debris avalanche deposit with a preserved volume of ∼50 km 3 . The grabens are mainly filled with high-K lava flows, ponded to thicknesses of as much as 400 m; this effusive volcanism culminated in the development of the Little Walker caldera over a relatively small part of the field. Trachydacite outflow ignimbrites from the caldera also became ponded in the larger graben-vent complex, where they interfingered with high-K lavas vented there, and escaped the graben-vent complex on its west margin to flow westward down two paleochannels to the western foothills. The Sierra Crest graben-vent system is spectacularly well exposed at the perfect structural level for viewing the controls of synvolcanic faults on the siting and styles of feeders, vents, and graben fills under a transtensional strain regime in an arc volcanic field.

Epeirogenic transients related to mantle lithosphere removal in the southern Sierra Nevada region, California: Part II. Implications of rock uplift and basin subsidence relations

We investigate the putative Pliocene–Quaternary removal of mantle lithosphere from beneath the southern Sierra Nevada region using a synthesis of subsidence data from the Great Valley, and geomorphic relations across the Sierra Nevada. These findings are used to test the results and predictions of thermomechanical modeling of the lithosphere removal process that is specific to the Sierra Nevada, as presented in an accompanying paper referenced here as Part I. Our most successful thermomechanical model and the observational data that it explains are further bundled into an integrated physiographic evolution–geodynamic model for the three-dimensional epeirogenic deformation field that has affected mainly the southern Sierra Nevada–San Joaquin Basin region as a result of underlying mantle lithosphere removal. The coupled Sierra Nevada mountain range and Great Valley basin are recognized as a relatively rigid block (Sierra Nevada microplate) moving within the San Andreas–Walker Lane dextral plate juncture system. Our analysis recognizes that the Sierra Nevada possessed kilometer-scale local and regional paleotopographic relief, and that the Great Valley forearc basin possessed comparable structural relief on its principal stratigraphic horizons, both dating back to the end of Cretaceous time. Such ancient paleorelief must be accounted for in considering late Cenozoic components of uplift and subsidence across the microplate. We further recognize that Cenozoic rock and surface uplift must be considered from the perspectives of both local epeirogeny driven by mantle lithosphere removal, and regional far-field–forced epeirogeny driven by plate tectonics and regional upper-mantle buoyancy structure. Stratigraphic relations of Upper Cretaceous and lower Cenozoic marine strata lying on northern and southern Sierra Nevada basement provide evidence for near kilometer-scale rock uplift in the Cenozoic. Such uplift is likely to have possessed positive, and then superposed negative (subsidence) stages of relief generation, rendering net regional rock and surface uplift. Accounting for ancient paleorelief and far-field–driven regional uplift leaves a residual pattern whereby ∼1200 m of southeastern Sierra crest rock and similar surface uplift, and ∼700 m of spatially and temporally linked tectonic subsidence in the southern Great Valley were required in the late Cenozoic by mantle lithosphere removal. These values are close to the predictions of our modeling, but application of the model results to the observed geology is complicated by spatial and temporal variations in the regional tectonics that probably instigated mantle lithosphere removal, as well as spatial and temporal variations in the observed uplift and subsidence patterns. Considerable focus is given to these spatial-temporal variation patterns, which are interpreted to reflect a complex three-dimensional pattern resulting from the progressive removal of mantle lithosphere from beneath the region, as well as its epeirogenic expressions. The most significant factor is strong evidence that mantle lithosphere removal was first driven by an east-to-west pattern of delamination in late Miocene–Pliocene time, and then rapidly transitioned to a south-to-north pattern of delamination in the Quaternary.

Epeirogenic transients related to mantle lithosphere removal in the southern Sierra Nevada region, California, part I: Implications of thermomechanical modeling

The putative Pliocene–Quaternary removal of mantle lithosphere from beneath the southern Sierra Nevada region (California, USA) is investigated by the iteration of thermal-mechanical models that incorporate and are tested against a range of data that are geologically observable, including rock uplift and basin subsidence data, structural and compositional data on crustal architecture, and a synthesis of seismic data that image lower crust–upper mantle structure of the region. The primary focus is testing model results with rock uplift and basin subsidence data. The initial state of our models recognizes that (1) the sub–Sierra Nevada batholith mantle lithosphere, including a substantial thickness of eclogitic cumulates that were produced during high magma flux arc activity, termed arclogite, was cooled to a conductive geotherm by amagmatic flat slab subduction at the end of the Cretaceous; and (2) the gravitationally metastable mantle lithosphere was thermally mobilized from beneath in the Neogene by the opening of an underlying slab window. Based on a detailed synthesis of appropriate rheologies of the multiphase system, a preferred class of models correctly predicts (1) the ca. 10 Ma inception of the Sierra Nevada microplate due to a lithospheric separation event along the eastern Sierra Nevada region as a result of the mobilization of the mantle lithosphere as a Rayleigh-Taylor instability; and (2) the subsequent delamination of the arclogite root of the Sierra Nevada batholith that appears to be in progress. Our preferred model also predicts focused rock uplift and basin subsidence resulting from delamination, both of which are anomalous to uplift and subsidence patterns of all other regions of the microplate. The rheology of the Great Valley crust is found to control rock uplift patterns across the Sierra Nevada, and tectonic subsidence in the Tulare Basin of the Great Valley. The Tulare Basin is uniquely situated over the region where the principal residual arclogite root remains attached to batholithic crust. The anomalous rock uplift and tectonic subsidence data are best satisfied by modeling a bulk rheology for the Great Valley crust that is similar to that of the Sierra Nevada batholith. These results are consistent with a recent synthesis of basement core and geophysical data showing that much of the Great Valley basement consists of the western Early Cretaceous zones of the Sierra Nevada batholith. The existence of this batholithic domain within the Great Valley subsurface is also in agreement with recent seismic data that resolve additional residual arclogite root materials along the base of the crust of this region.

Development of the Eastern California Shear Zone — Walker Lane belt: The effects of microplate motion and pre-existing weakness in the Basin and Range

The role that the motion of microplates has played in the development of the Eastern California Shear Zone in western North America is unclear. It was previously proposed that shear related to Baja California motion has propagated from the Gulf of California northward into Nevada. However, there is evidence that the formation of the Eastern California Shear Zone predates the inception of Baja California motion. Thus, either Baja California began to move northwestward prior to 6 Ma, or the Eastern California Shear Zone formed by shear strain resulting from different microplate motion than Baja California. The role of Sierra Nevada motion on the development of an adjacent shear zone remains untested. We present a numerical modeling study that examines the deformational response of western North America to Baja California and/or Sierra Nevada microplate motion. In particular we study if, and under what condition of microplate motion, shear strain can localize in the northern and southern part of the Eastern California Shear Zone (separated by the Garlock fault). We find that given the pre-existing weakness from Miocene Basin and Range extension (a lithospheric strength contrast or normal faults) shear strain from either the Baja California or Sierra Nevada microplates can lead to the formation of the Eastern California Shear Zone. Furthermore, the presence of a pre-existing weakness explains the earlier initiation of strike-slip faulting north of the Garlock fault. Our models suggest that today, Baja California microplate motion is the major player for present-day deformation rates along the Eastern California Shear Zone and was in the past 6 Myrs also a major driver of strain localization in southern Eastern California Shear Zone within the Mojave block.

Transtensional analyses of fault patterns and strain provinces of the Eastern California shear zone–Walker Lane on the eastern margin of the Sierra Nevada microplate, California and Nevada

Obliquity of the strain velocity field to the deformation zone boundary requires strain to be triaxial. At the plate boundary scale, fault geometries predicted by transtensional theory better explain observed fault patterns in the northern Eastern California shear zone–Walker Lane area than do 2D plane strain predictions. Structural provinces in the Eastern California shear zone–Walker Lane are defined by variations in the orientation of the eastern margin of the Sierra Nevada microplate, and include the Honey Lake–Pyramid Lake, Lake Tahoe, Mono Lake–Long Valley, Owens Valley, and Coso subprovinces. The local geometry of the transtensional zone boundary and the microplate transport direction determines orientations of the instantaneous strain axes for each province. Fault orientations predicted with respect to these axes are consistent with those observed in each structural province. The variation of zone geometry among predominantly coaxially dominated strain provinces over a consistent period of deformation does not result in dramatic changes in shape or orientation among finite strain ellipsoids. Although Quaternary faulting strongly reflects transtensional kinematics, the magnitude of estimated finite strain for each province suggests that present-day transtensional deformation has not accommodated large amounts of extension, vertical thinning, or strike-slip offset. K-values for all structural provinces plot in the k>1 constrictional field on the logarithmic Flinn (Ramsay) diagram, indicating non-plane, transtensional strain. Collectively, these theoretical applications and structural–tectonic observations have implications for evaluating kinematics and faulting during brittle non-plane strain deformation, and for the overall evolution of the transtensional Sierra Nevada–North America plate boundary.

New evidence for alternating effusive and explosive eruptions from the type section of the Stanislaus Group in the ‘Cataract’ palaeocanyon, central Sierra Nevada

High-K volcanic rocks of the Stanislaus Group record magmatic events that occurred when the Sierra Nevada microplate began to calve off from the western edge of the Nevadaplano at 10–9 Ma. Despite the fact that these rocks have been studied in the Sierra Nevada for 120 years, their stratigraphy is not well known, because they were largely deposited in a complicated palaeocanyon network that drained the Nevadaplano and are preserved on high peaks in roadless areas. We present new 1:6000 scale mapping and a detailed measured section in the Bald Peak–Red Peak area of the Sonora Pass, in the upper reach of the ‘Cataract’ palaeocanyon, or palaeo-Stanislaus river. The Bald Peak–Red Peak area was proposed as the type section for the Stanislaus Group in 1953 but had not been mapped or measured in sufficient detail to demonstrate stratigraphic relations. Our work demonstrates the following stratigraphy (from base to top): (1) Table Mountain Latite (TML) trachyandesite (latite) and basaltic trachyandesite (shoshonite) lava flows, with phenocrystic clinopyroxene and skeletal plagioclase and groundmass olivine, and minor olivine basalt and basaltic andesite lava flows; (2) Tollhouse Flat Member of the Eureka Valley Tuff (EVT) welded ignimbrite, with abundant phenocrystic biotite; (3) a single trachydacite lava flow with phenocrystic amphibole, referred to here as the Lava Flow Member of the EVT; (4) By-Day Member of the EVT welded ignimbrite, which lacks phenocrystic biotite; (5) Upper Member of the EVT, consisting of unwelded ignimbrite; and (6) a single, very thick (60 m) aphyric basaltic trachyandesite (shoshonite) lava flow that we assign to the Dardanelles Formation. Our work confirms for the first time that the Dardanelles Formation exists as a unit that overlies all members of the EVT, and that its geochemistry is the same as that of the TML in the type section. Our documentation of a trachydacite lava flow within the EVT in the central Sierra Nevada shows that widespread effusive volcanism alternated with the explosive volcanism, rather than merely preceding and following it, and that magmas of trachydacitic composition were erupted as lavas as well as pyroclastic flows. The preservation of all formations and members of the Stanislaus Group in the ‘Cataract’ channel suggests that it was an important palaeogeomorphic feature.

Step-over in the structure controlling the regional west tilt of the Sierra Nevada microplate: eastern escarpment system to Kern Canyon system

The Sierra Nevada and Great Valley are coupled, and behave as a semi-rigid microplate. The microplate formed as it was calved off the western edge of the Nevadaplano in the late Miocene, at which time westward regional tilting began. Tilting is controlled by west-side-up normal faulting primarily along the eastern Sierra escarpment system. Uplift and exhumation along the eastern Sierra are balanced by subsidence and sedimentation along the western Great Valley. The west tilt of the microplate is expressed by the west slope of a regional relict landscape surface that developed across much of the Sierra Nevada basement, and by the westward continuation of the surface as the basal Eocene nonconformity of the west-dipping Great Valley Tertiary section. The rigid behaviour of the microplate breaks down along its southern ∼100–150 km segment as expressed by seismicity, pervasive faulting and the development of a deep marine basin, the San Joaquin Basin (SJB), whose facies and palaeogeographic patterns diverge from regional patterns of the rest of the Great Valley. The disrupted state of the southern segment of the microplate resulted from its Late Cretaceous position above a regional lateral ramp in the underlying Franciscan-related subduction megathrust. The Kern Canyon fault system began its polyphase history as a complex oblique dextral shear zone above the megathrust lateral ramp. It was remobilized in the Neogene as an oblique transfer structure partitioning differential extension between the southern Sierra Nevada and the SJB. In Quaternary time, the Kern Canyon zone was again remobilized as a west-side-up normal fault system whose geomorphic and structural expressions are best developed south of ∼36.4° N. This normal fault system controls the west tilt of the relict landscape surface in the southern Sierra region, as well as the west dip pattern in the strata of the adjacent SJB. To the east of the Kern Canyon normal fault system, the relict landscape surface slopes continuously southwards from the high eastern Sierra into a low-lying, multiply extended terrane. Thus, from ∼36.4° N southwards, the west tilt along the western Sierra and the west dip of the adjacent Great Valley strata are controlled by the Kern Canyon system. Fresh normal scarps along the eastern Sierra escarpment system become more subdued and ultimately die out southwards from ∼36.4° N. Thus, currently, the controlling structure for the west tilt of the microplate steps westwards in the south from the eastern escarpment system over to the Kern Canyon system.

Transtension in the Brittle Field: The Coso Region, Southern California

Transtension in the upper crustal brittle field, at the lithospheric scale, involves some difficult kinematic problems that result from the non-coaxial component of bulk strain, which generates rotation about vertical axes and the internal deformation of rotating blocks. Plate boundary zone deformation in the non-plane-strain brittle field cannot involve the rotation of a mosaic of very large rigid crustal blocks. In transtension, constriction is achieved by simultaneous arrays of rotating normal faults and wrench faults, partitioned spatially and temporally, producing very complex polyphase brittle deformation. The Coso transtensional region at the southeastern margin of the Sierra Nevada-Great Valley block illustrates the complex synchroneity and polyphase superposition of rotating normal and wrench fault arrays in a constrictional strain field in the brittle regime, which generates a "continuum rubble" of small blocks.

Transtensional Deformation of the Brittle Crust: Field Observations and Theoretical Applications in the Coso-China Lake Region, Eastern Margin of the Sierra Nevada Microplate, Southeastern California

The focus of this work is the description of non-plane strain brittle deformation through field observations and theoretical applications. The seismically and volcanically active Coso—China Lake region in the transtensional Eastern California shear zone is an exemplary location for field observation of brittle structure. The region deforms in a constrictional strain field, well-described by earthquake focal mechanisms and GPS measurements, and further demonstrated by our field observations and outcrop-scale mapping. Fundamental geometric relationships in transtensional zones and tectonic reconstruction allows practical applications of theory using real transtension zone parameters, and expands our ability to describe triaxial strain in three dimensions. Theoretical instantaneous strain axes are constrictional and oriented xi WNW, yi NNE, zi vertical, similar to seismogenic and GPS derived instantaneous strain axes, and kinematic analysis of field data. Orientations of brittle structures in the field correspond reasonably well to theoretical predictions, with normal faults striking generally NNE, parallel to yi, dextral faults northwest to NNW, sinistral faults ENE, and fold axes and thrust faults WNW, parallel to xi. We calculate theoretical rates and amounts of shortening, elongation, and rotation for specific structures within the deformation zone; paleomagnetically-derived rotations in Pliocene volcanics fall within our predicted ranges. We subdivide the field area based on geometry and derive shapes and orientations of the instantaneous and finite strain ellipsoids and associated k-values (Northern Coso, k = 6.84; Southern Coso, k = 2.21). Our results indicate a less prolate, more coaxially-dominated, higher strain and strain rate in Southern Coso, and a more prolate, less coaxially dominated, lower strain and strain rate in Northern Coso, part of which is reflected by elevation and topography. We observe varying degrees of three-dimensional strain partitioning of brittle structures depending on scale, as is also seen in seismicity, and possible partitioning of rotations around vertical and horizontal axes. Multi-scale partitioning of strain and rotation, influenced by preexisting and reactivated structures, characterizes transtensional deformation at Coso, and is well described by kinematic analysis of structural field observations and regional theoretical applications and analyses.

Crustal deformation across the Sierra Nevada, northern Walker Lane, Basin and Range transition, western United States measured with GPS, 2000–2004

Global Positioning System (GPS) data collected in campaigns in 2000 and 2004 were processed and interpreted with other GPS data in the western Basin and Range province to provide new constraints on the rate, style, and pattern of deformation of the central and northern Walker Lane (WL), which lies near the western boundary of the Basin and Range. Across the central WL, near 38°N latitude, the velocities with respect to North America increase westward by ∼10 mm/yr inducing dextral shear. Farther north between 40° and 41°N latitude, a western zone of ∼7 mm/yr relative motion undergoes dextral shear, and an eastern zone of ≤3 mm/yr relative motion undergoes extension and shear. These data show that the northern WL is essentially a dextral shear zone experiencing minor net dilatation (ɛΔ = 2.6 ± 0.8 nstrain/yr). Near most Holocene normal faults, dilatation inferred from the velocity field is not greater than the uncertainties. However, near the central Nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts (ɛΔ = 23.0 ± 3.9 nstrain/yr), some of which is attributable to transient postseismic deformation following large historic earthquakes. A block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the Honey Lake, Warm Springs, east Pyramid fault system, and Mohawk Valley faults is ∼7 mm/yr. The WL is a zone whose width and dilatation rate increase northwestward, consistent with counterclockwise rotation of the Sierra Nevada microplate and transfer of deformation into the Pacific Northwest.

Thirty-Five-Year Creep Rates for the Creeping Segment of the San Andreas Fault and the Effects of the 2004 Parkfield Earthquake: Constraints from Alignment Arrays, Continuous Global Positioning System, and Creepmeters

We present results from differential Global Positioning System (gps) surveys of seven alignment arrays and four continuous gps sites along the creeping segment of the San Andreas fault. Surveys of four alignment arrays from the central creeping segment yield 33- to 36-year average minimum slip rates of 21–26 mm/yr. These rates are consistent with previous alignment array surveys spanning a 10-year period and with rates determined by creepmeters, indicating approximate steady- state creep along the central creeping segment for at least 35 years. Motion between continuous gps sites that span the central creeping segment is 28.2 ± 0.5 mm/yr for two sites that are 1 km apart and 33.6 ± 1 mm/yr for two sites that are 70 km apart. Slip rates therefore increase with distance from the creeping segment of the San Andreas fault. All rates reported here are significantly slower than the 39 ± 2 mm/yr rate predicted for motion between the Sierra Nevada–Great Valley block and the Pacific plate. Repeat surveys of three alignment arrays following the 2004 Parkfield earthquake demonstrate that its coseismic and short-term postseismic offsets decrease rapidly with distance from the epicenter, from 150 mm to 15 mm to <5 mm at respective distances of 9, 36, and 54 km to the northwest. Continuous gps data confirm that little coseismic and postseismic slip occurred along the central creeping segment. Geodetic and geologic slip rates are compared and different models for the accommodation of transcurrent deformation across the creeping segment are discussed.

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.