Geochemistry of the Great Valley Group: An Integrated Provenance Record

Sedimentary geochemistry of fine-grained strata of the Great Valley Group (GVG) in California documents a provenance signal that may better represent unstable, mafic minerals and volcanic clasts within sediment source regions than the provenance signal documented in the petrofacies and detrital zircon analysis of coarser sedimentary fractions. Geochemistry of the GVG provides an overall provenance framework within which to interpret sandstone petrofacies and detrital zircon age signatures. The geochemical signature for all Sacramento Valley samples records an overall continental arc source, with significant variation but no clear spatial or temporal trends, indicating that the geochemical provenance signal remained relatively consistent and homogenized through deposition of Sacramento basin strata. The San Joaquin basin records a distinct geochemical provenance signature that shifted from Early to Late Cretaceous time, with Lower Cretaceous strata recording the most mafic trace element geochemical signature of any GVG samples, and Upper Cretaceous strata recording the most felsic geochemical signature. These provenance results suggest that the early San Joaquin basin received sediment from the southern Sierran foothills terranes and intruding plutons during the Early Cretaceous, with sediment sources shifting east as the southern Sierran batholith was exhumed and more deeply eroded during the Late Cretaceous. The GVG provenance record does not require sediment sources inboard of the arc at any time during GVG deposition, and even earliest Cretaceous drainage systems may not have traversed the arc to link the continental interior with the margin. Because the GVG provenance signature is entirely compatible with sediment sources within the Klamath Mountains, the northern and western Sierran foothills belt, and the main Cretaceous Sierran batholith, the Klamath-Sierran magmatic arc may have formed a high-standing topographic barrier throughout the Cretaceous period

Cenozoic Tectonic Evolution of the Central Wassuk Range, Western Nevada, USA

The central Wassuk Range of western Nevada is ideally located to investigate the interplay of Basin and Range extension and Walker Lane dextral deformation on the margin of the Basin and Range province. To elucidate the Cenozoic evolution of the range, the author conducted geologic mapping, structural data collection and analysis, geochemical analysis of igneous lithologies, and geochronology. This research delineates a three-stage deformational history for the range. A ~15 Ma pulse of ENE-WSW directed extension at high strain rates (~8.7 Ma) was immediately preceded by the eruption of andesites and was accommodated by high-angle, closely spaced (1-2 km), east-dipping normal faults which rotated and remained active to low angles as extension continued. A post-12 Ma period of extension at low strain rates was accommodated by a second generation of normal faults and two prominent dextral strike slip faults which strike NW, subparallel to the dextral faults of the Walker Lane at this latitude. A new pulse of extension began at ~4 Ma and continues today. The increase in the rate of range-bounding fault displacement has resulted in impressive topographic relief on the east flank of the Wassuk Range and supports a shift in extension direction from ENE-WSW during the highest rates of Miocene extension to WNW-ESE today. The total extension accommodated across the central Wassuk Range since the Middle Miocene is \u3e 200%, with only a brief period of dextral fault activity during the Late Miocene. Data presented here suggest a local geologic evolution intimately connected to regional tectonics, from intra-arc extension in the Middle Miocene, to Late Miocene dextral deformation associated with the northward growth of the San Andreas fault, to a Pliocene pulse of extension and magmatism likely influenced by both the northward passage of the Mendocino triple junction and possible delamination of the crustal root of the southern Sierra Nevada

Modern Strain Localization in the Central Walker Lane, Western United States: Implications for the Evolution of Intraplate Deformation in Transtensional Settings

Approximately 25% of the differential motion between the Pacific and North American plates occurs in the Walker Lane, a zone of dextral motion within the western margin of the Basin and Range province. At the latitude of Lake Tahoe, the central Walker Lane has been considered a zone of transtension, with strain accommodated by dip-slip, strike-slip, and oblique-slip faults. Geologic data indicate that extension and strike-slip motion are partitioned across the central Walker Lane, with dip-slip motion resulting in E–W to ESE–WNW extension along the present-day western margin of the central Walker Lane since approximately 15 Ma, and dextral strike-slip motion across a zone further east since as early as 24 Ma. GPS velocity data suggest that present-day strain continues to be strongly partitioned and localized across the same regions established by geologic data. Velocity data across the central Walker Lane suggest a minimum of 2 mm/yr extensional strain focused along the western margin of the belt, with very little extension across either the central or eastern portions of the Walker Lane. These data indicate very little dextral motion across the central and western portions of the domain, with dextral motion of 3–5 mm/yr presently focused along a discrete zone of the eastern part of the central Walker Lane, coincident with existing, mapped strike-slip faults. Historic seismic data reveal little seismic activity in areas of Late Holocene dip-slip motion in the west or dextral motion in the east, suggesting a period of quiescence in the earthquake cycle and the likelihood of future activity in both areas. Based on this and previous studies, it is likely that a combination of pre-Cenozoic crustal structure, a relatively weak lithosphere beneath the Walker Lane, and long-term low stress ratios in the crust have permitted the long-term partitioning of dextral and extensional strain exhibited across the central Walker Lane. The present-day location of dextral strain in the central Walker Lane is subparallel with dextral deformation documented in the northern Walker Lane, suggesting that as strain continues to accumulate, these two discrete zones could become a continuous strike-slip system which will play a more important role in the future accommodation of relative Pacific–North American plate motion

Hornbrook Formation, Oregon and California: A Sedimentary Record of the Late Cretaceous Sierran Magmatic Flare-Up Event

Early Late Cretaceous time was characterized by a major magmatic flare-up event in the Sierra Nevada batholith and early phases of magmatism in the Idaho batholith, but the sedimentary record of this voluminous magmatism in the U.S. Cordillera is considerably less conspicuous. New detrital zircon U-Pb ages from the Hornbrook Formation in southern Oregon and northern California reveal a significant and sustained influx of 100–85 Ma detrital zircons into the broader Hornbrook region beginning ca. 90 Ma. Detrital zircon ages and hafnium isotopic compositions, combined with whole-rock geochemistry, suggest that sediment was largely derived from the Sierra Nevada, requiring uplift and erosion of the Sierra Nevada batholith during and immediately following the Late Cretaceous magmatic flare-up event. Sediment derived from the eroded arc may have been transported northward along the axis of the arc, between a western drainage divide along the arc crest and the rising Nevadaplano to the east. Although the Klamath Mountains and Blue Mountains Province present more proximal potential sources of Jurassic and Early Cretaceous detrital zircons in the Hornbrook Formation than the Sierra Nevada, Late Cretaceous deposition on the Klamath Mountains 80 km west of Hornbrook Formation outcrops, and Late Cretaceous deep-water deposition on the Blue Mountains in the Ochoco Basin suggest that these regions were the locus of subsidence and sedimentation, rather than erosion, during Late Cretaceous time. The limited outcrop extent of the Hornbrook Formation may represent only a sliver of a much larger Late Cretaceous Hornbrook basin system. Complete characterization of the episodic magmatic history of continental arcs requires integration of age distributions from the arc itself and from detrital zircons eroded from the arc; it is critical to recognize the potential of drainage systems to transport sediment to depocenters not directly linked to present-day arc exposures

Segmentation of the Wassuk Range Normal Fault System, Nevada (USA): Implications for Earthquake Rupture and Walker Lane Dynamics

Normal faults are commonly segmented along strike, with segments that localize strain and influence propagation of slip during earthquakes. Although geometry of segments can be constrained by fault mapping, it is challenging to determine seismically relevant segments along a fault zone. Because slip histories, geometries, and strength of linkages between normal fault segments fundamentally control the propagation of rupture during earthquakes, and differences in segment slip rates result in differential uplift of adjacent footwalls, we use along‐ strike changes in footwall morphology to detect fault segments and the relative strength of the mechanical links between them. We apply a new geomorphic analysis protocol to the Wassuk Range fault, Nevada, within the actively deforming Walker Lane. The protocol examines characteristics of footwall morphology, including range‐crest continuity, bedrock‐channel long profiles, catchment area variability, and footwall relief, to detect changes in strike‐parallel footwall characteristics. Results reveal six domains with significant differences in morphology that we use to identify seismically‐relevant fault segments and segment boundaries. We integrate our results with previous studies to determine relative strength of links between the six segments, informing seismic hazard assessment. When combined with recent geodetic studies, our results have implications for the future evolution of the Walker Lane, suggesting changes in the accommodation of strain across the region. Our analysis demonstrates the power of this method to efficiently detect along‐strike changes in footwall morphology related to fault behavior, permitting future researchers to perform reconnaissance assessment of normal fault segmentation worldwide

Provenance of the Pythian Cave Conglomerate, Northern California: Implications for Mid-Cretaceous Paleogeography of the U.S. Cordillera

Provenance analysis of middle Cretaceous sedimentary rocks can help distinguish between disparate tectonic models of Cretaceous Cordilleran paleogeography by establishing links between sediment and source, as well as between currently separated basins. This study combines new detrital zircon age data and compositional data with existing provenance data for the Pythian Cave conglomerate, an informally-named unit deposited unconformably on the eastern Klamath Mountains, to test possible correlations between the Pythian Cave conglomerate and similar-age deposits in the Hornbrook Formation and the Great Valley Group. These provenance results indicate that restoring Late Cretaceous clockwise rotation of the Blue Mountains adds a significant sediment source for Cretaceous basins previously associated with only the Klamath Mountains (e.g., the Pythian Cave conglomerate and Hornbrook Formation) or a combined Klamath-Sierran source (e.g., Great Valley Group). Comparison of the Pythian Cave conglomerate with the Klamath River Conglomerate and the Lodoga petrofacies suggests that the Pythian Cave conglomerate system was separate from the nearby Hornbrook Formation and was probably related to the Lodoga petrofacies of the Great Valley Group

The Impact of Inter-Bed Cohesion on Fold-Related Fracture Development, Stillwell Anticline, West Texas (USA)

The interpretation of fracture networks in contractional folds is challenging due to the range of factors that control fracture formation. We use outcrop-based analysis of fractures in plan-view pavements and in a 9-bed cross-sectional exposure to evaluate the fracture system within Cretaceous limestone layers of a Laramide fold in west Texas, the Stillwell anticline. Opening-mode extension fractures (veins) at high angles to bedding dominate the fracture population, although shear fractures and faults cut bedding at low angles within the forelimb. Analysis of extension fractures reveals NW-striking axial parallel and NE-striking axial-perpendicular fracture sets interpreted to have formed during contractional folding, a third N-striking fracture set formed during subsequent Basin and Range extension, and a fourth ESE-striking fracture formed due to unloading during exhumation. Fracture fill textures suggest that many fracture apertures increased during exhumation. The relative abundances of the four fracture sets and the intensity of each set vary from bed to bed in cross section. Because beds display no significant differences in mechanical strength and there is no correlation between bed thickness and fracture intensity, we attribute this bed-to-bed variability to differences in cohesion between beds. Bed decoupling, when combined with low extensional

Understanding a Critical Basinal Link in Cretaceous Cordilleran Paleogeography: Detailed Provenance of the Hornbrook Formation, Oregon and California

The Hornbrook Formation is a Cretaceous overlap assemblage that rests unconformably on accreted terranes and plutons of the Klamath Mountains in southern Oregon and northern California. The combined results of sandstone petrography, detrital zircon U-Pb age and Hf isotopic systematics, and whole-rock Nd analysis document an abrupt change in sediment sources for the Hornbrook Formation during the Late Cretaceous. The lower members of the Hornbrook Formation record provenance in the Klamath Mountains and the Sierran Foothills belt that is characterized by detrital zircon age distributions with large Jurassic and Early Cretaceous peaks (170-130 Ma) and positive zircon Hf and whole-rock Nd values. In contrast, upper members of the Hornbrook Formation include abundant sediment derived from the Cretaceous Sierran Batholith that is characterized by large Cretaceous peaks (120-85 Ma) in detrital zircon age distributions and less positive zircon Hf and whole-rock Nd values. A similar Late Cretaceous provenance shift is present in the Great Valley Group of California, which likely formed the southern continuation of the Hornbrook basin during deposition of the upper Hornbrook members. These provenance results may reflect changing plate kinematics along the U.S. Cordilleran margin during the Late Cretaceous, including extension and subsidence in the Klamath Mountains and Blue Mountains regions followed by rapid uplift of the main Sierra Nevada Batholith. Thus, the detailed provenance signature for the Hornbrook Formation presented here records regional tectonic events in the mid- to Late Cretaceous U.S. Cordillera, and suggests that the Hornbrook Formation and Great Valley Group shared similar sources but remained separate basins until mid- to Late Cretaceous time

Facies Architecture and Provenance of a Boulder-Conglomerate Submarine Channel System, Panoche Formation, Great Valley Group: A Forearc Basin Response to Middle Cretaceous Tectonism in the California Convergent Margin

Tectonic reorganization induced by a rapid increase in plate motion ­obliquity and rate beginning at ca. 100 Ma affected California’s Andean-style convergent margin, with concomitant changes in the accretionary prism of the Franciscan Complex, the Great Valley forearc basin, and the Sierran continental arc. Using facies analysis and a combined provenance approach, we suggest that this ca. 100 Ma tectonic signal is preserved in a Cenomanian (Upper Cretaceous) boulder-conglomerate outcrop along the San Luis Reservoir (SLR) in the southern Great Valley, which represents the thickest and coarsest deep-water deposit ever described in the Great Valley Group (GVG). We document a 1.8-km-thick by 4-km-long depositional-dip profile of an interpreted SE-directed (axial) submarine channel system that is part of a conglomeratic package that stretches 20 km along the east-central Diablo Range. Our facies analysis of the SLR area documents five facies associations within four aggradational channel complex sets, followed by regional abandonment. Sandstone petrography and mudrock geochemical data suggest a dissected continental Sierra Nevadan arc source. Conglomerate clast counts show abundant ophiolitic-type clasts that may be derived from the Coast Range Ophiolite and/or the Western Sierra Nevada Metamorphic Belt. Detrital-­zircon geochronology data also indicate western and central Sierra Nevadan sources; however, we interpret an anomalous (relative to other Cenomanian localities) 105–95 Ma zircon population to indicate the initial erosional products from the volcanic carapace associated with the Late Cretaceous magmatic flare-up within the eastern Sierran arc. This flare-up has been linked to an increase in arc-parallel plate motion that induced deformation along shear zones in the eastern Sierra Nevada, allowing for widespread plutonism. Our provenance interpretation makes the SLR area the earliest Upper Cretaceous GVG locality to receive significant detritus from the flare-up, effectively linking tectonic plate motion changes and coarse-grained, deep-water forearc sedimentation

The Birth of a Forearc: The Basal Great Valley Group, California, USA

The Great Valley basin of California (USA) is an archetypal forearc basin, yet the timing, structural style, and location of basin development remain controversial. Eighteen of 20 detrital zircon samples (3711 new U-Pb ages) from basal strata of the Great Valley forearc basin contain Cretaceous grains, with nine samples yielding statistically robust Cretaceous maximum depositional ages (MDAs), two with MDAs that overlap the Jurassic-Cretaceous boundary, suggesting earliest Cretaceous deposition, and nine with Jurassic MDAs consistent with latest Jurassic deposition. In addition, the pre-Mesozoic age populations of our samples are consistent with central North America sources and do not require a southern provenance. We interpret that diachronous initiation of sedimentation reflects the growth of isolated depocenters, consistent with an extensional model for the early stages of forearc basin development