Diablo Range Research Articles

Quaternary-Active Faults and the Role of Inherited Structures in the Sacramento-San Joaquin Delta, Western Central Valley, Northern California

Seismic sources and their associated hazards within the Sacramento-San Joaquin Delta region of north-central California are relatively poorly characterized as compared to other, more heavily studied regions of northern California, such as the San Francisco Bay Area. Here we present a synthesis of subsurface, bedrock geology, and geodetic datasets from the Delta and from the Coast Ranges and Diablo Range to the northwest and southwest, respectively. We integrate these data and our own surface geologic and geomorphic observations to present a comprehensive review of faults in the Delta that exhibit Quaternary activity. Structural geologic data from the surrounding region highlight the significant influence that Late Cretaceous-to-Paleogene forearc structures exert on the geometry and kinematics of major Quaternary-active structures within the Delta. These inherited structures — including the Pittsburg-Kirby Hills Fault, Midland Fault, and Great Valley Fault System — exhibit a range of geometries and kinematics. Analysis of geomorphology along these structures suggests that these structures combine to accommodate Quaternary strain across the Delta region. A clearer understanding of subsurface geometries and structural relationships, built upon the regional tectonic history, provides insight into modern deformation accommodated on older structures and helps inform interpretations of seismic hazard within the Delta.

Habitat suitability assessment for tule elk in the San Francisco Bay and Monterey Bay areas

While California’s statewide tule elk (Cervus canadensis nannodes) population has recovered from two or three individual survivors in the late 19th century, the subspecies exists today in numerous widely disjunct populations, leaving vast areas of the species’ former range uninhabited. Large unoccupied areas of historic tule elk range include the Santa Cruz Mountains and the northern Diablo and northern Santa Lucia ranges. Natural range expansion by existing populations into these areas is blocked by major highways and urban development; although, before considering tule elk translocations, it is necessary to assess the habitat suitability there. To this end, we fit a resource selection function (RSF) using generalized linear mixed models to GPS collar data collected from nearby radio collared tule elk and used several environmental GIS layers to capture important habitat characteristics. We fit the RSF in a habitat use versus availability framework with only linear and quadratic terms and used stepwise model selection ranked by AICc to maximize its generalizability, enabling transferability to our unoccupied study area. We also used k-fold cross validation to evaluate our RSF and found it predicted habitat within the San Luis Reservoir herd well. The fit habitat relationships mostly followed expectations based on tule elk ecology, including positive responses to herbaceous vegetation cover and waterbody proximity, and negative responses to high tree cover and high puma habitat suitability. Our RSF accurately predicted currently occupied elk habitat as suitable and found well over 500,000 ha (2,000 mi2) of suitable but unoccupied habitat throughout the northern Diablo Range, the inland and coastal sides of the Santa Cruz Mountains, and the northern Santa Lucia Range. Assuming translocations, and construction and improvement of highway wildlife crossings, our results support the potential for re-establishing tule elk in these regions, which are more coastal and mesic than the species’ current habitat in the central Diablo and northern Gabilan ranges.

Paleogeographic reconstruction of regional accretionary complex architecture, Franciscan Complex, northwestern San Francisco Bay Area, California, USA

Abstract The Franciscan Complex of western California, USA, the archetypal subduction accretionary complex, cannot serve as a model subduction accretionary complex unless its local-to-regional architecture is clearly understood. Yet, architectural details are not clearly understood in many regions, including the northwestern San Francisco Bay Area. Here, Cenozoicage, dextral strike-slip faulting on faults of the San Andreas System fragmented the original architecture, forming crustal blocks and juxtaposing fragments of accretionary rock of different tectonostratigraphy. One little-known Cenozoicage fault and block boundary, the Tamarancho Shear Zone, separates northeastern crustal blocks that are dominated by Franciscan rocks from southwestern blocks with significantly different Franciscan accretionary tectonostratigraphy. The northeastern blocks have abbreviated accretionary stacks with at least one blueschist-facies accretionary unit, whereas the southwestern blocks lack blueschist-facies accretionary units and have either a westward-and-downward–younging Franciscan tectonostratigraphy or thrustsheet stacks composed of partial sequences of ocean-plate stratigraphy rocks. The northwestern San Francisco Bay Area Franciscan Complex is bounded on the southwest by the San Andreas fault (sensu stricto) and on the northeast by the Petaluma Valley–Point Richmond–Silver Creek fault. Using paleogeographic reconstruction, the original Franciscan Complex accretionary architecture of the northwestern San Francisco Bay Area can be partially reconstructed by removing block separations on San Andreas System faults and enhanced by unfolding Cenozoic folds. Accretionary units of the northwestern San Francisco Bay Area Franciscan Complex were originally assembled ~190 km southeast of their present locations, west of the southern Diablo Range. Reconstruction of the accretionary complex in that location and considerations of tectonostratigraphy require that the Novato Block, located northeast of the Tamarancho Shear Zone, and the Mt. Tamalpais Block, to its southwest, be separated along or across strike in the reconstructed accretionary complex. Either dual subduction zone or faulted plate geometries produced the northwestern San Francisco Bay Area segment of the accretionary complex, and each model highlights the possibilities of along- or across-strike variations in the structure and history of the accretionary complex.

Review of considerations for restoration of tule elk to the San Francisco Peninsula and northern Monterey Bay counties of California

Successful translocations of tule elk (Cervus canadensis nannodes) have been conducted since the early 1900s, with their state population rising from a nadir of as few as three surviving individuals to about 500 when reintroductions began, and to over 5,700 by 2017. However, natural range expansion of extant populations is currently limited by heavily trafficked major highways and urban areas with dense human populations. We determined that the San Francisco Peninsula and northern Monterey Bay counties (the study area) offer 193,973 ha (479,308 acres) of protected open space, several orders of magnitude greater than coastal tule elk home range size. Habitat suitability is supported by abundant historical observer, museum, and archeological records of elk located in this region. The nearest elk population to the study area is in eastern Santa Clara County and has grown from 65 animals that were translocated to Mt. Hamilton in the Diablo Range from 1978–1981 to at least 90 in five–six separate herds counted by aerial and photographic surveys in 2019. United States (U.S.) Highway 101 and metropolitan San Jose remain barriers to western range extension. Translocation and/or construction of freeway over- and under-crossings may enable westward range expansion to a less arid region, contributing to increased resilience of tule elk to climate change, and bringing aesthetic, financial, and ecological benefits of this once native ungulate grazer to the area.

New Idria serpentinite protrusion, Diablo Range, California: From upper mantle to the surface

The New Idria serpentinite body in the Coast Ranges of California is a diapir that resulted from the interaction of the migrating Mendocino trench-ridge-transform fault triple junction, transpression, metasomatic fluids, and previously subducted oceanic crust and mantle. Northward propagation of the San Andreas fault progressively eliminated the original subduction zone, allowing seawater to penetrate into the formerly subducting abyssal peridotite mantle, triggering serpentinization. The associated physical changes in density, volume, and strength yielded an expanding, buoyantly rising serpentinite protrusion, facilitated by transpression along the San Andreas fault. Sedimentary facies and intrusion of minor cross cutting syenite and alkali basalt dikes indicate that the serpentinization-driven diapir buoyantly rose and widely breached the surface by <i>ca.</i> 14 Ma, attending migration of the Mendocino Triple Junction past the latitude of New Idria.

Unsaturated Flow Processes and the Onset of Seasonal Deformation in Slow‐Moving Landslides

AbstractPredicting rainfall‐induced landslide motion is challenging because shallow groundwater flow is extremely sensitive to the preexisting moisture content in the ground. Here, we use groundwater hydrology theory and numerical modeling combined with five years of field monitoring to illustrate how unsaturated groundwater flow processes modulate the seasonal pore water pressure rise and therefore the onset of motion for slow‐moving landslides. The onset of landslide motion at Oak Ridge earthflow in California’s Diablo Range occurs after an abrupt water table rise to near the landslide surface 52–129 days after seasonal rainfall commences. Model results and theory suggest that this abrupt rise occurs from the advection of a nearly saturated wetting front, which marks the leading edge of the integrated downward flux of seasonal rainfall, to the water table. Prior to this abrupt rise, we observe little measured pore water pressure response within the landslide due to rainfall. However, once the wetting front reaches the water table, we observe nearly instantaneous pore water pressure transmission within the landslide body that is accompanied by landslide acceleration. We cast the timescale to reach a critical pore water pressure threshold using a simple mass balance model that considers variable moisture storage with depth and explains the onset of seasonal landslide motion with a rainfall intensity‐duration threshold. Our model shows that the seasonal response time of slow‐moving landslides is controlled by the dry season vadose zone depth rather than the total landslide thickness.

Origin of Mélanges of the Franciscan Complex, Diablo Range and Northern California: An Analysis and Review

The Franciscan Complex of California is characterized in part by the presence of mélanges. In general, mélange origins are attributed to sedimentary, tectonic, or diapiric processes—or a combination of these. Published reviews list the main features of mélanges characteristic of each type of origin. In this review, particular diagnostic features typical of sedimentary, tectonic, and diapiric mélanges are used to assess 15 specific mélanges, which in some cases have been interpreted in contrasting ways in the literature. The data do not support the view that most Franciscan mélanges were formed by sedimentary processes, but rather that both tectonic and sedimentary processes are important. There is little evidence that diapirism contributed significantly to Franciscan mélange genesis. Tectonic features present in most mélanges of subduction accretionary complexes create challenges in assessing mélange-forming processes. Notably, although tectonic overprints commonly mask the primary diagnostic fabric of sedimentary mélanges, some diagnostic features—such as depositional contacts, fossils in mélange matrix, and interlayering of mélange and non-mélange units—are critical to recognition of mélanges of sedimentary origin.

Observations of Foothill Yellow-Legged Frog Predation by a Native Frog, Snake, and Giant Water Bug in a Central California Intermittent Stream

During the summers of 2015 and 2018, we observed predation on Foothill Yellow-legged Frogs (Rana boylii) by a giant water bug (Abedus indentatus), a California Red-legged Frog (Rana draytonii), and a Diablo Range Gartersnake (Thamnophis atratus zaxanthus) adjacent to 3 separate isolated pools along intermittent reaches of Coyote Creek, Santa Clara County, California, USA. To the best of our knowledge, our observations provide the first published record of California Red-legged Frog and giant water bug preying upon Foothill Yellow-legged Frogs. As pool habitat contracts over the course of the dry season, locally abundant Yellow-legged Frogs may be increasingly vulnerable to predation from a suite of aquatic and terrestrial predators.

Drivers of earthflow motion revealed by an 80 yr record of displacement from Oak Ridge earthflow, Diablo Range, California, USA

Research Article| September 21, 2018 Drivers of earthflow motion revealed by an 80 yr record of displacement from Oak Ridge earthflow, Diablo Range, California, USA Alexander L. Nereson; Alexander L. Nereson † 1Department of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95060, USA †anereson@gmail.com Search for other works by this author on: GSW Google Scholar Noah J. Finnegan Noah J. Finnegan 1Department of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95060, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Alexander L. Nereson † 1Department of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95060, USA Noah J. Finnegan 1Department of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95060, USA †anereson@gmail.com Publisher: Geological Society of America Received: 25 Jan 2018 Revision Received: 10 May 2018 Accepted: 27 Jul 2018 First Online: 21 Sep 2018 Online Issn: 1943-2674 Print Issn: 0016-7606 © 2018 Geological Society of America GSA Bulletin (2019) 131 (3-4): 389–402. https://doi.org/10.1130/B32020.1 Article history Received: 25 Jan 2018 Revision Received: 10 May 2018 Accepted: 27 Jul 2018 First Online: 21 Sep 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alexander L. Nereson, Noah J. Finnegan; Drivers of earthflow motion revealed by an 80 yr record of displacement from Oak Ridge earthflow, Diablo Range, California, USA. GSA Bulletin 2018;; 131 (3-4): 389–402. doi: https://doi.org/10.1130/B32020.1 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 SocietyGSA Bulletin Search Advanced Search Abstract Predicting the velocity of slow landslides over multidecadal time scales requires an understanding of the potential drivers of landslide motion, analytical frameworks or models that connect those drivers to kinematic response, and historical records of movement that have sufficient spatial and temporal resolution to test model predictions. Given that detailed long-term records of slow landslide movement are still relatively uncommon, we used a time series of aerial imagery to assemble an 80 yr (1937–2017) record of movement for the Oak Ridge earthflow, a slow-moving landslide in California’s northern Diablo Range. Landslide movement was unsteady and nonuniform during this period. We used this history of movement, visual evidence from imagery and the field, and a record of surface moisture balance (the Palmer Drought Severity Index) to evaluate the relative roles of possible drivers of movement. Drivers considered included: supply of mobile regolith from the landslide source zone, fluvial debuttressing from below, changes in climate forcing over time, and changes in failure plane orientation in space. Specifically, we found that spatial patterns of earthflow velocity within the transport zone, where the bulk of movement occurred, were controlled largely by the slope of the underlying failure plane, whereas temporal patterns were governed largely by climate-driven changes in surface moisture balance (PDSI) at the annual-decadal scale. Declines in sediment supply acted as a secondary control on temporal velocity variations over our study period; however, the influence of this driver likely grows at longer time scales. Evidence for transient waves of motion in response to fluvial debuttressing was limited to the toe, and continued monitoring of Oak Ridge earthflow is required to determine whether those perturbations in sediment flux will migrate further upslope. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Characterization of asbestiform glaucophane-winchite in the Franciscan Complex blueschist, northern Diablo Range, California

This paper presents a geologic and mineralogical investigation of asbestiform amphibole from blueschist in the Diablo Range, northern California. The analysis of fibers in air samples shows that the dominant blueschist amphibole composition ranges from glaucophane to subordinate winchite. In outcrop, blueschist amphibole exhibits a velvety luster, and its occurrence ranges from crosscutting veins to highly deformed foliated and lineated tectonites. TEM and SEM photographs reveal a highly fibrous habit typically associated with asbestiform amphiboles. Dimensional analysis reveals a mean fiber width of 0.27 μm, and lengths and aspect ratios are shorter than reported for commercially exploitable asbestos, with a mean length of 2.8 μm and mean aspect ratio of 11.5. The data are consistent with other research showing that the width population of fibers, and not length or aspect ratio, is the key indicator of the asbestiform habit. The blueschist data are compared to non-asbestiform hornblende amphibole in granitic rocks. The fiber concentration of the hornblende (458 MFG) is low compared to the blueschist amphibole (107,880 MFG) and chrysotile in serpentinite (196,066 MFG), indicating that under similar conditions, the exposure potential from cleaved particles could be several orders of magnitude lower than from asbestiform particles. The asbestiform habit of glaucophane is present at two other locations in the Franciscan Complex, suggesting that asbestiform blueschist amphibole may be characteristic within blueschist terranes, and rock formations containing asbestiform amphibole may be more common and widespread than previously assumed.

Spatial patterns in occupancy and reproduction of Golden Eagles during drought: Prospects for conservation in changing environments

ABSTRACT We used a broad-scale sampling design to investigate spatial patterns in occupancy and breeding success of territorial pairs of Golden Eagles (Aquila chrysaetos) in the Diablo Range, California, USA, during a period of exceptional drought (2014–2016). We surveyed 138 randomly selected sample sites over 4 occasions each year and identified 199 pairs of eagles, 100 of which were detected in focal sample sites. We then used dynamic multistate modeling to identify relationships between site occupancy and reproduction of Golden Eagles relative to spatial variability in landscape composition and drought conditions. We observed little variability among years in site occupancy (3-yr mean = 0.74), but the estimated annual probability of successful reproduction was relatively low during the study period and declined from 0.39 (± 0.08 SE) to 0.18 (± 0.07 SE). Probabilities of site occupancy and reproduction were substantially greater at sample sites that were occupied by successful breeders in the previous ...

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.

Geologic evolution of a Cretaceous tectonometamorphic unit in the Franciscan Complex, western California

ABSTRACTThe Franciscan Yolla Bolly terrane of the NE California Coast Ranges consists mainly of quartzose metagreywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals, including ubiquitous lawsonite. Some Yolla Bolly rocks also contain one or more of the newly grown phases, pumpellyite, aragonite, glaucophane, and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under physical conditions of ~200–300°C and ~8 kbar at subduction-zone depths approaching 30 km. Petrologically similar Franciscan metaclastic-rich map units – Yolla Bolly terrane-like rocks, here designated the ‘YB’ unit – crop out in the central and southern California Coast Ranges. Recently published detrital zircon U‒Pb SIMS and LA-ICPMS data for 19 ‘YB’ metagreywackes indicate maximum ages of formation as follows: ~110–115 Ma (8) in the NE California Coast Ranges; ~95–107 Ma (7) in the San Francisco Bay area + Diablo Range; and ~85–92 Ma (4) in the dextrally offset Nacimiento Block. These fault-bounded ‘YB’ strata do not constitute coeval parts of a single tectonostratigraphic unit. Instead the term tectonometamorphic is proposed for such time-transgressive map units. Based on the current and likely Cretaceous 30° angular divergence between NS-palaeomagnetic stripes of the Farallon oceanic plate and the NNW-trending California convergent margin, I infer that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust and its largely clastic sedimentary blanket may have resulted in progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagreywacke section. During the ~30 million year interval, ~115–85 Ma, the locus of ‘YB’ accretion, underflow, and tectonic regurgitation evidently moved SE along an ~1000 km stretch of the accretionary margin of western California.

Summer die-off of western pond turtle (Actinemys marmorata) along an intermittent coast range stream in central California

Abstract During late summer and fall 2014, we documented western pond turtle (Actinemys marmorata) mortality, as indicated by the presence of turtle shells, along a 3.7-km reach of Coyote Creek in the Diablo Range of central California. In total, we observed 39 western pond turtle shells scattered irregularly along our study reach. Shells were found in dry reaches adjacent to or close to pools containing live turtles, as well as in or adjacent to dry pools in isolated dry reaches. Ninety percent of shells observed contained no carcass, and several shells showed evidence of predation. Though the cause of mortality is unclear, our observations confirm that western pond turtles may experience high mortality during droughts, which could result in significant population decline. The presence of live turtles in refugial pools emphasizes the importance of protecting and managing permanent pools in the face of intensified drought conditions.

Designating tectonostratigraphic terranes versus mapping rock units in subduction complexes: perspectives from the Franciscan Complex of California, USA

Accretionary complex histories are broadly understood. Sedimentation in seafloor and trench environments on drifting subducting plates and in associated trenches, followed by (1) deformation and metamorphism in the subduction zone and (2) subsequent uplift at the overriding plate edge, result in complicated stratigraphic and structural sequences in accretionary complexes. Recognizing, defining, and designating individual terranes in subduction complexes clarify some of these complicated relationships within the resulting continent-scale orogenic belts. Terrane designation does not substitute for detailed stratigraphic and structural mapping. Stratigraphic and structural mapping, combined with radiometric and palaeontologic dating, are necessary for delineation of coherent, broken, and dismembered formations, and various mélange units, and for clarification of the details of subduction complex architecture and history. The Franciscan Complex is a representative subduction complex that has evolved through sedimentation, faulting, folding, and low-temperature metamorphism, followed by uplift, associated deformation, and later overprinted deformation. Many belts of Franciscan rocks are offset by strike-slip faults associated with the dextral San Andreas Fault System. In the Franciscan Complex, among the terrane names applied widely, are the ‘Yolla Bolly Terrane’ and the ‘Central Terrane’. Where detailed mapping and detrital zircon ages exist, data reveal that the two names have been applied to rocks of similar general character and age. In the northeastern Diablo Range, Franciscan Complex rocks include coherent units, broken and dismembered formations, and various types of mélanges, all assigned at various times to the Yolla Bolly and other terranes. The details of stratigraphic and structural history revealed by large-scale mapping and radiometric dating prove to be more useful in clarifying the accretionary complex history than assigning a terrane name to the rocks. That history will assist in resolving terrane assignment issues and allow discrimination of subduction-associated and post-subduction events, essential for understanding the overall history of the orogen.

Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust

Investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust, and here we report on our systematic fabric analyses of glaucophane, lawsonite, and epidote in naturally deformed blueschists from the Diablo Range and Franciscan Complex in California, and the Hida Mountains in Japan. Glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations (CPOs) characterized by a (100)[001] pattern. These characteristics, together with a bimodal distribution of grain sizes from some samples, possibly indicate the occurrence of dynamic recrystallization for glaucophane. Although lawsonite and epidote display high aspect ratios and a strong CPO of (001)[010], the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism. The P-wave (AVP) and S-wave (AVS) seismic anisotropies of glaucophane (AVP=20.4%, AVS=11.5%) and epidote (AVP=9.0%, AVS=8.0%) are typical of the crust; consequently, the fastest propagation of P-waves is parallel to the [001] maxima, and the polarization of S-waves parallel to the foliation can form a trench-parallel seismic anisotropy owing to the slowest VS polarization being normal to the subducting slab. The seismic anisotropy of lawsonite (AVP=9.6%, AVS=19.9%) is characterized by the fast propagation of P-waves subnormal to the lawsonite [001] maxima and polarization of S-waves perpendicular to the foliation and lineation, which can generate a trench-normal anisotropy. The AVS of lawsonite blueschist (5.6–9.2%) is weak compared with that of epidote blueschist (8.4–11.1%). Calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench-parallel and trench-normal seismic anisotropy beneath NE Japan, but not to that beneath the Ryukyu arc. Our results demonstrate, therefore, that lawsonite has a strong influence on seismic velocities in the oceanic crust, and that lawsonite might be the cause of complex anisotropic patterns in subduction zones.

Lagophylla diabolensis(Compositae–Madiinae), a New Hare-Leaf from the Southern Diablo Range, California

Lagophylla diabolensis is a new hare-leaf from the southern Diablo Range of Fresno, Monterey, and San Benito counties, California. Plants of the Diablo Range hare-leaf were previously included within L. dichotoma, which is treated here in a restricted sense to comprise plants from the Sierra Nevada foothills and eastern Great Central Valley. Lagophylla diabolensis differs morphologically from L. dichotoma by having consistently glandular distal foliage (glands clear to dark-purple), narrower cauline leaves, generally uniformly tawny stems, and smaller heads. The taxonomic significance of those morphological differences is corroborated by other evidence that L. diabolensis is more closely related to the widespread L. ramosissima than to L. dichotoma sensu stricto. The Diablo Range hare-leaf occurs as scattered colonies, often in clayey soils of grassy openings in oak-pine woodland below 1100 m elevation. Extreme rarity and paucity of recent collections of L. diabolensis and L. dichotoma in the current sense indicates that both species warrant conservation concern.

Rheological contrast between glaucophane and lawsonite in naturally deformed blueschist from Diablo Range, California

AbstractDeformation microstructures for a lawsonite blueschist from the New Idria serpentinite body, Diablo Range, are investigated to clarify rheological behaviors of glaucophane and lawsonite, which are main mineral assemblages of subducting oceanic crust at relatively cold geotherm. Developments of crystal‐preferred orientations (CPOs) with small grain size, irregular grain boundary and high aspect ratio of glaucophane indicate deformation mechanism as recovery and dynamic recrystallization possibly accommodated by dislocation creep, while lawsonite deforms by rigid body rotation based on euhedral grains with angular or straight grain boundaries. Higher aspect ratios, lower angle to foliation, and stronger CPOs of both minerals in the glaucophane‐rich layer rather than those in the lawsonite‐rich layer suggest the strain localization into the glaucophane‐rich layer. Additionally fabric strength (the degree of crystal alignment) and seismic anisotropy are higher in the glaucophane‐rich layer than that of the lawsonite‐rich layer, which is consistent with the microstructural analyses. All our results imply, therefore, the dominant role of glaucophane rather than lawsonite for rheological behavior and seismic anisotropy of blueschist.

Tectonic significance of low-temperature blueschist blocks in the Franciscan mélange at San Simeon, California

The blueschist-bearing Franciscan mélange exposed near San Simeon is very similar to the mélange terrane forming the Central Belt north of San Francisco. Epidote-bearing low-T blueschist blocks at San Simeon (n=3) are the same age as high-T garnet-bearing blocks in the northern, and Diablo Range mélange terranes (~155–150Ma). This age approximates the time of initiation of Franciscan subduction. During the initial stages of Franciscan subduction, part of the top volcanic sequence from the subducting oceanic plate and small volumes of overlying sediments were imbricated at the bottom of the overriding plate and metamorphosed under high-P conditions. This underplated material was variably retrograded as subduction refrigerated the forearc blocks. Actinolitic rinds formed at the contacts between blueschists and intercalated mantle under relatively static conditions as the base of the overriding plate became serpentinized. A model is proposed in which exhumation was facilitated by gravity-driven, seaward-oriented extensional thinning of the forearc prism caused by the decrease in subducting plate dip starting around 80Ma that caused the Laramide orogeny. As this occurred, blueschist and graphite-schist blocks were plucked from the bottom of the hanging wall, incorporated into the shale- and water-rich shear zone at the plate interface (subduction channel shear zone), and exhumed during the upward flow of mélange driven by the movement of the downgoing plate while subduction was still active.

U‐Pb ages of detrital zircons in Pacheco Pass metagraywackes: Sierran‐Klamath source of mid‐Cretaceous and Late Cretaceous Franciscan deposition and underplating

Using laser ablation ICP‐MS techniques, U‐Pb ages are reported for 338 detrital zircons separated from four Franciscan metagraywacke samples from imbricate thrust sheets cropping out in the Pacheco Pass area, east central California Coast Ranges. Structurally higher slabs are clastic quartz ± albite‐bearing rocks typified by neoblastic lawsonite + jadeitic pyroxene; the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz + albite. Studied specimens contain moderate amounts of phengite + titanite ± chlorite, stilpnomelane, carbonate, iron oxides, rock fragments, and carbonaceous matter. These Diablo Range clastic sediments were deposited in mid‐Cretaceous and Late Cretaceous time. The highest allochthon, unit I, was deposited after ∼102 Ma, unit IV was deposited after ∼90–102 Ma, and unit V was deposited after ∼86 Ma. Metagraywacke depositional‐accretionary ages within slab IV young upward, but ages of the slabs decrease progressively downward. In the four‐sample aggregate, most zircons have igneous ages falling in the 135–170 Ma range, with a smaller population at 85–120 Ma. The rocks contain rare zircons of Middle Proterozoic–Late Archean ages. A minor source in northern California and NW Nevada seems likely for the Pacheco Pass metagraywackes, but the Sierran‐Klamath calcalkaline arc provided most of the clastic debris. Judging by the volcanic nature of lithic clasts in these Eastern Belt metagraywackes, and because massive Sierran plutonism occurred at ∼85–120 Ma, the abundant 135–170 Ma zircons probably were derived chiefly from the eroding comagmatic volcanic arc rather than from the less voluminous Jurassic arc plutons.