Northern California Coast Ranges Research Articles

Erosion rates over millennial and decadal timescales at Caspar Creek and Redwood Creek, Northern California Coast Ranges

AbstractComparing millennial‐scale denudation rates from cosmogenic nuclides with decadal‐scale sediment yields can shed light on erosional processes and on the effects of land use on sediment delivery to streams. Detailed measurements of sediment fluxes in the Northern California Coast Ranges at Caspar Creek and Redwood Creek have provided estimates of physical erosion rates since 1963 and 1971, respectively. We used cosmogenic 10Be to measure millennial‐scale denudation rates averaged over 1400–8700 years at six catchments in Caspar Creek and four catchments in Redwood Creek. Our 10Be measurements at Caspar Creek imply denudation rates that are nearly spatially uniform across the entire catchment and average 0·09 ± 0·02 mm a−1. These millennial‐scale rates implied by cosmogenic 10Be are faster than physical erosion rates of 0·005 ± 0·001 mm a−1 to 0·046 ± 0·007 mm a−1 inferred from sediment flux measurements over the past few decades in the same catchments. At Redwood Creek, our cosmogenic 10Be measurements imply millennial‐scale denudation rates that vary across the catchment from 0·14 ± 0·03 mm a−1 to 0·44 ± 0·09 mm a−1, in contrast to physical erosion rates ranging from 0·038 ± 0·011 mm a−1 to 0·48 ± 0·09 mm a−1 derived from sediment flux measurements made over the past few decades at the same catchments. The decadal‐scale and millennial‐scale measurements tend to differ most at the smallest tributaries, but differ by less than a factor of three for the Caspar Creek and Redwood Creek catchments as a whole. These measurements suggest that denudation rates at Caspar Creek are slower than rock uplift rates of 0·3–0·4 mm a−1, implying that Caspar Creek is not in topographic steady state. Copyright © 2005 John Wiley & Sons, Ltd.

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex (SFVC) comprise three distinct petrological groups based on their whole-rock geochemistry: (1) oceanic tholeiites; (2) transitional alkali basalts and glasses; (3) high-Al, low-Ti tholeiites. Major and trace element, and Sr–Nd–Pb isotopic data indicate that the oceanic tholeiites formed as low-degree partial melts of normal mid-ocean ridge basalt (N-MORB)-source asthenosphere similar in isotope composition to the East Pacific Rise today; the alkalic lavas were derived from an enriched source similar to that of E-MORB. The high-Al, low-Ti lavas resemble second-stage melts of a depleted MORB-source asthenosphere that formed by melting spinel lherzolite at low pressures. Trace element systematics of the high-Al, low-Ti basalts show the influence of an enriched component, which overprints generally depleted trace element characteristics. Tectonic discrimination diagrams show that the oceanic tholeiite and alkali suites are similar to present-day basalts generated at mid-oceanic ridges. The high-Al, low-Ti suite resembles primitive arc basalts with an enriched, alkali basalt-like overprint. Isotopic data show the influence of recycled components in all three suites. The SFVC was constructed on a substrate of normal Coast Range ophiolite in an extensional forearc setting. The close juxtaposition of the MORB-like olivine tholeiites with alkali and high-Al, low-Ti basalts suggests derivation from a hybrid mantle source region that included MORB-source asthenosphere, enriched oceanic asthenosphere, and the depleted supra-subduction zone mantle wedge. We propose that the SFVC formed in response to collision of a mid-ocean ridge spreading center with the Coast Range ophiolite subduction zone. Formation of a slab window beneath the forearc during collision allowed the influx of ridge-derived magmas or the mantle source of these magmas. Continued melting of the previously depleted mantle wedge above the now defunct subduction zone produced strongly depleted high-Al, low-Ti basalts that were partially fertilized with enriched, alkali basalt-type melts and slab-derived fluids.

Toward using Margaritifera falcata as an indicator of base level nitrogen and carbon isotope ratios: insights from two California Coast Range rivers

Measurements of freshwater mussel tissue are potentially very useful for determining base-level isotopic values for food web studies in aquatic environments. As long-lived, filter-feeding organisms, mussels have the potential to spatially and temporally average the isotopic baseline signal. Following from earlier studies that focused on lake environments, this study investigates the stable carbon and nitrogen isotope ratios in tissue of the river dwelling freshwater mussel, Margaritifera falcata, in two extensively studied northern California coast range rivers, the South Fork Eel and Navarro. We highlight advantages and challenges for using riverine mussel isotopes as indicators of baselines. δ13C of primary producers is known to vary with habitat along the South Fork Eel channel, but our measurements show no such variations, demonstrating that riverine mussels do preserve a spatially averaged measure of instream derived food sources. Mean δ13C and δ15N are shown to be markedly different in the two rivers, reflecting differences in food sources and possibly watershed land use. We also found that δ15N of mussel tissue increased by approximately 2‰ with mussel age in both rivers. This suggests it is important to consider age and size effects when estimating baseline values from mussel tissues.

Southward-younging apatite (U–Th)/He ages in the northern California Coast Ranges due to a northward-migrating crustal welt

(U–Th)/He in apatite dating of Franciscan Formation samples from between 39°47′N and 39°08′N in the northern California Coast Ranges (NCCR) reveals a southward- and downward-younging pattern of ages (21.8 ± 0.6 Ma to 7.9 ± 0.9 Ma). These ages are interpreted to represent a northward or northeastward dipping fossil He partial retention zone that was tilted and exhumed by a northward-migrating pulse of crustal thickening that begins ∼150–200 km ahead of the migrating Mendocino Triple Junction (MTJ) [K.P. Furlong, R. Govers, Ephemeral crustal thickening at a triple junction: the Mendocino crustal conveyor, Geology 27 (1999) 127–130). The ∼8 Ma ages from the southern end of the sample transect are interpreted to come from rocks that were immediately below the He partial retention zone when crustal thickening began, ∼5 my before the MTJ passed by; these rocks have now reached the surface, ∼3 my after passage of the MTJ. The older ages from the central and northern portions of the sample transect are interpreted to represent rocks that were within the He partial retention zone prior to crustal thickening ahead of the MTJ, and which therefore record ages transitional from older subduction-related cooling and exhumation to more recent transform-related cooling and exhumation. In addition to providing support for the model of a migrating crustal welt associated with the MTJ, the existence of an essentially intact, gently tilted fossil partial retention zone in the northern California Coast Range raises the possibility of using isochronous surfaces as neotectonic structural markers in this geologically complex and heavily vegetated region.

Migration of the Mendocino triple junction and ephemeral crustal deformation: Implications for California Coast range heat flow

Thermal models have been constructed to demonstrate how regional crustal deformation and subsequent asthenospheric upwelling associated with the migration of the Mendocino triple junction affect the thermal regime of the northern California Coast Ranges. Ephemeral crustal thickening caused by the migration of the triple junction, coupled with the thermal effects of inflow of asthenosphere into the slab window, produces a heat flow signature that is consistent with the observed surface heat flow in the region. In addition the thermal models provide evidence that slab window temperatures are lower than previously assumed following the passage of the triple junction.

A Seasonal Precipitation and Stream Flow Hindcast and Prediction Study in the Western United States during the 1997/98 Winter Season Using a Dynamic Downscaling System

Abstract The authors present a seasonal hindcast and prediction of precipitation in the western United States and stream flow in a northern California coastal basin for December 1997–February 1998 (DJF) using the Regional Climate System Model (RCSM). In the seasonal hindcast simulation, in which the twice-daily National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis was used for the initial conditions and time-dependent boundary forcing, RCSM has simulated realistically the temporal and spatial variations of precipitation in California and stream flow in a northern California coastal basin. For the headwater basin of the Russian River in the northern California Coast Ranges, the Topography-Based Hydrologic Model (TOPMODEL) forced by observed daily precipitation resulted in a correlation coefficient of 0.88 between observed and simulated DJF stream flow. In the coupled stream flow hindcast, the authors obtained a correlation coefficient of 0.7 between simulated and...

Influence of Habitat Patchiness on Genetic Diversity and Spatial Structure of a Serpentine Endemic Plant

Abstract:Conservation of rare plant species often involves small, local populations dispersed within apparently suitable habitat. We used enzyme electrophoresis to study genotypic diversity in 32 populations ofCalystegia collina, a self‐incompatible clonal plant endemic to serpentine substrate in northern California's Coast Range. Genotypic diversity within a population was a good predictor of seed set by randomly marked flowers. Ramets from the most abundant genotypes in a population were most likely to produce flowers, but flowers from abundant genotypes were less likely to produce seed capsules than were flowers from rarer genotypes. These results are consistent with previous findings that reproductive success inC. collinais limited by the availability of compatible pollen. On small serpentine outcrops supporting only one or two populations,C. collinadid not show reduced genotypic diversity or heterozygosity compared with populations on large outcrops supporting many populations. Instead, genotypic diversity and outcrop size had strong but independent influences on reproductive success. Although large serpentine outcrops contain more populations and provide better conditions for flower and seed production, significant diversity of unique genotypes clearly exists in isolated serpentine outcrops. Our findings suggest that plant conservation strategies must take into account the natural distribution of populations. The effects of habitat fragmentation onC. collinaand other plant species that occur naturally in small, discrete patches may be unlike those that have been documented in more recently fragmented species.

Synthesis of Crustal Seismic Structure and Implications for the Concept of a Slab Gap beneath Coastal California

Compilation of seismic transects across the central and northern California Coast Ranges provides evidence for the widespread tectonic emplacement beneath the margin of a slab of partially subducted oceanic lithosphere. The oceanic crust of this lithosphere can be traced landward from the former convergent margin (fossil trench), beneath the Coast Ranges, to at least as far east as the Coast Range/Great Valley boundary. Comparison of measured shear and compressional wave velocities in the middle crust beneath the Hayward fault with laboratory measurements suggests that the middle crust is a diabase (oceanic crust). Both of these observations are consistent with recent models of the high heat flow and age progression of Neogene volcanism along the Coast Ranges based on tectonic emplacement (stalling) of young, hot oceanic lithosphere beneath the margin, but appear to contradict the major predictions of the slab-gap or asthenospheric-window model. Finally, the Neogene volcanism and major strike-slip faults in the Coast Ranges occur within the thickest regions (>14 km thick) of the forearc, suggesting that the locations of Cenozoic volcanism and faulting along the margin are structurally controlled by the forearc thickness rather than being determined by the location of a broad slab gap.

40Ar/39Ar dating of detrital micas from Oligocene-Pleistocene sandstones of the Olympic Peninsula, Klamath Mountains, and northern California Coast Ranges: provenance and paleodrainage patterns

Sandstones of the Eocene-Miocene Hoh assemblage of the Olympic Peninsula and Late Oligocene - Miocene Weaverville Formation (Klamath Mountains) were studied to determine if the Hoh sandstones could be tectonically transported equivalents of the Weaverville Formation. Distinct Hoh sandstone types exposed between La Push and the Hoh River include (1) highly brecciated and veined, quartz-poor, mica-poor volcaniclastic sandstone preserved in mélange blocks; and (2) relatively unveined and unbrecciated, quartzose micaceous sandstone preserved in mélange blocks and bounding turbidites. 40Ar/39Ar laser heating analyses of single crystals of detrital muscovite grains from quartzose Hoh sandstones yield Late Cretaceous - early Tertiary ages, consistent with contributions to Hoh detritus from the Idaho batholith. Volcaniclastic mélange block sandstones could be derived from older Tertiary volcanic terranes of the northeast Olympic Peninsula, or from Mesozoic accretionary terranes. Analyses of muscovites from the fluviatile Weaverville Formation of the Klamath Mountains, California, yield Pennsylvanian ages with a possible source within the Klamath Central Metamorphic terrane. No provenance link was detected between the Hoh assemblage and Weaverville Formation. Analyses of muscovites from Pliocene and Pleistocene sandstones of the Wildcat Group of the northern California Coast Ranges yield both Cretaceous - early Tertiary and Pennsylvanian ages, suggesting derivation from both local and distant sources. Although an ancestral "Snake River" paleodrainage system to the Klamath Mountains region was shutoff during Weaverville sedimentation, it may have been reestablished in the Late Miocene when Idaho-derived sediments were again transported to northwestern California.

<sup>40</sup>Ar/<sup>39</sup>Ar dating of detrital micas from Oligocene-Pleistocene sandstones of the Olympic Peninsula, Klamath Mountains, and northern California Coast Ranges: provenance and paleodrainage patterns

Sandstones of the Eocene-Miocene Hoh assemblage of the Olympic Peninsula and Late Oligocene - Miocene Weaverville Formation (Klamath Mountains) were studied to determine if the Hoh sandstones could be tectonically transported equivalents of the Weaverville Formation. Distinct Hoh sandstone types exposed between La Push and the Hoh River include (1) highly brecciated and veined, quartz-poor, mica-poor volcaniclastic sandstone preserved in mélange blocks; and (2) relatively unveined and unbrecciated, quartzose micaceous sandstone preserved in mélange blocks and bounding turbidites. 40Ar/39Ar laser heating analyses of single crystals of detrital muscovite grains from quartzose Hoh sandstones yield Late Cretaceous - early Tertiary ages, consistent with contributions to Hoh detritus from the Idaho batholith. Volcaniclastic mélange block sandstones could be derived from older Tertiary volcanic terranes of the northeast Olympic Peninsula, or from Mesozoic accretionary terranes. Analyses of muscovites from the fluviatile Weaverville Formation of the Klamath Mountains, California, yield Pennsylvanian ages with a possible source within the Klamath Central Metamorphic terrane. No provenance link was detected between the Hoh assemblage and Weaverville Formation. Analyses of muscovites from Pliocene and Pleistocene sandstones of the Wildcat Group of the northern California Coast Ranges yield both Cretaceous - early Tertiary and Pennsylvanian ages, suggesting derivation from both local and distant sources. Although an ancestral "Snake River" paleodrainage system to the Klamath Mountains region was shutoff during Weaverville sedimentation, it may have been reestablished in the Late Miocene when Idaho-derived sediments were again transported to northwestern California.

The Role of Convective Geothermal Systems in the Generation, Migration, and Entrapment of Oil: ABSTRACT

Modern convective geothermal systems (for example, at Yellowstone National Park) and their fossil equivalents, epithermal mineral deposits (like McLaughlin, California) have traditionally been considered poor petroleum prospects. The concentrated heat which drives these systems is commonly viewed as a negative influence: in other words, any oil initially present in or generated by these systems is quickly degraded to a useless carbonaceous residue. This is true in extreme cases, but numerous examples from the Great Basin, the northern California Coast Ranges, and elsewhere suggest that under certain circumstances, geothermal systems can efficiently generate, transport, and entrap significant quantities of producible petroleum. B.R.T. Simoneit has shown that oil can be hydrothermally generated in an {open_quotes} instant{close_quotes} of geologic time. It follows, then, that away from geothermal systems` high-temperature centers (or in wholly moderate-temperature systems), the encompassing shallow thermal anomalies can distill large volumes of oil from otherwise immature hydrocarbon source rocks. Transport of this newly-generated oil is enhanced by the buoyant upwelling of heated aqueous fluid. Porosity for oil transport and storage is created or increased by hydrothermal dissolution of rock-forming or secondary silicates and carbonates. Finally, geothermal {open_quotes}self-sealing{close_quotes} -- the deposition of secondary minerals (especially silica and clay) at the marginsmore » of a system -- can provide a very effective hydrocarbon seal. Lee Allison has noted the coincidence of igneous intrusions and oil reservoirs in Nevada. It is suggested that here and elsewhere, both igneous-related and amagmatic geothermal systems in otherwise favorable settings should be viewed as prime petroleum exploration targets.« less

Massive sulfide metallogenesis at a late Mesozoic sediment-covered spreading axis: Evidence from the Franciscan complex and contemporary analogues

The Island Mountain deposit, an anomalous massive sulfide in the Central belt of the Franciscan subduction complex, northern California Coast Ranges, formed during hydrothermal activity in a sediment-dominated paleo-sea-floor environment. Although the base of the massive sulfide is juxtaposed against a 500-m-wide melange band, its gradational upper contact within a coherent sequence of sandstone, siltstone, and mudstone indicates that hydrothermal activity was concurrent with turbidite deposition. Accumulations of sulfide breccia and clastic sulfide were produced by mass wasting of the sulfide mound prior to burial by turbidites. The bulk composition of sulfide samples (pyrrhotite rich; high Cu, As, and Au contents; radiogenic Pb isotope ratios) is consistent with a hydrothermal system dominated by fluid-sediment interaction. On the basis of a comparison with possible contemporary tectonic analogues at the southern Gorda Ridge and the Chile margin triple junction, we propose that massive sulfide mineralization in the Central belt of the Franciscan complex resulted from hydrothermal activity at a late Mesozoic sediment-covered ridge axis prior to collision with the North American plate.

Cenozoic volcanism in the California Coast Ranges: Numerical solutions

We present the results of numerical simulations of Cenozoic volcanism in the Coast Ranges of California, associated with the northward migration of the Mendocino Triple Junction (MTJ). Three aspects of the thermal evolution and magmatism in the wake of the MTJ are investigated: (1) mantle flow and thermal perturbation in the “slabless window”; (2) pressure‐release partial melting in the upwelling asthenosphere; and (3) thermally induced crustal anatexis. Numerical results show that upwelling of asthenosphere in the “slabless window” causes a drastic change in the thermal structure of the lithosphere and results in significant pressure‐release partial melting in the upwelling asthenosphere. Up to 4–5 km of basaltic magma may be produced. Most of this mafic magma is likely stored near the base of the crust. Thermally induced crustal anatexis mainly occurs in the deep crust (below 20 km), where a high degree (>50%) of partial melting may be predicted. Most of the crustal melt is probably generated within 1 m.y. after the passage of the MTJ. This work also suggests that the spatial and temporal distribution of Cenozoic volcanism in the California Coastal Ranges is directly related to the variation of the relative velocity between the North America and Pacific plates in the last 30 m.y. The voluminous volcanic rocks in the northern California Coast Ranges correlate to the relatively high velocity between these two plates in the last 10 m.y. The reduced volcanism in the central Coast Ranges may be related to the slow migration of the MTJ between 10 and 20 Ma.

The Stonyford Volcanic Complex: A Jurassic Seamount in the Northern California Coast Ranges

The Stonyford Volcanic Complex: A Jurassic Seamount in the Northern California Coast Ranges

Concordant paleolatitudes from ophiolite sequences in the northern California Coast Ranges, U.S.A.

Paleomagnetic data have been obtained from two ophiolite sequences in the northern California Coast Ranges: from Mount Diablo in the San Francisco Bay area and from Potter Valley, north of Clear Lake. The ophiolite exposed at Mount Diablo is part of the late Middle to Late Jurassic Coast Range ophiolite, and that exposed near Potter Valley is Late Jurassic to perhaps Early Cretaceous in age and occurs within the Franciscan assemblage. Data from the sheeted-dike complex at Mount Diablo show these rocks to be strongly overprinted, probably following uplift and erosion of the ophiolite. Samples whose primary remanent magnetization seems to be recovered yield a mean paleomagnetic pole at 30.7°N, 159.5°E with α 95 = 5.6°. A comparison of this pole with the Jurassic apparent polar wander path for North America indicates that the ophiolite has rotated 45° ± 7° counterclockwise relative to the craton and has not been latitudinally displaced. The diabase and pillow basalt in Potter Valley have not been strongly overprinted and data from those rocks yield a paleomagnetic pole at 79.0°N, 61.5°E with α 95 = 6.4°. This result indicates that the ophiolite at Potter Valley has rotated approximately 29° ± 8° clockwise, and has undergone little or no latitudinal displacement. Because of the predominantly northeastward transport of oceanic plates converging with the western margin of North America since middle Mesozoic time, the absence of appreciable northward displacement of either ophiolite fragment indicates that both formed close to the continental margin.

Distribution and seasonal occurrence of the hyporheic fauna in a northern California stream

The surface and hyporheic fauna of a second-order reach of a northern California Coast Range stream (Big Canyon Creek, Lake Co., CA, USA) was examined using substrate colonization samplers (i.e. hyporheic pots) during both the wet and dry seasons in the prevailing Mediterranean climate of the region. In terms of total number of macroinvertebrate taxa, the surface (0 cm to −5.0 cm within the stream substrate) level had higher richness than any level within the hyporheic (−5.1 cm to −15.0 cm, −15.1 cm to −25.0 cm, or −25.1 cm to −35.0 cm) from the beginning of the dry season (May) to the beginning of the wet season (October); during this period the surface was not subject to disturbance from wet-season storms. During the wet season, richness at the surface was similar to that observed at any of the three hyporheic levels examined. Macroinvertebrate density at the surface was substantially reduced during the wet season; however, mean surface densities always exceeded those found within the hyporheic zone (from 25% to 78% of total macroinvertebrate numbers were found at the surface). Seasonal fluctuations in abundance of total macroinvertebrates and density of many constituent populations were less within the hyporheic zone than at the surface. Apparently, early instars of abundant surface taxa do not penetrate the interstices, and substrate disturbance due to spates is less in deeper levels. Compared with many hyporheic faunas described from other temperate-zone sites, this hyporheic community shows reduced numerical dominance by the Chironomidae.

Possible depth limit for underplating by a seamount

The Cazadero metabasite allochthon is located within the central melange belt of the northern California Coast Ranges, U.S.A. It composed mainly of pillowed and massive mafic flows, hyaloclastite and minor pelagic sediments. It is about 3 × 15 km 2 in extent, and is in fault contact with Late Mesozoic fore-arc flysch deposits of the Great Valley sequence. Three metamorphic zones were mapped in the allochthon, as follows-in order of increasing metamorphic grade: the lawsonite, the pumpellyite and the epidote zone. CaNa pyroxenes are ubiquitous; they were used to estimate the P- T path, together with the oxygen isotope geothermometry of Taylor and Coleman. The result shows a bending point in the P- T path for blueschist facies metamorphism of the Cazadero allochthon. At shallow depths, lower grade metamorphism proceeded under predominant pressure-increase from 4 to 7 kbar at nearly constant T of about 150–200 °C, whereas after passing the bending point higher-grade metamorphism occurred with increasing temperature from 200 to 350 °C at nearly constant P of about 7–8 kbar. The bending point reflects the depth of transfer of underplating materials from the oceanic lithosphere and its overlying sediments to the hanging wall plate. Either seamount volcanics and pelagic sediments, or mixed packages of fragmented oceanic crust with trench-fill turbidites, were subducted beneath the North American plate. They were subjected to early lawsonite- and pumpellyite-zone metamorphism with very low geothermal gradient. We infer that when they reached about 20 km depth, underplating occurred and the metamorphosed rocks became part of the hanging wall. Heat conduction from the overlying hydrated mantle wedge caused a temperature increase at nearly constant depth. The earlier recrystallized seamount volcanics and pelagic sediments were then metamorphosed at a much higher geothermal gradient. The Cazadero allochthon has suffered extremely high P T subduction zone metamorphism and it preserves very well the record of Cretaceous subduction-underplating processes.

Jurassic volcanic glass from the Stonyford volcanic complex, Franciscan assemblage, northern California Coast Ranges

Hyaloclastite breccias that occur near the top of the Stonyford volcanic complex contain abundant volcanic glass and rare basaltic clasts that have not been altered significantly by hydrothermal processes or by subsequent metamorphism. They resemble hyaloclastites observed in the summit regions of young Pacific seamounts, and appear to have formed by submarine fire fountains, during which globules of the erupting lava were fragmented by thermal shock from contact with seawater. These breccias contain the first-reported occurrence of unaltered olivine (Fo 86 ) and plagioclase (An 63-80 ) in volcanic rocks of the Franciscan complex. The glasses are rich in K 2 O, Nb, Zr, Rb, and Sr relative to both normal and enriched mid-ocean ridge basalt (MORB), and low in Y, V, and Sc. Pillow lavas intercalated with the hyaloclastites and a crystalline basalt clast have similar compositional characteristics. These magmas are similar to alkalic and transitional basalts that are common in intraplate seamounts and along oceanic spreading centers associated with large ion lithophile element (LILE)-enriched mantle plumes. Other volcanic rocks of the Stonyford complex are LILE-rich tholeiitic basalts and ferrobasalts characteristic of intraplate seamounts. A heterogeneous source region that has at least two distinct end members is suggested by the Zr-Nb systematics: depleted suboceanic asthenosphere (normal MORB-type source) and enriched asthenosphere.

Extensional fault uplift of regional Franciscan blueschists die to subduction shallowing during the Laramide orogeny

Research Article| December 01, 1989 Extensional fault uplift of regional Franciscan blueschists die to subduction shallowing during the Laramide orogeny Scot W. Krueger; Scot W. Krueger 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar David L. Jones David L. Jones 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar Author and Article Information Scot W. Krueger 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 David L. Jones 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1989) 17 (12): 1157–1159. https://doi.org/10.1130/0091-7613(1989)017<1157:EFUORF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Scot W. Krueger, David L. Jones; Extensional fault uplift of regional Franciscan blueschists die to subduction shallowing during the Laramide orogeny. Geology 1989;; 17 (12): 1157–1159. doi: https://doi.org/10.1130/0091-7613(1989)017<1157:EFUORF>2.3.CO;2 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 Regional structural and stratigraphic relations in the northern California Coast Ranges require an episode of extensional tectonic unroofmg of coherent blueschist terranes of the Eastern belt of the Franciscan complex along a crustal-scale, low-angle, normal-fault system, here named the Coast Range detachment. This tectonic unroofmg occurred during the latest Cretaceous and early Tertiary, contemporaneous with the compressional Laramide orogeny of the North American interior. Dynamic models of accretionary wedges predict that shallowing of subduction, as has been inferred during the Laramide orogeny, will impart extensional stresses in the wedge above the subducting plate. We therefore suggest a single causal event—gradual shallowing of subduction in the latest Cretaceous and early Tertiary—to link the extensional fault uplift of regional Franciscan blueschists to Laramide compression in the North American interior. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Deformation of the Eastern Franciscan Belt, northern California

The late Jurassic and Cretaceous Eastern Franciscan belt of the northern California Coast Range consists of two multiply deformed, blueschist-facies terranes; the Pickett Peak and Yolla Bolly terranes. Four deformations have been recognized in the Pickett Peak terrane, and three in the Yolla Bolly terrane. The earliest recognized penetrative fabric, D 1, occurs only in the Pickett Peak terrane. The later penetrative fabrics, D 2 and D 3, occur in both the Yolla Bolly and Pickett Peak terranes. D 1 and D 2 apparently represent fabrics that formed during subduction and accretion of the terranes. Fabrics from both D 1 and D 2 are consistent with SW-NE movement directions with respect to their present geographic positions. D 3 postdates blueschist-facies metamorphism of the terranes and may be related to emplacement of the terranes to higher structural levels. A broad regional warping, D 4, is evident from the map pattern and folding of large metamorphosed thrust sheets. D 4 folds may be related to deformation associated with oblique convergence along the continental margin in late Cretaceous and (or) early Tertiary time.