Franciscan Complex Research Articles

Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California

We present new U/Pb ages for detrital zircons separated from six quartzose metagraywackes collected from different Franciscan Complex imbricate nappes around San Francisco Bay. All six rocks contain a broad spread of Late Jurassic–Cretaceous grains originating from the Klamath–Sierra Nevada volcanic-plutonic arc. Units young structurally downward, consistent with models of progressive underplating and offscraping within a subduction complex. The youngest specimen is from the structurally lowest San Bruno Mountain sheet; at 52 Ma, it evidently was deposited during the Eocene. None of the other metagraywackes yielded zircon ages younger than 83 Ma. Zircons from both El Cerrito units are dominated by ca. 100–160 Ma grains; the upper El Cerrito also contains several grains in the 1200–1800 Ma interval. These samples are nearly identical to 97 Ma metasedimentary rock from the Hunters Point shear zone. Zircon ages from this melange block exhibit a broad distribution, ranging from 97 to 200 Ma, with only a single pre-Mesozoic age. The Albany Hill specimen has a distribution of pre-Mesozoic grains from 1300 to 1800 Ma, generally similar to that of the upper El Cerrito sheet; however, it contains zircons as young as 83 Ma, suggesting that it is significantly younger than the upper El Cerrito unit. The Skaggs Spring Schist is the oldest studied unit; its youngest analyzed grains were ca. 144 Ma, and it is the only investigated specimen to display a significant Paleozoic detrital component. Sedimentation and subduction-accretion of this tract of the trench complex took place along the continental margin during Early to early–Late Cretaceous time, and perhaps into Eocene time. Franciscan and Great Valley deposition attests to erosion of an Andean arc that was active over the entire span from ca. 145 to 80 Ma, with an associated accretionary prism built by progressive underthrusting. We use these new data to demonstrate that the eastern Franciscan Complex in the northern and central Coast Ranges is a classic accretionary prism, where younger, structurally lower allochthons are exposed on the west, and older, structurally higher allochthons occur to the east, in the heavily studied San Francisco Bay area.

Literature Review of Unconsolidated Sediment in San Francisco Bay and Nearby Pacific Ocean Coast

A review of the geologic literature regarding sedimentation in the San Francisco Bay estuarine system shows that the main part of the bay occupies a structural tectonic depression that developed in Pleistocene time. Eastern parts, including San Pablo Bay and Suisun Bay, have had sedimentation throughout late Mesozoic and Tertiary. Carquinez Strait and the Golden Gate may represent antecedent stream erosion. Sedimentation has included estuarine, alluvial, and eolian deposition. The ages of estuarine deposition includes the modern high sea level stand and earlier Pleistocene interglacial periods. Sediment sources can be generally divided into the Coast Ranges, particularly the Franciscan Complex, and “Sierran.” Much of the estuarine system is floored by very fine sediment, with local areas of sand floor. Near the Golden Gate, sediment size decreases in both directions away from the deep channel. Bedforms include sand waves (submarine dunes), flat beds, and rock and boulders. These are interpreted in terms of dominant transport directions. Near the Golden Gate is an ebb-tidal delta on the outside (including San Francisco bar) and a flood-tidal delta on the inside (parts of Central Bay). The large tidal prism causes strong tidal currents, which in the upper part of the estuary are normally much stronger than river currents, except during large floods. Cultural influences have altered conditions, including hydraulic mining debris, blasting of rocks, dredging of navigation channels, filling of the bay, and commercial sand mining. Many of these have served to decrease the tidal prism, correspondingly decreasing the strength of tidal currents.

Stream capture and piracy recorded by provenance in fluvial fan strata

Stream capture and piracy in tectonically active regions have been described in geomorphic systems worldwide; however, few studies show the influence stream capture has on the rock record. We present an analysis of fluvial fan stratigraphy that developed as a result of multiple stream capture events, building a complex stratigraphic succession beneath the Lawrence Livermore National Laboratory (LLNL), California. The LLNL site is located in the southeast portion of the tectonically active Livermore Basin, a transpressional basin in the California Coast Ranges. Geomorphic evidence for this stream capture include: (1) the Arroyo Seco enters the basin from the south through an uplifted fault block, (2) south of this fault block lies an abandoned Arroyo Seco fluvial fan, (3) north of the fault block, in the Livermore Basin, Arroyo Seco built a 7-km 2 fluvial fan, apparently forcing the Arroyo Las Positas, a smaller stream that enters the basin from the east, northward around the Arroyo Seco fan, and (4) a knickpoint exists near the point of capture on Arroyo Seco. Stratigraphic evidence reflecting this shift in the Arroyo Seco position into the Livermore Basin was evaluated through a provenance study of 215 gravel units from 34 boreholes spaced evenly over the 2.6 km 2 LLNL site. The Arroyo Seco derives its sediment from both the Jurassic–Cretaceous Franciscan Assemblage and the Altamont Hills (which are comprised of Mesozoic Great Valley Group and Tertiary continental sediments). The Arroyo Las Positas drains only the Altamont Hills and thus lacks the Franciscan Assemblage-derived clasts. The origin of the individual gravel units was determined by the percentage of Franciscan Assemblage indicator pebbles (red chert, green chert and blueschist) in the samples. Through this analysis, we determined that high-percentage Franciscan Assemblage-derived clasts were present below a depth of approximately 35 m below the surface, low-percentage Franciscan Assemblage-derived clasts were present at depths between 35 m and 18 m, and high-percentage Franciscan Assemblage-derived clasts were present from depths of approximately 18 m to the surface of the fluvial fan. These results indicate that the Arroyo Seco flowed north and deposited sediments at the LLNL site, then was later absent from the basin at which time it formed a fan south of the fault block. During this absence of the Arroyo Seco, the Arroyo Las Positas, a westerly flowing stream, dominated the sediment supply at the LLNL site. The Arroyo Seco was then captured by a gully headward eroding through the uplifted fault block, redirecting the Arroyo Seco into the basin once again. This history of multiple stream captures created three stratigraphic units with alternating overall channel and paleoflow orientations.

BATHYSIPHON (FORAMINIFERIDA) AT PACHECO PASS, CALIFORNIA: A GEOPETAL, PALEOCURRENT, AND PALEOBATHYMETRIC INDICATOR IN THE FRANCISCAN COMPLEX

Abstract We document a new locality for the foraminiferid Bathysiphon at Pacheco Pass, California, a classic blueschist-facies location on the eastern edge of the Franciscan Complex. The nearly featureless, silicified tubes of Bathysiphon cf. aaltoi Miller, 1986 described here are the largest reported from the Franciscan Complex; they are <153 mm long and easily recognized in the field. The rarity of fossils in the Franciscan Subduction Complex has hindered field mapping and, thus, a better understanding of its stratigraphy and structure. Bathysiphon specimens can be used as geopetal indicators, since they occur parallel to bedding at the top of argillaceous layers, as paleocurrent indicators (showing a SSE trend at Pacheco Pass), and as paleobathymetric indicators, suggesting, by their large size, mid-to-lower bathyal depths (2000–4000 m).

The processes of underthrusting and underplating in the geologic record: structural diversity between the Franciscan Complex (California), the Kodiak Complex (Alaska) and the Internal Ligurian Units (Italy)

AbstractExisting studies on active subduction margins have documented the wide diversity in structural style between accretionary prisms, both in space and time. Together with physical boundary conditions of the margins, the thickness of sedimentary successions carried by the lower plate seems to play a key role in controlling the deformation and fluid flow during accretion. We have tested the influence of the subducting sedimentary section by comparing the structural style and fluid‐related structures of four units from three fossil accretionary complexes characterized by similar physical conditions but different subducting sediment thicknesses: (1) the Franciscan Complex of California, (2) the Internal Ligurian Units of Italy and (3) the Kodiak Complex, Alaska.Subducting plates bearing a thick sedimentary cover generally result in coherent accretion through polyphase deformation represented by folding and thin thrusting events, while underplating of sediment‐starved oceanic sections results in diffuse deformation and mélange formation. These two structural styles can alternate through time in a single complex with a long record of accretion such as Kodiak.The parallel analysis of the selected analogues show that although the volume of sediments carried by the lower plate determines different structural styles, deformation is strongly controlled by injection of overpressured fluids during underthrusting and accretion. Transient hydrofracturing occurs through the development of a system of dilatant fractures grossly parallel to the décollement zone. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Tectonic underplating of trench sediments beneath magmatic arcs: the central California example

We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare‐up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo‐depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180 km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion‐related trench sediment was shuffled from the trench to the sub‐arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high‐pressure/low‐temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite‐facies metamorphism and limited partial melting.

Jurassic and Cretaceous Gastropods from Hydrocarbon Seeps in Forearc Basin and Accretionary Prism Settings, California

Fourteen gastropod species from 16 Mesozoic hydrocarbon seep carbonate deposits of the Great Valley Group and Franciscan Complex in California are described. Two genera are new: Bathypurpurinopsis has a fusiform shell with a siphonal fold, and variable Paskentana has turbiniform or littoriniform shells with spiral and/or scaly sculpture and convex or shouldered whorls. Due to the lack of data on shell microstructure and protoconch morphology, many of our taxonomic assignments have to remain tentative at present. Species that are described as new include: Hokkaidoconcha bilirata, H. morenoensis, H. tehamaensis (Hokkaidoconchidae). Abyssochrysos? giganteum (Abyssochrysidae?), Paskentana globosa, P. berryessaensis. and Bathypurpurinopsis stantoni (Abyssochrysoidea, family uncertain). The total fauna represents a mixed bag of taxa that were: (i) widely distributed during the late Mesozoic (Amberleya); (ii) restricted to late Mesozoic seep carbonates in California (Atresius, Bathypurpurinopsis, Paskentana); and (iii) members of seep/deep-sea groups with a long stratigraphic range (abyssochrysids, hokkaidoconchids).

CLEAR LAKE SEDIMENTS: ANTHROPOGENIC CHANGES IN PHYSICAL SEDIMENTOLOGY AND MAGNETIC RESPONSE

We analyzed the sedimentological characteristics and magnetic properties of cores from the three basins of Clear Lake, California, USA, to assess the depositional response to a series of land use changes that occurred in the watershed over the 20th century. Results indicate that distinct and abrupt shifts in particle size, magnetic concentration/mineralogy, and redox conditions occur concurrently with a variety of ecological and chemical changes in lake bed sediments. This coincidence of events occurred around 1927, a datum determined by an abrupt increase in total mercury (Hg) in Clear Lake cores and the known initiation of open-pit Hg mining at the Sulphur Bank Mercury Mine, confirmed by 210Pb dating. Ages below the 1927 horizon were determined by accelerator mass spectrometry on 14C of coarse organic debris. Calculated sedimentation rates below the 1927 datum are approximately 1 mm/yr, whereas rates from 1927 to 2000 are up to an order of magnitude higher, with averages of approximately 3.5-19 mm/yr. In both the Oaks and Upper Arms, the post-1927 co-occurrence of abrupt shifts in magnetic signatures with color differences indicative of changing redox conditions is interpreted to reflect a more oxygenated diagenetic regime and rapid burial of sediment below the depth of sulfate diffusion. Post-1927 in the Oaks Arm, grain size exhibits a gradual coarsening-upward pattern that we attribute to the input of mechanically deposited waste rock related to open-pit mining activities at the mine. In contrast, grain size in the Upper Arm exhibits a gradational fining-upward after 1927 that we interpret as human-induced erosion of fine-grained soils and chemically weathered rocks of the Franciscan Assemblage by heavy earthmoving equipment associated with a road- and home-building boom, exacerbated by stream channel mining and wetlands destruction. The flux of fine-grained sediment into the Upper Arm increased the nutrient load to the lake, and that in turn catalyzed profuse cyanobacterial blooms through the 20th century. The resulting organic biomass, in combination with the increased inorganic sediment supply, contributed to the abrupt increase in sedimentation rate after 1927.

A revised kinematic model for the relative motion between Pacific oceanic plates and North America since the Late Cretaceous

The convergence between the Pacific, Farallon, and Kula oceanic plates and the North American plate has provided a driving force for tectonic processes that have shaped the western margin of North America. Yet reconstructions of plate motion in the Pacific basin have traditionally relied on the assumption of fixed hot spots. In the last decade, substantial Cretaceous to Paleogene motion between the Pacific and Atlantic hot spots has been recognized, providing motivation to reevaluate the history of relative motion between Pacific oceanic plates and North America by means of plate circuit reconstructions. This paper presents new kinematic models based on two alternative plate circuits linking the Pacific plate to North America since Late Cretaceous time. When compared to models relying on fixity of Pacific and Indo‐Atlantic hot spots, our reconstructions suggest that the Pacific‐Kula and Pacific‐Farallon ridges were ∼600–1000 km closer to the western margin of North America and hence that the Farallon and Kula plates were smaller in Late Cretaceous to middle Eocene time. These findings cast significant doubt on the viability of a configuration of the Farallon‐Kula ridge that juxtaposes the Kula plate with the southern part of the paleo‐North American margin during this time interval. The results of our new reconstructions also suggest more oblique convergence between the Farallon (or Kula) plate and North America from the Late Cretaceous to the middle Eocene related to the combination of dominantly northwest motion of the Pacific plate and steady southwest motion of North America. This provides an efficient mechanism for the coast‐parallel translation of some accreted terranes of North America. It may also explain the structure of geological units in western North America, such as the Central Belt of the Franciscan Complex, which show a dominant coast‐parallel fabric formed during an interval coeval with the episode of oblique convergence.

Three-Dimensional Seismic Attenuation Structure around the SAFOD Site, Parkfield, California

Abstract We present models of the three-dimensional (3D) seismic attenuation structure, both Q p and Q s , for a 16 km 2 area centered on the San Andreas Fault Observatory at Depth (SAFOD). The P - and S -wave t * -values used in the inversion were determined from local earthquake data recorded by seismic network and portable array stations within the Parkfield region by inverting arrival spectra for source parameters, t * , and site response. Two techniques for determining the site response, the joint and alternating methods, were compared and it was found that the alternating method significantly underestimated site response variations. The t * -values were inverted to obtain 3D frequency-independent Q p and Q s models using 3D V p and V s models and associated event locations. A shallow low- Q area ( Q p and Q s about 50–75) on the southwest edge of both models is attributed to the low-velocity Cenozoic sedimentary rocks that overlie the Salinian basement rock. A high- Q feature ( Q p and Q s about 250 to 300) abuts this area and is interpreted as the Salinian basement. Adjacent to the San Andreas fault (SAF) trace, on its southwest side, there is a low- Q feature ( Q p and Q s about 50–80) attributed to a wedge of sedimentary rocks; uniformly low Q p - and Q s -values suggest that the wedge is fluid rich. A low- Q basin feature ( Q p and Q s about 50–75) on the northeast side of the SAF is interpreted as a fluid rich zone. Beneath this area there is a high- Q feature ( Q p and Q s about 220–300), which may be caused by crack closure due to increased pressure with depth in the rocks of the Franciscan formation. Given these high Q -values, it seems unlikely that this area acts as a fluid pathway for fluids entering the fault zone from the east into the seismogenic zone of the SAF.

Mid-Cretaceous oceanic anoxic events in the Pacific Ocean revealed by carbon-isotope stratigraphy of the Calera Limestone, California, USA

The paleoceanographic history of the Pacific Ocean during the mid-Cretaceous is poorly constrained due to the loss of much of the contemporaneous Pacific seafloor to subduction and difficulties in recovering chert-rich sediments by ocean drilling. Pelagic sediments that were originally deposited in the Pacific Ocean but that have been subsequently accreted during subduction potentially provide an alternative paleoceanographic archive. This study presents micropaleontological and carbon-isotope data from the Calera Limestone, part of the classic Franciscan Complex exposed in Permanente Quarry, central California, USA. In the three stratigraphic sections studied, pelagic limestones with low organic-carbon contents are the dominant lithology. However, two stratigraphic intervals are recognized that contain organic carbon, and these date to the early Aptian and late Albian–early Cenomanian. These time intervals correspond to two mid-Cretaceous oceanic anoxic events (OAEs): early Aptian OAE1a (equivalent to the “Selli level”) and late Albian OAE1d (equivalent to the “Breistroffer event”). It is well established that both of these events were associated with significant carbon-isotope excursions, which are also shown to exist in the Calera Limestone. The record of OAE1a from the Calera Limestone complements ocean drilling records by providing further evidence for variability in the sedimentary and stratigraphic record of this event. The carbon-isotope data from the late Albian–early Cenomanian provide the first detailed chemobiostratigraphic record of this period for the Pacific Ocean, confirming that environmental change occurred at this time in the Pacific, possibly related to OAE1d.

Fertile abyssal peridotites within the Franciscan subduction complex, central California: Possible origin as detached remnants of oceanic fracture zones located close to a slow-spreading ridge

This paper describes lherzolite and harzburgite within shale-matrix mélanges of the late Mesozoic Franciscan Complex at the western margin of central California. The Cr# ( = Cr/(Cr + Al) atomic ratio) of spinels in the lherzolite is extremely low (0.11 to 0.13), plotting at the low Cr# end of the abyssal peridotite field, although the moderate to high Cr# of spinels in the harzburgite leaves two possible origins as forearc or abyssal peridotite. The relationships between the Fo and NiO contents of olivines, plot along the mantle olivine array and indicate the residual character of the lherzolite. A high Mg# ( = Mg/(Mg + Fe 2+) atomic ratio) for clinopyroxenes is one of the characteristics of the lherzolite, reflecting low-temperature Mg–Fe redistribution with olivine beneath the ocean floor at around 700 °C. The rare earth element patterns of clinopyroxenes in sampled lherzolite show simple light rare earth element-depleted patterns with flat middle and heavy rare earth elements patterns, resembling those obtained for peridotites dredged from the Vulcan Fracture Zone at the American–Antarctica Ridge. This mineral chemistry, in combination with a lack of association with gabbros and sheet–dike complexes, implies that the lherzolite and harzburgite represent fertile abyssal peridotites along oceanic fracture zones near a slow-spreading ridge; the peridotites experienced a low degree of melting during their history. This origin is in contrast to that of the Coast Range ophiolite, the most extensive ophiolitic belt in California, which formed in a supra-subduction zone setting.

Kinematics of Franciscan Complex exhumation: New insights from the geology of Mount Diablo, California

Three major lithotectonic units of an ancient arc-trench system are present in western California. In ascending structural order, they include: the Franciscan Complex, a remnant accretionary prism partly including high-pressure (blueschist-facies) metamorphic rocks in a subduction melange; the relatively unmetamorphosed Coast Range ophiolite; and the Great Valley Group forearc sediments. In the Mount Diablo study area east of San Francisco, the subduction complex and ophiolite are juxtaposed across the Coast Range fault, and together they represent a structurally attenuated section through the ancestral forearc crust exposed by uplift and folding of the late Cenozoic Mount Diablo anticline. The Coast Range fault is locally associated with a sheared zone in the lower part of the attenuated ophiolite. Shear-sense indicators in the serpentinite, such as S-C fabrics, rotated clasts, and slick-enfibers, record the deformation kinematics and consistently indicate that the ophiolite moved down in a normal sense relative to the Franciscan rocks in the modern reference frame. Based on these kinematic relations and metamorphic facies contrasts between the Franciscan and the Coast Range Ophiolite, as much as 6–18 km structural attenuation has occurred along the Coast Range fault. After restoration of vertical axis rotation and late Cenozoic fold deformation, the Coast Range fault dips at a low angle toward the northeast, and shear-sense data restore to top-to-the-northeast motion on the Coast Range fault, implying normal displacement of the ophiolite with respect to the structurally underlying Franciscan Complex. These relations are consistent with the hypothesis that the Franciscan rocks were exhumed by crustal attenuation along a low-angle fault system. These data suggest that the Coast Range fault dominantly accommodated normal displacement and ductile thinning, and that these processes were primarily responsible for the structural exhumation of high-pressure assemblages in the Franciscan Complex relative to the structurally overlying forearc crust.

Isotope geochemistry of mercury in source rocks, mineral deposits and spring deposits of the California Coast Ranges, USA

We present here the first study of the isotopic composition of mercury in rocks, ore deposits, and active spring deposits from the California Coast Ranges, a part of Earth's crust with unusually extensive evidence of mercury mobility and enrichment. The Franciscan Complex and Great Valley Sequence, which form the bedrock in the California Coast Ranges, are intruded and overlain by Tertiary volcanic rocks including the Clear Lake Volcanic Sequence. These rocks contain two types of mercury deposits, hot-spring deposits that form at shallow depths (< 300 m) and silica-carbonate deposits that extend to depths of 1000 m. Active springs and geothermal areas continue to precipitate Hg and Au and are modern analogues to the fossil hydrothermal systems preserved in the ore deposits. The Franciscan Complex and Great Valley Sequence contain clastic sedimentary rocks with higher concentrations of mercury than volcanic rocks of the Clear Lake Volcanic Field. Mean mercury isotopic compositions ( δ 202Hg) for all three rock units are similar, although the range of values in Franciscan Complex rocks is greater than in either Great Valley or Clear Lake rocks. Hot spring and silica-carbonate mercury deposits have similar average mercury isotopic compositions that are indistinguishable from averages for the three rock units, although δ 202Hg values for the mercury deposits have a greater variance than the country rocks. Precipitates from spring and geothermal waters in the area have similarly large variance and a mean δ 202Hg value that is significantly lower than the ore deposits and rocks. These observations indicate that there is little or no isotopic fractionation (< ± 0.5‰) during release of mercury from its source rocks into hydrothermal solutions. Isotopic fractionation does appear to take place during transport and concentration of mercury in deposits, however, especially in their uppermost parts. Boiling of hydrothermal fluids, separation of a mercury-bearing CO 2 vapor or reduction and volatilization of Hg (0) in the near-surface environment are likely the most important processes causing the observed Hg isotope fractionation. This should result in the release of mercury with low δ 202Hg values into the atmosphere from the top of these hydrothermal systems. Estimates of mass balance suggest that residual Hg reservoirs are not measurably enriched in heavy Hg isotopes as a result of this process because only a small amount of Hg (< 4%) leaves actively ore-forming systems.

A Bathysiphon (Foraminifera) ‘shell bed’ from the Cretaceous of northern California, USA: Example of a parautochthonous macro-skeletal deposit in deep-ocean turbidites

Shell beds consisting of concentrations of minimally transported, slightly damaged skeletal remains of indigenous organisms—comparable to bedded shelly accumulations of certain shallow-marine environments—have rarely been reported from truly deep-ocean turbidites. The general expectation is that shelly accumulations, when they do occur, ought to be derived from upslope sources and many kilometers away from the site of deposition. A Cretaceous thin-bedded turbidite in the Franciscan Complex of northern California, however, hosts a concentration of large specimens of the giant foraminiferan, Bathysiphon aaltoi, reflecting localized transportation and deposition in the original life habitat. The tests were derived from a densely populated thicket of the bathysiphonid probably located only a few metres/10s-of-metres away, decimated by a turbidity current that either overflowed an active submarine fan channel or spread outward from a suprafan lobe. As such, this unusual bathysiphonid-rich deposit can be viewed as a kind of deep-ocean level bottom ‘shell bed’.

Mélanges and ophiolites during the Pan-African orogeny: the case of the Bou-Azzer ophiolite suite (Morocco)

Abstract Since the discovery of ophiolite sequences, the Bou-Azzer inlier has been considered a key area for understanding the evolution of the northern margin of the West African craton during the Pan-African orogeny. For about 20 years, it had been commonly accepted that the Bou-Azzer inlier represents an accretionary mélange accreted onto the West African craton under blueschist metamorphic conditions, similar to the Franciscan Complex and the Sanbagawa facies series. This would imply that a low geothermal gradient was prevalent during the subduction of the Pan-African oceanic plate, and that the ocean was subducted with a high convergence rate. A reinvestigation of the metamorphic conditions by a thermodynamic approach shows that the ophiolite sequence of Bou-Azzer underwent HT greenschist metamorphic conditions instead of blueschist metamorphic conditions. We propose that the ophiolites of Bou-Azzer are not similar to the Sanbagawa facies series or to the Franciscan Complex, but bear similarities to the Albanian or Cyprus ophiolites, which represent dismembered ophiolite sequences overprinted by greenschist conditions.

Compressional and Shear-Wave Velocity versus Depth Relations for Common Rock Types in Northern California

Abstract This article presents new empirical compressional and shear-wave velocity ( Vp and Vs ) versus depth relationships for the most common rock types in northern California. Vp versus depth relations were developed from borehole, laboratory, seismic refraction and tomography, and density measurements, and were converted to Vs versus depth relations using new empirical relations between Vp and Vs . The relations proposed here account for increasing overburden pressure but not for variations in other factors that can influence velocity over short distance scales, such as lithology, consolidation, induration, porosity, and stratigraphic age. Standard deviations of the misfits predicted by these relations thus provide a measure of the importance of the variability in Vp and Vs caused by these other factors. Because gabbros, greenstones, basalts, and other mafic rocks have a different Vp and Vs relationship than sedimentary and granitic rocks, the differences in Vs between these rock types at depths below 6 or 7 km are generally small. The new relations were used to derive the 2005 U.S. Geological Survey seismic velocity model for northern California employed in the broadband strong motion simulations of the 1989 Loma Prieta and 1906 San Francisco earthquakes; initial tests of the model indicate that the Vp model generally compares favorably to regional seismic tomography models but that the Vp and Vs values proposed for the Franciscan Complex may be about 5% too high.

Contrasting early and late Mesozoic petrotectonic evolution of northern California

Devonian–Middle Jurassic terrane assemblies in the Klamath Mountains and Sierra Nevada Foothills consist chiefly of ophiolite-chert-argillite sequences. Mafic-ultramafic complexes are oceanic, whereas associated fine-grained deep-water terrigenous sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes. Coeval calc-alkaline arc rocks are volumetrically rare. Geologic and petrochemical relations suggest a rifted arc origin for Klamath mafic metavolcanic units inter-layered with distal turbidites in the 170–200 Ma North Fork terrane; detrital zircon U-Pb ages indicate that the clastic debris had a regional eastern Klamath source. The Eastern Hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby Eastern Klamath Antelope Mountain Quartzite. The seaward 200 Ma Rattlesnake Creek terrane is an ophiolitic melange with North Fork petrotectonic affinities. The North Fork–Eastern Hayfork–Rattlesnake Creek amalgam correlates with the Calaveras Complex and the outboard Jura-Triassic arc belt in the Sierran Foothills. Geochemical bulk-rock and zircon U-Pb age data support interpretation of the 200 Ma Jura-Triassic arc as an adjacent offshore mafic belt overlying a 300 Ma ophiolitic basement. These oceanic complexes were sutured against the Central Metamorphic Belt–Eastern Klamath–Feather River–Northern Sierra terrane backstop before deposition and deformation of the outboard Upper Jurassic Galice and Mariposa formations. Klamath-Sierran terrane assemblies reflect ∼230 m.y. of transpression-transtension involving only minor episodes of subduction, producing ubiquitous ophiolite-chert-argillite lithologies and rare felsic arc rocks. In contrast, the Late Jurassic to largely Cretaceous Klamath–Sierra Nevada quartzo-feldspathic volcanic-plutonic arc attests to massive calc-alkaline magmatism attending a strong eastward component of underflow by the Farallon plate. The coeval Galice-Mariposa formations, followed by the Cretaceous Great Valley forearc and Franciscan trench deposits, are first-cycle felsic debris shed mainly from the Klamath-Sierran arc. These units record ∼70 m.y. of rapid sialic crustal growth attending major periods of approximately margin-normal convergence. This profound transition in northern California included Devonian–Middle Jurassic rifting, drifting, and stranding of ophiolite-chert-argillite terranes along an adjacent curvilinear continental margin, then nearly head-on Cretaceous subduction that resulted in massive calc-alkaline igneous activity, the erosion of which generated the felsic Great Valley Group forearc basin and Franciscan Complex trench clastic sedimentary units.

Unique deformation processes involving the recrystallization of chrysotile within serpentinite: implications for aseismic slip events within subduction zones

AbstractSerpentinite bodies within the Franciscan Complex, a Mesozoic accretionary prism located in California, USA, display a unique form of deformation that involves the recrystallization of chrysotile and the formation of a block‐in‐matrix structure. The phacoidal‐shaped blocks have a preferred orientation, and result from the local replacement of serpentine minerals by chrysotile grains that are aligned parallel to ductile shear planes such as S–C foliation; ultimately, some of the rocks evolved into chrysotile schist. The relic blocks are also fragmented into multiple parts, with the spaces between fragments being infilled by recrystallized chrysotile. The low coefficient of friction of chrysotile means that this deformation process acts to suppress the frictional properties of the entire serpentinite body within the forearc mantle. This phenomenon can be attributed to the slip style that occurs in aseismic regions of subduction zones in areas shallower than the stability field of antigorite.

40Ar/39Ar Ages from Coherent, High-Pressure Metamorphic Rocks of the Franciscan Complex, California: Revisiting the Timing of Metamorphism of the World's Type Subduction Complex

In the Franciscan subduction complex of California, high-pressure metamorphic rocks have generally been divided into two groups, "high-grade blocks" and "coherent metamorphic rocks." High-grade blocks, typically discrete blocks centimeters to tens of meters in diameter, exhibit coarse metamorphic minerals and relatively high metamorphic grade, including blueschist, eclogite, and amphibolite assemblages, and are typically found encased within mélange matrix of distinctly lower grade. Coherent metamorphic rocks are generally of epidote blueschist and lower grade and consist of large, more or less intact thrust sheets of metasedimentary and metavolcanic rocks. Considerable age dating work has been completed on the high-grade blocks, but dating the metamorphism of the coherent units has proven very difficult because of the very fine grain size of their metamorphic minerals, which has precluded the preparation of pure mineral separates for isotopic analysis. In this paper, we report new white mica and amphibole 40Ar/39Ar step heating ages from mineral separates from several coherent units. These new ages include: (1) white mica ages of 120.5 Ma from the South Fork Mountain Schist; (2) white mica ages of approximately 132 Ma from the Skaggs Springs schist; (3) a white mica age of approximately 154 Ma from the coherent schists of Ward Creek; and (4) white mica ages of 146.5 Ma (eclogite) and 134 Ma (epidote blueschist) from the Willow Spring slab, a newly recognized 2.1 by 1.2 km (map view) coherent metabasite slab in the Panoche Pass area comprising rocks ranging from lawsonite blueschist to eclogite. We also obtained hornblende ages of about 156 and 159 Ma, respectively, from high-grade amphibolite blocks from McLaren Park and the Hunters Point Shear Zone in San Francisco. A review of the new and previous ages indicates that the ages of the high-grade blocks and the coherent metamorphic rocks partially overlap, strongly suggesting they formed in the same subduction zone. Moreover, our field mapping near Panoche Pass has identified several fault-bounded sheets of coarse-grained, coherent metamorphic rocks similar to the Willow Spring slab that display mineralogy identical to that of many highgrade blocks. Unlike many of the world's subduction-related metamorphic terranes, the Franciscan exhibits evidence for high-pressure metamorphism that spanned at least 80 m.y. in time, the product of subduction, accretion, and metamorphism of different packets of rock at different times. Although the Franciscan may be an exception among subduction complexes, attempts to define a single age of metamorphism in other high-pressure or ultrahigh-pressure terranes may need reconsideration.