ABSTRACT Recent seismic reflection data across the Japan Trench show that frontal accretion involves offscraping sediments on top of horsts and scooping-up sediment from grabens. However, seismic profiling does not illuminate the structure within the accretionary prism, and thus the processes of accretion and prism growth are unknown. Key data from scientific drilling at Integrated Ocean Drilling Program Site C0019 that penetrated the prism in the region of large displacement during the 2011 Tohoku earthquake support a model in which frontal accretion occurs by imbricate thrusting, folding, and stacking of thrust sheets that are composed of semicoherent-sediment strata. Using palinspastic restoration techniques, we conclude that out-of-sequence thrusting and duplex development during the underthrusting of horsts can form and displace hanging-wall ramps along the plate-boundary detachment, which helps to explain the formation of some unexpected tectonostratigraphic relations at C0019, such as the emplacement of a thick section of the youngest sediments at the base of the accretionary prism, and numerous juxtapositions of different-age sediments within the basal plate-boundary fault zone.

Abstract Active faults slip at different rates over the course of the seismic cycle: earthquake slip ( c . 1 m s −1 ), interseismic creep ( c . 10–100 mm year −1 ) and intermediate rate transients (e.g. afterslip and slow slip events). Studies of exhumed faults are sometimes able to identify seismic slip surfaces by the presence of frictional melts, and slow creep by textures diagnostic of rate-limited plastic processes. The Pasagshak Point Thrust preserves three distinct fault rock textures, which are mutually cross-cutting, and can be correlated to different strain rates. Ultrafine-grained black fault rocks, including pseudotachylyte, were formed during seismic slip on layers up to 30 cm thick. Well-organized S – C cataclasites 7–31 m thick were formed by slow creep, with pressure solution as a dominant, rate-limiting mechanism. These must have formed at strain rates consistent with long-term plate-boundary motion, but solution-creep healing acted to reduce porosity of the cataclasites and eventually restricted fluid connectivity such that creep by this mechanism could not continue. Disorganized, non-foliated, rounded clast cataclasites were formed at shear rates faster than solution creep and are interpreted as representing shear at intermediate strain rates. These could have formed during afterslip or delocalization of slip associated with an earthquake rupture. Supplementary material: Detailed map of Pasagshak Peninsula is available at http://www.geolsoc.org.uk/SUP18493 .

ABSTRACT Fluid flow out of the seafloor offshore Monterey Bay region is extensive. To date 16 major active and ancient, or dormant, seep sites have been identified and many of these sites are composed of smaller sites too numerous to map at a regional scale. These seeps have been identified by the presence of chemosynthetic communities that are primarily composed of chemoautotrophic organisms or by carbonate deposition and buildups. Of the 17 identified sites, 9 active cold seep sites support living chemosynthethic communities. Seven major dormant seep sites have been identified based upon the presence of carbonate deposits or buildups. Identified seep sites are primarily concentrated along fault trends associated with the boundary of the Salinian block or Palo Colorado-San Gregorio fault zone, and along the lower flanks and crests of tectonically uplifting slopes. A combination of transpressional squeezing and overburden pressures, vertical advection through hydrocarbon and organic-rich sediment, and seaward flow of meteoric waters supply fluids to the seep sites.

ABSTRACT An exceptional exposure of the San Gregorio Fault provides the opportunity for detailed observations of structural fabrics within an active fault zone. Where it is exposed in the intertidal zone in Moss Beach, California, the San Gregorio Fault juxtaposes different sedimentary lithologies within the Pliocene Purisima Fm. An approximately 10 meter-wide zone of clay-rich foliated gouge marks the fault. The damage zone is approximately 100 meters wide, and the distribution of deformation is heterogeneous across the fault zone. Structural fabrics in the northeast fault block include breccias and both microscopic and outcrop-scale shear zones; these record the effects of cataclasis on porous sandstones and conglomerates. Deformation in the mudstones of the southwest fault block is accommodated by an incipient scaly foliation as well as by numerous fractures and faults. Microstructural analyses indicate that the San Gregorio accommodates dextral strike-slip offset as well as a component of west-side up reverse motion. Evidence for the role of fluids in this fault zone includes field relations and geochemical data. Anomolous hydrocarbon content within the foliated fault gouge indicate that the fault is a migration conduit. Fluctuations in fluid pressure within this fault zone may help elucidate the mechanics and seismogenic potential of the San Gregorio Fault.

ABSTRACT Authigenic carbonate structures in Miocene biosiliceous sediments are well exposed near a late Miocene angular unconformity in coastal cliffs at Santa Cruz, California and closely resemble carbonate structures formed at modern seep sites on the seafloor, including the adjacent Monterey Bay. The Miocene vent structures show varied morphologies, including pipes (“chimneys”) and bedding-parallel slabs., but, unlike many modern seep carbonates, they lack an associated vent macrofauna. They are composed of low magnesium calcite which cements and partly replaces the host sediment, indicating that the structures formed below the sediment-water interface and not above the seafloor. Carbon and oxygen isotopic compositions suggest carbonate precipitation occurred in a low temperature pore fluid environment fairly near the seafloor, within the zone of bacterial sulfate reduction. The host rock, the Santa Cruz Mudstone, comprises interbedded siliceous mudstones and thin, brittle opal-CT porcelanite layers which show two dominant fracture sets, one striking N30°E, the other N60°W. Most of the carbonate vent structures occur within the porcelanite layers, and the orientations of many pipes and slabs parallel the strikes of the two fracture sets. This suggests that the fluids which precipitated the carbonates were channeled along fractures. This structural control and the proximity of the vent structures to an angular unconformity indicates that deformation was a major factor, creating fracture permeability and probably also causing tectonic compaction of sediments as well as expulsion of fluids. The main Miocene vent locality lies near three major fault zones (San Gregorio, Monterey Bay, and Ben Lomond), and we speculate that the deformation was related to tectonism on one or more of these faults. Among the unresolved issues is the time and burial depths at which the carbonate vent structures formed. Some evidence (e.g. preservation of opal-A diatoms in the calcite structures) favors carbonate precipitation prior to the opal-A to opal-CT phase transformation, while other evidence (e.g. lack of compaction around the carbonate structures) suggests precipitation occurred after or contemporaneously with the silica phase change. These conflicting scenarios might be reconciled if the silica phase transformation occurred relatively early at shallow burial depths in an environment of advecting fluids with low silica concentrations.

ABSTRACT Sandstone intrusions are widespread west of Santa Cruz, California and were emplaced during late Cenozoic tectonic deformation of this region. Among these is a very large and complex intrusion which is well exposed along the coastline at Yellow Bank Creek. Here, fluidized sands from the Miocene Santa Margarita Sandstone were injected upward into fractured biosiliceous rocks of the Santa Cruz Mudstone, probably due to faulting and seismic shaking. The complicated internal structure of this intrusion includes sedimentary xenoliths, fluidization structures, and secondary limonite staining. The latter likely occurred during oxidation by groundwater and produced conspicuous, complicated layering which serves to mask and confuse interpretations of the earlier-formed features. Among the earlier formed features are fluidization structures, comprising (1) flow banding which records injection of sands horizontally in silllike areas of the intrusion, and (2) heave structures which reflect mainly vertical injection of hydrocarbons and sands partially saturated with hydrocarbons into water-saturated sands. This latter type of injection appears to have occurred at a hydrocarbon front that was derived from either a localized petroleum accumulation or else from remnants of hydrocarbons that had mostly migrated updip prior to the clastic intrusion event. Dolomitic cementation occurred preferentially in the hydrocarbon-saturated sands due to degradation of the hydrocarbons. Paleotemperature estimates of the intrusive sandstones (by apatite fission track analysis) and of the host Santa Cruz Mudstone (by vitrinite reflectance) indicate maximum temperatures of about 60°C for the former, 50°C for the latter. Our data suggests that initial fluidization began in water-saturated sands of the bioturbated facies in the Santa Margarita Sandstone; following upward intrusion of these sands, fluidization and injection expanded into hydrocarbon-bearing sands within the cross-bedded facies of the same unit.

ABSTRACT Samples collected from the northern meander of Monterey Canyon, California, and the adjacent Soquel Canyon include Cretaceous granodiorites and middle Tertiary basaltic andesites and sandstones. Plagioclase separated from the granodiorite basement rocks from Soquel Canyon yielded an age of 79 ± 0.8 Ma and are isotopically similar to Salinia-terrane granitoids exposed on the Monterey Peninsula. The Soquel Canyon granodiorite is crosscut by mafic dikes that are basaltic andesite in composition. Plagioclase separated from one mafic rock has been dated at 23.7 ± 0.5 Ma, consistent with the middle Tertiary pulse of volcanism characteristic of this region. This mafic unit also intrudes an overlying sandstone unit forming a “peperite” texture resulting from contemporaneous volcanism and sedimentation. The peperite constrains the lithic-rich sandstone, which we propose to have been deposited in a sedimentary basin associated with local tectonic extension, to a late Oligocene and (or) early Miocene age. The artifacts of the sedimentary basin are truncated (and deformed) on the south by the Monterey Bay fault zone, and exposed within the northern meander of the Monterey Canyon. These new lithologies require a revision of the Neogene lithostratigraphy of Monterey Bay and may also be useful in linking the local volcanic, tectonic and sedimentary history to the complex tectonic development of central California during the middle Tertiary. We suggest that strike-slip or transtensional movement along the Monterey Bay Fault Zone opened a basin in late Oligocene and (or) early Miocene into which was deposited a coarse, lithic-rich (Vaqueros?) sandstone. The contemporaneous volcanism of basaltic andesite is alkalic in character and may have been a result of mantle upwelling within a slab window or local transtension along the major faults active during this period.