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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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ODP Site 858 (1)
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Pacific Ocean
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East Pacific
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Northeast Pacific
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Gorda Rise (1)
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Juan de Fuca Ridge (1)
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Mendocino fracture zone (1)
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North Pacific
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Northeast Pacific
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Gorda Rise (1)
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Juan de Fuca Ridge (1)
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Mendocino fracture zone (1)
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Santa Cruz Mountains (1)
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United States
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California
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Humboldt County California
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Santa Cruz County California
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Southern California (1)
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elements, isotopes
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carbon
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C-13/C-12 (2)
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isotope ratios (3)
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isotopes
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stable isotopes
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C-13/C-12 (2)
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O-18/O-16 (3)
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carbon
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Ocean Drilling Program
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Leg 139 (1)
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Leg 167
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ocean floors (1)
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Pacific Ocean
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North Pacific
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sediments
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Analysis of the December 1998 Santa Cruz Mountains, California, Earthquake Sequence
Abstract The extraordinarily well-preserved and well-exposed Semail ophiolite of northern Oman hosts several large plagiogranite intrusions in proximity to economic copper sulphide deposits of the Lasail mining district. A progression of isotopic, chemical and mineralogical transformations observed within the plagiogranites and high-level gabbros (HLG), and a comparison of these effects with those in the lowermost dykes of the immediately overlying sheeted dyke complex (SDC) tracks the evolution of hydrothermal fluids and the alteration of overlying dykes and pillow lavas during discharge of these fluids on the sea floor. The largest hydrothermal alteration aureoles, and the greatest extent of metamorphic veins and metasomatic replacement features, are found adjacent to the largest high-level plagiogranite bodies, beneath and adjacent to the major ore bodies in northern Oman. The ubiquitous presence of metamorphic actinolitic hornblende, sodic plagioclase, epidote and titanite in metabasalts within the high-temperature alteration zones points to the most likely mineralogical and structural controls on the development and evolution of the hydrothermal fluids. Depleted Cu contents of the adjacent crustal rocks and Cu enrichments above the plagiogranite intrusions demonstrate the redistribution of heavy metals adjacent to the complexes. Field relationships implicate the formation of both the epidosites and plagiogranites in the genesis of the ore deposits. An important process inferred from the field and geochemical data is the assimilation of previously hydrothermally altered basaltic and gabbroic country rocks by stoping into the magma chambers developed near the SDC-gabbro horizon in the ophiolite. We suggest that this process of combined assimilation-fractional crystallization, together with replenishment and recharge by injection and quenching of basaltic magma ‘pillows’ into these plagiogranite magma chambers (i.e. RAFC), plays a major role in the development of these composite intrusions.
Mass wasting, methane venting, and biological communities on the Mendocino transform fault
Anatomy and origin of carbonate structures in a Miocene cold-seep field
Hydrothermal alteration within the basement of the sedimented ridge environment of Middle Valley, northern Juan de Fuca Ridge
Carbonate Vent Structures in the Upper Miocene Santa Cruz Mudstone at Santa Cruz, California
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.
Igneous and Sedimentary Rocks from Monterey Canyon, California and Implications for Regional Tectonics
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.
Temporal variations in secondary minerals from Nazca plate basalts, diabases, and microgabbros
The mineralogy and chemistry of secondary phases observed in altered basalts from the Nazca plate provide a record of temporal variations in the processes that accompany the aging of oceanic crust. The earliest formed phase in vein and vesicle fillings is Fe- and Mg-rich, Al-poor saponite; it is the most abundant alteration mineral in younger rocks dredged from near the East Pacific Rise, and it lines many fractures and vesicles in much older pillow basalts recovered from the plate. Saponite formation is enhanced by low pH values, high water-rock ratios, and Mg- and Fe-rich solutions, conditions that persist until fractures and other pore spaces become closed and the hydrothermally induced circulation becomes negligible. During this phase of alteration, constituents derived from high-temperature leaching of underlying holocrystalline diabasic and gabbroic rocks are redistributed, both into secondary minerals found in overlying pillow basalts and into sea water. Holocrystalline, upper layer 2 basalts containing abundant smectite are richer in Mg, Na, K, and Ti and poorer in Ca relative to unaltered fresh glasses. During the waning of the hydrothermal system, lower temperatures and slightly alkaline and oxidizing solutions are indicated by the formation of calcite, ferric oxides and hydroxides, and celadonite. Calcite commonly fills all remaining void spaces in hydrothermally altered rocks. If such hydrothermally altered crust is re-exposed to oxygenated sea water by tectonic uplift or by other means, intense low-temperature oxidation can dramatically affect its bulk chemical composition and secondary mineralogy. Earlier formed saponites and ferrosaponites are destroyed and replaced by celadonite and ferric oxides, and bulk-rock compositions show pronounced losses in Mg and enrichments in Fe and K relative to fresh glasses. It is argued that secondary mineral assemblages in altered layer 2 crustal rocks are very sensitive indicators of conditions of alteration. With few exceptions, evidence of nonoxidative, hydrothermal alteration is ubiquitous in upper layer 2 crustal rocks recovered from many different locations on the Nazca plate.