No description available.

Glass Buttes, a Pliocene silicic volcanic complex within the High Lava Plains province of Oregon, was erupted approximately 5.0 to 5.8 million years ago. Geologic mapping revealed that the eastern portion of the complex is underlain by rhyolitic glass domes, flows and rare pyroclastis flows. Basalt flows are interlayered with and onlap the silicic glass. Younger basalt flows, erupted from local vents, overlie silicic glass and onlapping basalts. The eastern end of Glass Buttes is hydrothermally altered at the surface; a weak geothermal anomaly coincides with the altered areas. Alteration, localized by northwest trending normal faults, occurs primarily as opalite replacement of rhyolite glass with associated cinnabar, alunite, clay-rich vein material, hematite, and hyalite. Alteration paragenesis at the surface was defined, and physicochemical conditions during hydrothermal activity were inferred from alteration minerals and assemblages and trace element content of alteration minerals. Alteration identified in the subsurface is interpreted to be related to an older hydrothermal system. Carbonate, pyrite, quartz, and minor smectite and chlorite occur in vugs and fractures, and partially replace subsurface basalt. Abundant fine-grained disseminated pyrite occurs in permeable units. Pyrite separates from disseminations and veins within basalt and permeable glassy units contain up to 13 ppm Au. The pyrite samples are also anomalous with respect to arsenic and antimony.

The northern Los Angeles basin is influenced by two structural styles: the west-trending compressional Transverse Ranges to the north, and the strike-slip Peninsular Ranges to the south. The interaction of these two structural styles has resulted in a complex fold/fault belt at the northern margin of the Los Angeles basin, which deforms a variable sequence of late Miocene through Quaternary marine strata. Subsurface mapping of Quaternary marine gravels by electric-log correlation documents the latest phase of deformation in the northern Los Angeles basin. The Quaternary marine gravels are folded at the Wilshire arch, the Hollywood basin, the central trough, the Newport-Inglewood fault, and the Santa Monica fault. The west-plunging Wilshire arch, which follows Wilshire Boulevard east of the Newport- Inglewood fault, is a broad fold identified and named in this study. Deformation of the Wilshire arch, which is underlain and caused by the potentially-seismogenic Wilshire fault, began around 0.8 - 1.0 Ma. A fault-bend fold model, based on the shape of the Wilshire arch, indicates a dip-slip rate of 1.5 - 1.9 mm/yr for the Wilshire fault, whereas a three-dimensional elastic dislocation model indicates a right-reverse slip rate of 2.6 - 3.2 mm/year for the Wilshire fault. The finer-grained marine Pliocene strata include the late Pliocene to early Pleistocene Pico member, and the early Pliocene Repetto member, of the Fernando Formation. Thickness and lithology variations in the Pico and Repetto strata, which were influenced by syndepositional structures, indicate that the entire Pliocene and the latest Miocene were characterized by compression. The primary structure present throughout the Pliocene is a south-dipping monocline, which was underlain and caused by a deep reverse fault, dipping ~55 - 60° to the north, referred to here as the Monocline fault. Relative subsidence of the central trough resulted in deposition of up to 7000 ft (2135 m) of Pico strata, and up to 5000 ft (1525 m) of Repetto strata, compared to zero deposition on the monoclinal high. In the western part of the study area, the south-dipping monocline is interrupted by the secondary East Beverly Hills fold, which may be a rabbit-ear fold that accommodates excess volume by bedding-parallel slip. The East Beverly Hills fold was active in the latest Miocene through Pliocene, and was most active during early Pliocene Repetto deposition. In the eastern part of the study area, the monocline is interrupted by the Las Cienegas fold, which formed in the hangingwall of the Las Cienegas fault. The Las Cienegas fault was a normal fault in the late Miocene, and was reactivated in the Pliocene as a steep reverse fault. Folding and uplift on the Las Cienegas anticline occurred throughout the Pliocene, with the greatest amount occurring during lower and lower-middle Pico deposition. / Graduation date: 1994

Over 7000 sq. km of salt and six Plio-Pleistocene biostratigraphic horizons were mapped in the East Breaks and Garden Banks areas using a 12,000 km grid of seismic data and all obtainable well data. Structure mapping of allochthonous Jurassic salt and the six horizons (Globoquadrina altispira, Lenticulina 1, Angulogerina B, Hyalinea B, Trimosina A, and Sangamon Fauna) and isopachs of the intervals between these horizons revealed notable lateral variations in the area underlain by salt, in the degree of salt deformation, and in the size and thickness of associated intraslope basins. East of 94.5° W salt structures occupy 40% of the area and exhibit complex shapes that suggest a high degree of salt deformation. West of 94.5° W salt structures occupy 11% of the area and consist mostly of structurally simple salt stocks. A zone of high-offset north-south trending faults mark the transition between these two areas. Isopach maps of the six Plio-Pleistocene intervals (from 2.9 Ma to the present) reveal major shifts in the rates and locations of sediment accumulation. From 2.9 to 1.0 Ma. sediment-accumulation rates averaged only 0.8-1.3 mm/y with a maximum rate of 2.7 mm/y. From 1.0 to 0.69 Ma. sediment-accumulation rates averaged 5.8 mm/y with a maximum rate of 11.6 mm/y. This interval correlates to sediments deposited between the extinctions of Hyalinea balthica and Trimosina denticulata and recorded a major period of sediment loading/salt withdrawal between 1.0-0.69 Ma. From the end of this time to the present, sediment -accumulation rates averaged 1.7-2.1 mm/y with a maximum rate measured at 6.2 mm/y. Increased sediment influx during 1.0-0.69 Ma coincides with a major third order sea level lowstand and was focused in central Garden Banks. The restriction of such dramatically increased accumulation rates to this area suggests that sediment influx was accompanied by large-scale salt withdrawal. The increase in accommodation space created by salt withdrawal appears to be the most important factor affecting accumulation rates. Salt structural styles found on the upper continental slope are transitional between those found on the lower slope and those on the shelf. The shelf is dominated by isolated, individual salt stocks (km²) surrounded by kilometer thick sedimentary sections. The lower slope is dominated by broad, laterally continuous, allochthonous salt sheets (10³ km²) with moderate to thin sediment cover. The upper slope contains both of these structural styles plus intermediate size (10-10² km²) salt ridges and massifs. Observations made during this study suggest that differential sediment loading is the mechanism causing the changes in structural style. A Loading/Dissection model is presented to explain the formation of the three primary salt structural styles, their genetic relationship, and their observed distribution. Differential loading has dissected large salt sheets into numerous smaller and irregularly shaped ridges and stocks (like those found on the upper slope). Salt found on the upper slope originated in the Jurassic Louann Formation, but is now surrounded by Pleistocene age sediments. To achieve this relationship, it appears that some Jurassic salt has undergone at least two cycles of sediment loading and consequent diapirism. Salt/sediment relationships suggest that virtually all of the mapped salt is allochthonous. Repetitive sediment loading and salt structural development has not been previously documented and represents a step beyond the limits of current salt structural models. / text

Four multicyclic complex calderas and smaller ignimbrite shields located within the Altiplano Puna Volcanic Complex of the Central Andes (APVC) erupted 13000 km�� of magma within the last 11 Ma. One of the largest and most complex of these is the Cerro Guacha Caldera. Ar-Ar age determinations and paleomagnetic directions suggest that the Cerro Guacha Caldera was formed by two major eruptions, caldera collapse, resurgence cycles and several smaller eruptions. Two major ignimbrites (> 600 km��) are found with ������Ar-�����Ar from biotites and sanidines of 5.65 �� 0.01Ma for the 1300 km�� (magma volume) Guacha ignimbrite and 3.49 �� 0.01Ma for the 800 km�� Tara Ignimbrite. The last major eruption occurred on the western flank producing the 1.72 �� 0.02 Ma Puripica Chico Ignimbrite with a volume of approximately 10 km��. Characteristic remanent magnetization data (ChRM) for these ignimbrites show that the Guacha has reverse polarity, while the Tara is normally polarized and the magnetic fingerprints have allowed their current full extents to be identified. A conspicuous lineament of volcanic structures in the eastern part of the caldera, bordering a caldera moat, filled out welded ignimbrites and sedimentary lacustrine sequences suggest an earlier 60x40 km outer collapse associated with the Guacha explosive episode. A central graben formed on the Guacha welded ignimbrite is related to a first episode of resurgence. Evidence of a second 30 x15 km inner collapse includes offset of welded Guacha ignimbrites and alignment of lava domes associated with the Tara ignimbrite. A second resurgence episode is suggested by the presence of an uplifted central block consisting primarily of welded Tara ignimbrite. As a whole the three ignimbrites (Guacha, Tara and Puripica Chico) share the same petrological and geochemical characteristics: high-K series, compositional ranges from dacite to rhyolite, with andesitic members present as lavas (for the Guacha and Puripica Chico Ignimbrites) and as pumices (for the Tara Ignimbrite). Highest silica content is found in the Chajnantor dome. Rayleigh modeling for Ba, Rb and Sr suggests at least 60% of crystal fractionation to account for the compositional variation between the Guacha andesite and the Chajnantor dome. Dy/Hb ratio increases with time from the Guacha andesite to the Negreal andesite suggesting stabilization of garnet owing to crustal thickening. Fe-Ti exchange geothermometry for the Tara Ignimbrite yielded log fO��� values ranging from -13.06 to -13.38 and temperatures of 714�� to 801��C. Amphibole geobarometry yielded pressures ranging from 150 to 180 MPa equivalent to 5.3 and 6.4 km depth respectively for the Tara Ignimbrite; the pressures range between 133 to 242 MPa, equivalent to 5.0 to 9.2 km depth for the Guacha Ignimbrite. The zircon saturation method yielded saturation temperatures of 716�� and 705��C for the Guacha and Chajnantor dome respectively and 784��C for the Tara Ignimbrite. The zircon crystallization range for the magmas of the Cerro Guacha Caldera is 1.25 Ma for the Guacha Ignimbrite; 1.09 Ma for the Puripica Chico Ignimbrite and 0.95 Ma for the Tara Ignimbrite. Recycling of antecrystic zircons within the caldera magmas is continuos through time. / Graduation date: 2012