The Chairman welcomed new panel members Tanya Atwater, Casey Moore and Mike Purdy. He outlined the Agenda, and pointed out that the Panel should also consider appointment of a new Chairman for after the fall 1990 meeting as he will by that time have served for three years and will , in addition, be prevented from attending the Annual Meeting in early December 1990 by a commitment to field work in the Antarctic at that rime.

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.

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.

In the decollement zone of the Barbados accretionary prism, a 3-D seismic image exhibits patchy high-amplitude negative polarity reflections, which have been attributed to large overpressures confined to the fault zone. We collected laboratory P-wave velocity and porosity vs. pore pressure data, using core samples from and adjacent to the decollement zone at ODP Site 948. Logs constrain density and velocity through the decollement zone at Site 948. We use these data to calibrate the reflectivity of the fault zone to pore pressure through waveform and amplitude models of the fault plane reflections. Modeling of the positive polarity Site 948 reflection indicates that it can be explained by a lithologic boundary coincident with the decollement, without anomalous fault properties. By contrast, the dominantly-negative polarity waveform of the reflection [approx]2 km arcward (beneath Site 947) is best modeled by inserting a 16-19 m thick zone of extremely low impedance into the Site 948 impedance structure, with a gradational return to [open quotes]normal[close quotes] impedance just above the positive boundary. Relative amplitudes in this reflection indicate a larger impedance contrast than can be accounted for at sub-lithostatic fluid pressure, based on the core properties data. We conclude that lithostatic pore pressure withmore » attendant hydraulic dilation of the fault zone is required to generate the negative-polarity reflections. Mapping of these reflections thus delineates zones of elevated fluid content and zero effective stress in the fault zone.« less

Research Article| May 01, 1979 Cross section, Alaska Peninsula–Kodiak Island–Aleutian Trench: Summary ROLAND VON HUENE; ROLAND VON HUENE 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GEORGE W. MOORE; GEORGE W. MOORE 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. CASEY MOORE; J. CASEY MOORE 2Earth Sciences Board, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar CHRISTOPHER D. STEPHENS CHRISTOPHER D. STEPHENS 3U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information ROLAND VON HUENE 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 GEORGE W. MOORE 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 J. CASEY MOORE 2Earth Sciences Board, University of California, Santa Cruz, California 95064 CHRISTOPHER D. STEPHENS 3U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 Geological Society of America GSA Bulletin (1979) 90 (5): 427–430. https://doi.org/10.1130/0016-7606(1979)90<427:CSAPIT>2.0.CO;2 Article history First Online: 01 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 ROLAND VON HUENE, GEORGE W. MOORE, J. CASEY MOORE, CHRISTOPHER D. STEPHENS; Cross section, Alaska Peninsula–Kodiak Island–Aleutian Trench: Summary. GSA Bulletin 1979;; 90 (5): 427–430. doi: https://doi.org/10.1130/0016-7606(1979)90<427:CSAPIT>2.0.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 SocietyGSA Bulletin Search Advanced Search Abstract No Abstract Available. 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.

Research Article| April 01, 1979 Variation in strain and strain rate during underthrusting of trench deposits J. Casey Moore J. Casey Moore 1Earth Sciences, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar Author and Article Information J. Casey Moore 1Earth Sciences, University of California, Santa Cruz, California 95064 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1979) 7 (4): 185–188. https://doi.org/10.1130/0091-7613(1979)7<185:VISASR>2.0.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 J. Casey Moore; Variation in strain and strain rate during underthrusting of trench deposits. Geology 1979;; 7 (4): 185–188. doi: https://doi.org/10.1130/0091-7613(1979)7<185:VISASR>2.0.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 Both convergence velocity and the thickness of sediment entering a subduction zone affect the dynamics of deformation in this environment. Specifically, the steady-state trench model implies that at constant sediment influx and convergence angle, the strain of underthrust trench deposits varies as the first power of convergence rate for a given distance of underthrusting and as the square of convergence rate for a given duration of underthrusting. Trench deposits are probably disrupted during high-strain and high-strain-rate convergent regimes, whereas they may be more coherently deformed in relatively low-strain and low-strain-rate regimes. Variations from coherent to tectonically disrupted terranes may be interpreted in terms of changes in both convergence rate and influx of trench sediments. 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.

Cretaceous deep-water sedimentary rocks are discontinuously exposed or have been dredged, along 1,700 km of the outer continental margin of the Alaska Peninsula-Bering Sea shelf. In the Shumagin and Sanak Islands, on the continental shelf near the southwestern end of the Alaska Peninsula, the deep-sea sediments are comprised of monotonous sections of thin (4 cm) to thick (10 m) bedded sandstone and mudstone, showing grading, convolute lamination, groove and flute casts. The sandstone beds are lithic arenite with more than 40% volcanic-derived framework grains. Over 500 measurements of sole marking in the Shumagin and Sanak Islands show maxima End_Page 795------------------------------ toward the southwest and west-northwest respectively, with minor lateral feed from the north. This flysch sequence was deposited primarily by turbidity currents in an elongate trough supplied from a northerly volcanic source area. In the absence of a confining basement seaward of the flysch deposits, the original depositional basin is interpreted as an oceanic trench. These trench deposits were deformed initially in a semilithified state with the development of axial-plane salty cleavage. Fold axes parallel the existing continental shelf edge, trending northeast and west-northwest in the outer Shumagin and Sanak Islands, respectively. Folds are overturned seaward predominantly, axial surfaces dipping landward. Locally units may be described as broken formations, though no melanges are observed. The style of this early folding is consistent with, but not diagnostic of, gravity liding. Alternatively, the rocks may have been deformed by underthrusting at the trench inner wall. At rates of 10-13 to 10-14/sec (calculated assuming underthrusting), the trench sediments may have undergone strain hardening from increasing internal grain friction and cementation during dewatering. End_of_Article - Last_Page 796------------