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Stratigraphic and volcano-tectonic relations of Crater Flat Tuff and some older volcanic units, Nye County, Nevada
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Neogene tectonics and geomorphology of the eastern Uinta Mountains in Utah, Colorado, and Wyoming
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Late Cenozoic tectonism of the Sacramento Valley, California
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Revised instrumental hypocenters and correlation of earthquake locations and tectonics in the central United States
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The metamorphic and plutonic rocks of the southernmost Sierra Nevada, California, and their tectonic framework
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Tectonically controlled fan delta and submarine fan sedimentation of late Miocene age, southern Temblor Range, California
The Santa Margarita Formation in the southern Temblor Range, composed of conglomerate and subordinate sandstone, evolved as a large complex of fan deltas and submarine fans in late Miocene time. An 80 to 90-m.y.-old granitic basement of the Salinian block and an accompanying 23.5-m.y.-old volcanic field now located in the northern Gabilan Range and the Pinnacles area, respectively, were the primary source terranes. In general, the fan deltas crop out along the west side of the southern Temblor Range, whereas the proximal parts of the submarine fans crop out along the east side of the range. The fan deltas consist of subaerial topset beds and low-angle basinward-dipping subaqueous foreset beds. Strata interpreted to be topset beds are composed largely of conglomerate with thick to very thick horizontal beds and matrix-supported clasts. Most of the thick to very thick conglomerate beds are internally massive and disorganized. Strata interpreted as foreset beds are composed of thick-bedded, large-scale, low-angle, cross-stratified conglomerate and sandstone units which commonly are internally massive. Abundant molluskan macrofossils such as Ostrea and Pecten are present in the subaqueous foreset beds; many have been displaced downslope from their original site of deposition. Conglomerate- and sandstone-filled submarine canyons, through which coarse-grained detritus was transported to the adjacent submarine fans, locally have cut into the foreset beds of the fan deltas. These submarine canyon deposits are generally better stratified than adjacent foreset-bed deposits, and they consist of thick horizontal beds, internally massive or normally graded, arranged in fining- and thinning-upward sequences. Isolated and composite conglomerate- and sandstone-filled channels, which crop out on the east flank of the southern Temblor Range, are interpreted as proximal submarine-fan channel deposits. These channel-form conglomerate and sandstone deposits are characterized by thick, horizontal beds which are internally massive or normally graded containing division T a , and locally T b , of the Bouma sequence. Sparse calcareous foraminifers collected from diatomaceous interbeds suggest that these fan channels were deposited in upper bathyal water depths. Subaerial and regenerated subaqueous debris flows probably formed the bulk of the Santa Margarita fan delta and submarine fan system. Santa Margarita debris flows ranged from the mudflow variety to the cataclysmic debris-avalanche variety. The cogenetic Republic and Williams sandstones of local usage, located on the east side of the southern Temblor Range, are slightly older and finer grained than the Santa Margarita Formation. These units, containing well-graded sandstones, fining- and thinning-upward and coarsening- and thickening-upward sandstone sequences, thick-bedded tabular and channel-shaped sandstones, a mixture of shallow- and deep-water foraminifers, and a fan-shaped geometry in the subsurface, are interpreted as submarine fan deposits. Sedimentation associated with the Santa Margarita Formation was intimately related to the growing southern Temblor Range anticlinorium and the right-laterally shifting Salinian block along the San Andreas fault. Examples of control exerted on Santa Margarita sedimentation by the southern Temblor Range anticlinorium include the preferential accumulation of sediments along the flanks of the anticlinorium, thickening of strata on the downthrown side of the Recruit Pass fault and on flanks of selected anticlines, intraformational unconformities, and possible partial blockage of the eastward-prograding fan deltas by the Recruit Pass fault. East of the growing southern Temblor Range anticlinorium, the distal ends of the Santa Margarita submarine fans were deflected northwestward by the growing Buena Vista Hills anticline. Several examples of well-defined diachronous sedimentation, where conglomerates and sandstones of the Santa Margarita Formation occupy progressively higher stratigraphic levels in a northwest direction subparallel to the trace of the San Andreas fault, strongly imply that the Salinian basement terrane was shifting in a right-lateral sense during Santa Margarita sedimentation. Regional factors of importance in focusing conglomerate sedimentation on the southern Temblor Range locale for a 2- to 3-m.y. period in the late Mohnian were right-lateral oblique slip on the San Andreas fault, formation of the "big bend" in the San Andreas fault by left-lateral slip along the Garlock and White Wolf faults, and the partial overlap of the Salinian and Franciscan assemblage basement rocks.
Geology and tectonic history of the lower Cape Fear River valley, southeastern North Carolina
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Distribution, facies, ages, and proposed tectonic associations of regionally metamorphosed rocks in northern Alaska
Approximately half of the exposed bedrock in northern Alaska has been regionally metamorphosed. The most widespread metamorphic episode that affected northern Alaska occurred under low-grade, initially high-pressure (blueschist-facies) conditions during Mesozoic time. This episode is thought to have been related to the obduction of one or more oceanic terranes onto the continental margin of North America. Rocks whose metamorphism is considered to have been part of this major episode have an aerial distribution of approximately 10,000 km 2 in the southern Brooks Range, 5,000 km 2 across much of the Seward Peninsula, and 800 km 2 in the Ruby geanticline within the southeastern borderlands of the Yukon-Koyukuk basin. In the southern Brooks Range and on the Seward Peninsula, continental rocks experienced a clockwise pressure-temperature path that evolved during Middle Jurassic to late Early Cretaceous time from the low- to high-temperature subfacies of the blueschist facies and, finally, due to decreasing pressure, evolved to the greenschist facies. Metamorphism in the southern Brooks Range was associated with north-vergent compression along a south-dipping subduction zone that emplaced the oceanic rocks of the Angayucham terrane (represented by klippen of ultramafic rocks and prehnite-pumpellyite-facies metabasite, metatuff, metachert, and metasedimentary rocks) onto the continental margin. The present structural and metamorphic relation between the continental blueschistand greenschist-facies rocks and the structurally overlying lower temperature and pressure oceanic prehnite-pumpellyite-facies rocks to the south indicates that postmetamorphic or late metamorphic down-to-the-south, low-angle extensional faulting has dismembered the upper plate and removed much of the section that originally buried the blueschists. High-pressure metamorphism on the Seward Peninsula probably had a similar origin to that in the southern Brooks Range, but remnants of the overriding plate have not been identified, and the mechanism (such as left-lateral strike-slip faulting or oroclinal bending) by which the high-pressure rocks in the two areas were separated is not known. A significant difference between the thermal histories of these two metamorphic belts also is enigmatic: low-grade metamorphism on the Seward Peninsula was followed by intermediate-pressure amphibolite-facies and, locally, granulite-facies metamorphism as well as plutonism in mid-Cretaceous time, but no such high-temperature events have been documented within the high-pressure belt of the southern Brooks Range. In the Ruby geanticline, glaucophane, attesting to high-pressure metamorphism, is sporadically developed both within the continental rocks of the lower plate and, less commonly, near the base of the overlying oceanic thrust sheets. The direction from which the oceanic rocks were thrust and determination of which oceanic sheets were involved is unclear. Although the majority of the metamorphic episodes that affected northern Alaska occurred during the Mesozoic, older episodes have been documented or are suspected in a few areas. Late Proterozoic medium-grade metamorphism has been documented in a small area in the southwestern Brooks Range, Middle to Late Proterozoic or early Paleozoic metamorphism just south of the Ruby geanticline, and middle Paleozoic metamorphism in the eastern Brooks Range. High-temperature, high-pressure metamorphism of klippen in the Yukon-Tanana upland is tentatively considered to have occurred in early Paleozoic time. Metamorphic ages of units in several areas, particularly in the Ruby geanticline and the Yukon-Tanana upland, can be bracketed only between the probable Paleozoic age of their protoliths and the late Early Cretaceous age of postmetamorphic granitoids that intrude them.
Distribution, facies, ages, and proposed tectonic associations of regionally metamorphosed rocks in Southwestern Alaska and the Alaska Peninsula
The oldest dated metamorphic sequence in Alaska, the fault-bounded Kilbuck Terrane, consists of continental rocks that were metamorphosed under amphibolite-facies conditions during early Proterozoic (1.77 Ga) time. Proterozoic or early Paleozoic metamorphic ages are also possible for greenschist- and amphibolite-facies continental rocks in interior Alaska (Ruby and Nixon Fork terranes). Medium-grade metamorphism on the Alaska Peninsula accompanied intrusion of a Jurassic arc. North of Bristol Bay, low-grade, locally high-pressure Mesozoic metamorphism is attributed to the progressive underthrusting of a subduction complex beneath an oceanic arc followed by underthrusting of the Kilbuck Terrane beneath the subduction complex.
Structural geology and tectonics of the Orville Coast region, southern Antarctic Peninsula, Antarctica
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Geology of the Libby thrust belt of northwestern Montana and its implications to regional tectonics
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The Loma Prieta, California, earthquake of October 17, 1989: Tectonic processes and models
If there is a single theme that unifies the diverse papers in this chapter, it is the attempt to understand the role of the Loma Prieta earthquake in the context of the earthquake 'machine' in northern California: as the latest event in a long history of shocks in the San Francisco Bay region, as an incremental contributor to the regional deformation pattern, and as a possible harbinger of future large earthquakes. One of the surprises generated by the earthquake was the rather large amount of uplift that occurred as a result of the reverse component of slip on the southwest-dipping fault plane. Preearthquake conventional wisdom had been that large earthquakes in the region would probably be caused by horizontal, right-lateral, strike-slip motion on vertical fault planes. In retrospect, the high topography of the Santa Cruz Mountains and the elevated marine terraces along the coast should have provided some clues. With the observed ocean retreat and the obvious uplift of the coast near Santa Cruz that accompanied the earthquake, Mother Nature was finally caught in the act. Several investigators quickly saw the connection between the earthquake uplift and the long-term evolution of the Santa Cruz Mountains and realized that important insights were to be gained by attempting to quantify the process of crustal deformation in terms of Loma Prieta-type increments of northward transport and fault-normal shortening.
An outline of tectonic, igneous, and metamorphic events in the Goshute-Toano Range between Silver Zone Pass and White Horse Pass, Elko County, Nevada; a history of superposed contractional and extensional deformation
Seven kinds of fault-bounded tracts are described. One of the tracts provides a good example of Mesozoic contractional folding and faulting; six exemplify various aspects of Miocene extensional faulting. Massive landslide deposits resulting from Tertiary faulting are described. Mesozoic intrusive rocks and extensive exposures of Miocene volcanic rocks are described and dated. The age ranges of stratigraphic units were based on numerous conodont collections, and ages of igneous rocks were determined by argon/argon and fission-track methods. The geologic complexity of the Goshute-Toano Range provides opportunities for many additional productive structural studies.
Geology, age, and tectonic setting of the Cretaceous Sliderock Mountain Volcano, Montana
The Sliderock Mountain stratovolcano, part of the Upper Cretaceous continental magmatic arc in southwestern Montana, consists of volcaniclastic strata and basaltic andesite lava flows. An intrusive complex represents the volcano's solidified magma chamber. Compositional diversity within components of the volcano appears to reflect evolution via about 50 percent fractional crystallization involving clinopyroxene and plagioclase. 40Ar/39Ar indicate that the volcano was active about 78?1 Ma.
A geologic guide to Wrangell-Saint Elias National Park and Preserve, Alaska: A tectonic collage of northbound terranes
Wrangell-Saint Elias National Park and Preserve, the largest unit in the U.S. National Park System, encompasses near 13.2 million acres of geological wonderments. This geologic guide presents history of exploration and Earth-science investigation; describes the complex geologic makeup; characterizes the vast college of accretion geologic terranes in this area of Alaska's continental margin; recapitulates the effects of earthquakes, volcanoes, and glaciers; characterizes the copper and gold resources of the parklands; and describes outstanding locales within the park and preserve area. A glossary of geologic terms and a categorized list of additional sources of information complete this report.
Distal volcano-tectonic seismicity near Augustine Volcano
Clustered earthquakes located 25 km northeast of Augustine Volcano occurred more frequently beginning about 8 months before the volcano’s explosive eruption in 2006. This increase in distal seismicity was contemporaneous with an increase in seismicity directly below the volcano’s vent. Furthermore, the distal seismicity intensified penecontemporaneously with signals in geodetic data that appear to reveal a transition from magmatic inflation of the volcano to dike injection. Focal mechanisms for five events within the distal cluster show strike-slip-fault movement. Directly above the earthquake cluster, shallow (<5 km deep) folds and faults mapped using multichannel seismic-reflection data strike northeast, parallel to the regional structural grain. About 10 km northeast of Augustine Volcano, however, the Augustine-Seldovia arch, an important trans-basin feature, strikes west and intersects the northeast-striking structural zone. We propose that the fault causing the distal earthquake cluster strikes northwest, subparallel to the arch, and is a right-lateral strike-slip fault. Future earthquake monitoring might show whether increasing activity in the remote cluster can aid in making eruption forecasts.