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At least 757 records · Page 42Linked to original sources
USGS1998

Upper crustal structure and Neogene tectonic development of the California continental borderland

Multichannel seismic-reflection data, sonobuoy seismic-refraction data, and regional geology are used to define the upper crustal structure of the southern California continental borderland and to delineate the characteristics of the main lithotectonic belts of the region. The Catalina Schist belt is separated on its west side from the gently deformed Nicolas forearc belt by faults that have steep west dips and pronounced normal separations. On its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the Peninsular Ranges belt at the coastline near Oceanside. The Catalina Schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early Miocene time in conjunction with about 10° of clockwise rotation of the western Transverse Ranges belt. Part of the uplift of the Catalina Schist could have occurred on the detachment fault, but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt. A large amount of uplift is required, and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western Transverse Ranges and Nicolas forearc belts. Extension, accompanied by probable large amounts of right slip, continued in the borderland region during and after middle Miocene time. The later stage of extension was accompanied by rapid clockwise rotation of the western Transverse Ranges of at least 90°. Most of the borderland, including the belt of schist that was uplifted in early Miocene time, was further deformed into numerous basins and ridges during this stage of oblique extension. The primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the Pacific plate after it had become coupled to the Farallon plate system, which had previously been subducted beneath the borderland.

Geological Society of America Bulletin↗
USGS1998

Tsivat Basin conduit system persists through two surges, Bering Piedmont Glacier, Alaska

The 1993–1995 surge of Bering Glacier, Alaska, occurred in two distinct phases. Phase 1 of the surge began on the eastern sector in July, 1993 and ended in July, 1994 after a powerful outburst of subglacial meltwater into Tsivat Lake basin on the north side of Weeping Peat Island. Within days, jökulhlaup discharge built a 1.5 km 2 delta of ice blocks (25–30 m) buried in outwash. By late October 1994, discharge temporarily shifted to a vent on Weeping Peat Island, where a second smaller outburst dissected the island and built two new sandar. During phase 2, which began in spring 1995 and ended within five months, continuous discharge issued from several vents along the ice front on Weeping Peat Island before returning to the Tsivat Basin.

Alaska↗
USGS1998

Pleistocene to Holocene contrasts in organic matter production and preservation on the California continental margin

Organic matter in sediments from cores collected from the upper continental slope (200–2700 m) off California and southern Oregon shows marked differences in concentration and marine character between the last glacial interval (ca. 24–10 ka) and either Holocene time or last interstadial (oxygen isotope stage 3, ca. 60–24 ka). In general, sediments deposited during Holocene time and stage 3 contain higher amounts of marine organic matter than those deposited during the last glacial interval, and this contrast is greatest in cores collected off southern California. The most profound differences in stage 3 sediments are between predominantly bioturbated sediments and occasional interbeds of laminated sediments. The sediments are from cores collected within the present oxygen minimum zone on the upper continental slope from as far north as the Oregon-California border to as far south as Point Conception. These upper Pleistocene laminated sediments contain more abundant hydrogen-rich (type II) marine algal organic matter than even surface sediments that have large amounts of nonrefractory organic matter. The stable carbon-isotopic composition of the organic matter does not change with time between bioturbated and laminated sediments, indicating that the greater abundance of type II organic matter in the laminated sediments is not due to a change in source but rather represents a greater degree of production and preservation of marine organic matter. The presence of abundant, well-preserved organic matter supports the theory that the oxygen minimum zone in the northeastern Pacific Ocean was more intense, and possibly anoxic, during late Pleistocene time as a result of increased coastal upwelling that enhanced algal productivity.

California↗
USGS1998

Implications of seismic reflection and potential field geophysical data on the structural framework of the Yucca Mountain-Crater Flat region, Nevada

Seismic reflection and gravity profiles collected across Yucca Mountain, Nevada, together with geologic data, provide evidence against proposed active detachment faults at shallow depth along the pre-Tertiary–Tertiary contact beneath this potential repository for high-level nuclear waste. The new geophysical data show that the inferred pre-Tertiary–Tertiary contact is offset by moderate- to high-angle faults beneath Crater Flat and Yucca Mountain, and thus this shallow surface cannot represent an active detachment surface. Deeper, low-angle detachment surface(s) within Proterozoic-Paleozoic bedrock cannot be ruled out by our geophysical data, but are inconsistent with other geologic and geophysical observations in this vicinity. Beneath Crater Flat, the base of the seismogenic crust at 12 km depth is close to the top of the reflective (ductile) lower crust at 14 to 15 km depth, where brittle fault motions in the upper crust may be converted to pure shear in the ductile lower crust. Thus, our preferred interpretation of these geophysical data is that moderate- to high-angle faults extend to 12–15-km depth beneath Yucca Mountain and Crater Flat, with only modest changes in dip.

Nevada↗
USGS1998

Lithological and hydrological influences on ground-water composition in a heterogeneous carbonate-clay aquifer system

The influence of clay units on ground-water composition was investigated in a heterogeneous carbonate aquifer system of Miocene age in southwest Florida, known as the Intermediate aquifer system. Regionally, the ground water is recharged inland, flows laterally and to greater depths in the aquifer systems, and is discharged vertically upward at the saltwater interface along the coast. A depth profile of water composition was obtained by sampling ground water from discrete intervals within the permeable carbonate units during coring and by squeezing pore water from a core of the less-permeable clay layers. A normative salt analysis of solute compositions in the water indicated a marine origin for both types of water and an evolutionary pathway for the clay water that involves clay diagenesis. The chemical composition of the ground water in the carbonate bedrock is significantly different from that of the pore water in the clay layers. Dissolution of clays and opaline silica results in high silica concentrations relative to water in other parts of the Intermediate aquifer system. Water enriched in chloride relative to the overlying and underlying ground water recharges the aquifer inland where the confining clay layer is absent, and it dissolves carbonate and silicate minerals and reacts with clays along its flow path, eventually reaching this coastal site and resulting in the high chloride and silica concentrations observed in the middle part of the Intermediate aquifer system. Reaction-path modeling suggests that the recharging surficial water mixes with sulfate-rich water upwelling from the Upper Floridan aquifer, and carbonate mineral dissolution and precipitation, weathering and exchange reactions, clay mineral diagenesis, clay and silica dissolution, organic carbon oxidation, and iron and sulfate reduction result in the observed water compositions.

Geological Society of America Bulletin↗
USGS1998

Tectonic controls on magmatism in the Geysers-Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

Geological Society of America Bulletin↗
USGS1999

Recrystallization and anatexis along the plutonic-volcanic contact of the Turkey Creek caldera, Arizona

Unusual geologic and geochemical relations are preserved along the contact between intracaldera tuff and a resurgent intrusion within the 26.9 Ma Turkey Creek caldera of southeast Arizona. Thick intracaldera tuff is weakly argillically altered throughout, except in zones within several hundred meters of its contact with the resurgent intrusion, where the groundmass of the tuff has been variably converted to granophyre and unaltered sanidine phenocrysts are present. Dikes of similarly granophyric material originate at the tuff-resurgent intrusion contact and intrude overlying intracaldera megabreccia and tuff. Field relations indicate that the resurgent intrusion is a laccolith and that it caused local partial melting of adjacent intracaldera tuff. Geochemical and petrographic relations indicate that small volumes of partially melted intracaldera tuff assimilated and mixed with dacite of the resurgent intrusion along their contact, resulting in rocks that have petrographic and compositional characteristics transitional between those of tuff and dacite. Some of this variably contaminated, second-generation magma coalesced, was mobilized, and was intruded into overlying intracaldera rocks.

Arizona↗
USGS1999

Ground-water, large-lake interactions in Saginaw Bay, Lake Huron: A geochemical and isotopic approach

Delineating the nature and extent of ground-water inputs is necessary to understand the hydrochemistry of large lakes. Characterizing the interaction between ground water and large lakes (e.g., the Great Lakes) is facilitated by the use of geochemical and isotopic data. In this study, pore waters were extracted from sediment cores collected from Saginaw Bay and the surrounding Saginaw lowland area; the geochemistry and stable isotope signature of these pore waters were used to identify sources for the water and solutes. Cores from Saginaw Bay and the Saginaw lowland area yielded strong vertical gradients in chloride concentrations, suggesting that a high-chloride source is present at depth. The spatial distribution of cores with elevated chloride concentrations corresponds to the regional distribution of chloride in ground water. Most of the Saginaw lowland area cores contain water with significantly lower δ 18 O values than modern meteoric water, suggesting that the water had been recharged during a much cooler climate. The δ 18 O values measured in pore waters (from Saginaw Bay cores) containing high chloride concentrations are similar to modern meteoric water; however, values lighter than modern meteoric water are encountered at depth. Chloride:bromide ratios, used to distinguish between different chloride sources, identify formation brine as the likely source for chloride. Transport models indicate that a combination of advection and diffusion is responsible for the observed Saginaw lowland area pore-water profiles. Pore-water profiles in Saginaw Bay sediments are produced primarily by diffusion and require significantly less time to evolve. An upward flux of solutes derived from formation brine could occur elsewhere within the Great Lakes region and significantly affect the geochemical cycling of chloride and other contaminants (e.g., trace metals).

Michigan↗
USGS1999

Paleofluid-flow circulation within a Triassic rift basin: Evidence from oil inclusions and thermal histories

The migration of subsurface fluid flow within continental rift basins has been increasingly recognized to significantly affect the thermal history of sediments and petroleum formation. To gain insight into these paleofluid flow effects, the thermal history of the Taylorsville basin in Virginia was reconstructed from fluid-inclusion studies, apatite fission-track data, and vitrinite reflectance data. Models of thermal history indicate that the basin was buried to the thermal maximum at 200 Ma; a cooling event followed during which the eastern side of the basin cooled earlier and faster than the western side, suggesting that there was a differential uplift and topographically driven fluid flow. This hypothesis is supported by analyses of secondary oil and aqueous inclusions trapped in calcite and quartz veins during the uplift stage. Gas chromatograms of inclusion oils exhibit variable but extensive depletion of light molecular-weight hydrocarbons. The relative abundance of n-alkanes, petrographic observations, and the geological data indicate that the alteration process on these inclusion oils was probably neither phase separation nor biodegradation, but water washing. Water:oil ratios necessary to produce the observed alteration are much greater than 10000:1. These exceedingly high ratios are consistent with the migration of inclusion oils along with fluid flow during the early stages of basin evolution. The results provide significant evidence about the role of a subsurface flow system in modifying the temperature structure of the basin and the composition of petroleum generated within the basin.

Geological Society of America Bulletin↗
USGS1999

Importance of mechanical disaggregation in chemical weathering in a cold alpine environment, San Juan Mountains, Colorado

Weathering of welded tuff near the summit of Snowshoe Mountain (3660 m) in southwestern Colorado was studied by analyzing infiltrating waters in the soil and associated solid phases. Infiltrating waters exhibit anomalously high potassium to silica ratios resulting from dissolution of a potassium-rich glass that occurs as a trace phase in the rock. In laboratory experiments using rock from the field site, initial dissolution generated potassium-rich solutions similar to those observed in the field. The anomalous potassium release decreased over time (about 1 month), after which the dominant cation was calcium, with a much lower potassium to silica ratio.

Colorado↗
USGS1999

Northern San Andreas fault near Shelter Cove, California

The location of the San Andreas fault in the Shelter Cove area of northern California has been the subject of long-standing debate within the geological community. Although surface ruptures were reported near Shelter Cove in 1906, several subsequent workers questioned whether these ruptures represented true fault slip or shaking-related, gravity-driven deformation. This study, involving geologic and geomorphic mapping, historical research, and excavation across the 1906 rupture zone, concludes that the surface ruptures reported in 1906 were the result of strike-slip faulting, and that a significant Quaternary fault is located onshore near Shelter Cove. Geomorphic arguments suggest that the Holocene slip rate of this fault is greater than about 14 mm/yr, indicating that it plays an important role within the modern plate-boundary system. The onshore trace of the fault zone is well expressed as far north as Telegraph Hill; north of Telegraph Hill, its location is less well-constrained, but we propose that a splay of the fault may continue onshore northward for at least 9 km to the vicinity of Saddle Mountain.

California↗
USGS1999

Structural and petrologic evolution of the Lihue basin and eastern Kauai, Hawaii

The topography of the eastern part of the Hawaiian island of Kauai is dominated by the Lihue basin, a large (∼110 km 2 ) semicircular depression bounded by steep cliffs and partly filled by late rejuvenated-stage (or posterosional stage) volcanic material. As with other large, semicircular basins on ocean-island volcanoes, the subsurface geology and origin (e.g., structural collapse vs. fluvial erosion) of the Lihue basin are poorly understood. New analyses of samples collected from eastern Kauai and drill holes within the basin document several important features of the late-stage geologic evolution of Kauai. First, thick (>300 m) sequences of rejuvenated-stage Koloa Volcanics in the Lihue basin show systematic, basin-wide geochemical trends of increasingly incompatible elements with time, indicating a gradual decrease in the extent of partial melting of mantle sources with time. Second, beneath the rejuvenated-stage volcanics in the basin, a thin layer of postshield alkalic stage lavas (e.g., hawaiites and mugearites) overlies older shield-stage tholeiitic lavas of the Napali Member, indicating that the Lihue basin formed by structural collapse, not fluvial erosion. Third, a large (∼2–5 km 3 ) matrix-supported breccia, interpreted as deposits of one or more debris flows, is within the rejuvenated-stage volcanics throughout the basin, and correlates with surficial exposures of the Palikea Breccia west of the basin. Isotopic compositions of the bulk breccia are similar to those of tholeiites from the east side of Kauai, and distinct from those of west Kauai tholeiites. Clasts within the breccia are dominantly hawaiite and alkali gabbro. The source region of the breccia in the steep cliffs and highlands of the central massif to the west of the basin must contain magmatic products of an extensive postshield alkalic stage, including hawaiite flows and one or more large intrusive bodies or ponded sequences of alkali gabbro.

Hawaii↗
USGS1999

Late Pleistocene environments of the western Noatak basin, northwestern Alaska

Glacial Lake Noatak formed repeatedly during middle and late Pleistocene time as expanding glaciers from the DeLong Mountains blocked the Noatak River valley. Downcutting by the Noatak River has exposed thick sediment successions in bluffs up to 86 m high. Two river bluffs, Nk-26 and Nk-29A, contain correlative organic-rich flood-plain deposits that were formed during and after deposition of the Old Crow tephra at about the transition between oxygen isotope stage 6 and oxygen isotope stage 5, at the beginning of the last interglaciation. Both bluffs also contain older interglacial or interstadial flood-plain deposits of uncertain age.

Alaska↗
USGS1999

Pedogenic calcite as evidence for an early Holocene dry period in the San Francisco Bay area, California

Rainfall at the site of Union City, California, during early Holocene time appears to have been about half that of today, 470 mm/yr. We base this conclusion on detailed descriptions and particle-size analyses of 12 soil profiles and 1:20 scale logs of the fluvial stratigraphy in two 100-m-long, 5-m-deep excavations dug perpendicular to the axis of an alluvial fan along the Hayward fault. Subsidence and right-lateral movement along the fault allowed an offset stream to produce a nearly continuous alluvial record documented by 35 14 C ages on detrital charcoal. Bk (calcitic) horizons in paleosols developed in the fan suggest that a relatively dry climatic period occurred from 10 to 7 ka (calendar-corrected ages). The pedogenic calcite exists primarily as vertically oriented filaments and fine, cavernous nodules formed at ped intersections. Soils and paleosols formed before 10 ka or since 7 ka did not have Bk horizons. Bk horizons that were buried suddenly at 7 ka were overlain by leached zones averaging 41 ± 3 cm thick—about half the current depth of leaching.

California↗
USGS1999

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an “acoustic window” into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse.

Geological Society of America Bulletin↗
USGS1999

Active tectonics of the Seattle fault and central Puget sound, Washington - Implications for earthquake hazards

We use an extensive network of marine high-resolution and conventional industry seismic-reflection data to constrain the location, shallow structure, and displacement rates of the Seattle fault zone and crosscutting high-angle faults in the Puget Lowland of western Washington. Analysis of seismic profiles extending 50 km across the Puget Lowland from Lake Washington to Hood Canal indicates that the west-trending Seattle fault comprises a broad (4–6 km) zone of three or more south-dipping reverse faults. Quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault A, the northernmost fault, which forms the boundary between the Seattle uplift and Seattle basin. Analysis of growth strata deposited across fault A indicate minimum Quaternary slip rates of about 0.6 mm/yr. Slip rates across the entire zone are estimated to be 0.7–1.1 mm/yr.

Washington↗
USGS1999

Glacial Lake Lind, Wisconsin and Minnesota

Glacial Lake Lind developed in the pre–late Wisconsinan St. Croix River valley, Minnesota and Wisconsin, and lasted more than 1000 yr during the retreat of the Superior lobe at the end of the Wisconsinan glaciation. Lake Lind sediment consists primarily of red varved silt and clay, but also includes mud-flow deposits, nearshore silt (penecontemporaneously deformed in places), nearshore rippled sand, and deltaic sand.

Minnesota, Wisconsin↗
USGS2000

Late Holocene Peléan-style eruption at Tacaná volcano, Mexico and Guatemala: past, present, and future hazards

Tacaná volcano, located on the border between Mexico and Guatemala, marks the northern extent of the Central American volcanic chain. Composed of three volcanic structures, it is a volcanic complex that has had periodic explosive eruptions for at least the past 40 k.y. The most recent major eruption occurred at the San Antonio volcano, the youngest volcanic edifice forming the complex, about 1950 yr ago. The Peléan style eruption, issued from the southwest part of the dome, and swept a 30° sector with a hot block and ash flow that traveled about 14 km along the Cahoacán ravine. Deposits from this event are well exposed around the town of Mixcun and were therefore given the name of that town, the Mixcun flow deposit. The Mixcun flow deposit is, in the channel facies, a light gray, massive, thick (>10 m), matrix-supported unit with dispersed lithic clasts of gravel to boulder size, divisible in some sections into a variable number of flow units. The overbank facies is represented by a thin (<1 m), massive, matrix-supported unit. In both of these facies the deposit has disseminated charcoal, fumarolic pipes, and juvenile lithics with cooling joints. The Mixcun flow deposit contains clasts of (1) light gray, dense andesite, (2) dark gray, glassy and banded andesite, and (3) minor altered red andesite from the edifice, set in a matrix of sand and silt. The Mixcun flow deposit covers an area of at least 25 km 2 and has a minimum estimated volume of 0.12 km 3 . Basaltic-andesite inclusions (54% SiO 2 ) and various signs of disequilibrium in the mineral assemblage of the two-pyroxene andesitic products (60%–63% SiO 2 ) suggest that magma mixing may have triggered the eruption. Following deposition of the Mixcun flow deposit andesitic to dacitic (62%–64% SiO 2 ) lava flows were extruded and a dacitic dome (64.4% SiO 2 ) at the San Antonio summit formed. Syn-eruptive and posteruptive lahars flooded the main drainages of the Cahoacán and Izapa-Mixcun valleys in the area of the present city of Tapachula (population 250000) and the pre-Hispanic center of Izapa. Three radiocarbon ages date this event between A.D. 25 and 72 (range ±1σ, 38 B.C.–A.D. 216), which correlates with a halt in construction at Izapa (Hato phase of ca. 50 B.C.–A.D. 100), probably due to temporary abandonment of the city caused by lahars. Another similar event would produce extensive damage to the towns (population of about 68,000 people) now built upon the Mixcun flow deposit. The main summit of Tacaná volcano continues to show signs of fumarolic activity; the most recent period of activity in 1985–1986 culminated in a minor phreatic explosion.

Tapachula↗