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1,183 records · Page 66Linked to original sources
USGS1991

Late Quaternary stratigraphy and depositional history of the Long Island Sound basin

The stratigraphy of Late Quaternary geologic units beneath Long Island Sound (LIS) is interpreted from 3,500 km of high-resolution, seismic-reflection profiles supplemented by vibracore data. Knowledge gained from onshore regional geologic studies and previous offshore investigations is also incorporated in these interpretations. Glacial deposits overlie and nearly fill an Inner Lowland which is floored by crystalline rocks in the north and bounded to the south by an irregular, north-facing cuesta scarp in Cretaceous Coastal-Plain strata. The thick glacial section consists largely of sediment that was deposited in glacial Lake Connecticut during the retreat of the last (late Wisconsin) ice sheet. Ice-marginal lacustrine fan deposits and submerged extensions of the southeastern Connecticut recessional moraines mark positions of the ice margin during systematic northward retreat. When the ice margin stood at positions just north of the present shoreline of Connecticut (17.6-16.5 ka), glacial Lake Connecticut was slightly larger than Long Island Sound is today. Ice-marginal deltas and near-ice marginal fluviodeltaic deposits occur along coastal Connecticut. Delta-building was diachronous; progressively younger deltas record northwesterly retreat of the ice margin and lowering lake levels. Distally from the deltas, varied lake-clays (up to 150 m thick) drape the underlying topography. The lake spillway cut across the Harbor Hill-Roanoke Point-Fishers Island-Charlestown end moraine at its lowest point just west of Fishers Island (The Race). Lake levels fell as the spillway notch was erosionally deepened, and gradually the lakebed became subaerially exposed. A largely preserved channel system cuts the lake-deposit surface; fluvial channel-fill sediments are overlain by estuarine sediments in the channel system. Perhaps as early as 15 ka, but no later than 13 ka, the sea entered the Basin through the channel at The Race and quickly transgressed westward via the lakebed channel system. As the sea rose further and spread throughout the Basin, a wave-cut surface was formed. Extensive marine deltaic deposits, up to 40 m thick, overlie the wave-cut marine unconformity in the east-central part of the Basin. These sediments were derived from erosion of the drained bed of Lake Hitchcock, in the Connecticut valley to the north, and were deposited in a -40-m relative sea at about 12.4 ka. Where quiet waters prevail, marine mud generally less than 15 m thick blankets the older deposits of the Basin. Elsewhere, especially in eastern LIS, tidal currents are actively reworking and transporting glacial and postglacial deposits.

Connecticut, New York↗
USGS2014

The Lepanto Cu–Au deposit, Philippines: A fossil hyperacidic volcanic lake complex

Hyperacidic lakes and associated solfatara in active volcanoes are the expression of magmatic gas expansion from source to surface. Here we show for the first time, that the vein system that comprises the ~ 2 Ma high-sulfidation, Lepanto copper–gold deposit in the Mankayan district (Philippines) was associated with a contemporary hyperacidic volcanic lake complex—possibly the first such lake recognized in the geological record. A 15–20‰ difference in sulfur isotopic composition between barite and sulfides and sulfosalts in the vent fumarole encrustations supports the interpretation that SO 2 -rich volcanic gas vented into the base of the lake and marginal to it and ties the mineralization directly to magmatic gas expansion, fracture propagation, and mineralization that occurred through a series of decompression steps within the feeder fracture network. These data confirm that crater lake environments such as Kawah Ijen (Java, Indonesia) provide modern day analogs of the Lepanto and other high sulfidation Cu–Au depositing environments. We also provide extensive analysis of sulfosalt–sulfide reactions during vein formation within the hyperacidic lake complex. Pyrite ± silica deposited first at high temperature followed by enargite that preserves the vapor–solid diffusion of, for example, antimony, tin, and tellurium into the vapor from the crystallizing solid. Subsolidus, intra-crystalline diffusion continued as temperature declined. Pyrite and enargite are replaced by Fe-tennantite in the lodes which initially has low Sb/(Sb + As) atomic ratios around 13.5% close to the ideal tennantite formula, but evolves to higher ratios as crystallization proceeds. Fumarole encrustation clasts and sulfosalts in the lake sediment are more highly evolved with a larger range of trace element substitutions, including antimony. Substitution of especially Zn, Te, Ag, and Sn into tennantite records metal and semi-metal fractionation between the expanding magmatic gas and deposited sulfide sublimates provides a rare insight into the fate of metals and semi-metals in the shallower parts of fracture arrays that feed modern hyperacidic lakes. These data support a growing understanding of the formation of high-sulfidation gold deposits as the consequence of single-phase expansion of gas from magmatic-gas reservoirs beneath the surface of active volcanoes without the intervention of a later aqueous fluid including groundwater. Aggressive sulfide–sulfosalt reactions, including pitting and the almost complete dissolution of earlier minerals, are persistent characteristics of the vein assemblages and precious metals typically occur late in pits or along brittle fractures. These characteristics support a hypothesis of mineral deposition at temperatures of the order of 600 °C in contrast to available fluid inclusion data from enargite that record temperatures following phase transitions in the sulfosalt during the retrograde devolution of the deposit in the presence of groundwater.

Benguet↗
USGS2011

Eruptive history of South Sister, Oregon Cascades

South Sister is southernmost and highest of the Three Sisters, three geologically dissimilar stratovolcanoes that together form a spectacular 20km reach along the Cascade crest in Oregon. North Sister is a monotonously mafic edifice as old as middle Pleistocene, Middle Sister a basalt-andesite-dacite cone built between 48 and 14ka, and South Sister is a basalt-free edifice that alternated rhyolitic and intermediate modes from 50ka to 2ka (largely contemporaneous with Middle Sister). Detailed mapping, 330 chemical analyses, and 42 radioisotopic ages show that the oldest exposed South Sister lavas were initially rhyolitic ~50ka. By ~37ka, rhyolitic lava flows and domes (72-74% SiO 2 ) began alternating with radially emplaced dacite (63-68% SiO 2 ) and andesite (59-63% SiO 2 ) lava flows. Construction of a broad cone of silicic andesite-dacite (61-64% SiO 2 ) culminated ~30ka in a dominantly explosive sequence that began with crater-forming andesitic eruptions that left fragmental deposits at least 200m thick. This was followed at ~27ka by growth of a steeply dipping summit cone of agglutinate-dominated andesite (56-60.5% SiO 2 ) and formation of a summit crater ~800m wide. This crater was soon filled and overtopped by a thick dacite lava flow and then by >150m of dacitic pyroclastic ejecta. Small-volume dacite lavas (63-67% SiO 2 ) locally cap the pyroclastic pile. A final sheet of mafic agglutinate (54-56% SiO 2 ) - the most mafic product of South Sister - erupted from and drapes the small (300-m-wide) present-day summit crater, ending a summit-building sequence that lasted until ~22ka. A 20kyr-long-hiatus was broken by rhyolite eruptions that produced (1) the Rock Mesa coulee, tephra, and satellite domelets (73.5% SiO 2 ) and (2) the Devils Chain of ~20 domes and short coulees (72.3-72.8% SiO 2 ) from N-S vent alignments on South Sister's flanks. The compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the South Sister locus throughout its lifetime. South Sister is part of a reach of the Cascades unusually active in the last 50kyr, characterized by high vent density, N-S vent alignments, and numerous eruptive units of true rhyolite (≥ 72% SiO 2 ) that distinguishes it from much of the Quaternary Cascade arc; these are eruptive expressions of the complex confluence of arc and intraplate magmatic-tectonic regimes.

Journal of Volcanology and Geothermal Research↗
USGS2008

Eruptive history and tectonic setting of Medicine Lake Volcano, a large rear-arc volcano in the southern Cascades

Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that < 6% of the ∼ 2000 km 2 of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km 3 , giving an overall effusion rate of ∼ 1.2 km 3 per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H 2 O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.

Journal of Volcanology and Geothermal Research↗
USGS1997

Crustal structure along the west flank of the Cascades, western Washington

Knowledge of the crustal structure of the Washington Cascades and adjacent Puget Lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region. We present a model for crustal velocity structure derived from analysis of seismic refraction/wide-angle reflection data collected in 1991 in western Washington. The 280-km-long north-south transect skirts the west flank of the Cascades as it crosses three tectonic provinces including the Northwest Cascades Thrust System (NWCS), the Puget Lowland, and the volcanic arc of the southern Cascades. Within the NWCS, upper crustal velocities range from 4.2 to 5.7 km s-1 and are consistent with the presence of a diverse suite of Mesozoic and Paleozoic metasediments and metavolcanics. In the upper 2-3 km of the Puget Lowland velocities drop to 1.7-3.5 km s-1 and reflect the occurrence of Oligocene to recent sediments within the basin. In the southern Washington Cascades, upper crustal velocities range from 4.0 to 5.5 km s-1 and are consistent with a large volume of Tertiary sediments and volcanics. A sharp change in velocity gradient at 5-10 km marks the division between the upper and middle crust. From approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from ???6.0 to 7.2 km s-1. These high velocities do not support the presence of marine sedimentary rocks at depths of 10-20 km beneath the Cascades as previously proposed on the basis of magnetotelluric data. Crustal thickness ranges from 42 to 47 km along the profile. The lowermost crust consists of a 2 to 8-km-thick transitional layer with velocities of 7.3-7.4 km s-1. The upper mantle velocity appears to be an unusually low 7.6-7.8 km s-1. When compared to velocity models from other regions, this model most closely resembles those found in active continental arcs. Distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect. In the NWCS and Puget Lowland, most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5-10 km depth and continues to 20-30 km depth. The region of transition between the NWCS and the Puget Lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two. Earthquakes within the Cascades are generally shallower (0-20 km) and are dominated by events associated with the Rainier Seismic Zone.

Washington↗
USGS2017

Fluid-driven uplift at Long Valley Caldera, California: Geologic perspectives

Since persistent seismicity began in the Sierra Nevada adjacent to Long Valley caldera in 1978–1980, intracaldera unrest has been marked by (1) episodes of uplift totaling ~ 83 cm, centered on the middle Pleistocene resurgent dome, and (2) recurrent earthquake swarms along a 12-km-long segment of the caldera's ring-fault zone that is contiguous with both the dome and the Sierran seismogenic domain. Others have attributed the recent unrest to magmatic intrusion(s), but it is argued here that evidence for new magma is lacking and that ongoing uplift and ring-fault-zone seismicity are both promoted by ascent of aqueous fluid released by second boiling of the residue of the enormous Pleistocene rhyolitic reservoir terminally crystallizing at depths ≥ 10 km. For 2 Myr, eruptive vent clusters migrated southwestward from Glass Mountain to Mammoth Mountain. There has been no eruption on the resurgent dome since 500 ka, and since 230 ka volcanism has been restricted to the caldera's west moat and contiguous Sierran terrain, both outside the structural caldera. High-temperature hydrothermal activity in the central caldera waned after ~ 300 ka, cooling the Pleistocene rhyolitic focus to the extent that drilling on the resurgent dome found mid-caldera temperature to be only 100 °C and isothermal at depths of 2–3 km. Beneath most of the resurgent dome, there is little seismicity at any depth, no emission of magmatic CO 2 or other magmatic gases, no elevated 3 He/ 4 He ratios, and only normal to below-normal heat flow. Most of the 75-km-long ring-fault zone is likewise aseismic, excepting only the 12-km segment contiguous with the extracaldera seismogenic domain in the Sierra. Since 1980, the Sierran seismicity has released 3.6 times more cumulative seismic energy than have intracaldera earthquakes. The caldera seismicity is not driven by stresses associated with the adjacent uplift but, instead, by the extracaldera tectonic stressfield. Sierran seismicity activated the directly contiguous south-moat segment of the ring-fault zone, which had originated in the caldera-forming eruption at 767 ka and everywhere else remains sealed. Hypocenter relocation studies of 1000s of earthquakes along the seismic segment have resolved recurrent upward-migrating swarms within networks of cryptic faults, apparently triggered by rapidly ascending pulses of high-pressure low-viscosity aqueous fluid. Entering the brittle crust at depths of 8–10 km, such fluid is just what should be expected from second boiling of the late-stage CO 2 -poor rhyolitic residue. The fluid provides the pressure source above the apex of the crystallizing caldera-wide pluton and then escapes laterally to the newly reactivated southern segment of the ring-fault zone, its only available permeable pathway, where it mediates the ongoing south-moat seismicity.

California↗
USGS2002

High-resolution aeromagnetic mapping of volcanic terrain, Yellowstone National Park

High-resolution aeromagnetic data acquired over Yellowstone National Park (YNP) show contrasting patterns reflecting differences in rock composition, types and degree of alteration, and crustal structures that mirror the variable geology of the Yellowstone Plateau. The older, Eocene, Absaroka Volcanic Supergroup, a series of mostly altered, andesitic volcanic and volcaniclastic rocks partially exposed in mountains on the eastern margin of YNP, produces high-amplitude, positive magnetic anomalies, strongly contrasting with the less magnetic, younger, latest Cenozoic, Yellowstone Plateau Group, primarily a series of fresh and variably altered rhyolitic rocks covering most of YNP. The Yellowstone caldera is the centerpiece of the Yellowstone Plateau; part of its boundary can be identified on the aeromagnetic map as a series of discontinuous, negative magnetic anomalies that reflect faults or zones along which extensive hydrothermal alteration is localized. The large-volume rhyolitic ignimbrite deposits of the 0.63-Ma Lava Creek Tuff and the 2.1-Ma Huckleberry Ridge Tuff, which are prominent lithologies peripheral to the Yellowstone caldera, produce insignificant magnetic signatures. A zone of moderate amplitude positive anomalies coincides with the mapped extent of several post-caldera rhyolitic lavas. Linear magnetic anomalies reflect the rectilinear fault systems characteristic of resurgent domes in the center of the caldera. Peripheral to the caldera, the high-resolution aeromagnetic map clearly delineates flow unit boundaries of pre- and post-caldera basalt flows, which occur stratigraphically below the post-caldera rhyolitic lavas and are not exposed extensively at the surface. All of the hot spring and geyser basins, such as Norris, Upper and Lower Geyser Basins, West Thumb, and Gibbon, are associated with negative magnetic anomalies, reflecting hydrothermal alteration that has destroyed the magnetic susceptibility of minerals in the volcanic rocks. Within Yellowstone Lake, which is mostly within the Yellowstone caldera, aeromagnetic lows also are associated with known hydrothermal activity in the lake. Many of the magnetic lows extend beyond the areas of alteration and hot springs, suggesting a more extensive currently active or fossil hydrothermal system than is currently mapped. Steep magnetic gradients, suggesting faults or fractures, bound the magnetic lows. This implies that fractures localize the hot springs. Magnetic gradient trends reflect the mapped Basin and Range structural trends of north and northwest, as well as northeasterly trends that parallel the regional trend of the Snake River Plain and the track of the Yellowstone hot spot which follow the Precambrian structural grain. These trends are found both at small scales such as in hydrothermal basins and at more regional fault scales, which suggests that the regional stress field and reactivated older structures may exert some control on localization of hydrothermal activity. ?? 2002 Elsevier Science B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS2012

Geology of the Mount St. Helens area: Record of discontinuous volcanic and plutonic activity in the Cascade Arc of southern Washington

The Quaternary edifice of Mount St. Helens volcano was built upon a deeply eroded terrane of gently folded and altered volcanic and plutonic rocks that represent the core of the Tertiary Cascade magmatic arc. These rocks constitute an east dipping homoclinal sequence, several kilometers thick, of subaerially erupted mafic to silicic flows and volcaniclastic strata; K‐Ar ages from this section range from about 28 to 23 Ma (late Oligocene and earliest Miocene), which corresponds to an apparent lull in Cascade volcanism to the north of Mount Rainier. Volcanism was essentially continuous during this period of time, and neither a well‐defined base nor top of the section is exposed within the mapped area. Basalt and basaltic andesite dominate the lower part of the mid‐Tertiary section, whereas andesitic and dacitic rocks comprise most of the upper part. This section was intruded by numerous mafic to silicic dikes, sills, and irregular plutonic bodies, most no more than a few million years younger than their host rocks, and subjected to pervasive burial metamorphism and widespread hydrothermal alteration. Large areas of hornfelsed rock surrounding even relatively small intrusions indicate that the proportion of plutonic rock becomes significantly greater at shallow depth beneath the existing erosion surface. A large granitic pluton intruded the mid‐Tertiary section north of Spirit Lake at about 21 Ma. The Earl porphyry copper deposit occurs within the pluton but appears too young (17 Ma) to be genetically related to it. In contrast to the rather continuous and voluminous Oligocene to early Miocene activity, volcanism since then in the Mount St. Helens area has been localized and Volumetrically minor. Products of three younger eruptive periods have been recognized: a sequence of 15 m.y. old pyroxene andesite flows resting unconformably on mid‐Tertiary strata south of Mount St. Helens, widespread shallow dikes and sills of pyroxene andesite between 10 and 8 m.y. old, and compositionally diverse rocks erupted during the past 3 m.y. The Quaternary lavas are more potassic than the Tertiary lavas and typically contain phenocrysts of hornblende and biotite, which are absent from the older rocks. A number of Tertiary structures define a broad NNE trending zone that may reflect a deep‐seated lithospheric flaw that has controlled the locus of Cascade magmatism in southern Washington for the past 25 m.y. Mount St. Helens lies within this zone at the intersection of the NNW striking St. Helens seismic zone (SHZ) and an ENE trending alignment of Pleistocene silicic plug‐domes. No surface breakage has been detected along the SHZ, which is apparently very young. The linear zone of silicic vents is probably controlled by a fault that has been interpreted from seismic records to occur directly beneath the volcano. This zone parallels the directions of regional maximum horizontal compressive stress and North America/Juan de Fuca plate convergence. Mount St. Helens is an example of a low‐volume tectonically controlled magmatic system in an early stage of development.

Washington↗
USGS2010

Analysis of nonvolcanic tremor on the San Andreas Fault near Parkfield, CA using U.S. Geological Survey Parkfield Seismic Array

Reports by Nadeau and Dolenc (2005) that tremor had been detected near Cholame Valley spawned an effort to use UPSAR (U. S. Geological Survey Parkfield Seismic Array) to study characteristics of tremor. UPSAR was modified to record three channels of velocity at 40–50 sps continuously in January 2005 and ran for about 1 month, during which time we recorded numerous episodes of tremor. One tremor, on 21 January at 0728, was recorded with particularly high signal levels as well as another episode 3 days later. Both events were very emergent, had a frequency content between 2 and 8 Hz, and had numerous high-amplitude, short-duration arrivals within the tremor signal. Here using the first episode as an example, we discuss an analysis procedure, which yields azimuth and apparent velocity of the tremor at UPSAR. We then provide locations for both tremor episodes. The emphasis here is how the tremor episode evolves. Twelve stations were operating at the time of recording. Slowness of arrivals was determined using cross correlation of pairs of stations; the same method used in analyzing the main shock data from 28 September 2004. A feature of this analysis is that 20 s of the time series were used at a time to calculate correlation; the longer windows resulted in more consistent estimates of slowness, but lower peak correlations. These values of correlation (peaks of about 0.25), however, are similar to that obtained for the S wave of a microearthquake. Observed peaks in slowness were traced back to source locations assumed to lie on the San Andreas fault. Our inferred locations for the two tremor events cluster near the locations of previously observed tremor, south of the Cholame Valley. Tremor source depths are in the 14–24 km range, which is below the seismogenic brittle zone, but above the Moho. Estimates of error do not preclude locations below the Moho, however. The tremor signal is very emergent but contains packets that are several times larger than the background tremor signal and lasts about 5 s. These impulsive wavelets are similar to low-frequency earthquakes signals seen in Japan but appear to be broader band rather than just higher in low-frequency energy. They may be more appropriately called high-energy tremor (HET). HET signals at UPSAR correlate well with the record of this event from station GHIB of the HRSN borehole array at Parkfield and HETs typically have a higher cross-correlation coefficient than the rest of the tremor event. The amplitudes of a large HET are consistent with a magnitude of 0.1 when compared with a M2.3 event that had about the same epicenter. Polarizations of the tremor episode at UPSAR are mostly just north of east. Both linearity and azimuth evolve over time suggesting a change in tremor source location over time and linearity is typically higher at the HETs.

California↗
USGS2003

Deformation near the Casa Diablo geothermal well field and related processes Long Valley caldera, Eastern California, 1993-2000

Regional first-order leveling lines, which extend from Lee Vining, CA, to Tom's Place, CA, have been surveyed periodically since 1957 by the U.S. Geological Survey (USGS), the National Geodetic Survey (NGS), and Caltrans. Two of the regional survey lines, or leveling networks, intersect at the Casa Diablo geothermal well field. These leveling networks, referenced to a distant bench mark (C916) near Lee Vining, provide time-series vertical control data of land-surface deformation that began around 1980. These data are also useful for delineating localized subsidence at Casa Diablo related to reservoir pressure and temperature changes owing to geothermal development that began in 1985. A comparison of differences in bench-mark elevations for five time periods between 1983 and 1997 shows the development and expansion of a subsidence bowl at Casa Diablo. The subsidence coincides spatially with the geothermal well field and temporally with the increased production rates and the deepening of injection wells in 1991, which resulted in an increase in the rate of pressure decline. The subsidence, superimposed on a broad area of uplift, totaled about 310 mm by 1997. The USGS established orthogonal tilt arrays in 1983 to better monitor deformation across the caldera. One tilt array (DBR) was established near what would later become the Casa Diablo geothermal well field. This array responded to magmatic intrusions prior to geothermal development, tilting away from the well field. With the start of geothermal fluid extraction in 1985, tilt at the DBR array reversed direction and began tilting into the well field. In 1991, geothermal power production was increased by a factor of four, and reservoir pressures began a period of steep decline. These changes caused a temporary three-fold increase in the tilt rate. The tilt rate became stable in 1993 and was about 40% lower than that measured in 1991-1992, but still greater than the rates measured during 1985-1990. Data from the local leveling networks spanning the well field and the bounding graben were analyzed for several 2-year periods (1993-1995, 1995-1997, and 1997-1999). Annual rates of change across the normal faults bounding the graben have steadily decreased for each 2-year period between 1993 and 1999, reflecting the slowing decline in geothermal reservoir pressure. Horizontal control data from a two-color electronic distance meter (EDM) defined the lateral extent of subsidence at Casa Diablo. The EDM and leveling data elucidate the localized effect of the shallow source of subsidence and the broader effect of the deeper magmatic inflation source. Data from bench marks common to both the vertical and the horizontal control networks were used to assess the effect of subsidence on the EDM base station (CASA). Modeling of geodetic data collected during periods of little or no magmatic inflation indicated that the CASA two-color EDM station is being drawn toward the well field at a rate of 3-5 mm/yr. ?? 2003 Elsevier B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗
USGS2012

The geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Journal of Geophysical Research E: Planets↗
USGS2012

A seismic refraction survey of the Imperial Valley Region, California

The U.S. Geological Survey conducted an extensive seismic refraction survey in the Imperial Valley region of California in 1979. The Imperial Valley is located in the Salton Trough, an active rift between the Pacific and North American plates. Forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0.5–1 km. More than 1300 recording locations were occupied, and more than 3000 usable seismograms were obtained. We analyzed five profiles using a standard ray‐tracing program, constructed a contour map of reduced travel times from our most widely recorded shot point, and modeled an existing gravity profile across the Salton Trough. Results are itemized: (1) All models have in common a sedimentary layer ( V p = 1.8–5.0 km/s), a “transition zone” ( V p = 5.0–5.65 km/s), a basement ( V p = 5.65 km/s in the Imperial Valley, 5.9 km/s on the bordering mesas), and subbasement ( V p = 7.2 km/s). (2) The sedimentary layer ranges in thickness along the axis of the Salton Trough from 3.7 km (Salton Sea) to 4.8 km (U.S.‐Mexican border). On the bordering mesas it is quite variable in thickness. (3) The “transition” zone is about 1 km thick in most places. In the Imperial Valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement. On the bordering mesas, however, there is a discontinuity at the top of this zone. (4) There are apparently two types of basement. On the bordering mesas, basement is crystalline igneous and metamorphic rocks. In the Imperial Valley, basement is mostly lower‐greenshist‐facies sedimentary rocks, based primarily on the smooth transition in character from sediment to basement arrivals, the low value of basement velocity, and the fact that deep (4 km) wells in the valley penetrate only the upper part of the known Cenozoic stratigraphic column for the Salton Trough. (5) The subbasement, or intermediate crustal layer, ranges in depth along the axis of the Salton Trough from 16 km (Salton Sea) to 10 km (U.S.‐Mexican border). Gravity modeling requires that this layer deepen and/or pinch out beneath the bordering mesas and mountain ranges. Based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the Imperial Valley, the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust. (6) Several structures are seen that affect basement, transition zone, and deeper parts of the sedimentary layer. They include a scarp along the Imperial fault, as much as 1 km down to the northeast, and a scarp passing roughly along the topographic boundary between the Imperial Valley and the bordering mesa to the west, as much as 3½ km down to the east. We interpret the latter scarp to be the suture, or rift boundary, between the older crystalline basement on the mesa and the younger metasedimentary basement in the Imperial Valley. (7) On a contour map of reduced travel time from our most widely recorded shot point, subtle patches of early arrivals among otherwise late arrivals in the central Imperial Valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150°C. Apparently the Salton Trough is a location where new crust is being generated. As the rift opens, mafic intrusive rocks fill it from below as sedimentary rocks fill it from above. Rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth (metasedimentary basement in the Imperial Valley) and thus consolidates the new crust.

California↗
USGS2012

Television observations from Surveyor 3

A total of 6315 pictures were taken by the television camera on Surveyor 3 after the lunar landing. These pictures have provided much new information about the location of the landing site on the moon, the detailed topographic and geologic characteristics of the lunar surface, and the appearance of the earth as seen from the moon, both during eclipse of the sun and during partial direct illumination by the sun. Surveyor 3 landed in a subdued crater slightly more than 200 meters in diameter, which has a low rounded rim and is about 15 meters deep. The spacecraft is situated on the east wall of the crater, about half way between the center of the crater and the rim crest. The spacecraft is inclined 14.7° ± 1.0° toward the west. The selenographic coordinates of the landing site are 2.94°S latitude, 23.34°W longitude, relative to selenodetic control adopted by the Aeronautical Chart and Information Center. Small morphologic elements of the landing site include small craters, linear ridges and troughs, and fragmental debris. The craters and fragmental debris resemble those observed at the Surveyor 1 landing site, both in distribution of shape and in distribution of size. Most of the craters in Surveyor 1 and 3 pictures are inferred to be of impact origin. Their size-frequency distribution corresponds to the distribution that would be produced by repetitive bombardment by meteoroids, a bombardment sufficiently prolonged that the crater population has reached a steady state or has come to equilibrium. Some of the craters observed at the Surveyor 3 landing site are inferred to be of secondary impact origin, and some probably have been formed either by subsidence or by drainage of fragmental debris into cracks or fissures in the subsurface. Fragmental debris at the landing site is inferred to have been derived primarily by the same process of repetitive bombardment that produced the majority of craters. The inferred volumetric size-frequency distribution of fragments, derived from the observed size distribution of fragments on the surface, is similar to the distribution that would be produced by repetitive bombardment of coherent rocks by meteoroids with a mass-frequency distribution like that found from observed meteors and recovered meteorites on earth. Two prominent strewn fields of blocky debris were observed around two craters, 13 and 15 meters across, at the Surveyor 3 site. The 13-meter crater has a sharp raised rim, and the 15-meter crater has a more subdued rounded rim. The blocks associated with the subdued crater have twice as high a mean roundness as the blocks associated with the raised-rim crater, and they are much more deeply buried. The size-frequency distribution function for the fragments in each of the strewn fields of blocks resembles the size-frequency distribution for fragments ejected by impacts in strong rock, such as Meteor Crater, Arizona. Most of the fragments at the surface of the Surveyor 3 landing site are evidently part of a layer of fragmental material of low cohesion that is at least 1 meter thick along the upper parts of the wall of the crater in which Surveyor 3 landed and may be much thicker near the center of the crater. The evidence from the Surveyor 3 pictures suggests that this layer of debris, or regolith, is subject to downslope creep or mass movement. Creep is probably caused by seismic shaking, due mainly to near and far impact events and perhaps due partly to internal lunar seismicity. Disturbances of the lunar surface produced by Surveyor 3, like those produced by Surveyor 1, exposed material at depths of a few centimeters or less that was darker than the material at the surface. The albedo of the fine-grained fragmental debris is probably 20 to 30% lower at depths of only a fraction of a millimeter than it is at the optically observed surface. All coarse fragments protruding above the general level of the surface have a higher albedo than the fine-grained matrix of the surface. These general photometric relationships can be explained if it is assumed that the surfaces of the particles in the shallow lunar subsurface tend to become coated with a dark substance; the term proposed for this hypothetical substance is ‘lunar varnish’ On the protruding surface of blocks and coarse fragments the lunar varnish is scrubbed off by the processes that cause rounding. The exposed surfaces of fine particles on the lunar surface are similarly affected, but, because they are mixed relatively rapidly with particles just beneath the surface, the process is incomplete and the fine-grained material exposed at the lunar surface, therefore, has a lower albedo than blocks and other large fragments. Coating of particles by lunar varnish evidently takes place just beneath the surface. The estimated normal luminance factor (normal albedo) of an undisturbed part of the lunar surface next to footpad 2 of the Surveyor 3 spacecraft is 8.5%. An area of the lunar surface disturbed by the surface sampler has an estimated normal luminance factor of 6.6%, and fine-grained material placed on footpad 2 by the surface sampler has an estimated normal luminance factor of 7.6%. The errors in all these estimates may be as high as 25% because of uncertainties of correction required for light scattered from the camera mirror. Preliminary search for color differences, by color reconstitution methods, revealed no determinable differences in color among various coarse blocks, the fine-grained matrix of the surface, or fine-grained material disturbed by the surface sampler. Surveyor 3 pictures of the eclipse of the sun by the earth revealed a bright region in the refraction halo surrounding the earth, which was correlated with the position of the sun, and a series of bright beads that occurred over regions of the earth largely clear of clouds. Clouds tend to occult the refracted rays of the sun, most of which pass through the lower part of the atmosphere at the limb; the beads occurred in the depressions in the optical silhouette of the earth. Preliminary reduction of the color of the refracted light showed that the brightest region, near the position of the sun, exhibited a correlated color temperature close to 4800°K. The color temperature tended to be lower for light that followed paths of greater atmospheric absorption. Preliminary analysis of Surveyor 3 pictures of the partly illuminated earth revealed colors similar to the colors recorded from orbit by the Mercury and Gemini astronauts.

Journal of Geophysical Research↗