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At least 1,333 records · Page 74Linked to original sources
USGS1996

Origin of Bermuda's clay-rich Quaternary paleosols and their paleoclimatic significance

Red clayey paleosols that are chiefly the product of aerosolic dust deposition are interbedded in the Quaternary carbonate formations of the Bermuda oceanic island system. These paleosols provide a basis for reconstructing Quaternary atmospheric circulation patterns in the northwestern Atlantic. Geochemical analyses were performed on representative paleosol samples to identify their parent dust source. Fine-grained fractions were analyzed by energy-dispersive X ray fluorescence to determine trace element (Zr, Y, La, Ti, and Nb) concentrations and to derive geochemical signatures based on immobile element ratios. These ratios were compared with geochemical signatures determined for three possible sources of airborne dust: (1) Great Plains loess, (2) Mississippi River Valley loess, and (3) Saharan dust. The Zr/Y and Zr/La ratios provided the clearest distinction between the hypothesized dust sources. The low ratios in the paleosol B horizons most closely resemble Saharan dust in the <2-μm size class fraction. Contributions from the two North American loessial source areas could not be clearly detected. Thus Bermuda paleosols have a predominantly Saharan aerosolic dust signature. Saharan dust deposition on Bermuda during successive Quaternary glacial periods is consistent with patterns of general circulation models, which indicate that during glacial maxima the northeast summer trade winds were stronger than at present and reached latitudes higher than 30°N despite lower-than-present sea surface temperatures in the North Atlantic.

Journal of Geophysical Research D: Atmospheres↗
USGS1996

Detection and monitoring of H2O and CO2 ice clouds on Mars

We have developed an observational scheme for the detection and discrimination of Mars atmospheric H 2 O and CO 2 clouds using ground-based instruments in the near infrared. We report the results of our cloud detection and characterization study using Mars near IR images obtained during the 1990 and 1993 oppositions. We focused on specific wavelengths that have the potential, based on previous laboratory studies of H 2 O and CO 2 ices, of yielding the greatest degree of cloud detectability and compositional discriminability. We have detected and mapped absorption features at some of these wavelengths in both the northern and southern polar regions of Mars. Compositional information on the nature of these absorption features was derived from comparisons with laboratory ice spectra and with a simplified radiative transfer model of a CO 2 ice cloud overlying a bright surface. Our results indicate that both H 2 O and CO 2 ices can be detected and distinguished in the polar hood clouds. The region near 3.00 μm is most useful for the detection of water ice clouds because there is a strong H 2 O ice absorption at this wavelength but only a weak CO 2 ice band. The region near 3.33 μm is most useful for the detection of CO 2 ice clouds because there is a strong, relatively narrow CO 2 ice band at this wavelength but only broad “continuum” H 2 O ice absorption. Weaker features near 2.30 μm could arise from CO 2 ice at coarse grain sizes, or surface/dust minerals. Narrow features near 2.00 μm, which could potentially be very diagnostic of CO 2 ice clouds, suffer from contamination by Mars atmospheric CO 2 absorptions and are difficult to interpret because of the rather poor knowledge of surface elevation at high latitudes. These results indicate that future ground-based, Earth-orbital, and spacecraft studies over a more extended span of the seasonal cycle should yield substantial information on the style and timing of volatile transport on Mars, as well as a more detailed understanding of the role of CO 2 condensation in the polar heat budget.

Journal of Geophysical Research E: Planets↗
USGS1996

Extension across Tempe Terra, Mars, from measurements of fault scarp widths and deformed craters

Two independent methods, with no common assumptions, have been used to estimate the extension across the heavily deformed Tempe Terra province of the Tharsis region of Mars. One method uses measurements of normal fault scarp width with average scarp slope data for simple grabens and rifts on Mars to estimate the fault throw, which, combined with sparse fault dip data, can be used to estimate extension. Formal uncertainties in this method are only slightly greater than those in other methods, given that the total uncertainty is dominated by the likely uncertainty in the fault dip (assumed to be 60° ± 15°). Measurement of normal fault scarp widths along two N25°–50°W directed traverses across Tempe Terra both yield about 22 ± 16 km of extension (or ∼2% strain across the northern traverse and nearly 3% across the southern one). About three quarters of the extension has occurred during the two main phases of Tharsis-related deformation from Middle/Late Noachian to Early Hesperian and from Late Hesperian to Early Amazonian, with more extension closer to the center of Tharsis during the first phase. Extension across the region was also determined by measuring the elongation and elongation direction of all ancient Noachian impact craters without ejecta blankets, which predate most of the deformation. Results have been corrected for initial non circularity of craters, established from similar measurements of young (post deformation) impact craters, yielding a statistically significant mean strain of 1.96 ± 0.35% in a N38° ± 10°W direction across Tempe Terra (extension of ∼20 ± 4, comparable in magnitude and direction to the average result from the scarp measurement method). Both methods indicate an average extension for single normal fault scarps (and shortening across wrinkle ridges for the crater method) of ∼100 m. The agreement between the results of the two independent methods in overall extension and average single normal fault extension argues that the average scarp slope and fault dip data in the fault scarp width method accurately represent the actual extension across the observed structures. This conclusion supports existing geometric and kinematic models for structural features on Mars. A preliminary estimate of the total circumferential extension around Tharsis (at a radius of ∼2500 km) is roughly 60 ± 42 km; total hoop strain is about 0.4% distributed heterogeneously (Tempe Terra is the most highly strained region on Mars).

Journal of Geophysical Research E: Planets↗
USGS1997

Sedimentary history and mass flow structures of Chryse and Acidalia Planitiae, Mars

Geologic mapping and crater counting in Chryse and Acidalia Planitiae (GAP) reveal five major sedimentary deposits of Hesperian to Early Amazonian age, including (1) a mass flow deposited during the Early Hesperian near Deuteronilus Mensae (northeast of the map region) that may have resulted from the carving of Kasei Valles, >3000 km southwest of the exposed part of the deposit; (2) knobby plains material consisting of channel (likely; from Simud and Tiu Valles and possibly Ares and Shalbatana Valles) and mass-wasting deposits in central and eastern CAP; (3) material largely from Maja and Ares Valles emplaced in at least western and southern CAP (outcrops in southern Chryse Planitia developed thermokarst); (4) a thin mass flow covering much of southern Chryse Planitia that emanated from Simud and Tiu Valles; and (5) a thick, extensive (perhaps >3500 km across) mass flow deposit in central and northern CAP derived from accumulation and backflow of the preceding thin mass flow or perhaps melting of polar deposits. Other possible deposits may not be recognizable owing to burial by younger materials or a lack of morphologic signature. Various associated landforms appear to be consistent with the mass flow interpretations, including lobate and linear scarps along deposit edges, fractures related to desiccation of thick sediments, troughs, and ridges near the edges of the deposit indicative of secondary mass movement and deformation, pitted domes and fissure-fed flows possibly formed by sedimentary (mud) eruptions, and longitudinal channel grooves perhaps formed by roller vortices. No convincing evidence for paleoshorelines or stagnant ice sheets is found in CAP. These findings suggest that mass flow and hyperconcentrated flooding may have been the predominant processes of outflow-channel dissection in CAP. Elsewhere in the northern plains, similar landforms are prevalent. The mass flow interpretation does not require either multiple episodes of extraordinarily high water-discharge rates achieved by freeing huge volumes of water from the crust, repetitive recycling of immense volumes of water into highland aquifers at the heads of Chryse channels, or profound climate change. Mars Pathfinder will most likely land on and inspect the surface of the thin mass flow that originated from the canyons of Simud and Tiu Valles.

Journal of Geophysical Research E: Planets↗
USGS1997

Selection of the Mars Pathfinder landing site

The Mars Pathfinder spacecraft will land on a depositional fan near the mouth of the catastrophic outflow channel, Ares Vallis (19.5°N, 32.8°W). This site offers the prospect of analyzing a variety of rock types from the ancient cratered highlands, intermediate-age ridged plains, and reworked channel deposits. Analyses of these rocks by Pathfinder instruments will enable first-order scientific questions to be addressed, such as differentiation of the crust, the development of weathering products, and the nature of the early environment, as well as their subsequent evolution on Mars. Constraints imposed by (1) spacecraft and rover designs (which are robust), (2) entry, descent, and landing, (3) scientific potential at various sites, and (4) safety were important considerations in site selection. Engineering constraints require a 70 km by 200 km smooth, flat (low slopes) area located between 10° and 20°N that is below 0 km elevation, with average radar reflectivity, little dust, and moderate rock abundance. Three regions on Mars are between 10° and 20°N and below 0 km elevation: Chryse, Amazonis, and Isidis-Elysium. Science considerations favor sites at the mouths of outflow channels (grab bag sites offer an assay of rock types on Mars), highland sites (early crustal differentiation and climate), and sites covered with dark (unoxidized) material. Sites are considered safe if they are clearly below 0 km elevation, appear acceptably free of hazards in high-resolution (<50 m/pixel) Viking orbiter images and have acceptable reflectivity and roughness at radar wavelengths, thermal inertia, rock abundance, red to violet ratio, and albedo. Recent 3.5-cm wavelength radar observations were used to verify elevation, reflectivity, and roughness within the landing ellipses. Three sites meet all of these criteria: Ares Vallis, Tritonis Lacus, and Isidis. Although Isidis appears to be safer than Tritonis and Ares, the greater scientific potential at Ares Vallis resulted in its selection. Comparisons of the Grand Coulee (channel) and the depositional Ephrata Fan of the Channeled Scabland in eastern Washington, with Ares Vallis and its depositional fan also suggest the Ares Vallis landing site is safe and scientifically interesting.

Journal of Geophysical Research Planets↗
USGS1997

The imager for Mars Pathfinder experiment

The imager for Mars Pathfinder (IMP), a stereoscopic, multispectral camera, is described in terms of its capabilities for studying the Martian environment. The camera's two eyes, separated by 15.0 cm, provide the camera with range‐finding ability. Each eye illuminates half of a single CCD detector with a field of view of 14.4×14.0° and has 12 selectable filters. The ƒ/18 optics have a large depth of field, and no focussing mechanism is required; a mechanical shutter is avoided by using the frame transfer capability of the 512×512 CCD. The resolving power of the camera, 0.98 mrad/pixel, is approximately the same as the Viking Lander cameras; however, the signal‐to‐noise ratio for IMP greatly exceeds Viking, approaching 350. This feature along with the stable calibration of the filters between 440 and 1000 nm distinguishes IMP from Viking. Specially designed targets are positioned on the Lander; they provide information on the magnetic properties of wind‐blown dust, measure the wind vectors, and provide radiometric standard reflectors for calibration. Also, eight low‐transmission filters are included for imaging the Sun directly at multiple wavelengths, giving IMP the ability to measure dust opacity and potentially the water vapor content. Several experiments beyond the requisite color panorama are described in detail: contour mapping of the local terrain, multispectral imaging of the surrounding rock and soil to study local mineralogy, viewing of three wind socks, measuring atmospheric opacity and water vapor content, and estimating the magnetic properties of wind‐blown dust. This paper is intended to serve as a guide to understanding the scientific integrity of the IMP data that will be returned from Mars starting on July 4, 1997.

Journal of Geophysical Research E: Planets↗
USGS1997

A numerical investigation of choked flow dynamics and its application to the triggering mechanism of long-period events at Redoubt Volcano, Alaska

We use numerical simulations of transonic flow through a crack to study the dynamics of the formation of shock waves downstream from a nozzle-like constriction inside the crack. The model solves the full set of Navier-Stokes equations in two dimensions via an explicit multifield finite difference representation. The crack walls are assumed to be perfectly rigid, and elastic coupling to the solid is not considered. The simulations demonstrate how the behavior of unsteady shock waves near the walls can produce recurring step-like pressure transients in the flow, which in turn induce resonance of the fluid-filled crack. The motion of the shock waves is governed primarily by smooth, low-amplitude pressure fluctuations at the outlet of the crack. The force induced on the walls scales with the amplitude of the shock, which is a function of the magnitude of the inlet pressure, aperture of the constriction, and thickness of the boundary layer. The applied force also scales in proportion to the spatial extent of the shock excursion, which depends on the fluctuation rate of outlet pressure. Using the source parameters of long-period (LP) events at Redoubt Volcano, Alaska, as a guide for our simulations, we infer that coupling of the shock to the walls occurs for crack inlet to outlet pressure ratios p i /p o >2.31 and that the position of the shock front becomes most sensitive to outlet pressure fluctuations for flow regimes with p i /p o >2.48. For such regimes, fluctuations of outlet pressure of up to ±0.5 MPa at rates up to 3 MPa/s are sufficient to induce pressure transients with magnitudes up to 12.5 MPa over 0.1–2.5 m of the walls within ∼0.5 s. These flow parameters may be adequate for triggering the LP events in the precursory swarm to the December 14, 1989, eruption of Redoubt. According to the flow model the recurrence rate and amplitudes of L.P events are inferred to be a manifestation of the response of a shallow hydrothermal reservoir to the sustained injection of superheated steam from a magma column roofing below this reservoir.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Post seismic deformation associated with the 1992 Mω = 7.3 Landers earthquake, southern California

Following the 1992 M ω =7.3 Landers earthquake, a linear array of 10 geodetic monuments at roughly 5‐km spacing was established across the Emerson fault segment of the Landers rupture. The array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it. The array was surveyed by Global Positioning System 0.034, 0.048, 0.381, 1.27, 1.88, 2.60, and 3.42 years after the Landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation. The temporal behavior is described roughly by a short‐term (decay time 84±23 days) exponential relaxation superimposed upon an apparently linear trend. Because the linear trend represents motions much more rapid than the observed preseismic motions, we attribute that trend to a slower (decay time greater than 5 years) postseismic relaxation, the curvature of which cannot be resolved in the short run (3.4 years) of postseismic data. About 100 mm of right‐lateral displacement and 50 mm of fault‐normal displacement accumulated across the geodetic array in the 3.4‐year interval covered by the postseismic surveys. Those displacements are attributed to postseismic, right‐lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace. The right‐lateral slip amounted to about 1 m directly beneath the geodetic array, and the fault‐normal displacement is apparently primarily a consequence of the curvature of the rupture. These conclusions are based upon dislocation models fit to the observed deformation. However, no dislocation model was found with rms residuals as small as the expected observational error.

California↗
USGS1997

Multiphase groundwater flow near cooling plutons

We investigate groundwater flow near cooling plutons with a computer program that can model multiphase flow, temperatures up to 1200°C, thermal pressurization, and temperature‐dependent rock properties. A series of experiments examines the effects of host‐rock permeability, size and depth of pluton emplacement, single versus multiple intrusions, the influence of a caprock, and the impact of topographically driven groundwater flow. We also reproduce and evaluate some of the pioneering numerical experiments on flow around plutons. Host‐rock permeability is the principal factor influencing fluid circulation and heat transfer in hydrothermal systems. The hottest and most steam‐rich systems develop where permeability is of the order of 10 −15 m 2 . Temperatures and life spans of systems decrease with increasing permeability. Conduction‐dominated systems, in which permeabilities are ≤10 −16 m 2 , persist longer but exhibit relatively modest increases in near‐surface temperatures relative to ambient conditions. Pluton size, emplacement depth, and initial thermal conditions have less influence on hydrothermal circulation patterns but affect the extent of boiling and duration of hydrothermal systems. Topographically driven groundwater flow can significantly alter hydrothermal circulation; however, a low‐permeability caprock effectively decouples the topographically and density‐driven systems and stabilizes the mixing interface between them thereby defining a likely ore‐forming environment.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

A test of the double-shearing model of flow for granular materials

The double-shearing model of flow attributes plastic deformation in granular materials to cooperative slip on conjugate Coulomb shears (surfaces upon which the Coulomb yield condition is satisfied). The strict formulation of the double-shearing model then requires that the slip lines in the material coincide with the Coulomb shears. Three different experiments that approximate simple shear deformation in granular media appear to be inconsistent with this strict formulation. For example, the orientation of the principal stress axes in a layer of sand driven in steady, simple shear was measured subject to the assumption that the Coulomb failure criterion was satisfied on some surfaces (orientation unspecified) within the sand layer. The orientation of the inferred principal compressive axis was then compared with the orientations predicted by the double-shearing model. The strict formulation of the model [ Spencer , 1982] predicts that the principal stress axes should rotate in a sense opposite to that inferred from the experiments. A less restrictive formulation of the double-shearing model by de Josselin de Jong [1971] does not completely specify the solution but does prescribe limits on the possible orientations of the principal stress axes. The orientations of the principal compression axis inferred from the experiments are probably within those limits. An elastoplastic formulation of the double-shearing model [ de Josselin de Jong , 1988] is reasonably consistent with the experiments, although quantitative agreement was not attained. Thus we conclude that the double-shearing model may be a viable law to describe deformation of granular materials, but the macroscopic slip surfaces will not in general coincide with the Coulomb shears.

Journal of Geophysical Research B: Solid Earth↗
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↗
USGS1997

A dynamic balance between magma supply and eruption rate at Kilauea volcano, Hawaii

The dynamic balance between magma supply and vent output at Kilauea volcano is used to estimate both the volume of magma stored within Kilauea volcano and its magma supply rate. Throughout most of 1991 a linear decline in volume flux from the Kupaianaha vent on Kilauea's east rift zone was associated with a parabolic variation in the elevation of Kilauea's summit as vent output initially exceeded then lagged behind the magma supply to the volcano. The correspondence between summit elevation and tilt established with over 30 years of data provided daily estimates of summit elevation in terms of summit tilt. The minimum in the parabolic variation in summit tilt and elevation (or zero elevation change) occurs when the magma supply to the reservoir from below the volcano equals the magma output from the reservoir to the surface, so that the magma supply rate is given by vent flux on that day. The measurements of vent flux and tilt establish that the magma supply rate to Kilauea volcano on June 19, 1991, was 217,000±10,000 m 3 /d (or 0.079±0.004 km 3 /yr). This is close to the average eruptive rate of 0.08 km 3 /yr between 1958 and 1984. In addition, the predictable response of summit elevation and tilt to each east rift zone eruption near Puu Oo since 1983 shows that summit deformation is also a measure of magma reservoir pressure. Given this, the correlation between the elevation of the Puu Oo lava lake (4 km uprift of Kupaianaha and 18 km from the summit) and summit tilt provides an estimate for magma pressure changes corresponding to summit tilt changes. The ratio of the change in volume to the change in reservoir pressure ( dV / dP ) during vent activity may be determined by dividing the ratio of volume erupted to change in summit tilt ( dV / d tilt ) by the ratio of pressure change to change in summit tilt ( dP / d tilt ). This measure of dV / dP , when combined with laboratory measurements of the bulk modulus of tholeitic melt, provides an estimate of 240±50 km 3 for the volume of Kilauea's magma reservoir. This estimate is much larger than traditional estimates but consistent with seismic tomographic imaging and geophysical modeling of Kilauea's magma system.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Seismic-geodynamic constraints on three-dimensional structure, vertical flow, and heat transfer in the mantle

Joint inversions of seismic and geodynamic data are carried out in which we simultaneously constrain global-scale seismic heterogeneity in the mantle as well as the amplitude of vertical mantle flow across the 670 km seismic discontinuity. These inversions reveal the existence of a family of three-dimensional (3-D) mantle models that satisfy the data while at the same time yielding predictions of layered mantle flow. The new 3-D mantle models we obtain demonstrate that the buoyancy forces due to the undulations of the 670 km phase-change boundary strongly inhibit the vertical flow between the upper and lower mantle. The strong stabilizing effect of the 670 km topography also has an important impact on the predicted dynamic topography of the Earth's solid surface and on the surface gravity anomalies. The new 3-D models that predict strongly or partially layered mantle flow provide essentially identical fits to the global seismic data as previous models that have, until now, predicted only whole-mantle flow. The convective vertical transport of heat across the mantle predicted on the basis of the new 3-D models shows that the heat flow is a minimum at 1000 km depth. This suggests the presence at this depth of a globally defined horizon across which the pattern of lateral heterogeneity changes rapidly.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Depth dependence of earthquake frequency-magnitude distributions in California: Implications for rupture initiation

Statistics of earthquakes in California show linear frequency-magnitude relationships in the range of M 2.0 to M 5.5 for various data sets. Assuming Gutenberg-Richter distributions, there is a systematic decrease in b value with increasing depth of earthquakes. We find consistent results for various data sets from northern and southern California that both include and exclude the larger aftershock sequences. We suggest that at shallow depth (∼0 to 6 km) conditions with more heterogeneous material properties and lower lithospheric stress prevail. Rupture initiations are more likely to stop before growing into large earthquakes, producing relatively more smaller earthquakes and consequently higher b values. These ideas help to explain the depth-dependent observations of foreshocks in the western United States. The higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock. At greater depth (9–15 km), any rupture initiation is more likely to continue growing into a larger event, so there are fewer foreshocks. If one assumes that frequency-magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake, then a small ( M 2) rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a M 5.5 or larger event, compared to the same small rupture initiation at 0 to 3 km.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north-south within the magmatic regions and northwest-southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest-southeast dextral shear of the Walker Lane belt to the east-west extension characteristic of the Cascade arc in central Oregon. Faulting along north-south strikes in the volcanically active areas indicates the east-west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

The roles of time and displacement in velocity-dependent volumetric strain of fault zones

The relationship between measured friction μ A and volumetric strain during frictional sliding was determined using a rate and state variable dependent friction constitutive equation, a common work balance relating friction and volume change, and two types of experimental faults: initially bare surfaces of Westerly granite and rock surfaces separated by a 1 mm layer of less than 90 μm Westerly granite gouge. The constitutive equation is the sum of a constant term representing the nominal resistance to sliding and two smaller terms: a rate dependent term representing the shear viscosity of the fault surface (direct effect), and a term which represents variations in the area of contact (evolution effect). The work balance relationship requires that μ A differs from the frictional resistance that leads to shear heating by the derivative of fault normal displacement with respect shear displacement, d δ n / d δ s . An implication of this relationship is that the rate dependence of d δ n / d δ s contributes to the rate dependence of μ A . Experiments show changes in sliding velocity lead to changes in both fault strength and volume. Analysis of data with the rate and state equations combined with the work balance relationship preclude the conventional interpretation of the direct effect in the rate and state variable constitutive equations. Consideration of a model bare surface fault consisting of an undeformable indentor sliding on a deformable surface reveals a serious flaw in the work balance relationship if volume change is time-dependent. For the model, at zero slip rate indentation creep under the normal load leads to time-dependent strengthening of the fault surface but, according to the work balance relationship, no work is done because compaction or dilatancy can only be induced by shearing. Additional tests on initially bare surfaces and gouges show that fault normal strain in experiments is time-dependent, consistent with the model. This time-dependent fault normal strain, which is not accounted for in the work balance relationship, explains the inconsistency between the constitutive equations and the work balance. For initially bare surface faults, all rate dependence of volume change is due to time dependence. Similar results are found for gouge. We conclude that μ A reflects the frictional resistance that results in shear heating, and no correction needs to be made for the volume changes. The result that time-dependent volume changes do not contribute to μ A is a general result and extends beyond these experiments, the simple indentor model and particular constitutive equations used to illustrate the principle.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

CRUST 5.1: A global crustal model at 5° x 5°

We present a new global model for the Earth's crust based on seismic refraction data published in the period 1948-1995 and a detailed compilation of ice and sediment thickness. An extensive compilation of seismic refraction measurements has been used to determine the crustal structure on continents and their margins. Oceanic crust is modeled with both a standard model for normal oceanic crust, and variants for nonstandard regions, such as oceanic plateaus. Our model (CRUST 5.1) consists of 2592 5° × 5° tiles in which the crust and uppermost mantle are described by eight layers: (1) ice, (2) water, (3) soft sediments, (4) hard sediments, (5) crystalline upper, (6) middle, (7) lower crust, and (8) uppermost mantle. Topography and bathymetry are adopted from a Mandard database (ETOPO-5). Compressional wave velocity in each layer is based on field measurements, and shear wave velocity and density are estimated using recently published empirical V p -V s and V p -density relationships. The crustal model differs from previous models in that (1) the thickness and seismic/density structure of sedimentary basins is accounted for more completely, (2) the velocity structure of unmeasured regions is estimated using statistical averages that are based on a significantly larger database of crustal structure, (3) the compressional wave, shear wave, and density structure have been explicitly specified using newly available constraints from field and laboratory studies. Thus - this global crustal model is based on substantially more data than previous models and differs from them in many important respects. A new map of the thickness of the Earth's crust is presented, and we illustrate the application of this model by using it to provide the crustal correction for surface wave phase velocity maps. Love waves at 40 s are dominantly sensitive to crustal structure, and there is a very close correspondence between observed phase velocities at this period and those predicted by CRUST 5.1. We find that the application of crustal corrections to long-period (167 s) Rayleigh waves significantly increases the variance in the phase velocity maps and strengthens the upper mantle , velocity anomalies beneath stable continental regions. A simple calculation of crustal isostacy indicates significant lateral variations in upper mantle density. The model CRUST 5.1 provides a complete description of the physical properties of the Earth's crust at a scale of 5° × 5° and can be used for a wide range of seismological and nonseismological problems.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Stratigraphic contrasts and tectonic relationships between Carboniferous successions in the Trans-Alaska Crustal Transect corridor and adjacent areas, northern Alaska

The Carboniferous succession along the Trans-Alaska Crustal Transect (TACT) corridor in the Atigun Gorge area of the central Brooks Range consists of the Kayak Shale (Kinderhookian) and the Lisburne Group (Kinderhookian through Chesterian). The Kayak Shale is at least 210 m thick; it is chiefly black, noncalcareous shale with several limestone beds of pelmatozoan-bryozoan packstone and formed in an open-marine setting. The Lisburne Group is a carbonate rock succession about 650 m thick and consists mainly of skeletal packstone, wackestone, and milestone which contain locally abundant calcispheres, ostracodes, algae, and sponge spicules; it accumulated largely in a shallow water platform environment with restricted circulation. This restriction was probably produced by a coeval belt of skeletal sand shoals recognized 70 km to the west in the Shainin Lake area. Significant and apparently abrupt shifts in the age and lithofacies of Carboniferous strata occur across the central and eastern Brooks Range. These shifts are most marked in a zone roughly coincident with what is interpreted by many workers to be the leading edge of the Endicott Mountains allochthon. Notable lithologie contrasts are also observed, however, between sections in the northern and southern parts of the Endicott Mountains allochthon. This suggests that considerable tectonic shortening has taken place within the allochthon, as well as between it and parautochthonous rocks to the northeast. The Carboniferous section near Mount Doonerak is more similar in age and lithofacies to coeval sections in the central Brooks Range that are considered allochthonous than to parautochthonous sections to the northeast.

Journal of Geophysical Research B: Solid Earth↗