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USGS2012

Hydrous carbonates on Mars?: Evidence from Mariner 6/7 infrared spectrometer and ground‐based telescopic spectra

Absorption features at 2.28 and 5.4 μm identified in Mariner 6/7 infrared spectrometer and terrestrial telescopic spectra are consistent with the spectra of hydrous magnesium carbonates such as hydromagnesite and artinite. Spectral characteristics of these hydrous carbonates are different from those of the anhydrous carbonates, as the former do not have the strong spectral features typically associated with anhydrous carbonates such as calcite and siderite. Theoretical mixing indicates that, depending on the type of hydrous carbonate, 10–20 wt % can be incorporated into the regolith without contradicting the spectral observations or the Viking x ray fluorescence chemical analysis. Hydrous carbonates form as weathering products of mafic minerals in the presence of H 2 O and CO 2 , even in the Antarctic. Their formation as evaporite minerals from either original magmas or hydrothermally altered rocks is consistent with the Martian environment, provided liquid water is or has been at least transiently present. On Earth, formation of hydrous Mg carbonates is associated with the production of amorphous iron oxides, which is consistent with both the environment and the inferred surface mineralogy of Mars. These minerals are about 60 wt % H 2 O, CO 3 , and OH; if they are abundant everywhere at the 10% level, then about 6% of the surface weight could be volatiles bound in this type of mineral. Although the stability of hydrous carbonates in a Martian environment is uncertain, there may be kinetic factors inhibiting the dehydration of these minerals, which may persist metastably in the current environment. Although the spectroscopic evidence for anhydrous carbonates is scant, the possible presence of hydrous carbonates provides an appealing mechanism for the existence of carbonates on Mars.

Journal of Geophysical Research↗

USGS2012

Venus cartography

The entire surface of the planet Venus is being mapped at global and regional scales (1:50,000,000 through 1:1,500,000) with synthetic aperture radar (SAR), radar altimeter, and radiometer measurements of physical properties from the Magellan spacecraft. The mapping includes SAR image mosaics, shaded relief maps, and topographic contour overlays made from altimetry data and by radargrammetric methods. Methods used include new techniques of radar image processing that became operational as a result of the Magellan mission. The area being mapped (at a resolution of 75 m/pixel) is roughly equivalent to that of Earth, including seafloors. The mapping is designed to support geologic and geophysical investigations.

Journal of Geophysical Research E: Planets↗

USGS2012

Superposed local and regional paleostresses: fault-slip analysis of Neogene extensional faulting near coeval caldera complexes, Yucca Flat, Nevada

Numerous reduced stress tensors are computed by multiple inversions of 906 temporally and spatially partitioned fault-slip data from the Yucca Flat region in the southwest Nevada volcanic field to constrain the Neogene paleostress and faulting history and to investigate how the regional tectonic stress field was affected by local caldera magmatism. Perturbed, shallow (<400 m), pre-11 Ma paleostress configurations, determined west and northwest of present (post-11 Ma) Yucca Flat basin, existed during mild extensional faulting and are attributed to superposition of transient caldera-magmatic stresses on the regional stress field. Northwest of Yucca Flat a progressive shift in least principal stress (σ 3 ) directions near known calderas located 5–15 km to the west occurred under a normal-slip stress state during caldera development between about 15 and 13 Ma. A brief (∼0.5 m.y.) change to a strike-slip stress state occurred at about 13 Ma and was accompanied by small-offset, quasi-conjugate strike-slip faulting. This stress state was most distinct, relative to a normal-slip state, near calderas where stress solutions and fault relations indicate closer affinities to a reverse-slip state. Inferred 11.6–11.45 Ma paleostress tensors indicate radial tension associated with either initial caldera collapse or local post-collapse topographic modification of the stress field. Post-11 Ma normal-slip stress tensors are associated with normal- and oblique-slip faults that accommodated subsidence and eastward extension of Yucca Flat basin away from the caldera complexes. These tensors do not indicate stress modifications due to residual caldera-related effects and thus were used to infer post-11 Ma regional stress changes. The stress field has rotated as much as 65° clockwise since 11 Ma during extensional development of Yucca Flat basin, with most of the rotation and extension occurring before about 8.5 Ma. Results suggest that shallow magmatism and caldera development can strongly alter extensional tectonic stress fields, fault patterns, and slip directions in the uppermost crust out to distances of roughly two magma chamber radii away from a magma body.

Journal of Geophysical Research↗

USGS2012

Interseismic uplift at the Nankai subduction zone, southwest Japan, 1951–1990

Uplift as a function of time from 1951 through 1990 has been deduced from annual mean sea level measured at 15 tide gages along the Nankai subduction zone, southwest Japan. The recurrence interval for rupture of the Nankai subduction zone is about 100 years, and the most recent rupture was in late 1946. Thus the 1951–1990 uplift record covers most of the first half of the earthquake cycle. The precision of the uplift record is better than could be obtained currently by annual Global Positioning System (GPS) measurements. The pre‐1960 uplift record shows rapid deformation that appears to terminate in 1959. After 1959 the uplift record is remarkably linear in time: Significant curvature in the uplift‐versus‐time plots is detected at only three of the 15 tide gages. The inferred uplift rates are not quantitatively consistent with the predictions of either the viscoelastic coupling or elastic half‐space models of subduction, but the agreement is sufficient to suggest that the causative physical mechanisms have been identified. The immediate postseismic response is controlled by the propagation of slip downward along the downdip extension of the coseismic rupture, and the interseismic response is caused by the accumulation of a slip deficit on the main thrust zone. Asthenosphere relaxation is not required to explain the observations.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Seismic velocity structure and composition of the continental crust: A global view

Seismic techniques provide the highest‐resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high‐pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle ( Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5‐km depth intervals, both in the form of histograms and as an average velocity‐depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5‐km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low‐grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m 3 , and the average SiO 2 content is 61.8%.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Geodetic monitoring of the southern San Andreas Fault, California, 1980-1991

Five geodetic arrays (10 to 40 km aperture) located along the San Andreas fault have been surveyed frequently (several times in most years) over the 1980–1991 interval to detect possible fluctuations in the deformation rate. In each survey of an array the distances between the same four to seven pairs of geodetic monuments were measured. The distances measured (with corresponding standard deviation) range from 8.4 km (3.4 mm) to 38 km (8.2 mm). The data are displayed as plots of measured distance as a function of time. Linear fits in such plots furnish a satisfactory representation of the data. That is, the scatter of the data about the linear fits is within the range expected for the estimated standard deviations in measurement. However, there are coherent low‐amplitude (within the observational error) fluctuations apparent in some of the measured distances. Those fluctuations need not be tectonic effects but rather may be either random patterns or artifacts introduced by systematic drift in instrument calibration or wander of the geodetic monuments. We conclude that the measurements are consistent with steady deformation of the arrays over the 1980–1991 interval, and we find no convincing evidence in the data for fluctuations in the rate of deformation.

California↗

USGS2012

Estimation of rod scale errors in geodetic leveling

Comparisons among repeated geodetic levelings have often been used for detecting and estimating residual rod scale errors in leveled heights. Individual rod-pair scale errors are estimated by a two-step procedure using a model based on either differences in heights, differences in section height differences, or differences in section tilts. It is shown that the estimated rod-pair scale errors derived from each model are identical only when the data are correctly weighted, and the mathematical correlations are accounted for in the model based on heights. Analyses based on simple regressions of changes in height versus height can easily lead to incorrect conclusions. We also show that the statistically estimated scale errors are not a simple function of height, height difference, or tilt. The models are valid only when terrain slope is constant over adjacent pairs of setups (i.e., smoothly varying terrain). In order to discriminate between rod scale errors and vertical displacements due to crustal motion, the individual rod-pairs should be used in more than one leveling, preferably in areas of contrasting tectonic activity. From an analysis of 37 separately calibrated rod-pairs used in 55 levelings in southern California, we found eight statistically significant coefficients that could be reasonably attributed to rod scale errors, only one of which was larger than the expected random error in the applied calibration-based scale correction. However, significant differences with other independent checks indicate that caution should be exercised before accepting these results as evidence of scale error. Further refinements of the technique are clearly needed if the results are to be routinely applied in practice.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Rapid extension recorded by cooling‐age patterns and brittle deformation, Naxos, Greece

The metamorphic core complex exposed as the island of Naxos in the Aegean Sea records an unusually complete sequence of structures developed as a result of continental extension. The structures formed during Miocene rise and cooling from ductile, upper amphibolite facies and anatectic conditions to brittle near‐surface conditions beneath the Naxos detachment fault. Top‐to‐the‐north ductile fabrics in the footwall, which initially developed during amphibolite facies prograde metamorphism, were overprinted by a succession of north directed, normal sense lower‐temperature brittle structures as the footwall was tectonically unloaded and unroofed. Pseudotachylite and cataclasite formation, brittle faulting, alteration, and erosion of the footwall occurred during continued slip and tectonic denudation. Neogene conglomerate and megabreccia, in part derived from exhumation of this footwall, lie structurally above the peripheral Naxos fault. Published K‐Ar and 40 Ar/ 39 Ar ages for hornblende, white mica, and biotite in the footwall decrease northwestward; apparent ages 17–50 Ma in the southeast correspond to areas of low metamorphic grade where preextension argon was partially retained. Published ages 16–10 Ma in higher‐grade rocks of the domal core in the north are cooling ages that for each of the three minerals show a component of younging in the NNE direction of extension. Assuming this is the direction of unroofing, we interpret the rate of this younging as the fault slip rate as the footwall rocks moved >20 km SSW relative to their hanging wall along the base of the Naxos detachment fault. The calculated rates of slip average 5–8 mm/yr, comparable to maximum rates reported in the Basin and Range province.

Naxos↗

USGS2012

Frictional slip of granite at hydrothermal conditions

Sliding on faults in much of the continental crust likely occurs at hydrothermal conditions, i.e., at elevated temperature and elevated pressure of aqueous pore fluids, yet there have been few relevant laboratory studies. To measure the strength, sliding behavior, and friction constitutive properties of faults at hydrothermal conditions, we slid laboratory granite faults containing a layer of granite powder (simulated gouge). Velocity stepping experiments were performed at temperatures of 23° to 600°C, pore fluid pressures P H 2 O of 0 (“dry”) and 100 MPa (“wet”), effective normal stress of 400 MPa, and sliding velocities V of 0.01 to 1 μm/s (0.32 to 32 m/yr). Conditions were similar to those in earlier tests on dry granite to 845°C by Lockner et al. (1986). The mechanical results define two regimes. The first regime includes dry granite up to at least 845° and wet granite below 250°C. In this regime the coefficient of friction is high (μ = 0.7 to 0.8) and depends only modestly on temperature, slip rate, and P H 2 O . The second regime includes wet granite above ∼350°C. In this regime friction decreases considerably with increasing temperature (temperature weakening) and with decreasing slip rate (velocity strengthening). These regimes correspond well to those identified in sliding tests on ultrafine quartz. We infer that one or more fluid-assisted deformation mechanisms are activated in the second, hydrothermal, regime and operate concurrently with cataclastic flow. Slip in the first (cool and/or dry) regime is characterized by pervasive shearing and particle size reduction. Slip in the second (hot and wet) regime is localized primarily onto narrow shear bands adjacent to the gouge-rock interfaces. Weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide, or from a dependence of strength on gouge particle size. Major features of the granite data set can be fit reasonably well by a rate- and temperature-dependent, three-regime friction constitutive model (Chester, this issue). We extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural, slipping faults for sliding rates as low as 31 mm/yr. We do this for two end-member cases. In the first case, pore pressure is assumed hydrostatic at all depths. Shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data, while at depths below about 9–13 km (depending on slip rate), strength becomes less sensitive to depth but sensitive to slip rate. In the second case, pore pressure is assumed to be near-lithostatic at depths below ∼5 km. Strength is low at all depths in this case (<20 MPa, in agreement with observations of “weak” faults such as the San Andreas). The predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm/yr, in rough agreement with the seismic-aseismic transition depth observed on mature continental faults. These results highlight the importance of fluid-assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry, large displacements, higher concentrations of phyllosilicates, and time-dependent fault healing.

Journal of Geophysical Research↗

USGS2012

Effects of geothermal development on deformation in the Long Valley Caldera, eastern California, 1985-1994

Long Valley caldera in east central California has been the site of crustal unrest in the form of seismicity and ground deformation in response to magmatic inflation since 1980. Uplift of the resurgent dome has totaled ∼0.6 m over the 1975–1992 period. Within this region of uplift, and near the southwestern edge of the resurgent dome, is the Casa Diablo area, which experienced relative subsidence of 0.17 m between 1985 and 1992. Geothermal fluid production began at Casa Diablo in 1985 to supply a binary electric power plant; currently, three such plants generate a total of about 40 MW of electricity. The plants are supplied by 170°C water pumped from depths near 150m; all the produced water is reinjected at cooler temperatures at depths near 600 m. Analyses of data from (1) regional leveling lines and a network of bench marks in the geothermal well field area, (2) tilt observations from an L‐shaped array of bench marks, and (3) calculations relating pressure and temperature changes to subsidence indicate that relative subsidence at Casa Diablo results mainly from reductions in pressure in the shallow production reservoir and overlying formations and reductions in temperature in the underlying injection reservoir. Secondary effects on deformation are caused by offsets along the Taylor‐Bryant fault bounding the west side of the well field.

California↗

USGS1996

Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Interseismic deformation along the San Andreas Fault in southern California

Eight trilateration networks located along the San Andreas fault in southern California have been surveyed 8 to 19 times within a 14- to 17-year interval between 1971 and 1992. The data, measurements of distances between the same 10 to 32 pairs of geodetic monuments within a network in each of the surveys, have been corrected for coseismic offsets from nearby earthquakes calculated from dislocation models. The corrected data, a total of 2027 measurements, are displayed in the form of plots of measured distance versus time for each of the 167 lines measured. The hypothesis that the interseismic deformation is steady is tested by examining whether deviations from linear fits to the data in the plots are within the range expected for observational error. A significant deviation from steady deformation is found only for the network located neax Palmdale, California. In that network, many of the measurements made in the survey of early 1982 deviate from the trend defined by the measurements in other surveys. The deviations are not of the form (fixed proportional error) one would generally expect from systematic survey error. If the survey of early 1982 is excluded, the remaining data at Palmdale are consistent with steady deformation. Thus the apparent strain event observed near Palmdale in 1982 was transient.

Journal of Geophysical Research↗

USGS2012

Principal component analysis of interseismic deformation in southern California

Eight trilateration networks along the San Andreas fault in southern California have been surveyed repeatedly (8 to 19 times) within an approximately 15-year interval between 1971 and 1992. The data for each network were analyzed by principal component analysis to represent the observed changes in line length as a superposition of individual modes: L ij - L i = ∑ k A ik C k ( t j ), where L ij is the length of the i th line measured at the time t j , L i is the average length of the i th line, and A ik C k ( t j ) is the k th mode. Each mode is described by a common time function Ck(tj) that is evaluated only at the times t j of the surveys, and the participation of each line in that mode is described by a constant amplitude A ik for that line. For all networks the deformation is largely described by the first mode, and the first-mode time function is essentially linear in time. That is, the deformation in each of the networks appears to be steady. Two anomalies in the accumulation of deformation were observed: In the network near Palmdale a transient strain event occurred in the second mode in early 1982, and in the network within Cajon Pass, fluctuations in the accumulation of deformation somewhat greater than would be attributed to measurement error were found. Although both anomalies are formally significant given the estimated measurement error, I am not confident that either anomaly is real. The principal conclusion is that the accumulation of interseismic deformation in the southern California trilateration networks is steady within the precision of measurement.

Journal of Geophysical Research↗

USGS1996

Holocene slip rate of the Hayward fault at Union City, California

Measured offsets of well-dated alluvial fan deposits near the Masonic Home in Union City constrain Holocene slip rate of the Hayward fault between 7 and 9 mm/yr. Our best minimum geologic slip rate over the past 8.27 ± 0.08 kyr (i.e., 8270 years) is 8.0 ± 0.7 mm/yr. A steep stream (its channel cut into bedrock) flows southwest out of the East Bay Hills, crosses the fault, and deposits its load on an alluvial fan. We cut two 5-m-deep, fault-parallel trenches 20–30 m southwest of the main fault through the crest of the fan. Walls of the trenches reveal a series of nested distributary channel fills. These channels had cut into old surfaces that are indicated by paleosols developed on flood silts. We distinguished many channel fills by their shape, clast size, flow direction, elevation, and relation to paleosols, enabling us to correlate them between both trenches. Two distinct episodes of fan deposition occurred during the Holocene. Reconstructing the apex positions of these fan units indicates that about 42 ± 6 m and 66 ± 6 m of fault slip has occurred since their inceptions at about 4.58 ± 0.05 ka, and 8.27 ± 0.05 ka, respectively. We lowered the age and age uncertainty of the younger unit from earlier reports based on new multiple radiocarbon dates. The 4.58 ka slip rate of 9.2 ± 1.3 mm/yr is not significantly different at 95% confidence from the 8.27 ka slip rate of 8.0 ± 0.7 mm/yr. Because current regional strain rates are fully consistent with Neogene plate tectonic rates (Lisowski et al., 1991) and the historic surface rate of creep in Union City is only 4.7 ± 0.1 mm/yr (Galehouse, 1994), the larger, ≥8 mm/yr, Holocene slip rate implies that strain is now accumulating on a locked zone at depth. The 8 mm/yr rate is probably minimal because earlier trenching evidence nearby implies that some unknown additional amount of fault deformation occurs outside of the narrow fault zone assumed in measuring slip. Lienkaemper et al. (1991) suggest that the fast creep rate of 9 mm/yr, measured near the southern end of the Hayward fault, may underestimate the deep slip rate, because 1868 surface slip occurred there in addition to the continuing fast creep. If the historic deep slip rate equals the long-term rate, then the 9 mm/yr creep rate reflects the minimum seismic loading rate of the Hayward fault better than the ≥8 mm/yr Holocene rates do.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Strain accumulation across the central Nevada seismic zone, 1973–1994

Five trilateration networks extending for 280 km along the central Nevada seismic zone (1915 Pleasant Valley, M = 7.3; 1954 Dixie Valley, M = 6.8; 1954 Stillwater, M = 6.8; 1954 Rainbow Mountain, M = 6.6; 1954 Fairview Peak, M = 7.1; and 1932 Cedar Mountain, M = 7.2) have been surveyed 6 times since 1973 to determine deformation along the zone. Within the precision of measurement the deformation appears uniform along the zone and is described by the principal strain rates 0.036±0.008 μstrain/yr N60°W±3° and −0.031±0.008 μstrain/yr N30°E±3°, extension reckoned positive. The observed strain rates are consistent with simple, right‐lateral, tensor shear at the rate of 0.033 μstrain/yr across a shear zone striking N15°W. This central Nevada shear zone appears to be the northward continuation of the eastern California shear zone. The orientation of the strike‐slip and normal‐slip ruptures within the central Nevada seismic zone are consistent with principal stress axes parallel to the measured principal strain rate axes. Space‐based geodetic measurements (very long baseline interferometry) indicate that the relative motion accommodated across the Basin and Range province west of Ely, Nevada, is about 9.1±1.5 mm/yr N16°W±8° (Dixon et al., 1995.) Notice that the right‐lateral shear zone postulated to explain deformation in the central Nevada seismic zone is properly oriented to accommodate that relative motion. However, a 135‐km effective width of the shear zone would be required to accommodate all of the 9.1 mm/yr relative motion at the strain rates observed in the Nevada seismic zone; only about 3 mm/yr of that relative motion is accommodated within the span of the trilateration networks.

California, Nevada↗

USGS1996

Crustal velocity field near the big bend of California's San Andreas fault

We use geodetic data spanning the 1920–1992 interval to estimate the horizontal velocity field near the big bend segment of California's San Andreas fault (SAF). More specifically, we estimate a horizontal velocity vector for each node of a two-dimensional grid that has a 15-min-by-15-min mesh and that extends between latitudes 34.0°N and 36.0°N and longitudes 117.5°W and 120.5°W. For this estimation process, we apply bilinear interpolation to transfer crustal deformation information from geodetic sites to the grid nodes. The data include over a half century of triangulation measurements, over two decades of repeated electronic distance measurements, a decade of repeated very long baseline interferometry measurements, and several years of Global Positioning System measurements. Magnitudes for our estimated velocity vectors have formal standard errors ranging from 0.7 to 6.8 mm/yr. Our derived velocity field shows that (1) relative motion associated with the SAF exceeds 30 mm/yr and is distributed on the Earth's surface across a band (>100 km wide) that is roughly centered on this fault; (2) when velocities are expressed relative to a fixed North America plate, the motion within our primary study region has a mean orientation of N44°W ± 2° and the surface trace of the SAF is congruent in shape to nearby contours of constant speed yet this trace is oriented between 5° and 10° counterclockwise relative to these contours; and (3) large strain rates (shear rates > 150 nrad/yr and/or areal dilatation rates < −150 nstr/yr) exist near the Garlock fault, near the White Wolf fault, and in the Ventura basin.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Tholeiitic‐alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km 3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice‐impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset‐bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset‐bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H 2 O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H 2 O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice‐impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile‐rich alkalic melts, causing them to rise to the top of the chamber and erupt.

British Columbia↗

USGS1996

The 1992 M=7 Cape Mendocino, California, earthquake: Coseismic deformation at the south end of the Cascadia megathrust

We invert geodetic measurements of coseismic surface displacements to determine a dislocation model for the April 25, 1992, M = 7 Cape Mendocino, California, earthquake. The orientation of the model slip vector, which nearly parallels North America-Juan de Fuca relative plate convergence, and the location and orientation of the model fault relative to the offshore Cascadia megathrust, suggest that the 1992 Cape Mendocino earthquake is the first well-recorded event to relieve strain associated with the Cascadia subduction zone. We use data from three geodetic techniques: (1) the horizontal and vertical displacements of 13 monuments surveyed with the Global Positioning System, corrected for observed horizontal interseismic strain accumulation, (2) 88 section-elevation differences between leveling monuments, and (3) the uplift of 12 coastal sites observed from the die-off of intertidal marine organisms. Maximum observed displacements are 0.4 m of horizontal movement and 1.5 m of uplift along the coast. We use Monte Carlo techniques to estimate an optimal uniform slip rectangular fault geometry and its uncertainties. The optimal model using all the data resolves 4.9 m of slip on a 14 by 15 km fault that dips 28° SE. The fault extends from 1.5 to 8.7 km in depth and the main-shock hypocenter is close to the downdip projection of the fault. The shallowly dipping fault plane is consistent with the observed aftershock locations, and the estimated geodetic moment is 3.1 × 10 19 N m, 70% of the seismic moment. Other models that exclude leveling data collected in 1935 and 1942 are more consistent with seismological estimates of the fault geometry. If the earthquake is characteristic for this segment, the estimated horizontal slip vector compared with plate convergence rates suggests a recurrence interval of 140 years, with a 95% confidence range of 100–670 years. The coseismic uplift occurred in a region that also has high Quaternary uplift rates determined from marine terrace studies. If repeated ruptures of this southernmost segment of the Cascadia megathrust are responsible for the Quaternary uplift, a comparison of the coseismic uplift with coastal uplift rates suggests a recurrence interval of 200–400 years. Thus comparing horizontal and vertical coseismic to long-term deformation suggests a recurrence interval of about 100–300 years for M = 7 events at the south end of the Cascadia megathrust.

Journal of Geophysical Research B: Solid Earth↗