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At least 127 records · Page 7Linked to original sources
USGS2012

Paleozoic paleomagnetism and northward drift of the Alexander terrane, southeastern Alaska

Paleozoic limestone, graywacke, sandstone, milestone, red beds and volcanic rocks of the Alexander terrane, southeastern Alaska, have yielded six paleomagnetic pole positions after thermal and alternating-field demagnetization. These poles are from sample groups of late Middle Ordovician, Late Ordovician, Devonian, Late Devonian, and early and late Carboniferous age. To test various tectonic models for the structural development of this part of western North America, the paleomagnetic results are compared to those for the North American craton. It is found that the observed inclination and declination values deviate significantly from the values predicted for the present-day position of the Alexander terrane (55.5N, 133.5W). Better matching can be obtained for a paleoposition of the terrane at about 40N, 120W, in the present position of western Nevada and northeastern California. In addition, an in situ 25° clockwise rotation of the terrane is required to restore it to its original position.

Journal of Geophysical Research Solid Earth↗
USGS2012

Stress measurements at depth in the vicinity of the San Andreas fault. Implications for the magnitude of shear stress at depth

Using the hydraulic fracturing technique, we have made a systematic series of in situ stress measurements in wells drilled near the San Andreas fault. In an attempt to provide constraints for the magnitude of shear stress on the San Andreas fault at depth we have measured both the variation of stress with distance from the fault in relatively shallow (∼230 m) wells and the variation of stress with depth in a ∼1-km-deep well located 4 km from the fault. The shallow wells are located along profiles roughly perpendicular to the fault in the western Mojave desert near Palmdale and in central California where the fault is creeping. In both areas the direction of maximum compression was found to be approximately 45° from the local trend of the San Andreas. The two stress profiles show very similar results: (1) shear stress (on planes parallel to the San Andreas) increases with distance from the fault, more markedly in the western Mojave, (2) the far-field shear stress at ∼200 m depth is ∼50 bars, and (3) the horizontal principal stresses as well as shear stress increase with depth more rapidly in the wells farthest from the fault. The ∼1-km-deep well, also located in the western Mojave desert, shows increases of both horizontal principal stresses and shear stress with depth. Shear stress increases from about 25 bars at 150–300 m to about 80 bars at 750–850 m. Although this rapid increase of shear stress with depth suggests that the mean shear stress on the fault at seismogenic depths exceeds several hundred bars, the principal stresses increase with depth in a steplike manner. As this may be a near-surface effect, extrapolation of the measurements to much greater depths may not be warranted.

Journal of Geophysical Research Solid Earth↗
USGS2012

Heat flow and energetics of the San Andreas fault zone

Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000-km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m −2 ) throughout the 550-km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200-km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near-lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of these basal tractions, the average shearing stress in the seismogenic layer would increase with distance from the main fault, and the seismogenic layer would offer substantial resistance to plate motion even though resistance on the main fault might be negligible. These speculative models have testable consequences.

Journal of Geophysical Research Solid Earth↗
USGS2012

Some constraints on levels of shear stress in the crust from observations and theory

In situ stress determinations in North America, southern Africa, and Australia indicate that on the average the maximum shear stress increases linearly with depth to at least 5.1 km measured in soft rock, such as shale and sandstone, and to 3.7 km in hard rock, including granite and quartzite. Regression lines fitted to the data yield gradients of 3.8 MPa/km and 6.6 MPa/km for soft and hard rock, respectively. Generally, the maximum shear stress in compressional states of stress for which the least principal stress is oriented near vertically is substantially greater than in extensional stress regimes, with the greatest principal stress in a vertical direction. The equations of equilibrium and compatibility can be used to provide functional constraints on the state of stress. If the stress is assumed to vary only with depth z in a given region, then all nonzero components must have the form A+Bz , where A and B are constants which generally differ for the various components. This implies that the deviatoric stress changes linearly with depth, and the general solution also allows the directions of the horizontal principal stresses to change monotonically with depth. Solutions to the equations, a churning stress to vary with both z and x , were fit to the observations of Zoback and Roller, who measured stress along a horizontal profile near Palmdale, California. The results indicate that the average shear stress in the upper 8 km of the fault zone is about 3.5 MPa less than the shear stress in the far field, but this far field term, which is part of the solution and has the form A + Bz , cannot be evaluated using the existing constant depth data.

Journal of Geophysical Research Solid Earth↗
USGS2012

Tectonic stresses in the lithosphere: constraints provided by the experimental deformation of rocks.

The strengths of rocks clearly place an upper limit on the stress that can be sustained by the upper half of the lithosphere. Laboratory data on rock rheology are generally lacking at intermediate temperatures and pressures on the important rock types expected in the lithosphere, so a definitive accounting of the strength distribution with depth in the upper lithosphere is presently unattainable. Analogies are drawn between the fragmentary strength data on silicates at intermediate temperature and the more extensive experimental data on marble and limestone, and several tentative conclusions are drawn: First, brittle processes, such as faulting and cataclasis, are expected to control rock strength at low pressures and temperatures. The strengths associated with these brittle mechanisms increase rapidly with increasing effective pressure and are relatively insensitive to temperature and strain rate. Second, the transitions between brittle and ductile processes occur at critical values of the least principal stress σ 3 . I suggest that the concept of the deformation mechanism map of Ashby (1972) be extended to brittle-ductile transitions by normalizing the applied differential stress σ by σ 3 , i.e., the transitions occur at critical values of σ/σ 3 . Third, the high temperature flow law of olivine, widely applied to problems involving flow in the asthenosphere, is inappropriate to the conditions of temperature found in the lithosphere, because different dislocation flow mechanisms dominate at low to intermediate temperatures. The fragmentary rheological data suggest the following rheological structure of the lithosphere where it is inelastically deforming: A rapid pressure-driven increase in rock strength with depth culminates with a shear strength maximum of up to 8 kbar at depths that depend on the state of stress and on the temperature distribution. A review of the mechanisms of weakening associated with water suggests that water weakening effects are probably not important in the oceanic lithosphere but are likely to be controlling in the continental crust.

Journal of Geophysical Research Solid Earth↗
USGS2012

Lithospheric loading by the 1896 Riku-u earthquake, northern Japan: Implications for plate flexure and asthenospheric rheology

Under favorable circumstances the time-dependent aseismic deformation resulting from the loading of the lithosphere by the stress drop of large dip slip earthquakes can be used to determine both the effective elastic plate thickness and the asthenospheric viscosity. The deformation has several similarities with the deflection of the lithosphere by surface loads and with movements due to postglacial rebound. Level changes obtained in the 80 years since the M = 7.5, 1896 Riku-u earthquake, an intraplate thrust event in northern Honshu, provide convincing evidence that asthenospheric readjustments are responsible for the observed movements. Leveling surveys crossing the zone of surface faulting have been repeated five times since 1900 and delineate a localized depression that has subsided at a continually decreasing rate. The depression is centered close to the 1896 faulting, and its shape and width, about 75 km, are matched by our model using a plate thickness of 30 km. The decaying subsidence rate constrains the viscosity of the uppermost asthenosphere to be 1×10 20 P. A linear viscous rheology matches the observed decay quite well, although measurements are sparse during the several decades following the earthquake.

Journal of Geophysical Research Solid Earth↗
USGS2012

Elastic moduli of rock glasses under pressure to 8 kilobars and geophysical implications

Shear and longitudinal velocities were measured by the ultrasonic phase comparison method as a function of pressure to 8 kbar on synthetic glasses of basalt, andesite, rhyolite, and quartz composition and on natural obsidian. Velocities of most of the glasses decrease anomalously with pressure, but increasingly more-normal behavior occurs with decrease in SiO 2 content. The pressure derivatives of rigidity and bulk modulus increase linearly, from −3.39 to −0.26 and from −5.91 to +2.09, respectively, with decrease in SiO 2 content from 100 to 49%. The change from negative to positive in the pressure derivatives of both moduli and observed at Poisson's ratio of about 0.25 is consistent with the Smyth model for the anomalous elastic behavior of glass. If the temperature in the upper mantle is about 1500°C, tholeiitic basalt would be molten in accordance with the partial melt explanation for the low-velocity zone; at 1300°C and below, basalt would be in the glassy state, especially if more felsic than tholeiite. At a temperature of 1370°C and at 30 kbar, reasonable values for the upper mantle at 100 km depth, the basalt glass of this study would have a viscosity of about 10 13 P. On the basis of the theory of viscoelasticity the glass would support shear wave propagation at frequencies above 0.1 Hz. Under such conditions of PT, 10 to 30% basalt glass in a matrix either of eclogite or olivine would reduce the seismic velocities by 3 to 9% and could also account for the values observed in the low-velocity zone.

Journal of Geophysical Research Solid Earth↗
USGS2012

Geodetic measurement of horizontal deformation across the Rio Grande rift near Socorro, New Mexico

Trilateration surveys of a geodetic network across the Rio Grande rift near Socorro, New Mexico, in 1972, 1973, 1976, and 1979 have failed to detect any significant strain accumulation. The surveys place an upper bound (95% confidence limit) of 1 mm/a (a = years) on east-west spreading cross the rift in 1972–1979. There is marginal evidence from triangulation for an episode of east-west spreading across the rift within the interval 1954–1972. The trilateration network lies on the south flank of an uplift caused by magma intrusion into a midcrustal sill during this century according to Reilinger and Oliver. The horizontal deformation induced by sill inflation is sufficiently small that continued uplift during 1972–1979 cannot be excluded by the observed absence of significant horizontal deformation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Preseismic and coseismic deformation associated with the Coyote Lake, California, earthquake

The Coyote Lake earthquake ( M L = 5.9; August 6, 1979; epicenter about 100 km southeast of San Francisco) occurred on the Calaveras fault within a geodetic network that had been surveyed annually since 1972 to monitor strain accumulation. The rupture surface as defined by aftershocks is a vertical rectangle 20 km in length extending from a depth of 4 km to about 12 km. The observed deformation of the geodetic network constrains the average slip to be about 0.33 ± 0.05 m right lateral. Although the geodetic data furnished an exceptionally detailed picture of the preearthquake deformation, no significant premonitory anomaly associated with the Coyote Lake earthquake can be identified.

Journal of Geophysical Research Solid Earth↗
USGS2012

Electrical properties of granite with implications for the lower crust

The electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature. Minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure, structurally bound water, oxygen fugacity, and other parameters. The effect of sulfur fugacity may be important but is experimentally unconfirmed. In addition to changing the magnitude of electrical properties, the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties. With increasing temperature, changes in water content retain large, but lessened, effects on electrical properties. Near room temperature, a monolayer of water will decrease the electrical resistivity by an order of magnitude. Several weight-percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5. At elevated temperatures just below granitic melting, a few weight-percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2. Above the melting temperature (650° to 1100°C depending upon water pressure), a few weight-percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy. Remarkably, the few weight-percent water must be present as free water. Experiments with hydrated hornblende schist (with structural water) indicate an electrical resistivity very similar to that for dry granite. The implications of these results, together with the findings of deep magnetic sounding and magnetotelluric surveys, suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Galapagos Spreading Centre at 86° W: A detailed geothermal field study

We report here measurements of the heat flow field of the Galapagos Spreading Center on crust of age less than 1.0 m.y. The 443 measurements in an area of about 570 km 2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near-axis conductive heat flux. The intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern, with circulation apparently continuing beyond the limits of our survey. The areal average of the conductive heat flux is 7.1 ± 0.8 HFU (295 ± 33 mW/m 2 ), about one-third the heat flux predicted by plate models. The remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis. The pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography. Sharp lateral gradients in the heat flow, greater than 10 HFU/km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys, may indicate a local concentration of the circulation by surface fault systems and/or variable sediment thickness.

Journal of Geophysical Research Solid Earth↗
USGS2012

The accommodation of relative motion at depth on the San Andreas fault system in California

Plate motion below the seismogenic layer along the San Andreas fault system in California is generally assumed to occur by aseismic slip along a deeper extension of the fault. It is also possible that below the seismogenic layer, deformation is distributed laterally over a zone. Several observed features of the San Andreas fault in California have implications about the mode of accommodation of relative motion along the plate boundary beneath the seismogenic zone: the shallow depth of all earthquakes in California, the depth to which coseismic slip occurred during the 1906 San Francisco earthquake, the broad zone of strain accumulation, the broad heat flow anomaly, and the existence of widely separated parallel faults. The observations strongly imply that below the seismogenic zone, relative motion is distributed over a zone and occurs by inelastic flow rather than by aseismic slip on discrete fault planes. The existence of multiple faults further suggests that tractions at the base of the brittle layer are significant over time periods of years to hundreds of years.

Journal of Geophysical Research Solid Earth↗
USGS2012

Strain accumulation across the Denali fault in the Delta River canyon, Alaska

Deformation along the Denali fault in the Delta River canyon was determined from geodetic surveys in 1941/1942, 1970, 1975, and 1979. The data were best for the 1975–1979 interval; in that period the average strain accumulation was essentially pure right lateral shear at a rate of 0.6 ± 0.1 μrad/a (a is years) (engineering shear) across a vertical plane striking N87°E. The plane of maximum shear is rotated about 30° counterclockwise from the local strike of the Denali fault but closely coincides with the strike of a major linear segment of the fault that begins 50 km farther west. The deformation between 1941/1942 and 1970 is consistent with a similar rate of strain accumulation if one removes the coseismic strain step contributed by the 1964 Alaska earthquake. The 1970–1975 deformation is poorly defined owing to uncertainties in the 1970 survey, but the strain accumulation during that period is certainly much less than during the 1975–1979 interval. The 1975–1979 strain accumulation is interpreted by means of a dislocation model which suggests that the Denali fault in the vicinity of the Delta River Canyon behaves as a leaky transform fault (i.e., a source of spreading as well as lateral slip). The block south of the Denali fault appears to be moving westward (parallel to the strike of the Denali fault west of the 147th meridian) at the rate of about 20 mm/a relative to the North American plate. Because the linear segment of the Denali fault east of the 147th meridian (along which the geodetic network is located) strikes N63°W, accommodation of the westward motion of the southern block requires some spreading within the fault zone as well as right lateral slip on the fault.

Journal of Geophysical Research Solid Earth↗
USGS2012

Tectonic setting for ophiolite obduction in Oman

The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the northern boundary of the Arabian-African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the Tethyan Sea, Gondwana Land continued its dispersal, and the Arabian-African plate drifted northward about 10°. These events combined with the opposite rotation of Eurasia and Africa initiated the closing of the Tethyan during the Late Cretaceous. At the early stages of closure, downwarping of the Arabian continental margin combined with the compressional forces of closure from the Eurasian plate initiated obduction of the Tethyan oceanic crust along preexisting transform faults, and still hot oceanic crust was detached along oblique northeast dipping thrust faults. Amphibolites developed at the base of the detached hot peridotite as it was thrust southward over oceanic volcanic and sedimentary rocks. Plate configurations combined with palinspastic reconstructions show that subduction and attendant large-scale island arc volcanism did not commence until after the Tethyan sea began to close and after the Samail ophiolite was emplaced southward across the Arabian continental margin. The Samail ophiolite nappe now rests upon a melange consisting mainly of pelagic sediments, volcanics, and detached fragments of the basal amphibolites which in turn rest on autochthonous shelf carbonates of the Arabian platform. Laterites and conglomerates with reworked laterites on the eroded upper surface of the ophiolite indicate a period of emergence prior to the deposition of shallow water Maestrichtian carbonates. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just north of the present Jaz Murian depression in Iran.

Journal of Geophysical Research Solid Earth↗
USGS2012

Sr isotopic tracer study of the Samail ophiolite, Oman

We have measured Rb and Sr concentrations and Sr isotopic compositions in 41 whole-rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite, diabase dikes, and gabbro and websterite dikes within the metamorphic peridotite. Ten samples of cumulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have mean 87 Sr/ 86 Sr ratios and standard deviations of 0.70314 ± 0.00030 and 0.70306 ± 0.00034, respectively. The dispersion in Sr isotopic composition may reflect real heterogeneities in the magma source region. The average Sr isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern midocean ridge basalt. The 87 Sr/ 86 Sr ratios of noncumulate gabbro, plagiogranite, and diabase dikes range from 0.7034 to 0.7047, 0.7038 to 0.7046, and 0.7037 to 0.7061, respectively. These higher 87 Sr/ 86 Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with seawater. Mineral separates from dikes that cut harzburgite tectonite have Sr isotopic compositions which agree with that of cumulate gabbro. These data indicate that the cumulate gabbro and the different dikes were derived from partial melting of source regions that had similar long-term histories and chemical compositions.

Journal of Geophysical Research Solid Earth↗
USGS2012

An assessment of the accuracy of the geodetic measurements that define the southern California uplift

Examination of the charge that the geodetic measurements which define the southern California uplift are seriously flawed by height-dependent systematic errors indicates that this charge is unfounded. Our review of those factors designed to minimize the effects of rod error shows that systematic errors attributable to rod miscalibration are trivial and would tend to cancel during the particularly pertinent period 1955–1965. Comparisons between the results of measurements in which the correlation between topography and signal is generally poor reveal large aseismic tilts in a number of places within and around the margins of the uplift. Especially significant in this context are the results of preuplift and postuplift levelings over routes characterized by diverse length, topography, and atmospheric conditions that produce closely matching, temporally equivalent heights for a representative bench mark within the uplift. Statistical analyses of the short-wavelength components of uplift signal and elevation along a frequently repeated survey line indicate variable correlations between the two. However, the spatial and temporal patterns of these correlations are inconsistent with their attribution to height-dependent systematic errors. Statistically significant short-wavelength correlations along this survey line are reasonably explained in part as due to real movement associated with differentially subsiding bench marks, and there is a strong likelihood that this movement tends to dominate many of the correlations.

Journal of Geophysical Research Solid Earth↗
USGS2012

Geodetic strain measurements in Washington

Two new geodetic measurements of strain accumulation in the state of Washington for the interval 1972–1979 are reported. Near Seattle the average principal strain rates are 0.07 ± 0.03 μstrain/yr N 19°W and −0.13 ± 0.02 μstrain/yr N71°E, and near Richland (south central Washington) the average principal strain rates are −0.02 ± 0.01 μstrain/yr N36°W and −0.04 ± 0.01 μstrain/yr N54°E. Extension is taken as positive, and the uncertainties quoted are standard deviations. A measurement of shear strain accumulation (dilatation not determined) in the epoch 1914–1966 along the north coast of Vancouver Island by the Geodetic Survey of Canada indicates a marginally significant accumulation of right-lateral shear (0.06 ± 0.03 μrad/yr) across the plate boundary (N40°W strike). Although there are significant differences in detail, these strain measurements are roughly consistent with a crude dislocation model that represents subduction of the Juan de Fuca plate. The observed accumulation of strain implies that large, shallow, thrust earthquakes should be expected off the coast of Washington and British Columbia. However, this conclusion is not easily reconciled with either observations of elevation change along the Washington coast or the focal mechanism solutions for shallow earthquakes in Washington.

Journal of Geophysical Research Solid Earth↗
USGS2012

An interpretation of episodic slip on the Calaveras fault near Hollister, California

Data from a precision multi-wavelength distance measuring (MWDM) instrument, located near Hollister, California, have been analyzed in terms of strike-slip faulting in the region covered by the network. Four episodes of deformation that are readily identifiable in the MWDM data for the year following September 1975 can be modeled as slip on the Calaveras fault. Creepmeter data tend to support this interpretation, since the creep events and the observed episodes of deformation appear to be correlated in a temporal sense. Lower bounds on the moment for each episode of slip can be determined from the MWDM data by the technique of linear programing. The results indicate that aseismic slip is the dominant mechanism for strain release, since the combined moment of 1.16 × 10 24 dyn cm for the four episodes far exceeds the moment of all of the earthquakes that occurred during the year. Creepmeter data and geologic evidence that are taken in conjunction with the lower bound of the moment indicate that the depth of aseismic slip for some of the episodes of fault slip could extend to below the greatest depth of earthquake hypocenters for this region of central California.

Journal of Geophysical Research Solid Earth↗