Physical Sciences and Mathematics Commons^™

Teleseismic Evidence For A Low-Velocity Body Under The Coso Geothermal Area, P. Reasenberg, W. Ellsworth, A. Walter

United States Geological Survey: Staff Publications

Teleseismic P wave arrivals were recorded by a dense array of seismographs tations located in the Coso geothermal area, California. The resulting pattern of relative residuals reveals an area showing approximately 0.2-s excess travel time that migrates with changing source azimuth, suggesting that the area is the 'delay shadow' produced by a deep, low-velocity body. Inversion of the relative residual data for three-dimensional velocity structure determines the lateral variations in velocity to a depth of 22.5 km beneath the array. An intense low-velocity body, which coincides with the surface expressions of late Pleistocene rhyolitic volcanism, high heat flow, and hydrothermal …

Faulting Patterns In North-Central Nevada And Strength Of The Crust, Mary Lou Zoback, Mark D. Zoback

United States Geological Survey: Staff Publications

NNE normal fault trends characterize much of the northern Basin and Range province. These faults make sharp bends to NNW and ENE trends in north- central Nevada in the vicinity of a mid-Miocene rift characterized by a zone of diabase dike swarms, graben-filling flows, and a coinciding aeromagnetic anomaly. Despite a roughly 45 ° change in the least principal stress direction since mid-Miocene time, pre-existing NNW- and ENE-trending faults in the vicinity of the rift accommodated the extension whereas regionally, major crustal blocks were faulted along a NNE trend, approximately perpendicular to the modern least principal stress direction. An assumed …

State Of Stress In The Conterminous United States, Mary Lou Zoback, Mark D. Zoback

United States Geological Survey: Staff Publications

Inferring principal stress directions from geologic data, focal mechanisms, and in situ stress measurements, we have prepared a map of principal horizontal stress orientations for the conterminous United States. Stress provinces with linear dimensions which range between 100 and 2000 km were defined on the basis of the directions and relative magnitude of principal stresses. Within a given province, stress orientations appear quite uniform (usually within the estimated range of accuracy of the different methods used to determine stress). Available data on the transition in stress direction between the different stress provinces indicate that these transitions can be abrupt, occurring …

The Effect Of Dissolution Of Volcanic Glass On The Water Chemistry In A Tuffaceous Aquifer, Rainier Mesa, Nevada, Art F. White, Hans C. Claassen, Larry Benson

United States Geological Survey: Staff Publications

Geochemistry of ground water associated with the Tertiary tuffs within Rainier Mesa, southern Nevada, was investigated to determine the relative importance of glass dissolution in controlling water chemistry. Water samples were obtained both from interstitial pores in core sections and from free-flowing fractures. Cation compositions showed that calcium and magnesium decreased as a function of depth in the mesa, as sodium increased. The maximum effect occurs within alteration zones containing clinoptilolite and montmorillonite, suggesting these minerals effectively remove bivalent cations from the system.

Comparisons are made between compositions of ground waters found within Rainier Mesa that apparently have not reacted …

Stress Measurements At Depth In The Vicinity Of The San Andreas Fault: Implications For The Magnitude Of Shear Stress At Depth, Mark D. Zoback, Hiroaki Tsukahara, Stephen Hickman

United States Geological Survey: Staff Publications

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 …

Actinocyclus Ingens Var. Nodus : A New, Stratigraphically Useful Diatom Of The Circum-North Pacific, Jack G. Baldauf, John A. Barron

United States Geological Survey: Staff Publications

Actinocyclus ingens var, nodus Baldauf, n, var., is a morphologically distinct variety of the Miocene diatom A. ingens Ranray. The last occurrence of this new taxon approximates the top of the lower Middle Miocene Denticula Iauta Zone in the circum-North Pacific. Its first occurrence appears to be in the lower part of the D. fauta zone and possibly lies close to the Lower Miocene-Middle Miocene boundary.