Constraints on the current stress regime of the actively extending northern Basin and Range province are provided by deformation data (focal mechanisms and fault slip studies), hydraulic fracturing in situ stress measurements, borehole elongation (“breakouts”) analyses, and alignment of young volcanic vents. The integrated data indicate significant variations both in principal stress orientations and magnitudes. An approximately E-W least principal stress direction appears to characterize both the eastern and western margins of the Basin and Range province, whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented NW to N60°W. The contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins. Active deformation along the southeastern and western province margins is characterized by a combination of strike-slip and normal faulting. Focal mechanisms along northeastern province margin (Wasatch front) and in central Nevada indicate a combination of normal and oblique-normal faulting. Temporal, regional, and depth-dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles. However, some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated. Evidence for significant temporal variation (or multiple cycles of variation) in relative stress magnitude comes from the Sierran front-Basin and Range boundary region where recent earthquakes are predominantly strike slip, whereas the profound relative vertical relief across the Sierra frontal fault zone in the last 9–10 m.y. implies a normal faulting stress regime. Using the best data on stress orientation, relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements. Analysis of well-constrained slip vectors in the Owens Valley, California, area indicate that large temporal variations in the magnitude of the approximately N-S oriented maximum horizontal stress are required to explain dominantly dip-slip and strike-slip offsets on subparallel faults. Similar faulting relations are observed throughout much of the boundary zone between the Basin and Range-Sierra Nevada (including the Walker Lane belt). Along the eastern province margin in the Wasatch front area in Utah, available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of <4–5 km. Earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth. Furthermore, superimposed sets of young fault striae along a segment of the Wasatch fault also indicate temporal variations of relative stress magnitudes. Sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion, thermally induced flow, and the influence of the San Andreas plate boundary. Although difficult to date accurately, the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle (1000–5000 years) and multiple earthquake cycles (10,000+ years).

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State of stress and modern deformation of the Northern Basin and Range Province

Abstract Estimates of the in situ stress tensor in sedimentary basins can be inferred from information gathered during the drilling and logging of hydrocarbon exploration wells. Principal stress magnitudes can be obtained from information provided by density logs, drilling histories, leak-off test results and pore pressure measurements. Horizontal principal stress orientations are indicated by borehole breakouts recorded on four-arm dipmeter logs. The stress magnitude estimates are comparable with values obtained from hydraulic fracturing and the orientation data provide a level of detail hitherto unobtainable. The information obtained assists in designing hydraulic fractures, in secondary recovery programmes and in obtaining optimum drill bit performance. It also contributes to understanding lithospheric plate movements, crustal subsidence and tectonic deformation.

Investigating stress regimes in sedimentary basins using information from oil industry wireline logs and drilling records

Physical and Hydrologic-Flow Properties of Fractures: Las Vegas, Nevada - Zion Canyon, Utah - Grand Canyon, Arizona - Yucca Mountain, Nevada, July 20-24, 1989 (Field Trip Guidebook T385)

Fractures have been characterized for fluid-flow, geomechanical, and paleostress modeling at three localities in the vicinity of drill hole USW G-4 at Yucca Mountain in southwestern Nevada. A method for fracture characterization is introduced that integrates mapping fracture-trace networks and quantifying eight fracture parameters: trace length, orientation, connectivity, aperture, roughness, shear offset, trace-length density, and mineralization. A complex network of fractures was exposed on three 214- to 260-m 2 pavements cleared of debris in the upper lithophysal unit of the Tiva Canyon Member of the Miocene Paint-brush Tuff. The pavements are two-dimensional sections through the three-dimensional network of strata-bound fractures. All fractures with trace lengths greater than 0.2 m were mapped and studied.

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Characterizing fractured rock for fluid-flow, geomechanical, and paleostress modeling: Methods and preliminary results from Yucca Mountain, Nevada

Borehole breakouts are caused by unequal stress concentrations around a borehole resulting in shear failure of the borehole wall and creating hole elongation perpendicular to the maximum horizontal in situ stress. Breakouts in Tertiary sedimentary rocks were identified in 10 holes in the West Coalinga, Cantua Creek, and West Haven oil fields in the western San Joaquin Valley. The deeper and more reliable (1.5 to 3.7 km) breakouts were oriented northwest, indicating northeast compressive stress, perpendicular to the major fold axes in the San Joaquin Valley and at a 70°–90° angle to the major strike-slip faults in the Coast Ranges. This stress direction is consistent with a reverse slip component on northwest-trending faults in the region and with the focal mechanisms reported for the May 1983 Coalinga earthquake. The existing literature on stress measurements in central and southern California indicates that on a regional scale, the stress regime is transitional between thrust and strike-slip faulting. A hydraulic fracture test in the Kettleman Hills oil field at 3 km depth indicates that this stress regime exists in the upper crust near Coalinga as well.

Stress orientations from well bore breakouts in the Coalinga Region

This paper documents the borehole spalling pattern in relation to lithology, geologic structure, stress, and depth of two Nevada Test Site (NTS) areas, Yucca Flat and Pahute Mesa. Downhole movies and stereo photographs from 58 large-diameter test holes at NTS reveal a strong NW-SE elongation due to sidewall spalling. This trend is found in Yucca Flat and Pahute Mesa, which have distinctly different geologies, and is parallel to the direction of regional extension. The directionality is due to tangential stress concentration at the wellbore, which has a peak value along the azimuth of minimum horizontal regional stress. The percentage of borehole length affected by spalling varies from 5% near the surface to 30% at depths over 500 m. The severity of spalling is also a function of lithology; however, geologic discontinuities apparently have little effect on the orientation and occurrence of elongations. Directional spalling was observed in relatively shallow alluvial deposits, suggesting the persistence of horizontal stress differences at shallow depths. If the elongations are truly controlled by the deviatoric stress field, this implies that the regional stress direction is fairly uniform throughout NTS. A simple relation is given for the in situ stress and material strength conditions associated withmore » borehole elongation. 24 references, 13 figures, 2 tables.« less

J.E. Springer

Papers

Borehole elongation and its relation to tectonic stress at the Nevada Test Site