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T. P. Barnhard

Publications and source records attributed to T. P. Barnhard.

5 recordsLinked to original sources
USGS1993

Aspects of three-dimensional strain at the margin of the extensional orogen, Virgin River depression area, Nevada, Utah, and Arizona

The Virgin River depression and surrounding mountains are Neogene features that are partly contiguous with the little-strained rocks of the structural transition to the Colorado Plateau province. This contiguity makes the area ideally suited for evaluating the sense, magnitude, and kinematics of Neogene deformation. Analysis along the strain boundary shows that, compared to the adjacent little-strained area, large-magnitude vertical deformation (both uplift and depression, locally exceeding 10 km) greatly exceeds extensional deformation and that significant amounts of lateral displacement approximately parallel the province boundary. Extensional kinematics, the directions of lateral displacements, and the position and orientation of uplifts and one major depression are such that isostatic rebound following tectonic denudation is an unlikely direct cause of the strong vertical structural relief adjacent to the strain boundary. Instead, the observed structures are first-order features defining a three-dimensional strain field produced by approximately east-west extension, vertical structural attenuation, and extension-normal shortening. All major structural elements of the strain-boundary strain field are also found in the adjacent Basin and Range. Especially evident is a discontinuous and inhomogeneous distribution of vertical structural relief resulting from uplift and tilting of the footwalls on convex-upward normal faults, combined with formation of extension-parallel folds. Deformation resulted in a bewilderingly complex mixture of coeval folds, normal faults, strike-slip faults, and, in some areas, reverse faults, thrust faults, and thrust faults reactivated as extensional detachment faults. These complexly mixed structures exhibit a tectonically important aspect of scale independence, ranging from micro- structures to mountain-range scale. Many intensely extended domains appear to reflect a process of surface-relief minimization above uplifted and tilted blocks. Some intensely extended zones are depth-limited by relatively undeformed rocks above, and intact tilted blocks below, and are best viewed as subhorizontal zones of accommodation for the uplift and tilt of subjacent blocks. Whether or not the intense deformation extends to the surface, its depth distribution commonly reflects lithologic or paleostructural controls. Whatever the cause for the concentration of extensional strain, many gently dipping zones are more accurately viewed as local features recording small to moderate extension-parallel translations than a regionally interconnected detachment faults recording enormous extension-parallel translations. Although the total Neogene strain field in the Nevada-Utah-Arizona tricorner area has unique aspects, other parts of the Basin and Range contain generally similar structures that presumably formed from similar processes. These processes include lateral mass transfer, probably by viscoelastic flow, of the material needed to inflate areas of strong uplift and to translate blocks laterally by tectonic rafting. Directions of mass transfer vary but are commonly both parallel and normal to the direction of extension or parallel to the boundaries of extended areas. Much of the strong vertical structural relief probably reflects boundary-condition or localized perturbations (in the form of restraints or releases) of the lateral flow system. These conclusions are consistent with modern concepts of the flexural strength and rheological stratification of the crust in regions undergoing extension.

Geological Society of America Bulletin↗
USGS1988

Map of fault scarps formed in unconsolidated sediments, Tooele 1 degree by 2 degrees quadrangle, northwestern Utah

This report is one of a series of studies of western Utah (Anderson and Bucknam, 1979; Bucknam and Anderson, 1979b), eastern Nevada (Barnhard, 1985), and central Nevada (Wallace, 1979) that shows the distribution, relative age, and amount and extent of surface rupture on Quaternary fault scarps. Previous paleoseismicity studies in the Tooele 1°x2° quadrangle include reports by Gilbert (1928), Bucknam (1977), Anderson and Miller (1980), Bucknam and others (1980), and Everitt and Kaliser (1980). Geologic and geophysical data pertinent to paleoseismicity of the Tooele 1°x2° quadrangle can be found in reports by Mikulich and Smith (1974), Cook and others (1980), Smith and Bruhn (1984), and Arabasz and others (1987). The present study is a continuation of mapping fault scarps in 1°x2° quadrangles in the Basin and Range province; the purpose is to determine the youngest surface-faulting age and distribution of fault scarps for earthquake-hazards analysis. The accompanying map shows 10 areas containing late Quaternary fault scarps formed on unconsolidated sediments in the Tooele 1°x2° quadrangle, northwestern Utah. The faulted sediments include sandy, pebbly, to bouldery colluvium, alluvium, and lacustrine deposits. They are inferred to have been deposited in Quaternary time during the latest phase of basin-filling sedimentation. Because the map shows only scarps on unconsolidated sediments, it does not show all faults that could produce earthquakes. A comprehensive map of all Quaternary faults would also include those in bedrock and at the contact of bedrock and alluvium, as well as Quaternary faults that appear only as lineaments in surficial deposits. Initial study involved photointerpretation of high-quality 1:60,000-scale black and white aerial photographs of the map area; offsets as small as 1 m could be detected from these photos if the faults cut planar surfaces of considerable extent. A map of suspected fault scarps, identified from the photointerpretation, was compiled at a scale of 1:250,000 and served as a guide for subsequent field studies in conjunction with a similar map from Bucknam (1977). We have chosen not to connect discontinuous segments of aligned fault scarps unless there is compelling evidence of continuous surface rupture. Field studies consisted of (1) confirming scarps as fault related and deleting from the preliminary map those features that were not fault related or that had no surface offset (lineaments), (2) searching for stratigraphic indicators of amount and age of offset, and (3) measuring surface profiles according to procedures described in Bucknam and Anderson (1979a). We profiled selected fault scarps where they were sufficiently continuous and unmodified by fluvial deposition or erosion. In the early morning or late afternoon when the sun was at 1ow angles, additional field searches were made for scarps resulting from ground ruptures that had not been identified during the photo interpretation phase of the study. No additional scarps were found. Techniques that yield the relative age of fault scarps can be used as a guide to the frequency and location of relatively large earthquakes. For large regions, geologically determined rates of seismic activity in the Basin and Range province are generally in good agreement with rates determined from historic seismicity (Bucknam and Algermissen, 1984). Fault-scarp studies such as this have been used as a guide in locating boundaries of seismic source zones used in probabilistic earthquake hazard estimates of the Basin and Range province (Thenhaus and Wentworth, 1982; Bucknam and Thenhaus, 1983). We will first discuss the geologic setting of the map area and attributes of fault-scarp morphology, and then discuss the individual fault zones.

Utah↗