Wanda J. Taylor

Relations between hinterland and foreland shortening: Sevier orogeny, central North American Cordillera

The tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens. The classic central Cordilleran foreland fold-and-thrust belt in the United States (Late Jurassic to early Tertiary Sevier belt) and the more internal zones to the west (central Nevada thrust belt) provide data critical to understanding the developmere of internal and external parts of orogens. The Garden Valley thrust system, part of the central Nevada thnkst belt, crops out in south-central Nevada within a region generally considered to be the hinterland of the Jurassic to Eocene Sevier thrust belt. The thrust system consists of at least four principal thrust plates composed of strata as young as Pennsylvanian in age that are unconformably overlain by rocks as old as Oligocene, suggesting that contraction occurred between those times. New U/Pb dates on intrusions that postdate contraction, combined with new paleomagnetic data showing significant tilting of one area prior to intrusion, suggest that regionally these thrusts were active before -85-100 Ma. The thrust faults are characterized by long, relatively steeply dipping ramps and associated folds that are broad and open to close, upright and overturned. Although now fragmented by Cenozoic crustal extension, individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike. We correlate the structurally lowest thrust of the Garden Valley thrust system, the Golden Gate-Mount Irish thrust, southward with the Gass Peak thrust of southern Nevada. This correlation carries the following regional implications. At least some of the slip across Jurassic to mid-Cretaceous foreland thrusts in southern Nevada continues northward along the central Nevada thrust belt rather than noaheastward into Utah. This continuation is consistent with age relations, which indicate that thrusts in the type Sevier belt in central Utah are synchronous with or younger than the youngest thrusts in southern Nevada. This in turn implies that geometrically similar Sevier belt thrusts in Utah must die out southward before they reach Nevada, that slip along the southem Nevada thrusts is partitioned

Shallow shear velocity and seismic microzonation of the urban Las Vegas, Nevada, basin

Las Vegas Valley has a rapidly growing population exceeding 1.5 million, subject to significant seismic risk. Surveys of shallow shear velocity performed in the Las Vegas urban area included a 13-km-long transect parallel to Las Vegas Blvd. (The Strip), and borehole and surface-wave measurements of 30 additional sites. The transect was completed quickly and economically using the refraction microtremor method, providing shear velocity versus depth profiles at 49 locations. The lowest velocities in the transect, nehrp d class, are near intrabasin faults found near Interstate 15 and Lake Mead Blvd. Calcite cementation of alluvium (a.k.a. caliche) along the Las Vegas Strip elevates Vs30 values to 500–600 m/sec, nehrp c class. Our transect measurements correlate poorly against geologic map units, which do not predict the conditions of any individual site with accuracy sufficient for engineering application. Some usda soil map units do correlate, and Vs30 predictions based on measurements of soil units match transect measurements in the transect area. Extending soil-map predictions away from the area of dense measurement coverage generally failed to predict new measurements. Further, for several test sites the predictions were not conservative, in that the soil model predicted higher Vs30 than was later measured (predicting lesser potential ground motion). Subsurface information is needed to build a Vs30 model extending predictions throughout Las Vegas Valley. A detailed stratigraphic model built by correlating >1100 deep well logs in Las Vegas predicts Vs30 better than surface maps, but again only in parts of the Valley well-measured for velocity. The stratigraphic model yields good predictions of our transect Vs30 measurements. It is less accurate, although at least conservative, when extended to sites away from the transect.

Connection between igneous activity and extension in the central Mojave metamorphic core complex, California

The development of metamorphic core complexes and associated low-angle detachment faults commonly is intimately associated with synextensional igneous activity. In most areas studied to date, the relation of magmatism to extension is obscured by imprecise dating and by the overprint of later tectonic events. We present data from the early Miocene central Mojave metamorphic core complex (CMMCC) which indicate that extension was accompanied by igneous activity, as reflected by prekinematic, synkinematic, and postkinematic plutons and coeval volcanic rocks deposited in the associated extensional basins. The principal intrusion is an early Miocene granite pluton exposed in outcrops across an area greater than 400 km2. Dikes adjacent to the pluton are common in the Mitchel Range, at The Buttes, and at Fremont Peak. The overall orientation of the pluton and associated dikes is west-northwest, roughly perpendicular to the extension direction. Results of U-Pb analyses on zircon from two pluton and two dike samples yield ages of 20 to 23 Ma. Two other dike samples yield inconclusive results. Synextensional basins formed by detachment faulting during the core complex development. Rocks in these basins compose the Jackhammer and Pickhandle formations and filled an elongate, NW trending trough more than 50 km long. The 4oAr/39Ar ages for tuff beds are as old as 23.8 +_ 0.3 Ma near the base of the lower Pickhandle Formation and as young as 21.3 +_ 0.5 Ma in the uppermost lower Pickhandle. Hence volcanism and plutonism are coeval. The diversity of intrusive relations relative to the timing and development of the mylonitic fabric in the CMMCC precludes any simple cause-and-effect relationship between magmatism and extensional deformation. Rather, magmatism and extension may have been localized at a releasing bend in a transfer-fault system which links extension in the CMMCC with extension in the Colorado River area to the east.

Characterizing anomalous ground for engineering applications using surface-based seismic methods

Shallow seismics are in demand today in tectonically active regions to characterize and classify sites for earthquake response studies. The surface-based seismic methods are the most widely used for this purpose. In developed areas, the passive-source methods, also known as microtremor methods, are popular because of their efficiency and because the available frequency content is appropriate to determine an average shear-wave velocity for the upper 30 m. This information is required by the International Building Code, which is used by many municipalities in the US and elsewhere.

Temporal changes in fault strike (to 90°) and extension directions during multiple episodes of extension: An example from eastern Nevada

In areas with prolonged extensional histories or multiple episodes of extension, temporally distinct extensional fault systems, including dip-slip-normal, oblique-slip, and/or strike-slip faults, may form. These fault systems may be spatially distinct, overlapped, or completely superimposed, and different fault systems may have different fault strikes and extension directions. Although fault strike is not a very sensitive indicator of principal stress directions, changes through time in fault strike of as much as 90° exceed the uncertainty. Overprinted extensional fault systems of different ages with significantly different (to 90°) fault strikes and extension directions provide strong evidence of changes in the regional stress field with time. Tens of millions of years of extension occurred in the Basin and Range Province of the United States and many temporally distinct fault sets formed through that time. We document at least four extensional episodes and the fault kinematics of each, where possible, in the central Hiko Range, eastern Nevada. The four Cenozoic extensional events are prevolcanic, synvolcanic, postvolcanic Miocene–Pliocene (?), and postvolcanic Pliocene (?)–Quaternary. The prevolcanic faults strike north-south and are interpreted as footwall faults to the regional Eocene–Oligocene Snake-Stampede extensional system, which caused approximately east-west extension. The synvolcanic faults are late Oligocene–Miocene age and strike east-west. Although they are rare and have minor offset, they accommodated north-south–directed extension. Postvolcanic faults, part of the Timpahute lineament, are approximately east-west– striking normal and northeast-striking oblique-slip faults. They also accommodated approximately north-south extension during Miocene time. The northeast-striking left-normal oblique-slip fault set formed as new faults and records a change between north-south and east-west extension in Miocene time. Later east-west extension occurred along the Pliocene (?) to Quaternary Hiko fault zone. These four overprinted extensional systems show two separate changes in normal fault strike of ∼90°, from north-south to east- west and from east-west back to north-south. Consequently, the extension direction similarly changed through time. We suggest that causes for the changes in direction of upper crustal extension include a change from stresses created by (1) plate interactions, which generated approximately east-west extension, to (2) mantle motions, which generated approximately north-south extension, and (3) back again. The extension related to mantle motions is associated with a band of volcanism that passed across the northern Basin and Range Province, moving generally from north to south, during the protracted period of extension. Therefore, a change from plate boundary– or interaction-derived stresses to mantle- or volcanism-related stresses and back again is possible.

The late Paleozoic Southwestern Laurentian Borderland

A broad region of late Paleozoic tectonism along the Laurentian margin from central Nevada (United States) to Sonora (Mexico), including offshore western terranes, is herein termed the Southwestern Laurentian Borderland (SLaB). Carboniferous–middle Permian sinistral translation within the SLaB explains a wide range of apparently disparate geologic observations, including (1) diachronous spatial distribution of regional structures and continental- margin sedimentary basins; (2) sub-regional unconformities and associated structures in central Nevada and Sonora; (3) displacement of the southwest part of the early Paleozoic passive margin, which now comprises the Caborca block in Sonora; and (4) distribution from central Nevada to Sonora of allochthonous lower Paleozoic deep-water sandstones with distinctive detrital zircons derived from northern Laurentia. Northward latitudinal translation of Laurentia by ~3000 km proposed by recent plate-tectonic models provides a mechanism for displacement of the continent relative to outboard western lithospheric domains. The SLaB was a sinistral transpressive plate boundary active from Mississippian to middle Permian time. As defined here, it directly followed the Antler collisional event and ended prior to the Sonoma orogeny.