Bennie W. Troxel

Death Valley bright spot: A midcrustal magma body in the southern Great Basin, California?

A previously unrecognized midcrustal magma body may have been detected by COCORP deep seismic reflection profiles in the Death Valley region of the southern Great Basin. High-amplitude, relatively broad-band reflections at 6 s (15 km) are attributed to partially molten material within a subhorizontal intrusion. This “bright spot” extends laterally at least 15 km beneath central Death Valley. A moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000-yr-old basaltic cinder cone. The fault zone is inferred to have been a magma conduit during the formation of the cinder cone. Vertical variations in crustal reflection character suggest that the Death Valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and/or intruded lower crust. The Death Valley bright spot is similar to reflections recorded by COCORP in 1977 in the Rio Grande rift, where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20-km depth.

Late Cenozoic history and styles of deformation along the southern Death Valley fault zone, California

Late Cenozoic deposits in the southern Death Valley region have been offset ∼35 km by right-lateral, strike-slip faulting on the southern Death Valley fault zone since Miocene time. Virtually all slip took place prior to ∼1 m.y. ago along western traces of the fault zone. During the past 1 m.y., the eastern traces of the fault zone have been active and characterized by oblique slip, with a lateral component of only a few hundred metres. Movement along these eastern traces has formed normal faults and gentle-to-isoclinal folds that have uplifted fan gravel and lacustrine sediments as much as 200 m above the modern alluvial fan surface. Surveying of the longitudinal profile of the Amargosa River, which flows within the eastern traces of the fault zone, suggests that vertical deformation continues today. The 35 km of right-lateral offset, which is based on matching offset alluvial fan gravel with its source area, refines earlier estimates of 8 to 80 km of movement for the southern Death Valley fault zone, and it is consistent with the geometry of a pull-apart basin model for central Death Valley. Causes for the observed differences in the style and timing of movement of the eastern and western subzones are not well understood. The study area, however, is located a few kilometres north of the intersection of the southern Death Valley and Garlock fault zones. The Garlock fault zone changes its sense of movement from left-lateral strike-slip to east-vergent thrusting against the southern Death Valley fault zone. The resulting compression may have caused the shift in activity and the change in style of deformation along branches of the southern Death Valley fault zone.

Limitations on Right-Lateral, Strike-Slip Displacement, Death Valley and Furnace Creek Fault Zones, California

Sedimentary units of late Precambrian and Cambrian age in the southern Death Valley–Kingston Range region of eastern California contain several through going linear features with configurations that apparently place a several-mile limit on the total right-lateral displacement that may be assigned to the Death Valley–Furnace Creek fault system. Much greater magnitudes have been suggested previously. Precambrian paleogeologic contacts, recorded in the northeastward truncation of successively older units of the Pahrump Group by the unconformity at the base of the Noonday Dolomite, can be traced discontinuously as nearly straight features from the Kingston Range west-northwestward to the Panamint Range, about 75 miles. A belt of talc mineralization, which also trends west-northwestward, is marked by a crudely linear arrangement of deposits reflecting the distribution of a dolomitic facies of a carbonate unit in the lower part of the Pahrump Group. An “algal” dolomite unit, which forms the lower part of the Noonday Dolomite, thins to the southwest and terminates along a line that can be traced northwestward for about 25 miles along the east margin of Death Valley to Virgin Spring Wash and probably thence westward to the southern part of the Panamint Range. The plots of these linear features, all of which apparently cross either the Death Valley fault zone or Furnace Creek fault zone, or both, indicate that no more than 5 and 2 miles, respectively, of right-lateral, strike-slip movement has occurred on these zones. Indeed, the position and orientation of the lines require no lateral displacement at all for their explanation. Similar limitations on movement along Death Valley fault zone seem to be imposed by the presence, on the east side of Death Valley, of a wedge of shale and graywacke within the Noonday Dolomite. The wedge thins and becomes progressively finer-grained northeastward. It also contains a basal conglomerate with clasts of the “algal” dolomite and the Kingston Peak Formation. These features suggest a source area west of southern Death Valley. Right-lateral, strike-slip movement of 10 miles or more on the Death Valley fault zone, if restored, would anomalously juxtapose the clastic wedge with a terrane in the Panamint Range where the “algal” dolomite is preserved. The possibility that large-scale, right-lateral displacement in the region of central and southern Death Valley has been accomplished by crustal flexing, as advocated by some geologists, remains unsupported by the available structural data and is difficult to reconcile with the configurations of the geologic lines.

TECTONICS OF DEATH VALLEY REGION, CALIFORNIA

The north-northwest-trending Death Valley and Furnace Creek right-lateral slip fault zones border the Black Mountains, California. Other tectonic features in the Death Valley region appear compatible with this strike-slip strain system of northeast-southwest shortening and northwest-southeast relative extension. Accordingly, some of the Basin and Range structures of Tertiary to Recent age may be characterized by strike-slip rather than dip-slip faulting, and result from compressional rather than tensional dynamics.

Strata of Late Precambrian-Cambrian Age, Death Valley Region, California-Nevada

The Noonday Dolomite, Johnnie Formation, Stirling Quartzite, Wood Canyon Formation, and Zabriskie Quartzite, which seem best assigned to the late Precambrian-Cambrian interval, are a conformable and predominantly detrital part of the stratigraphic record of the Death Valley region. They can be traced from type localities in the Spring Mountains and in the well-known section in the Nopah Range southwestward to the Salt Spring Hills and Silurian Hills in southern Death Valley and southeastward to the Kingston Range-Winters Pass area. Satisfactory correlation of these widespread formations is facilitated by (1) regional persistence of most of the members as originally recognized in the Nopah Range, (2) the distinctive lithologic character of the Noonday Dolomite, and (3) the presence of two marker units--an oolite and a volcanic ash(?)--high in the Johnnie Formation. Isopachous contouring of the stratigraphic interval from the top of the Zibriskie Quartzite to the base of the Noonday Dolomite appears to outline a late Precambrian-Cambrian marine trough. The major axis of this trough strikes north-northwest and is approximately coincident with the present Amargosa Valley and southern Death Valley. Along this axis, the combined thickness of these strata or their apparent equivalents decreases progressively from nearly 12,000 feet in the Funeral Mountains to only a few hundred feet in the Marble Mountains 160 miles toward the south. The isopachous lines drawn near the southeastward bend in the Garlock fault zone are nearly parallel with this major break, and the projected zero contour is about 10 miles southwest of it. This pattern strongly suggests that the structural element south and southwest of the Garlock fault, generally referred to as the Mojave block, existed as a topographic high in late Precambrian-Cambrian time.

Tertiary extensional features, Death Valley region, eastern California

The southeastern part of the Death Valley region (Fig. 1) displays two remarkable structural features: turtlebacks (Curry, 1938) and the Amargosa chaos (Noble, 1941). The changing ideas during the past half-century about the origin of these features reflect the growth of understanding of the major aspects of Basin and Range tectonics. Although these features were initially believed to be related to thrust faulting, a consensus now exists that they are different aspects of widespread Tertiary extension associated with the development of the Basin and Range province. The evidence upon which this historical debate is based is discussed in the site descriptions presented herein.

Thematic Mapper and field investigations at the intersection of the Death Valley and Garlock fault zones, California

Analysis of processed images and detailed field investigations have provided significant information concerning the late-Pliocene and Quaternary evolution of the intersection of the Garlock and Death Valley fault zones. The imagery was used to determine patterns of sedimentation and age relationships on alluvial fans and to determine the geometry, styles of deformation, and relative ages of movements on major and minor faults in the study area. The field investigation often confirmed the inferences drawn from the images and provided additional tectonic and geomorphologic data about the Quaternary deformation of the region. All the data gathered in the course of this project support the contention that the Garlock fault zone terminates in the Avawatz Mountains and that the Death Valley fault zone continues south of the intersection for at least 50 km, forming the eastern boundary of the Mojave province.