46983-AC8
Mechanisms of Crustal Extension: A Structural and Geochronological Study in the Death Valley Region of Eastern California
With apologies for submitting this first annual report well into the second year of the project, I am able to claim important progress on each element of the research. Very detailed structural mapping and kinematic analysis have been completed along the complex but discontinuously exposed Sheephead fault at Sheephead Mountain, California and its purported continuation in the Amargosa Valley, along with field checking of existing published mapping in the southern Black Mountains and Alexander Hills. Samples of Paleoproterozoic gneiss collected from the southern Black Mountains on the eastern flank of Death Valley have been partially processed for 40Ar/39Ar thermochronology, and samples of mostly extrusive intermediate-composition volcanic rocks collected from Sheephead Mountain. The latter suite is being prepared for 40Ar/39Ar geochronology as part of an undergraduate project during the summer of 2009 (Marissa Tremblay, Barnard College). Fieldwork has also been completed at locations along the Northern Death Valley-Furnace Creek strike-slip fault, with a view to constructing regional geological cross-sections, and evaluating key piercing points for the estimation of offset.
The character, slip sense and geological significance of the Sheephead fault have been resolved after decades of debate in the geological literature. The Sheephead fault is a bookshelf-style, west- to northwest-striking, right-slip fault zone composed of right-stepping oblique left-slip faults, and with a down-to-the-north component. Continuity to the west (Virgin Spring area of the southern Black Mountains) and to the southeast (Amargosa Valley) beneath younger cover is supported by the similar structural style of faulting in those areas. The fault was active mainly in late Miocene to early Pliocene time. Our minimum estimate of offset is ~6 km, based upon the correlation of misaligned folds that may once have been contiguous. In the absence of clear-cut piercing points, related to the generally different ages of rocks juxtaposed by the fault, our maximum estimate of offset based on geometrically constrained scaling of block rotation within the fault zone is ~18.5 ± 8.5 km.
These data favor crustal extension accommodated by a series of discrete normal faults and intervening tilted fault blocks. Although right slip on the Sheephead fault was accompanied by northwest-directed extension on either side, that deformation is kinematically incompatible with extensional transport to the northwest across the structure. These details cast doubt on purported large-scale displacement (tens of kilometers) in the hanging wall of the Amargosa “detachment” fault of the Black Mountains, and more generally on the rolling hinge model – a widely accepted explanation for the exhumation of crystalline rocks from depth in the Black Mountains. The Amargosa fault cannot have been a continuous structure while the Sheephead fault was active. Extreme crustal extension (< 400%) is inconsistent with our new data.
A re-evaluation of piercing points for the northwest-striking Northern Death Valley-Furnace Creek fault suggests right slip of ~26-47 km, and perhaps a little more than that towards the northwest. Taking crustal extension within juxtaposed blocks into account, we infer ~33-54 km of extension across Death Valley, equivalent to an increase of ~40-100% in the original line length.
A series of manuscripts is currently in preparation. Two, on the Sheephead fault and Northern Death Valley-Furnace Creek fault, will be included in the Ph.D. dissertation of Byrdie Renik, to be completed and defended by late summer, 2009. A third on the thermochronology and geochronology will depend on analyses to be undertaken during the summer and fall.
Byrdie Renik has accepted a job with Exxon-Mobil. A second Ph.D. student, Rafael Almeida, passed his oral qualifying exam. in spring, 2009, and is working on a comparable project on the eastern side of the central Basin and Range Province, east of Las Vegas, Nevada. Summer intern Lila Neiswanger (Columbia College) will be processing some of Rafael’s samples for isotope geochemistry. We are using Nd and Pb isotopes to evaluate the key piercing point for estimating crustal extension in this transect – the correlation of clasts of 1.45-billion-year-old rapakivi granite in middle Miocene avalanche breccias at Frechman Mountain with a purported source at Gold Butte, 65 km to the east. Preliminary geochronological and isotopic data suggest that this long-accepted interpretation may be incorrect, and therefore that crustal extension estimates in this area also may be too large. A third Ph.D. student, Marc Van Keuren, will join my group in the fall, with an extensional tectonics project yet to be defined.
As originally proposed, we envisaged a three-year project. Two years of funding were provided. I request a one-year extension of the project (to August 31, 2010) to permit completion of the planned analytical work for the Death Valley region.
