Structural, thermal, and paleomagnetic constraints on the tectonic evolution of the Black Mountains crystalline terrane, Death Valley region, California, and implications for extensional tectonism

The Black Mountains, Death Valley, California, represent "new real-estate" developed as a result of extensional tectonism, synrift intrusion, and tectonic denudation. Unroofing of the crystalline complex has resulted in the preservation of a subhorizontal, undulatory fault as the modern erosion surface, forming a series of topographic and structural antiforms known as the Death Valley turtlebacks. Geologic characteristics suggest the range represents a pre-extensional crustal section unroofed along a westerly dipping detachment zone. Faulted and brecciated strata on the eastern portion of the range overly a gently east dipping detachment fault. The fault is locally intruded out by hypabyssal plutons and its easterly dip reflects eastward rotation associated with unroofing.;Thermochronologic (Ar/Ar and fission track) and geobarometric data provide time-depth constraints on the Miocene intrusive and unroofing history. Cooling histories reveal a diachronous cooling pattern of decreasing ages toward the northwest. The data suggest denudation of the western portion of the range of 10-15 km. Considering the current distance of the structurally deepest samples away from east-tilted Tertiary strata in the southeastern Black Mountains, these data indicate an average initial dip of the detachment system of 20;Paleomagnetic data from Miocene intrusions yield a dual polarity magnetization whose in situ directions are discordant with an expected late Cenozoic reference direction. The data suggest large-scale clockwise rotation after the core detached from stable terrane to the west, interpreted as oroflexure related to right-lateral shear along the Death Valley fault zone. Hanging wall Miocene-Pliocene strata are folded into a syncline with an axial surface coplanar with the axial surface of a synform in the underlying detachment. This suggests the turtlebacks are Tertiary folds (antiforms). Involvement of Miocene plutons in a thick zone of footwall tectonites subparallel to the turtleback surfaces support Tertiary folding. Folding may cause deactivation of the detachment and the formation of a younger, more planar fault system. The present Black Mountains frontal fault may represent this out-stepping of a normal-fault system away from a deactivated folded detachment.