CONSTRAINTS ON LARGE-MAGNITUDE EXTENSION OF DEATH VALLEY, CALIFORNIA: INSIGHT FROM THE WORLD’S YOUNGEST RAPAKIVI GRANITES

Death Valley has been interpreted as having moderate to extreme amounts of extension. Large-magnitude slip is supported by reconstruction of the sediment source of the Miocene Eagle Mountain Formation for ~100 km of displacement (Niemi et al., 2001). The applicability of this marker was questioned by Christie-Blick et al. (2007). Additional reconstruction markers are needed to support, refine, or refute large magnitude extension.

A holistic approach is needed using as many reconstruction markers as possible. Pre-Miocene faults (Pennsylvanian to Late Cretaceous), Mesozoic plutons and Proterozoic Pahrump Group rocks show components of large-magnitude dextral and extensional offset in the Death Valley region.

An additional potential reconstruction feature is a set of middle Miocene rapakivi granites. The Little Chief granitic porphyry (LCG) of the Panamint Range is in the hanging wall of the Death Valley extensional system, whereas the porphyritic granite of Kingston Peak (GKP) of the Kingston Range is in the footwall. These two plutons have similar K-Ar and Ar-Ar ages, textures, intrusive structures, geochemical signatures and country rock units. The major difference between these plutons is their relative depth of emplacement, with the LCG being shallower. Reconstruction of these two plutons into a contiguous body yields a similar vector to the Eagle Mountain Formation’s.

This potential reconstruction marker is explored via modern crystallization age determinations using U-Pb CA-TIMS of zircons. Preliminary data for the GKP has a Th-corrected ²⁰⁶Pb/²³⁸U age of 12.203 ±0.019 Ma for the northern phase and 12.124 ±0.052 Ma for the southern phase, compared to 11.342 ±0.053 Ma for the southern and 11.305 ±0.025 Ma for the northern phases of the LCG. These new data are dissimilar but within the possible range of a cooling pluton. The relative age pattern of northern and southern phases rules out a simple matching of the plutonic bodies for a reconstruction marker. The LCG is interpreted to have intruded westward from a steeply dipping E-W fracture zone. A remnant of this fracture zone may be a set of granitic inclusions preserved along the along the contact of the northern and southern phases of the LCG. Further work is needed to determine the crystallization age of the inclusions in the LCG and the geometry of the western GKP.