METAMORPHIC PATHS RECORD THE CONTRACTION AND THE EXTENSION THAT LED TO FORMATION OF THE DEATH VALLEY TURTLEBACKS

The Death Valley turtlebacks have attracted study since the 1930’s when they were first described as fault-bounded, basement-cored antiforms. Concepts for the turtlebacks evolved as more data became available but by the 1990’s field studies together with U-Pb dating had established that most of the plutons were Neogene, syn-extensional intrusions, and that the high-grade metamorphism was at least in part Neogene. Those field studies established that second sillimanite conditions (sill+kspar) were reached during formation of the main fabric, with basement thoroughly remobilized with cover. What was poorly resolved, however, was the relative importance of Mesozoic vs Cenozoic metamorphism as well as the pressure conditions of the metamorphic episodes. Our study resolves this problem and illustrates the importance of many eyes focused on sampling in an area where previous limited sampling had established that likely lithologies occurred for comprehensive thermobarometric studies. Specifically, our sampling at Mormon Point turtleback led to discovery of a rich record of the PT paths. A long-lived, moderate pressure assemblage initially developed during Mesozoic contraction with peak PT conditions of 695C and 0.72GPa based on garnet cores with bt+plg +/- ky inclusions. Garnet rims record comparable temperatures (660-685C) but lower pressures of 0.53-0.61GPa, which are consistent with the observed migmatitic texture and sill +kfeld. Together these data support the hypothesis that these rocks initially were deeply buried by Mesozoic thrust systems and then heated to melting conditions in the latest Mesozoic-Paleogene. This is consistent with the presence of Eocene pegamatites. Partial exhumation, probably by post-contractional erosion, was followed by Neogene emplacement of a syn-extensional intrusive sheet above the turtlebacks that reheated the rocks to nearly 700C but at lower pressures. In general, these data support the conclusion that the Black Mountains lay beneath the Panamint Mountains prior to the Death Valley extension, but beg the question of why extensive Jurassic and Late Cretaceous plutonism has not yet been observed in the Black Mountains.