THERMAL HISTORY OF THE BLACK MOUNTAINS, DEATH VALLEY, CALIFORNIA

Wright and Troxel first appreciated “turtlebacks” of deep crustal rocks in the Black Mountains (Wright et al., Geology, 1974). In personal conversations Bennie would describe how: (1) the region had “structural tiers” with deeper crustal rocks that deformed in the ductile field below those in the brittle field, (2) there were impressive folds around the southern end of the Black Mountains, and (3) they had to map in the Funeral Range before they could make sense of the Black Mountains. Since that time the Funeral and Black Mountains have been recognized as essentially core complexes with metamorphic and igneous units bounded by low-angle detachment faults beneath Miocene and younger volcanic and sedimentary deposits. The Funeral Range lower plate has been well-documented to contain Mesozoic high-grade metamorphic rocks and even crustal melt products, as well as having undergone pre-Cenozoic exhumation. The Black Mountain lower plate, however, is largely known for Miocene intrusions into Proterozoic gneisses. Previous workers (e.g., Holm and Wernicke, Geology, 1990) have pointed out that the Miocene exhumation of the Black and Funeral Mountains postdated earlier exhumation, and Late Cretaceous-through-Paleogene pegmatite bodies and local fold-fault relationships of the same age have been identified in the Black Mountains (Miller and Friedman, Geology, 1999; Miller, Geology, 2003). Yet, the degree to which the Black Mountains was part of a larger orogenic system of elevated thermal gradients that persisted from the Late Cretaceous into the Paleogene remained uncertain. We (Lima et al., Geology, 2018) have reported new zircon U-Pb laser ablation depth profiling ages that show that the Mormon Point turtleback hosts felsic intrusions of Late Cretaceous age and that the surrounding gneisses were affected by Late Cretaceous-through-Paleogene heating. Thus, the Black Mountains are part of a broader system of elevated thermal conditions across the western US Cordillera. This system may either restore to one deep orogenic root or comprise a series of lithospheric “boudins”. In either case, the elevated regional thermal condition facilitated the maintenance of high elevations across the region and persisted for >35myr despite its position above the subducting Farallon plate.