EFFICIENT FEEDBACK BETWEEN LATE CENOZOIC EXTENSION AND MAGMATISM IN THE DEATH VALLEY EXTENSIONAL REGION, CALIFORNIA

The Death Valley extensional region (DVER) has experienced multiple distinct episodes of late Cenozoic magmatism and deformation; however, their temporal and spatial relationship has not been fully understood. We present high-precision CA-ID-TIMS ²⁰⁶Pb/²³⁸U zircon ages for eight DVER plutons to resolve this. Initial ²³⁰Th-corrected ²⁰⁶Pb/²³⁸U zircon ages show nearly continuous plutonism between 13.2 and 10 Ma. Early plutonism was characterized by emplacement of shallow granitic stocks (Kingston Range Granite, Little Chief Granite, and granite of Rabbit Holes Spring). This was followed by voluminous intrusive magmatism at ca. 10.5 Ma, reflected in contemporaneous zircon dates from the Willow Spring Diorite, Smith Mtn Granite, and quartz monzonite of Gold Valley. A younger pulse of plutonism occurs at ~8.2 Ma with emplacement of compositionally diverse intrusions (granite of Deadman Pass and diorite of Furnace Site). DVER plutons are constructed incrementally over > 100 kyr and in rare cases instantaneously, at least within the age resolution of CA-ID-TIMS. The youngest zircon ages typically overlap with or are shortly followed in time by published cooling ages suggesting that plutons cooled rapidly and are syn-extensional.

Integrating DVER pluton geochronology with documented large-scale faults allows us to create a simple tectonic reconstruction. The timing between plutonism and deformation events suggests that voluminous intrusive magmatism in DVER likely facilitated extensional reactivation of a segment of the low to moderately west-dipping Kingston Range-Halloran Hills detachment, known as the Amargosa-Black Mountains detachment fault. Magmatism was concentrated in a narrow zone parallel to the strike of the fault. In detail, our reconstruction shows that ca. 13.2–10 Ma plutons follow a westward-younging trend, with a possible northward component, and that younger ~8.2 Ma plutonism shifted farther northward during the onset of NW-directed dextral transtension. The role of magmatism is largely ignored in most DVER tectonic models, yet our results indicate that there is efficient feedback between late Cenozoic extension and magmatism and that the latter affected the location and magnitude of subsequent deformation events.

Prepared in part by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-865117