ELUCIDATING DRIVING FORCES OF EOCENE EXTENSIONAL TECTONISM IN THE BASIN AND RANGE THROUGH THE LENS OF THE TITUS CANYON FORMATION, DEATH VALLEY, CALIFORNIA

Geologic mapping, measured sections, and geochronologic data elucidate the tectono-stratigraphic development of the Titus Canyon extensional basin in Death Valley, California, and provide new constraints on the age of the Titus Canyon Formation, one of the earliest syn-extensional deposits in the central Basin and Range. Detrital zircon maximum depositional ages (MDAs) and compiled ⁴⁰Ar/³⁹Ar ages indicate that the Titus Canyon Formation spans 40(?)–30 Ma, consistent with an inferred Duchesnean age for a unique assemblage of mammalian fossils in the lower part of the formation. The Titus Canyon Formation preserves a shift in depositional environment from fluvial to lacustrine at ca. 35 Ma, which along with a change in detrital zircon provenance, may reflect both the onset of local extensional tectonism and climatic changes at the Eocene–Oligocene boundary. Our data establish the Titus Canyon basin as the southernmost basin in a series of late Eocene extensional basins that formed along the axis of the Sevier orogenic belt. The distribution of lacustrine deposits in these Eocene basins defines the extent of a low-relief orogenic plateau (Nevadaplano) that occupied eastern Nevada at least through Eocene time. As such, the age and character of the Titus Canyon Formation implies that the Nevadaplano extended into the central Basin and Range, ~200 km farther south than previously recognized. Development of the Titus Canyon extensional basin in the central Basin and Range precedes local Farallon slab removal by ca. 20 Ma, drawing in to question the role of Farallon slab removal in driving the initiation of Basin and Range extension. Revised chronologies of extensional basin formation from southern California to southern Idaho, combined with thermochronometric evidence, delineate an orogen-wide late Eocene extensional event, implying that plate boundary stress changes due to decreased convergence rates in Eocene time are a likely geodynamic drivers of early extensional collapse of the Basin and Range.