NEW GEOCHRONOLOGIC AND GEOLOGIC MAPPING CONSTRAINTS ON THE EXTENT AND TIMING OF DEVELOPMENT OF THE ROGERSON GRABEN ALONG THE TRACK OF THE YELLOWSTONE HOTSPOT, IDAHO-NEVADA

The Neogene N-trending, ~15 km-wide, Rogerson graben is a prominent extensional structure emanating from the track of the Yellowstone hotspot at the confluence of the western and eastern Snake River plains. From the hotspot track in Idaho, the graben has been traced 80 km southward to a position north of the Jurassic Contact pluton in Nevada. Previous workers recognized that the graben filled with hot-spot-derived ignimbrites, and began to form at 10 Ma coincident with peak eruptions and the inception of subsidence of the central and eastern parts of the Snake River Plain. This work also established that the graben has an east-dipping master fault with fault slip continuing to the Pliocene. Our new mapping, in and south of, the Contact pluton reveals that the graben extends another ~50 km southward into Knoll basin, with the pluton acting as a transfer zone across which the graben steps to the west and has a west (rather than east) -dipping master fault. The mapping also indicates that the grabens west-bounding fault cuts the ~13–11 Ma Cougar Point Tuff and an undated, stratigraphically higher basalt flow. To better constrain the age of this fault we utilized U/Pb ages of detrital zircons to assess maximum depositional age (MDA) of sandstone above and below the basalt in conjunction with an ⁴⁰Ar/³⁹Ar incremental heating experiment on groundmass from the basalt. The basalt sample yielded a plateau age of 8.76 ± 0.27 Ma, and underlying and overlying sandstone beds yield single-grain MDAs of 10.1 ± 0.3 and 9.1 ± 0.5 Ma, respectively, which are consistent with the age of the basalt. These data indicate the fault postdates ~9 Ma. This new data, in conjunction our previously reported field and geochronologic data on the grabens master fault, indicates that development of the graben south of the pluton took place largely between ~9–8 and 3 Ma. This study recognizes that Rogerson Graben is ~130 km long with a transfer zone across the Contact pluton, and furthermore is interconnected with the regional Knoll-Ruby fault system that extends as far south as the Ruby-East Humboldt metamorphic core complex.