Preliminary findings from a low temperature thermochronometry survey of the Salmon River Mountains, Idaho.

Abstract:

The landscape response to mountain building is multiphase, extending beyond periods of magmatism, uplift, and faulting into the protracted decline of high relief topography. It is in these later stages that landscapes provide a singular record of the geodynamics associated with the end of orogeny. The Salmon River Mountains of central Idaho erode the exhumed Mesozoic Atlanta Lobe of the Idaho Batholith as well as Challis plutons intruded during an Eocene bout of volcanism. Previous studies show this region experienced uplift and exhumation through the late Mesozoic into the Eocene. However, the present-day landscape is inconsistent with such antiquity. Low relief uplands incised by the Salmon River suggest these mountains may be the product of a rejuvenating landscape uplifted by post-Miocene epeirogeny. Here, we present preliminary apatite (U-Th)/He thermochronometry data that constrain the Eocene to present cooling history of bedrock from the Salmon River Mountains. Apatite Helium dates range from 20 to 35 Ma and show a decreasing trend toward the south. These results suggest cooling rates have been non-uniform across the study area since the Eocene. Such a pattern may be the result of northward tilting of interior Idaho by dynamic forces originating from the Snake River Plain. When compared with forward models of cooling given different geomorphic scenarios, our results suggest the Salmon River Mountains may not be as young as they seem. Instead, apatite Helium dates on the order of ~30 Ma at the base of the Salmon River Canyon are most easily explained if the present-day landscape was largely formed by the end of the Eocene. Preliminary results do not rule out the possibility of a younger landscape, however, and ongoing analyses should reveal the full picture of landscape evolution of central Idaho.