DETRITAL ZIRCONS FROM MIOCENE TO RECENT SEDIMENTS OF THE WESTERN CORDILLERAN OROGEN: OREGON-IDAHO GRABEN AND SNAKE RIVER PLAIN, IDAHO

New U-Pb detrital zircon SHRIMP geochronologic data from six 30- to 60-grain random samples from Miocene to Recent volcaniclastic and fluvio-lacustrine sands of western Idaho and eastern Oregon track drainage reversal related to formation of the mid-to late Miocene Oregon-Idaho graben (OIG), a north-south extensional feature truncated by the northwest-trending western Snake River Plain (WSRP).

Two-mica arkosic sands of the Sucker Creek Fm (ca. 15 Ma) located 18 km south of Adrian, Oregon, are dominated by 45-52 Ma Challis Volcanic grains and 80-105 Ma Idaho batholith grains with a small population of 250-260 Ma grains from Permo-Triassic accreted terranes. This provenance signature is similar to the ca. 9 Ma Table Rock sandstone in the Boise foothills and to the modern Boise River, suggesting that a south-draining paleo-Boise River system existed prior to formation of the OIG around 15 Ma. The provenance is distinct from streams draining the modern Owyhee Mtns, which lack Challis-age grains. Recycled Paleozoic and Proterozoic grains from the miogeocline are notably absent, confirming the mid-Miocene continental divide was west of the present Wood River and Pioneer Mountains, and that pre-Challis unroofing had already removed sedimentary cover from the Atlanta lobe of the Idaho batholith. Mid-Miocene drainage from the central Idaho thrust belt carried recycled zircons from the miogeocline southward or eastward, not into the WSRP and OIG.

Upper Miocene, 12 to 5? Ma synvolcanic sands within the OIG are dominated by 8-17 Ma locally derived volcanic grains and 38-42 Ma late Eocene grains, likely from northern Nevada. Zircons from central Idaho, such as Challis Volcanics and Idaho batholith grains, are absent. Therefore, a drainage reversal after 12 Ma is suggested, corresponding with the underfilled stage of the OIG, and with <10 Ma Miocene sands deposited by north- and west-flowing streams sourced in local rhyolitic volcanic highlands of the Owyhee-Humboldt and Bruneau-Jarbidge volcanic centers.

Sands from the Pliocene overfilled stage of the WSRP match modern Snake River provenance, recording migration of the continental divide to a position within the Idaho-Wyoming thrust belt. These data are consistent with previous studies documenting eastward migration of the continental divide and tumescent hotspot bulge.