EASTWARD MIGRATION OF THE CONTINENTAL DIVIDE IN IDAHO TRACKED BY DETRITAL ZIRCONS IN MIDDLE MIOCENE TO RECENT (16 TO 0 MA) SEDIMENTS OF THE SNAKE RIVER PLAIN

Detrital zircon (60-random grain) samples from the modern Snake River and its tributaries, the Oregon-Idaho graben and the western Snake River Plain demonstrate eastward migration of the continental divide, consistent with predictions of the Yellowstone-Snake River Plain hotspot model of Pierce and Morgan (1992; GSA Mem. 179).

Middle to late Miocene (16 to 10 Ma) sediments of the Oregon-Idaho graben, the Table Rock sandstone of the Boise foothills, and the modern Boise River contain similar detrital zircon spectra, defined by large populations of 100 to 70 Ma grains of the Atlanta lobe of the Idaho batholith, and 52 to 42 Ma grains from the Challis magmatic event, plus sparse 250 Ma grains from Permo-Triassic accreted terranes.

Late Miocene (7.5 Ma) Oregon-Idaho graben sediments contain distinctive 42 to 35 Ma late Eocene zircon grain populations from northern Nevada, which are distinctly younger than the Challis population from north of the Snake River Plain. Prior to this time, northern Nevada presumably drained southward, blocked from the Snake River by the Bruneau-Jarbidge and Twin Falls volcanic fields and associated highlands.

Significant numbers of recycled Paleoproterozoic Yavapai-Mazatzal (1780 to 1650 Ma), syn-Belt Supergroup volcanic (1450 to 1400 Ma) and Mesoproterozoic Grenville (1250 to 1000) grains are present in the Modern Snake River, and in western Snake River Plain sediments starting about 5 Ma. This suggests that thrust sheets of the Cordilleran thrust belt, that contain Mesoproterozoic, Neoproterozoic and Paleozoic strata that host these recycled grains, were east of the continental divide until the end of the Miocene, and that the Snake River as we see it today is a Pliocene feature.

This work demonstrates the power of detrital zircon studies in tracking drainage reversals, and in providing identifying “barcodes” for specific geologic provinces.