Detrital Zircons Constrain Western North American Drainage Paths over the Last 10 m.y

Fluvial sand contains detrital zircon (DZ) grains, each with a robust U-Pb radiometric clock, which provide “born-on” dates that track a sand’s ultimate magmatic provenance.  Zircons can be multiply recycled, so the most useful grains come directly from a local source.  DZ evidence supports the following:  1. Before about 7 Ma, the eastern Snake River drained eastward into the Missouri/Mississippi and/or the Green/Colorado.  2.  The Late Miocene Bear River may have been integrated with streams of the Bonneville basin, as well as with the east-draining ancestral Snake.   3. In the last 7 m.y. the northern continental divide has migrated eastward, from a former position within the Idaho batholith.  4. During much of the Late Miocene extent of Lake Idaho, the western Snake River had southern headwaters in the Humboldt River basin and an outlet through northern California.  5.  After about 3 Ma, this western Snake became integrated into the Salmon/Columbia, through Hells Canyon.  6.  The Colorado River was integrated into Gulf of California by about 5 Ma, but before that its drainage may have been northward through eastern Nevada and Idaho.  7.  No pre-Pleistocene connections can be established between the upper Rio Grande and the upper Colorado.