BIRTH AND EVOLUTION OF THE VIRGIN RIVER: POST 5 MA UPLIFT OF THE WESTERN COLORADO PLATEAU

The uplift history of the Colorado Plateau has been debated for over a century with still no unified hypotheses for the cause, timing, and rate of uplift. ⁴⁰Ar/³⁹Ar dating of semi-continuous basaltic volcanism over the past ~6 Ma within the Virgin River drainage system, southwest Utah and southern Nevada, provides a way to calibrate differential river incision and compare patterns of basaltic volcanism migration, mantle velocity structure, channel steepness, lithology, incision history and the birth and evolution of the Virgin River.

New detrital sanidine ages constrain the arrival of the Virgin River to a maximum depositional age of 5.8 Ma. Incision magnitudes and rates of the Virgin River show a stair-step increase in bedrock incision as the river crosses multiple N-S trending normal faults. Average calculated rates are 23 m/Ma in the Lake Mead block, 85 m/Ma in the combined St. George and Hurricane blocks, and 338 m/Ma in the Zion block. Block-to-block differential incision adds cumulatively such that the Zion block has been deeply incised ~1.1 km (~315 m/Ma) over 3.6 Ma relative to the Colorado River confluence. We test two hypotheses: 1) observed differential incision magnitudes and rates along the Virgin River system are a measure of mantle-driven differential uplift of the Colorado Plateau relative to sea level over the past ~5 Ma. 2) Observed differential river incision relates to river integration across previously uplifted topography and differential rock types with no post 5 Ma uplift.

Strong correlations exist between high channel steepness (k_sn) and low mantle velocities throughout the Virgin River drainage while weaker correlations exist between high k_sn and resistant lithologies. Basaltic volcanism, which has migrated at a rate of ~18 km/Ma parallel to the Virgin River between ~13 and 0.5 Ma suggests a possible mantle-driven mechanism for the combined observations of differential uplift across faults and additional young Colorado Plateau epeirogenic uplift tracked by headward river propagation. Thus, we interpret the Virgin River to be a < 4.5 Ma disequilibrium river system responding to ongoing upper mantle modification and related basalt extraction, which is driving ~ 1.1 km of young uplift of the western Colorado Plateau.