LATE-MIOCENE TO PLEISTOCENE EXHUMATION ALONG THE UPPER ARKANSAS RIVER VALLEY SEGMENT OF THE RIO GRANDE RIFT IN SOUTHERN COLORADO

We present new apatite (U-Th-Sm)/He (AHe) low temperature thermochronology results from six vertical sampling transects in the upper Arkansas River valley in Colorado. The transects were collected in the footwall of the east-directed normal fault system of the Rio Grande Rift (RGR), which bounds the Sawatch Range, CO. Spanning elevations from ~2700 m in the valley to over 4000 m at the summits, the transects begin at Mount Shavano, just north of the Poncha Pass fault transfer zone and continue northward to Mount Princeton, Mount Columbia, Mount Belford, Mount Elbert and lastly Galena Mountain, located just north of Leadville, CO. Collected in the ~36 Ma (Fridrich et al., 1998) Mount Princeton Batholith, preliminary ages from Mount Princeton increase in elevation, from ~2 Ma at the range front to ~20 Ma at the peak. At Mount Belford, samples collected in Proterozoic igneous rock have AHe ages that range from ~6 Ma at low elevations to ~13 Ma at the peak. These late Miocene to Pliocene ages are absent at Mount Elbert, where we observe ~11 Ma ages at the lowest elevations and ages ranging from 30-38 Ma in the rest of the transect. Together, these results suggest that exhumation in the Sawatch segment of the RGR initiated in the middle Miocene (~10 Ma) and continues to the present at rates around 0.5 – 1 mm/yr. Such an exhumation history is both more recent and more rapid than observed in the Gore Range, to the north of our study, where exhumation initiated in the Oligocene and continued until ~7 Ma (Landman and Flowers, 2013). The observation of late Miocene to Pliocene ages in multiple transects and at a variety of elevations is inconsistent with interpretations that relate the young ages near the range-bounding fault to hydrothermal fluid circulation. Rather, we suggest that our data indicate significant rates of post-Pliocene exhumation on the RGR. Additional analysis of samples from these transects may help refine length-displacement scaling relationships for the Sawatch segment of the RGR, and may potentially delineate temporal or spatial patters in the initiation and rate of faulting along the Rio Grande Rift.