LITHOSPHERIC HYDRATION GRADIENT AND ELEVATED OLIGOCENE HEAT FLOW ACROSS THE TRANSITION BETWEEN THE NORTH AMERICAN CORDILLERA AND CRATONIC INTERIOR (Invited Presentation)

The High Plains straddle the transition between the North American Cordillera and cratonic interior. The gradual eastward decrease of elevations across this region corresponds to gradients in surface heat flow, lithospheric thickness, and seismic wavespeeds. These observations imply a gradient in lithospheric modification, but its extent, timing, and character remain unclear. To address this problem we use low temperature thermochronometry of Triassic sandstone samples from an east-west transect across the High Plains of northern New Mexico and the Texas panhandle to resolve geotherm evolution and unroofing. Ten samples yield apatite (U-Th)/He (AHe) and apatite fission-track (AFT) dates that increase eastward, from 12±4 to 53±21 Ma and from 28±3 to 225±13 Ma, respectively. Time-temperature simulations of these data indicate that minimum post-depositional peak temperatures were attained during or prior to early Oligocene time and decrease eastward from ~125 to 75 °C. Samples at the eastern and western ends of the transect record substantial Oligocene cooling, while samples at the center also record a middle Miocene cooling event. When combined with available stratigraphic, geologic, and thermochronologic information, the results suggest that Oligocene heat flow values ~30 mW/m² higher than today were achieved 300 km east of the topographically abrupt surface expression of the Cordilleran front. Westward-deepening Oligocene erosion of 650 to 1800 m was followed by a middle Miocene decrease in geothermal gradients accompanied by <1 km of erosion. We propose that the Oligocene thermal anomaly, subsequent erosion, and distribution of Tertiary volcanism across this region may have been caused by a gradient in Laramide-age lithospheric hydration, followed by melt infiltration, warming, and associated surface uplift after Farallon slab removal. Thus, the modern surface elevation gradient mimics a transitional lithospheric boundary between the North American Cordillera and cratonic interior through processes that may commonly modify cratonic lithosphere during orogenesis.