ELEVATED MIOCENE GEOTHERMS ON THE HIGH PLAINS OF EASTERN NEW MEXICO AND THE TEXAS PANHANDLE FROM APATITE FISSION-TRACK AND (U-TH)/HE THERMOCHRONOMETRY

The Rio Grande rift and southern Rocky Mountains are characterized by high elevation, high heat flow and low density mantle, contrasting with the eastward decay in elevation and increased lithospheric thickness toward the cratonic interior of North America. Resolving the nature of the transition between these regions on the High Plains is important for characterizing the eastward encroachment of Sevier-Laramide contraction, mid-Tertiary magmatism, and Late Tertiary magmatism/extension. We used apatite fission-track (AFT) and (U-Th)/He thermochronometry for twelve Triassic sandstone samples collected in a region stretching from the eastern Rio Grande rift to the High Plains (encompassing the eastern half of New Mexico to the Texas panhandle) to address this problem. The AFT data define the base of an eastward tilted middle Cenozoic partial annealing zone (PAZ, 110 C isotherm) on the High Plains, with dates that range from late Oligocene (ca. 28 Ma) near Santa Fe, to Triassic (>200 Ma) in the Texas panhandle. The middle Cenozoic AFT PAZ intersects the land surface near Santa Rosa, NM, and is at a depth of ~840 m in a well near Logan, NM. Apatite (U-Th)/He dates are Miocene (12 to 21 Ma) well onto the High Plains, and define the base of the apatite He partial retention zone near Tucumcari in eastern New Mexico, ~80 km east of Santa Rosa. Together the data suggest that Miocene geothermal gradients on the western High Plains were > 40 °C/km, significantly higher than typical “cratonic” geotherms. The results document heating and unroofing coincident with middle Cenozoic magmatism and extension significantly further east than previously recognized, and may require intrusive magmatic inputs in areas lacking extrusive equivalents to induce the observed effects.