Detrital zircon U-Pb-He double dating constraints on Laramide hinterland magmatism, tectonism, exhumation, and sediment routing – insights from the Tornillo Basin, West Texas

Abstract:

Improved constraints on the magmatic, tectonic, exhumation, and sediment routing evolution of the SW US and NW Mexico during Laramide-Mexican orogenesis would yield key insight to hinterland tectonic controls on continental-scale sediment routing systems. Detrital zircon (DZ) U-Pb-He double dating allows linkage of provenance and source terrane thermal history by obtaining U-Pb crystallization and (U-Th)/He cooling ages from single DZ grains. Derived lag times (He date – depositional age) and closure intervals (Δt, U-Pb age – He date) can be leveraged to quantify hinterland exhumation history. DZ U-Pb provenance results show that Upper Cretaceous-Eocene strata in the Tornillo Basin of W Texas sourced via axial and transverse rivers from Laramide and Mesozoic Cordilleran magmatic arcs, Proterozoic basement, and Jurassic-Cretaceous sedimentary rocks exposed in the inverted Border Rift and Sevier-Laramide thrust belts. We present 96 new DZ U-Pb-He double dates from 7 Upper Cretaceous-Eocene sandstone samples. DZ (U-Th)/He dates comprise Late Cretaceous-Eocene (67%, n = 64/96) and Middle Jurassic-Early Cretaceous (30%, n = 29/96) modes. Short (~3 Myr for plutonic zircons) and intermediate (~21 Myr for volcanic zircons) lag times and closure intervals (~10 Myr for plutonic zircons; ~1 Myr for volcanic zircons) show that Laramide zircons were sourced from rapidly exhuming plutonic rocks and volcanic rocks of variable ages, consistent with combined Laramide magmatism and tectonic uplift. Cordilleran plutonic zircons comprise intermediate lag times (~20 Myr) and closure intervals (~20 Myr) that suggest slower exhumation. Jurassic zircons with intermediate lag times (~23 Myr) and long closure intervals (~77 Myr) were sourced from Nazas arc rocks exhumed during Sevier-Laramide tectonism, whereas long lag times (~63 Myr) and closure intervals (~45 Myr) reflect erosion from Jurassic-Cretaceous sedimentary units of the Border Rift. These results demonstrate that sediment routing through the Tornillo Basin was the result of rapid exhumation of Laramide magmatic rocks, basement and sedimentary rocks exhumed during Laramide tectonism and Border Rift inversion, and protracted exhumation of the Cordilleran arc, and thus elucidate the nature of the SW US-NW Mexico landscape during Laramide-Mexican orogenesis.