Zircon Geochronology of Volcanic Rocks from the Trans-Pecos Orogenic Belt, Western Texas: Timing the Cessation of Laramide Folding, Uplift, and Post Flat-Slab Ignimbrite Flare Ups

This study refines regional differences in the cessation of Laramide compressional deformation and the subsequent initiation of episodic slab-asthenosphere influenced ignimbrite-rhyolite volcanism in West Texas by utilizing LA-ICP-MS U-Pb zircon geochronology. Ages have been obtained for volcanic rock samples that occur both above and below the angular unconformity demarcating compressional deformation structures below and a lack of contractional structures above, as well as the maximum depositional ages for Tertiary siliciclastic formations present below the unconformity. By assessing the provenance of the detritus age-spectra from the siliciclastic formations the eastward migration of the magmatic arc, subsequent uplift, and arrival of the flat-slab to the Texas-New Mexico border (~47 Ma) and beyond has been tracked by assessing the increasing abundances more proximal detritus in each progressively younger formation. The volcanic results suggest cessation of shortening associated with Laramide-style deformation and uplift in the northern Trans-Pecos region occurred by ~38-36.5 Ma (Eocene), but persisted in the south e.g., in the Big Bend National Park region until ~31 Ma (Oligocene). Pre-Basin and Range Tertiary volcanism and compressional deformation associated with the Laramide orogeny in the Trans-Pecos region of West Texas is attributed to: 1) the subduction of thick, buoyant oceanic crust on the Farallon slab causing a major slab flattening period that resulted in magmatic arc volcanic activity progressively migrating eastward to Trans-Pecos Texas; 2) Laramide compression, uplift, and accelerated erosion directly preceding or synchronous with arrival of the flat-slab; 3) a quiescent period or gap in volcanism upon arrival of the flat-slab associated with an extensive Farallon-North American lithosphere contact; and, 4) renewed volcanism usually in the form of ignimbrite flare-ups associated with the transition to steep slab and subsequent rollback. The marked age difference in post-angular unconformity ignimbrite deposits from north to south documented here is attributed to the early abutment and steepening of the flat-slab against a pronounced ancient cratonic lithospheric mantle keel east of the Davis Mountains in the north followed by rollback. A later related steepening perhaps facilitated by tearing of the slab in the southern Trans-Pecos area likely occurred with no direct collision of the flat-slab against a keel because of a less buoyant subducted lithosphere and less pronounced or absent cratonic lithospheric mantle keel to the south.