TEXAS THREE STEP – THE EXHUMATION AND EXPOSURE HISTORY OF THE LLANO UPLIFT ALONG THE LAURENTIA AND GULF OF MEXICO RIFT MARGINS

The Llano Uplift of central Texas consists of Grenvillian metasedimentary and igneous rocks (~1288-1233 Ma) and late orogenic granites (~1120 Ma). These rocks are unconformably overlain by a condensed section of Cambro-Ordovician sandstone and limestone, Pennsylvanian limestone and shale, and Cretaceous limestone. While major unconformities separate these tectono-stratigraphic packages, the long-term exhumation and exposure history of the Llano Uplift from the Mesoproterozoic to the Cenozoic remains relatively poorly understood. This study presents new zircon and apatite (U-Th)/He data from across the Llano Uplift as well as detrital zircon U-Pb-He double dates of Cenozoic strata surrounding the Llano Uplift to elucidate its long-term cooling exhumation and exposure history. Zircon (U-Th)/He (ZHe) ages cluster between 800-850 Ma in the western Llano Uplift and between 570-770 Ma in the eastern Llano Uplift. ZHe ages show no [U]effective or grain size correlations, suggesting rapid cooling and exhumation during Cryogenian and Edicaran-Cambrian Rodinia rifting as well as implying post-rift residence in the upper crust (<4 km) and limited late Paleozoic burial in the Ouachita foreland. Apatite (U-Th)/He (AHe) ages show relatively uniform ages across the uplift clustering between 150-160 Ma, recording the exhumation of the Llano Uplift as the rift flank during initiation of Gulf of Mexico opening. These data constrain the timing/magnitude of Mesozoic unroofing prior to the Cretaceous transgression and burial (<2 km). Lastly, detrital zircon U-Pb-He double dating of Cenozoic strata surrounding the Llano Uplift point to Early Miocene exhumation and erosion of Llano basement rocks and likely the entrenchment of rivers due to epeirogenic uplift. Detrital zircons with Grenville U-Pb ages and Cryogenian-Edicaran ZHe ages, diagnostic of Llano Mesoproterozoic basement, are not present in Paleogene sedimentary rocks of central Texas, but appear in Early Miocene strata. The combination of low-T thermochronometry and detrital U-Pb-He double dating paints a picture of the Llano Uplift as a crystalline basement block that was exhumed during both Rodinia and Gulf of Mexico rifting and remained shallow in the crust throughout the late Precambrian and Phanerozoic, and experienced early Cenozoic epeirogenic uplift.