TAPPING TEXAS INTO PLACE: A PARAUTOCHTHONOUS INDENTER MECHANISM FOR THE ANCESTRAL ROCKY MOUNTAINS OROGEN (Invited Presentation)

Tectonic models seeking to explain late Paleozoic intraplate deformation of the Ancestral Rocky Mountains (ARM) orogen, as much as 1000 km from time-equivalent mobile margins of Laurentia, suffer from several shortcomings that include: (1) incorrectly interpreted subduction polarity of the closing Rheic Ocean; (2) implicit assumption that all phases of the diachronous Alleghanian-Ouachita collisional orogen contributed equally to intraplate deformation; (3) unconvincing evidence for orogenic buttressing along the western and southwestern Laurentian margins; and (4) resulting inadequate explanation of stress propagation far into the Laurentian continental lithosphere. To remedy these shortcomings, we propose that a parautochthonous lithospheric indenter, termed here the Texas block, drove ARM deformation. Unlike the neighboring Precordillera terrane, the Texas block failed to separate from Laurentia, but experienced only modest extension on scattered faults during early Cambrian Rodinia breakup. Geologic evidence of rift extension includes juvenile early-middle Cambrian (~539-509 Ma) alkalic magmatism that roughly defines the margins of the block and contemporaneous syndepositional normal faults in SW Colorado, the Grand Canyon region, and southern New Mexico, which just preceded the final Sauk transgression at ~505 Ma. Hard collision of Gondwanan terranes with the corner of the Texas promontory, recorded by rapid subsidence of the Fort Worth and Kerr–Val Verde proforeland basins, imperfectly returned the Texas block to its approximate pre-breakup position in Atokan-Demoinesian time, coincident with dominant ARM basement uplift. Cambrian extensional structures within and flanking the Texas block were reactivated to form various ARM uplifts, including the Diablo, Pedernal, Uncompahgre, and Arbuckle-Wichita uplifts and the Central Basin Platform, among others, which inherited their diverse basement margins from the Cambrian faults. "Intraplate" deformation, a result of tapping the Texas block back into place, was largely complete by mid-early Permian time, as signaled by late Wolfcampian to Leonardian onlap onto many former ARM uplifts.