Grenville Tectonics In Texas: A Diachronous Orogeny

Complex deformation near the "Grenville" (Llano) Front in west Texas occurred ~65 m.y. after deformation ceased in the Llano Uplift of central Texas, the Texas Grenville orogen core. These later orogenic events are interpreted to result from complex plate interactions along the edge of an indentor subsequent to continental collision in the Llano region.

The Llano Uplift records collision of a southern continent and island arc with North America. Early high P metamorphism at 1147–1128 Ma results from subduction of the continental margin, which was followed by regional polyphase deformation at medium P. Uplift and cooling were rapid, potentially related to slab break-off, and were accompanied by low P metamorphism and plutonism from 1119 to 1070 Ma.

In west Texas, northward propagating progressive continental convergence is recorded at 1057-1035 Ma with late out-of-sequence-thrusts uplifting and juxtaposing rocks that deformed at different crustal levels at 1000 - 980 Ma. Rocks near Van Horn, TX, record polyphase folding and ductile shearing with NW transport in a dextral transpression zone and medium P metamorphism at 1057-1035 Ma that decreases northwestward approaching the foreland. In the foreland, polyphase folding with northward transport at mid/lower greenschist facies conditions and cross-cutting oblique dextral slip along brittle/ductile WNW-trending faults occurred at depth, and thrusting at the surface formed the synorogenic Hazel conglomerates. Uplift of the medium P metamorphic rocks at 1000-980 Ma occurred along the brittle out-of-sequence Streeruwitz thrust and other subsidiary thrusts. This deformation marks a change to NNE to NE directed transport. Lastly a series of complex domes and basins formed, most likely as a result of transcurrent motion on variously oriented faults. The timing and complex history of deformation is best explained by continued subduction along the continental margin after continental collision in the Llano region and deformation along the edge of an indentor.