GRENVILLE-AGE (1060-980 MA) DEFORMATION PARTITIONING NEAR FRONTAL THRUSTS IN THE CARRIZO MOUNTAIN GROUP, TRANS-PECOS TEXAS

Recent mapping of the ~1.33 Ga Carrizo Mountain Group (CMG), of Trans-Pecos Texas, in the “keystone” exposure of the NW Carrizo Mountains, shows how Grenville deformation in the hinterland was kinematically linked with that in the foreland. Despite significant changes in structural trends, dextral transpression was most common through time, and across the orogen.

Structural styles change cratonward (NW) across the CMG, approaching its contact with the foreland basin along the Streeruwitz thrust. Rocks exposed in the S Carrizo Mountains and further SE underwent Gt- to Sta-grade polyphase deformation as local combinations of mylonitic shearing and hm- to km-scale upright & recumbent folds. In the central Carrizos, Epi-Amph facies rocks form hm- to km-scale, upright to NW-verging F₁ folds, display limited refolding, and include local, D₁ and D₂, oblique dextral + reverse mylonitic zones

In contrast with this more penetrative deformation, in the Bt-grade rocks of the NW Carrizos, deformation was largely stratigraphically controlled, partitioned along contacts and into multilayer units (phyllite + thin quartzite and rhyolite beds) sandwiched between thick, upright, rhyolite and quartzite layers. Reclined, hm-scale F₁ isoclines with E-trending axial planes and moderately SSE-plunging axes occur mostly in the Mineral Creek Formation, above the ≤ 300 m-thick Carrizo Peak Metarhyolite. Below this metarhyolite, ENE-striking, top-to-the-NNW D₁ mylonites occur within the upper 100 m of another, lower rhyolite, ≤ 275 m thick, along its contact with the phyllite and quartzite of the Bass Canyon Formation. D₂ deformation occurs mainly as a NE-striking, SE-dipping S₂ foliation, parallel to structural trends further SE, that cuts across all units.

The mismatch between structural trends in the NW Carrizos and those to SE appears to be controlled by a deep structure that has also controlled structures in the Grenville foreland, and throughout the Phanerozoic.