DEFORMATIONAL ANALYSIS OF THE MESOPROTEROZOIC SIXMILE ANTIFORM AND FLANKING SHEAR ZONES, SOUTH-CENTRAL LLANO UPLIFT, TEXAS

In the Llano Uplift, Grenvillian metamorphic rocks record a polyphase deformational and metamorphic history. Detailed mapping across the gently SE-plunging Sixmile antiform, Oxford and Enchanted Rock quadrangles, provides additional constraints on deformational kinematics and chronology. This regional, open fold is cored by Valley Spring Domain (VS) quartzo-feldspathic gneisses, with Packsaddle Schist Domain (PS) rocks (mostly graphite, biotite, and amphibole rich schists) along its flanks. Deformation appears moderate in the core of the antiform, and the fold is bounded by mylonitic western (“Cottonwood”) and eastern (“Oxford”) shear zones at or near the PS/VS contact.

At least five deformational events have affected this area. D1 is represented by a regional, penetrative NW foliation S1 (S2 of some other workers), formed during dynamothermal metamorphism. Associated with D1 are tight to isoclinal folds (F1), tectonic eclogite bodies, and PS transport. D2 created the two shear zones. The Oxford SZ has a NE foliation (S2) contrary to regional NW strike; S-C fabrics; retrogressed eclogite bodies; L-S tectonites; and prominent stretching lineations (L2) that in places trend roughly parallel to earlier F1 fold axes. The Cottonwood SZ has quasi-plastic mylonites with rotated porphyroclasts indicating dextral shear with NW transport of the overlying PS rocks, and sigmoidal schist bodies. An overprinting crenulation in some SZ rocks marks D3. Regional D4 folding created the Sixmile antiform. Late Paleozoic extension (D5) formed an E-W graben with an internal drag-fold-syncline of Paleozoic rocks.