Grenville-Age Tectonic Evolution, Western Llano Uplift, Central Texas

J. Levine¹, B. Hunt², S. Mosher^1
1 Department of Geological Sciences, University of Texas at Austin, TX
² Currently with the Barton Springs/Edwards Aquifer Conservation District, Austin, TX 

The central Texas Llano Uplift records polyphase deformation and dynamothermal metamorphism associated with Grenville-age orogenesis. This paper documents the tectonic evolution of the western Llano Uplift through detailed structural mapping and kinematic analysis of gneisses and schists along a 20-kilometer transect of the Llano River in Mason and Llano Counties, Texas.

Six phases of deformation (D1-D6) produced local- to regional-scale folding (F₁-F₅) and fabrics (S₁-S₅) under uppermost amphibolite facies conditions. D₁-D₃ are constrained between 1254 +6/-4 Ma and 1126 +5/-4 Ma; D₄ and D₅ between 1126 +5/-4 Ma and 1076 ± 5 Ma. D₁ and D₂ produced isoclinal folds with a well-developed axial planar foliation parallel to compositional layering (S₀). F₂ folds plunge easterly and S₁/S₂ fabrics dip north to southeast. F₃ folds are tight with southeast plunging fold axes and an S₃ foliation sub-parallel to S₁/S₂ fabrics. F₄ and F₅ folds are open folds that refold all previous folds on an outcrop- to regional-scale and contain axial planar fabrics. F₄ folds have northeast striking axial planes, whereas F₅ folds have northwest striking axial planes with southeast plunging fold axes. Local late extension and shear zones (D₆) occurred, partly, synchronous with granite dikes dated by U-Pb geochronology at ca. 1076 Ma. Granitic and pegmatitic intrusions increase from west to east.

The eastern and western portions of the uplift have opposite senses of structural stacking and differing kinematics in all but the D₅ structures. This suggests the eastern uplift is dominated by arc-continent collision and the western uplift by continent-continent collision.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004