Contrasting Grenville-aged tectonic histories across the Llano Uplift, Texas: New evidence for deep-seated high-temperature deformation in the western uplift

Abstract

Models of doubly vergent orogens provide an excellent proxy for the Llano Uplift of central Texas, a Grenville-aged belt that consists of two portions with different structural styles, metamorphic grades, degrees of partial melting, and opposite directions of tectonic transport. Six phases of deformation at amphibolite-facies conditions are recorded in both portions of the uplift as a result of continent-continent and arc-continent collision. However, in the western uplift, field mapping and thin section analysis of microstructures and metamorphic mineral assemblages provide evidence for temperatures above the second sillimanite isograd and partial melting during both early and late deformation. These observations correlate well with numerical models for the Alps, which have identified a prowedge and retrowedge in the crust above a subducting slab. The western uplift is coincident with the retrowedge, located at greater depth in the orogenic pile, leading to greater temperatures and more melting as well as opposite vergence from the prowedge. A lack of discrete shear zones and opposite structural stacking and vergence in the western uplift, coupled with apparently greater temperatures and more widespread partial melting, suggest that the western uplift has a different tectonic history from the eastern uplift. Most notably, this study documents widespread and pervasive partial melting during uniformly distributed deformation, as well as abundant granitic intrusions during latest deformation in the western uplift. Analogue models readily accommodate these observations from both eastern and western parts of the uplift in the form of a bivergent orogenic wedge.