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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, fi eld 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 identifi ed 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.

The fault boundary between the western and eastern Blue Ridge (WBR-EBR) in the southern Appalachians separates Mesoproterozoic basement rocks and their cover from Neoproterozoic to Paleozoic accreted rocks. Several northeast striking faults delineate the boundary, including the Gossan Lead shear zone in northwestern North Carolina. Varying tectonic interpretations of WBR-EBR boundary include a premetamorphic fault, an Acadian dextral strike-slip fault, or an Alleghanian fault. We use field-based, microstructural, and theromochronometric analyses to determine the conditions, kinematics, and timing of deformation, in order to distinguish among competing hypotheses for the Gossan Lead shear zone. This comprehensive approach has allowed us to attribute a number of new and previously observed tectonic fabrics to specific orogenic events; key relationships necessary to the study of multiply deformed tectonic margins. Detailed mapping and microstructural analysis of the Gossan Lead shear zone document a several kilometer-wide mylonitic zone with kinematic indicators that record dominantly top-to-the-NW thrust motion, with local strike-slip and normal sense indicators. Dynamically recrystallized quartz and feldspar constrain a range of deformation conditions from amphibolite to greenschist facies. Two unaltered lineation-forming amphiboles from mylonitic amphibolites record 40 Ar/ 39 Ar cooling ages of 347-345 Ma, and a mylonitized metagraywacke records a muscovite 40 Ar/ 39 Ar cooling age of 336 Ma. These data are consistent with dominantly NW directed thrusting along the Gossan Lead shear zone at amphibolite to greenschist facies conditions, and rapid cooling in the Middle Mississippian. We suggest these data support overprinting and/or reactivation of an earlier structure along this complexly deformed boundary by 336 Ma. LEVINE ET AL.3500