TECTONIC WEDGING IN THE NORTHERN VERMONT APPALACHIANS: AN EMPLACEMENT MODEL FOR THE ALBITIC CORE ROCKS OF THE GREEN MOUNTAIN ANTICLINORIUM

The Green Mountain anticlinorium (GMA) consists of a middle greenschist garnet-albite core and a lower greenschist non-albitic cover. Kinematic and strain analyses of the Prospect Rock Fault (PRF), separating the core and cover, suggest that tectonic wedging played a significant role in juxtaposing these lithotectonic units during the D1 phase of the Taconic Orogeny. The PRF has a consistently west over east, hinterland directed, sense of shear prior to the Acadian Orogeny. The evolution of fabrics in the fault zone shows a transition from simple shear strain dominance to pure shear dominance culminating in an oblate finite strain ellipsoid for the western PRF. Quartz rods, folded S1 inclusion trails, and relic grains all support a simple shear dominated D1. Rf/Phi analyses on pre-D2 albites indicate that following the D1 emplacement of the albitic wedge, there was significant flattening along the present cover-core contact. The D2 flattening is recorded in the dissolution of pre-D2 albite porphyroblasts on stress facing sides, and the recrystallization of material into pressure shadows paralleling a shallowly dipping S2. The change in shear strain reflects the evolving trajectory of the wedge from horizontal emplacement to exhumation.

Exhumation of the wedge was partially accommodated by late D2 down to the east motion along the Burgess Branch Fault Zone (BBFZ) located east of the axis of the GMA. The P-T contrast between the upper and lower plate of the BBFZ cannot be fully accommodated through motion along the BBFZ. The strain history shows minimal extensional strain restricted to late D2 and early strain history suggestive of flattening parallel to S2. The P-T contrast is best explained through the wedging of high pressure rocks into the low grade cover rock during D1-D2 where a shallow roof fault was progressively over-steepened and underwent late D2 down to the east brittle extension.