PROTRACTED EXHUMATION ALONG THE FOOTWALL OF THE EASTERN BORDER FAULT OF THE HARTFORD BASIN, CT USA

New field mapping in the Durham Quadrangle in south-central Connecticut, has identified exhumed fault rock structures associated with oscillating brittle-ductile deformation in the footwall of the Eastern Border fault (EBf). Here we report a suite of structures and fabrics ranging from undeformed pegmatite dikes to mylonitic schists and gneisses, to mylonites, phyllonites, and annealed and foliated that reflect continuous deformation during exhumation from lower crust to near surface conditions. Regional anatectic conditions are preserved in ubiquitous granite pegmatites that filled fractures. Many pegmatites in straight walled dikes were subsequently folded and stretched into boudins, documenting brittle fracture followed by high-grade ductile deformation.

In the easternmost areas of the quadrangle strongly foliated amphibolite-facies gneisses reflect ductile deformation with fold hinges plunging shallowly SW. These rocks are dominantly plagioclase gneiss with layers of (calc-alkaline) amphibolite. In high strain localities amphibolites show sub-horizontal amphibole and feldspar lineations. At slightly lower grades open folds in amphibolite are locally cut by kinematically-linked thrust faults. Where fluids allowed retrogression reaction weakening produced biotite dominated mylonitic schists with quartz veins and ribbons. These mylonitic schists grade westward from biotite dominant to mixed muscovite + biotite mylonitic schists. Nearest to the EBf a muscovite-quartz phyllonite suggests top to the southeast (reverse) sense of shear.

Also near the fault, foliated cataclastites suggest the comminution of feldspar in a brittle to ductile stick-slip setting near the bottom of the seismic zone. This foliation was then folded into shallowly dipping folds with axes striking northeast consistent with NW-SE maximum shortening direction.

At near surface (zeolite facies) conditions deformation continued as slickensided brittle faults. The faults found in syn-extensional Mesozoic cover sandstones of the EBf hanging wall trend ~140 with a dip of 65 NW with normal dip-slip dominant motion. Thus the footwall of the EBf preserves a suite of structures formed during exhumation and the transition from Alleghanian collision to Mesozoic extension.