DEFORMATION AND MINERALIZATION HISTORY OF THE MESOZOIC EASTERN BORDER FAULT, HARTFORD BASIN, CONNECTICUT AND MASSACHUSETTS

The western edge of the Mesozoic, half-graben Hartford Basin is defined by a group of normal faults collectively known as the Eastern Border Fault (EBF) that strike north-northeast and dip between 25° and 40° west. These faults juxtapose Paleozoic metamorphic rocks against basin fill sedimentary and volcanic rocks. By examining existing geologic maps of the region it is interpreted that the EBF consists of at least eight individual faults with arc-shaped surface traces between 10 and 40 km long that stretch from the southern end of the Hartford basin at Long Island Sound and its northern terminus near Greenfield, Massachusetts.

We have analyzed and sampled four EBF outcrops. Three of these expose transects across the fault's damaged zone in Paleozoic basement rocks, but do not expose sedimentary or volcanic rocks within the basin. Paleozoic rocks contain mylonitic ductile deformation textures across a zone 20-200 m wide that are progressively cut by semi-brittle and brittle deformation near each exposure's western edge (i.e. closer to the core of the fault). Brittle fault rocks at each exposure contain sporadic to pervasive silica and calcite mineralization that precipitated concurrently with fault slip. Multiple cross-cutting quartz and calcite veins suggest that mineralization alternated between carbonate and silica. Early-formed veins are fine-grained and are highly deformed by crystal plastic flow and cataclasis, and are cut by later, coarser-grained, less-strained veins.

The fourth fault EBF site analyzed exposes both Paleozoic basement rock and Mesozoic basin-fill basalt (on the east and west sides of CT Highway 77 respectively). Field relations and sedimentation patterns in this area suggest that basalt eruption and sedimentation occurred concurrently with EBF slip and that the EBF defined a significant topographic scarp. Fault splays cutting basalt contain quartz and calcite mineralized gouge and breccia. Semi-brittle deformation textures in quartz and calcite veins (including crystal plastic flow with grain boundary migration crystallization and cataclasis) cut cataclastically deformed basalt. This suggests burial and heating of the basalt during faulting, with deformation at temperatures up to ~300°C, prior to exhumation to the currently exposed structural level.