SHEAR ZONE EVIDENCE FOR EPISODIC UPLIFT ALONG A MESOZOIC WESTERN BORDER FAULT, HARTFORD BASIN, MASSACHUSETTS

Recent detailed analyses of fault breccia, chloritic microbreccia, cataclasites, and ultracataclasites present in the Devonian Hatfield pluton of western Massachusetts provide new evidence for the formation and evolution of the Mesozoic Hartford rift basin in this region. Previous gravity studies (Burger and Honeycutt, 1995) inferred the presence of more than 1.25 km of Mesozoic sedimentary deposits immediately adjacent to these shear zones. The juxtaposition of pervasive brittle shear zones and fault breccia in crystalline rocks alongside a thick sedimentary sequence suggests faulting along the western margin of the basin was an important factor in basin evolution. These shear zones display a definite sequence of slightly more ductile zones (dynamically recrystallized quartz) offset by more brittle zones (no dynamically recrystallized quartz). The younger, more brittle zones, also contain abundant hydrothermal epidote and are themselves cut by a two-meter thick fault breccia containing clasts exceeding 15 centimeters in diameter and pervaded by hydrothermal silica. These latest deposits contain fluorite and barite and most likely are coeval with nearby Mesozoic ore deposits. Shear zone mineralogy and offset history suggest the block of crystalline rock bordering the rift valley along its western margin experienced episodic uplift in the Mesozoic with displacements well in excess of one kilometer. Both shear zone and gravity observations support fault displacements along the western margin of the Massachusetts portion of the Hartford Basin that were at least one-third as extensive as along the presently exposed eastern border fault.