IDENTIFICATION OF A MESOZOIC METAMORPHIC CORE COMPLEX IN SOUTHEASTERN NEW ENGLAND

A symmetric pattern of Mesozoic extensional faults has been identified in SE New England. Mesozoic faulting of rocks here is well established by normal motion along the west-dipping Eastern Border fault zone (EBF) of the Hartford basin. However, during earlier Alleghanian shortening reverse motion along this fault zone must have accommodated shortening, leading to the exhumation of Acadian hanging wall rocks, as confirmed by cooling ages in W Conn.. The Bloody Bluff-Lake Char fault zone (BBLCfz) has a similar history of reactivation. The Putnam-Nashoba terrane (PNt) in the hanging wall of this fault overrode the Boston Avalon terrane (BAt) along the BBLCfz beginning in the Carboniferous. Sediments were shed onto the fault’s foreland Narragansett Basin and with continued shortening the PNt overrode these footwall rocks producing Alleghanian metamorphism. Subsequent reverse motion of this fault led to Km-scale differential exhumation in the western BAt. This exhumation exposed a west-facing metamorphic field gradient from lowest greenschist to kyanite grade, requiring significant normal reactivation (down to the NW) motion on the BBLCfz (Goldstein, 1989) that also produced the Mesozoic Middleton basin in Mass. (Kaye 1983). A pattern of decreasing amphibole cooling ages north and south of South Killingly CT (Wintsch et al. 1992) requires Km-scale synformal warping of the BAt with an axis that coincides with the major WSW-plunging synclinal axis of the Narragansett basin. Uniform biotite Ar ages of ~250 Ma across this zone constrain the warping to the latest Permian, implicating near N-S shortening at that time. In contrast to these faults other Alleghanian metamorphic field gradients southeast of the Beaverhead fault (Bhf) increase to the east. In the Tiverton area this gradient requires normal motion on a SE dipping Bhf and explains the fault’s truncation of footwall isograds in southern RI. A second east-facing metamorphic field gradient exists in the New Bedford area separated from the gradient in the Tiverton area by a proposed SE-dipping normal fault passing through the village of Westport. Together the above four faults flank an axial high in western RI with extension both east and west. This pattern reveals an early Mesozoic core complex in response to early extension preceding the opening of the Atlantic.