DEXTRAL TRANSPRESSION AND OBLIQUE CRUSTAL EXTRUSION IN THE ALLEGHANIAN OF EASTERN NEW ENGLAND

Much of the deformation in eastern New England associated with the assembly of Pangaea was accommodated along reactivated and new ductile fault zones between the Bronson Hill and coastal New England terranes. We propose that Acadian dextral transpression in this wide zone persisted into the Alleghanian of the late Paleozoic. Shortening in this convergent zone was opposed by Laurentian crust, a then cool and strong (far field) mechanical buttress with its eastern margin approximately coincident with the Connecticut River in northern New England, but subsequently overlain by the Hartford basin to the south. The oblique convergence of Gondwana against the outboard margin of the New York promontory caused shortening between the Laurentian buttress and the peri-Gondwanan terranes from coastal Maine south to Rhode Island. Southwest-directed transpression at ~45° to this buttress caused oblique crustal extrusion toward the southeast of the warm and soft crystalline rocks as thrust nappes. This deformation partitioning explains the apparent coincidence of dextral Alleghanian transpression in coastal New England with southeast verging thrust nappes. This interpretation has been long in coming because of the difficulty of confidently discriminating between Acadian and Alleghanian fabrics and metamorphism. However, new 40Ar/39Ar data and kinematic observations from the field support this hypothesis by suggesting dextral transpression and southeast vergent thrusting. Argon release spectra from muscovite from eastern Massachusetts and from phyllonites along the buttress in Vermont and Connecticut show the increasing dominance of white mica with crystallization ages of <300 Ma overprinting muscovite with Devonian cooling ages.