HOW THE ALLEGHANIAN OROGENY REVEALS INSIGHTS ON THE ORIGIN OF THE EARLY MESOZOIC HARTFORD BASIN

The case for the broad terrane model to explain the present distribution of Early Mesozoic basins in eastern North America has been building for several decades. Our collective observations on rocks east and west of the Hartford basin (HB) of CT have coalesced to support this case and to put the evolution of this basin into a larger areal and temporal context. First, the Alleghanian orogeny in SE New England and east of the HB involved shortening and loading of crystalline thrust sheets in an out-of-sequence stack from Rhode Island west to the Bronson Hill terrane (BHT) immediately east of the present basin. These slices were all verging south and east on NW-dipping listric faults. One of these listric faults was to become the eastern border fault of the HB; it lifted the western margin of Peri-Laurentia over the BHT while keeping the thrust surface dipping moderately to the west. Migration of thrust-sheet development farther west of the HB was arrested by the Grenville crustal buttress that abruptly thickens near Cameron’s line in western Conn.(Long et al. AGU 2018). However, deformation of Peri-Laurentian rocks east of this buttress and west of the present basin is abundant. Alleghanian shortening increased the amplification of several Acadian domes and caused the development of retrograde Carb.. to Perm. schistosities during exhumation. Late Paleozoic westward tilting of a belt of Ordovician to Devonian schistose rocks extending NNE of Bridgeport produced a metamorphic field gradient from staurolite to sillimanite grade that is supported by the SE younging of 40Ar/39Ar cooling ages. Permian strike-slip movement along the East Derby shear zone and thinning of rocks to the east in the Orange-Milford belt was accompanied by extensive retrograde metamorphism. Clockwise rotation of shortening from a NW to nearly N direction during the final assembly of Pangea slowed the exhumation, lowered the slope of the eroding Peri-Laurentian surface, and led to the abrupt transition of that surface from erosional to depositional in the Triassic with no intervening weathered horizon. 40Ar/39Ar ages of detrital muscovite confirm a dominant eastern provenance for the arkosic sediments that filled the HB. Thus a broad westward slope of eastern Laruentia that established a ‘broad terrane’ was probably established by early Alleghanian times.