EXTREME NEO-ACADIAN CRUSTAL SHORTENING AND VERTICAL/LATERAL EXTRUSION IN NEW ENGLAND DUE TO OBLIQUE COLLISION/TRANSPRESSION ACCOMMODATED BY LITHOSPHERIC DELAMINATION

The Palmer Zone of Transpression (PZoT) within the Bronson Hill (BHZ) and western Central Maine zones (CMZ) formed in response to oblique convergence between Avalon-Meguma and Laurentia in the mid-late Paleozoic, resulting in crustal shortening of at least 5:1 between Maine and southern New England. Orogen normal flattening/reverse displacement and orogen parallel elongation/dextral displacement were each strongly partitioned within the PZoT into zones dominated by pure shear (orthogneisses) and simple-shear (paragneisses). All plutons within the PZoT/BHZ/CMZ are deformed (flattened, boudinaged) by the regional transpression system. All metapelites contain foliations/lineations that are synkinematic with high-grade Sil-Kfs-Grt-Bt+Crd assemblages or post-kinematic with retrograde Grt+Kfs => Sil-Bt assemblages. Both observations indicate shortening continued after crustal thermal conditions peaked. Geochronology shows that the preponderance of deformation across the PZoT/BHZ/CMZ system in Massachusetts occurred continuously throughout the Carboniferous. The style and timing of deformation extended west of the CMZ into the Greenwich syncline and Monson Gneiss of the BHZ. West of the Monson Gneiss, deformation is characterized by a regional sinistral shear system and monazite ages show a temporal overlap with dextral transpression to the east, with the bulk of ages ranging from ~325 Ma to ~275 Ma. This is consistent with dextral transpression, extrusion, and north-directed lateral escape of a segment of the middle crust in the Carboniferous. The extreme crustal shortening of the BHZ-CMZ from Maine to southern New England, pre-transpressional magmatism, and high-grade metamorphism appear to have occurred in the absence of subduction. We propose that crustal thickening in response to shortening that commenced at ca. 370 Ma resulted in lithospheric delamination and mantle advective heating of the crust. Loss of the crustal lithosphere resulted in crustal heating, high-grade metamorphism (the central Mass. Metamorphic high), and magma generation that peaked at 360-350 Ma. Delamination created space for continued shortening that was also accommodated by vertical extrusion of middle to upper crust. Heating led to softening of the lower to middle crust to accommodate lateral extrusion.