TRANSPRESSION, OROGEN-PARALLEL EXTRUSION, AND THE FORMATION OF MANTLED GNEISS DOMES

There are two north-south trending sets of gneiss domes in the New England Appalachians. The western belt contains thirteen domes that expose either Laurentian basement rocks or 475 Ma rocks of the Shelburne Falls arc (SFA). The eastern belt contains twenty-one gneiss domes cored by either Avalonian (?) crust or 450 Ma rocks of the Bronson Hill arc (BHA). The dome belts are flanked by Silurian to Early Devonian basins that formed just before the Acadian orogeny: the Connecticut Valley trough (CVT) and the Central Maine terrane (CMT). During the Acadian, km-scale nappes from the CVT and CMT were transported westward over the regions now occupied by the gneiss domes. The domes are elongated north-south and surrounded by high-strain zones (HSZ) that separate the core gneisses from the mantling units. The nappes were refolded during doming and units were dramatically thinned or omitted in the HSZ. In the Chester dome in Vermont, sense of shear indicators suggest that rocks above the high-strain zone were displaced southwest relative to rocks below it. P-T paths of rocks from below the HSZ in the Chester dome indicate decompression of several kbars during metamorphism, whereas rocks above the HSZ record nearly isobaric conditions. This pattern is consistent with normal-sense displacement between the core of the dome and its mantling sequence during Acadian metamorphism.

The domes occur in a region where the Acadian orogen is atypically narrow. Mechanical decoupling along the HSZ occurred between the quartz-feldspar-rich core gneisses of the domes and the overlying nappes of mica-rich metasediments. Pressure estimates of approximately 10 kbar indicate significant tectonic loading in the core of the Chester dome, presumably during the westward transport of nappes from the CVT. East-west shortening and vertical thickening of the quartz-feldspar-rich gneisses below the nappes would have required enormous work against gravity. It was energetically more favorable for the gneisses to be extruded northward where the orogen was wider and not as dramatically thickened. During extrusion the core gneisses cut upsection into the nappes of Silurian rocks and the thickness of the Lower Paleozoic section was dramatically reduced. The New England gneiss domes owe their north-south elongation and streamlined shapes to this flow pattern.