ACADIAN TRANSPRESSION, EXTRUSION, AND GNEISS DOME FORMATION IN THE NEW ENGLAND APPALACHIANS

Two N-S trending sets of gneiss domes in New England dominate the western Acadian belt. One belt is west of the Connecticut Valley trough (CVT) and is cored by fragments of Laurentian basement and rocks of the Early Ordovician Shelburne Falls arc (SFA). The other is west of the Kearsarge-Central Maine trough (KCMT) and is cored by Late Proterozoic gneisses and rocks of the Late Ordovician Bronson Hill arc (BHA). The CVT and KCMT contain Silurian and Devonian metasediments that were rapidly deposited just before collision between Laurentia and Avalon. Collision caused dramatic shortening of the Silurian and Devonian basin deposits expressed as large-scale, west-verging nappes, which were overprinted during the doming phase of the Acadian orogeny. The domes are commonly elongated N-S and some preserve evidence for shear zones between the quartzo-feldspathic gneisses in the cores and the overlying metasediments. The western Acadian belt is narrow (~50 km) in Conn., Mass., southern Vt., and southern N.H., but widens considerably in northern Vt., northern N.H., and Maine.

Crustal shortening during collision between Laurentia and Avalon appears to have been greatest in southwestern New England and to have decreased northward. Middle to upper crustal rocks were extruded westward as nappes from the CVT and KCMT over the SFA and BHA, respectively. In contrast, more rigid lower crustal blocks, including rocks of the SFA, BHA, and fragments of Laurentian basement, were extruded northward relative to Laurentia. Early crustal thickening by nappe formation may have made extrusion of rigid blocks more energetically favorable than horizontal shortening in the lower crust. During northward extrusion the quartzo-feldspathic blocks moved upward relative to the overlying metasediments producing extensional shear zones with NNE-SSW displacement. Ongoing E-W compression lead to N-S elongation and steep dips on the E and W limbs of the domes. Extrusion also produced differential uplift recorded by large and abrupt E-W variations in P conditions of Acadian metamorphism.