How have Terrane Accretion, Rifting, and Recent Mantle Dynamics Shaped the Lithosphere beneath the New England Appalachians?

Paul Karabinos, Williams College
Yantao Luo, Yale University
Roberto Masis Arce, Rutgers University
Kimberly Espinal, Yale University
Laura Webb, University of Vermont
Maureen Long, Yale University
Vadim Levin, Rutgers University
Yvette Kuiper, Colorado School of Mines

Abstract

Li et al. (2018) used data from the Earthscope Transportable Array (70 km station spacing) to propose a dramatic 15 km change from a deeper Moho in the west to shallower in the east in southwestern New England. The Seismic Experiment for Imaging Structure beneath CT (SEISConn) employed 15 seismic stations with approximately 11 km spacing in northern CT and RI to create a more detailed image of the lithosphere. We are now creating two more transects of closely spaced seismic stations in northern MA and from western VT to southern ME through the White Mountains of NH. The goals of this project are to image the crust and mantle beneath New England to search for features created during the Paleozoic accretion of Gondwanan-derived terranes and the formation of Pangea, the Mesozoic rifting of Pangea, and by recent mantle dynamics. We are also investigating the possibility that crustal shortening during later orogenic events displaced terrane boundaries in the shallow crust relative to the lower crust and upper mantle. Results from SEISConn and preliminary results from the western portion of the MA transect reveal that 45 km thick Grenville crust in western New England abuts 30 km thick crust approximately 25 km west of the easternmost surface exposures of Laurentian rocks. Furthermore, there appears to be a zone of overlap in Moho west of the step where eastern crust and its uppermost mantle was thrust over western crust. Other features include a strong positive velocity gradient (PVG) beneath the Mesozoic basin, and several west-dipping PVGs in the crust and mantle beneath CT. A prominent west-dipping feature in the mantle may be a slab subducted during a Paleozoic orogeny. Preliminary 40Ar/39Ar dates from white mica and biotite in fault zones indicate that the Green Mountain and Berkshire massifs were thrust westward during the Silurian and Devonian, suggesting that the Taconic orogen was displaced during later collisions. A vital question is the extent to which accreted terranes remain anchored to their underlying mantle lithosphere. The Northern Appalachian Anomaly (NAA) is a region of unusually hot mantle characterized by low seismic anisotropy centered below the White Mountains of New Hampshire, and possibly extending as far south as the eastern part of the SEISConn transect. The NAA may be related to recent mantle dynamics, but its origin is controversial, as is its relationship to post-rift uplift and magmatism in the White Mountains.

Session

Session 6: Advances in Geology, Geochronology, Geophysics