Tomographic inversion for the seismic velocity structure beneath northern New England using seismic refraction data

Abstract

The seismic velocity structure of the crust and the upper mantle beneath northern New England was investigated from the travel time data of the 1984 Maine Seismic Refraction Profile and the 1988 Ontario‐New York‐New England Seismic Refraction Profile seismic refraction experiments by using tomographic inversion techniques. Strong lateral seismic velocity variations were found in the upper 15 km of the crust, and these variations are correlated with regional geologic and tectonic structures. The Grenville basement in the Adirondack Mountains of eastern New York is characterized by an unusually high P wave velocity of 6.7 km/s in the upper 15 km of the crust. This high‐velocity basement extends eastward below western Vermont to a depth between 15 and 20 km from the surface. More typical upper crustal P wave velocities of 5.6 to 6.4 km/s were generally found in central and southern Maine and in New Hampshire, and the anisotropic upper crustal zone west of Penobscot Bay in Maine was imaged by locally slower velocities. The correlation of the inversion velocity structure with the tectonic features of northern New England shows that the Appalachian mountains in northern New England may extend, tectonically, to at least 20 km deep in the crust. The velocity distribution at 20 km depth shows three northeast elongate juxtaposed belts which correspond to the Connecticut Valley‐Gaspe Synclinorium, the Bronson Hill Anticlinorium and the Kearsarge‐Central Maine Synclinorium, respectively. The Connecticut Valley‐Gaspe and Kearsarge‐Central Maine Synclinoria have relatively high velocities from 6.94 to 7.12 km/s while the Bronson Hill Anticlinorium has relatively low velocities from 6.52 to 6.75 km/s in the lower crust. Another low‐velocity area of 6.52 km/s at 20 km depth is imaged beneath the White Mountain Batholith in central New Hampshire and may be a lower crustal expression of this batholith. Upper mantle velocity structures were obtained for the Grenville basement, the Appalachians in northern New England and the Avalon terrane in southeastern Maine from the long‐range seismic refraction data. Generally, the older Grenville lithosphere (about 1100 Ma) in Quebec has higher velocity in the upper mantle than does the younger Avalon terrane (no older than 650 Ma) in the southeastern part of northern New England. The data suggest that the velocity of the upper mantle beneath the old North American craton increases stepwise with observable velocity discontinuities in depth, while that beneath the Avalon terrane does not increase through the first 40 km of the upper mantle. The data analysis also shows that the upper mantle suture between the Grenville basement and the Avalon terrane occurs in a narrow belt beneath the Appalachian mountains of central Maine and New Hampshire. The did of the Moho discontinuity beneath northern New England is modeled as nearly flat as 0.7° in the N57°W direction and as steep as 1.2° beneath the Grenville basement.