Major geological features and lateral variation of crustal structure in southern New England

Abstract

Rg dispersion studies in southern New England (SNE) provide a basis for comparing lateral variation in crustal structure with geological features. Rg waves reveal lateral variations at depths ranging from near the surface down to a few kilometers. The similarity between Rg results and those of other geophysical studies suggests that variations revealed by Rg studies penetrate into the middle of the crust, if not deeper. The Waterbury Dispersion Region (WDR), named after a local geological structure, is characterized by the highest Rg group velocities in SNE (∼2.9–3.1 km/s at a period of 1 s). These higher velocities may be due to rocks enriched in oceanic type crust and/or granulite facies metamorphic rocks. The Hartford and New Haven dispersion regions (HDR and NHDR) comprise the northern and southern parts of the Mesozoic Hartford Rift Basin, respectively. The HDR is characterized by the lowest group velocities in SNE (∼2.1–2.4 km/s at a period of 1 s). These lower velocities appear to be related to relatively thick Pleistocene glacial sediments overlying a thick sequence of Mesozoic sedimentary rocks. Higher velocities in the NHDR are attributed to a thinner cover of glacial sediments and a thinner package of sedimentary rocks. These younger rocks overlie metamorphic basement which is, on average, closer to the surface in the NHDR than in the HDR. The Bronson-Avalon Dispersion Region (BADR), east of the Hartford Rift Basin, represents several terranes that are geologically diverse but, at the scale of this study, not differentiated by Rg dispersion. The seismic structure beneath the BADR appears to be quite homogeneous; group velocities in this region range from about 2.6 to 2.8 km/s at a period of 1 s. The BADR is interpreted to consist of stacked layers of gently dipping to nearly flat-lying thrust sheets. The Torrington Dispersion Region (TDR) lies north of the WDR and west of the HDR and has group velocities similar to those of the BADR.