Evidence for a lithospheric mantle suture between Laurentia and the Appalachian Province beneath Connecticut using Sp receiver functions

Abstract:

Connecticut’s tectonic history is well established, with the Taconic, Gander, and Avalonia terranes accreting onto Laurentia and then failed rifting forming the Hartford Basin, as Pangea began to break apart. While the surface geology of New England is well understood in terms of Wilson Cycling, the same cannot be said of the mantle lithosphere. Recent increases in station coverage confirm the presence of a slow S-wave velocity anomaly imaged beneath New England, termed the Northern Appalachian Anomaly (NAA), which may be indicative of more recent dynamic processes affecting the region.

Our study looks for evidence of the impact from orogenesis and post orogenic magmatic processes on lithospheric structure beneath Connecticut using Sp receiver functions. Data from 76 seismic stations, including from the SEISConn seismic experiment (data from 2015-2019), were used in the analysis. Common conversion point stacked results show a pronounced negative velocity gradient at ~80 to 100 km depth in the western half of the study area (-75° to -73°W), which shallows to roughly 60 km depth beneath the eastern half of the study area (-73° to -71°W). West of -73°W the negative phase is large and robust, while amplitudes east of -73°W are typically weaker and/or more variable. The change in amplitude and negative phase depth coincides with a step-over in crustal thickness imaged with Ps receiver functions by Li et al. (2018).

A direct comparison of our results to the regional S-wave tomography model of Yang and Gao (2018) indicates that our negative phase is the LAB. Our results indicate that the LAB exhibits a stepwise change in behavior from west to east. This change in LAB character correlates exceeding well with the surface expression of the Taconic/Gander boundary suggesting that the present-day lithospheric structure imaged at depth is inherited from prior tectonic events, specifically collision of Laurentia with the Appalachian Province, and that subsequent tectonism, such as rifting, and more recent dynamic processes have not overprinted this structure. However, this explanation requires the boundary between the lithosphere and asthenosphere to remain unchanged over 440 MYA which is difficult to explain in light of our current understanding of the thermal evolution of the lithosphere-asthenosphere boundary.