Collaborative Research: How have orogenesis, rifting, and recent mantle dynamics shaped the lithosphere beneath the New England Appalachians?

Continents represent a key component of the Earth’s plate tectonic system. Plate tectonics appears to be unique among all planetary bodies in our solar system and plays a crucial role in making our planet hospitable to life. Continents are formed and modified by fundamental plate tectonic processes, including volcanism, mountain-building, collisions with other continental masses, and rifting or breakup of supercontinents. Understanding the evolution of continental lithosphere (that is, the crust and uppermost mantle beneath continents) through time remains one of the most important challenges in Earth science. This project will study the processes that have shaped the continental lithosphere beneath the New England Appalachians through geologic time using a multi-disciplinary approach that includes seismic imaging of the crust and upper mantle, the analysis of geological structures, and dating the motion on ancient faults. The investigators will carry out detailed imaging using seismic waves from distant earthquakes measured at seismometers deployed in two linear arrays (one across New York and Massachusetts, the other across Vermont, New Hampshire, and Maine). They will pair their seismic imaging work with studies of geologic structures exposed at the surface that will provide essential context on its plate tectonic history. The integration of information obtained from both approaches will illuminate the processes that have shaped the lithosphere beneath New England through the geologic past, opening a window into how continental lithosphere evolves as a part of the Earth’s plate tectonic system. This project aims to understand how rifting, subduction, and terrane accretion have shaped lithospheric structure beneath New England: Does the lithosphere preserve a record of subduction and accretion or does lithospheric structure heal over time? How do subsequent accretion events overprint the structure of earlier collisions? How did Mesozoic rifting affect the structure of Paleozoic accretions? Are recent to ongoing mantle processes modifying the lithospheric structure? The investigators will analyze data from the NEST (New England Seismic Transects) experiment, a deployment of ~25 broadband seismometers that is configured to resolve crustal targets on length scales that are appropriate for direct linkage with geological structures. They will apply a suite of imaging techniques (receiver function analysis, body wave travel time analysis, wavefield migration imaging, and SKS splitting measurements) that will provide a comprehensive view of both the isotropic and anisotropic structure of the New England crust and upper mantle, including the region underlain by the so-called Northern Appalachian Anomaly (NAA). The U-Pb geochronology work will focus on the identification and analysis of xenocrystic (inherited) cores in zircon grains from arc plutonic and volcanic rocks from the Shelburne Falls and Bronson Hill arcs, in order to understand the history and timing of Taconic (and later) orogenic processes. The investigators will also conduct field work and structural analyses to investigate the extent and relative timing on frontal thrusts of basement massifs and the Taconic Mountains in western New England and New York, and the Connecticut Valley border fault system in central New England. They will constrain the timing of deformation by collecting and dating samples via 40Ar/39Ar geochronology from these regions, in order to determine how widespread reactivation of old faults (and displacement on new faults) has been throughout the region. The project emphasizes undergraduate involvement in cross-disciplinary research and field work, particularly by participants from groups that have been historically underrepresented in Earth science, enabled by collaboration among faculty at a liberal arts college and research institutions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.