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Constraints on the thickness, transitional boundaries, and composition of Earth's crust are pivotal in studying its formation and evolution. We use data from 132 seismic installations throughout the northeastern US to explore how tectonic events, such as orogenesis and rifting, have altered the crust of the northeastern US and southeastern Canada, and to distinguish between Laurentia and the Appalachian terranes. We include data from seismic installations from the NEST and SEISConn experiments, spanning the Laurentia–Appalachian boundary, and present estimates of crustal thickness,V_p/V_s, and thickness of the transition between crustal and mantle rocks using Ps receiver functions. We find some first-order differences between Laurentia and Appalachian terranes, with Laurentia exhibiting thicker crust (c.39 v.c.33 km) and a broader crust–mantle transition thickness (c.3 v. <1.5 km). AverageV_p/V_svalues are similar between Laurentia (c.1.77) and Appalachian terranes (c.1.74); however, we identify anomalousV_p/V_sin a few regions, including highV_p/V_saround the Adirondack Mountains and lowV_p/V_sin southern New England. The southern New England region is also anomalous in terms of its systematically thinner crust and sharper crust–mantle transition, which may be a consequence of the formation and collapse of the Acadian altiplano during the mid-to-late Paleozoic. 

The New England Appalachians provide a fascinating window into a host of fundamental geological problems. These include the modification of crustal and mantle lithospheric structure via orogenesis, terrane accretion, and continental rifting, the evolution of individual terranes through processes such as channel flow and ductile extrusion, and the causes and consequences of the Northern Appalachian Anomaly (NAA), a prominent geophysical anomaly in the upper mantle. Recent and ongoing deployments of dense seismic arrays in New England are providing images of the crust and upper mantle in unprecedented detail, allowing us to address both new and longstanding science questions. These deployments include the Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn, 2015-2019), the New England Seismic Transects (NEST, 2018-present), and the GEology of New England via Seismic Imaging Studies (GENESIS, 2022-present) arrays. Here we present results from these experiments that are shedding new light on the tectonic evolution of New England and the ways in which structures and processes in the upper mantle can affect the structure of the overlying lithosphere. These include detailed new images of crustal architecture beneath central and southern New England, including a sharp transition from thick (~48 km) crust Laurentia terranes to thin (~32 km) crust beneath Appalachian terranes. The character of this offset beneath the SEISConn and NEST arrays suggests an overlap of two Moho boundaries, forming an overthrust-type structure that may have resulted from reactivation of faults during the compression and shortening associated with the formation of the hypothesized Acadian Altiplano. Beneath SEISConn, there is evidence for multiple relict structures preserved in the lithosphere from past episodes of terrane accretion and suturing, as well as anisotropic layering that constrains the kinematics of past lithospheric deformation events. Beneath the NEST line in central New England, we infer a relatively shallow (~80 km) lithosphere-asthenosphere boundary above the NAA upper mantle geophysical anomaly, providing evidence for lithospheric thinning above a presumed asthenospheric upwelling. Finally, preliminary results suggest layered crustal anisotropy beneath the GENESIS array, perhaps corresponding to a past episode of channel flow in the mid-crust. 

Throughout her career, Professor Sharon Mosher has been a pioneer in the structural analysis of polydeformed rocks and regions. Her work on the evolution of superposed rock fabrics in complexly deformed areas, for example, has greatly improved our ability to determine how faults, shear zones, and orogens evolve over time. Traditionally, sequences of foliations, mineral lineations, folds, and other structural elements have been interpreted in terms of discrete, multiphase deformation events. However, alternative interpretations where structural sequences result from a single, progressive event also are common, especially where changes in stress fields or flow parameters result in non-steady deformation. Here, in honor of Professor Mosher, we present examples of three different types of structural sequences that formed in large seismogenic faults and shear zones in SW New Zealand and southern California. These examples illustrate the different ways in which multiple generations and styles of rock fabrics develop and become preserved in zones of localized deformation. The first example is from a large fault zone located inboard of the Puysegur subduction zone in Fiordland, New Zealand. This zone displays several generations of superposed fabrics that record a history of repeated reactivations over a few tens of millions of years. A second set of examples, from both Fiordland and southern California, illustrates how non-steady deformation can result in parallel ductile and brittle fabrics, including veins of pseudotachylyte, that formed during a single, progressive shearing event. The third example, also from Fiordland, shows how parallel rock fabrics in a large, lower crustal shear zone formed diachronously across a large region as the inboard and outboard belts of the Mesozoic Median batholith converged. Each of these examples displays different structural relationships among rock fabrics in the field. To decipher their histories, we combined structural data with 40Ar/39Ar and U-Pb (zircon, titanite) geochronology. The examples illustrate the utility of combining field observations with both direct and indirect isotopic 

Many large fault zones record multiple reactivations that can be difficult to resolve and interpret in the field. Here, we use examples from Vermont and New Zealand to illustrate how structural data combined with 40Ar/39Ar geochronology can be used to reconstruct fault reactivation histories and interpret their possible origins. In SW New Zealand, the Spey-Mica Burn fault zone parallels a transpressive boundary between the Pacific and Australian plates. Integrated structural and 40Ar/39Ar data obtained from pseudotachylyte, mylonite, and other fault rocks allow us to distinguish successive phases of faulting (i.e., reactivations) from cases where different styles of brittle and ductile deformation occurred simultaneously (or nearly so) in the fault zone. Apparent age spectra from multiple minerals show age gradients that reveal four reactivations spanning ~20 Ma. The style and timing of these events correlate well to times of increased convergence rate and collisions between oceanic ridge segments and a nearby trench. Fault zones in NW Vermont also record different styles of reactivation. The Hinesburg Thrust (HT), which juxtaposes Late Proterozoic-Early Cambrian rift clastic rocks against Ordovician carbonate rocks of the Champlain Valley belt, includes a ~30 m thick zone of mylonite that is cut by a cataclastic fault and deformed by folds. 40Ar/39Ar data suggest the mylonite formed during the Ordovician Taconic orogeny and later was folded into a series of domes and basins during the Late Silurian-Devonian Acadian orogeny. Farther west, the Champlain thrust fault (CT) juxtaposes Cambrian dolostones against Ordovician calcareous shales. Superposed faults within the foot wall of the CT show a progressive change in movement direction from W-directed thrusting, to NW-directed thrusting, to N-S slip, and NE-SW slip. These changing slip directions appear to reflect wholly Taconic motion along a north-dipping lateral ramp between Burlington and Shelburne where the CT cuts up section to the south. Acadian reactivation of the CT appears restricted to late folding similar to the HT. These examples highlight the utility of combining structural data with 40Ar/39Ar geochronology to unravel slip histories in continental fault zones and to distinguish among the different styles and origins of fault reactivation.