Thesis advisor: Seth C. Kruckenberg / The Potters Pond migmatite domain (PPMD) is a heterogeneous zone of migmatites located ~10 km southwest of Cascade, Idaho within the western Idaho shear zone (WISZ). The PPMD is the only known exposure of migmatites within the WISZ over its ~300 km length, occurring where the shear zone orientation changes from 020° south to 000° north of the migmatite domain. Structural mapping within the PPMD has identified multiple generations of migmatite with varied structural fabrics. Leucosome layers were sampled from distinct migmatite localities and morphologies (e.g., metatexite, diatexite) to determine the timing and duration of partial melting in the PPMD. U-Pb age determinations of zircon by means of LA-ICP-MS document two periods of protracted migmatite crystallization during the Early and Late Cretaceous. Early Cretaceous (ca. 145 to 128 Ma) migmatite crystallization ages are coeval with the collision and suturing of oceanic terranes of the Blue Mountains province with North America, and the formation of the Salmon River suture zone (SRSZ). Migmatite crystallization ages from ca. 104 to 90 Ma are associated with Late Cretaceous dextral transpression in the WISZ. Field observations and geochronology of cross cutting leucosome relationships are interpreted to record deep crustal deformation and anatexis associated with formation of the SRSZ, subsequently overprinted by solid-state deformation and renewed anatexis during the evolution of the WISZ. These data are the first direct evidence of the synmetamorphic fabric related to the SRSZ east of the initial Sr 0.706 isopleth, and that the WISZ is a temporally distinct overprinting structure. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Dense array seismic techniques can be applied to multiple types of seismic data to understand regional tectonic processes via analysis of crustal velocity structure, imaging reflection surfaces, and calculating high-resolution hypocenter locations. The two regions presented here include an intraplate seismogenic fault zone in Virginia and a steep cratonic margin in eastern Oregon and Idaho. The intraplate seismicity study in Virginia consisted of using 201 short-period vertical-component seismographs, which recorded events as low as magnitude -2 during a period of 12 days. Dense array analysis revealed almost no variation in the seismic velocity within the hypocentral zone, indicating that the aftershock zone is confined to a single crystalline-rock terrane. The 1-2 km wide cloud of hypocenters is characterized by a 29° strike and 53° dip consistent with the focal mechanism of the main shock. A 5° bend along strike and a shallower dip angle below 6 km points toward a more complex concave shaped fault zone. The seismic study in Idaho and Oregon was centered on the inversion of controlled-source wide-angle reflection and refraction seismic P- and S-wave traveltimes to determine a seismic velocity model of the crust beneath this part of the U.S. Cordillera. We imaged a narrow, steep velocity boundary within the crust that juxtaposes the Blue Mountains accreted terranes and the North American craton at the western Idaho shear zone. We found a 7 km offset in Moho depth, separating crust with different seismic velocities and Poisson's ratios. The crust beneath the Blue Mountains terranes is consistent with an intermediate lithology dominated by diorite. In the lower crust there is evidence of magmatic underplating which is consistent with the location of the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is thicker and characterized by a felsic composition dominated by granite through most of the crust, with an intermediate composition layer in the lower crust. This sharp lithologic and rheologic boundary strongly influenced subsequent deformation and magmatic events in the region. / Ph. D. / Dense array seismic techniques involve using many instruments deployed closely together to record natural or man-made ground shaking. These techniques can be applied to different types of seismic data to understand the regional composition and behavior of the Earth’s crust, and identify locations where earthquakes have occurred. The two regions presented here include a zone in Virginia known to have small earthquakes and a location in eastern Oregon and Idaho where younger crust meets older crust across a very steep boundary. The seismicity study in Virginia consisted of using 201 instruments to record earthquake aftershocks with very small magnitudes during a period of 12 days. Dense array analysis techniques revealed almost no variation in the speed that seismic waves travel within the zone of aftershocks, indicating that the aftershocks are confined to a single crystalline-rock region. The 1-2 km wide zone of sub-surface aftershock locations is consistent with the rupture orientation of the main earthquake on 23 August 2011. The zone’s slightly concave shape indicates a complex region of rock movement. The seismic study in Idaho and Oregon was centered on analyzing seismic waves that are generated by explosions and travel through the crust, bending and reflecting when they pass through variations in the rock. We imaged a narrow, steep boundary that juxtaposes the younger Blue Mountains crust and the older North American craton at the western Idaho shear zone (WISZ). We found a sharp ~7 km step between the thicknesses of the two regions and different seismic velocities on either side of the boundary. The crust beneath the Blue Mountains terranes is consistent with an intermediate rock type dominated by diorite. In the lower crust there is evidence of a layer that is consistent with un-erupted material from the Columbia River Basalts. The continental crust east of the WISZ is thicker and dominated by granite through most of the crust, with an intermediate composition layer in the lower crust. This sharp boundary strongly influenced subsequent deformation and magmatic events in the region.