Growth and deformation of oceanic lithosphere: Case studies from Atlantis Bank, Southwest Indian Ridge, and the Baker terrane, northeastern Oregon

This dissertation consists of two case studies which examine processes of oceanic crustal growth and deformational in two separate environments and at two different stages along the plate tectonic cycle. The first part investigates processes of crustal growth and tectonic denudation in slow-spreading oceanic crust. We use U-Pb zircon geochronology to date the timing of lower crustal accretion, and (U-Th)/He zircon thermochronology to constrain timescales and rates of cooling and hydrothermal fluid flow at the Atlantis Bank, Southwest Indian Ridge. Geochronologic results indicate that ∼75% of zircon ages agree with their predicted magnetic ages, whereas up to 25% are statistically older than predicted. We argue that these older components originally crystallized at depths of ∼5-18 km in the axial lithosphere and were subsequently uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank. We also report 26 new (U-Th)/He zircon ages, which constrain the timescales and rates of lower crustal cooling on Atlantis Bank. The majority of the (U-Th)/He zircon ages (∼73%) record anomalously young ages, indicating cooling through ∼200°C well outside the rift valley. We interpret these anomalously young ages as recording a thermal heating event associated with localized high-temperature (>300°C) hydrothermal fluid flow during a period of transtension.; The second aspect of my dissertation examines the tectonic development of the Baker terrane, a long-lived accretionary complex/forearc, which developed in association with subduction-related island arc terranes of the paleo-Pacific ocean. The northern margin of the Baker terrane is characterized by an imbricate fault zone consisting of slices of subduction-related, meta-igneous and metasedimentary rocks faulted into chert-argillite melange. The metaplutonic slabs crystallized between 226 and 231 Ma and are characterized by low initial 87Sr/86Sr (0.7033--0.7034) and negative initial epsilon Nd values (+7.7 to +8.5). In contrast, the chert-argillite melange has initial 87Sr/86Sr values ranging from 0.7073--0.7094 and initial epsilonNd values between -4.7 and -7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the far-traveled, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary may be analogous to the ongoing Molucca Sea collisional zone.