Congratulations to Nikolas Midttun who successfully defended his dissertation on Tuesday January 4th, 2022.

Advisor: Nathan Niemi 

Abstract:

Continental lithosphere exhibits complex deformation across broad regions, challenging simplistic tectonic models of how plate boundaries behave. As geologists have attempted to build improved models that explain observed patterns of deformation, they have uncovered a wide array of geodynamic mechanisms and material properties that may control the rates, styles, and magnitudes of continental deformation. In most study areas, we now have an excess of hypothesized controls and a dearth of data with which to test these competing models. This dissertation presents three field studies of continental deformation that apply a diverse set of investigative approaches to characterize deformation, then link that deformation to the most likely geodynamic causes. Two studies are from the Basin and Range extensional province, addressing debates around the relative role of plate boundary stresses and internal gravitational potential energy gradients in driving extensional tectonism over the Cenozoic. In Chapter 2, I use field mapping, stratigraphy, detrital zircon geochronology, and structural observations to build a basin history for the Titus Canyon Formation, one of the earliest syn-extensional deposits in the central Basin and Range. Our results from the Titus Canyon Formation indicate that it may record extension related to a ~40 Ma slowdown in plate convergence along western North America. In Chapter 4, I apply multiple low-temperature thermochronometers to a suite of Jurassic plutons in the northern Basin and Range to decipher their exhumational histories. Our data reveal not only the character of Neogene extensional tectonism in the northern Basin and Range, but also changes in Cretaceous-Paleogene exhumation that may be related to contractional tectonism of the Sevier orogeny and construction of the Nevadaplano. Chapter 5 study examines the recent tectonics of the down-going plate in the Caucasus collision, the Lesser Caucasus and Eastern Pontides mountains. We apply low-temperature thermochronology to estimate the tectonic response to the initiation of collision at 5 Ma, with the goal of understanding the partitioning of strain between the two colliding plates. Our results indicate that the collision is highly asymmetric, with the majority of shortening in the down-going plate being accommodated by strike-slip faulting, in contrast with reverse faulting that characterizes deformation in the overriding plate. Together these studies illustrate the rich complexity of continental deformation, and the wide variety of geodynamic mechanisms that must be considered when building models of how deformation evolves in old continental lithosphere.