Development and application of coupled K-feldspar PB isotopic and 40AR/39AR methods to reconstruct late cretaceous/cenozoic deformation of the California margin
Keywords: Geological and Environmental Sciences.

The evolving plate boundary along the California margin over the last 100 m.y. has dramatically impacted the geologic evolution of the western United States. Laramide flat-slab subduction extinguished Sierra Nevada arc magmatism as trench-derived sediments (i.e., Rand and related schists) were underthrust and accreted at 25-30 km depth beneath the southernmost Sierra Nevada and NW Mojave Desert (SSN/MD) during the Late Cretaceous. During this time, the southern California forearc margin was severely disrupted. Breachment of the mid-Cretaceous batholith in the southern California region ultimately permitted depositional systems to extend eastward and permit extraregional sediment from the continental interior to reach the margin during the Eocene. Subsequently, between 30-25 Ma, the North American and Pacific plate boundary was established. The resulting development of a northwest-propagating transform margin produced "slab gaps" beneath the SSN/MD that triggered mantle upwelling, volcanism, and extension of the crust. Each of these significant changes has imprinted a thermal history record in the SSN/MD crust that can be interrogated through integrated detrital and basement thermochronology. Modal detrital phases such as K-feldspar can yield significant thermal history and provenance information but have been under-utilized. The beneﬁts of using K-feldspar in detrital studies are twofold: K-feldspar 40Ar/39Ar has shown promise as both a basement and detrital thermochronometer. Moreover, its propensity to incorporate common Pb into its crystal structure allows its Pb isotope composition to be used as a provenance tool. The well-established extreme variation in Pb isotopic values and thermal fingerprints of tectonic processes across the western United States can be exploited to reconstructing the southern California margin. In this dissertation, I develop and apply basement and detrital K-feldspar coupled 40Ar/39Ar and Pb isotopic analysis to evaluate the Late Cretaceous/Cenozoic evolution of the California margin. In Chapter 1, I demonstrate viability of K-feldspar coupled 40Ar/39Ar and Pb isotopic analysis by quantitatively investigating the eﬀects of laser fusion during 40Ar/39Ar analyses of K-feldspar upon subsequent LA-ICP-MS measurement of Pb isotopic ratios. I also show the feasibility of using LA-ICP-MS to generate statistically significant datasets. In Chapter 2, I describe how I have developed a regionally extensive database of new Pb isotopic values in K-feldspar for the southwestern United States basement. I furthermore evaluate the utility of this database by demonstrating how it can be applied to evaluate paleodrainage evolution during the early Eocene to early Miocene along the SSN/MD margin. In Chapter 3, I develop a database of new 40Ar/39Ar thermal history results from mid-Cretaceous plutonic rocks throughout the SSN/MD region. I also develop single-crystal incremental heating methods to detrital K-feldspar from derivative Eocene-Miocene strata. Coupled interpretation of the basement and detrital results is used to assess the Late Cretaceous thermal evolution of the SSN/MD region. Collectively, these results constrain the magnitude and timing of Laramide subduction refrigeration of the southern California margin. Unimodal and identical detrital K-feldspar age distributions and thermal histories from the forearc strata confirm subduction refrigeration extended into the shallow crust and affected a broad region of the Mojave Desert province. Final exhumation of the deepest rocks in the western Mojave in the early Miocene was ultimately due to northwestward migration of the Mendocino triple junction along the California margin. Collectively, these case studies demonstrate how these new analytical and interpretative methods can be profitably applied to future regional deformation and tectonically driven re-organization of drainage patterns.