Abstract
Chapter 1: Sedimentary basins record crustal-scale tectonic processes related to the construction and demise of orogenic belts, making them an invaluable archive for the reconstruction of the evolution of the North American Cordillera. In southwest Montana, USA, the Renova Formation, considered to locally represent the earliest accumulation following Mesozoic–Cenozoic compressional deformation, is widespread but remains poorly dated, and its origin is debated. Herein, we employed detrital zircon U-Pb and (U-Th)/He double dating and sanidine 40Ar/39Ar geochronology in the context of decimeter- scale measured stratigraphic sections in the Renova Formation of the Muddy Creek Basin to determine basin evolution and sediment provenance and place the basin-scale record within a regional context to illuminate the lithospheric processes driving extension and subsidence. The Muddy Creek Basin is an extensional half graben in southwest Montana that is ∼22 km long and ∼7 km wide, with a >800-m-thick sedimentary package. Basin deposition began ca. 49 Ma, as marked by multiple ignimbrites sourced from the Challis volcanic field, which are overlain by a tuffaceous fluvial section. Fluvial strata are capped by a 46.8 Ma Challis ignimbrite constrained by sanidine 40Ar/39Ar dating. An overlying fossiliferous limestone records the first instance of basinal ponding, which was coeval with the cessation of delivery of Challis volcanics–derived sediment into the Green River Basin. We attribute initial ponding to regional drainage reorganization and damning of the paleo–Idaho River due to uplift and doming of the southern Absaroka volcanic province, resulting in its diversion away from the Green River Basin and backfilling of the Lemhi Pass paleovalley. Detrital zircon maximum depositional ages and sanidine 40Ar/39Ar ages show alternating fluvial sandstone and lacustrine mudstone deposition from 46 Ma to 40 Ma in the Muddy Creek Basin. Sediment provenance was dominated by regionally sourced, Challis volcanics–aged and Idaho Batholith–aged grains, while detrital zircon (U-Th)/He (ZHe) data are dominated by Eocene cooling ages. Basin deposition became fully lacustrine by ca. 40 Ma, based on an increasing frequency of organic-rich mudstone with rare interbedded sandstone. Coarse-grained lithofacies became prominent again starting ca. 37 Ma, coeval with a major shift in sediment provenance due to extension and local footwall unroofing. Detrital zircon U-Pb and corresponding ZHe ages from the upper part of the section are predominantly Paleozoic in age, sourced from the Paleozoic sedimentary strata exposed in the eastern footwall of the Muddy Creek detachment fault. Paleocurrents shift from south- to west-directed trends, supporting the shift to local sources, consistent with initiation of the Muddy Creek detachment fault. Detrital zircon maximum depositional ages from the youngest strata in the basin suggest deposition continuing until at least 36 Ma. These data show that extension in the Muddy Creek Basin, which we attribute to continued lithospheric thermal weakening, initiated ∼10 m.y. later than in the Anaconda and Bitterroot metamorphic core complexes. This points to potentially different drivers of extension in western Montana and fits previously proposed models of a regional southward sweep of extension related to Farallon slab removal.Chapter 2: The sedimentary record plays a key role in our understanding of the orogenic cycle, from thrusting and crustal thickening to extension and topographic demise. Within the North American Cordillera, the basinal record related to orogenic collapse has often been overlooked as an avenue to study extensional processes, especially when related to metamorphic core complexes. We present decimeter-scale stratigraphy, detrital zircon U-Pb and sanidine 40Ar/39Ar geochronology to reconstruct the evolution of the Deer Lodge Valley, situated in the hanging-wall of the Anaconda metamorphic core complex, providing insights into the coupled sedimentary and structural evolution of the MCC. Deer Lodge Valley stratigraphy is dominated by section-scale coarsening upward sequences with mudstone and siltstone overlain and eroded into by pebble-cobble conglomerate, with an overall basin-ward fining. These lithofacies stacking patterns and calculated maximum depositional ages indicate deposition in the Deer Lodge Valley was dominated by early to middle Eocene stream flow-dominated, prograding alluvial fans, with metamorphic core complex exhumation and extension driving basin formation and deposition. Provenance analyses suggest sediment was locally sourced from the adjacent core complex highlands, and that each alluvial fan had a distinct point source, recording the gradual exhumation of the footwall. This detailed stratigraphic and geochronologic record of the Deer Lodge Valley shows that metamorphic core complex-related basins share a set of depositional characteristics that match closely with those proposed for a supradetachment basin model. This study has shown that it is crucial to consider the syntectonic depositional record in tandem with the basement thermochronologic record to gain a holistic, time-transgressive understanding of MCC formation, and more broadly, orogenic collapse.
Chapter 3: Metamorphic core complexes provide a rare glimpse into thermomechanical processes in the lithosphere and play a substantial role in the evolution of the crust. The North American Cordillera contains a northwest trend of metamorphic core complexes, which have been extensively studied using a variety of bedrock thermochronologic and thermal history modeling techniques. An often-overlooked dataset in determining the timing and driving mechanisms behind core complex formation is the synextensional basin record, which not only preserves a unique archive of source-region evolution, but also preserve the initial surface response to high-magnitude, localized extension. Herein, we present new (U-Th)/He thermochronology and HeFTy inverse thermal history models on previously U-Pb dated detrital zircon samples from the Deer Lodge Valley, situated in the hanging-wall of the Anaconda metamorphic core complex, east of the Flint Creek Range. We produced 63 detrital zircon (U-Th)/He analyses ranging from 10.8 Ma to 612.5 Ma with 67% of grains producing Paleocene-Eocene cooling ages, and > 50% of grains yielding lag times < 10 m.y. HeFTy inverse thermal history models show all samples remain below the zircon partial retention zone until ca. 70 Ma, then rapidly cooled between ca. 70 – 55 Ma. When compared to preexisting basement thermochronologic data within the Anaconda MCC, multiple thermochronometric systems are cooling through the partial retention zone prior to the previously recognized onset of exhumation and extension within the Anaconda Range. We interpret there to be an earlier period of exhumation-related cooling ca. 65 – 55 Ma. We do not believe these Paleocene cooling ages with <10 m.y. lag times can be explained solely through cooling of Cretaceous batholiths in the footwall. Exhumation and cooling in the core complex is attributed to syncontractional, gravitational collapse of thermally weakened lithosphere. Continued eastward propagation of the Helena Salient and Sevier-style thrusting produced lateral gradients in gravitational potential energy, while the emplacement of multiple Cretaceous batholiths in the footwall thermally weakened already thickened crust, leading to focused high-magnitude extension within the Anaconda MCC. The newly documented cooling and exhumation of the Anaconda MCC aligns with other early phases of cooling found within the Northern Belt of MCCs, the majority of which have preserved synextensional basin records. To fully understand the timing and driving mechanisms behind MCC formation, it is crucial to utilize all records associated with MCC deformation.