Theses and Dissertations at Montana State University (MSU)
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Item The Montana Alps: kilometer-scale recumbent folding and tectonic attenuation in the Anaconda Range, southwestern Montana(Montana State University - Bozeman, College of Letters & Science, 2022) Neal, Bryce Alan; Chairperson, Graduate Committee: Andrew K. Laskowski; This is a manuscript style paper that includes co-authored chapters.The Eocene Anaconda metamorphic core complex (AMCC) is the most recently documented metamorphic core complex in the North American Cordillera. While much work has focused on constraining the nature and timing of core complex extension, earlier deformation preserved in its footwall is not as well understood. The AMCC footwall contains an anomalously thin, mid-crustal section of Mesoproterozoic Belt Supergroup and Paleozoic strata. While the tectonic nature of this thinning is generally accepted, the mechanisms behind it remain enigmatic. Geologists from the Montana Bureau of Mines and Geology hypothesize that footwall strata were attenuated along the upper limb of the Fishtrap recumbent anticline (FRA), a kilometer-scale NW-verging recumbent fold exposed throughout the southwestern AMCC footwall. New geologic mapping in the Carpp Ridge 7.5' quadrangle and U-Pb geochronology better constrain the nature and timing of tectonic attenuation in this complex area. Two generations of folds deformed rocks in the quadrangle: F 1 recumbent folds with S 1 axial planar fabrics associated with the FRA, and F 2 upright folds with S 2 axial planar fabrics that refold the FRA. These deformations are likely Late Cretaceous in age based on dates from cross-cutting intrusions, although a foliation sub-parallel to S 1 in a 51.87 Ma granodiorite stock in the FRA hinge suggests localized Eocene deformation. Elsewhere in the field area, the same granodiorite crosscuts S1 fabrics. F 1 folds and S 1 fabrics transpose, attenuate, and omit Belt strata in the southeastern portion of the quadrangle, suggesting that recumbent folding is intimately associated with tectonic attenuation. Further, west-vergent F 1 and F 2 folds may be decoupled from regional east-vergent tectonics and instead related to gneiss-doming in the ~75-74 Ma Lake of the Isle shear zone. Gneiss-doming and associated development of the FRA may have been driven by widespread decompression of the Cordilleran middle crust during Late Cretaceous time, perhaps in response to delamination of lithospheric mantle or arc-root foundering beneath the Cordilleran magmatic arc of SW Montana.Item Deformation and metasomatism of the Qomolangma Formation: a geochemical and microstructural analysis of the summit limestone, Mount Everest, Nepal(Montana State University - Bozeman, College of Letters & Science, 2015) Corthouts, Travis Leo; Chairperson, Graduate Committee: David R. LagesonThis study is based on a suite of new samples that represent a structural transect across the Qomolangma Formation, an Ordovician limestone that comprises the uppermost ~125 meters of Mount Everest. Past studies posit Everest's summit limestone to be mildly sheared and unmetamorphosed; however, this study shows that the Qomolangma Formation has endured more strain and metamorphism than previously thought. Ti-in-biotite and Ti-in-quartz geothermometry, electron backscattered diffraction analysis, and geochemical analysis of tourmaline has been used to infer the metamorphic and deformational history of this formation. Results show that the Qomolangma Formation has experienced significant ductile shear throughout, though samples preserve a stronger shear fabric toward the top of the formation. Furthermore, our data show a gradation in the degree of metamorphism across the Qomolangma Formation, increasing toward the base of the unit. Samples collected from the structural top of the formation (Summit samples) have a penetrative foliation with significant grain size reduction of calcite (~8 microns) and yield temperature estimates of > or = 250°C. In contrast, samples from the base (South Summit samples) are distinguished by an increase in recrystallized grain size of calcite (~66 microns) and yield temperatures estimates of 500-600°C. Another important difference between Summit and South Summit samples is aggregates of the coexisting minerals muscovite, chlorite, tourmaline, rutile and biotite found throughout South Summit samples. Geochemical analysis of tourmaline suggests these minerals crystallized from metasomatic fluids (hydrothermal fluids) released from leucogranite bodies emplaced into rocks structurally subjacent to the summit limestone. As a result, metasomatic fluids caused localized metamorphism at the base of the Qomolangma Formation, producing the significant increase in metamorphic grade observed in South Summit samples. It is interpreted that the fabric preserved in Summit samples was ingrained during initial thrust faulting in the Eocene to Oligocene, whereas the fabric and metamorphic grade observed in South Summit samples is the result of detachment faulting and leucogranite emplacement in the Early Miocene. Lastly, microstructural observations suggest that metasomatic fluids may have promoted faulting on the Qomolangma detachment, a splay of the South Tibetan Detachment believed to be at the base of the Qomolangma Formation.Item Amphibolite to granulite facies metamorphism and deformation in the Jerome Rock Lakes area, Spanish Peaks, Montana(Montana State University - Bozeman, College of Letters & Science, 2015) Chadwick, Jesse Socorro; Chairperson, Graduate Committee: David W. MogkStructural analysis, bedrock mapping, and quantitative thermobarometry reveal a protracted metamorphic history for the multiple lithologies present in the Jerome Rock Lakes area of the Spanish Peaks, located in the northern Madison Range, southwest Montana. Volumetrically-dominant quartzofeldspathic gneisses are primarily of the tonalite-trondhjemite-granodiorite (TTG) suite and record multiple episodes of high-temperature deformation. Coarse-grained hornblende in association with K feldspar-bearing leucosome indicates locally vapor-present melting. Clinopyroxene-bearing metabasic enclaves in the quartzofeldspathic gneisses record the subsolidus hydration of clinopyroxene porphyroclasts to hornblende and subsequent dehydration melting of hornblende in the transitional granulite facies to produce garnet coronae and clinopyroxene neoblasts under vapor-absent conditions. Variably migmatized garnet-sillimanite schists record amphibolite facies metamorphism at ca. 5.5 kbar and 640 °C, with extensive melt production via melting of biotite. Inclusions of staurolite in kyanite-bearing metaquartzite are inferred to indicate the presence of modern thicknesses of continental crust and a geothermal gradient similar to Phanerozoic values. Infrequent layers containing tschermakitic amphibole layers may represent the residuum of vapor-present melting of biotite and indicate local variation in the activity of water. Migmatized garnet amphibolites record peak upper-amphibolite to transitional granulite facies metamorphism at ca. 10 kbar and 800 °C. A clockwise P-T path of Alpine character is proposed for the lithologies of the Jerome Rock Lakes area, with peak pressures in the kyanite field attained before peak temperatures, followed by rapid, steeply adiabatic to isothermal decompression. Early isoclinal folds developed during prograde metamorphism and were rotated into the regional foliation. Progressive deformation resulted in the formation of later isoclinal folds coplanar with foliation. Peak temperatures were attained after cessation of deformation, indicated by undeformed leucosomes. A second episode of deformation produced outcrop-scale parallel folds trending north-south. The Precambrian tectonic history of the northern Madison Range is inferred to have terminated in a continental collision with northwest-southeast directed shortening that produced steeply-dipping, NNE-trending regional foliation. This style is generally consistent with that observed elsewhere in the Spanish Peaks and in the Tobacco Root Range, generated by the ca. 1.7 Ga collision of western North America with the Medicine Hat craton.Item The structural geology and tectonic history of the northern Flint Creek Range, western Montana(Montana State University - Bozeman, College of Letters & Science, 1984) Baken, Jeffrey FrankItem Laramide basement deformation in the northern Gallatin Range and southern Bridger Range, southwest Montana(Montana State University - Bozeman, College of Letters & Science, 1987) Miller, Erick W. B.