Browsing by Subject "Structural geology"
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Item Cenozoic deformation history of the Andean plateau in southern Peru : stratigraphic, structural, and geochronologic constraints(2015-08) Perez, Nicholas David; Horton, Brian K., 1970; Gulick, Sean; McQuarrie, Nadine; Steel, Ron; Stockli, Daniel FCenozoic shortening in the central Andes of southern Peru was accommodated by thin- and thick-skinned deformation that governed hinterland/foreland basin dynamics, the timing and location of exhumation, and development of modern high topography. A new line length balanced cross section estimates 130 km of shortening (38%) across the Eastern Cordillera and Subandean Zone. I propose the location of a pre-Andean graben in the Eastern Cordillera, and a kinematic model that links selectively inverted basement-involved normal faults to shallow detachments that accommodate thin-skinned deformation across the orogen. New U-Pb zircon geochronology from synrift deposits establishes Triassic age deposition, and suggests compartmentalized rift basins were filled with local Eastern Cordillera sediment sources. Eocene exhumation in the Eastern Cordillera represents reactivation of Triassic normal faults and the onset of Andean deformation. In-sequence deformation was transferred from the Eastern Cordillera to the Altiplano by the thin-skinned Central Andean Backthrust Belt and induced flexural subsidence in the Ayaviri hinterland basin beginning at ~30 Ma. Facies analyses, sediment provenance, geochronology, and structural mapping define multiple phases of basin reorganization that are temporally correlative with motion along basin margin thrust faults. Major middle Miocene reorganization of the Ayaviri basin is linked to ~17 Ma out-of-sequence thrust fault motion in the Western Cordillera. Oligocene-Miocene hinterland basin evolution in the northern Altiplano was driven by thrust tectonics. U-Pb detrital zircon geochronology from Cretaceous through Cenozoic stratigraphy in hinterland and foreland basins record distinct provenance differences since the Cretaceous. This is the detrital record of either an inherited structural high in the Eastern Cordillera that predated Eocene shortening and created two depocenters with distinct provenance, or lateral provenance variations across a large retroarc foreland basin. Existing K/Ar, ⁴⁰Ar/³⁹Ar, and new zircon (U-Th)/He thermochronology suggest Eocene-Oligocene exhumation in the Eastern Cordillera was synchronous ~400 km along strike. New apatite (U-Th)/He data from the Eastern Cordillera demonstrate a change to localized, diachronous exhumation and uplift events in the Miocene-Pliocene. Apatite (U-Th)/He thermochronology demonstrates onset of deformation in the Subandean Zone by ~15 Ma, after shortening and exhumation in the Eastern and Western Cordillera ceased.Item Epithermal vein and carbonate replacement mineralization related to caldera development, Cunningham Gulch, Silverton, Colorado(1984-05) Hardwick, James Fredrick, 1955-; Kyle, J. RichardEpithermal vein and carbonate replacement deposits in Cunningham Gulch are located within the western San Juan Tertiary volcanic field in southwestern Colorado. The Pride of the West epithermal vein system is hosted within the intracaldera facies of the Sapinero Mesa Tuff, a voluminous ash-flow tuff that erupted from and resulted in the formation of the San Juan Caldera at 28 mybp. The Pride of the West vein system is developed along a radial fracture formed during resurgence of the San Juan Caldera prior to eruption of the Crystal Lake Tuff (27.5 mybp). This eruption led to the concomitant collapse of the Silverton Caldera, nested within the larger San Juan Caldera. The Pride of the West, Osceola, and Little Fanny mines are positioned near the intersection of the Pride radial fracture system and the buried structural margin of the San Juan Caldera, suggesting that ore concentration was controlled by this structural setting. Large limestone blocks of the Mississippian Leadville Formation are incorporated into the intracaldera fill volcanics in the mine area. These blocks appear to have been engulfed within mudflow breccias of the Tertiary San Juan Formation (32.1 mybp). They were then emplaced in their present structural position within a caldera-collapse breccia which caved from the oversteepened margin of the San Juan Caldera. Regional propylitic alteration of the hosting volcanics to a chlorite-calcite-pyrite assemblage preceded vein-associated alteration and mineralization. The veins are enveloped by a narrow phyllic alteration assemblage of quartz, sericite, illite, kaolinite, and pyrite. The veins are comprised of sphalerite, galena, chalcopyrite, pyrite, hematite, magnetite, quartz, pyroxmangite, calcite, and minor barite. Substantial bodies of replacement ore are present where the vein structures intersect the limestone blocks; the mineral assemblages of the replacement deposits are identical to those of the feeding vein structures. Commonly, replacement textures are spectacular concentrations, especially the "zebra ore" which primarily consists of regularly spaced, alternating bands of sulfides and quartz. These "zebra" laminations are stratigraphically controlled and appear to represent replacement of a depositional or diagenetic fabric. Main ore-stage mineralization began with widespread deposition of quartz with or without pyrite, followed by sphalerite, chalcopyrite, and galena. Post ore-stage brecciation and silicification events are evident and were followed by deposition of calcite and minor barite during the waning stages of the hydrothermal system. The distributions of Fe, Mn, Pb, and Ca suggest a lateral component of fluid flow from northwest the southeast, away from the structural margin of the Silverton Caldera. Fluid inclusion data from both vein and replacement-type sphalerite and quartz indicate that mineral deposition occurred over a range of 200 to 312°C (mean 243°C) from solutions containing 1 to 5% total salts. The high base metal to precious metal content of the ore, the phyllic alteration assemblage, and the temperature and composition of the ore-forming fluid indicate that the mine workings are within the lower portion of a fossil geothermal system.Item Fold-related brittle structures and associated strain in a limestone bed of the Carmel Formation, San Rafael Swell, Utah(2015-12) Laciano, Peter Joseph; Marrett, Randall; Cloos, Mark; Ukar, EstibalitzThe San Rafael Swell (SRS) is a basement-cored Laramide uplift located in central-eastern Utah. The SRS is bounded on the east by a 70 km long monocline, a fault-propagation fold, with excellent exposure of sedimentary strata including the Carmel Formation. This monocline is an ideal natural laboratory for studying brittle deformation associated with folding. Qualitative and quantitative observations for brittle structures in a limestone bed near the base of the Carmel Fm. were made in a wide range of bedding dip, curvature, and fold domains. Kinematic data was collected for 2942 structures (1865 veins, 746 stylolites, 314 faults) in 30 locations in order to calculate principal directions of strain. Additionally, data was collected along 71 scanlines at 19 of those locations in order to estimate structure intensities and strain magnitudes. Dekameter-displacement thrust faults, acting as ramps between inferred layer-parallel faults, accommodate orders of magnitude more strain than all other observed brittle structures. These faults are only found in segments of the monocline where bedding dip is high, but curvature is low, which provides strong evidence that limb rotation more strongly controls strain magnitudes than layer bending in the SRS. The trishear model effectively predicts SRS monocline geometry, specifically observed limb thickening, broad, curved hinges, and progressively rotating limb. This is likely due to the dominance of thick, homogeneous rock packages, such as the Navajo Sandstone, in the SRS monocline. In contrast, strain localization within the Carmel Fm. is poorly predicted by trishear: there is strong evidence of flexural slip, and folding induced structure orientations and calculated principal strain directions remain consistent relative to bedding. These strain directions are inconsistent with trishear forward models produced by workers such as Zuluaga et al. (2014) that do not stay consistent relative to bedding. These divergences are likely due to the fact that trishear is a kinematic model that assumes rock homogeneity, while the Carmel Fm. is stratigraphically and mechanically heterogeneous. Because this heterogeneity appears to have a strong effect on strain localization, kink band models likely better estimate strain localization in the Carmel limestone bed as well as other layers in folded heterogeneous strata. The monocline’s interpreted transition from layer-parallel shortening to extension at the steepest locations in the monocline, and thus at most advanced stage of folding, enabled estimation of the dip of the basement fault beneath the SRS as ~30°. This shallow dip contrasts with the steep dip (~60°) assumed for the SRS by Zuluaga et al. (2014) and observed in the Kaibab uplift (Huntoon and Sears, 1975; Tindall, 2000), but is consistent with a recent estimation of 20-40° for the SRS by Davis and Bump (2009) using trishear modeling.Item From rifting to collision : the evolution of the Taiwan Mountain Belt(2013-05) Lester, William Ryan; McIntosh, Kirk D.; Lavier, Luc LouisArc-continent collisions are believed to be an important mechanism for the growth of continents. Taiwan is one of the modern day examples of this process, and as such, it is an ideal natural laboratories to investigate the uncertain behavior of continental crust during collision. The obliquity of collision between the northern South China Sea (SCS) rifted margin and Luzon arc in the Manila trench subduction zone allows for glimpses into different temporal stages of collision at different spatial locations, from the mature mountain-belt in central-northern Taiwan to the 'pre-collision' rifted margin and subduction zone south of Taiwan. Recently acquired seismic reflection and wide-angle seismic refraction data document the crustal-scale structure of the mountain belt through these different stages. These data reveal a wide rifted margin near Taiwan with half-graben rift basins along the continental shelf and a broad distal margin consisting of highly-extended continental crust modified by post-rift magmatism. Magmatic features in the distal margin include sills in the post-rift sediments, intruded crust, and a high-velocity lower crustal layer that likely represents mafic magmatism. Post-rift magmatism may have been induced by thermal erosion of lithospheric mantle following breakup and the onset of seafloor spreading. Geophysical profiles across the early-stage collision offshore southern Taiwan show evidence the thin crust of the distal margin is subducting at the Manila trench and structurally underplating the growing orogenic wedge ahead of the encroaching continental shelf. Subduction of the distal margin may induce a pre-collision flexural response along the continental shelf as suggested by a recently active major rift fault and a geodynamic model of collision. The weak rift faults may be inverted during the subsequent collision with the continental shelf. These findings support a multi-phase collision model where the early growth of the mountain belt is driven in part by underplating of the accretionary prism by crustal blocks from the distal margin. The wedge is subsequently uplift and deformed during a collision with the continental shelf that involves both thin-skinned and thick-skinned structural styles. This model highlights the importance of rifting styles on mountain-building.Item Host rock fracture analysis: applying the deformation mechanics associated with shallow igneous intrusion to the fracture bridging theory, mckinney hills laccolith, big bend national park(Texas Tech University, 2005-08) Zimmerman, Nathan Marcus; Yoshinobu, Aaron S.; Lehman, Thomas; Gurrola, HaroldA detailed structural analysis of fractures, faults, and cleavage within the Cretaceous and Tertiary siliclastic host rocks along the semi-circular northwestern aureole of the McKinney Hills laccolith was performed to understand the relationship between regional tectonic strain and local emplacement-related strain, since the late Mesozoic. Local and regional effects of deformation are evident in the field that wrap around the laccolith (e.g. deflected host rocks, fractures) and cut through the laccolith (e.g. normal faults). Approximately 2000 fracture orientations, collected primarily from sandstone in host rock exposures within 5 structural domains and 6 stratigraphic stations in the Pen and Aguja Formations that span the western flank of the intrusion, reveal three different types of fractures in the host rocks based on bedding layer morphology and cross-cutting relationships: 1) cross-cutting, 2) confined, and 3) densely spaced. The most common type of fracture, 2) confined to a lithologic layer, are the opening mode I, extensional type fracture where no displaced markers indicate shear (e.g. slickenlines, calcite veins, displaced fossils, etc.). The confined sandstone fractures commonly either arrest at shale layers or bridge shale layers where the fracture surface continues into the overlying sandstone layer with no apparent fracture in the shale. Fracture orientations collectively analyzed in rose diagrams and stereonets reveal regional modes of deformation as well as local strain from laccolith emplacement. In all sub-domains and six measurement stations in the Pen and Aguja Formations, a primary NNW-trending and subordinate NE-trending fracture sets that coincides with predicted early Basin and Range and late Basin and Range extensional fracture directions for the Trans-Pecos region of West Texas, respectively. Theoretical laccolith emplacement-related fracture sets trend both sub-parallel to bedding strike and bedding dip. Secondary- and tertiary-trending sets of fractures that coincide with theoretical emplacement-related, extensional fracture trends occur in sub-domains IA/B, IIA/B, IIIA, IVA/B, and VC, and the six measurement stations. The fracture bridging model mimics commonly observed conditions of fracturing in the Pen and Aguja Formations, in which a fracture confined to a sandstone layer prior to laccolith emplacement is modeled. A layered model is utilized where closing tractions applied to the crack surface in the shale layer approximate mechanical differences in rock properties. Using a stress intensity analysis at the fracture tip the role to which a fracture will propagate as a function of depth within a laccolithic overburden is explored. A layer-parallel, tensile stress, associated with flexure (i.e. stretching and bending) of the McKinney Hills laccolithic overburden, acting as a bent elastic plate, is derived and applied to the fracture bridging theory. A subsequent plot of assumed values fall well within the critical sandstone failure envelope, using previously determined, mean fracture toughness values. Therefore, both orientation and numerical analyses confirm that smaller tertiary sets of fractures mapped in the Pen and Aguja Formations must be strain locally imposed by laccolith emplacement.Item Host rock fracture analysis: Applying the deformation mechanics associated with shallow igneous intrusion to the fracture bridging theory, mckinney hills laccolith, big bend national park(2005-08) Zimmerman, Nathan Marcus; Yoshinobu, Aaron S.; Lehman, Thomas; Gurrola, HaroldA detailed structural analysis of fractures, faults, and cleavage within the Cretaceous and Tertiary siliclastic host rocks along the semi-circular northwestern aureole of the McKinney Hills laccolith was performed to understand the relationship between regional tectonic strain and local emplacement-related strain, since the late Mesozoic. Local and regional effects of deformation are evident in the field that wrap around the laccolith (e.g. deflected host rocks, fractures) and cut through the laccolith (e.g. normal faults). Approximately 2000 fracture orientations, collected primarily from sandstone in host rock exposures within 5 structural domains and 6 stratigraphic stations in the Pen and Aguja Formations that span the western flank of the intrusion, reveal three different types of fractures in the host rocks based on bedding layer morphology and cross-cutting relationships: 1) cross-cutting, 2) confined, and 3) densely spaced. The most common type of fracture, 2) confined to a lithologic layer, are the opening mode I, extensional type fracture where no displaced markers indicate shear (e.g. slickenlines, calcite veins, displaced fossils, etc.). The confined sandstone fractures commonly either arrest at shale layers or bridge shale layers where the fracture surface continues into the overlying sandstone layer with no apparent fracture in the shale. Fracture orientations collectively analyzed in rose diagrams and stereonets reveal regional modes of deformation as well as local strain from laccolith emplacement. In all sub-domains and six measurement stations in the Pen and Aguja Formations, a primary NNW-trending and subordinate NE-trending fracture sets that coincides with predicted early Basin and Range and late Basin and Range extensional fracture directions for the Trans-Pecos region of West Texas, respectively. Theoretical laccolith emplacement-related fracture sets trend both sub-parallel to bedding strike and bedding dip. Secondary- and tertiary-trending sets of fractures that coincide with theoretical emplacement-related, extensional fracture trends occur in sub-domains IA/B, IIA/B, IIIA, IVA/B, and VC, and the six measurement stations. The fracture bridging model mimics commonly observed conditions of fracturing in the Pen and Aguja Formations, in which a fracture confined to a sandstone layer prior to laccolith emplacement is modeled. A layered model is utilized where closing tractions applied to the crack surface in the shale layer approximate mechanical differences in rock properties. Using a stress intensity analysis at the fracture tip the role to which a fracture will propagate as a function of depth within a laccolithic overburden is explored. A layer-parallel, tensile stress, associated with flexure (i.e. stretching and bending) of the McKinney Hills laccolithic overburden, acting as a bent elastic plate, is derived and applied to the fracture bridging theory. A subsequent plot of assumed values fall well within the critical sandstone failure envelope, using previously determined, mean fracture toughness values. Therefore, both orientation and numerical analyses confirm that smaller tertiary sets of fractures mapped in the Pen and Aguja Formations must be strain locally imposed by laccolith emplacement.