The Eastern Mexican Alkaline Province (EMAP) is a belt of Tertiary alkaline igneous intrusions extending from Coahuila to Veracruz. Lithologies range from felsic to mafic and from silica-saturated to undersaturated. Mineralization includes F ± Be associated with felsic lithologies, Mo ± Cu related to porphyries and pegmatites, Fe-oxide ± Cu skarn and Au associated with dioritic-monzonitic intrusions, REE ± Th related to carbonatites, and base metal ± Ag peripheral to igneous centers of diverse types. Two recurring features of the EMAP include (1) andradite-grossular garnet + magnetite skarn and endoskarn and (2) sodic-calcic alteration. These features resemble Fe-oxide(Cu-Au-REE) systems (e.g. Olympic Dam-type deposits), whose origins are controversial but increasingly associated with external saline fluids. The presence of Mesozoic evaporites in eastern Mexico supports the likelihood of evaporitic fluid involvement. However, observations that (1) mineralization styles are correlated with magma type and (2) certain alteration styles (e.g. K-silicate) are reminiscent of alteration by magmatic fluids, suggest that hydrothermal systems of eastern Mexico involve magmatic fluids as well.

The structure of the northern Colorado Plateau is dominated by a series of fault-bounded basement-cored uplifts: the Kaibab, Circle Cliffs, Miners Mountain, San Rafael Swell, Monument, and Uncompahgre uplifts. Outcrop-scale structures, including jointed Eshelby inclusions, stylolites, echelon vein arrays, deformation bands and meso-scale faults, suggest that the Monument and San Rafael Swell were formed by NW-SE-directed shortening while the other four uplifts were formed by SW-NE-directed shortening. Elastic modeling demonstrates that the simultaneous shortening in two directions is a result of deformation by a three-dimensional stress field where the magnitude of σ₂ approaches that of σ₁. Geologically, σ₁ is attributed to coupling between the horizontally-subducting Farallon slab and the North American plate, while the elevated values of σ₂ are attributed to far-field stresses generated by the topographically high Sevier thrust belt, located on the western edge of the Plateau. Mechanically, basment-cored uplift systems are controlled by the relative strengths of the uplift-bounding faults, which is determined by several factors, including dip, coefficient of friction, and pore-fluid pressure. Reverse slip may raise a fault's effective strength by building topography and thus increasing the weight of the hangingwall. A model is developed in which faults "slip strengthen," and promoting transfer of deformation to other, now weaker faults and broadening the region of deformation. In the broader picture, tectonic force may be applied to the edge of the system, (e.g., continental collision) or by basal traction (e.g., flat-slab subduction). In the former case shear stress on the basal detachment retards shortening. Total basal shear force (stress multiplied by area) grows with distance toward the foreland at the expense of tectonic force available to drive slip on fault ramps. Assuming quasi-uniform crustal strength, the greatest deformation develops in the hinterland where tectonic force is highest. Alternatively, tectonic force may be applied as a basal traction (as in the Laramide orogeny), in which case basal shear drives shortening and total shear force/deformational intensity grow with distance toward the foreland. This offers a natural explanation for why the Colorado Plateau, which occupies the hinterland of the Laramide orogen, is the least deformed part of the orogen.

Field relationships between the coeval Ischichuca and Chanares Formations in the southern part of the Ischigualasto rift basin, northwestern Argentina, combined with mineralogical, geochemical, and petrographic analyses demonstrates that the climate was semi-arid to semi-humid during the Middle Triassic. Although climate changes (sediment and water supply) controlled the types of sediment that accumulated in the basin, facies distribution is controlled by differential subsidence in the rift basin. Asymmetrical subsidence is inferred from the change in thickness of the rocks perpendicular to the western basin-bounding Valle Fertil fault. Braidplain and fluvially-dominated lacustrine deltaic deposition occurred closest to the Valle Fertil fault. The rivers drained the surrounding highlands composed of Precambrian metamorphic rocks and Paleozoic plutons. Fluvial systems tended towards the topographic minimum, in this case, the lake, which is 10 km from the Valle Fertil fault. The lack of Gilbert-style deltas suggests that the fluvial systems entering the lake were not on a steep gradient and the lake was not very deep. The widespread lateral extent and thin palustrine deposits and B horizons of moderately well-developed Vertisols along the southern and eastern margins illustrates slow aggradation of material. Vertic features in paleosols, pedogenic mud chips in fluvial and lacustrine deposits, and clastic-organic and carbonate-organic couplets in lacustrine facies indicate seasonality of precipitation. The Corg/Stotal ratios, presence of Unionids, charophytes, and detrital clay minerals, and lack of desiccation features or saline and saline alkaline minerals indicate the lake was fresh. Significant coal swamp deposition occurred along the lake margins. The combination of Vertisols, coals, and lack of pedogenic carbonate in the paleosols suggests the mean annual precipitation in the Ischigualasto Basin was between 120--250 cm. The carbon isotopic composition of terrestrial organic matter in fluvial and lacustrine deposits and Vertisols documents that plants were subjected to significant water stress.

The origin of pedogenic salts in the Atacama Desert has long been debated. Possible salt sources include in situ weathering at the soil site, local sources such as aerosols from the adjacent Pacific Ocean or salt-encrusted playas, and extra-local atmospheric dust. To identify the origin of Ca and S in Atacama soil salts, we determined δ ³⁴S and ⁸⁷Sr/⁸⁶Sr values of soil gypsum/anhydrite and ⁸⁷Sr/⁸⁶Sr values of calcium carbonate along three east-west trending transects in the Atacama. Our results demonstrate the strong influence of marine aerosols on soil gypsum/anhydrite development in areas where marine fog penetrates inland. In areas where the Coastal Cordillera is >1200 m, however, coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced. Salts in inland soils appear to originate from eolian redistribution of playa salts that are precipitated from evaporated ground water. This ground water has acquired its dissolved solids from water-rock interactions (both thermal and low-temperature) along flowpaths from recharge areas in the Andes. The spatial distribution of high-grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local dust, supporting arguments for an atmospheric source of nitrate. Ground water in the Atacama is derived from precipitation in the High Andes (>3500 m) that infiltrates soils and then flows down the Pacific slope of the Andes to feed aquifers within the hyperarid core of the Atacama Desert. At many locations, ground water surfaces and creates springs, marshes, and wetlands. In order to track late Quaternary fluctuations in ground-water recharge, paleowetland deposits at eight separate locations (between 18°-26°S) were mapped and dated. Over 200 AMS ¹⁴C dates on a variety of materials provide firm age control on these deposits. Replication of time-stratigraphic units from an assortment of hydrologic settings and varying distances from recharge areas in the Andes show that ground-water systems are responding to regional changes in climate and that response times are probably short (<1000 years). Results suggest that the wettest period represented by deposits was during the late Glacial/early Holocene (∼16-9.5 ka B.P.) and that a moderately wet period occurred during the mid-Holocene (8--3 ka B.P.). Major drops in Atacama water tables, due to regional drought, occurred between 9.5-8 and ∼3 ka B.P. The late Holocene was characterized by generally lower water tables than during the mid-Holocene and subject to more frequent water table drops. Fluctuations in tropical Pacific Sea Surface Temperatures, the Walker Circulation, and ENSO variability is thought to be the major control on precipitation over this region during the late Quaternary.

Tectonic shortening within the Himalayan fold-thrust belt in Nepal has been accommodated by southward displacement of large thrust sheets. Most workers focus on the impact that the Main Central, Main Boundary, and Main Frontal thrusts have had on the orogen's structural, thermal, and geomorphic evolution. However, mapping across Nepal, has revealed the presence of the Ramgarh thrust, which is another orogen-scale thrust. The Ramgarh thrust, which had previously been recognized in India and far-western Nepal, occurs within Lesser Himalayan zone rocks, and accommodates a magnitude of shortening similar to that of the Main Central thrust. This dissertation focuses on the structural and tectonic significance of the Ramgarh thrust. Minor details notwithstanding, the structural characteristics of the Ramgarh thrust remain consistent along the ∼800 km width of the fold-thrust belt in Nepal. At current levels of erosion, the Ramgarh thrust is always exposed in a hanging-wall flat on footwall flat thrust relationship with other Lesser Himalayan zone rocks, and also with overlying rocks carried by the Main Central thrust. Mapping along a north-south transect in central Nepal has permitted the construction of a balanced cross-section, which shows that the fold-thrust belt has accommodated a minimum of 489 km of tectonic shortening. A large proportion of which was accommodated by slip on the Ramgarh thrust. Integrating structural constraints provided by mapping and the cross-section with existing thermochronologic, thermobarometric, and foreland basin provenance datasets yields a kinematic model for the structural evolution of the fold-thrust belt. Recognition of the structural relationship between the Ramgarh and Main Central thrusts also permits new insight into the nature of the Main Central thrust. Structural mapping combined with Nd isotope studies from the vicinity of the Ramgarh and Main Central thrusts in Langtang National Park suggest that the Main Central thrust can be defined as a relatively narrow tectonostratigraphic contact, and not as a broad, poorly defined, shear zone. Additionally, much of the volume of highly strained rocks in the footwall of the Main Central thrust may be genetically related to deformation on faults (including the Ramgarh thrust) that lie structurally below the Main Central thrust.

Growth strata analysis and detrital-zircon geochronology are useful applications of stratigraphy to tectonic problems. Whereas both tools can contribute to kinematic analyses of supracrustal rock bodies, growth strata are also useful for analyzing the influence of tectonics on stratigraphic architecture. This study reports: (1) a conceptual model for growth strata development; (2) stratigraphic and kinematic analyses of growth strata architectures from growth structures in southeastern Utah, the Gulf of Mexico, and northeastern Spain; and (3) the detrital-zircon geochronology of the Salinian block of central coastal California. Kinematic sequence stratigraphy subdivides growth strata into kinematic sequences that are separated by kinematic sequence boundaries. Kinematic sequences can be further partitioned into kinematic domains based on the termination patterns of strata within a kinematic sequence. Salt-related fluvial growth strata from the Gulf of Mexico and southeastern Utah contain stratigraphic architectures that are unique to different kinematic domains. Offlap kinematic domains contain fluvial strata indicative of high slopes, low accommodation rates, and strong structural influence on paleocurrent direction. Onlap kinematic domains contain fluvial strata indicative of moderate slopes, high accommodation rates, and decreased structural influence on paleocurrent direction. The stratigraphic architecture of alluvial-fan thrust-belt growth strata in northeastern Spain does not display a marked correlation with kinematic domain, and is most easily interpreted using existing models for autocyclic alluvial-fan evolution. Detrital-zircon (U-Pb) geochronologic data from basement and cover rocks of Salinia suggest that Salinia originated along the southwestern margin of North America, likely in the vicinity of the Mojave Desert. The presence of Neoproterozoic and Late Archean detrital zircons in Salinian basement rocks also suggest that Salinian sediments were recycled from miogeoclinal sediments of the western margin of North America.

The Late Triassic Ischigualasto Formation, located in northwestern Argentina, was deposited during the late Norian Age in a continental rift valley. Detailed field descriptions revealed a connection between sedimentation rates and a change in the mean annual precipitation rates. The mean annual precipitation in the lower half of the Ischigualasto Formation is estimated at >760 mm and the upper half is estimated at <760 mm to 1400mm. Increases in the mean annual precipitation affected the architecture of the alluvial deposits such as decreased paleosol profile thickness and increased channel interconnectedness. Avulsion frequencies were calculated from paleosol successions sandwiched between channel sandstones and showed a decrease in the avulsion frequency through time as the Ischigualasto Formation was deposited. Since these channel sandstones were deposited along the axis of the a rift valley and avulsion should increase with increases in fault movement along the main rift valley fault, it is proposed here that the avulsion frequency suggests that there is a decrease in fault activity and extension in the upper half of the Ischigualasto Formation. Initial models of the Ischigualasto Basin have been compared to field data collected and a newer model of the active rift systems, and after review, a new revised model is proposed. The new model suggests that there are three main phases of rift development that are related to the amount of crustal extension: sub-basin, single basin, and passively subsiding basin phases. Positions of lava flows within the basin fill were also considered as signs of local volcanic activity; these are absent in the upper portions of the basin fill. Chronostratigraphic frameworks developed for the entire basin fill also support a decrease in the sedimentation rate through time during the deposition of the basin fill.

Here we report new and previously published Re-Os and trace elemental data on gold as well as coeval sulfides and oxides from various Archean to Alphanumeric gold deposits. When possible, measured concentrations and isotopic ratios were used to determine geochronology as well as the likely source reservoir of the gold mineralization. Rhenium-osmium ages of some gold and related minerals are consistent with existing geochronology and in some cases better constrain genetic models of mineralization. The initial Os isotopic composition is also preserved within some ore minerals and reflects the importance of both crustal and mantle material in the generation of specific deposits. However, many of the gold deposits show evidence of post-crystallization Re-Os disturbance, and so age and source information are not preserved. In these cases, it is likely that hydrothermal fluids added or removed Re or Os subsequent to primary mineralization. Preservation of age favors minerals with high Re and Os concentrations and/or minerals from monocyclic gold deposits without multiple influxes of fluids. More specifically, gold and pyrite from the 2890--2710 Ma Witwatersrand basin, typically form ca. 3000 Ma isochrons with chondritic initial 187Os/188Os values. The Os concentrations of the gold range from approximately 2 to over 4000 ppb and are significantly elevated compared to other gold deposits. The older age, mantle source and high concentration of the gold support modified paleo-placer models with minor hydrothermal modification but not significant deposition of gold via hydrothermal fluids of any age. Other gold deposits have low average Os concentrations ranging from approximately 20 ppt to 1 ppb and Re/Os ratios generally similar to average continental crust. The initial 187Os/188Os of these deposits vary but seem to have more crustal signatures than the Witwatersrand deposits. Data support gold deposition from leaching of gold and Os via hydrothermal fluids from crustal lithologies. Os concentration and Re/Os data of gold show systematic variation with the age of the deposit. These trends may result from a decrease in the efficiency of partial melts to extract juvenile gold and Os from the cooling mantle and the increasing importance of recycling gold from pre-existing continental crust.

Gastropod shells are commonly preserved in the geologic record, but are often avoided for radiocarbon dating because some taxa incorporate ¹⁴C-deficient carbon during shell formation. To determine the potential of minute (3-10 mm in length) gastropods for ¹⁴C dating, we collected specimens en vivo from alluvium dominated by Paleozoic carbonates and adjacent to springs with ¹⁴C-deficient water present at the surface. We found that at least three taxa, Pupilla blandi, Euconulus fulvus, and Succineidae, do not incorporate ¹⁴C-deficient carbon during shell formation. We ¹⁴C dated fossil shells from these taxa recovered from the Coro Marl, a paleowetland deposit in the San Pedro Valley of southern Arizona. Our results indicate that the timing of high water table conditions in the valley occurred between >28 and 15.5 ka, which is consistent with moist conditions found in other paleoclimate records from the American Southwest. The summit area of Mauna Kea, Hawaii was covered intermittently by ice caps during the Late Pleistocene. The maximum extents of the last two ice caps (older and younger Makanaka) were similar, reaching ∼800 m below the summit. We have developed a new chronology for these glaciations using cosmogenic ³⁶Cl which shows that the ice caps began retreating from their maximum extents at 23.3 ± 2.3 ka and 13.0 ± 0.9 ka, respectively, broadly coincident with the last glacial maximum and the Younger Dryas chronozone. The potential for using in situ cosmogenic ¹⁴C to determine surface exposure ages of Holocene landforms, quantify erosion rates, and decipher complex exposure histories when used in conjunction with other cosmogenic nuclides is well known. Before this potential can be realized, however, protocols for isolating and extracting in situ ¹⁴C must be developed. Analytical techniques have been developed previously to isolate in situ ¹⁴C from quartz and carbonate. Although these minerals can be found in most places on Earth, they are usually absent from basaltic terrains. To fill this gap, we conducted numerous chemical pretreatment experiments and step-heated extractions aimed at isolating and extracting in situ ¹⁴C from olivine. Our results suggest that step-heated extractions alone may be sufficient to isolate in situ ¹⁴C from olivine.

The structure of basement-cored uplift mountains is fundamentally controlled by the geometry of the basement and the basement unconformity. In the Sierra de Hualfin, an uplift located in the Sierras Pampeanas of NW Argentina, the well-exposed basement unconformity displays along-strike structural variations. The Sierra de Hualfin is composed of a core of Ordovician granitoid overlain by and thrust over Tertiary sedimentary rocks. The range is uplifted by an east-directed thrust fault, creating a steep range-front on the east and a gently dipping backlimb on the west. In the central region, however, basement and cover rocks in the backlimb are folded. Cross sections across the uplift and a three-dimensional computer model show that the variations along strike are due to differences in the geometry of the underlying fault or faults that deform the basement unconformity. Analysis of the folded basement unconformity on the west side of Sierra de Hualfin shows that brittle processes of faulting, cataclasis and fracture foliation help the basement attain a folded form. Pervasive and continuous joint fractures, and an unconformity-parallel fracture foliation at the surface, set up a condition for "flow" of granitic materials. Additionally, in order for basement to fold, the tip of the underlying fault must be located within the basement before it begins propagating towards the surface. Using the knowledge gained from the detailed structural studies, two regional cross sections were constructed in the region of Sierra de Hualfin, showing spatial and geometric relations between the adjacent uplifts, and the thrust systems of the eastern Puna Plateau. The cross sections show detachment levels at 16 km and ∼27 km for uplifts in the northern Sierras Pampeanas. Thrust faults of the northern Sierras Pampeanas, and of the eastern edge of the Puna can be interpreted as part of the same east-directed thrust system, with intervening west-directed backthrust development. The combination of the modeling of Sierra de Hualfin and the regional cross sections with detachments at 16 to 27 km depth suggests that basement-cored uplifts can be formed by gently-dipping faults detaching in the middle crust and near the base of the crust.