The early Laramide tectonic history and Proterozoic metamorphic history of the northeastern Colorado Front Range were examined using kinematic data from minor faults at 25 locations, and U-Th/He dating of apatite from 2 samples (3 unsuccessful) supported by optical petrography, X-ray maps and geothermometry. The role of strike-slip faulting in Laramide uplift of the eastern flank of the northern Front Range was analyzed through kinematic analysis of 97 minor (<100 m trace) faults. The dominant fault population was oriented approximately perpendicular to bedding, with lineations sub-parallel to bedding. Rotating bedding to horizontal showed these faults to have the pattern of a strike-slip conjugate set. Unfolded left-lateral faults have an average orientation of (287, 87N) with lineations to (287, 01); right-lateral faults have an orientation of (065, 88S) with lineations to (245, 00). The timing of motion on these faults postdates 98 Ma deposition of Dakota group sandstones, and predates the folds that rotated them (68 Ma from the age of synorogenic conglomerates). The conclusion is that strike-slip motion was active during the earliest Laramide. The principal strain axes from these faults, after rotation, give an average shortening axis orientation of (276, 03) and an average extension direction of (006, 02). The calculated shortening axis orientation is consistent with that of later Laramide deformation, supporting the hypothesis that strike-slip deformation occurred in the northeast Front Range during the earliest Laramide. Analysis of minor faults on part of the eastern flank of the northern Front Range shows that strike-slip faulting was a locally important deformation mechanism at the onset of the Laramide orogeny. Principal strain axis analysis suggests that the regional tectonic regime was one of east-west shortening and north-south extension prior to the onset of the main phase of Laramide deformation, at which time the regional strain field rotated to one of east-west shortening and vertical extension. Twenty-seven thin-sections of Big Thompson Canyon metapelites were petrologically characterized prior to selection for mineral separation and U-Th/He analysis of apatite. All samples show late high-temperature static recrystallization that has partially recovered prior fabrics. At high grades, sillimanite porphyroblasts overgrow all fabrics. X-ray maps and geothermometry were also conducted to enhance the characterization of the sample suite. Garnet-biotite phase equilibria indicate that initial prograde metamorphism took place at approximately 550 °C. Apatites separated from five of the twenty-seven samples analyzed in thin section were analyzed for U-Th/He thermochronometry. Three samples returned no plausible results; one sample returned one plausible age; and one sample returned three similar and plausible ages. Basement rocks cooled through the closure temperatures for helium and fission-tracks in apatite nearly simultaneously, at about 55 Ma. These temperatures, 40 and 60 °C respectively, correspond to depths of ~ 1.5 and 2.5 km. Laramide exhumation of the Northern Front Range was very rapid. Estimates of minimum magnitude of exhumation during Laramide time may need to be increased from ~2 km to ~3 km. / text

A field study of positive inversion is conducted to describe associated structural fabrics and to infer kinematic development of the Palisades Monocline, Grand Canyon, Arizona. These features are then compared to sand, clay and solid rock models of positive inversion to test model results and improve understanding of inversion processes. The N40W 90 oriented Palisades fault underlying the monocline has experienced northeast-southwest Precambrian extension and subsequent northeastsouthwest Laramide contraction. The magnitude of inversion is estimated to be 25% based on vertical offset across the fault, although this does not account for flexure or horizontal shortening. The preferred N50W 90 joint and vein orientation and N50W 68 NE and SW conjugate normal faults are consistent with the Palisades fault and northeastsouthwest extension. The N45E 90 joint orientation and approximately N40W 28 NE and SW conjugate thrust faults are consistent with northeast-southwest contraction. The deformation is characterized by three domains across the fault zone: 1) the hanging wall, 2) the footwall, and 3) an interior, fault-bounded zone between the hanging wall and footwall. Extensional features are preserved and dominate the hanging wall, contractional features define footwall deformation, and the interior, fault-bounded zone is marked by the co-existence of extensional and contractional features. Extension caused a master normal fault and hanging wall roll-over with distributed joints, veinsand normal faults. During inversion, contraction induced reverse reactivation of existing hanging wall faults, footwall folding and footwall thrust-faulting. Precambrian normal slip along the master normal fault and subsequent Laramide reverse slip along the new footwall bounding fault created an uplifted domain of relatively oldest strata between the hanging wall and footwall. Physical models of co-axial inversion suggest consistent development of the three domains of deformation described at the Palisades fault, however the models often require magnitudes of inversion greater than 50%. Although vertical block motion during horizontal compression is not predicted directly by the Mohr-Coulomb criterion, physical models and analytical solutions (incorporating Mohr- Coulomb criterion) suggest maximum stress trajectories and near vertical failure above high angle basement faults that compare favorably with the Palisades fault zone.

Multiple generations of normal faults dismembered, tilted, and exposed thicknesses of up to 15 km of the upper crust in portions of central and southern Arizona. This extension, variable in distribution and magnitude, was superimposed on the axis of the Laramide magmatic arc and dismembers many porphyry copper systems, allowing for detailed study of vertical and lateral zonation of alteration around these centers. This study examines tilted fault blocks containing portions of porphyry systems across Arizona, focusing on hydrothermal alteration deep and distal in these systems (3+ km paleodepth) to develop a more complete understanding of porphyry occurrences as larger geochemical systems. This study focuses on Na-Ca and coarse muscovite alteration in the roots of Laramide porphyry copper systems across Arizona (Ajo, Sierrita, Kelvin-Riverside, Mt. Grayback, Granite Mountain, Charleston, Globe-Miami, Sycamore Canyon, Copper Basin, Texas Canyon, and Copper Creek), provides a detailed study of Middle Jurassic coarse muscovite alteration at Luhr Hill in the Yerington district, Nevada, and documents the structural and hydrothermal evolution of the Ajo mining district in southwestern Arizona. Most areas in this study are interpreted to be highly extended, highly eroded, or both, allowing for study of deep hydrothermal alteration. Na-Ca alteration has been previously documented extensively along the Jurassic arc of the southwestern United States but less widely known in younger plutons, notably of Laramide age in Arizona. Coarse muscovite alteration previously has rarely been documented in porphyry copper systems, and this study shows that coarse muscovite alteration is likely present in systems where root zones are exposed at surface. Na-Ca alteration also is present in many in Laramide porphyry systems, though volumetrically minor, as no Laramide system contains more than a few volume perfect Na-Ca alteration in a given hydrothermal system. Na-Ca alteration in Laramide systems can include Ca, Na-Ca and Na alteration but is dominated by Na alteration (epidote-albite-chlorite ± actinolite). At Ajo, both Na-Ca and coarse muscovite alteration are present within the district due to superposition of temporally unrelated hydrothermal alteration, coupled with complex extensional deformation. This study shows that both Na-Ca and coarse muscovite alteration are more common in Laramide porphyry copper systems than previously recognized, that Na-Ca alteration is most commonly developed as shallower Na alteration (albite-epidote-chlorite ± actinolite), common deeper Na-Ca alteration (oligoclase-actinolite-epidote), and rare, deep Ca alteration (oligoclase-diopside-actinolite ± garnet ± epidote). Na-Ca alteration is commonly less voluminous in Laramide systems than documented in systems along the Jurassic arc. Coarse muscovite alteration, commonly termed greisen, occurs structurally below and commonly postdates potassic alteration and likely formed from late-stage, low-temperature, magmatic-hydrothermal fluids. Coarse muscovite alteration associated with more silicic magmatic compositions is developed at shallower depths and contains muscovite with higher trace element contents, coarse muscovite alteration with more variable mineral assemblages, and coarse muscovite veins that are better mineralized. Coarse muscovite alteration (greisen) occurs as the main mineralized veins at the tops of evolved metaluminous to peraluminous granites in W-Sn systems, as well mineralized veins in the cores and tops of Mo-Cu porphyry systems, and as poorly mineralized veins in the roots of porphyry copper systems. Detailed understanding of coarse muscovite alteration in a given district can, therefore, can be an indicator of depth or petrologic affiliation of a system. These results provide a better understanding of late-stage magmatic-hydrothermal alteration and hydrothermal alteration associated with the incursion of external fluids into the root zones of porphyry copper systems.

Geologic mapping (1:24,000 scale) of the Ice Cave Peak quadrangle, Uintah and Duchesne Counties, Utah has produced a better understanding of the geologic structures present in the quadrangle and has increased our understanding of faulting in northeastern Utah. Map units in the quadrangle range in age from late Neoproterozoic to Quaternary and include good exposures of Paleozoic rocks (Mississippian to Permian), limited exposures of Mesozoic rocks, and good exposures of Tertiary strata (Duchesne River Formation and Bishop Conglomerate) deposited during uplift of the Uinta Mountains. Lower Mississippian strata along the south flank of the Uinta Mountains have typically been mapped as Madison Limestone. Our preliminary mapping suggested that the Madison could perhaps be subdivided into an upper unit equivalent to the Deseret Limestone, and a lower unit separated by a phosphatic interval equivalent to the Delle Phosphatic Member of the Deseret Limestone found farther west. Upon further investigation, we propose not extending the use of Deseret Limestone, with the equivalent to the Delle Phosphatic Member at its base, into the south-central Uinta Mountains. Microprobe analysis revealed no phosphorus in thin sections of this unit. Instead, the unit is composed almost entirely of calcite and dolomite. A zone of northwest-trending faults, called the Deep Creek fault zone, occurs mainly east of the Ice Cave Peak quadrangle. However, our mapping shows that this fault zone extends into the quadrangle. These faults are both strike-slip and normal/oblique faults as documented by mapping and kinematic indicators and cut the folded hanging-wall sedimentary rocks above the Uinta Basin-Mountain boundary thrust fault. These faults may be part of an en echelon fault system that is rooted in the Neoproterozoic and reactivated during Laramide deformation above a possible transfer zone between segments of the buried boundary thrust.

This dissertation contains two studies that investigate the Mesozoic and Cenozoic tectonics of the western USA. The first study investigates shortening in the Sevier thrust belt of northeast Utah and southwest Wyoming. Cross section restoration suggests that there was &sim;8-14 km of pre-Absaroka-thrust shortening above the Jurassic Preuss salt detachment (PSD), but not below it, in the hanging wall of the Absaroka thrust. Reflection seismic data show that the Crawford thrust is not offset along the PSD, indicating that the additional shortening on the Absaroka plate was transferred east before main movement on the Crawford thrust. Integration of surface and subsurface geology suggests slip from the Willard or Lost Creek thrust was transferred several tens of kilometers east along the PSD between &sim;102-90 Ma.The second study investigates the low-temperature thermochronology of the Laramide Ranges. We dated 91 borehole and surface samples from basement-cored uplifts of the Rocky Mountain foreland (Wind River, Beartooth, Bighorn and Laramie Ranges), and the Uncompahgre Uplift, using the apatite (U-Th)/He system. (U-Th)/He ages generally increase with increasing elevation. Most samples show age dispersion of tens to hundreds of Myr. Several samples show correlations between (U-Th)/He age and effective U concentration (eU = [U] + 0.235[Th]), indicating that radiation damage has affected (U-Th)/He age. Many surface and near-surface samples have (U-Th)/He ages that are older than apatite fission-track ages.Forward and inverse modeling using a radiation damage diffusion model showed that (U-Th)/He ages may be widely dispersed, and may be older than apatite fission-track ages within a fossil partial retention zone. Most samples, however, do not exhibit the predicted (U-Th)/He age-eU correlation. We show that the effects of grain size can obscure (U-Th)/He age-eU correlations. Best-fit thermal histories from the inversion of age-eU pairs were extrapolated to other elevations to create model age-elevation plots. "Too-old" (U-Th)/He ages that are not within a fossil partial retention zone are likely due to He implantation from high-eU phases. Inverse modeling of (U-Th)/He age data suggests that rapid exhumation within the Laramide province began earlier in the Bighorn Mountains (before &sim;71 Ma) than the Beartooth Range (before &sim;58 Ma).

Detailed mapping (1:24,000), measured sections, and clast counts in conglomerates of the Duchesne River Formation and Bishop Conglomerate in the Vernal NW quadrangle in northeastern Utah reveal the middle Cenozoic stratigraphic geometry, the uplift and unroofing history of the eastern Uinta Mountains, and give evidence for the pulsed termination of Laramide uplift. The Unita Mountains are an EW-trending reverse fault bounded and basement-cored, Laramide uplift. The oldest unit of the Duchesne River Formation, the Eocene Brennan Basin Member, contains 80-90% Paleozoic clasts and <20% Precambrian clasts. Proximal to the Uinta uplift the conglomerates of this member are dominated by Paleozoic Madison Limestone clasts (70-90% of all clasts). Farther out into the basin, Paleozoic clasts still dominate in Brennan Basin Member conglomerates, but chert clasts are more abundant (up to 43%) showing the efficiency of erosion of the carbonate clasts over a short distance (~5 km). Conglomerates in the progressively younger Dry Gulch Creek, Lapoint, and Starr Flat members show a significant upward increase in Precambrian clasts with 34-73% Uinta Mountain Group and 8-63% Madison Limestone. Duchesne River Formation has a significant increase in coarse-grained deposits from the southern parts of the quadrangle (20-50% coarse) to the northern parts (75% coarse) nearer the Uinta uplift. The lower part of the Duchesne River Formation exhibits a fining upward sequence representing a tectonic lull. Clast count patterns show that pebbly channel deposits in the south maintain similar compositions to their alluvial fan counterparts. To the north, the fine-grained Lapoint and Dry Gulch Creek members of the Duchesne River Formation appear to pinch out completely. This can be explained by erosion of these fine-grained deposits or by lateral facies shifts before deposition of the next unit. Starr Flat Member conglomerates were deposited above Lapoint Member siltstones and represent southward progradation of alluvial fans away from the uplifting mountain front. Similarities in composition and sedimentary structures have caused confusion surrounding the contact between the Starr Flat Member and the overlying Bishop Conglomerate. Within the Vernal NW quadrangle, we interpret this contact as an angular unconformity (the Gilbert Peak Erosion Surface) developed on the uppermost tilted red siltstone of the Starr Flat Member sometime after 37.9 Ma. Stratigraphic and structural relationships reveal important details about the development of a Laramide mountain range: 1) sequential unroofing sequences in the Duchesne River Formation, 2) progradation of alluvial fans to form the Starr Flat Member, 3) and the unconformable nature of the Gilbert Peak Erosion Surface lead to the conclusion that there were at least 3 distinct episodes of uplift during the deposition of these formations. The last uplift episode upwarped the Starr Flat Member constraining the termination of Laramide uplift in the Uinta Mountains to be after deposition of the Starr Flat Member and prior to deposition of the horizontal Bishop Conglomerate starting at about 34 Ma. This, combined with 40Ar/39Ar ages of 39.4 Ma from the Dry Gulch Creek and Lapoint member, show that slab rollback related volcanism was occurring to the west while the Uinta Mountains were being uplifted on Laramide faults. These new 40Ar/39Ar ages constrain the timing of deposition and clarify stratigraphic relationships within the Duchesne River Formation; they suggest a significant unconformity of as much as 4 m.y. between the Duchesne River Formation and the overlying Bishop Conglomerate, which is 34-30 Ma in age, and show that Laramide uplift continued after 40 Ma in this region.

Subsidence is one of the main factors controlling the stratigraphy and overall stratal architecture in tectonically active basins. This was particularly important in the Western US Cordilleran foreland and Laramide basins when some other controls were minor, e.g. reduced eustatic fluctuations in the late Cretaceous greenhouse period. The first part of the dissertation examines the upper Campanian Williams Fork Clastic Wedge (WFCW) in southern Wyoming and northern Colorado, through an outcrop and subsurface database. The WFCW built out from the Sevier orogenic belt like earlier clastic wedges, but its large-scale geometry changed as basement involved Laramide structures partitioned it. At the center of the WFCW there is an extensive fluvial sandstone sheet, the Canyon Creek Member of the Ericson Formation. From its proximal to distal reaches (~200 km) there is a first order trend of stratigraphic thickening and net-to-gross reduction, and a change from braided to meandering depositional style. These trends are caused by isostatic rebound of the foreland basin during periods of relative quiescence in the Sevier orogenic belt and by the eastward migration of dynamic subsidence. However, this long spatial trend was markedly modified by differential subsidence across Laramide-style structures. The Campanian age initiation of the Laramide structures appears to be earlier than the Maastrichtian to Paleogene age commonly attributed to the initiation of this orogeny. The second part of this research focuses on the transgressive limb of the WFCW, particularly on two sandstone bodies isolated in marine mudstones in the uppermost Almond Formation. The sandstone bodies previously interpreted as lowstand shoreline deposits are re-interpreted as transgressive shelf ridges generated by tidal currents and storm waves. There are limited examples of ancient tidal shelf ridges published and no facies model was described. Using Almond Fm. outcrops and examples from the literature, the diagnostic characteristics of storm and tidal shelf ridges are presented. The third part of the dissertation investigates the effects of differential subsidence on the large scale stratigraphic infill of a foreland basin through a geometric model and a series of flume experiments. The mathematical model and flume experiments show that despite constant allogenic forcing, three distinct autogenic responses in stratal architecture, associated with the imposed tectonic and sediment supply conditions are possible. The first response was “autoretreat”, where shoreline migration switched from initial progradation to retrogradation. The second response was progradation followed by constant aggradation. The third response was maintained progradation with a markedly accelerating rate, a new autogenic behavior termed “shoreline autoacceleration”. / text

The Laramide Rocky Mountains in western U.S.A is an important topographic feature in the continental interior, yet its formation and evolution are poorly constrained. This study uses the oxygen and strontium isotope geochemistry of freshwater bivalve fossils from six Laramide basins in order to reconstruct the spatial evolution of the paleotopography and Precambrian basement erosion in late Cretaceous-early Eocene. In addition it uses the sedimentology, detrital zircon U-Pb geochronology, and isotope paleoaltimetry of early Eocene sedimentary strata to constrain the tectonic setting, paleogeography and paleoclimate of the Wind River basin. Annual and seasonal variation in ancient riverwater δ¹⁸O reconstructed from shell fossils shows that the Canadian Rocky Mountains was 4.5±1.0 km high in late Cretaceous-early Paleocene, and the Laramide ranges in eastern Wyoming reached 4.5±1.3 km high, while the ranges in western Wyoming were 1-2 km high in late Paleocene. The ⁸⁷Sr/⁸⁶Sr ratios of riverwaters reconstructed from the same fossils show that Proterozoic metamorphic carbonates in the Belt-Purcell Supergroup were not exposed in the Canadian Rocky Mountains during Late Cretaceous-early Paleocene, but that Precambrian silicate basement rock was exposed and eroded in the Laramide ranges during late Paleocene-early Eocene. The sedimentary environment of the early Eocene Wind River basin changed from gravelly fluvial and/or stream-dominated alluvial fan to low-sinuosity fluvial systems. Tectonic uplift of the Washakie and Wind River Range in early Eocene formed the modern paleodrainage system, although the elevation of the basin floor was only ~500 m high at that time, and early Eocene paleoclimate is more humid than modern climate.

In the Basin and Range province of southwestern North America, Oligocene and Miocene normal faults are superimposed upon the Late Cretaceous-early Tertiary magmatic arc. This study examines tilted fault blocks containing dismembered pieces of porphyry systems, including pieces below and peripheral to ore bodies, that are exposed at the modern surface. Features in the magmatic-hydrothermal porphyry systems are used to place constraints on the style of extension in Arizona, and reconstructions of extension are used to examine the deep and peripheral portions of porphyry systems to provide a more complete understanding of porphyry systems as a whole. The Eagle Pass, Tea Cup, and Sheep Mountain porphyry systems of Arizona are examined in this study. In all the study areas, previous interpretations of the style of extension involved strongly listric normal faults. However, similar amounts of tilting observed in hanging wall and footwall rocks, as well as structure contour maps of fault planes, require that down dip curvature on faults was minimal (<1°/km. Instead, extension is shown here to have occurred as sets of nearly planar, "domino-style" normal faults were superimposed upon one another, including in the Pinaleño metamorphic core complex. Reconstructions of Tertiary extension reveal that sodic (-calcic) alteration is occurs 2-4 km peripheral to, and greisen alteration is found structurally below and overlapping with, potassic alteration. In addition, a preliminary reconstruction of extension across the Laramide magmatic arc reveals that the geometry, as revealed by known porphyry systems, is of similar scale to that of other magmatic arcs. These results help further the debate surrounding competing models of continental extension, and combine with previous work to provide a more complete understanding of the geometries of Arizona porphyry systems at the district and arc scale.

The long-term (>1 Ga) thermal histories of cratons are enigmatic, with geologic data providing only limited snapshots of their evolution. We use zircon (U-Th)/He (zircon He) thermochronology and age composition correlations to understand the Proterozoic-Phanerozoic thermal history of Archean Wyoming province rocks exposed in the northern Laramide ranges of western North America. Zircon He ages from the Wind River Range (54 dates) and Bighorn Mountains (32 dates) show negative correlations with effective uranium (eU), a proxy for radiation damage. Zircon dates from the Bighorns are between 960 Ma (low-eU) and 20 Ma (high-eU) whereas samples from the Wind Rivers are between 582 Ma (low-eU) and 33 Ma (high-eU). We applied forward modeling using the zircon radiation damage and annealing model ZrDAAM to understand this highly variable dataset. A long-term t-T path that is consistent with the available geologic constraints successfully reproduced age-eU correlations. The best fit to the Wind Rivers data involves two phases of rapid cooling at 1800-1600 Ma and 900-700 Ma followed by slower cooling until 525 Ma. During the Phanerozoic, these samples were heated to maximum temperatures between 160 and 125 degrees C prior to Laramide cooling to 50 degrees C between 60 and 40 Ma. Data from the Bighorn Mountains were successfully reproduced with a similar thermal history involving cooler Phanerozoic temperatures of similar to 115 degrees C and earlier Laramide cooling between 85 and 60 Ma. Our results indicate that age-eU correlations in zircon He datasets can be applied to extract long-term thermal histories that extend beyond the most recent cooling event. In addition, our results constrain the timing, magnitude and rates of cooling experienced by Archean Wyoming Province rocks between recognized deformation events, including the >1 Ga period represented by the regionally-extensive Great Unconformity.