ABSTRACT The Laramide foreland belt comprises a broad region of thick-skinned, contractional deformation characterized by an anastomosing network of basement-cored arches and intervening basins that developed far inboard of the North American Cordilleran plate margin during the Late Cretaceous to Paleogene. Laramide deformation was broadly coincident in space and time with development of a flat-slab segment along part of the Cordilleran margin. This slab flattening was marked by a magmatic gap in the Sierra Nevada and Mojave arc sectors, an eastward jump of limited igneous activity from ca. 80 to 60 Ma, a NE-migrating wave of dynamic subsidence and subsequent uplift across the foreland, and variable hydration and cooling of mantle lithosphere during slab dewatering as recorded by xenoliths. The Laramide foreland belt developed within thick lithospheric mantle, Archean and Proterozoic basement with complex preexisting fabrics, and thin sedimentary cover. These attributes are in contrast to the thin-skinned Sevier fold-and-thrust belt to the west, which developed within thick passive-margin strata that overlay previously rifted and thinned lithosphere. Laramide arches are bounded by major reverse faults that typically dip 25°–40°, have net slips of ~3–20 km, propagate upward into folded sedimentary cover rocks, and flatten into a lower-crustal detachment or merge into diffuse lower-crustal shortening and buckling. Additional folds and smaller-displacement reverse faults developed along arch flanks and in associated basins. Widespread layer-parallel shortening characterized by the development of minor fault sets and subtle grain-scale fabrics preceded large-scale faulting and folding. Arches define a regional NW- to NNW-trending fabric across Wyoming to Colorado, but individual arches are curved and vary in trend from N-S to E-W. Regional shortening across the Laramide foreland was oriented WSW-ENE, similar to the direction of relative motion between the North American and Farallon plates, but shortening directions were locally refracted along curved and obliquely trending arches, partly related to reactivation of preexisting basement weaknesses. Shortening from large-scale structures varied from ~10%–15% across Wyoming and Colorado to <5% in the Colorado Plateau, which may have had stronger crust, and <5% along the northeastern margin of the belt, where differential stress was likely less. Synorogenic strata deposited in basins and thermochronologic data from basement rocks record protracted arch uplift, exhumation, and cooling starting ca. 80 Ma in the southern Colorado Plateau and becoming younger northeastward to ca. 60 Ma in northern Wyoming and central Montana, consistent with NE migration of a flat-slab segment. Basement-cored uplifts in southwest Montana, however, do not fit this pattern, where deformation and rapid inboard migration of igneous activity started at ca. 80 Ma, possibly related to development of a slab window associated with subduction of the Farallon-Kula Ridge. Cessation of contractional deformation began at ca. 50 Ma in Montana to Wyoming, followed by a southward-migrating transition to extension and flare-up in igneous activity, interpreted to record rollback of the Farallon slab. We present a model for the tectonic evolution of the Laramide belt that combines broad flat-slab subduction, stress transfer to the North American plate from end loading along a lithospheric keel and increased basal traction, upward stress transfer through variably sheared lithospheric mantle, diffuse lower-crustal shortening, and focused upper-crustal faulting influenced by preexisting basement weaknesses.

Early Pennsylvanian (309–318 Ma) paleocave sediments hosted in the Mississippian (345–359 Ma) Leadville Limestone were partly derived from long-distance (>2000 km) source areas. In addition to showing the importance of long-distant dust transport in cave sediments, because these paleocave deposits are derived from loess, their presence may document the earliest terrestrial signature of the late Paleozoic ice age in North America. The Leadville Limestone was subject to karst processes following late Mississippian eustatic sea-level fall, including formation of phreatic tubes, breakout domes, tower karst (kegelkarst), solution valleys (poljes), sinkholes (dolines), solution-enhanced joints (grikes), surficial flutes (rillenkarren), and solution pans (kamenitzas). In the Leadville Limestone, speleothems are interbedded with karst breccias and fluvial cave sediments. The overlying Pennsylvanian Molas Formation is a loessite (eolian siltstone) composed of angular quartz silt with ferruginous kaolinite rims. The U-Pb ages of accessory zircons indicate that the source areas for the eolian silt are from the peri-Gondwanan terranes and Grenville Province of eastern and southern North America, which are ~2000 km to the east. There is also a provenance signature from the rising Ancestral Rocky Mountains. The evidence suggests dust trapping on land surfaces by paleokarst topography, moisture, and vegetation. Weak paleosols in the Molas Formation suggest relatively rapid rates of dust accumulation. The high porosity and low bulk density of modern loess soils make them susceptible to groundwater piping. This mechanism may have facilitated redeposition of the Molas Formation loess into karst passageways, to be remobilized by later hydrologic events. The paleocave sediments in the Leadville Limestone can be linked to the overlying loess in the Molas Formation by compositional and textural matches. Facies analysis of the paleocave sediments documents episodic hydrologic events, producing a sequence of inundites and debrites separated by mud drapes with mud cracks. These event deposits are interbedded with flowstones and dripstones. Cave sediments are increasingly utilized as archives of geologic change. Recognition that dust is a significant component of cave sediments highlights the inherited properties from distant source areas, land-atmosphere transfer processes, land-surface deposition processes, and resedimentation processes into the karst system.

Abstract Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed ‘monotonous intermediates’, provide a unique opportunity to investigate the evolution of crustal magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite magmas as a function of temperature. The experiments (200 MPa, 720–1100°C) span the inferred pre-eruptive conditions of FCT magmas, and are shown to provide the best match to the chemical and physical properties of the erupted magmas at 790±10°C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of magma, and the contrasted behaviour of water-saturated v. water-undersaturated magmas. In both cases, however, there is a broad interval of temperature (200°C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system, such that small perturbations to the heat content of the system (e.g. by cooling or new magma injections) results in very little change in magma properties. The crystal content in this interval is 55–65 wt%, which is close to the phenocryst content (40–55 wt%) of monotonous intermediates. We propose that crystal-rich magmas tend to settle in this ‘petrological trap’, changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate magma to accumulate in the shallow crust until such time as the net buoyancy force of these crystal-rich magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.

Abstract The early Mesoproterozoic (ca. 1400 Ma) is an enigmatic time in the tectonic evolution of southern Laurentia. Circa 1400 Ma granites within Laurentia and multiple other continents have distinctive geochemistry consistent with crustal extension or mantle upwelling. In the southwestern United States, these granites are commonly foliated and are often spatially associated with km-scale ductile shear zones. Deformation is attributed to intracontinental tectonism driven by active convergence along the distal southern margin of Laurentia. The recent discovery of deformed and metamorphosed, ca. 1450 Ma sedimentary rocks in northern New Mexico has strengthened the case for regional deformation and orogenesis. However, important questions remain about the tectonic significance of these events and how to reconcile tectonic models with granite petrology at the regional to global scale. This trip focuses on the protolith age of Proterozoic metasedimentary rocks and the kinematics, timing, and tectonic significance of deformation, magmatism, and metamorphism for the Mesoproterozoic across different crustal levels in the southern Rocky Mountains to highlight the ongoing questions and controversies regarding the Mesoproterozoic tectonic setting of Laurentia. This field trip will examine some of the diverse and most recently discovered evidence for ca. 1400 Ma orogenesis in the southern Rocky Mountains. We hope this trip will promote new interest and discussion about the Mesoproterozoic tectonic evolution of Laurentia. We will visit multiple outcrops in the Wet Mountains of southern Colorado and the Picuris Mountains of northern New Mexico. Stops in the Wet Mountains are arranged from north to south to examine contrasting styles of ca. 1400 Ma deformation with increasing paleodepth across the tilted Proterozoic crustal section. In the Picuris Mountains, we focus on detrital zircon geochronology and revisions to the lithostratigraphy of Paleoproterozoic and recently documented Mesoproterozoic metasedimentary rocks, the nature of regional metamorphism, and the style of deformation, ca. 1450–1400 Ma.

Potential field data (gravity and magnetic measurements) are both useful and cost-effective tools for many geologic investigations. Significant amounts of these data are traditionally in the public domain. A new magnetic database for North America was released in 2002, and as a result, a cooperative effort between government agencies, industry, and universities to compile an upgraded digital gravity anomaly database, grid, and map for the conterminous United States was initiated and is the subject of this paper. This database is being crafted into a data system that is accessible through a Web portal. This data system features the database, software tools, and convenient access. The Web portal will enhance the quality and quantity of data contributed to the gravity database that will be a shared community resource. The system's totally digital nature ensures that it will be flexible so that it can grow and evolve as new data, processing procedures, and modeling and visualization tools become available. Another goal of this Web-based data system is facilitation of the efforts of researchers and students who wish to collect data from regions currently not represented adequately in the database. The primary goal of upgrading the United States gravity database and this data system is to provide more reliable data that support societal and scientific investigations of national importance. An additional motivation is the international intent to compile an enhanced North American gravity database, which is critical to understanding regional geologic features, the tectonic evolution of the continent, and other issues that cross national boundaries.