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.

The Late Jurassic (157–150 Ma) Morrison Formation of the Western Interior of the United States contains abundant altered volcanic ash. On the Colorado Plateau, this formation accumulated behind and downwind of a subduction-related volcanic arc along the western margin of North America. The ash in these distal fallout tuffs probably drifted eastward from coignimbrite ash clouds related to collapse calderas. Altered volcanic ash is particularly abundant in the Brushy Basin Member of the upper part of the Morrison Formation. In one 110-m-thick section in eastern Utah, 35 separate beds were deposited in a 2.2 m.y. period. Alteration occurred when glassy volcanic ash fell into fluvial and lacustrine environments, where it was diagenetically altered to various mineral assemblages but most commonly to smectitic clay. Periodically, ash fell into saline, alkaline lakes, and diagenetic alteration of the glassy ash produced a crudely zoned deposit on the Colorado Plateau. Altered volcanic ash beds in the outermost part of the lacustrine deposits are argillic (with smectitic clay), whereas zeolitic (clinoptilolite, analcime) and feldspathic (K-feldspar and albite) alteration dominates the interior zones. Feldspathic ash layers contain secondary silica, and consequently immobile element (e.g., Al, Ti, and high field strength elements) abundances were strongly diluted in these rocks. In contrast, the argillic ash beds experienced strong SiO 2 depletion, and, as a result, they are enriched in the relatively immobile elements. The compositions of the zeolitic ash beds are intermediate between these two extremes and experienced the least alteration. As a result of these changes, immobile element concentrations are less reliable than ratios for determining the original magmatic composition of the ash. Most of the altered ash (regardless of type) was also depleted in water-soluble elements like the alkalies, U, and V. The latter two elements were oxidized during diagenesis of the ash, became soluble, and were partially leached away by groundwater. Locally, U and V in groundwater were reduced upon contact with organic materials and formed important ore deposits. Several aspects of the mineralogy and geochemistry of the altered volcanic ash beds yield information about their original magmatic compositions. The volcanic ash beds typically have small phenoclasts of quartz, sanidine, plagioclase, biotite, zircon, apatite, and Fe-Ti oxides. Titanite is present in ∼40% of the ash beds; pyroxene and amphibole were found in less than 5%. Phenocryst assemblages, mineral compositions, inferred high f O 2 , rare earth element patterns, and immobile element ratios all suggest the parent magmas for the altered tuffs were subduction-related dacites and rhyolites. Small numbers of tuffs have Fe-rich biotite, amphibole, and/or clinopyroxene; both pyroxene and amphibole are alkali rich. These tuffs lack titanite, but some contain anorthoclase and F-rich apatite. Combined with enrichments in Nb and Y, these features show some tuffs had an A-type character and were related to some type of within-arc extension. Paleowind directions, and distribution, radiometric ages, and compositions of the volcanic ash beds and of plutons in the western United States suggest that the most likely eruption sites were in the subduction-related Jurassic magmatic arc, which extended across western Utah and central Nevada and southward into the Mojave of California and southern Arizona (present-day coordinates). Pb isotopic compositions show that at least some of the ash was erupted from magma systems (now exposed as plutons) in the Mojave Desert. We conclude that a brief ignimbrite flare-up from 157 to 150 Ma, but focused on the time period from 152 to 150 Ma, in this region may have been driven by slab steepening and conversion to a strike-slip boundary after a preceding phase of folding and thrusting. The presence of ash beds with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was geologically a transitional time in North America when subduction was changing to transtensional movement along the western plate boundary.

Volcanic clasts incorporated in the lower portion of the Tertiary Santa Fe Group sedimentary rocks of the Culebra graben, San Luis Basin, Colorado, provide constraints on the timing of regional tectonic events by provenance determination. Based on currently exposed volcanic terrains, possible clast sources include Spanish Peaks and Mount Mestas to the east, the San Juan volcanic field to the west, and the Thirtynine Mile volcanic field, a remnant of the Central Colorado volcanic field, to the north and east of the San Luis Basin. Provenance was determined by a variety of geochemical, mineral chemical, and geochronologic data. Large porphyritic Santa Fe Group volcanic clasts are potassic with a wide compositional range from potassic trachybasalt to rhyolite. The whole-rock chemistry of the Culebra graben clasts is similar to that of the Thirtynine Mile and San Juan volcanic fields. Culebra graben amphibole and biotite chemistry is generally consistent with that of rocks of the San Juan volcanic field, but not with Spanish Peaks samples. Trace-element data of Culebra graben volcanic clasts overlap with those of the San Juan and Thirtynine Mile volcanic fields, but differ from those of the Mount Mestas. Thermobarometric calculations using mineral chemistry suggest that many Culebra graben rocks underwent a three-stage crystallization history: ~1120 °C at 7–10 kbar, ~1100 °C at 2.3–4.6 kbar, and hornblende formation ~800 °C at 3 kbar. Within the Culebra graben clasts, zircon rim U-Pb geochronologic systematics as well as amphibole and biotite 40 Ar/ 39 Ar plateau data yield ages ranging from 36 to 29 Ma. These ages are consistent with ages of the Thirtynine Mile volcanic field (36–27 Ma) and the Conejos Formation of the San Juan volcanic field (35–29 Ma), but predate Spanish Peaks (ca. 27–21 Ma) and Mount Mestas (ca. 25 Ma). Based on these data, Spanish Peaks and Mount Mestas are excluded as potential source areas for the Santa Fe Group volcanic clasts in the Culebra graben. The San Juan volcanic field is also an unlikely source due to the distance from the depositional site, the inconsistent paleo-current directions, and the pressure-temperature conditions of the rocks. The most likely scenario is that the Central Colorado volcanic field originally extended proximal to the current location of the Culebra graben and local delivery of volcanic clasts was from the north and northeast prior to the uplift of the Culebra Range and Sangre de Cristo Mountains.