Abstract The timing and nature of early deformation in the Rio Grande Rift remains poorly constrained. We present evidence for the earliest structural signature of rift extension in the Sangre de Cristo Range, southern Colorado, based on new geologic mapping, structural analysis, rock magnetic data, and thermochronology. These analyses focus on the ∼30.0 Ma granite of Chokecherry Canyon, which hosts discrete low‐angle mylonitic shear zones and a distributed, gently SW‐dipping protomylonitic fabric. Incremental stretching axes, stretching lineations, and Kmax magnetic lineations plunge gently WSW. Quartz microstructures and crystallographic orientations indicate dominantly coaxial strain in the protomylonite and general shear in the discrete shear zones. Quartz c‐axis opening‐angle thermometry suggests deformation at ∼420°–540°C. Thermal modeling of 40 Ar/ 39 Ar K‐feldspar data indicates rapid postmagmatic cooling below the brittle–plastic transition, supporting shear‐zone formation immediately after emplacement. Slow cooling from ∼20 to 13 Ma was followed by renewed rapid cooling at ∼13 Ma, interpreted as the onset of extensional exhumation along the Sangre de Cristo fault system. These results show that extension in the northern Rio Grande Rift was active by ∼30 Ma, earlier than previously recognized. We propose a two‐stage model for northern Rio Grande Rift evolution: Stage I (30–23 Ma) records ENE–WSW extension localized in low‐angle mylonitic shear zones associated with mid‐crustal intrusions; Stage II (≤18 Ma) reflects brittle high‐angle normal faulting, focused exhumation, and rift narrowing. Stage I magmatism and deformation along the western range front likely established crustal weaknesses that guided later fault development.

Abstract The Sangre de Cristo Range in southern Colorado records a complex tectonic history that includes Late Cretaceous–Eocene Laramide contraction and Oligocene–Quaternary Rio Grande rift extension. We present new thermochronologic data ( 40 Ar/ 39 Ar, fission‐track, (U‐Th)/He) and thermal history models that provide insight into this polyphase history and spatiotemporal patterns of extensional exhumation. Our data indicate that the Alvarado fault, which bounds the northeastern flank of the range, initiated as a NE‐dipping reverse fault during the early stages of the Laramide orogeny and was subsequently reactivated as a normal fault in the early Miocene. The onset timing of rapid cooling associated with extensional exhumation systematically youngs southwestward across the range from 20–16 Ma along the northeastern flank, 18–14 Ma near the range crest, and 14–10 Ma along the southwestern flank. We attribute this exhumation pattern to earlier onset of normal slip on the Alvarado fault, with extension shifting to the SW‐dipping Sangre de Cristo fault system along the southwestern flank of the range and adjacent San Luis Basin at 14–10 Ma. The southwestern flank of the range was exhumed from temperatures of ∼240 ± 25°C at the onset of Miocene rapid cooling, consistent with ∼7–9 km of displacement across the Sangre de Cristo fault system. Thermal history models indicate limited cooling between the Oligocene inception of rift extension and the Miocene onset of rapid cooling, suggesting that development of rift flank topography and adjacent extensional basins was primarily associated with Miocene normal faulting.

The Sangre de Cristo Range in southern Colorado exposes some of the deepest Cenozoic structural levels in the Rocky Mountain region, including mylonitic shear zones associated with both the Laramide orogeny and Rio Grande rift. We investigated the relation between Laramide contraction and Rio Grande rift extension with detailed geologic mapping, kinematic analysis, and geochronometry in a 50 km(2) area centered on the Independence Mine shear zone (IMSZ). The 15-100-m-thick IMSZ is one of several shallowly to moderately (similar to 45 degrees +/- 20 degrees) W-SW-dipping brittle-plastic shear zones along the western flank of the range. These shear zones display microstructural evidence of initiation as top-NE contractional mylonite zones, consistent with regional Laramide kinematics, which have been pervasively overprinted by shear fabrics indicating top-SW extensional reactivation. Both top-NE and top-SW shear fabrics involve cataclasis and quartz dislocation creep, although top-SW shear is more commonly localized along phyllosilicate-lined shear bands. Shear zones are hosted predominately within Proterozoic gneiss, and contain abundant chlorite and white mica derived from alteration of hornblende and feldspar, which indicates that weakening driven by fluid reactions played an important role in localizing strain. Extensional overprinting appears to be most pervasive along more steeply dipping portions of shear zones and where secondary phyllosilicates form an interconnected weak phase, which suggests that reactivation was primarily controlled by geometry and rheological contrasts inherited from contraction. One top-SW shear zone adjacent to the IMSZ cuts a late Oligocene gabbro stock, and monazite grains synkinematic with top-SW shear in the IMSZ yielded late Oligocene to Early Miocene U-Th-Pb dates that correspond with initiation of the Rio Grande rift. Reactivation of weak reverse faults may represent an important structural control during initial extension in the middle crust, prior to slip along the high-angle Sangre de Cristo normal fault system.

The Buck Creek ultramafic complex is a fragment of oceanic cumulate emplaced into the lower Laurentian continental crust during Ordovician Taconic subduction. We document olivine fabrics in the relatively pristine dunites preserved from peak metamorphic conditions of ∼850°C and 1.0–1.4 GPa confining pressure. Mineral assemblages and microstructures indicate nearly anhydrous conditions at peak metamorphism and the activity of dislocation creep with minor evidence of grain boundary sliding. Grain size piezometry indicates stress conditions of ∼17–25 MPa. Analysis of crystallographic preferred orientations (CPO) and intracrystalline misorientations indicate the primary activity of the [001](010) slip system leading to the development of B‐type olivine fabrics. We suggest that the Buck Creek dunites formed as ocean crust cumulates and were partially subducted beneath the Laurentian continental crust to deformation conditions similar to those in the shallow mantle wedge. We document that deformation at Buck Creek occurred at lower differential stress conditions and lower water content than typically associated with B‐type CPOs, broadening the range of known conditions in which these fabrics may form.

Limestones of the Whiskey Canyon Member of the Gray Mesa Formation (Middle Pennsylvanian, early Desmoinesian age) of central New Mexico were examined for evidence of aridity. All samples contained abundant chert, both massively bedded and nodular. The chert and limestone display abundant angular detrital quartz (<= 20 mu m) and fine sand grains with corroded surfaces interpreted to have formed by the corrosion of a disordered crystalline lattice caused by eolian abrasion. The original angular quartz grains show morphological characteristics of eolian dust deposited in a marine environment; that is, they are eolo-marine sediment. Microcrystaline quartz (chert) was reprecipitated following dissolution of all or part of the detrital quartz grains. Consequently, dissolution of the eolo-marine dust provided the source of silica for the enclosed chert. Detrital zircon U-Pb ages restricted to two well-defined age peaks in the limestone suggest a local source of siliciclastic input, whereas a broad distribution of ages in the chert beds implies an expanded source region, consistent with sediment transport by wind. The presence of large amounts of eolo-marine dust in the Gray Mesa limestones indicates that the climatic changes accompanying the Atokan-Desmoinesian transition in central Pangea, indicated by changes in wetland floras and geological features, were widespread across the tropical region of the time.

Laboratory experiments demonstrate that intragranular water exerts an important control on deformation within quartz, causing weakening and promoting plasticity. The role of water in natural quartz deformation, however, remains unclear, as recent studies find an inverse relationship between water content and the magnitude of plastic strain. Furthermore, little work has investigated the effects, if any, of water on the relative activity of various slip systems in quartz. We focus on a naturally strained quartzite from the Antietam Formation of the Blue Ridge in Virginia, USA. Quartz water content ranges from < 50 to > 2000 ppm H2O. Water content and crystallographic data were correlated for 968 grains, enabling us to explore the relationship between water content and quartz crystallographic preferred orientation (CPO) patterns. "Dry" (< 150 ppm H2O) and "wet" (> 500 ppm H2O) subsets show distinct CPOs; c axes of dry grains define a cross girdle oriented perpendicular to the extension direction (x), whereas c axes of wet grains are concentrated along the perimeter of the pole figure. All water content subsets show grains clustered near the direction of maximum shortening (z), consistent with activity of the basal < a > slip system. The cross girdle in the driest grains suggests activity of prism < a > and possibly rhomb < a >, whereas the orientation of the wettest grains implies a contribution from prism [c] slip. These slip system interpretations are supported by analyses of intragranular misorientations. These results indicate that water content impacts the relative activity of various slip systems in natural quartz, potentially affecting application of the quartz opening angle thermometer.

Two lithologically distinctive Mississippian facies tracts (MFTs) are described from Lower and Middle Mississippian strata (Kinderhookian and Osagean North American Stages) in southeast Laurentia (SEL). The MFTs extend from the central Appalachian Basin westward across the Cincinnati Arch, through the Eastern (Illinois) Basin, onto the Burlington shelf in central Missouri and into the Western Interior (Forest City) Basin, USA. The MFTs developed under radically different climatic regimes. Kinderhookian climate cycles ranged from 3rd order humid to 4th and 5th order humid-subhumid alternation. These climate fluctuations controlled Kinderhookian sediment flux, evidenced by fluvial-deltaic sandstones, coal beds, and pro-deltaic terrestrial-organic-matter-enriched marine black shales. A dramatic climate shift coincided with the Kinderhook-Osage boundary, from 3rd order cool-humid to 3rd order warm-arid conditions, causing cessation of deltaic sedimentation and the onset of eolian sedimentation at that boundary. This abrupt climate reorganization is reflected in 3rd, 4th, and 5th order continental and eolo-marine loessites that replaced fluvial-deltaic facies. Eolianites, evaporites, and calcareous protosols indicate Osagean aridity. Consequently, we reject the deltaic depositional paradigm for Osagean siliciclastic facies. Osagean eolian sediment consisted mainly of quartz silt with significant pristine (unaltered) feldspar silt (similar to <= 10%). Pristine feldspars are consistent with an arid sedimentary source that lacked significant chemical weathering under an arid paleoclimate. Dissolution of the chemically reactive disordered lattice of eolian-abraded quartz in subarkosic loess served as the predominant source of silica for massive amounts of biotic and abiotic chert in Osagean eolo-marine sediments. The <= 20 mu m fraction of quartz dust is particularly susceptible to dissolution and re-precipitation as microcrystalline quartz (chert). We conclude that tectonic, eustatic, and climatic (allocyclic) processes all exerted some control on deposition; tectonics and eustasy controlled accommodation space, whereas paleoclimate changes (cycles), driven predominantly by orbital forcing, were the principal control on sediment supply and lithostratigraphy.