Laramide Time Research Articles

The peridotite deformation cycle in cratons and the deep impact of subduction

Xenoliths play a crucial role in interpretation of mantle deformation and geochemistry. The classic work of Mercier and Nicolas (1975) introduced the concept of the peridotite deformation cycle, which connected observed microstructures to a physical sequence of deformation. We revisit Mercier and Nicolas' original concept, bringing in new constraints using large area EBSD maps and associated microstructural datasets, analysis of water contents in nominally anhydrous minerals, and trace element chemistry of pyroxenes and garnets. We apply these techniques to a well-characterized suite of peridotite xenoliths from the Eocene-age Homestead and Williams kimberlites in the northwestern Wyoming Craton. Pyroxene water content and trace element mineral chemistries reveal ubiquitous hydrous metasomatism beneath the craton, most likely linked to the Cenozoic Laramide Orogeny. Homestead xenoliths primarily exhibit coarse protogranular and equigranular textures, B-type olivine fabrics, and generally elevated mineral water contents compared to Williams. Xenoliths from Williams are strongly deformed, with porphyroclastic and transitional textures containing annealed olivine tablets, mostly A-type olivine fabrics, and generally lower mineral water contents. As a whole, mantle from Homestead to Williams reflects a cratonic scale deformation cycle that likely initiated in Laramide times and lasted until the end of orogeny in the Eocene. At Williams, evidence for a rapid deformation “sub-cycle” within the main deformation cycle is preserved in the tablet-bearing xenoliths, corresponding to the enigmatic “transitional” texture of Mercier and Nicolas (1975). Our results suggest that this texture reflects interruption of the main deformation cycle by processes possibly related to a rapidly forming lithospheric instability and generation of the kimberlite magma – offering a new interpretation of this ambiguous peridotite texture. Collectively, our results incorporate typically disparate geochemical and textural datasets on xenoliths to shed new insights into how metasomatism, volatiles, and deformation are connected in the deep cratonic lithosphere.

In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900°C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700°C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab.In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes.

Mapping the 3D extent of the Stillwater Complex, Montana—Implications for potential platinum group element exploration and development

Geophysical models characterize the exposed and interpreted buried extent of the Stillwater Complex, critical for understanding the origin of the layered mafic intrusion and its associated high-grade platinum group element resources. The 3D models, constrained by gravity, magnetic, xenolith, seismic, borehole, and rock property data indicate that the likely maximum extent of the Stillwater Complex beneath Phanerozoic cover is ~10 times greater than its outcrop, ~2240 km2. The thickness values are poorly constrained but vary from ~7000 to 12,000 m, depending on crustal and mantle density variations and depths to the top of the lower crust and mantle. This thickness may include dense metasedimentary units of the basin into which the Stillwater Complex intruded. Using the modeled thickness results in a volume estimate of ~24,700 km3, albeit poorly constrained. New analyses of xenoliths from the Cretaceous Sliderock and Suzie Peak intrusions produce ages of 2706–2716 Ma, corresponding to the age of the Stillwater Complex, and 2813 Ma, corresponding to the age of Archean gneissic basement. Seismic reflectors in inferred Archean crystalline basement, possibly including the Stillwater Complex, dip ~25–30° north, with segments dipping as much as 70° north. Layered reflectors beneath the Phanerozoic sedimentary section and above the inferred Archean crystalline basement may represent metasedimentary units, perhaps a southern extension of the Mesoproterozoic Belt Basin. The potential field models and the seismic reflection data suggest that the Stillwater Complex was dipping northward prior to deposition of Cambrian strata, perhaps uplifted in the late Archean or Proterozoic as previously proposed, and that during Laramide times, faulting and intrusions highly disrupted the complex. Temperature measurements from boreholes help constrain the depths of feasible mining of the complex.

Protracted magmatism and magnetization around the McClure Mountain alkaline igneous complex

AbstractThe McClure Mountain–Iron Mountain igneous complex is an alkalic intrusive center in the northern Wet Mountains of southern Colorado. It was emplaced in early Cambrian time into gneissic/granitic 1.75–1.45 Ga Proterozoic host rocks. Numerous dikes are associated with the complex, primarily along the western side. Although the main intrusive nepheline-syenite body is well dated, the ages of the surrounding dikes are poorly known. Crosscutting relationships and poorly defined K-Ar dates suggest that the dikes are younger than the main intrusion. Paleomagnetic samples were collected from dikes associated with the McClure Mountain igneous complex. Geochronologic samples were also collected from two dikes sampled for their paleomagnetism. We obtained U-Pb zircon ages of 526 ± 8 Ma for a lamprophyric extracomplex dike and 483 ± 2 Ma for a trachytic extracomplex dike. These ages suggest either multistage or protracted dike intrusion around the ca. 524 Ma McClure Mountain complex. Our paleomagnetic data are consistent with previously published results. Dikes of the complex primarily exhibit southeast and shallow paleomagnetic directions, with variable declinations. Results from several baked contact tests indicate that the magnetizations are secondary. A steeply inclined magnetization is pervasive and was acquired over a protracted interval from late Laramide time to the present day.

Comment on: “Geochronology and geothermometry of the Laramide metamorphism in the Cambrian metabasalts from the Cerro Rajón Formation, Caborca region, northwest Mexico” by A. J. Barrón-Díaz, F. A. Paz-Moreno, D. P. Miggins and A. Iriondo

In their paper, Barrón-Díaz et al. (2019a) present new geochronological and thermobarometric data of rocks of the Bamori Ranch area, Caborca region, Sonora, Mexico. The authors interpret the greenschist facies of Neoproterozoic and Paleozoic strata as a result of contact metamorphism close to supposed extended granodioritic intrusions that emplaced during Laramide time (90–40 Ma). In our opinion, this interpretation is poorly constrained. Based on previous geological studies as well as a re-evaluation of presented thermobarometric data, we propose that greenschist facies metamorphism and 40Ar-39Ar ages described by the authors can be the result of a regional metamorphism event that developed during the Laramide orogeny in northwestern Sonora.

A shifting rift—Geophysical insights into the evolution of Rio Grande rift margins and the Embudo transfer zone near Taos, New Mexico

We present a detailed example of how a subbasin develops adjacent to a transfer zone in the Rio Grande rift. The Embudo transfer zone in the Rio Grande rift is considered one of the classic examples and has been used as the inspiration for several theoretical models. Despite this attention, the history of its development into a major rift structure is poorly known along its northern extent near Taos, New Mexico. Geologic evidence for all but its young rift history is concealed under Quaternary cover. We focus on under­standing the pre-Quaternary evidence that is in the subsurface by integrating diverse pieces of geologic and geophysical information. As a result, we present a substantively new understanding of the tectonic configuration and evolution of the northern extent of the Embudo fault and its adjacent subbasin. We integrate geophysical, borehole, and geologic information to interpret the subsurface configuration of the rift margins formed by the Embudo and Sangre de Cristo faults and the geometry of the subbasin within the Taos embayment. Key features interpreted include (1) an imperfect D-shaped subbasin that slopes to the east and southeast, with the deepest point ∼2 km below the valley floor located northwest of Taos at ∼36° 26′N latitude and 105° 37′W longitude; (2) a concealed Embudo fault system that extends as much as 7 km wider than is mapped at the surface, wherein fault strands disrupt or truncate flows of Pliocene Servilleta Basalt and step down into the subbasin with a minimum of 1.8 km of vertical displacement; and (3) a similar, wider than expected (5–7 km) zone of stepped, west-down normal faults associated with the Sangre de Cristo range front fault. From the geophysical interpretations and subsurface models, we infer relations between faulting and flows of Pliocene Servilleta Basalt and older, ­buried basaltic rocks that, combined with geologic mapping, suggest a revised rift history involving shifts in the locus of fault activity as the Taos subbasin developed. We speculate that faults related to north-striking grabens at the end of Laramide time formed the first west-down master faults. The Embudo fault may have initiated in early Miocene southwest of the Taos region. Normal-oblique slip on these early fault strands likely transitioned in space and time to dominantly left-lateral slip as the Embudo fault propagated to the northeast. During and shortly after eruption of Servilleta Basalt, proto-­Embudo fault strands were active along and parallel to the modern, NE-aligned Rio Pueblo de Taos, ∼4–7 km basinward of the modern, mapped Embudo fault zone. Faults along the northeastern subbasin margin had northwest strikes for most of the period of subbasin formation and were located ∼5–7 km basinward of the modern Sangre de Cristo fault. The locus of fault activity shifted to more northerly striking faults within 2 km of the modern range front sometime after Servilleta volcanism had ceased. The northerly faults may have linked with the northeasterly proto-Embudo faults at this time, concurrent with the development of N-striking Los Cordovas normal faults within the interior of the subbasin. By middle Pleistocene(?) time, the Los Cordovas faults had become inactive, and the linked Embudo–Sangre de Cristo fault system migrated to the south, to the modern range front.

Mantle-derived guyanaite in a Cr-omphacitite xenolith from Moses Rock diatreme, Utah

Guyanaite, naturally occurring β-CrOOH, has been identified in a xenolith of Cr-rich omphacitite from the Moses Rock diatreme in the Navajo Volcanic Field of the southwestern United States. It occurs as the dominant phase in small clusters of accessory minerals, intergrown with kosmochlor-rich omphacite, zincian chromite, eskolaite, and carmichaelite. The assemblage is interpreted as the result of metasomatism of chromite-bearing serpentinite by slab-derived fluids during subduction of the Farallon Plate in Laramide time. At the time of entrainment of the xenolith, the rock was undergoing prograde metamorphism, with guyanaite dehydrating to eskolaite plus water. This reaction, and the coeval dehydration of the inferred accompanying host serpentinites (which would have been much more volumetrically significant), provided water for hydration of the subcontinental upper mantle, contributing to uplift of the Colorado Plateau. Recognition of guyanaite as a component of a subducted slab supports recent proposals, based on laboratory experiments, that high-pressure polymorphs of other common crustal oxy-hydroxide minerals such as boehmite and goethite (i.e., high-pressure δ-AlOOH and ɛ-FeOOH) can also transport and store water in the mantle.

Structural and microstructural evolution of the Rattlesnake Mountain Anticline (Wyoming, USA): New insights into the Sevier and Laramide orogenic stress build-up in the Bighorn Basin

The Rocky Mountains in western US provide among the best examples of thick-skinned tectonics: following a period of thin-skinned tectonics related to the Sevier orogeny, the compressional reactivation of basement faults gave birth to the so-called Laramide uplifts/arches. The Bighorn basin, located in Wyoming, is therefore a key place to study the transition from thin- to thick-skinned tectonics in orogenic forelands, especially in terms of microstructural and stress/strain evolution. Our study focuses on a classic Laramide structure: the Rattlesnake Mountain Anticline (RMA, Wyoming, USA), a basement-cored anticline located in the western part of the Bighorn basin. Stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures, fault-slip data and calcite twinning paleopiezometry. Most of the fractures are related to three main tectonic events: the Sevier thin-skinned contraction, the Laramide thick-skinned contraction, and the Basin and Range extension. Serial balanced cross-sections of RMA and displacement profiles suggest that all thrust faults were coeval, evidencing strain distribution in the basement during faulting. The comparison of RMA with another structure located in the eastern edge of the Bighorn basin, i.e. the Sheep Mountain Anticline (SMA), allows to propose a conceptual model for the geometric and kinematic evolution of Laramide-related basement-cored anticlines. Finally, the stress evolution is reconstructed at both the fold scale and the basin scale. We show that the evolution of stress trends and magnitudes was quite similar in both structures (RMA and SMA) during Laramide times (thick-skinned tectonics), in spite of different stress regimes. During Sevier (thin-skinned tectonics) and post-Laramide times, stress trends and fracture patterns were different in these two structures. These results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes, which were likely controlled by the structural style.

History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau

The Rocky Mountain orogenic plateau has the highest mean elevation and topographic relief in the contiguous United States. The mean altitude exceeds 2 km above sea level and relief increases from 30 m in the river valleys of the Great Plains to more than 1.6 km deep in the canyons and basins of the Rocky Mountains and Colorado Plateau. Despite over a century of study, the timing and causes of elevation gain and incision in the region are unclear. Post-Laramide development of relief is thought to either result from tectonic activity or climatic change. Interpretation of which of these causes dominated is based upon reconstruction of datums developed from, and supported by, paleoelevation proxies and interpretations of landscape incision. Here we reconstruct a datum surface against which regional incision can be measured in order to evaluate late Cenozoic tectonic and climatic infl uences. The distribution, magnitude, and timing of post-Laramide basin fi lling and subsequent erosion are constrained by depositional remnants, topographic markers, and other indicators across the region. We suggest that post-Laramide basin fi lling resulted from slow subsidence during Oligocene to Miocene time. Incision into this basin fi ll surface began in late Miocene time and continues today. The pattern of incision is consistent with control by localized extensional tectonism superimposed upon regional domal surface uplift. Localized extension is associated with the projection of the Rio Grande Rift into the central Rockies, and the domal uplift generally coincides with the position of buoyant mantle anomalies interpreted at depth. If the magnitudes of incision directly refl ect magnitudes of surface elevation gain, they are less than can be resolved by existing paleoelevation proxy methods. In addition, the combination of post-Laramide subsidence followed by regional surface uplift reduces the net magnitude of surface elevation change since Laramide time.

Emergence of the Canadian Rockies and adjacent plains: A comparison of physiography between end-of-Laramide time and the present day

The landscape of the Canadian Rockies in southern Alberta is not a direct result of constructional processes; that is, the ridges and peaks have not been pushed into the positions in which we see them today. Tectonic activity provided original elevation but not mountains: at the end of Laramide time, what are now the front ranges and foothills of the Rockies comprised a high-elevation upland of relatively low relief. The present mountain physiography is the result of 55–60 million years of post-orogenic differential erosion, in which more resistant rocks have been left at higher elevations than less-resistant rocks. The Canadian Rockies and the foothills are developed in a thin-skinned, thrust-and-fold belt created during the Laramide Orogeny; the adjacent Interior Plains cut across foreland basin sediments derived from the mountains. The mountains currently consist of large parts of ridges of well-indurated Paleozoic and, locally, Proterozoic rock alternating with valleys developed in soft Mesozoic clastic rock. In the foothills, where the soft Mesozoic rock is at the surface, relief is subdued, but ridges of more-resistant sandstone rise above shaley lowlands. The plains are relatively flat but also contain erosional outliers of higher paleo-plains-surfaces. Numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the Laramide Orogeny, while the western plains have lost at least 2 km, requiring that the local relief of the mountains and foothills that we see is erosional in origin. Local physiography is adjusted to lithology: the mountains have high relief because the exposed sub-Mesozoic rocks can hold up high, steep slopes, whereas the foothills have low relief because the underlying Cretaceous rocks cannot hold up high, steep slopes. The east-facing escarpment at the mountain front is a fault-line scarp along a low-angle thrust. Mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt, and physiography of the belt at the end of Laramide time (60–55 Ma) depended mainly on whether Mesozoic or Paleozoic/Proterozoic rocks were exposed at the surface at that time. A reconstruction using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges at the eastern edge of the Rocky Mountains were mostly or perhaps entirely covered with Mesozoic rocks and despite that high elevation had a hilly, not mountainous, character. The main ranges, in the central Rocky Mountains, were in part stripped of Mesozoic cover by then and more mountainous. Treeline was higher then, and the thrust belt may have been largely or entirely vegetated. Generation of modern relief in the front ranges, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-Laramide time. The Interior Plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments. Although some of the present low local relief of the plains results from weakness of underlying Cretaceous/Tertiary rocks, the low relief is probably largely related to the process of denudation.

Segmentation of the Laramide Slab—evidence from the southern Sierra Nevada region

During the latest Cretaceous-early Paleogene Laramide orogeny, the lithosphere beneath the southernmost Sierra Nevada batholith and the adjacent Mojave Desert region batholith was sheared off and displaced deeper into the mantle. The lithosphere beneath the greater Sierra Nevada batholith to the north was left intact until mid-Miocene time, when fragments of it were entrained as volcanic xenoliths. The Laramide slab was evidently segmented into a shallow flat segment to the south and a deeper segment to the north. Shearing off of the upper mantle to the south was followed by the tectonic underplating of schists derived from Franciscan subduction complex and possibly forearc basin protoliths. The overlying batholithic crust was deformed, deeply denuded, and tectonically breached westward across its forearc region while the schists were underplated. Westward breachment resulted from a combination of west-directed thrusting and extensional collapse. The westernmost breached rocks were tectonically removed by a combination of trench linked transform and subduction erosion processes. Subsequent tectonic erosion by the Neogene San Andreas transform has left the Salinia and the San Gabriel terranes as dispersed residual fragments of the westward breached arc segment. The shallow slab segment appears to have been ;500 km in width, measured along the plate edge. To the south, the slab descent appears to have remained deep beneath the Peninsular Ranges batholith. Classic Laramide structures of the craton are concentrated in a corridor that corresponds to the shallow slab segment as defined by plate edge relations and the corresponding trajectory of Farallon‐North American relative plate motions when viewed on a pre-Neogene palinspastic base. The plate interior is suggested to have been deformed first by end loading as the shallow slab segment initially descended beneath the plate edge, and then by greater basal traction components as the shallow segment progressed beneath the cratonic region. The subcontinental mantle lithosphere beneath the cratonic deformation zone remained intact through Laramide time. Similar segmented slab topologies have been resolved beneath the modern Andean orogen where the shallow segments are correlated with the subduction of aseismic ridges. A number of researchers have suggested that the Laramide orogeny arose from the subduction of a counterpart of the Hess-Shatsky large igneous province of the northwest Pacific basin. Fragments of rock assemblages that are correlative to the Hess-Shatsky province that were accreted to the Franciscan complex in Laramide time support this view. Recognition of the resulting shallow slab segment and its trajectory beneath North America explains the geographic focusing of Laramide deformation, relative to the rest of the Cordilleran orogen, and the relationships between plate edge and plate interior deformational regimes.

Reconstructing the ancestral Yellowstone plume from accreted seamounts and its relationship to flat-slab subduction

Recent geodynamic analyses have emphasized the relationship between modern flat-slab subduction zones and the overriding of buoyant oceanic crust. Although most models for the evolution of the Late Mesozoic–Cenozoic Laramide orogeny in the southwestern United States involve flat-slab subduction, the mechanisms proposed are controversial. An examination of the geological evolution of the 60–50-Ma Crescent terrane of the Coast Ranges indicates that it was formed in a shallowing-upward Loihi-type oceanic setting culminating in the eruption of subaerial lavas. Plate reconstructions indicate that the Crescent terrane was emplaced into ca. 20-Ma crust, and the presence of subaerial lavas implies an uplift due to the plume of ca. 4.2 km, which we use to calculate a minimum buoyancy flux of 1.1 Mg s −1, similar to that of the modern Yellowstone plume. Published paleomagnetic data indicate that the Crescent terrane was formed at a paleolatitude similar to that of the Yellowstone plume. The Crescent seamount was accreted within 5 My of the cessation of plume magmatism. Plate reconstructions indicate that it would have originated about 750 km to the west of the North American plate margin if it developed above a fixed Yellowstone plume, and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement. We interpret the Crescent terrane as due to the ancestral Yellowstone plume. Such a plume would have generated an elongate swell and related plateau that would have been overridden by the North American margin. Taken together, the relationship between flat-slab and overriding of oceanic plateau in Laramide times would have been analogous to the relationship between modern Andean flat-slab subduction zones and the Juan Fernandez and Nazca oceanic plateaus.

Stratigraphic assessment of the Arcelia–Teloloapan area, southern Mexico: implications for southern Mexico's post-Neocomian tectonic evolution

Stratigraphic assessment of the “Tierra Caliente Metamorphic Complex” (TCMC) between Arcelia and Teloloapan in southern Mexico, based on photo interpretation of Landsat Thematic Mapper images and field mapping at the 1:100,000 scale, tests different tectonic evolution scenarios that bear directly on the evolution of the southern North American plate margin. The regional geology, emphasizing the stratigraphy of a portion of the TCMC within the area between Arcelia and Teloloapan is presented. Stratigraphic relationships with units in adjacent areas are also described. The base of the stratigraphic section is a chlorite grade metamorphic sequence that includes the Taxco Schist, the Roca Verde Taxco Viejo Formation, and the Almoloya Phyllite Formation. These metamorphic units, as thick as 2.7 km, are covered disconformably by a sedimentary sequence, 2.9 km thick, composed of the Cretaceous marine Pochote, Morelos, and Mexcala Formations, as well as undifferentiated Tertiary continental red beds and volcanic rocks. The geology may be explained as the evolution of Mesozoic volcanic and sedimentary environments developed upon attenuated continental crust. Our results do not support accretion of the Guerrero terrane during Laramide (Late Cretaceous–Paleogene) time.

Late Laramide dolomite recrystallization of the Husky Rainbow "A" hydrocarbon Devonian reservoir, northwestern Alberta, Canada: paleomagnetic and geochemical evidence

The Rainbow Field is in reefal carbonates of the Middle Devonian Keg River Formation in the Western Canada Sedimentary Basin. Alternating field and thermal step demagnetization was done on specimens from unoriented core from a vertical, an inclined, and a horizontal well core in the dolomite reservoir. Although they had no viscous remanent magnetization component to use for orientation, most specimens had a well-defined characteristic remanent magnetization (ChRM) that resides in single to pseudosingle domain magnetite. By rotating the mean ChRM direction around its core axis, a small circle can be generated for each core and the small circles intersect in the true ChRM direction of D = 168°, I = -73.5o (α95 = 5.8°, k = 32.2). Its paleopole of 164°E, 83°N (A95 = 10°) defines a Tertiary age with one sigma limits of Middle Eocene to Middle Miocene age. Petrographic examination defines four generations of dolomite. Matrix dolomite has 60-100 µm diameter crystals that were later recrystallized to 200-400 µm. Dolomite cements are represented by vug-filling coarse dolomite (100-200 µm) and saddle dolomite (1000 µm). All four generations of dolomite give similar δ18O values of -10.7 to -16.5‰ (Peedee Belemnite, PDB), δ13C values of +0.7 to +3.2‰ (PDB), and Sr isotopic ratios of 0.70826 to 0.70846 that do not match the expected Middle Devonian carbonate or seawater values. We interpret these data to indicate that mixed pre-Laramide basinal fluids, heated by burial during the Laramide Orogeny, were present during late Laramide time when the dolomites were recrystallized and (or) precipitated prior to petroleum migration and accumulation in the Rainbow "A" reservoir. Thus the combined use of paleomagnetism, geochemistry, and petrography has been proven to be a useful technique to date and identify dolomitization events and pathways for the migration of hydrocarbons.

Regional tectonics and fracture patterns in the Fall River Formation (Lower Cretaceous) around the Black Hills foreland uplift, western South Dakota and northeastern Wyoming

Abstract The Fall River Formation around the Black Hills uplift is pervasively fractured by layer-perpendicular joints. Systematic joints in the formation maintain consistent orientations over large areas and are commonly abutted by later-formed fractures, resulting in an orthogonal pattern. There are two major systematic sets, trending northeast and northwest, and one minor set trending north-south. The first two sets define two major fracture domains in the study area. The northwest joint set occupies a southern domain where it is the sole systematic fracture set. The northeast joint set is pervasively established throughout the northern domain, where northwest and north-south fracture sets are also developed in well-defined sectors. There is no genetic or spatial relationship between joint sets and local Laramide monoclines or folds of the region. Instead, the stratigraphic record indicates that joint development originated early in the lithification history of Fall River sandstones. Jointing occurred in response to local and regional extensional stresses that pervaded the northern and southern domains as a result of recurrent movement on basement faults that parallel the regional lineament system and surface structural zones throughout the region. Major uplift of the Black Hills and local fold development during Laramide time merely resulted in passive rotation of the early formed systematic and non-systematic joints.

Kinetics of the Smectite to Illite Transformation in the Denver Basin: Clay Mineral, K-Ar Data, and Mathematical Model Results (1)

Clay mineral data, potassium-argon (K-Ar) ages, and mathematical model simulations indicate the important kinetic factors controlling the smectite to illite transformation (illitization) in Upper Cretaceous Mowry and Niobrara core and outcrop bentonites from the Denver basin. The data also provide additional information on the thermal history of the basin. Data show that both the percentages of illite layers and the K-Ar ages of mixed layer illite-smectite (I/S) increase with increasing depths of burial, and this trend was simulated mathematically employing a very simplified burial model with a fifth-order overall kinetic expression for the formation of illite. For most of the basin, the data is compatible with I/S having been formed in response to increased temperature from progressive burial, because the oldest K-Ar ages of I/S are from the deepest buried I/S along the basin axis. In addition, these data qualitatively suggest that I/S even as small as <0.1 micrometer retains radiogenic argon under Denver basin burial temperatures. I/S separated from drill core bentonites from the Wattenberg gas field, astride the basin axis and known to have been subjected to an anomalously high temperature history, are the most illitic and among the oldest measured from core in this study. The ages of illitization (about 60 Ma) are coincident with the Laramide time of structural development of the basin and uplift of the Front Range. The reconstructions of the burial history, and the modelling of the timing and extent of illitization, suggest that the area of the Wattenberg gas field was an upper Cretaceous depocenter, an area of relatively deeper burial along the basin axis. However, the data can not rule out the possibility that these sediments at and near the Wattenberg gas field were heated not only by burial but also by i neous and related hydrothermal activity associated with the Colorado mineral belt during the Laramide uplift (70-45 Ma) of the Front Range. Likewise, the anomalously illitic I/S in outcrop bentonites along the Front Range adjacent to the Wattenberg gas field may have been formed in response either to deep burial and/or by Laramide-age secondary heating superimposed on burial.

Uncertainties and implications of the Late Cretaceous and Tertiary position of North America relative to the Farallon, Kula, and Pacific Plates

We present updated global plate reconstructions and calculated uncertainties of the Pacific, Kula, and Farallon/Vancouver plates relative to North America for selected times since 68 Ma. Improved magnetic data from the Indian Ocean decrease the uncertainties in. the global plate circuit approach; these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots. Major differences between these results and those of others are due to our use of more detailed Africa‐North America reconstructions, separate Vancouver and Farallon plate reconstructions, and the assumption of a rigid Antarctica plate during Cenozoic time. The uncertainties in the relative positions of the Pacific and North America plates since the time of anomaly 7 (26 Ma) range up to ±100 km in position, or from 1 to 3 m.y. in time. If the Mendocino triple junction initiated at about 28.5 Ma, its position would have been at 31.3°N ± 130 km relative to fixed North America. Unacceptable overlap of oceanic crust of the Pacific plate with continental crust of western North America in the anomaly 10 (30 Ma) reconstruction is a minimum of 340±200 km along an azimuth of N60°E and may be accounted for by Basin and Range extension. Pacific‐North America displacement in the past 20 Ma is found to be considerably less than that calculated by fixed hotspot reconstructions. Farallon (Vancouver)‐North America convergence velocity decreased greatly between the times of anomalies 24 and 21 (56 to 50 Ma), prior to the 43 Ma age of the Hawaiian‐Emperor bend and the often quoted 40 Ma “end” of the Laramide orogeny. A change in direction of Farallon‐North America convergence occurred sometime between 50 and 42 Ma and also may not correlate with the time of the Hawaiian‐Emperor bend. The lack of data from subducted parts of the Farallon and Kula plates permits many possibilities regarding the position of the Kula‐Farallon ridge, the age of subducted crust, or the position of oceanic plateaus during the Laramide orogeny, leaving open the question of the relationship between plate tectonic scenarios and tectonic style during Laramide time. Displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed North America during the interval between anomalies 30/31 and 18 (68 to 42 Ma) are found to be: Pacific plate, 1700±200 km northward; Farallon plate, 3200±400 km northeastward; Vancouver plate, 3000±400 km northeastward; Kula plate, if attached to the Pacific plate after A24 time, 2500±400 km northward.

Tectonic origin of fractures for fissure vein emplacement in the Boulder Batholith and adjacent rocks, Montana

In the Boulder batholith and adjacent rocks, the dominant and most productive metalliferous fissure veins trend approximately east and dip steeply. Other steep veins, generally narrower, trend northwest, northeast, and rarely north. Fissure veins also occur locally in thrust faults in volcanic rocks along the west side of the batholith. Fractures along which these veins were emplaced were probably initiated by a regional compressional stress field during Laramide (Late Cretaceous-early Tertiary) time. The batholith was emplaced during the Late Cretaceous and is synorogenic with respect to the surrounding Laramide thrust and fold belt, which is characterized by east-west compression. Postorogenic rocks (Eocene and younger) do not host major fissure vein deposits.Fractures utilized by east-trending metalliferous veins are parallel to the axis of maximum compressional stress and thus appear to be extensional; this may be the reason these veins are widest and longest. Northeast- and northwest-trending veins were probably emplaced along conjugate shears; these veins are narrower than east-trending veins. Veins that trend north are less common and probably formed along release fractures perpendicular to the principal axis of compressional stress. These veins are narrower than many of those that trend east.Mechanical behavior of various rock units during deformation had an important effect on ore deposition. The longest, widest, and most productive fissure veins occur in competent, brittle rocks (Precambrian crystalline basement, batholith intrusions, and Elkhorn Mountains Volcanics). Ductile rocks (Mesozoic shales and Belt Supergroup argillites) do not commonly contain major fissure veins, except where these rocks have been contact metamorphosed and have become more brittle. Paleozoic strata, mainly carbonates, are of intermediate ductility and are hosts to fissure veins in only a few districts adjacent to the batholith.

Tectonostratigraphic Analysis of Powder River Basin, Wyoming: ABSTRACT

The Powder River basin of northeastern Wyoming is a basin of the broken foreland between the stable interior of the craton and the Cordilleran orogenic belt. The formation of the basin reflects the complex depositional history of both regions. The rock strata within the basin may be divided into tectonostratigraphic units that allow interpretation of basin evolution within a plate-tectonic context. During Precambrian to Late Devonian time, the area occupied by the present-day basin was transitional between the craton and the rifted margin of the continental shelf to the west. The basin area occupied a complex setting peripheral to foreland basins and the stable continental interior during the Antler orogeny (Devonian-Mississippian), Humboldt orogeny (Pennsylvanian-Permian), Sonoman orogeny (Permian-Triassic), and Nevadan stage of the Cordilleran orogeny (middle Late Jurassic). After the beginning of the Sevier orogeny in the Late Jurassic, the area was within the Cretaceous retroarc foreland basin. The first indication of subsidence within the basin is late in Laramide time (Paleocene); subsidence continued through at least early Eocene time. Stream drainages in Paleocene to ear y Eocene time were dominated by rising highlands, which produced a north-flowing trunk stream. During much of the Paleogene, this drainage was connected to the Wind River basin on the southwest. A-type subduction of the cratonic margin beneath the rising magmatic arc in the western batholithic belt, following collision of exotic terranes during a period of increased movement of the craton relative to the Pacific plate, was the most probable cause for events of the Cordilleran orogeny. Laramide-style basement-block uplifts express the extension of the thin-skinned tectonics in the western orogenic belt eastward into the thicker lithosphere of the foreland along the craton margin, possibly during overriding of an aseismic ridge on the Pacific plate by western North America. Uplift and erosion of the basin since early Eocene time may be due to thermotectonic uplift around the apices of the Snake River plain and the Rio Grande rift. End_of_Article - Last_Page 855------------

The Crownpoint, New Mexico, earthquakes of 1976 and 1977

Abstract In 1976 and 1977, two unusually deep earthquakes of M L 4.6 and 4.2, respectively, occurred near the town of Crownpoint in northwestern New Mexico. Both events were felt extensively in the Four Corners region with maximum reported Modified Mercalli intensities of VI. These earthquakes are two of the larger earthquakes observed within the Colorado Plateau and are among the strongest earthquakes reported in New Mexico outside the Rio Grande rift. Relocations of the events placed them within the Chaco slope along the southern portion of the San Juan basin—an area that appears to have exhibited a low level of historical seismicity and tectonism possibly since Laramide times. The most significant aspect of the 1976 and 1977 Crownpoint earthquakes is their focal depths of 41 and 44 km, respectively. Such depths would locate the events in the lowermost crust in the vicinity of the Moho in sharp contrast to the upper crustal origin of most intraplate earthquakes. Although located within the Colorado Plateau, revised fault plane solutions exhibit normal faulting along northwest-trending planes and northeast-southwest-trending minimum compression, which are characteristic of Basin and Range-Rio Grande rift extension. This observation suggests that this portion of the Colorado Plateau is located within a tectonic transition zone between the Basin and Range-Rio Grande rift and Colorado Plateau stress provinces.