Laramide Foreland Research Articles

Tectonic Evolution of Sevier-Laramide Foreland Structures from Latest Jurassic Through the Eocene: ABSTRACT

Sevier-Laramide overthrusting was generated by both relative North American (NA)-Farallon and absolute NA plate motions. The magmatic arc thermal axis (MATA), adjacent thermally weakened hinterland metamorphic core (HMC), and mechanical anisotropies in the upper crust contributed to variations in thrust belt development. Latest Jurassic to earliest Cretaceous west-northwest-directed absolute motion of NA plate collapsed the Cordillera toward the east-southeast, causing southeast to east vergent thrusting along the hinterland and Sevier thrust belt. Relative NA-Farallon and absolute NA plate motions increased dramatically in the mid-Cretaceous between 105 and 85 m.y.B.P. causing the MATA-HMC to migrate eastward. Rapid west-northwest absolute motion of NA collapsed the Co dillera toward the east-southeast, causing major southeast to east vergent thrusting along the Sevier thrust belt. Absolute NA motion slowed after 85 m.y.B.P., but increasing relative NA-Farallon motion forced shallowing of Farallon subduction angle and further eastward migration of the MATA-HMC. This allowed relative plate convergence stresses to be transferred into the Laramide foreland (LF). Latest Cretaceous-Paleocene (72-56 m.y.B.P.) rapid (13 cm/yr, 5 in./yr), east-northeast-directed NA-Farallon plate convergence created northwest to north-trending, southwest to east or west vergent overthrusting and west-northwest-trending sinistral faulting in the LF from southern Arizona to northern Montana. Extremely rapid (15 cm/yr, 6 in./yr), north-northeast-directed NA-Farallon plate converg nce in the Eocene (56-43 m.y.B.P.) generated intense northwest-trending, southwest vergent overthrusting in southern Arizona, north-trending dextral faulting in the southern LF, and east-west-trending north to south vergent overthrusting in the northern LF. End_of_Article - Last_Page 501------------

Laramide Foreland Thrust Faulting in Southwestern Montana: ABSTRACT

Two major schools of though exist on the nature of Rocky Mountain foreland (Laramide) deformation: (1) advocates of the classical fold-thrust model of R. R. Berg, including variations and plate-tectonics settings by J. K. Sales, J. D. Lowell, A. W. Bally, and others; and (2) advocates of the classic upthrust or drape-fold model of J. J. Prucha and others, with recent vigorous support based on field and model studies by D. W. Stearns and his students. The first school concentrates on lateral-compressive End_Page 532------------------------------ deformation and the second vertical, or nearly vertical tectonics, as the principal deformation mechanism. The main battleground involves the floored by Archean crystalline rocks. This province includes much of southwestern Montana, west of commonly recognized limits of Laramide foreland deformation and south of the Helena salient of the Cordilleran overthrust belt. In this area, west-northwest of Yellowstone National Park, thrust faults and other structural features indicative of compressive deformation are widespread and have been mapped in the Snowcrest Range by M. R. Klepper and in the Greenhorn Range by J. B. Hadley. These two ranges expose the Snowcrest structural terrane: the complexly deformed steeper limb of the asymmetric Laramide Blacktail-Snowcrest massif. This massif, like the Madison-Gallatin uplift farther east, has been broken and stretched by Tertiary extension faults, principally subparallel and behind the major range-front thrusts and probably listric to these thrusts. Current geologic and gravity studies in the southwestern part of the Snowcrest structural terrane extend the zone of Laramide foreland thrust faulting into the southwest Montana reentrant of the overthrust belt. Here foreland thrust faulting is chiefly Late Cretaceous in age, and thrust faulting occurs along the Greenhorn lineament, a zone of crustal weakness active since Paleozoic time within the northwestern part of the Wyoming province. The presence of foreland thrust faulting has broad implications for oil and gas potential of the southwestern Montana and adjacent foreland. End_of_Article - Last_Page 533------------

Eocene Paleotectonics and Sedimentation in the Rocky Mountain-Colorado Plateau Region: ABSTRACT

End_Page 1331------------------------------The Laramide orogeny (c. 80 to 40 m.y.B.P.), which culminated during early Eocene time, resulted in the development of numerous uplifts and basins in the foreland of the western United States. Uplifts are assignable to three general classes: (1) Cordilleran thrust belt uplifts, (2) basement-cored, fault-bounded uplifts of the classic Laramide Rocky Mountains, and (3) monocline-bounded uplifts of the Colorado Plateau. Basins were also of three types: (1) Green River type--large equidimensional to elliptical basins bounded on three or more sides by uplifts and commonly containing lake deposits, (2) Denver type--asymmetrical, synclinal downwarps with a related uplift along one side, and (3) Echo Park type--narrow, highly elongate basins with through drainage and of strike-slip origin. Gr en River-type basins exhibit quasiconcentric zonation of facies, in contrast to the unidirectional, proximal-to-distal facies tract of Denver-type basins. Facies distribution in Echo Park-type basins is complex and often difficult to reconstruct due to faulting, erosional truncation, and cover. The prevalence of en echelon structures in the deformed zone east of the Colorado Plateau, and evidence for significant crustal shortening north of the plateau, suggest that the major structural features of the Laramide foreland were produced by large-scale, north-northeastward translation of the relatively rigid Colorado Plateau block. The magnitude of this motion, as indicated by dextral offset of lineaments which cross the eastern margin of the plateau and by the amount of crustal shortening in the Wyoming province, may be as great as 65 to 120 km (40 to 75 mi). This translation probably resulted from the interaction of relatively competent Colorado Plateau lithosphere with the underlying, gently dipping Farallon plate, which was being overridden by the western United States in Lar mide time. Evidence for increased strain rates in early Eocene time includes: (1) markedly higher rates of deposition and sand/shale ratios in the Gulf Coast geosyncline (Wilcox Group), (2) formation of several new basins in the southern Rocky Mountains in which Eocene deposits rest unconformably on pre-Cenozoic rocks, and (3) the generally coarser and more arkosic nature of Eocene sediments, as compared to older Laramide deposits, in many areas throughout the foreland. The early Eocene culmination of Laramide tectonism appears to result from two factors. First, the subducted Farallon plate achieved its shallowest dip at about 55 m.y.B.P., resulting in increased viscous coupling with the overriding continental lithosphere. Second, changing spreading-center geometries in the Labrador Sea, Norwegi n Sea, and Arctic Ocean caused the maximum horizontal stress direction to shift to a northeasterly orientation, causing the Colorado Plateau block to increasingly decouple from the craton along north-trending wrench faults in the southern Rocky Mountains. Translation of the Colorado Plateau to the north-northeast during Laramide time resulted in a series of transpressive uplifts and basins along its eastern margin and large-scale crustal shortening in the Wyoming province to the north. End_of_Article - Last_Page 1332------------

Foreland Deformation--A Critique of Cause: ABSTRACT

The cause of foreland deformation has been argued for nearly 100 years, and despite definitive stratigraphy, superb exposure, and extensive seismic and well data, mechanism and geometry remain elusive. It is necessary to separate arguments for cause from arguments that only support a particular geometric interpretation. The following statements are presented for discussion and are not advanced as the answer. (1) Foreland deformation is caused by end-load buckling in a plate tectonic setting driven by abnormally shallow subduction. (2) The crust is active (competent) unit, and plastic subcrust is a passive cushion allowing great vertical component, though a slight slope forced on it by subducting slab may have aided telescoping of crust above. (3) No strictly vertical cause can allow basins to go down as much as they did, and active (causal) intrusion under uplifts only is not likely. (4) While reverse faults and modest shortening predominate, fault attitudes can End_Page 1353------------------------------ have greater diversity than in thin-skinned regime because plastic substratum allows easy vertical motion; nearly vertical faults are allowable. (5) The basement does fold, but with difficulty and in broad wavelength. (6) Because of broad arching, a exists well within basement over uplifts, allowing high-level features, such as Rattlesnake Mountain, to be bounded by high-angle normal faults. (7) This same neutral surface forces out-of-the-basin crowding, causing steep flanks of most basin folds to face toward adjacent uplift. (8) The Hafner approach illustrates diversity of curved faults that can be generated in a vertically sinusoidally loaded beam, and which can be generated equally well in an end-loaded sinusoidally buckled beam, as long as it sits on a passive plastic substratum. (9) The Sanford model is excellent for depicting fault configuration generated in sediments above a high-angle fault. (10) Faults such as Dinosaur Monument can be seen to steepen downward, but my models suggest that they go listric at their lower transition with plastic substratum. (11) The COCORP trace of Wind River fault indicating a nearly planar 35° dipping fault most of way through crust is probably real; arguments that you cannot see a fault of granite against granite do not apply. (12) Gravity highs over uplifts, models, and later collapsed uplifts speak for a flexed and slab configuration and against a buoyant root configuration. (13) Thrusting from several different directions appears not to be a when viewed in context of jostled slabs. (14) Blocky corners do place limits on amount of thrust and strike-slip translation. (15) The argument for pure verticality to solve a presume space problem in Piney Creek structure loses validity as soon as bounding tears are allowed to stray from a perfectly vertical dip. (16) The argument at Elk Mountain that dip of bounding structure must be at least as low-angle as degree of overturning of sedimentary panels is wrong (proven by Five Springs). (17) Sales' eastward crowding of Colorado Plateau and development of a Wyoming couple north of it still seems cogent. (18) The Chapin and Cather, and Gries subdivision into movement phase also appears to be correct. (19) If horizontal compression is a reality, Stone must be correct in principle; there have to be logical connecting structures. (20) The crust can transmit stress over great distances because it is weak enough; southeast Asia tectonics require greater distances of stress transmittal than Laramide foreland tectonics. End_of_Article - Last_Page 1354------------

Tectonic Significance of Ross Pass Fault Zone, Central Bridger Range, Montana: ABSTRACT

The Ross Pass fault zone (RPFZ) in the central Bridger Range marks the boundary between Proterozoic Belt Supergroup rocks to the north and Archean metamorphic rocks to the south, and may represent the overlap of two Laramide styles of deformation: thin-skinned fold and thrust deformation to the north and basement-involved foreland deformation to the south. The fault zone consists of three northwest-trending, northeast-dipping, oblique-slip thrust faults with varying amounts of displacement. The middle thrust, the Pass fault, is the most extensive within the zone; to the west, the Proterozoic LaHood Formation is faulted against Archean quartzo-feldspathic gneisses and amphibolites and overlying Middle and Upper Cambrian strata. Displacement decreases eastward as the fault offsets the lower Paleozoic section and dies out within the upper Paleozoic strata. This variation in displacement is caused by folding within the hanging wall strata along the fault plane during thrusting. The folded strata are again offset by another smaller thrust to the northeast, the Peak fault, which may have originated as an out-of-the-syncline thrust. The Dry Fo k fault, southwest of the Pass fault, is largely an intraformational thrust within the Middle Cambrian Meagher Limestone which formed in response to buttressing against the Archean foreland block to the south. South of the RPFZ, both the Archean and the overlying Phanerozoic strata have been deformed into a large asymmetric, steeply dipping, eastward verging fold typical of basement-involved foreland structures. The structural relationships along the RPFZ developed as the southeast margin of the Montana salient of the Cordilleran fold and thrust belt impinged on the northern margin of the Laramide foreland province. The relatively small displacement within the thrusts indicates that the RPFZ is within or near the leading edge of the Montana salient. The RPFZ may be an extension of the inferred ancestral Willow Creek fault zone to the west, which was the southern margin of the Proterozoic Belt basin. Laramide compressional stresses exploited this long active zone of weakness and resulted in the impingement of fold and thrust structures upon a foreland uplift. End_of_Article - Last_Page 1333------------

Structural Evolution of Chihuahua Tectonic Belt: ABSTRACT

The Chihuahua tectonic belt is a Laramide foreland fold and thrust belt that is in the initial phase of exploration. It can be compared to the Southern Overthrust belt prior to the Pineview discovery in 1975. The part within the United States is approximately 300 mi (483 km) long and extends from southeast of El Paso to Presidio, Texas. The frontal thrusts of the belt lie against the Diablo platform. The stratigraphic sequence involved in thrusting is Permian to Lower Cretaceous. Thrusting has been accomplished by decollement of Jurassic and Permian evaporates. The Malone Formation (Jurassic) is the proximal part of a fan-delta complex and contains marine, subtidal, intertidal, and supratidal facies. Two distinctive locally derived conglomerate facies are present: (1) clast-supported, bimodal, dolomite-pebble conglomerate (sheet-flood facies); and (2) a matrix-supported, dolomite-pebble conglomerate that lacks sorting (debris-flow facies). Probable correlatives of the Malone Formation occur at Sierra del Kilo and Sierra de la Alcaparra in Chihuahua 60 mi (97 km) to the southwest. By using new surface-mapping and surface-sampling plus interpretation of seismic and gravity data, a sequence of tectonic events can be inferred: (1) Late Triassic(?) faulting and formation of the Chihuahua trough; (2) Laramide folding and thrusting; (3) late Laramide en echelon left-lateral strike-slip folding; and (4) basin-and-range faulting. End_of_Article - Last_Page 548------------

Structure of the Laramide Wind River Uplift, Wyoming, from Cocorp deep reflection data and from gravity data

The question of the structure of the Wind River uplift, a Laramide foreland structure in Wyoming, has been answered by Consortium for Continental Reflection Profiling (Cocorp) deep crustal reflection profiling and by gravity interpretation. Wyoming uplifts are asymmetrical anticlines whose steep limb is commonly cut by a reverse fault or thrust. If the thrust continues into the crust with low dip, the uplift is caused by horizontal movements; and if the fault steepens into a high‐angle reverse fault in depth, the uplifts are caused by vertical movement. The Wind River uplift is the largest of the Laramide structures in Wyoming, and the thrust fault has 14 km of vertical separation and 26 km of horizontal separation. About 168 km of crustal reflection profiling has been completed by Cocorp across the uplift. The Wind River thrust fault generates a continuous reflection from near the surface to a depth of about 24 km, and the thrust appears as a complex zone of faults. Apparent dip of the thrust is 30°–38°, and true dip may be up to 48°. Interpretation of deep crustal reflections is complicated by the presence of multiple reflections. A structure whose complexity approaches that of basement outcrops is found in the deep crust. Gravity modeling also suggests that the fault dips moderately and that dense rocks are uplifted in the deep crust. The Moho does not seem to be presently displaced by faulting. The crust in the Wind River uplift began to deform by large‐scale folding and then broke and faulted as a rigid slab. Faulting is caused by crustal shortening from compression that is related to plate movements.

Diastrophic Behavior around the Bighorn Basin

A striking feature of the mountain belt around the Bighorn Basin is the reversal of the direction of overthrusting and the asymmetry of folding in transverse segments of the ranges. Not only do these segments of alternating southwest and northeast thrusting characterize the ranges, but the asymmetry of each continues in minor anticlines well into or completely across the adjoining basins. Five such transverse strips are described. Deformation was that of the Laramide foreland in which the pre-Cambrian rocks and structures, relatively thinly covered, were a more important determining factor than in the geosynclinal folding farther west. The greatest crustal shortening was northeast-southwest. In the basement complex there were two types of yielding: (1) horizontal shearing along two sets of near-vertical fractures, each at about 45° to the shortening, breaking the foreland in places into blocklike units; and (2) vertical movements giving ranges and basins with various individual departures from the general Rocky Mountain alignment. In the adjustment to space requirements under horizontal compression, transverse strips of the ranges and basins moved differently. The minor folds have a regional northwest-southeast trend more or less regardless of the trends of the main ranges and basins which were outlined earlier. This minor folding was relatively shallow; near the surface easiest relief was upward, and the folds were aligned normal to the greatest shortening. Although the basement deformation did not greatly influence the trend of these lesser folds, it did determine the direction of their asymmetry.