Layer Parallel Shortening Strain A Key To The Sevier Laramide Deformational Sequence In The Tendoy Range Southwestern Montana

Layer-parallel-shortening (LPS) strain records the first strain experienced by rocks during tectonic events. LPS indicators preserved in vertical to overturned fold limbs along the leading edge of the fold-thrust belt in southwestern Montana are used to determine which deformation came first there: Sevier or Laramide. LPS-strain was analyzed for 22 samples collected in the Dixon Mountain area from different beds containing ooids, Pentacrinus, or brachiopods within a 580-meter stratigraphic interval. Field orientations of strain ellipses were then adjusted for: 1) trend and plunge of fold axes; 2) rotation of thrust sheets relative to the trend of the Sevier-emplaced, overturned Deadwood Gulch syncline; and 3) bedding dip. Retrodeformed orientations of 15 samples show an average shortening direction of 2̃27o, consistent with Sevier shortening (2̃13o); 4 samples show a shortening direction of 2̃68o; and 3 samples show a shortening direction of 1̃71o, perhaps indicating Sevier shortening across a lateral ramp.

The Little Water syncline is a complex structural feature within the Tendoy thrust sheet which formed during two temporally overlapping deformational episodes during the Late Cretaceous. The NNW-trending Four Eyes Canyon thrust and the structurally lower Tendoy thrust are associated with NE-SW-shortening related to the Cordilleran (Sevier-style) fold-thrust belt. The Tendoy thrust forms the structural front of the Cordilleran fold-thrust belt in southwestern Montana and its detached, NNW-trending overturned limb (of the Little Water syncline) was over-ridden by the Four Eyes Canyon thrust. Earlier research primarily used stratigraphic evidence coupled with geometric interpretations to suggest that: 1) clasts found in the syn-tectonic (Cretaceous) Beaverhead Formation were derived from the Four Eyes Canyon thrust; 2) Laramide-style deformation occurred before emplacement of the Tendoy thrust sheet; and 3) that the Tendoy thrust was then emplaced over the syn-tectonic Beaverhead Formation. The NE-trending (approximately 30o), overturned northern flank (at nearly right angles to the detached overturned limb) of the Little Water syncline has long been argued to be a structural feature associated with NW-SE-shortening (1̃20o) generally related to the Laramide Snowcrest thrust system. In this study of the Little Water syncline, I have used layer-parallel-shortening (LPS) strain-indicators (fossils and pellets) to obtain the initial direction of shortening preserved in: 1) the NNW-trending, detached, vertical-to-overturned, western limb; 2) the steeply-dipping-to-overturned, northwestern limb; and 3) the moderately W-dipping, upright, N-trending eastern limb to determine the initial deformation (Sevier or Laramide) that affected the syncline. When bedding is unfolded, LPS strain: at 11 of 14 sites is consistent with NE-SW-shortening (221o) associated with the Sevier orogeny; at 2 sites is consistent with N-S-shortening; and at 1 site is consistent with E-W-shortening. No sites have LPS strain consistent with NW-SE-shortening. Hence Sevier LPS strain occurred before Laramide deformation. The 3 sites (sites 1, 2, and 5) where LPS strain lies outside of Sevier-shortening (221o +20o), but not within the range of Laramide shortening (1̃10o-290o) likely represent Sevier LPS strain affected by local factors not accounted for in the retro-deformational sequence used to restore bedding to horizontal in this study. .

Grain-scale volume loss is an important process in the deformation of a fold-thrust belt. Prior to activation of slip on fault surfaces, and initiation of layer buckling in folds, instability is recorded by thickening of parallel bedding surfaces, grain boundary migration, and stress-induced solution transfer of mineral grains. Volume loss recorded by these mechanisms is significant but is an afterthought in any type of cross-section restoration. This research has three main objectives. Firstly, to estimate the amount of penetrative strain (PS) accommodated in weakly deformed sandstones along four E-W transects, from the thick-skinned Colorado Front Range into the Denver-Julesburg Basin. The methods employed to accomplish this were petrographic analysis, analog modeling, and cross-section restoration. Next, the distribution and mechanisms by which PS is accommodated in thick-skinned versus thin-skinned deformation belts were compared. Analog models simulating Laramide deformation helped to gain insight into strain partitioning by mimicking the change in crustal architecture from the Sevier belt into Laramide foreland basins. Finally, the study measured how changes in PS correlate with changes in physical parameters such as porosity and rebound strength. Additional factors, such as distance from major fault structures, were considered. Results from samples within the Denver-Julesburg Basin record PS between 8% and 12%. An early hypothesis suggested PS would correlate strongly with porosity and rebound strength and would decay with increasing distance from faults. This research indicates a weak correlation between these parameters when all samples are incorporated into the analysis. A robust correlation is noted when values for these parameters are averaged by formation or transect. PS values are limited by saturation of pressure solution shortening from grain impingement alone, while porosity values fluctuate with surface weathering. Analog models record decreased PS with increasing depth in the sedimentary section due to the presence of a rigid basement. Finally, cross-section restorations along Horsetooth Reservoir, the northernmost transect in the field area, and Rt. 34, near Loveland, record tectonic shortening values of 10% and 12.2%. Integration of tectonic shortening values, and PS values derived from analog models, estimate 0.79 and 0.36 km of PS related volume loss across the transects.

"In this well-illustrated book, Hildebrand expands upon his model for the development of the North American Cordillera detailed in Special paper 457. Starting with an overview of Cordilleran geology he goes on to provide an in depth look at how the Rubian ribbon continent was assembled. He integrates the complex geology of the Cordillera into an actualistic model involving arc magmatism, arc-continent collision, slab failure magmatism, and transcurrent motion in both Rubia and the western North American margin. While much of the focus is on the assembly of the Rubian ribbon continent, Hildebrand explores its interactions with North America during the Sevier and Laramide events and concludes that North America was the lower plate in both"--Provided by publisher.

The Himalayan mountain belt, which developed during the India–Asia collision starting about 55 Ma ago, is a dramatically active orogen and it is regarded as the classic collisional orogen. It is characterized by an impressively continuous 2500 km of tectonic units, thrusts and normal faults, as well as large volumes of high-grade metamorphic rocks and granites exposed at the surface. This constitutes an invaluable field laboratory, where amazing crustal sections can be observed directly in very deep gorges. It is possible to unravel the tectonic and metamorphic evolution of litho-units, to observe the mechanisms of exhumation of deep-seated rocks and the propagation of the deformation. Himalayan tectonics has been the target of many studies from numerous international researchers over the years. In the last 15 years there has been an explosion of data and theories from both geological and geophysical perspectives. This book presents the results of integrated multidisciplinary studies, including geology, petrology, magmatism, geochemistry, geochronology and geophysics, of the structures and processes affecting the continental lithosphere. These processes and their spatial and temporal evolution have major consequences on the geometry and kinematics of the India–Eurasia collision zone.

With its thickness of more than 15 km of strata, covering some 200,000 km2, the Belt basin displays one of the planet's largest, best-exposed, most accessible, and best-preserved sequences of Mesoproterozoic sedimentary and igneous rocks. This volume focuses on research into this world-class province; kindles ideas about this critical era of Earth evolution; and covers aspects of the basin from its paleontology, mineralogy, sedimentology, and stratigraphy to its magmatism, ore deposits, geophysics, and structural geology.

GEOLOGICAL FIELD TECHNIQUES The understanding of Earth processes and environments over geological time is highly dependent upon both the experience that can only be gained through doing fieldwork, and the collection of reliable data and appropriate samples in the field. This textbook explains the main data gathering techniques used by geologists in the field and the reasons for these, with emphasis throughout on how to make effective field observations and record these in suitable formats. Equal weight is given to assembling field observations from igneous, metamorphic and sedimentary rock types. There are also substantial chapters on producing a field notebook, collecting structural information, recording fossil data and constructing geological maps. Geological Field Techniques is designed for students, amateur enthusiasts and professionals who have a background in geology and wish to collect field data on rocks and geological features. Teaching aspects of this textbook include: step-by-step guides to essential practical skills such as using a compass-clinometer, making a geological map and drawing a field sketch; tricks of the trade, checklists, flow charts and short worked examples; over 200 illustrations of a wide range of field notes, maps and geological features; appendices with the commonly used rock description and classification diagrams; a supporting website hosted by Wiley-Blackwell is available at www.wiley.com/go/coe/geology