ABSTRACT Shales exhibit a wide range of textures, compositions, and mechanical properties, which are interlinked by their diagenetic history. During hydraulic fracturing of shales, the matrix is subjected to shear deformation, which may create microfractures and enhance hydrocarbon transport from nanoscale, organic matter (OM)-hosted pores to the larger, induced fracture network. To study the nanoscale response to shear deformation of shale pore systems with different diagenetic histories, we deformed shale samples from a formation in the Northern Rocky Mountains and the Eagle Ford Group in Texas, using confined compressive strength tests. N 2 and CO 2 adsorption were performed to quantify fracture effects on pore morphology including pore size distribution, porosity, surface area, and surface fractal dimensions. Most samples increased their gas adsorption quantity, pore volume, and surface area after failure. The surface fractal dimensions were less sensitive to shear deformation. Results show that varying nanometer-to-micron-scale fracture patterns are in part caused by contrasting rock fabrics that are preconditioned by their distinctive diagenetic histories. For example, fractures tend to propagate along the OM laminae, whereas others cut across OM grains and access OM pores. Other possible mechanisms for porosity increase include the deformation of relatively uncemented clay aggregates and contrasting amounts of intra-OM pores between samples. Thus, the mechanisms for syn-deformational porosity changes at the micro scale are highly dependent on diagenetic history, particularly the maturation of OM, and the cementation history relative to clay content.

Abstract Rheological inheritance occurs when older metamorphic and deformational fabrics impact the mechanics of younger tectonic provinces, such as occurs in extensional provinces developed on sites of previous orogenesis. The Funeral and Black Mountains from the Death Valley region of the US Basin and Range provide the opportunity to study such rheological inheritance. The Funeral Mountains expose shear zones containing high-grade metamorphic fabrics and evidence for synkinematic, decompression-driven melt of Late Cretaceous, orogenic origin. Quartz < c >- and [ a ]-axes patterns from the shear zones correlate with high-temperature slip systems. The quartz microstructures were formed via grain-boundary migration, and these are overprinted by high-strain layers of mixed-phase aggregates that underwent grain boundary sliding. Reaction textures from the Funeral Mountains illustrate that much of the fabric development post-dates melting, but locally involved melt–rock reactions. In contrast with the Funeral Mountains, the basement complex in the Black Mountains preserves few peak-metamorphic textures, largely owing to the overprinting by Cenozoic magmatism and deformation. However, local relicts of high-grade deformational fabrics yielding Late Cretaceous-through-Eocene magmatic zircon ages are overprinted by greenschist grade fabrics. Using outcrop and microstructural (including electron backscatter diffraction) observations, and thermodynamic modelling, we detail how segregation of melt products during orogenic partial melting resulted in chemically isolated compositional domains, favouring localization via the formation of fine-grained retrograde fabrics. We propose a conceptual model that builds on our results wherein the heterogeneous distribution of peak, orogenic metamorphic phases and melt products governs lower crustal strength and fabric evolution during extension. The Wilson Cycle may be sensitive to rheological inheritance as the width of continental margins formed during rifting will be sensitive to the fabrics and compositions formed during collision.

Abstract A prevailing hypothesis for the central Cascade Range of Washington State is that it underwent regional extension or transtension during the Eocene. This hypothesis is based on the idea that kilometers-thick, clastic, Eocene formations were deposited syntectonically in local basins. Our mapping and structural analysis indicate that these formations are preserved in fault-bounded, regional synclines, not in separate depositional basins. Thus, the type area for the hypothesis, the so-called Chiwaukum graben, is here renamed the Chiwaukum Structural Low. The Eocene arkosic Chum-stick Formation, which was thought to have been syntectonically deposited in the graben, is the proximal equivalent of the Roslyn Formation 25 km southwest of the graben. Because the name “Roslyn Formation” has precedence, the name “Chumstick Formation” should be abandoned. Additionally, several areas previously mapped as Chumstick Formation in the Chiwaukum Structural Low probably are parts of the older Swauk Formation and younger Wenatchee Formation. The southwestern boundary of the Chiwaukum Structural Low includes the Leav-enworth fault zone, which consists of postdepositional, northwest-striking reverse faults with adjacent northwest-striking folds. The reverse faults place the regionally extensive early-Eocene, arkosic Swauk Formation over the mid-Eocene, arkosic Chumstick Formation. A diamictite, which previously was placed in the Chumstick Formation and inferred to have been syntectonically derived from the Leavenworth fault zone, is part of the older Swauk Formation. We mapped a 0.6–1-km-thick conglomerate-bearing sandstone as a robust marker unit in the Chumstick Formation; instead of being spatially related to the bounding faults, this unit has a >30 km strike length around the limbs of folds in the structural low. The northwest-striking reverse faults and fold hinges of the structural low are cut by north-striking strike-slip faults, which likely are late Eocene to Oligocene; these north-south faults partially bound the structural low. The Eocene folds and faults were reactivated by deformation of the Miocene Columbia River Basalt Group; this younger folding largely defines the regional map pattern, including the structural low. A model to account for the above characteristics is that all of the Eocene formations, not just the Roslyn Formation, are kilometers thick and are remnants of regional unconformity-bounded sequences that were deposited on the Eocene margin of this part of North America. Their present distribution is governed by younger faults, folds, and erosion. Thus, the Eocene to Recent history of the central Cascade region is characterized not by crustal extension, but by episodes of folding (with related reverse faults) and strike-slip faulting.

Abstract Eocene sedimentary and volcanic rocks on the eastern flank of the Cascade Range consist of five regional, unconformity-bounded formations of the Challis synthem. These formations define a series of northwesterly striking folds. Five anticlines are 9 to 28 km apart, have pre-Tertiary crystalline rocks in their cores, high-angle reverse faults on their steeper northeastern limbs, and pass down-plunge into more gentle folds in the Neogene Columbia River Basalt Group (CRBG). Such northwesterly trending folds extend from east of the Columbia River across the Cascade Range to the Puget Lowland. The Chiwaukum graben and Swauk basin, which heretofore were thought to be local, extensional, depositional basins, are, instead, the major northwesterly trending synclines in this series of folds. The Eocene formations were preserved, not deposited, in these synclines. Dextral, N-S faults cut the reverse faults and the pre-CRBG portion of some of the folds. The post-CRBG folds control the regional distribution of the Eocene formations. The Cascade Range is a southerly plunging, post-CRBG anticline. Clasts in the Thorp Gravel indicate that this anticline began to rise ca. 4 Ma. The anticline has an amplitude of ∼3.5 km, and it causes the plunges of the northwesterly striking post-CRBG folds. The northerly and northwesterly post-CRBG folds form a regional interference pattern, or “egg-crate,” that dominates the present topography of Washington State.