The late Cenozoic extension in the Rio Grande rift of north-central New Mexico was predominantly accommodated by the north-south–trending Pajarito and Sangre de Cristo normal faults and the intervening east-northeast–striking predominantly strike-slip Embudo fault. Using this segment of the rift as our primary example, we have analyzed a series of three-dimensional nonlinear elastic-plastic finite-element models to assess the role of mechanical interactions between pairs of en echelon rift-scale listric normal faults in the evolution of intervening relay zones. The model results demonstrate that under orthogonal extension and an overall plane-strain deformation, relay zones may evolve in a three-dimensional strain field and along non-coaxial strain paths. The extent of non-plane strain and non-coaxial deformation depends on the fault overlap to spacing ratio, the relative orientations of the bounding faults, and the structural position within the relay zone. The model-derived minimum compressive stress vectors within the relay zone are oblique to the regional extension direction throughout the deformation. Within the Rio Grande rift of north-central New Mexico, the occurrence of northerly striking Neogene faults suggestive of east-west extension in the Española and the San Luis Basins, geographic variations in the vertical-axis rotations from paleomagnetic studies, and secondary fault patterns are consistent with the near-surface variations in the strain field predicted by the model. The model suggests that interaction between the Pajarito and the Sangre de Cristo faults may have played a major role in the evolution of this segment of the rift.

Abstract The Sevier fold-thrust belt and Laramide foreland comprise two interrelated mountain systems that formed during subduction-related orogenesis along the Cordillera margin of western North America. This field trip integrates field observations from across the two mountain systems with results of recent detailed structural and paleomagnetic studies to develop a tectonic model for evolution of these two classic belts and their relations to plate dynamics. Within the Sevier belt, regional structural relations, synorogenic sedimentation, patterns of internal strain in limestones, and paleomagnetically determined vertical-axis rotations and anisotropy of magnetic susceptibility (AMS) in red beds are examined to better understand processes that lead to systematic curvature in thrust belts. Widespread early layer-parallel shortening (LPS) was accommodated by spaced cleavage, fracture sets, minor folds, and minor faults. LPS directions are subperpendicular to structural trends of systematically curved, thin-skin thrust sheets of the Wyoming salient, reflecting a combination of primary dispersion about an average E-W direction and secondary rotation during thrusting. Rotation was concentrated along the front of a forward propagating wedge, where tectonic stress transmitted from the hinterland, topographic-related stresses, and along strike variations in sedimentary thickness, lithology, and fault strength led to curved thrust slip and differential shortening. Within the Laramide foreland, structural styles of basement-cored arches, sedimentation in basins, paleostress/strain patterns, and combined AMS and paleomagnetism of red beds are examined to test models of foreland deformation and relations to flat slab subduction. Limited LPS was accommodated mostly by minor faults with conjugate wedge and strike-slip geometries. Estimated paleostress directions have a regional WSW-ENE average, but vary from perpendicular to acute to variably trending, thick-skin basement-cored arches. Steep forelimbs display more complex relations, including younger fault sets that developed during evolving stress states and localized, limited vertical-axis rotations. Variations in arch trends and LPS directions are interpreted to partly reflect basal traction during flat-slab subduction beneath thick cratonic lithosphere, combined with spatial-temporal variations in stress/strain fields related to basement heterogeneities and evolving fault systems.

In map pattern, fault traces in fold-thrust belts (FTBs) are typically curvilinear, often composed of a series of salients and recesses, and in three-dimensions, faults have non-planar geometries. The three-dimensional nature of fault surfaces has important implications for the overall kinematics of FTBs. We report quartz c -axis textures, grain-shape data, field measurements, and microstructural data from rock adjacent to the fault in a salient-recess pair along the Moine thrust. Within the salient, relict grains demonstrate low to moderate strains and weak to undeveloped quartz c-axis textures. On a Flinn diagram, grain shape data cluster around the k = 0.75 line. Within the recess, the footwall samples >35 m normal distance (nd) from the fault trace have quartz c-axis textures that indicate a general flattening strain. Grain shape data from these deformed relict grains yield lower k-values and higher strains than those seen within the adjoining salient. Samples closer to the thrust (within 8 m nd), also within the recess, are completely recrystallized and exhibit asymmetric c-axis patterns (some with vorticity parallel maxima) and Regime 3 type microstructures. The long axes of these recrystallized quartz grains are gently plunging and oriented sub-perpendicular to the regional transport direction. Within the hanging wall, quartz overgrowths form radially about opaque grains within the foliation plane, further supporting the inference of general flattening strains within the recess. The lateral variation seen between the salient and the recess reflects the variable kinematics within non-planar thrust systems and therefore thrust fault kinematics in general.

The Provo salient of the Sevier fold-thrust belt in central Utah has a prominent arcuate shape in map view with the thrust traces strongly convex toward the foreland. We use the Provo salient as an example for understanding the kinematics of the formation of salients. The kinematic information from the preserved mesoscopic and microscopic structures related with the Cretaceous Sevier orogeny are used to infer two superimposed phases of deformation that define the early and the late directions of transport for formation of the Provo salient. These new three-dimensional kinematic results, together with available data from the Sheeprock thrust sheet and along the Leamington transverse zone, are used to distinguish between five end-member models of salient formation; namely, “bow and arrow rule,” “orocline,” “divergent transport,” “tear fault boundaries, and “lateral or oblique ramp boundaries.” The tectonic transport directions from the long-axis orientations of the finite strain ellipsoids yield trends generally parallel to the overall W-E transport direction (080°–117°) in the middle of the Provo salient, in the Midas, East Tintic, and Tintic Valley thrust sheets, with e 1 /e 3 axial ratios of 1.19–1.36. The Charleston-Nebo thrust sheet, in the foreland portion of the Provo salient, shows consistent NE (026°–062°) transport directions and axial ratios ranging from 1.19 to 1.46. Along the Leamington transverse zone, the long-axes orientations from strain ellipsoids trend E to ESE (091°–122°), and strain axial ratios range from 1.13 to 1.50. Octahedral strain values (ε s ) are relatively greater in the hinterland thrust sheet (viz. Sheeprock thrust sheet; ε s = 0.12–0.83) than other thrust sheets (ε s = 0.13–0.43). Analysis of fracture populations of the late (open) fractures within the Provo salient demonstrates that the late tectonic transport directions trend almost parallel to the early W-E transport directions in the middle of the salient, indicating that they remained constant during successive phases of crustal shortening. However, analyses of fracture populations from two different periods of fractures (viz. fractures with slickenlines and late open fractures) based on cross-cutting relationships indicate that late stage transport directions changed temporally along the edges of the salient. Overall, the variations in directions of both the early and the late transport show a divergent pattern. These variations in transport direction in different parts of the Provo salient suggest the possibilities of either lateral or oblique ramp boundaries or divergent transport. Detailed three-dimensional kinematic study along the Leamington transverse zone shows that the original E-W transport direction is progressively changed by interaction with the oblique ramp, suggesting that a model of modification at lateral or oblique ramp boundaries best explains the structure and kinematics at the edges of the Provo salient. In contrast, three-dimensional strain in the Sheeprock thrust sheet shows stretching perpendicular to the transport direction in the back end of the sheet and stretching parallel to the transport direction at the front of the sheet, indicating the possibility of divergent transport. We suggest that the Sheeprock thrust sheet evolved by divergent transport in the hinterland portion of the salient without any interaction with lateral or oblique ramp structures during relatively strong convergence, while areas of the frontal thrust sheets (e.g., Charleston-Nebo thrust) experienced a different mode of salient formation with W-E transport being modified by lateral or oblique ramp boundaries during relatively weak convergence.

Abstract Deformation in fault zones is commonly characterized by grain-scale microfracturing, with microcrack density typically increasing toward the middle of the zone. The cracks can form under a wide variety of conditions and need to be used with great caution in making tectonic interpretations, particularly in areas with a complex history of fault reactivation. Microcracks may be intragranular (contained within single grains) or intergranular (with a length of several grain diameters). Intragranular cracks formed under dominantly plastic deformation conditions are crystallographically controlled and may not be directly related to regional stresses. Intragranular cracks formed during initial fracturing under cataclastic conditions develop only in grains that are optimally oriented to the deforming stresses. Intergranular cracks form during progressive cataclasis as intragranular cracks grow to join one another: they may develop as transgranular cracks that cut across several grains or as grain-boundary cracks. Once formed, microcracks may be preserved in a variety of ways (e.g. sintering, healing, cementation) depending on postdeformation conditions, and may be distinguished from one another on the basis of microstructural characteristics. Distinguishing between successive generations of microcracks in areas of fault reactivation is particularly important in determining the deformation history and obtaining deformation conditions. For example, Proterozoic quartzites collected from the central Utah Sevier belt have undergone multiple episodes of contractional deformation followed by Basin-and-Range extension. The use of polarized and dark-field optical microscopy and scanning electron microscopy allows microcracks related to the separate episodes of deformation to be distinguished on the basis of morphology, mode of preservation and consistent cross-cutting relationships. Variations in microcrack density and volume of cataclasized rock for the different generations of microcracks are used to establish the patterns of overprinting during fault reactivation. Anastomosing zones of intense deformation formed during successive episodes of faulting may not coincide with one another, as grain-size reduction and cementing during each episode hardens the zones, causing deformation to shift to adjoining weaker rock. However, the fault zone as a whole is a sufficiently large inhomogeneity that it is reactivated during successive faulting events.

The Wind River Mountain Range is one of the most prominent Laramide uplifts of the Rocky Mountains deformed foreland. Precambrian rocks in the core of the Wind River anticline were deformed inhomogeneously along a three-dimensional network of brittle and brittle-ductile deformation zones, which are present at all scales, allowing shortening of the basement core by an amount equivalent to that of the Phanerozoic cover. Along the northeast flank of the anticline where the cover sequence was thin (< 5 km), the underlying basement was deformed under brittle conditions, forming an anastomosing network of brittle deformation zones. The Torrey Creek zone is one such large brittle zone that is well exposed. It probably grew into a major zone because of its location between two different lithologies. Shearing within the zone occurred by distributed slip on numerous gently-dipping conjugate fractures with unidirectional slip. The zone has a complex deformation history of successive phases of shearing (involving grain-size reduction by stable and unstable fracturing, gouging, and wear) and dilation (involving vein precipitation) that allowed it to grow in thickness with increasing displacement. Both the development of and sliding on a network of such deformation zones require a significant amount of energy, and are important components of the total deformation in the Wind River Mountains.

Comparisons of geometric relations, microtextures, and the nature of fluid-rock interaction for deformed basement rocks in parts of the Rocky Mountain foreland, Wyoming, and the Sevier orogenic belt, northern Utah, reveal regional variations in structural style. Basement within the Wind River Range, Wyoming, is deformed by large-scale reverse faults and by intervening major brittle deformation zones (BDZs). Slip along faults and BDZs produced 30% regional shortening by bulk pure shear. Basement away from BDZs is cut by widely spaced fractures, but fracture intensity increases toward BDZs, and complex fracture networks occur along BDZ margins. BDZs contain anastomosing zones of breccia and cataclasite that developed by accumulation of fine-grained matrix due mostly to intergranular cracking and production of wear fragments along shear fractures. Small amounts of chlorite, clay, and calcite record local alteration along major BDZs. Cataclasis appears to be the dominant deformation mechanism, although very fine-grained foliated cataclasite along major faults may record a switch to pressure solution, particulate flow, or grain boundary sliding. Basement within the northern Wasatch Range, Utah, is deformed by large-scale imbricate thrust faults, by intervening networks of major ductile deformation zones (DDZs), and locally by networks of minor DDZs. Slip along thrusts and DDZs produced 60% shortening, with regional-scale simple shear and significant components of pure shear within individual thrust sheets. Basement rock away from DDZs is cut by widely spaced fractures and displays limited alteration. A transition zone along DDZ margins displays variable alteration concentrated along complex fracture and microcrack networks. Variably deformed grains and mineral-filled veins record temporally overlapping cataclasis, fluid-rock interaction, and plastic deformation. DDZs contain phyllonite that formed by accumulation of fine-grained matrix due to pervasive alteration, plastic deformation, and periodic microcracking. Quartz in DDZs is highly strained and recrystallized, and feldspar is almost completely altered to foliated aggregates of mica. Cataclasite is mixed with phyllonite along major thrust fault zones. Concentration of deformation within BDZs and DDZs reflects overall strain softening along these zones relative to the host rock. Relations between displacements and thicknesses of zones record growth of BDZs and DDZs with time, and may reflect periods of strain hardening during progressive deformation. The behavior of any particular zone is determined by a number of competing strain softening and hardening processes that are related to grain size reduction, fluid-rock interaction, and geometric evolution of internal structures.

Abstract This site is located in Shenandoah National Park on the Appalachian Trail near the small parking area at mile 39.1 on the Skyline Drive (Fig. 1). It is accessibly by any vehicle including bus. Some agility is required in the “rock-scrambling” necessary to view part of the exposure. NOTE: Collecting is not allowed in Shenandoah National Park without special permission.

A comparison of mesoscopic and microscopic structures in the Idaho-Utah-Wyoming thrust belt and the Rocky Mountain deformed foreland in Wyoming reveals regional variations in structural style and a decrease in regional shortening from the thrust belt (~60 percent shortening) to the foreland (~30 percent shortening). Deformation in the thrust belt is thin-skinned and is achieved by regional-scale simple shear in the sedimentary cover, which is separated from the basement by a regional décollement. Detailed studies on the Crawford thrust sheet show that large-scale shortening in the cover was produced by thrust faults and associated folds, while internal shortening was achieved by pressure solution (recorded by spaced cleavage), plastic deformation (recorded by deformed fossils), and cataclasis (recorded by contraction faults). In the foreland, both basement and cover are deformed by regional-scale pure shear, although this may be the secondary effect of even larger, lithospheric-scale thrusting. Detailed studies in the Wind River Mountains (of the Wyoming foreland) show that large folds in the cover are directly related to major deformation zones in the basement. Localized strain softening along these zones allowed large displacements by cataclastic and diffusional processes, while basement blocks between zones underwent only minor deformation by fracturing and faulting. Minor internal shortening in the cover was produced by contraction faults, buckle folds, and rare tectonic stylolites. The regional change in structural style from the thrust belt to the foreland may reflect a change in the physical conditions of deformation, caused (at least partly) by significant differences in the thickness of the sedimentary cover in the two areas.

In the Moine thrust zone, Proterozoic (Moine) metasedimentary rocks of the Caledonian belt are carried over the foreland sequence of Archaean to Proterozoic (Lewisian) gneiss, upper Proterozoic (Torridonian) sandstones, and the unconformable sequence of Cambro-Ordovician shelf quartzites and limestones. The thrust structures occur on all scales, from minor duplex zones a few centimeters across to large thrust sheets on a kilometer scale. The thrust sequence is generally piggy-back; thus, the easternmost (highest level) thrust formed first. The thrust direction was toward N65°W ±10°. The lower thrust sheets have produced folds by the stacking of imbricate slices, although in Eriboll on the north coast, buckle folds are common in the hanging wall and footwalls of minor thrusts. The higher thrust sheets contain buckle folds, which locally are sometimes large-scale structures and generally oblique to thrust transport. This suggests that the buckle folds formed by differential movement, with the northern part of the thrust zone moving farthest to the west-northwest. Textural studies suggest that this movement in the north occurred at a slower rate, under more ductile conditions. Back thrusts and break-back faults developed at tip zones where the resistance to movement and/or fault propagation was high. The amounts of displacement estimated from offsets of Lewisian structures are 35 to 45 km. Balanced cross sections in the region between Foinaven and Assynt show shortening values from 35 km + to 55 km + . The Foinaven imbricates do not affect the lowest Cambrian strata or their basement; that is, the basement rocks must continue back some 55 km beneath the Moines, the distance equivalent to the restored middle to upper Cambrian. This implies that any crustal ramp to the Moine thrust zone must lie more than 55 km east of the present outcrop of the thrust. However, off the north coast of Scotland, deep seismic reflection profiles show moderately dipping reflections much farther to the northwest, and if these represent the crustal-scale ramp, this ramp must be offset by a major tear or transfer fault along the north Scottish coast. In the southern part of Assynt, in the central part of the thrust zone, there are several extensional fault systems as well as a late (extensional?) fault at the local base of the Moines that cuts across earlier thrust and extensional faults. These extensional movements suggest a gravity-spreading mechanism for some Caledonide thrusting. The timing of thrusting can be bracketed by the 430-Ma age for the Borralan igneous intrusion, which predates most major thrust movements, and by the Devonian age of molasse deposits. It is difficult to find the driving mechanism for the Moine thrust at this time.

Fault rocks can be studied by charting how undeformed rocks near a fault transform into mylonitic or cataclastic tectonites, or by examining rock masses at different points along a fault to determine how changes in temperature, pressure, etc. affected the fault’s history. Both approaches have merit in thrust belts because thrust faults form under a range of conditions and may evolve along several different paths. Using the first approach, we distinguish two fault zone types analogous to Means’ (1984) two types of shear zones: Type I fault zones grow in thickness as movement on the fault increases; Type II fault zones initiate as zones of localized deformation, and deformation becomes further localized as displacement increases. Both Type I and Type II fault zones occur in the Appalachian fold-and-thrust belt. The second approach shows that fault rocks from the thrust zone beneath the southern Appalachian Blue Ridge and that beneath the Bay of Islands ophiolite evolved in similar ways, despite differences in rock types and local structural history. Three conclusions emerge from our survey of fault rocks from thrust faults: (1) rocks from both external and internal thrust zones may deform by fracturing or by plastic flow, and may alternate between those modes as local physical conditions change; (2) fault zones with large displacement nearly always weaken with continued displacement; (3) fluid phases are critically important to the softening processes, which accommodate large displacements in both external and internal thrust zones.

Tectonic models of the Taconian orogeny in western New England must account for the rapid metamorphism of the Taconic klippen after thrusting. The most likely source of heat for this metamorphism is an overlying hot thrust sheet of accretionary wedge material, which overrode the continental margin of ancient North America, culminating in a continent-island arc collision. Thermal calculations indicate that rapid conductive heat transfer from such a sheet is possible. The dimensionless Peclet number suggests that conductive heat transfer is faster than, or operates at rates comparable to, advective heat transfer due to thrusting over a distance of at least 6 km from a thrust surface. Thus, syntectonic heating of footwall rocks below a major thrust surface is important and must be taken into account in tectonic models. The continental margin thrust system (CMTS) in western New England may have formed as a set of duplexes under a main roof thrust separating the CMTS from the overriding thrust sheet of accretionary wedge material and above a main floor thrust along which the CMTS was transported over autochthonous continental margin rocks. Thrust sheets in this system are composed of Middle Proterozoic Grenville basement and/or upper Proterozoic to Middle Ordovician cover rocks of a western shelf sequence or eastern slope-rise sequence. According to this duplex model, thrust faults tended to develop sequentially toward the foreland, in the transport direction. The relative timing of thrusting and metamorphism is an important constraint on tectonic models, but metamorphism is not a reliable datum with which to compare the timing of events in different parts of the thrust belt. As an example, synmetamorphic thrusting in the eastern internal part of the belt may have preceded brittle faulting to the west near the foreland. P-T paths of rocks from different thrust sheets separated by major faults will be qualitatively different, and detailed petrologic studies to determine and compare P-T paths from different thrust sheets may be useful in identifying faults along which the greatest displacement has occurred.

This chapter explores the critical role that kinematics plays in the construction and analysis of geological cross sections. The structures on any admissible cross section must arise from relative displacements that are consistent with reasonable deformation kinematics. Sections that violate this constraint are physically impossible. The deformation kinematics can be derived from a displacement field, but the scale at which the displacement field is analyzed affects our perceptions of the movement of rocks in the cross section. Microscopic displacement fields associated with grain-scale deformation may be derived by the standard techniques of finite strain analysis, while macroscopic displacement fields may be derived from the geometry of map-scale cross sections in those regions that have undergone uniform area strain. Physical compatibility requires that the two scales be linked. In regions of uniform area strain, the displacement fields at the two levels may be linked through elementary vector analysis. Finite strain data indicate that the central Appalachians suffered uniform area strain. Elementary vector analysis of a blind autochthonous roof duplex in the central Appalachians shows that: (1) the faulted stiff layer and its overlying roof layer have separate displacement fields; and (2) restoration of structure sections across regions of uniform area strain requires that sections be constructed approximately parallel to the finite strain trajectory of maximum shortening. Another way to link the microscopic and macroscopic deformation kinematics is the use of “loose lines” in the deformed and undeformed states. Loose lines drawn on geological cross sections predict the shear strains at any point within a thrust sheet. These shear strains, derived solely from the geometry of structures in the cross section, are independent data that can be compared to measured strains, providing an additional constraint for a cross section. Loose line analysis can also offer insight to the sequence of faulting in a cross section and the geometry of subsurface structures. Macroscopic kinematic analysis, using vector analysis and loose lines, shows that the “excess section” technique for predicting depth to detachment and finding the initial section length contains implicit assumptions about the kinematics of deformation. Use of this technique without examining the boundary conditions may lead to inadmissible or incorrect cross sections. The problem is perhaps most acute in blind thrust terranes where use of the excess section technique has led to significant underestimates in the amount of shortening in these terranes. Kinematic analysis suggests that “kinematic admissibility” is an additional criterion that can constrain geological cross sections. Since lines drawn on a section have kinematic significance, it is possible to test a section for kinematic admissibility by attempting to pass from its undeformed state, produced by “balancing” the section, to the deformed state by the process of “forward modeling.” This test is applied to several examples from the literature, and it is demonstrated that the proposed solutions can be rejected because they fail to meet the test of kinematic admissibility.