Structural geology has emerged as an integrative, synthetic science in the past 50 years, focused on deciphering the history preserved in the rock record and determining the processes of rock deformation. Owing to the nature of structural geology, studies focus on historical elements, such as structural inheritance and tectonic history, and increasingly involve theoretical, process-based approaches. The strength of the field is that it uses these historical- and process-based approaches simultaneously in order to determine the three-dimensional architecture, kinematic evolution, and dynamic conditions of lithospheric deformation over a wide range of spatial and temporal scales. In this contribution we focus on significant progress made in understanding shear zones, fault zones, intrusions, and migmatites, both as individual features and as systems. Intrinsic to these advances are insights into the strain history, specifically through the temporal evolution of geologic structures. Increasingly sophisticated geochronological techniques have advanced the field of modern structural geology by allowing age determinations to be linked to rock microstructure and deformational fabrics, from which displacement rates and strain rates can be estimated in some settings. Structural studies involving new approaches (e.g., trenching), and integrated with geomorphology and geodesy, have been applied to study active geologic structures in near surface settings. Finally, significant progress has been made in constraining the rheology of naturally deformed rocks. These studies generally rely on results of experimental deformation, with microstructural analyses providing the connection between naturally deformed rocks and results of experiments. Integration of field-based observations, laboratory-derived rheological information, and numerical models provide significant opportunities for future work, and continues the tradition of simultaneously using historical- and process-based approaches.

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.

Strains thought to be associated with movement of a thrust sheet over a ramp are recorded by cleavages and syntectonic fibers within a thrust slice of the Hamburg sequence north of Reading, Pennsylvania. The three cleavages and bedding are coaxial with the regional east-northeast strike, and three related sets of fibers are found in pressure shadows on microscopic, near-spherical pyrite framboids. Viewed east-northeast at XZ sections, the oldest fiber set in the pressure shadows begins with a short segment at ∼40° to the slaty cleavage (S 1 ), but smoothly reorients clockwise to parallelism with the gently dipping S 1 . A crenulation cleavage (S 2 ) crudely fans a broad antiform in S 0 and S 1 , but consistent north-northwest-side-up offsets across cleavage planes throughout the fold, and localized development of an extremely stylolitic character to S 2 in the core of the fold, suggest that this cleavage preceded folding. Fibers of the first set curve sharply but continuously into fibers of the second set, which are parallel to S 2 . A third cleavage (S 3 ) crenulates S 2 and exhibits consistent top-toward-south-southeast offsets. Short fibers parallel to S 3 are found on framboids in zones between S 2 lamellae, but within the S 2 lamellae, long fibers in cylindrical pressure shadows together with other features indicate a volume gain, subhorizontal extension during F 3 development. The progression through these three deformation phases fits well with the strains expected during the movement of a thrust sheet: (1) on a flat; (2) through a closing bend onto a ramp; and (3) through an opening bend onto a higher flat.

The Jacks Mountain anticlinorium is a high, continuous structure in the Pennsylvania Valley and Ridge Province midway between the Blue Mountain and Allegheny structural fronts. Evolution of its northwest limb proceeded through a number of structural stages that may be characteristic of the first-order anticlines of the middle Appalachians. Prior to major folding, faults rising to the northwest off the proposed Antes-Coburn detachment developed in sequence as the Stone Mountain duplex, the Bearpen Hollow thrust and the Potlicker Flat thrust, all cutting Silurian rocks. These were later passively folded to northwest dips during growth of the anticlinorium above the contracting and imbricating Cambro-Ordovician duplex. The northwest limb near the Stone Mountain duplex underwent later-stage layer-parallel extension and steep out-of-sequence reverse or strike-slip faulting, which were caused by the larger amounts of limb rotation and fold flattening at this part of the anticlinorium. Faults associated with this later event have mineralized pressure-solved breccias, suggesting that different deformation conditions prevailed. Evaluation of the poorly exposed Antes-Cobura detachment has not provided structural evidence of a systematic transport direction, but allows interpretation of this zone as both an early floor thrust for imbricates rising to the northwest into overlying Silurian rocks and a later boundary zone between parts of the stratigraphic section undergoing unequal layer-parallel shortening.

The Blue Ridge and Great Valley of western Virginia are part of a detached master thrust sheet that extends through the central-southern Appalachian change of trend and has a root zone situated east of the Blue Ridge under the Piedmont. The mapped Pulaski fault and North Mountain fault crop out in the Great Valley as splay faults terminating in thrust tip anticlines and merge at depth with the buried master detachment floored primarily in Upper Ordovician Martinsburg Shales. Overall transposition of the thrust sheet from the root zone indicates that as much as 32 km (20 mi) of displacement may be translated westward into the Valley and Ridge proper and Allegheny Plateau as initial cover shortening above the Martinsburg Shale. Within the Great Valley and into the Roanoke recess at the juncture of the central and southern Appalachians, much of the cover shortening of this master sheet is accommodated by the Pulaski and North Mountain faults. From northeast to southwest, movement on the outcropping and buried master segment of the North Mountain fault decreases, and the surface fault terminates in southern Rockbridge County. Displacement on the Pulaski fault increases from northeast to southwest, and we infer that it assumes the decreasing displacement on the outcropping and buried segments of the North Mountain fault by displacement transfer.

Precambrian crystalline basement of the Appalachian Blue Ridge deforms inhomogeneously by developing relatively narrow ductile deformation zones (DDZs). The Paleozoic sedimentary cover develops open to tight folds and penetrative fabrics. A transition between these two styles occurs at the base of the sedimentary cover in the Early Cambrian Chilhowee quartzites of the central Appalachians and in the Late Proterozoic arkosic sandstones of the southern Appalachians. On a mesoscopic scale, the transition zone sediments show tight to isoclinal folds with highly deformed overturned limbs analogous to mesoscopic (1 cm to 10 m wide) DDZs in crystalline basement. Deformation zones in the basement cut across the basement/cover contact and feed into the overturned limbs of tight folds. On a microscopic scale, both arkoses and granitic basement rocks show thin (5 mm) DDZs characterized by grain-size reduction and alteration of feldspars to quartz and mica. The actual style and symmetry of deformation varies with metamorphic grade, proximity to major thrust faults, and amount of tectonic shortening. In the Grandfather Mountain area of the southern Blue Ridge Province, sets of low-dipping DDZs close to major thrust faults approximate a simple shear deformation field. In the central Appalachians of northern Virginia, similar simple shear deformation features are observed close to major thrust faults, but sets of DDZs define a flattening plane perpendicular to tectonic transport direction higher up within the thrust sheets.

New outcrops created during the 1983 draining of Watauga Lake within the Mountain City window exposed the Little Pond Mountain thrust zone, marked by more than 200 m of Max Meadows-type carbonate breccia. The breccias are derived from the lower Rome Formation of the hanging wall, which is thrust 12 km over younger Rome beds. The upper boundary of the fault zone is gradational, beginning with intact shales and dolostones that become progressively disaggregated by boudinage and disharmonie folding, grading into a thick zone of polymict breccia in contact with Rome shales in the footwall. The brecciation is thrust related and tied to a particular stratigraphie horizon. Extreme competency contrast between brittlely deformed dolostones and shales, and interlayered, plastically deformed calcitic laminites is inconsistent with the current mineralogy and suggests the presence of weak evaporite-rich layers during deformation. Within the fault zone, these weak beds grade into the breccia matrix. Boudinaged dolostones and shales form clasts in a breccia mixed by mesoscopic isoclinal folding. Raindrop prints, ubiquitous mudcracks, and evaporite crystal molds in the lower Rome Formation are consistent with evaporative depositional environments. The breccias also exhibit features of evaporite solution-collapse breccias, including sedimentary cavity fillings. Pétrographie evidence for vanished evaporites includes anhydrite inclusions, evaporite crystal molds, and chert nodules pseudomorphous after anhydrite. A sparry calcite mosaic that apparently replaced evaporite laminites also forms the breccia matrix. The evaporite hypothesis is supported by interbedded dolostone and anhydrite discovered in the subsurface at the base of the Rome. The Watauga Lake breccias are postulated to be the result of décollement thrusting within a dolostone-anhydrite sequence at the base of the Rome Formation, producing a polymict evaporite-matrix breccia, which after deformation, underwent local solution collapse and widespread replacement of anhydrite by calcite. The Max Meadows breccias have long been considered unique, but a review of published work shows that these rocks and occurrences at Watauga Lake are identical in many ways to Rauhwacken (cornieules) of Europe and similar carbonate breccias in Nevada, the northern U.S. and Canadian Rockies, Ireland, and southern England, all of which have been interpreted as deformed carbonate-evaporite sequences. There are also similarities to carbonate breccias in Nova Scotia, northern Michigan, and the foreland of the Canadian Rockies that are purely the result of evaporite dissolution. These comparisons show that, in a given occurrence, thick carbonate breccias with similar diagenetic histories may originate from either décollement thrusting, evaporite dissolution, or a combination of the two processes.

Controversy is common concerning the sequence of thrust fault imbrication on the scale of one or several quadrangles. Regional thrusting sequences in young orogenic belts are generally from the hinterland to the foreland. This is contrary to the previously proposed regional progression of thrusting for the southern Appalachian Valley and Ridge province. This paper uses cutoff-line maps to systematically examine some of the map patterns and cross-sectional interpretations used as evidence for the foreland-to-hinterland sequence of thrusting. Idealized examples of cutoff-line maps and cross-sectional patterns for both truncated structures and stair-stepped structures can be compared with observed map patterns and previously proposed cross-sectional interpretations. This provides critical evidence for interpreting the map data. Additional critical observations can be made as to the extent that faults may be folded by underlying structures, rather than truncating them. Overall, the cutoff-line approach and the folded fault approach document that the truncated folds expected in map-pattern for a foreland-to-hinterland thrust sequence do not occur in the east Tennessee area. Folded faults and westward-younging cutoff-line patterns indicate that later faults were in front of, and beneath, earlier ones in a hinterland-to-foreland sequence.

A late Paleozoic fault duplex forms a structural culmination in the Blue Ridge-Piedmont thrust plate near Cartersville, Georgia. The duplex contains at least three major lens-shaped horses, stacked vertically and bounded by faults that branch from the sole of the Blue Ridge-Piedmont plate. The duplex telescopes older Paleozoic structures and metamorphic fabrics that are related to Taconic thrusting of the Blue Ridge over the North American shelf. The duplex, embedded in the sole of the Blue Ridge-Piedmont plate, may have been detached from the area of the Pine Mountain window in the central Georgia Piedmont, and horizontally displaced 130 km during formation of the Valley and Ridge fold and thrust belt.

Thrusts in the crystalline core of the southern Appalachians formed by both ductile and brittle mechanisms during three or more major Paleozoic deformational-thermal events (Taconic, Acadian, Alleghanian), in contrast to thrusts in the foreland which formed primarily as brittle faults during the Alleghanian. Early prethermal peak thrusts formed in the crystalline core, then were subsequently thermally overprinted and annealed. Thrusts that formed late in a metamorphic-deformational sequence have maintained a planar geometry. Many of these thrusts, such as the Brevard and Towaliga faults, were later reactivated in either the ductile or brittle or both realms, possibly involving both dip-slip and strike-slip motion. The thrusts framing the Pine Mountain and Sauratown Mountains windows formed both pre- and post-thermal peak. The pre-thermal peak Box Ankle thrust in the Pine Mountain window is a structurally lower fault, whereas the window is flanked externally by the post-thermal peak Towaliga (northwest) and Goat Rock (southeast) faults. Conversely, in the Sauratown Mountains the brittle Hanging Rock thrust frames an inner window beneath the older Forbush thrust. Here a downward and outward propagating sequence is suggested for the development of thrusts. North American basement rocks are involved in both the Pine Mountain and Sauratown Mountains windows, and basement and cover behave as a homogeneously coupled mass with respect to strain. Consequently, the only factor that controlled the siting of early thrusts may have been the depth to the ductile-brittle transition zone. The frontal Blue Ridge thrust was the last formed in the Blue Ridge-Piedmont thrust sheet although the Cartersville-Miller Cove thrust is a slightly older Alleghanian thrust than the Great Smoky fault.

Three different styles of plane- or nearly plane-roofed duplexes have been obtained using a computerized kinematic model of thrust-ramp anticlines. A thrust duplex consists of a group of fault-bounded slices (horses) with common lower (floor) and upper (roof) fault boundaries. The model assumes constant bed thickness and bed length parallel to the thrust ramp, in the forelimb of the anticline and everywhere that bedding is horizontal. Fault horse formation is accompanied by shear parallel to the new fault ramp. The model has been used to generate three different styles of plane- or nearly plane-roofed duplexes. In the only style having a perfectly planar roof, the fault horses are emplaced on the footwall upper flat and dip toward the foreland. The two hinterland-dipping duplex styles obtained do not have perfectly planar roofs. The first style has nearly planar faults within the duplex and can be developed with a minimum of two horses of intermediate spacing. The second style has faults that curve asymptotically into the roof and floor thrusts in a snakehead geometry and requires a minimum of four or more relatively closely spaced horses. Widely spaced horses having a common upper detachment result in a bumpy-roofed duplex with a roof deformed into anticlines and synclines.

Frontal ramps constitute vital clues to the mechanics of low-angle thrusting, but their origins have not yet been established. Several critical elements of the kinematics, however, now have been deciphered for a classic locale of the Wyoming thrust belt. They demonstrate that the frontal ramp of the Hogsback thrust sheet in the Kemmerer region might have developed by shear fracture, and that the origin of the ramp is not related to the folding of the Lazeart syncline, even though this major fold is adjacent to the ramp. Palinspastic analyses of highly constrained balanced cross sections demonstrate that the Lazaert syncline almost certainly originated during thrusting at the leading edge of the Absaroka thrust. The east limb was nearly horizontal when the west limb was overturned by Absaroka thrusting. The Lazeart syncline subsequently was carried piggyback as the upper plate of the Hogsback thrust moved over the frontal ramp. The east limb of the syncline then was rotated parallel to the frontal ramp by displacements along upper plate imbricates in the ramp region. Almost all of these imbricates formed in the break-back sequence; i.e., sequentially in the hinterland direction. They formed at the ramp when fault slip of the Hogsback plate ceased or decreased substantially along the flat east of the ramp; i.e., locking of the upper plate occurred within the ramp region itself after part of the plate had moved over the ramp. Locking at frontal ramps appears to be a potentially important phenomenon in the mechanics of low-angle thrusting.

Thrust faults are typically discontinuous. Based on the relative positions of adjacent fault segments, the discontinuities along thrust faults can be classified into two major groups: along-strike and down-dip. Adjacent fault segments are linked by transfer structures such as secondary dip-slip faults, tear faults, folds, cleavages, and pull-apart openings. Duplex structures are compressional down-dip discontinuities associated with echelon thrust faults with relatively large overlaps. Duplex structures in general, and cleavage duplexes in particular, are analyzed by calculating stresses due to interacting echelon mode II cracks. The results indicate that there are significant increases in the maximum compressive stress, mean stress, and maximum shear stress at the stepover area. The orientations of the planes upon which the largest compressive stresses act in the model are approximately consistent with the orientations of the cleavage planes in a few duplexes described in the literature.