Abstract Burnsville Cove in Bath and Highland Counties (Virginia, USA) is a karst region in the Valley and Ridge Province of the Appalachian Mountains. The region contains many caves in Silurian to Devonian limestone, and is well suited for examining geologic controls on cave location and cave passage morphology. In Burnsville Cove, many caves are located preferentially near the axes of synclines and anticlines. For example, Butler Cave is an elongate cave where the trunk channel follows the axis of Sinking Creek syncline and most of the side passages follow joints at right angles to the syncline axis. In contrast, the Water Sinks Subway Cave, Owl Cave, and Helictite Cave have abundant maze patterns, and are located near the axis of Chestnut Ridge anticline. The maze patterns may be related to fact that the anticline axis is the site of the greatest amount of flexure, leading to more joints and (or) greater enlargement of joints. Many of the larger caves of Burnsville Cove (e.g., Breathing Cave, Butler Cave-Sinking Creek Cave System, lower parts of the Water Sinks Cave System) are developed in the Silurian Tonoloway Limestone, the stratigraphic unit with the greatest surface exposure in the area. Other caves are developed in the Silurian to Devonian Keyser Limestone of the Helderberg Group (e.g., Owl Cave, upper parts of the Water Sinks Cave System) and in the Devonian Shriver Chert and (or) Licking Creek Limestone of the Helderberg Group (e.g., Helictite Cave). Within the Tonoloway Limestone, the larger caves are developed in the lower member of the Tonoloway Limestone immediately below a bed of silica-cemented sandstone. In contrast, the larger caves in the Keyser Limestone are located preferentially in limestone beds containing stromatoporoid reefs, and some of the larger caves in the Licking Creek Limestone are located in beds of cherty limestone below the Devonian Oris-kany Sandstone. Geologic controls on cave passage morphology include joints, bedding planes, and folds. The influence of joints results in tall and narrow cave passages, whereas the influence of bedding planes results in cave passages with flat ceilings and (or) floors. The influence of folds is less common, but a few cave passages follow fold axes and have distinctive arched ceilings.

Abstract: Salt is mechanically weaker than other sedimentary rocks in rift basins. It commonly acts as a strain localizer, and decouples supra- and sub-salt deformation. In the rift basins discussed in this paper, sub-salt faults commonly form wide and deep ramp synclines controlled by the thickness and strength of the overlying salt section, as well as by the shapes of the extensional faults, and the magnitudes and slip rates along the faults. Upon inversion of these rift basins, the inherited extensional architectures, and particularly the continuity of the salt section, significantly controls the later contractional deformation. This paper utilizes scaled sandbox models to analyse the interplay between sub-salt structures and supra-salt units during both extension and inversion. Series 1 experiments involved baseline models run using isotropic sand packs for simple and ramp-flat listric faults, as well as for simple planar and kinked planar faults. Series 2 experiments involved the same fault geometries but also included a pre-extension polymer layer to simulate salt in the stratigraphy. In these experiments, the polymer layer decoupled the extensional and contractional strains, and inhibited the upwards propagation of sub-polymer faults. In all Series 2 experiments, the extension produced a synclinal hanging-wall basin above the polymer layer as a result of polymer migration during the deformation. During inversion, the supra-polymer synclinal basin was uplifted, folded and detached above the polymer layer. Changes in thickness of the polymer layer during the inversion produced primary welds and these permitted the sub-polymer deformation to propagate upwards into the supra-salt layers. The experimental results are compared with examples from the Parentis Basin (Bay of Biscay), the Broad Fourteens Basin (southern North Sea), the Feda Graben (central North Sea) and the Cameros Basin (Iberian Range, Spain).

Abstract: Fault-related folds are common structural features found at a variety of scales in extensional settings, and have been recognized in both outcrop and subsurface studies. However, the detailed geometry and origin of complex 3D folds adjacent to normal faults are poorly known, and, in some cases, are interpreted to be due to strike-slip tectonics and post-rift contraction. Here we examine the 3D geometry of seismic-scale folds in a rift margin – the Red Sea – and discuss the interrelationship between the growth of normal faults and the development of their related folds. Detailed field mapping of the NW Red Sea rift system has shown that the rift margin is dominated by two large extensional fault systems formed by a series of linked NNW-, north–south- and NNE-striking fault segments. These linked segments exhibit distinct zigzag fault patterns and combine to form a number of NNW-trending faults that dip NE with dominant hanging-wall stratal dips to the SW. Hanging-wall stratal dips define 3D extensional fault-related synclinal folds in pre- and early synrift strata. The hanging-wall synclines are kilometre-scale, gently doubly plunging, with curved axial surface traces orientated sub-parallel to the bounding faults. Field data demonstrated that these folds are formed by along-strike variations in fault displacements, and they form transverse synclines combined with hanging-wall extensional fault-propagation folds. The complex 3D geometry of the hanging-wall synclines is the result of the along-strike segment linkage. Adjacent to the bounding faults, the stratal dips are sub-parallel to the faults as a result of extensional fault-propagation folding controlled by highly anisotropic pre-rift strata. Palaeo-strain analyses of fault-slip data, together with analysis of the fold geometry, clearly indicate that the faulting and folding in the NW Red Sea are formed by pure NE–SW extension during the Late Oligocene–Miocene rifting, and that contraction or strike-slip tectonics need not be invoked.

Abstract The Baraboo Quartzite contains numerous well-preserved sedimentary structures that enable interpretation of environments of deposition. Included are various types of cross-stratification, reactivation surfaces, and tidal bedding. The bulk of the Baraboo was deposited under tidal influence. The remainder represents deposition in a braided stream system. The origin of the tremendous volume of quartz sand remains somewhat unknown.

Abstract The Proterozoic Cuddapah Basin of south India is one of the most important Purana basins of Peninsular India. It hosts the Cuddapah Supergroup and the Kurnool Group of rocks. The structure of the Cuddapah Basin reveals that it forms the frontal part of a larger fold–thrust belt, named the Cuddapah fold–thrust belt (CFTB). The CFTB formed in response to the amalgamation of the Prince Charles Mountains–Rayner Complex of Antarctica with the Krishna province of India during the formation of the Rodinia Supercontinent. The CFTB, bounded by the Nellore–Khammam schist belt and Eastern Ghats terrane in the east and Peninsular Gneissic complex of the Dharwar Craton to the west, includes two frontal thrusts and foreland of an orogen. The frontal Nallamalai thrust structurally separates the basin into two blocks – the eastern Nallamalai fold belt and the western foreland. A model of evolution of the CFTB has been proposed. The CFTB, forming the front-most segment of a larger orogen associated with the intercratonic deformation related to the formation of the Rodinia Supercontinent, is the result of fault-propagation folding, forming an overturned anticline–syncline pair at the tip of the propagating Velikonda thrust which later cuts through the common limb of the fold pair.

The Columbia River system is one of the great river systems of North America, draining much of the Pacific Northwest, as well as parts of the western United States and British Columbia. The river system has had a long and complex history, slowly evolving over the past 17 m.y. The Columbia River and its tributaries have been shaped by flood basalt volcanism, Cascade volcanism, regional tectonism, and finally outburst floods from Glacial Lake Missoula. The most complex part of river development has been in the northern part, the Columbia Basin, where the Columbia River and its tributaries were controlled by a subsiding Columbia Basin with subtle anticlinal ridges and synclinal valleys superimposed on a flood basalt landscape. After negotiating this landscape, the course to the Pacific Ocean led through the Cascade Range via the Columbia Trans-Arc Lowland, an ancient crustal weakness zone that separates Washington and Oregon. The peak of flood basalt volcanism obliterated the river paths, but as flood basalt volcanism waned, the rivers were able to establish courses within the growing fold belt. As the folds grew larger, the major pathways of the rivers moved toward the center of the Columbia Basin where subsidence was greatest. The finishing touches to the river system, however, were added during the Pleistocene by the Missoula floods, which caused local repositioning of river channels.

Abstract A Marcellus-Burket/Geneseo field trip in the Appalachian Valley and Ridge features both brittle and ductile structures. The degree to which these structures have developed depends on both lithology, which is a function of the stratigraphic architecture of the Devonian Appalachian Basin and position relative to the foreland during the Alleghanian Orogeny. Joints are best developed in the black shales and the units immediately above with the J 2 joint set most prominent in the Brallier Formation just above the Burket/Geneseo Formation. Faults are seen in the form of cleavage duplexes and bedding-parallel slip accompanying flexural-slip folding. Cleavage duplexes are found in the Marcellus whereas bedding-parallel slip is more common in the overlying Mahantango Formation and further up the section in the Brallier Formation. Layer-parallel shortening decreases from greater than 50% to approximately 10% when crossing the Jacks Mountain–Berwick Anticline structural front from the hinterland portion to the foreland portion of the Valley and Ridge. Disjunctive cleavage, the primary mechanism for layer-parallel shortening, is best developed in carbonates whereas pencil cleavage is best developed in shales.

Orphans are exotic, internally complex, detached duplexes within windows of far-traveled thrust sheets. They were amalgamated to the base of the overriding sheet as allochthonous structures. Internal stratigraphy differs significantly from both super- and subjacent rocks of the enveloping thrust, thus indicating that they originated from a distal ramp location where the stratigraphy was intermediate between that of super- and subjacent rocks. They characteristically occur in windows elongated perpendicular to direction of thrust transport. These complex, antiformal stack types of duplexes include a bounding window-roof fault within the overriding thrust sheet that is antiformal and commonly overturned forelandward, whereas the basal fault of orphans is the smoothly curved primary thrust of the overriding sheet in which they are encapsulated. Within an orphan, structurally higher elements are typically part of an inverted sequence of strata that were duplexed after being overturned and may now be antiformal, thus conforming with the antiformal roof fault. Structurally lower elements are typically part of an upright sequence of the same orphan strata and may be anticlinal. The presence of the same strata in the lower upright and higher inverted elements of an orphan suggests derivation from the overturned footwall syncline of a distal ramp that was detached from the footwall and amalgamated to the overriding thrust sheet. Orphans, which individually contain internally different stratigraphic sequences, may be stacked together. Orphans with older strata lie toward the hinterland and those with progressively younger strata lie successively toward the foreland, suggesting their derivation from successive dismembered footwall ramps separated by intervening flats as the thrust climbed upward. Recognition of orphans has serious implications for balancing cross sections and modeling original fold-fault geometry. Orphans result from structural modification of footwall ramps with overturned footwall synclines, the modification of which changes the structure to apparent fault-bend folds through ramp-smoothing processes that involve reducing ramp angle through dismemberment and removal of the footwall syncline.

Analysis of regional joint orientations, vein mineral paragenesis, and fluid inclusion microthermometry provides valuable insights into the deformation history of the Central Appalachians. These data indicate that deformation occurred as a continuum from hinterland to foreland, and they reflect a progressive rotation in shortening direction. However, separate deformation milestones punctuate the history. Prior to North Mountain thrust emplacement, the rocks in the Valley and Ridge Province were fractured under a NNW-directed shortening. The emplacement of the North Mountain thrust sheet occurred under a more NW-directed shortening and resulted in rapid loading of the Paleozoic section in the footwall by 7–10 km of rock that extended 20–30 km toward the hinterland from the North Mountain thrust ramp. The loading of the Paleozoic section resulted in overpressuring of Cambrian Waynesboro décolle-ment and the development of the Valley and Ridge duplex, and the formation of the Adams Run–Cacapon Mountain anticlinorium in the northern part of the study area. Growth of this anticlinorium shed more sediments toward the foreland, thereby driving addition horse formation. Continued NW- and then WNW-directed shortening resulted in the formation of the Wills Mountain duplex under the load of the North Mountain thrust sheet and sediments derived from the Adams Run–Cacapon Mountain anticlinorium. The growth of the Wills Mountain duplex resulted in duplication of the Cambrian–Ordovician carbonate section with coeval syntectonic erosion, shedding 3–4 km of sediments into the plateau, and triggering the formation of several thrusts in the Cambrian–Ordovician carbonate section. A final E-W–directed shortening event resulted in extensive regional mesoscale fracturing.

The Qaidam Basin constitutes a major portion of the northeastern Tibetan Plateau, and an understanding of its tectonic development will help decipher how the Tibetan Plateau was formed. This study presents key subsurface data in conjunction with observations and analysis of the stratigraphic and sedimentary evolution to reconstruct Cenozoic tectonic history of the Qaidam Basin. We show that Late Cretaceous–Paleocene deposits of the southwestern Qaidam Basin can be well correlated with their counterparts of the southwestern Tarim Basin, implying that the two regions were originally connected or were in the same depositional basin during that period of time. The Qaidam Basin commenced subsiding due to crustal shortening in the Eocene, and it has subsequently evolved into an independent basin since the Miocene. The main depocenter was noticeably persistent in the middle of the western Qaidam Basin from Eocene to Miocene time, and then it shifted to the east. On the basis of spatial stratigraphic correlation and restoration of sedimentary processes, we surmise that there existed a proto–Qaidam Basin during the Paleogene, where the Suhai and Kumukol Basins represent its northern and southern margins, respectively. The Suhai and Kumukol Basins were subsequently isolated from the Qaidam Basin as a result of basinward thrusting in basin-margin areas. We suggest that the Qaidam Basin was generated as a result of crustal buckling or folding, manifesting itself as a synclinal depression. The crustal folding model can account for a number of observations, including localization of the depocenter in the middle of the basin, nearly concomitant deformation on the southern and northern sides of the Qaidam Basin, occurrence of major high-angle reverse faults at basin margins, and generation of adjacent intermontane Suhai and Kumukol Basins. It is further shown that the western Qaidam Basin experienced three distinct stages: the first stage was characterized by a simple synclinal depression; the second stage was marked by occurrence of reverse faults at inflection points of the megafold and continuous subsidence in the middle of the basin; and the third stage featured intra-basinal deformation and uplift. The eastern Qaidam Basin underwent a diverse evolution and became the main depositional area in the Quaternary. A tectonic model is accordingly advanced to illustrate Cenozoic tectonics of the Qaidam Basin.

Volcanic and granitic rocks of the Jack River igneous field were erupted and emplaced in the suture zone between the accreted Wrangellia composite terrane and the former margin of southern Alaska. The volcanic rocks unconformably overlie Jurassic-Cretaceous shale and sandstone of the Kahiltna assemblage and include 100–300 m of basalt, basaltic andesite, and andesite lava flows overlain by a rhyolite unit that consists of over 900 m of lava flows and pyroclastic deposits. Seven basaltic and rhyolite lava samples yield 40 Ar/ 39 Ar ages ranging from 56.0 ± 0.3 to 49.5 ± 0.3 Ma. Two granitic samples yield 40 Ar/ 39 Ar ages of 54.6 ± 0.4 and 62.7 ± 0.4 Ma. These age dates indicate that the onset of Jack River magmatism at ca. 62.7 Ma coincided with the terminal phase of terrane accretion and continued after accretion to at least 49.5 Ma. The volcanic rocks range between tholeiitic and high-K calc-alkaline series and show a bimodal distribution with respect to silica (dacite is absent). The Jack River basalts are tholeiitic, have rare earth element and high field strength element ratios that are in the range between Pacific enriched mid-ocean-ridge basalts and Hawaiian ocean-island basalts (e.g., La/Yb = 5.0–8.4; Nb/Zr = 0.07–0.11), and have a within-plate geochemistry (e.g., Ti/V >50; high Zr/Y). All of the Jack River volcanic rocks exhibit some degree of enrichment in large ion lithophile and/or fluid mobile elements (e.g., Cs, Ba, Th, U, K, and Pb), although the basalts have low ratios between large ion lithophile and high field strength elements (e.g., Ba/Nb as low as 32.7 and Pb/Nb of 0.28–0.35). The granitic rocks (granites to granodiorites) are strongly depleted in the heavy rare earth elements, and most samples exhibit characteristics of adakites (e.g., Al 2 O 3 >15 weight %, Yb = 0.6–1.2 ppm, Y = 5.5–12.5 ppm, and Sr/Y = 20.4–66.2). The Jack River basalts were derived from partial melts of a mantle source that was more enriched than depleted mid-ocean-ridge basalt mantle and that ranged toward an enriched mantle (EM-I-type) composition.The basalts then evolved by assimilation and fractional crystallization to form intermediate magmas. Rhyolite magmas were formed later as anatectic melts of upper crustal argillaceous rocks (Kahiltna assemblage), resulting in the bimodal volcanism. The granitic adakite magmas may have formed by melting of garnet-bearing metamorphosed sedimentary rocks (meta-Kahiltna assemblage) that formed lower crustal rocks in the suture zone. Although the Jack River igneous rocks do exhibit some arc-like geochemical characteristics (e.g., elevated large ion lithophile elements), they differ from calc-alkaline arc rocks in that (1) they are a bimodal volcanic suite; (2) the rhyolites are not comagmatic with the basaltic and intermediate rocks; (3) the basalts and andesites have higher TiO 2 (>1.5 weight %) than is typical for arc basalts and andesites; (4) the basalts do not exhibit depletion of high field strength elements (e.g., Ta and Nb) with respect to large ion lithophile elements; (5) the basalts have an intraplate geochemical affinity; and (6) adakites are present. These characteristics show that the geochemistry of postcollisional suture zone magmatism can be transitional between calc-alkaline arc and intraplate magmatism. The Jack River volcanic field is deformed into a broad, northeast-trending syncline, which is crosscut by small-scale brittle faults that include northwest- and west-trending normal-slip and oblique-slip faults, and a southeast-dipping reverse fault that places Kahiltna assemblage rocks over the Jack River volcanic rocks. The pattern of Jack River deformation is consistent with right-lateral simple shear along the Denali fault system and indicates an episode of post-49.5 Ma strike-slip along the McKinley strand of the Denali fault. The Jack River rocks, therefore, record the magmatic response to terrane accretion and the kinematics of margin-parallel transport of an accreted terrane assemblage after it was sutured to the continental margin.

Abstract Centralia is in Pennsylvania’s western middle anthracite field, a large synclinorium in Columbia and Schuylkill Counties. Centralia residents set fire to a landfill at the edge of town in 1962, thereby igniting the Buck Mountain coal bed. Laurel Run is in Pennsylvania’s northern anthracite field, on the northwest-dipping limb of the Wyoming Valley syncline. In 1915, a miner’s abandoned carbide lamp started a fire at Laurel Run, igniting the Red Ash, Top Red Ash, and Bottom Ross coal beds. The Centralia and Laurel Run fires are burning out of control. Subsidence and the venting of toxic gases have destroyed large sections of each community. Because the Centralia fire started in the hinge zone of an anticline separating two synclines in the Western Middle Field, it spread in four directions. The Laurel Run fire occurred on one limb of a syncline, limiting its spread to two directions. At Centralia, the steeper-dipping beds permitted the fire to reach a greater depth more rapidly than at Laurel Run. In addition, the point of origin and steeper dip at Centralia make this fire more difficult to control, even though only one coal bed is burning. A historical and sociological comparison of both communities shows that the people of Laurel Run had greater access to political power and more experience as a community in dealing with crises. Laurel Run secured more government support in combating the fire than Centralia did and so emerged from the fire as a more socially intact community. The present state of each fire further underscores how different geologic settings and social conditions can lead to different outcomes.

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.

A map of the principal grabens and half-grabens of the Neuquén Basin in the northeastern part of Neuquén Province, Argentina, was constructed based on two-dimensional (2-D) and three-dimensional (3-D) seismic information. The synrift structure of the region is characterized by high angle NW-trending normal faults. Two populations can be distinguished: one with an azimuth of 140° and the other one with an azimuth of 105°. The principal half-grabens are accommodated by NE-dipping normal faults. The principal NW-trending faults are less than 20 km long and are either crosscut by or terminate in transfer zones trending to the NE. The transfer zones are either faults or zones where the principal NW-trending faults loose slip or terminate. The principal normal faults were active until the deposition of the Lower Upper Jurassic Tordillo Formation. Subsequently, only a few faults related to differential subsidence over the half-grabens remained active. The typical structures of the region are smooth anticlines and synclines that affect the sag facies of Neuquén Basin. The anticlines developed over basement highs and the synclines over graben basins. The synclines are explained in this study as resulting from differential subsidence over the half-grabens. This differential subsidence could have been a continuous process that began in the synrift stage of the basin. A distinct element model was created to analyze the differential subsidence process and geometry. No evidence was found to support the hypothesis that tectonic inversion is an important process in this region. The main unconformities can be explained by the differential subsidence process.