Добірка наукової літератури з теми "Thrust forelimb and backlimb"

Abstract The NW Borneo deep-water fold-and-thrust belt, offshore Sabah, southern South China Sea, contains a structurally complex region of three to four seafloor ridges outboard of the shelf-slope break. Previous studies have suggested the seafloor ridges formed either above shale diapirs produced by mass movement of overpressured shales (i.e., mobile shale) or above an imbricate fold-and-thrust array. Here, we performed tectonostratigraphic analyses on a petroleum industry three-dimensional (3-D) seismic volume that imaged the full growth stratal record. We show fold growth history, deformation styles, along-strike structural variabilities, and synkinematic sedimentation during triangle zone–style fold growth. Nine seismic horizons within growth strata were mapped and correlated to petroleum industry seismostratigraphy. Synkinematic sedimentation interactions with growing folds and near-surface strains were analyzed from seismic attribute maps. We interpret that the seafloor structures were formed by imbricate thrusts above multiple detachments. We estimate ∼8 km minimum shortening since the late Miocene ca. 10 Ma. The folds show oversteepened fold forelimbs, back-rotated backlimbs, and forward-vergent (NW to NNW) “blind” thrust ramps that terminate within the growth strata. Fold cores show evidence of internal shear. Immature folds show detachment fold geometries, whereas mature folds show forelimb break thrusts, type I triangle zones, and rotated forward-vergent roof thrusts. Thrust linkages spaced ∼10 km apart were exploited as thrust top synkinematic sedimentation pathways; the linkages also partition near-surface strains. Our comprehensive, three-dimensional documentation of triangle zone fold growth and sedimentation in a deep-water fold belt highlights internal shear, multiple detachments, and opposite thrust vergence; mobile shales are not required to explain the deformation.

SUMMARYSoftshell turtles (Family Trionychidae) possess extensive webbing between the digits of the manus, suggesting that the forelimb may serve as an effective thrust generator during aquatic locomotion. However, the hindlimb has previously been viewed as the dominant propulsive organ in swimming freshwater turtles. To evaluate the potential role of the forelimb in thrust production during swimming in freshwater turtles, we compared the forelimb morphology and three-dimensional forelimb kinematics of a highly aquatic trionychid turtle, the spiny softshell Apalone spinifera, and a morphologically generalized emydid turtle, the red-eared slider Trachemys scripta. Spiny softshells possess nearly twice as much forelimb surface area as sliders for generating drag-based thrust. In addition, although both species use drag-based propulsion, several aspects of forelimb kinematics differ significantly between these species. During the thrust phase of the forelimb cycle, spiny softshells hold the elbow and wrist joints significantly straighter than sliders, thereby further increasing the surface area of the limb that can move water posteriorly and increasing the velocity of the distal portion of the forelimb. These aspects of swimming kinematics in softshells should increase forelimb thrust production and suggest that the forelimbs make more substantial contributions to forward thrust in softshell turtles than in sliders. Spiny softshells also restrict forelimb movements to a much narrower dorsoventral and anteroposterior range than sliders throughout the stroke, thereby helping to minimize limb movements potentially extraneous to forward thrust production. These comparisons demonstrate considerable diversity in the forelimb kinematics of turtles that swim using rowing motions of the limbs and suggest that the evolution of turtle forelimb mechanics produced a variety of contrasting solutions for aquatic specialization.

AbstractWhere primary porosity and permeability of a rock are unfavourable for hydrocarbon production, fractures can improve reservoir potential by enhancing permeability. Higher fracture intensity may create a better-connected fracture network, improving fractured-reservoir quality. Investigations into the controls on fracture intensity commonly conclude that either structural or lithological factors have the greatest influence on fracture abundance. We use the Swift Reservoir Anticline in northwestern Montana to investigate how fracture intensity varies throughout the structure and determine that although structural factors do influence fracture intensity, lithology is the main control at outcrop.The Swift Reservoir Anticline exposes bedding surfaces of the Mississippian Castle Reef Formation dolomite. Field data indicates that fracture intensity is highest in the fold forelimb, decreasing into the backlimb except in outcrops of coarse dolomite where fracture intensity is low, regardless of structural position. Field fracture intensity correlates with whole-rock quartz, kaolinite and porosity percentages. We suggest porosity and composition influence bulk-rock mechanical properties, which, in turn, control the fracture intensity at outcrop. Fracture intensity has a stronger relationship with lithological than structural factors, therefore we suggest that the key to predicting fracture intensity in the subsurface here is understanding how lithology varies spatially.

AbstractThe Teton anticline and adjacent structures, in the Sawtooth Range, Montana, USA, are fractured in such a way that may be taken as a model for fractures propagating during buckle folding. However, advances in understanding both the process of folding in forelands and the evolution of fracture patterns found within these folds suggest that it is time to reinterpret the nexus between fracturing and folding within these classic structures. With the benefit of seismic lines, the Teton anticline is best described as a fault-propagation fold. Joint propagation initiated with the formation of two major sets whose orientation is controlled by pre-folding, regional stresses. Two more joint sets propagated in local stress fields, developed in response to anticline growth. Some early joints were reactivated as wrench faults during amplification and tightening of the anticlines. The fracture sets identified are consistent with: (a) propagation in a regional stress field, which may be related to stretching in the Sawtooth Range orocline; and (b) tangential longitudinal strain of the backlimb and forcing or trishear of the forelimb during anticline development. Thus, we suggest that fracture networks across folded structures should be interpreted and characterized in the light of the geological history of the entire system.

Abstract. In the Spanish Pyrenees, the Sant Corneli-Bóixols thrust-related anticline displays an outstandingly preserved growth strata sequence. These strata lie on top of a major unconformity exposed at the anticline's forelimb that divides and decouples a lower pre-folding unit from an upper syn-folding one. The former consists of steeply dipping to overturned strata with widespread bedding-parallel slip indicative of folding by flexural slip, whereas the syn-folding strata above define a 200 m amplitude fold. In the inner and outer sectors of the forelimb, both pre- and syn-folding strata are near vertical to overturned and the unconformity angle ranges from 10 to 30°. In the central portion of the forelimb, syn-folding layers are gently dipping, whereas the angular unconformity is about 90° and the unconformity surface displays strong S–C shear structures, which provide a top-to-foreland slip sense. This sheared unconformity is offset by steeply dipping faults, which are at low angles to the underlying layers of the pre-folding unit. Strong shearing along the unconformity surface also occurred in the inner sector of the forelimb, with S–C structures providing an opposite, top-to-hinterland slip sense. Cross-cutting relationships and slip senses along the pre-folding bedding surfaces and the unconformity indicate that regardless of its orientation, layering in the pre- and syn-folding sequences of the Sant Corneli-Bóixols anticline were continuously slipped. This slipping promoted an intense stress deflection, with the maximum component of the stress tensor remaining at low angles to bedding during most of the folding process.

Folds of northern Iraq are considered integral part for the Western Zagros Fold – Thrust Belt. The growth of these folds was due to inversion displacement on inherited listric faults. This research deal with the relationship between the folds vergency and the faults that propagated folds, where that the dip of the back limb (gentle limb) for the fold is parallel to the thrust fault surface that propagated the fold, and the vergency of the fold determined by the forelimb (steep limb) situation. As a results, the folds of the high folded zone and of the western part of the low folded zone showed suture ( N and NE) vergency and foreland (S and SW) vergency, while the eastern part of the low fold zone showed foreland (S and SW) vergency only. The appearance of the suture and foreland vergency within the high folds considered as indication to the high tectonic development conformable with the location of these folds in the Iraqi Zagros Fold Belt, while the appearance of the suture and foreland vergency in the western part of the low folded zone attributed to the more tectonic development of this part in comparison with the eastern part of the zone that there folds appeared foreland vergencies only, or to the influence of the evaporite beds for Fatha formation in this part. 
 
 http://dx.doi.org/10.25130/tjps.25.2020.031

Field investigations in the western part of the Cape Smith Belt outlined four fault-bounded lithological assemblages tectonically overlying foliated to gneissic granitoids of the Archean Superior Province. These assemblages comprise sequences of pillowed volcanic rocks, sedimentary rocks of pelitic and psammitic composition, and volcaniclastic rocks. They are juxtaposed along the Lanyan Lake Fault against a structurally thickened sequence of mafic to ultramafic pillowed volcanic suite belonging to the Chukotat Group. The occurrence of volcaniclastic horizons in the uppermost levels of the Chukotat Group may indicate a northward facies transition from sea-floor volcanism to arc sedimentation, the latter corresponding to the Parent Group. A major pluton intruding the upper Chukotat Group, if assigned to the younger Narsajuaq intrusive suite, provides support for an 1844 – 1826 Ma link between two tectonic domains, formerly considered "suspect." These domains lie on either side of the Bergeron Fault in the east and central parts of the Cape Smith Belt. This fault, formerly interpreted as extending to Hudson Bay, was not recognized in this work. Thrust faulting, involving three kilometre-thick imbricate slices enclosing the Superior Province, was followed by the development of the Cape Smith Synclinorium with overturning of its northern limb, forelimb faulting, and large-scale folding along northwest-trending axes.

Abstract Geological framework; Geological setting: The Venezuela Andes or Merida Andes (fig. 1) extend from the Colombian border in the SW to Barquisimeto in the NE, and constitute a basement uplift exceeding 5,000 m near Merida (Pico Bolivar). This young chain is bordered to the W by the Maracaibo foredeep basin, and to the E by the Barinas-Apure foreland basin. The Bocono fault divides the Andean Belt in two parts along a NE-SW direction. This shows that the uplift of the Andes is contemporaneous with an oblique translation. In the study area, located on the northwestern flank near Maracaibo basin, three major structures are present: in the E, the N-S senestral strike slip Valera-Rio Momboy fault, in the S the E-W dextral strike slip Pinango fault and, in the center, the SW-NE striking Las Virtudes thrust verging toward NW. Lithologic and stratigraphic formations (fig. 4): The Las Virtudes Fault separates two different structural zones. In the SE, overthrust units are made of crystalline basement, Paleozoic substratum and preorogenic sedimentary formations (Cretaceous-Eocene). The foredeep flexural basin, located NW, is filled by synorogenic molasses (Neogene and Quaternary), largely developed within the Betijoque Fm. (Upper Miocene to Pliocene in age) which reaches a thickness of 5000 m. Structure of the northwestern Andean flank; Las Virtudes Fault and its thrust slice zone: Near Las Virtudes village (fig. 5, 6-2), this thrust is systematically associated with a narrow overturned foredeep depobelt (Cretaceous to Neogene in age). These slices are unknown elsewhere in the Andean Chain and represent the terminal faulted part of the thrust drag. However, where this slice zone is missing (central and northeastern part of the study area), the Las Virtudes Fault is not clearly documented: its throw decreases rapidly and it is possible that the fault disappears northeastward. Andean unit: Near the main strike slip faults, NE trending SE verging reverse faults develop (fig. 6-5). In central and northeastern parts, the throw of the reverse faults increases toward the Valera Fault. It seems that reverse faults are horsetail of this major strike slip fault (fig. 5). Internal part of the northwestern Andean foredeep basin: The foredeep sedimentary formations generally dip toward the NW. Associated to the lack of some formations, tilted anticlines toward the SE are observed (fig. 6-3 and 6-7), and indicate the vicinity of decollement levels in the foredeep, located in Luna-Colon, Pauji and Betijoque Fm.. Seismic profiles show (fig. 7) that the major decollement level of the foredeep is located in La Luna and Colon Fms. [Audemard, 1991; De Toni and Kellogg, 1993; Colletta et al., 1997]. Crustal architecture and timing of the deformation: Several stages can be distinguished in the building of the Andes. Development of an intracutaneous thrust wedge: The first effects of the Andean phase during Miocene are the development of an intracutaneous thrust wedge [Price, 1986]. The lower flat is located in the basement and the upper one in Cretaceous formations. The transport direction is NW. The foredeep develops on the forelimb of this structure and collects detrital products coming from erosion of the first (oldest) reliefs. Decollements in the foredeep basin could be contemporaneous with this major overthrust. Their origin could be due to radius of curvature differences within the thick sedimentary formations (fig. 8). Las Virtudes Fault and backthrusting: Las Virtudes Fault is one of the last events of this part of the Andean Belt. During Plio-Pleistocene, the continental crust breaks with a dip of 35 degrees SE. The Andean unit overthrusts the foredeep basin. Some of the foredeep decollements could still be active and form, together with Andean basement, a triangle zone. Las Virtudes Fault throw reaches 5 km between Las Virtudes and Monte Carmelo villages (fig. 8A). It decreases southwestwards and the back thrusts are probably younger. Northeastwards the throw decreases and eventually disappears (fig. 8B). In the same time the back thrust throws increase. Both seem to be contemporaneous. Conclusions: This structural model explains the basement occurrence in front of the Las Virtudes Fault on seismic profiles and allows to restore correctly the different northwestern flank structures of the Venezuela Andes. These structures can be explained by the conjugate movements of a NW verging intracutaneous thrust wedge and strike slip faults which create a SE verging triangular area (fig. 5). The Andean overthrust is transferred in the Falcon zone along the Valera fault. In the northeastern part of the Maracaibo block, the Valera and Bocono strike slip faults limit the Trujillo block (fig. 10) which moves towards the North during Neogene and Quaternary times.

The purpose of this study was to determine the source of postural instability in labyrinthectomized cats during lateral head turns. Cats were trained to maintain the head in a forward orientation and then perform a rapid, large-amplitude head turn to left or right in yaw, while standing freely on a force platform. Head turns were biomechanically complex with the primary movement in the yaw plane accompanied by an ipsilateral ear-down roll and nose-down pitch. Cats used a strategy of pushing off by activating extensors of the contralateral forelimb while using all four limbs to produce a rotational moment of force about the vertical axis. After bilateral labyrinthectomy, the initial components of the head turn and accompanying postural responses were hypermetric, but otherwise similar to those produced before the lesion. However, near the time of peak yaw velocity, the lesioned cats produced an unexpected burst in extensors of the contralateral limbs that thrust the body to the ipsilateral side, leading to falls. This postural error was in the frontal (roll) plane, even though the primary movement was a rotation in the horizontal (yaw) plane. The response error decreased in amplitude with compensation but did not disappear. We conclude that lack of vestibular input results in active destabilization of balance during voluntary head movement. We postulate that the postural imbalance arises from the misperception that the trunk was rolling contralaterally, based on signals from neck proprioceptors in the absence of vestibular inputs.

AbstractReconstructing the swimming capabilities of extinct marine tetrapods is critical for unravelling broader questions about their palaeobiology, palaeoecology and palaeobiogeography. Ichthyosaurs have long been the subject of such investigations because, alongside cetaceans, they are one of the few tetrapod lineages to achieve a highly specialized fish-like body plan. The dominant locomotory mode for the majority of derived, post-Triassic ichthyosaurs is hypothesized to have been caudal fin-driven propulsion. Limb-based swimming has however been suggested for some highly autapomorphic forms, such as the Cretaceous genus Platypterygius, which has a remarkably robust humeral morphology and exceptionally broad paddle-like limbs. To evaluate this atypical lifestyle model, we conducted a comprehensive comparative osteological assessment of Platypterygius in relation to extant marine mammals, whose analogous skeletal frameworks provide a structurally compatible selection of alternate propulsive strategies. Based on a proxy exemplar of the most completely known species, P. australis from the Early Cretaceous of Australia, the propodial shape, absence of functional elbow/knee joints, tightly interlocking carpals, hyperphalangy and extreme reduction of the pelvic girdle are most similar to cetaceans as opposed to pinnipeds or dugongs. There is no obvious structural consistency with aquatic mammals that use sustained forelimb-driven swimming. The exceptionally broad fore-paddle (a product of hyperdactyly) and extensive humeral muscle insertions might therefore have had a cetacean-like role in enhancing manoeuvrability and acceleration performance. We conclude that, despite its atypical features, P. australis was most likely similar to other ichthyosaurs in using lateral sweeps of the tailfin to generate primary propulsive thrust.

Le long des marges actives, la croissance de rides anticlinales chevauchantes et les processus tectoniques associés sont souvent cités comme étant l'une des causes principales entrainant des déstabilisations de pente et du transport en masse de sédiments au dos des prismes de subduction. Les dépôts associés (MTDs) sont très variés, ne serait-ce que le long d'une même marge, et leur nature, origine et expression peuvent témoigner de l'évolution tectonostratigraphique des bassins sédimentaires liés à la subduction (e.g., bassins perchés). Ce travail présente une analyse haute résolution des caractéristiques et mécanismes de mise en place des sédiments déstabilisés en examinant des MTDs miocènes affleurant dans la partie interne émergée de la marge Sud-Hikurangi (Île du Nord, Nouvelle-Zélande). Des données régionales de sismique réflexion marine ont aussi été utilisées afin d’analyser les géométries et architectures de plus grande échelle. Les résultats témoignent de l'importance des rides structurales dans le contrôle du remplissage sédimentaire des bassins. Différents styles de MTDs sont générés en fonction de leur position structurale (forelimb et backlimb) et à des moments spécifiques du développement des rides et des bassins perchés. Ceci suggère que les MTDs sont de puissants marqueurs tectonostratigraphiques. Ici, ils ont aidé à reconstruire, à des périodes clés, l'évolution de deux bassins et de la marge Hikurangi elle-même. Cette étude offre de nouvelles perspectives sur les interactions entre la déformation et la sédimentation pouvant être essentielles pour la compréhension de l’évolution des marges actives, de leurs risques géologiques et pour leur exploration<br>Along active margins, the prevalence of thrust ridges and tectonic processes (e.g., uplift, slope oversteepening) is generally called out as one of the main recurrent reasons for generating slope failures and mass wasting on subduction complexes. The resulting mass-transport deposits (MTDs) are often seen to vary strongly along a single margin and therefore, this research work proposes to investigate their nature, origin and significance in the frame of the tectonostratigraphic evolution of subduction-related sedimentary basins (e.g., trench-slope basins [TSBs]). Here, we present high-resolution outcrop-scale insights on both the characteristics and mechanisms of emplacement of the failed sediments by examining thrust-related MTDs from the Miocene cropping out in the emerged southern portion of the Hikurangi subduction margin (eastern North Island of New Zealand). Regional offshore seismic reflection data are also used to offer a broader overview and understanding of these systems through the study of the larger scale geometries and architectures. Results show the role and importance of the thrust ridges in controlling the TSB infilling. Different styles of MTDs are generated from different structural positions (forelimb and backlimb) and at specific times of thrust-ridge and TSB development. This suggests that MTDs are powerful tectonostratigraphic markers. Here, they help to unravel the evolution of two TSBs and more largely of the Hikurangi Margin at key periods. This study provides new insights on the close interplays between deformation and sedimentation, understandings of which may be key for geohazard, exploration and geodynamic predictions along active margins