Discrete Thrust Research Articles

Automatic Continuous Thrust Control for Supersonic Transport Takeoff Noise Reduction

Advanced takeoff trajectories are proposed for supersonic transport noise reduction by capitalizing on excess engine thrust and improved aerodynamic efficiency at higher takeoff speeds. These novel trajectories use i) automatic continuous control of thrust, ii) increased takeoff speed, and iii) reduced cut-back altitude, compared to conventional pilot-initiated discrete thrust cut-back procedures currently used for subsonic transport. In this paper, we develop an optimal control framework to assess the attributes of effective takeoff trajectories for supersonic transport that yield minimum noise levels. We quantify the noise reduction potential of advanced takeoff trajectories for the eight-passenger, 55-metric-ton, Mach-1.4 NASA Supersonic Technology Concept Airplane. For the aircraft examined, these advanced takeoff trajectories enable a cumulative certification noise reduction of 10.6 EPNdB, which is insufficient to meet current subsonic transport noise limits.

Fixed-time attitude control for aircraft with strongly constrained actuators

This paper investigates a novel fixed-time attitude control method for trans-medium aircraft to ensure proper contact between the fuselage and the new medium before transmitting from different environmental mediums. How to achieve rapid corrective attitude adjustment from improper contact by utilizing the Reaction Control System with multiple discrete thrust gears that possess discrete thrusts with limited output has become a great challenge. To solve this problem, a novel fixed-time attitude controller is proposed. Firstly, a non-singular segmented terminal sliding mode surface is constructed to achieve a fixed convergence time and avoid the singularity problem. Subsequently, a command regulator is designed to cope with the derivation introduced by the strongly constrained actuators, which consists of the approximate thrust model and an auxiliary system based on a double power function. Furthermore, the stability analysis indicates the fixed time convergence stability of the closed-loop system and provides a clear calculation of the settling time. Finally, numerical comparison results and Monte Carlo simulations verify that the attitude can converge in a predetermined time from different initial states.

What steers the “folding to faulting” transition in carbonate-dominated seismic fold-and-thrust belts? New insights from the Eastern Southern Alps (Northern Italy)

Several parameters steer the modes of shortening of carbonate-dominated fold-and-thrust belts from incipient- (layer parallel shortening, buckle folds) to evolved deformation stages (verging folds, discrete thrusts). In this study, we address the spatial and temporal evolution of contractional structures within carbonate-dominated fold-and-thrust belts by documenting the geometry, kinematics and structural architecture of the San Donato-Costa Thrust Zone, a splay of the regional Belluno Thrust of the seismically active Eastern Southern Alps (Northern Italy). Deformation is there accommodated by a variety of features ranging from open and upright to tight and verging folds cut by later thrusts. An integrated structural analysis indicates inherited primary features to have effectively steered the deformation style of the thrust and its immediate hanging wall and footwall. We propose an evolving deformation scenario initially governed by the inherited lithological features and localised pressure solution, then by the geometry of folds accommodating progressive shortening and, finally, by thrusting. The folding-faulting transition occurs when fold forelimbs dip ∼80° and the ratio between the dip angle of fore- and back limbs becomes ∼3.3. These geometric boundary conditions control the mechanical behaviour of carbonate multilayer successions during orogenic shortening in fold-and-thrust belts, assisting the partitioning between seismic and aseismic deformation.

Passive Structural Control on Skarn Mineralization Localization: A Case Study from the Variscan Rosas Shear Zone (SW Sardinia, Italy)

The case study presented here deals with the Pb-Zn-Cu skarn ores hosted in the Rosas Shear Zone (RSZ), a highly strained domain located in the external zone of the Sardinian Variscan chain. The RSZ is characterized by several tectonic slices of Cambrian limestones within a strongly folded and foliated Cambrian-Ordovician siliciclastic succession, intruded by late Variscan granites and mafic dykes. Based on geological mapping, structural and microscope analyses, our results show that the skarn ores in the RSZ are an example of passive structurally controlled mineralization. The RSZ was structured close to the brittle–ductile transition and, once exhumed to shallower crustal levels, acted as plumbing system favoring a large-scale granite-related fluid circulation. The paragenesis and the mineralization style of the skarn vary slightly according to the peculiarity of the local structural setting: a tectonic slice adjacent to the mafic dyke; an intensely sheared zone or a discrete thrust surface.

Sedimentary and structural controls on seismogenic slumping within mass transport deposits from the Dead Sea Basin

Comparatively little work has been undertaken on how sedimentary environments and facies changes can influence detailed structural development in slump sheets associated with mass transport deposits (MTDs). The nature of downslope deformation at the leading edge of MTDs is currently debated in terms of frontally emergent, frontally confined and open-toed models. An opportunity to study and address these issues occurs within the Dead Sea Basin, where six individual slump sheets (S1–S6) form MTDs in the Late Pleistocene Lisan Formation. All six slumps, which are separated from one another by undeformed beds, are traced towards the NE for up to 1km, and each shows a change in sedimentary facies from detrital-rich in the SW, to more aragonite-rich in the NE. The detrital-rich facies is sourced predominantly from the rift margin 1.5km further SW, while the aragonite-rich facies represents evaporitic precipitation in the hyper saline Lake Lisan. The stacked system of MTDs translates downslope towards the NE and follows a pre-determined sequence controlled by the sedimentary facies. Each individual slump roots downwards into underlying detrital-rich layers and displays a greater detrital content towards the SW, which is marked by increasing folding, while increasing aragonite content towards the NE is associated with more discrete thrusts. The MTDs thin downslope towards the NE, until they pass laterally into undeformed beds at the toe. The amount of contraction also reduces downslope from a maximum of ~50% to <10% at the toe, where upright folds form diffuse ‘open-toed’ systems. We suggest that MTDs are triggered by seismic events, facilitated by detrital-rich horizons, and controlled by palaeoslope orientation. The frequency of individual failures is partially controlled by local environmental influences linked to detrital input and should therefore be used with some caution in more general palaeoseismic studies. We demonstrate that MTDs display ‘open toes’ where distributed contraction results in upright folding and shortening rather than distinct thrusts. Such geometries may account for some of the contraction that is apparently missing when balancing seismic sections across large off shore MTDs.

Mid‐Pleistocene thin‐skinned glaciotectonic thrusting of the Aberdeen Ground Formation, Central Graben region, central North Sea

ABSTRACTThis paper presents the results of a high‐resolution 2D seismic survey of mid‐Pleistocene glaciogenic sediments in the Central Graben region of the central North Sea. Sediments have undergone major glaciotectonic thrusting and folding associated with the repeated southerly advance of a mid‐Pleistocene ice sheet. The total observed length of the thrust‐stacked section is approximately 5–6 km, comprising a series of discrete thrust slices, which range in length from &lt; 300 m to &gt; 700 m. The basal detachment of the thrust complex occurs at a depth of ca. 220 m below the sea bed within the upper Aberdeen Ground Formation. A thin‐skinned glaciotectonic model involving proglacial to ice‐marginal glaciotectonic thrusting followed by post‐tectonic deposition is proposed. Initial ice advance led to the over‐pressurizing of groundwater within a laterally extensive sand sheet in the upper Aberdeen Ground Formation, promoting the formation of a major décollement surface at the base of the developing thrust‐stack. Over‐pressurization of the groundwater system is thought to have occurred in response to rapid ice advance, suggesting that the development of large‐scale thrust complexes may be associated with surge‐type behaviour. The proposed model evidences complex dynamics of mid‐Pleistocene ice sheets within the central North Sea.

The Physics of H-Darrieus Turbine Starting Behavior

This paper provides a resolution to the contradictory accounts of whether or not the Darrieus turbine can self-start. The paper builds on previous work proposing an analogy between the aerofoil in Darrieus motion and flapping-wing flow mechanisms. This analogy suggests that unsteadiness could be exploited to generate additional thrust and that this unsteady thrust generation is governed by rotor geometry. Rotors which do not exploit this unsteadiness will not self-start. To confirm the hypothesis, unsteady effects were measured and then incorporated into a time-stepping rotor analysis and compared to experimental data for self-starting wind turbines. When unsteady effects were included, the model was able to predict the correct starting behavior. The fundamental physics of starting were also studied and parameters that govern the generation of unsteady thrust were explored, namely, chord-to-diameter and blade aspect ratios (ARs). Further simulation showed that the Darrieus rotor is prone to be locked in a deadband where the thrust is not continuous around a blade rotation. This discrete thrust is caused by the large variation in incidence angle during startup, making the Darrieus blade ineffective during part of the rotation. The results show that unsteady thrust can be promoted through an appropriate selection of blade aspect and chord-to-diameter ratios, therefore self-starting rotors may be designed. A new definition of self-starting is also proposed.

Tethered towing using open-loop input-shaping and discrete thrust levels

Abstract Asteroid retrieval, satellite servicing, and debris removal concepts often rely on a thrusting vehicle to redirect and steer a passive object. One effective way to tow the object is through a tether. This study employs a discretized tether model attached to six degree-of-freedom end bodies. To reduce the risk of a post-burn collision between the end bodies, discrete thrust input shaping profiles are considered including a Posicast input and a bang-off-bang thrust profile. These input shaping techniques attain desirable collision avoidance performance by inducing a tumbling or gravity gradient motion of the tethered formation. Their performance is compared to an earlier frequency notched thruster profile.

Analytical guidance for spacecraft relative motion under constant thrust using relative orbit elements

Proximity control of modern nano-spacecraft often relies on low and discrete thrust engines that are characterized by low consumption, and generate on-off force profiles. New guidance solutions must take into account the nature of this type of orbital engines. This paper introduces novel analytical guidance solutions for spacecraft relative motion considering continuous, on-off thrust, and using relative orbit elements as a geometrical representation of the dynamics. The solutions provide the relative state vector at any given time, accommodating any thrust magnitude along the three directions of the relative frame, as well as generic activation times and durations. Relative orbit elements geometrically interpret key aspects of the relative motion, including for example, the relative ellipse size, and the evolution of its center in time. The new solutions provide the guidance designer with a direct visualization of the thrust effects on the relative motion geometry, offering new possibilities for analytical guidance in the presence of continuous thrust engines, such as low thrust engines on nano-spacecraft. The paper presents the analytical solutions, and tests their effectiveness using a sample thrust profile based on input-shaping, previously developed by one of the authors using classical Cartesian coordinates. The use of relative orbit elements shows substantial benefits and added simplicity with respect to Cartesian-based approaches, holding the promise for straightforward onboard spacecraft implementation. The software developed for this research will be available open source11〈http://www.riccardobevilacqua.com/links.html〉., to be used by spacecraft guidance designers as trajectory design tool.

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of >5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thick-skinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U–Pb age determinations (2637–2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from >140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.

Cenozoic tectonic evolution of Asia: A preliminary synthesis

Asia has been a major testing ground for various competing models of continental deformation due to its relatively well-understood plate boundary conditions in the Cenozoic, exceptional exposure of active structures, and strain distribution, and widespread syn-collisional igneous activity as a proxy for the thermal state of the mantle and crust. Two Cenozoic orogens dominate the continent: the Himalayan–Tibetan orogen in the east induced by the India–Asia collision and the Turkish–Iranian–Caucasus orogen in the west induced by the Arabia–Asia collision. The development of the two orogens was accomplished by shortening in the early stage followed by strike-slip faulting and extension in the late stage. In the Himalayan–Tibetan orogen, shortening across two discrete thrust belts at 55–30 Ma in southern and northern Tibet created a large intracontinental basin (the Paleo-Qaidam basin) in between. Subsequent crustal thickening and a possible thermal event in the mantle (e.g., convective removal of central Tibetan mantle lithosphere) may have raised the elevation of this early intra-plateau basin up to ~ 2–3 km to its current height. Collision between India and Asia also caused lateral extrusion of southeast Asia between 32 Ma and 17 Ma. The latest stage of the India–Asia collision was expressed by north-trending rifting and the development of trench-facing V-shaped conjugate strike-slip faults in central Mongolia, central Tibet, eastern Afghanistan and southeast Asia. In the Turkish–Iranian–Caucasus orogen, early crustal thickening in the orogenic interior began at or prior to 30–20 Ma. This style of deformation was replaced by strike-slip faulting at ~ 15–5 Ma associated with further northward penetration of Arabia into Asia, westward extrusion of the Anatolia/Turkey block, and rapid extension across the Sea of Crete and Sea of Aegean. The late stage extension in both orogens was locally related to extensional core-complex development. The continental-margin extension of east Asia was developed in two stages: initially in a widely distributed zone that has an east-west width of 500–800 km during 65–35 Ma, which was followed by localized extension and opening of back-arc basins associated with the development of spreading centers at 32–17 Ma (e.g., Japan Sea or East Korea Sea, Bohai Bay, and South China Sea). Opening of the back-arc basins could be induced by (1) rapid eastward migration of the western Pacific trench system or (2) oblique subduction of Pacific plate beneath Asia that had produced a series of en echelon right-slip primary shear zones linking with back-arc spreading centers oriented obliquely to the strike of the nearby trench. Since ~ 15 Ma, the eastern margin of Asia became contractional in the east–west direction, as indicated by the collapse of back-arc basins in the western Pacific and the development of fold-thrust belts along the eastern continental margin. Coeval with the contraction is widespread east–west extension in Siberia, North China, and the Tibetan plateau. The above observations can be explained by a change in boundary condition along the eastern margin of Asia that allowed the thickened Asian continent to spread eastward, causing east-west extension in its trailing edge and east-west compression in its leading edge. In west Asia, continental-margin extension started at about 25–20 Ma in the Aegean and Cretan regions, which was associated with a rapid southward retreat of the Hellenic arc. The complex evolution of Cenozoic deformation in Asia may be explained by a combined effect of temporal changes in plate boundary conditions, thermal evolution of the upper mantle perturbed by collisional tectonics, and the built-up of gravitational energy through crustal thickening and thermal heating. Although the past research in Asia has treated the India–Asia and Arabia–Asia convergence as separate collisional processes, their interaction may have controlled the far-field Cenozoic deformation in Asia. The most pronounced result of this interaction is the creation of a northeast-trending 300–400-km wide and > 1500-km long zone of northwest-striking right-slip faults, which extends from the Zagros thrust belt in the south to western Mongolia in the north and links with the active Tian Shan and Altai Shan intracontinental orogens. Cenozoic deformation and coeval igneous activity spatially overlap with one another in the Himalayan–Tibetan and Turkish–Iranian–Caucasus orogens. A large Cenozoic magmatic gap exists between Tibet in the south and Mongolia in the north where Cenozoic deformation has not been associated with any coeval igneous activity. Finally, Cenozoic igneous activity is always associated with Jurassic–Cretaceous magmatic arcs, suggesting a causal relationship between the early arc magmatism and later syn-collisional magmatism.

Fuel-Optimal Maneuvers for Constrained Relative Satellite Orbits

This research investigates strategies to enable a deputy satellite to hover within a defined volume fixed in the vicinity of a chief satellite in a circular orbit for an extended period of time. Previous research developed initial methodologies for maintaining restricted teardrop hover orbits that exist in a plane fixed within the chief's local reference frame. These methods use the natural drift of the deputy satellite in the relative frame and impulsive thrust to keep the deputy in a bounded volume relative to the chief, but do not address fuel optimality. This research extends and enhances that work by finding the optimal trajectories produced with discrete thrusts that minimize fuel spent per unit time and stay within the user-defined volume, thus providing a practical hover capability in the vicinity of the chief. The work assumes that the Clohessy―Wiltshire closeness assumption between the deputy and chief is valid. Using the new methodology developed in this work, feasible closed- and nonclosed-relative orbits are found and evaluated based on a fuel criterion and are compared with an easily calculated continuous-thrust baseline. It is shown that in certain scenarios (generally corresponding to a smaller total time of flight) a discrete-thrust solution provides a lower overall fuel cost than a continuous-thrust solution. A simple check is proposed that enables the mission planner to make the correct strategy choice.

Fold evolution and drainage development in the Zagros mountains of Fars province, SE Iran

ABSTRACTA central question in structural geology is whether, and by what mechanism, active faults (and the folds often associated with them) grow in length as they accumulate displacement. An obstacle in our understanding of these processes is the lack of examples in which the lateral growth of active structures can be demonstrated definitively, as geomorphic indicators of lateral propagation are often difficult, or even impossible to distinguish from the effects of varying lithology or non‐uniform displacement and slip histories. In this paper we examine, using the Zagros mountains of southern Iran as our example, the extent to which qualitative analysis of satellite imagery and digital topography can yield insight into the growth, lateral propagation, and interaction of individual fold segments in regions of active continental shortening. The Zagros fold‐and‐thrust belt contains spectacular whaleback anticlines that are well exposed in resistant Tertiary and Mesozoic limestone, are often &gt;100 km in length, and which contain a large proportion of the global hydrocarbon reserves. In one example, Kuh‐e Handun, where an anticline is mantled by soft Miocene sediments, direct evidence of lateral fold propagation is recorded in remnants of consequent drainage patterns on the fold flanks that do not correspond to the present‐day topography. We suggest that in most other cases, the soft Miocene and Pliocene sediments that originally mantled the folds, and which would have recorded early stages in the growth histories, have been completely stripped away, thus removing any direct geomorphic evidence of lateral propagation. However, many of the long fold chains of the Zagros do appear to be formed from numerous segments that have coalesced. If our interpretations are correct, the merger of individual fold segments that have grown in length is a major control on the development of through‐going drainage and sedimentation patterns in the Zagros, and may be an important process in other regions of crustal shortening as well. Abundant earthquakes in the Zagros show that large seismogenic thrust faults must be present at depth, but these faults rarely reach the Earth's surface, and their relationship to the surface folding is not well constrained. The individual fold segments that we identify are typically 20–40 km in length, which correlates well with the maximum length of the seismogenic basement faults suggested from the largest observed thrusting earthquakes. This correlation between the lengths of individual fold segments and the lengths of seismogenic faults at depth suggest that it is possible, at least in some cases, that there may be a direct relationship between folding and faulting in the Zagros, with individual fold segments underlain by discrete thrusts.

Evidence of iceberg‐ploughing in a subaqueous ice‐contact fan, glacial Lake Rinteln, NW Germany

The Coppenbrügge subaqueous ice‐contact fan complex of early Saalian age is about 10 km long and up to 10 km wide and is composed of offset‐stacked fan clinothems that are transgressive‐regressive sequences formed during an overall lake level rise. The individual fan bodies consist of coarse gravel in the ice‐proximal part, passing distally into sandy facies and showing large‐scale foreset bedding. The iceberg scour recognized in an open‐pit outcrop is up to 1.5m deep, up to 2m wide and cut in undisturbed mid‐fan deposits. The scour‐fill can be traced laterally for about 15m and consists of sheared sand and, in the frontal zone, of downbent overlying strata surrounded by a zone of deformed sediments. The deformed sediment produced by the iceberg keel's shearing of the trough walls is a sand mass containing angular soft‐sediment clasts that show internal folds and fractures. The basal surface of the deformed sediment is a nearly horizontal shear plane, steepening up laterally as a discrete thrust and showing a flat‐ramp‐flat geometry. The scour was formed by the iceberg keel's ploughing the substrate and pushing the sediment sideways and frontally, forming a ridge of deformed sediments at the trough end. This ridge was concurrently eroded by an accompanying meltwater underflow which apparently developed a horseshoe system of scouring vortices around the grounded iceberg. The current's scour was filled with massive, non‐stratified sand deposited rapidly from turbulent suspension. The iceberg eventually broke up and its keel part was buried. As these ice fragments gradually melted, the space was closed by normal faulting and downbending of overlying strata. The collapsing scour‐fill became partly liquified, and the resulting water‐escape structures cut the normal faults and the overlying deposits. Though produced chiefly by tangential shear strain, iceberg‐ploughing features are readily distinguishable from other glaciotectonic deformations. They can serve as a diagnostic criterion for glaciolacustrine or glaciomarine environments and the distinguishing of ice‐contact subaqueous fans from ice‐contact deltas in the stratigraphic record.

Optimal Control of Interplanetary Trajectories Using Electrical Propulsion with Discrete Thrust Levels

This work proposes a solution to the optimal closed-loop trajectory control of interplanetary missions, with solar-electric propulsion system that provides discrete thrust levels. The solution is either suitable to the case of discrete multi-level electrical engine, or a cluster of several unidirectional engines. The objective is to guide the spacecraft from an initial state to a final state (position and velocity) at an exact given time, with minimum fuel expenditure. The optimization is based on the approach of optimal trajectory: given a nominal trajectory (close to, but not necessarily the optimal one), the objective is to control the spacecraft along an optimal neighboring trajectory. In this approach, when a deviation from the planned trajectory is developed, a new neighboring optimal trajectory is calculated to obtain the required end-condition, while beginning at the current point. Using this approach, we assure lower fuel consumption, comparing to another control policy that tries to maintain uncompromisingly a nominal optimal trajectory. The paper presents the development of the control law for this problem. An efficient algorithm for the numerical solution of the optimal controller is proposed and demonstrated for two examples: correction of satellite orbit around the moon, and an interplanetary rendezvous mission to Mercury.

The Caledonian deformation of the Brabant Massif and the Early Palaeozoic in northeast Germany: compared

The Caledonian deformation of the Brabant Massif and the Rügen Early Palaeozoic has been compared by structural analysis including strain determinations, X-ray texture analysis, illite crystallinity and vitrinite reflectivity. Both areas have experienced weak internal deformation under anchimetamorphic to very low-grade metamorphic conditions. The deformation of the Brabant Massif is characterized by (a) a pronounced diagenetic foliation which resulted from compaction and weak extensional tectonics; (b) tilting or weak folding of the Cambrian strata around the Cambrian/Ordovician boundary without internal deformation; (c) a main phase of Caledonian deformation in the Lower Devonian which was associated with open folding and an axial-plane pressure solution cleavage; (d) bending of Caledonian structures from northwest–southeast to northeast–southwest. The internal deformation appears homogeneous with decreasing strain values towards upper structural levels. The strain ellipsoid is predominantly prolate with the main axis of extension parallel to the bedding–cleavage intersection lineation. It is interpreted as the result of superposition of compaction, tectonic shortening and volume loss through pressure solution. The Rügen Early Palaeozoic is less deformed and less metamorphosed under anchizonal conditions. While the upper structural level is almost undeformed, towards the deeper structural levels a spaced and then a continuous slaty cleavage is developed which is successively crenulated by a fracture cleavage. The internal deformation appears inhomogeneous due to several thrust zones with somewhat higher strain values. Outside the thrust zones strain values increase towards lower structural levels, but not significantly. The strain ellipsoid is prolate with a strongly varying orientation of the X-axis. The structural inventory of both areas is comparable with other fold-and-thrust belts. While the Rügen Palaeozoic represents an imbricated thrust-fan of an upper structural level with discrete thrusts and inhomogeneous strain, the Brabant Massif represents a deeper structural level dominated by folding and more homogeneous internal deformation.

Ductile Shear As A Cause of Inverted Metamorphism: Example From the Nepal Himalaya: A Discussion

Hubbard (1996) postulated the cause of well-known development of kyanite as well as other index minerals representing dominant metamorphism was metamorphic inversion in a section of the eastern Nepal Himalaya due to the intense ductile shearing either preor synchronous with, but outlasted the deformation because of paucity of growth of metawithin the Main Central Thrust (MCT) zone and the overlying Tibetan Slab on the basis of geologimorphic minerals across the main tectonic fabric (Hubbard 1996, p. 496). cal and textural data (cf. Hubbard 1989). The main thesis of her recent paper revolves around the fact The metamorphic belts of the NW Himalaya and the eastern Nepal are characterized by (i) ductile that the distribution of inverted metamorphic isograds is confined to intensely sheared, ,5 km thick shear zones of high strain revealing S-C fabric and other shear structures having a top-to-south/southzone of the MCT, which either post-dated or outlasted predominant metamorphic assemblages (p. west vergence, not only in the basal parts but also in the Tibetan Slab at higher structural levels; 495). Conceptually, right-way-up metamorphic iso(ii) growth of metamorphic minerals is dominantly preto synchronous, but occasionally outlasting grads in the young orogenic belts like the Himalaya can be very efficiently inverted due to intense the main deformation; (iii) lack of large-scale overturned or recumbent folds in many sections, except shearing in a broad ductile shear zone. We wish to point out that the model, proposed by Hubbard Zanskar (Searle et al. 1992), and the Beas Valley, Himachal Pradesh (Frank et al. 1977); and (iv) pres(1996) for the eastern Nepal, is the same in principle as ours for the northwestern Himalayas pubence of inverted metamorphism in the lower and middle sequences and a normal sequence in the lished in Geology in 1993. Earlier, Hubbard (1989) fixed the upper boundary upper parts near the Tethyan sedimentary cover. Numerous structures having predominantly topof the 3–5 km thick MCT zone from this area arbitrarily, as she has documented that ‘‘The upper and to-southwest sense of ductile shearing have been frequently documented across many tectonic units lower boundaries of the MCT zone are not distinct, so for the sake of discussions the author defines of the Lesser Himalaya, HHC, and Tethyan Sedimentary Zone (Brunel 1986; Treloar and Rex 1990; boundaries that include the majority of high-strain zone’’ (p. 20). Ductile shear zones were also obPatel et al. 1993) and can, therefore, be correlated with the intense ductile shearing irrespective of the served by her in the overlying Tibetan Slab, where ‘‘Gneisses of the Tibetan slab are foliated though presence of the discrete thrusts or wide thrust zones (Jain and Anand 1988). foliation is somewhat less penetrative than probable coeval mylonitic fabric within the MCT zone’’ The ductile shear model, postulated by Hubbard (1996), bears strong structural similarities to the (p. 21), indicating synchrony of two dominant planar structures in this region. Within the MCT zone, one given earlier by Jain and Manickavasagam (1993), as it is evident from the simplified version she recorded that ‘‘The apparent metamorphic grade across the MCT zone gradually increases toof our model, where effects of post-metamorphic/ thrusting, folding, and other zones of high strain wards structurally higher levels’’ (p. 20), where fibrolite and sillimanite are developed along the folihave not been considered (figure 1). Keeping in mind the regional geological and tectonic variaation as well as in the Tibetan Slab (Hubbard 1989); thus an apparent structural and metamorphic grations that may be expected over the distance of about 1000 km from the NW Himalaya, it is likely dation persists near the upper contact of the MCT zone with the Tibetan Slab. Within the MCT zone, that the growth of metamorphic minerals may be diachronous within the Higher Himalayan shear zone. One of the very basic fact of the NW Hima1 Manuscript received February 1, 1997.

Early Palaeozoic foreland thrusting and basin reactivation at the Palaeo-Pacific margin of the southeastern Australian Precambrian Craton: a reappraisal of the structural evolution of the Southern Adelaide Fold-Thrust Belt

Regional and detailed structural mapping, kinematic analysis and balancing and restoration of cross sections has lead to a re-interpretation of the Adelaide Fold Belt in South Australia. In this belt sedimentary rocks of the Late Proterozoic Adelaidean and Early Cambrian Normanville and Kanmantoo sequences were deposited during at least two episodes of subsidence related to major crustal attenuation. Contraction and crustal thickening accompanied by granitoid intrusions are related to the Cambro-Ordovician Delamerian orogeny. During this event both basins were reactivated (“inverted”) and the sedimentary rocks are now incorporated in a WNW-verging foreland fold and thrust belt at the margin of the Proterozoic southeast Australian craton. Maximal shortening of the orogen was around 55%. Within the Adelaidean basin, shortening is dominantly accommodated by major mylonitic shear zones and reverse faults. In the Cambrian Kanmantoo basin, several thrusts are demonstrably reactivated growth faults, across which thickness changes of Cambrian sedimentary rocks are revealed by balancing and restoration of cross sections. Owing to the steep easterly dips of these faults, in the western part of the Kanmantoo basin lateral shortening of up to 58% is largely accommodated by intense folding and fold axial-planar flattening strain. Further east, strain magnitudes wane and the overall shortening of around 30% is accommodated by both, discrete thrust zones and regional folds. The basin reactivation along the Australian part of Gondwana's palaeo-Pacific craton margin reflects the rapid transition from passive to active tectonism, which possibly is a consequence of the disintegration of the formerly conjugate margins of Gondwana and Laurentia.

Cenozoic thrust emplacement of a Devonian batholith, northeastern Brooks Range: Involvement of crystalline rocks in a foreland fold-and-thrust belt

Involvement of crystalline rocks in thrusting near the foreland basin of a fold-and-thrust belt is relatively uncommon. In the northeastern Brooks Range, the Devonian Okpilak batholith was thrust northward and structurally elevated above adjacent foreland basin deposits during Cenozoic fold-and-thrust deformation. The batholith may have acted initially as a regional structural buttress, but a drop in the basal detachment surface to greater depth south of the batholith resulted in northward transport of the batholith. Shortening within the batholith was accommodated by (1) the development of discrete thrust slices bounded by ductile shear zones, (2) simple shear and development of penetrative mesoscopic and microscopic fabrics throughout the batholith, or both. The Mississippian Kayak Shale, a regional detachment horizon at the base of the overlying cover sequence, is depositionally thin or absent adjacent to the batholith. Thus, most of the cover sequence remained structurally coupled to the batholith during thrusting and was shortened by the development of penetrative structures.

Transition from near‐surface thrusting to Intrabasement Decollement, Schlinig Thrust, eastern Alps

West directed thrusting along the Schlinig thrust amounts to a minimum displacement of 45 km. A discrete plane of brittle thrusting is observed in the frontal part, where the Ötztal unit overrides the sediments of the Engadine Dolomites. Both shortening of the more ductile Lower and Middle Triassic and very substantial stretching by domino‐type normal faults in the brittle Upper Triassic dolomites are contemporaneous with, and caused by, thrusting. Extension is related to extrusion of parts of the sediments in the footwall of the Schlinig thrust. A relatively rapid transition into an intrabasement shear zone, which is as much as 2‐km‐thick, occurs farther to the east in a region where a paleotemperature of 300°C is indicated for Cretaceous metamorphism. Still farther to the east, localized shearing is transformed into a wider area affected by large‐scale folding. This second transition corresponds to estimated temperatures in excess of 550°C. Since the changes in structural style and deformation mechanisms toward the hinterland go hand in hand with increasing metamorphic grade, we regard thrusting as contemporaneous with metamorphism. This view is supported by geochronological data. The age of thrusting and metamorphism (90 Ma) indicates very rapid heating after the deposition of the youngest sediments (Cenomanian) involved in Cretaceous deformation. Our data suggest that localized detachment along a discrete thrust plane and within broader mylonite zones cannot always be traced indefinitely to greater depth. The transformation of localized shearing into large scale folding indicates a fundamentally different deformational style for lower crustal levels.