Thrust Planes Research Articles

The formation of a geometrical ridge network by the surge-type glacier Kongsvegen, Svalbard

Traditionally, geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge (‘crevasse-fill’ ridges). They are widely regarded as a characteristic landform of glacier surges. Understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record. The geometrical ridge network at the surge-type glacier Kongsvegen in Svalbard, does not form by crevasse filling. The networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin. The transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice. The thrusts were apparently formed during a glacier surge in 1948. Longitudinal ridges form through the meltout of elongated pods of debris, which on the glacier surface are subparallel to the ice foliation and pre-date the surge. This work adds to the range of landforms associated with glacier surges.

Transpressive structures in the Neoproterozoic Ariab-Nakasib Belt, northeast Sudan: evidence for suturing by oblique collision

The Ariab-Nakasib Belt (ANB) represents a Neoproterozoic suture zone, separating the oldest juvenile microplates of the Arabian-Nubian Shield, namely the Haya Terrane in the south and the Gebeit Terrane in the north. The Ariab-Nakasib fold-and-thrust belt was generated above a south-southeast dipping subduction zone during the oblique closure of a pre-existing oceanic basin. The northern boundary of the suture zone is defined by nappe remnants of the Oshib-Amur-Igarairi melange zone, which were obducted and transported to the northwest and north-northwest, respectively, along gently southeast dipping thrust planes. In contrast, the microplate collision produced a zone of strain accumulation in the southern part of the ANB creating isoclinal folds with steep foliations ( D 1 D 2 ), transpressive structures, and late thrusts. Continued oblique collision led to a refolding event (D 3) in the southwestern ANB during which the resulting strain was mainly taken up by the development of dextral superposed shear folds. The same stress regime caused the early structures in the Oko Shear Zone (OSZ), notably the north-northwest trending upright folds and the conjugate (125°–0°) ductile strike slip-faults, whereas the main faults in the OSZ that trend 150°–160° belong to a younger tectonic event. These younger faults in the OSZ are related to the collision of East and West Gondwana. The youngest, cross-cutting, faults are interpreted as a result of the Miocene opening of the Red Sea.

Particle motions in a physical model of shallow angle thrust faulting

A physical model of thrust faulting has been constructed out of elastic foam rubber. The thrust wedge has an angle of 25°., and is forced from the back by a steel-hydraulic system of effectively infinite rigidity, analogous to the force exerted by tectonic plates. The observed particle motions show many features different from those commonly assumed in dislocation models of subduction thrusts. Interface waves associated with fault opening propagate along the thrust plane (from the back) and temporarily decouple the overlying hanging wall plate from the foot wall. Because of the geometry of the reflecting fault boundary, and the free surface (also reflecting), energy is trapped in the wedge and, as a consequence, the particle motions and energy in the hanging wall are much greater than in the foot wall. The interface wave and the energy trapped in the upper plate propagate up the wedge and break out at the toe of the thrust with a spectacular increase in motions. If this model is analogous to the real earth, it suggests that some common assumptions in dislocation modeling may be incorrect. The model may explain apparent missing energy radiated to teleseismic distances, the anomalously low number of empirical Green’s functions needed to model teleseismicp waves, and the evidence of intense shaking on the hanging wall toe of some thrust fault earthquakes. The results support the suggestion that interface waves may explain the lack of high frictional heat generation from thrust faults, and may explain the ‘paradox of large overthrusts’. The results also suggest that the seismic hazard of great subduction thrust earthquakes and continental shallow angle thrust faults might in some cases be seriously underestimated.

A reassessment of the age and form of the Broadford Gabbro, Isle of Skye: new field evidence from Creag Strollamus

Synopsis The Broadford Gabbro has been reinterpreted as younger than the Outer Granites of the Eastern Red Hills Centre. Extrusive pyroclastic rocks, on the northern side of Creag Strollamus, are bound below by a weathered surface of one of the Outer Granites and above by a planar contact with the Broadford Gabbro. Other evidence includes chilled margins, back veining and relationships with members of the regional dyke swarm. It is believed that the Broadford Gabbro has the form of a conformable sheet that has utilised a pre-existing thrust plane. An apparent ‘dyke’ of gabbro extending across the NE shoulder of Creag Strollamus is interpreted as a tabular projection from the sheet. A possible feeder dyke has a NW–SE orientation. parallel to the trend of the regional dyke swarm and to the axis of the Corran Lodge Granite.

Structural evolution of the northern end of the Oman Ophiolite and enclosed granulites

A combined structural field mapping, microstructural and petrographic study has been undertaken to better understand the origin and evolution of the northernmost part of the Oman Ophiolite (Khawr Fakkan massif) in the framework of an oceanic spreading centre. Emplacement of a sheet of granulite facies metamorphic rocks into the peridotites occurred at temperatures at which the peridotites deformed ductily. The metamorphic rocks define the southern extension of a block boundary which separates an eastern peridotite part displaying high-temperature fabrics from a western one with porphyroclastic to mylonitic fabrics. Granitoid intrusions representing partial melts from the metamorphic rocks occur only in the ophiolitic part overlying the metamorphic rocks, thus delineating the same boundary between the two mantle blocks. This high-temperature boundary is not visible on satellite or aerial photographs. The granulite facies foliations and lineations in the metamorphic rocks can be correlated with porphyroclastic textures developed at decreasing temperatures in the associated peridotites and to the obduction of the ophiolite. The final emplacement of the metamorphic rocks in their present position was accompanied by retrograde hydration, decompression and continuous deformation at temperatures ranging from 800 to 300°C. The hydration is extended along the Wadi Ham fault. Two models are proposed for the emplacement of the metamorphic rocks. In the first model the protolith of the metamorphic rocks was deposited in a pull-apart basin along a transform fault and then subducted along the transform fault after reversal from transtension to transpression. In this model, the ophiolite obduction occurred later along another thrust plane. In the second model, the protolith material descends along an intraoceanic subduction zone and is emplaced into the ophiolite hanging wall along a secondary thrust caused by transpressive or rotational movements during the ophiolite obduction. In this case the ophiolite obduction occurred along the same intraoceanic subduction plane.

Structural evolution of the eastern Balkans (Bulgaria)

The Balkans are an E-W trending thrust belt mainly of Palaeogene age, generated by a left-lateral transpression with maximum stress-oriented N30°E. The transpressive nature of the Balkans may be deduced from the en echelon pattern of folds and thrusts which is due to oblique shortening with respect to pre-existing Mesozoic E-W trending crustal anisotropies between the Moesia Platform and the Balkans Basin. The front of the eastern Balkans is marked by the Chudnite Skali-Ajtos thrust. During the Neogene and Quaternary the Balkans were inverted by right-lateral transtensional tectonics which cross-cut and dismembered the thrust belt. The main transtensional segment is the southern margin of the Balkans (e.g. the Emine Line). The steep transtensional faults disconnected the thrust planes of the Balkans from the thrusts of the Srednogorie to the south. Grabens are in general asymmetrical, with master faults dipping 80-40° south-westward and probably flattening into the middle crust. The structural evolution of the Balkans may also be traced into the Black Sea, but the structural trends gradually change from the E-W trend to a NW-SE strike, indicating pure Palaeogene compression and more recent Neogene extension inverting and cross-cutting the compressive features. It is also interesting to note that the Neogene tensional or transtensional inversion is not cylindrical; it exhibits undulations of graben and transfer faults controlled by the pre-existing compressive fabric. The eastern Balkans and their continuation in the Black Sea are morphologically and structurally deeper and they plunge eastward and south-eastward towards the Black Sea. This is due both to the deeper position of the Balkans de´collement planes in the Black Sea and to the thermal subsidence along the Black Sea continental margin.

The Subthrust Trap beneath the Western Foothills of the Eastern Cordillera, Colombia: ABSTRACT

LASMO started exploration in the Espinal Block within the Upper Magdalena Valley of Colombia in 1987. Since then six exploration wells have been drilled within the block which have resulted in four New Field Wildcat successes. Two of these successes have been in subthrust configurations. This presentation shall describe the exploration and appraisal techniques used to delimit the Venganza Field which was the first subthrust field discovery made in Colombia. The first stages of our exploration for the subthrust trap were conventional seismic surveying, and structural modelling using the excellent geological outcrop data. These led to the formulation of the Venganza prospect which comprised tilted Late Cretaceous Guadalupe sandstones overthrusted by stratigraphically older Villeta shales. The Venganza Prospect was drilled in 1991 and encountered the reservoir as expected directly beneath the thrust plane, and produced oil at 5800 barrels/day on test. Subsequent to the Venganza-1 discovery, further seismic data were acquired and processed using pre-stack depth migration. These new data {open_quotes}verified{close_quotes} the Venganza geological model, and formed the basis for appraisal drilling. The Venganza-2 appraisal well, drilled 4 km to the north of the successful exploration well, was dry and indicated that the subthrust trap configuration was much more subtlemore » than first anticipated. The presentation shall then demonstrate the evolution of our understanding of the subthrust trap configuration and the evolution of our search techniques for it.« less

Geological remarks on the relationships between extension and convergent geodynamic settings

In convergent geodynamic settings, extensional and compressional tectonics occur contemporaneously or in different times and places, but they are deeply linked to each other in a number of different ways. The main types of extensional regimes here considered are for west-dipping subduction zones: (1) back-arc extension (e.g., Tyrrhenian Sea); (2) uplift and related extension due to asthenospheric wedging (e.g., Apennines); (3) extension at the subduction hinge (e.g., Marianas); (4) extension due to increasing length of arcs (e.g., Apennines). For east-, northeast- or north-northeast-dipping subduction zones: (5) extension induced by collapse of orogens (e.g., Alps, Himalayas); (6) extension induced by differential lithosphere-asthenosphere coupling in the hanging wall of a subduction zone (e.g., Aegean rift); (7) rifting induced by orogenic roots anchored into the mantle (e.g., Atlantic and Tethys). Other types may be considered: (8) inversion of plate motion, from convergent to divergent relative plate vectors due to external velocity fields (e.g., Basin and Range); (9) transtension may occur along oblique ramps of thrust arcs due to differential forward propagation of thrusts planes (e.g., southern arm of the Barbados accretionary prism?); transtension and transpression also occur contemporaneously along an undulate strike-slip setting, or extension can accompany foreland dipping duplexes; (10) apparent extension due to down-section cut of thrust planes along subduction zones. Each type of extension generates different shape, depth and relationships between hanging wall and footwall of the associated normal faults and shear zones.

Structural variation and landward vergence at the toe of the Oregon accretionary prism

The Oregon margin near 45°N spans a regional transition in structural style from seaward vergence in the south to landward vergence in the north. This variation probably reflects a regional change in both sediment type and rate of deposition that affects the potential for overpressure in the sediments. Structural style within the survey area shows a gradual northward transition from seaward to landward vergence and to lower slopes within the landward vergent area, suggesting a northward decrease in basal shear stress. Superimposed on this gradational variation are abrupt changes in structural style that correlate with NW striking strike‐slip faults in the Cascadia Basin. Because sediments thicken toward the east, translation along the strike‐slip faults results in juxtaposition of sediments with different physical properties and loading histories. In addition, the faults themselves may act as fluid conduits, resulting in stepwise changes in pore pressure on the décollement and concomitant change in structural style across the faults. Although the Oregon‐Washington margin is dominated by landward vergence, landward vergence has not been adequately explained by theoretical models or replicated in experimental models because of a simple omission in the boundary conditions of the underlying conceptual model. Landward vergence requires not only low basal shear stress but also an arcward dipping décollement (and to a lesser degree, a relatively strong wedge). In order for landward vergence to predominate, these three factors must combine in such a way that the backward verging thrust planes are favored.

Self‐similar cataclasis in the Saltville thrust zone, Knoxville, Tennessee

Fault rocks from the Saltville thrust zone, Knoxville, Tennessee, display a fractal geometry of clast size over 3 orders of magnitude. The cataclastic fractal geometry occurs at each magnification in different clast size classes and at combined magnifications. The mean of the fractal dimension (D) measured at each of the optical photomicrographs is generally smaller than that of the scanning electron microscopy images because of the smaller clast density in the optical sections. The fractal dimensions measured on randomly selected areas of the sections cut parallel to the thrust and normal to the thrust along the dip and strike, show a normal distribution with its mean, median, and mode that correlate closely with the dimension of ideal, fractal cataclasis (DI) based on the Sierpinski carpet model. The cataclasis was a statistical random, isotropic, and homogeneous fractal process that deformed the carbonates similarly parallel and normal to the thrust plane and in different parts of the thrust zone.

Microstructures and evolution of folds in SC-mylonites from Dudatoli-Almora crystallines of Garhwal Himalaya

In the Lesser Garhwal Himalaya, the North Almora Thrust separates the overlying medium-grade Dudatoli-Almora crystallines of Precambrian age from the unmetamorphosed to partly metamorphosed rocks of the Garhwal Group of Late Precambrian age. The crystalline nappe sheet consists of flaggy to schistose quartzites, granite gneisses and garnetiferous mica schist members in an ascending order. In different localities. different members of the Dudatoli-Almora crystallines are exposed along the thrust plane. Southwest of Adbadri fine-grained mylonitized schistose quartzites of Dudatoli-AImora crystallines are in contact with the underlying metabasites of the Garhwal Group. The mylonitized schistose quartzites consist of alternating thick (1 to 2m) quartzite and thin (10 to 20cm) micaceous quartzite bands. The micaceous quartzites can be further differentiated into alternating quartz-rich (0-5 to 2.0 cm thick) and mica-rich (0.2 to 1.0 cm thick) layers. In the quartzites the C-surfaces are parallel to the S-surfaces defined by the alternating quartz-rich and mica-rich layers. Further, the S-surfaces exhibit almost similar folds with multiple wavelengths where the axial planes are nearly parallel and enveloping surfaces are oblique to the lithological layering. The evolution of these folds has been envisaged in three phases of deformation on the basis of field evidence, fold geometry and microstructures.

Lateral variation in geometry of thrust planes and its significance, as studied in the Shumar allochthon, Lesser Himalayas, eastern Bhutan

The features mapped indicate a SSE-striking displacement vector for the Shumar thrust of eastern Bhutan. Sections parallel to this direction show that the frontal “flat” segment of the thrust plane, along with the subthrust and the hanging-wall rocks, has been folded into a synform. A longitudinal section, however, shows a cuspate-lobate shape (broad concave-upward curves meeting along sharp crests) of the thrust plane, which laterally cuts up through the subthrust rocks. The broad salients and sharp re-entrants defining the multiple lobate pattern of the thrust trace is due to the cuspate-lobate lateral curvature, which is a primary feature of the thrust plane. Each cusp, combined with the forward or backward inclination of the thrust plane imparts a scoop-like shape, several of which are joined by transverse ridges parallel to the displacement vector. The S-surfaces, transposed parallel to the limbs and axial surfaces of the isoclinal “early” folds in the allochthon, have been progressively reoriented parallel to the curviplanar thrust surface. “Later” post-thrusting folds superposed on these initially curved surfaces have curved hinges which trend at high angles to the displacement direction. As a result, the bent hinges are congruent to the laterally curved thrust surface below. The evolution of the Shumar thrust can be explained by lateral propagation along concave-upward trajectories. Thrust fronts propagating from multiple nuclei subsequently coalesced to form a single thrust surface. The ridges along which the thrusts coalesce have the form of lateral ramps.

5344625 Precious metal recovery process from sulfide ores

A section through the Neoproterozoic Mozambique Belt of Tanzania exposes western foreland (Archaean Tanzania Craton and Palaeoproterozoic Usagaran Belt), marginal (Western Granulites) and eastern, internal (Eastern Granulites) portions of the orogen. The assembly of granulite nappes at ca. 620 Ma displays westward emplacement along an eastward deepening basal decollement and forward propagation of thrusts, climbing from the deep crust to the surface. This goes along with eastward increase of syntectonic temperatures, derived from prevalent deformation mechanisms, and eastward decrease of the kinematic vorticity number. Distinctly different pressure - temperature paths with a branch of isothermal decompression (ITD) in Western Granulites and isobaric cooling (IBC) in Eastern Granulites reflect residence times of rocks within lower crustal levels. Western Granulites, exhumed rapidly at the orogen margin, display ITD and non-coaxial fabrics. Eastern Granulites in the internal orogen portions escaped from rapid exhumation and show IBC and co-axial flow fabrics. The vertical variation of structural elements, i.e. basement — cover relations within the Eastern Granulites, shows decoupling between lower and middle crust with horizontal west — east stretching in the basement and horizontal west — east shortening in the cover.A model of hot fold nappes [Beaumont, C., Nguyen, M.H., Jamieson, R.A., Ellis, S., 2006. Crustal flow modes in large hot orogens. In: Law, R.D., Searle, M.P., Godin, L., (eds). Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications. vol. 268, 91–145] is adopted to explain flow diversity in the deep crust. The lower crust represented by Eastern Granulite basement flowed coaxially outwards (westward) in response to thickened crust and elevated gravitational forces, supported by a melt-weakened, viscous channel at the crustal base. Horizontal flow with rates faster than thermal equilibration gave rise to isobaric cooling. Simultaneously the mid crust (Eastern Granulite cover) was shortened when hot fold nappes moved along upward climbing thrust planes. Western Granulites preserved isothermal decompression through exhumation by thrusting and coeval erosion at the orogen front.Two different styles define the Neoproterozoic East African Orogen between northern Egypt and southern Mozambique. The Arabian Nubian Shield in the north is classified as small and cold orogen in which thin — skinned thrusting was associated with lateral extrusion. The Central Mozambique Belt in Tanzania/Southern Kenya is classified as large and hot orogen characterized by thick-skinned thrusting and assembly of large granulite nappes.

The influence of pre-existing thrust faults on normal fault geometry in nature and in experiments

Relations between normal faults and pre-existing thrust faults are classically described in terms of three basic situations: normal faults can cross-cut thrust faults; they can branch out from thrust faults at depth on a de´collement level, or they can entirely reactivate thrust planes. The mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the ‘brittle’ rocks and silicone putty an interlayered de´collement. The models underwent compression, producing thrust faults with variable dips, followed by extension. Three possible ways of interaction are described here: (a) no interaction occurs in the case of low-dip thrust faults (<32 ° ± 1 °) and normal faults are developed independently, displaying a listric geometry; (b) branching at depth on the de´collement level occurs when dip of the thrust faults reaches 32 ° ± 1°. In this case, the dip of the normal faults, whose geometry becomes planar, decreases with increasing thrust dip. We suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre-existing fault zone, where a drop in the friction coefficient is likely to occur; (c) reactivation occurs in brittle material when dip of the pre-existing fault exceeds 41° ± 1°.

Structure and tectonics of the northern North Sea: new insights from deep penetration regional seismic data

Abstract Recently acquired, deep penetration regional seismic data provide important new insights into the structure and tectonic evolution of the northern North Sea. Improved data quality permits better understanding of fault geometries and salt tectonics in the graben areas and reveals the pre-Tertiary geology of the surrounding platforms for the first time. This paper presents seismic data from five structurally distinct areas. (1) South Viking Graben . This relatively narrow graben formed the principal sedimentary basin during the Mesozoic period and contains a thick Triassic-Cretaceous succession. Low-angle, eastward-dipping faults developed at the western basin margin control a relatively simple half-graben geometry. The Beryl Embayment is notable for the local rotation of Triassic-Jurassic strata independent of the underlying basal Permian reflectors, indicating local Mesozoic salt movement in this area. Faulted and anticlinal highs east of the graben axis may also record limited salt tectonics. (2) Horda Platform . This area comprises a series of eastward-dipping rotated Mesozoic fault blocks. Mesozoic extensional faults developed as splays in the hanging wall of a major westward-dipping deep detachment surface. (3) East Shetland Platform . This area formed a structural high throughout the Mesozoic era. Regional seismic data provide a clear image of the geometry of a Palaeozoic basin, thought to represent the northward continuation of the Orcadian Basin recognized onshore in mainland Scotland, Orkney and Shetland. The sedimentary sequence is probably of Devonian (-Carboniferous?) age and contains major unconformities, which may permit subdivision of this succession into tectono-stratigraphic units. The contact with the underlying basement is characterized by a distinctive change in seismic character, traceable over the entire area. The deep structure of the East Shetland Platform is dominated by westward- or northwestward-dipping low-angle basement faults, interpreted as thrust surfaces. This structural grain is associated with the local development of eastward-vergent folds interpreted as hanging wall anticlines developed in the Upper Palaeozoic cover sequence. These structures provide evidence of a Late Palaeozoic deformation phase. (4) Unst Basin . Lying to the west of the East Shetland Basin and bounded by a faulted horst on its eastern margin, the Unst Basin is underlain by a spectacular low angle, westward-dipping detachment surface. Roll-over into this fault is interpreted as the result of extensional roll-back of an older (?Caledonian) thrust plane. The fault geometry may indicate structural analogies with the West of Shetlands Province. (5) East Shetland Basin . The structure of this area is characterized by the development of extensive, westward-dipping fault blocks underlain by low-angle, eastward-dipping extensional faults traceable to great depth. The seismic data indicate thickening of the sedimentary succession in the footwalls of these faults, consistent with Jurassic faulting of a pre-existing Triassic basin.

Integrated Basin Chemical Modelling Redefines the Geothermal Evolution of the Arkoma Basin, Oklahoma, USA: ABSTRACT

Amoco utilized a variety of exploration technologies to become the economic success leader (EPs > 65%) in the thrusted Pennsylvanian Spiro sandstone play of the Arkoma basin, Oklahoma. It was found early in exploitation analysis that conventional thermal-history methodologies gave convicting results about the overall geothermal evolution of the basin. Within the thrusted terrain, vitrinite reflectance and bottom-hole temperature data give abnormally low paleo-temperatures. Apatite fission track analysis was unusable for sub-surface samples due to extremely high paleo-temperatures (>110{degrees}C at 450m until 50Mya). Integrated Basin Chemical Modelling (MCM) was used to determine timing of critical events (thrusting, tinting of maximum paleo-temperature, generation, expulsion and diagenetic, evolution). Preliminary modelling results indicate that maximum temperature post-dates thrusting of the Ouachitas - eliminating the necessity to palinspastically restore individual thrust sheets to determine spatial relationships of traps to migration pathways. Complex hydrologic flow is concentrated within the porous Spiro reservoir section and along thrust planes. IBCM modelling indicates the critical flow direction reverses several times during basin evolution. The Arkoma basin is divided into two distinct thermal provinces, each experiencing a separate, complex thermal history. Fission track analysis of surface samples indicates two periods of regional cooling - Late Paleozoic and Late Cretaceousmore » to Early Tertiary. Timing of these cooling events correlates to flow reversals indicated from IBCM modelling. Hydrologic flow reversals indicate multiple periods of migration/charge and the possibility of subtle stratigraphic traps formed from diagenetic changes within the prospective reservoir intervals.« less

The North Tyrrhenian–Northern Apennines post‐collisional system: constraints for a geodynamic model

ABSTRACTThe North Tyrrhenian–Northern Apennines post‐collisional system (NTAS) has been analysed on the basis of marine reflection profiles and bibliographic sources with the aim of providing an up‐to‐date geotectonic and evolutive framework consistent with structural data. The continental rifting began to affect the most internal (western) side of the system in the middle (?)‐upper Miocene, while the most external (eastern) zones were touched by rifting only in Pleistocene times. The birth and development of extensive, asymmetric basins took place with a progressive speeding up of rifting towards the external sectors.A diachronous, regionally extended and eastward rejuvenating unconformity, recognizable within the Neogene–Quaternary basins, marks the passage from the syn‐rift to the post‐rift tectonic regime. While the first is everywhere dominated by extension, the second one produces vertical movements. Reflection terminations related to this unconformity are interpreted in terms of vertical tectonic component. Three zones that experienced different post‐rift vertical movements are thus recognized in the NTAS: subsidence in the North Tyrrhenian deep water region; tectonic stability or alternation of moderate subsidence and uplift in the continental shelf and western Tuscany, strong uplift in eastern Tuscany.The updated structural data demonstrate that the NTAS upper crust is crosscut by both Tyrrhenian‐dipping and Apennine‐dipping low‐angle detachment planes, above which the upper crustal blocks rotated and experienced extensional transport along two opposite directions: to the west and to the east, respectively. The coexistence of opposite‐dipping crustal surfaces bears two fundamental implications for the NTAS geodynamics. First it stresses the importance of the anti‐Apennine transfer faults, since they separate crustal blocks of opposite extension polarity. Secondly, it testifies that the extension tectonics not only re‐utilized former crustal thrust planes, but also gave origin to new significant E‐ and NE‐dipping detachment planes.Plan and crossview geometries of faults, together with considerations on the role of the metamorphic core complexes, point to an interpretation of the NTAS structure that is more consistent with the ‘anastomosing shear’ model rather than the 'simple shear' or the ‘delamination’ ones.

The Variscan fold-thrust belt of southeast Ireland and its control on early Mesozoic extension and deposition: a method to predict the Sherwood Sandstone

Abstract The Sherwood Sandstone Group (SSG), of Early Triassic Scythian age, contains attractive fluvial and aeolian sandstone reservoirs in the basins between Ireland and Britain (Morecambe Bay gas field, 6.5TCF) and in southern England (Wytch Farm oil field, 300 MMBBL). While the SSG is very thick in the northern portions of the Mesozoic basin system, it is thinner and generally less well preserved in the southern portions of the Mesozoic basin system which are underlain by the Carboniferous aged Variscan fold-thrust belt. Previously it has been suggested that the SSG was deposited during a time of rifting, and consequently its variable presence was attributed to wells penetrating positions within fault blocks that were high and devoid of syn-rift deposition. Data examined in this study from the East Irish Sea Basin and from the South Celtic Sea Basin, however, suggest that the SSG was deposited in palaeotopographic lows, the positions of which were controlled by erosion around massifs and resistant thrust belt hangingwalls that formed the palaeotopography. Furthermore, the evidence presented suggests that rifting occurred during the deposition of the Upper and Middle Triassic Mercia Mudstone Group (MMG), which is often defined as a post-rift succession. Successful prediction of the areas of SSG deposition depends on the ability to reconstruct the Early Triassic palaeotopography as it was controlled by the Carboniferous thrust belt morphology. A reconstruction of the Variscan thrust belt is presented, which is based on onshore surface geology, thrusts and transfer faults from published onshore mapping, published offshore thrust plane reflector mapping, gravity data showing lineaments and indications of granite plutons, and a set of observations from well and seismic data. Some thrusts are postulated by observations of reactivation during Mesozoic extension. A set of predictive tools has been developed for identifying the presence of the SSG. Determination of the presence of the SSG on seismic reflection data is often difficult. Where the data quality is poor, we postulate that there is a better chance of finding the SSG where halites are found in the overlying MMG. This empirical relationship is believed to occur because rifts developed over SSG foreland and intermontane valleys were near sea-level, allowing marine incursions and salt precipitation. By contrast, rifts that developed in high topographic areas were above sea-level and not subject to marine incursion. Additionally, the degree of deformation seen in the pre-Triassic section on seismic data can be used to predict the position of the Variscan Front and thus the topographically low, sand-prone areas overlying the Carboniferous foreland basin. A palaeogeographic map is presented, incorporating the analysis of all of these datasets and utilizing a comparison to the topography of the exposed Ouachita Mountains of the southern USA.

Late Neogene kinematics of intra‐arc oblique shear zones: The Petilia‐Rizzuto Fault Zone (Calabrian Arc, Central Mediterranean)

The kinematics of intra‐arc shear zones play a key role in the secondary shaping of orogenic arcs such as the Calabrian Arc (central Mediterranean). Comparison of the Neogene structural development of the Petilia‐Rizzuto Fault Zone and the basement structure of the bordering Sila massif reveals that the fault zone is the surface expression of a deep NW–SE trending sinistral crustal oblique shear zone. This shear zone continues over a length of more than 130 km across the northern segment of the Calabrian Arc and shows a post‐Eocene sinistral displacement of about 50 km. The late Neogene forearc basin development and syndepositional tectonics along the fault zone are reconstructed in great detail by analyzing the middle Miocene‐Recent tectonic sequence stratigraphy. A strike‐slip cycle can be recognized whereby the subsequent activity of Riedel shears, tensional faults, and P shears, positive flower structures and principle displacement wrench faults, can accurately be traced in time. Observed phenomena are discussed in terms of the activity of a conjugate system of oblique thrust zones within the growing accretionary complex. The evolution of special types of thrust belt basins is illustrated. These include oblique thin‐skinned pull‐apart basins, oblique rhomboidal “harmonica” basins, and “detached slab” basins (new terms introduced here), evolving one into the other. A new feature illustrated is the recurrent basin inversion which generated passive roof duplexes through back‐shear motion and out‐of‐sequence thrusting along the wedge. The fault patterns and the style of inversion tectonics imply an E–W directed axis of effective compressive stress in this part of the arc. This resulted from an interaction of (1) local E–W directed compression related to a differential displacement of two parallel segments of the arc (generated by the migration to the southeast of the Calabrian Arc and opening of the Tyrrhenian backarc basin); (2) alternating NW–SE directed compression and extension (related to pulsating thrust wedge dynamics with phases of accretion and underthrusting respectively) and (3) regional, compressive interplate stress (middle Messinian‐middle Pliocene). All structures are overprinted by post middle Pleistocene extensional faulting (related to rapid uplift of intra‐arc massifs) and reversal along thrust planes and transcurrent faults. This extensional collapse reflects isostatic adjustments in response to plate rupture which was provoked by regional compressive stress.

Constraints on Himalayan deformation inferred from vertical velocity fields in Nepal and Tibet

Spirit leveling data from the Nepal Himalaya between 1977 and 1990 indicate localized uplift at 2–3 mm/yr in the Lesser Himalaya with spatial wavelengths of 25–35 km and at 4–6 mm/yr in the Greater Himalaya with a wavelength of ≈40 km. Leveling data with significantly sparser spatial sampling in southern Tibet between 1959 and 1981 suggest that the Himalayan divide may be rising at a rate of 7.5±5.6 mm/yr relative to central Tibet. We use two‐dimensional dislocation modeling methods to examine a number of structural models that yield vertical velocity fields similar to those observed. Although these models are structurally nonunique, dislocation models that satisfy the data require aseismic slip rates of 2–7 mm/yr on shallow dipping faults beneath the Lesser Himalaya and rates of 9–18 mm/yr on deep thrust faults dipping at ≈25°N near the Greater Himalaya. Unfortunately, the leveling data cannot constrain long‐wavelength uplift (&gt;100 km) across the Himalaya, and unequivocal estimates of aseismic slip in central Nepal are therefore not possible. In turn, the poor spatial density of leveling data in southern Tibet may inadequately sample the processes responsible for the uplift of the Greater Himalaya. Despite these shortcomings in the leveling data, the pattern of uplift is consistent with a crustal scale ramp near the Greater Himalaya linking shallow northward dipping thrust planes (3–6°) beneath the Lesser Himalaya and southern Tibet. Aseismic slip on the potential rupture surface of future great earthquakes beneath the Nepal Himalaya south of this ramp appears not to exceed 30% of the total convergence rate between India and southern Tibet resulting in an accumulating slip deficit of 13±8 mm/yr.