Ramp Anticlines Research Articles

Kinematics of Cenozoic Shortening of the Hotan Anticline Along the Northwestern Margin of the Tibetan Plateau (Western Kunlun, China)

AbstractKinematic constraints on the Cenozoic deformation along the northwestern edge of the Tibetan Plateau remain limited. Combining surface geological data and seismic profiles, we document the structural geometry and kinematics of the large‐scale east‐west striking Hotan anticline, along the foothills of the Western Kunlun Range. Four new balanced cross sections are constructed, and the temporal evolution of deformation is deciphered from the exceptionally well‐imaged growth strata at the front of the fold. This anticline results from a broad fault‐bend fold, subsequently deformed by a footwall duplex. The total shortening across the whole structure is relatively constant along strike, from ∼40 to ∼35 km. However, the shortening accommodated by the duplex varies laterally from ∼50–40% to 0% of the total shortening. Two distinct successive patterns of growth strata are recognized and are interpreted to be representative of deformation on the ramp anticline, followed by deformation on the duplex. The onset of deformation initiated by ∼16 Ma. Deformation of the underlying duplex began at ∼12 Ma to the west and subsequently propagated eastward. From these results, we determine a shortening rate of ∼5 mm/yr from ∼16 to ∼8–9 Ma across the Hotan thrust system, followed by a significant decrease in shortening rate, possibly down to <0.5 mm/yr. We explore the significance of this deceleration of deformation at the scale of the Western Kunlun foothills and at a broader regional scale as it may point to a regional kinematic reorganization by the late Miocene.

Early Cenozoic Eurekan strain partitioning and decoupling in central Spitsbergen, Svalbard

Abstract. The present study of field, petrological, exploration well, and seismic data describes backward-dipping duplexes comprised of phyllitic coal and bedding-parallel décollements and thrusts localized along lithological transitions in tectonically thickened Lower Devonian to lowermost Upper Devonian; uppermost Devonian–Mississippian; and uppermost Pennsylvanian–lowermost Permian sedimentary strata of the Wood Bay and/or Wijde Bay and/or Grey Hoek formations; of the Billefjorden Group; and of the Wordiekammen Formation, respectively. The study shows that these structures partially decoupled uppermost Devonian–Permian sedimentary rocks of the Billefjorden and Gipsdalen groups from Lower Devonian to lowermost Upper Devonian rocks of the Andrée Land Group and Mimerdalen Subgroup during early Cenozoic Eurekan deformation in central Spitsbergen. Eurekan strain decoupling along these structures explains differential deformation between Lower Devonian to lowermost Upper Devonian rocks of the Andrée Land Group and/or Mimerdalen Subgroup and overlying uppermost Devonian–Permian sedimentary strata of the Billefjorden and Gipsdalen groups in central–northern Spitsbergen without requiring an episode of (Ellesmerian) contraction in the Late Devonian. Potential formation mechanisms for bedding-parallel décollements and thrusts include shortcut faulting and/or formation as a roof décollement in a fault-bend hanging wall (or ramp) anticline, as an imbricate fan, as an antiformal thrust stack, and/or as fault-propagation folds over reactivated or overprinted basement-seated faults. The interpretation of seismic data in Reindalspasset indicates that Devonian sedimentary rocks of the Andrée Land Group and Mimerdalen Subgroup might be preserved east of the Billefjorden Fault Zone, suggesting that the Billefjorden Fault Zone did not accommodate reverse movement in the Late Devonian. Hence, the thrusting of Proterozoic basement rocks over Lower Devonian sedimentary rocks along the Balliolbreen Fault and fold structures within strata of the Andrée Land Group and Mimerdalen Subgroup in central Spitsbergen may be explained by a combination of down-east Carboniferous normal faulting with associated footwall rotation and exhumation, and subsequent top-west early Cenozoic Eurekan thrusting along the Billefjorden Fault Zone. Finally, the study shows that major east-dipping faults, like the Billefjorden Fault Zone, may consist of several discrete, unconnected (soft-linked and/or stepping) or, most probably, offset fault segments that were reactivated or overprinted to varying degrees during Eurekan deformation due to strain partitioning and/or decoupling along sub-orthogonal NNE-dipping reverse faults.

Application of geomorphic indices to Address the foreland Himalayan tectonics and landform deformation- Matiali-Chalsa- Baradighi recess, West Bengal, India

In the Himalayan foreland basin of the Tista-Jaldhaka system of North Bengal, the presence of Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT) along an E-W direction has been reanalysed in the context of tectonic instability especially along the 17.93 km stretch of the Kurti River. It is a small rain-fed stream that originated in the piedmont zone near the Samsing scarp (lineament trending E-W). This work discusses in detail the control of active Matiali, Chalsa, and Baradighi faults that has been determined by the river morphometric aspects, deformed landform as terraces, and the valley incision mechanism over time. We document not only the fault scarps as MBT near Matiali and HFT near Chalsa, but also decipher the Baradighi fault as a local manifestation of tectonism with well-exposed scarps. A ramp anticline (Synformal zone I) exists between the northernmost part of the Matiali scarp (MBT) and towards the south near the Chalsa scarp (HFT). However, to the south of Chalsa, lies another depression zone (Synformal zone II) that extends up to the Baradighi scarp, exposed in the left (eastern) side of Kurti River. Changes in geomorphic indices such as long profile analysis, drainage basin asymmetry, hypsometric curve, valley floor width to height ratio (Vfwh), concavity index, transverse topographic symmetry factor, and sinuosity index along the Kurti River have been evaluated. Uplift-erosion cycle with valley incision between the MBT-HFT and HFT-Baradighi fault scarp connotes that the foreland tectonics emerged to be the recent evolutionary phases in the tecto-morphological study.

Deformation patterns of upper Quaternary strata and their relation to active tectonics, Po Basin, Italy

AbstractDespite increased application of subsurface datasets below the limits of seismic resolution, reconstructing near‐surface deformation of shallow key stratigraphic markers beneath modern alluvial and coastal plains through sediment core analysis has received little attention. Highly resolved stratigraphy of Upper Pleistocene to Holocene (Marine Isotope Stage 5e to Marine Isotope Stage 1) alluvial, deltaic and coastal depositional systems across the southern Po Plain, down to 150 m depth, provides an unambiguous documentation on the deformation of previously flat‐lying strata that goes back in time beyond the limits of morphological, historical and palaeoseismic records. Five prominent key horizons, accurately selected on the basis of their sedimentological characteristics and typified for their fossil content, were used as highly effective stratigraphic markers (M1 to M5) that can be tracked for tens of kilometres across the basin. A facies‐controlled approach tied to a robust chronology (102 radiocarbon dates) reveals considerable deformation of laterally extensive nearshore (M1), continental (M2 and M3) and lagoon (M4 and M5) marker beds originally deposited in a horizontal position (M1, M4 and M5). The areas where antiformal geometries are best observed are remarkably coincident with the axes of buried ramp anticlines, across which new seismic images reveal substantially warped stratal geometries of Lower Pleistocene strata. The striking spatial coincidence of fold crests with the epicentres of historic and instrumental seismicity suggests that deformation of marker beds M1 to M5 might reflect, in part at least, syntectonically generated relief and, thus, active tectonism. Precise identification and lateral tracing of chronologically constrained stratigraphic markers in the 14C time window through combined sedimentological and palaeoecological data may delineate late Quaternary subsurface stratigraphic architecture at an unprecedented level of detail, outlining cryptic stratal geometries at the sub‐seismic scale. This approach is highly reproducible in tectonically active Quaternary depositional systems and can help to assess patterns of active deformation in the subsurface of modern alluvial and coastal plains worldwide.

Eastern Boulder-Weld fault zone, Colorado: A gravity slide with pop-up structures

The Boulder-Weld fault zone, located southeast of Boulder, Colorado, is about 10 km (6 mi) wide, 34 km (21 mi) long, and involves at least 335 m (1100 ft) of upper Cretaceous sedimentary rocks. It affects the Cretaceous upper Pierre Shale, Fox Hills Sandstone, and the coal–bearing lower Laramie Formation. This study is a detailed examination of the eastern portion of the fault zone which consists of undisturbed areas separated by three long, narrow, fault-bounded uplifts that have received a variety of interpretations over the years. The fault zone geometry is determined from 21 closely spaced cross sections that use more subsurface data than previous studies, incorporate the elevations of the major economic coal seam derived from a published composite structure-contour map, and are area balanced using area-depth-strain (ADS) analysis. The most common structural style is a pop-up structure in which the uplifts are bounded on both sides by reverse faults. At larger-displacement the pop-ups are at the tip of the ramp and a second fault has formed close to the base of the ramp. A few sections show simple ramp anticlines developed above listric thrusts. The lower detachment for all structures is the distinctive Kp2 marker in the upper Pierre Shale. ADS analysis of the best-controlled uplifts shows that the uplifts are area balanced and confirms the lower detachment to be near Kp2. The structures are interpreted to have formed as a gravity slide because they formed in a break-back sequence, a characteristic of gravity gliding, and because the transport direction is approximately down the current southeast dip of the Kp2 detachment.

Active tectonics in the Moulay Idriss Massif (South Rifian Ridges, NW Morocco): New insights from geomorphic indices and drainage pattern analysis

The Moulay Idriss Ramp anticlines of the South Rifian Ridges are located in the front of the Moroccan Rif Belt (southern branch of the Arc of Gibraltar) and have accommodated the tectonic escape of this Alpine chain as an active edge, in response to the converging Eurasian and African plates. Nevertheless, recent evidences of this landscape tectonic activity remain poorly defined. We investigated the variations in tectonic activity and evaluated its influence on the landscape evolution, through the application of geomorphic indices including: the mountain front sinuosity, valley floor width-to-height ratio, drainage basin asymmetry factor, hypsometric curves, normalized steepness index, longitudinal river profile, and χ as an indicator of drainage network disequilibrium. Morphotectonic study shows that fault related folds, in the area, produce differential uplift with a relatively high degree of tectonic activity along the eastern part of the massif. The doming process accounts for the elevation of the area, in the Dehar en Nsour salt dome, and this in turn, could be explained as being promoted by the Nzala des Oudayas fault system. On the other hand, the Moulay Idriss fault exerts a strong influence on the morphology of the Fert El Bir anticline inducing the Khoumane river deflection, while other streams exhibit rejuvenation and water divide migration. These new data elucidate the recent tectonic evolution of folds and strike-slip fault systems in the southern front of the Gibraltar-Rif southern edge, which contributes to the movement of the Rif belt with respect of the stable Africa, and sheds new light on the ‘‘Meknes’’ earthquake, confused with the contemporaneous 1755 Lisbon earthquake.

Detachment fold vs. ramp anticline: a gravity survey in the southern Pyrenean front (External Sierras)

The Santo Domingo Anticline (External Sierras, Southern Pyrenees), which separates the Jaca piggyback basin from the Ebro foreland basin, is a key structure of the Pyrenees. Its geometry has been interpreted both as a detachment fold and as a hangingwall anticline associated with an underlying thrust. In this paper, we present the results from a gravity survey and 2.5D gravity modelling carried out around the Santo Domingo Anticline. Density measurements indicate a sharp density contrast between the Triassic evaporites‐mudstones in the core of the anticline and the sedimentary sequence (limestones and sandstones) at its limbs. Gravity anomalies together with 2.5D gravity models allow to discern the along‐strike structural changes. From east to west, we document a change from the ramp‐associated fold to the detachment anticline. The capabilities and limitations of the gravimetric method for the determination of fold geometry are also discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Thrust tectonics in the central part of the External Hellenides, the case of the Gavrovo thrust

Thrust faulting plays an important role in the structural deformation of Gavrovo and Ionian zones in the central part of the ‘External Hellenides’ fold-and-thrust belt. The Skolis mountain in NW Peloponnese as well as the Varassova and Klokova mountains in Etoloakarnania are representative cases of ramp anticlines associated with the Gavrovo thrust. Surface geology, stratigraphic data and interpretation of seismic profiles indicate that it is a crustal-scale thrust acted throughout the Oligocene time. It is characterized by a ramp-flat geometry and significant displacement (greater than 10 km). Out of sequence thrust segmentation is inferred in south Etoloakarnania area. Down flexure and extensional faulting in the Ionian zone facilitated the thrust propagation to the west. The thrust emplacement triggered halokenetic movement of the Triassic evaporites in the Ionian zone as well as diapirisms that were developed in a later stage in the vicinity of the Skolis mountain.

Seismic geomorphology of cretaceous megaslides offshore Namibia (Orange Basin): Insights into segmentation and degradation of gravity-driven linked systems

This study applies modern seismic geomorphology techniques to deep-water collapse features in the Orange Basin (Namibian margin, Southwest Africa) in order to provide unprecedented insights into the segmentation and degradation processes of gravity-driven linked systems. The seismic analysis was carried out using a high-quality, depth-migrated 3D volume that images the Upper Cretaceous post-rift succession of the basin, where two buried collapse features with strongly contrasting seismic expression are observed. The lower Megaslide Complex is a typical margin-scale, extensional-contractional gravity-driven linked system that deformed at least 2 km of post-rift section. The complex is laterally segmented into scoop-shaped megaslides up to 20 km wide that extend downdip for distances in excess of 30 km. The megaslides comprise extensional headwall fault systems with associated 3D rollover structures and thrust imbricates at their toes. Lateral segmentation occurs along sidewall fault systems which, in the proximal part of the megaslides, exhibit oblique extensional motion and define horst structures up to 6 km wide between individual megaslides. In the toe areas, reverse slip along these same sidewall faults, creates lateral ramps with hanging wall thrust-related folds up to 2 km wide. Headwall rollover anticlines, sidewall horsts and ramp anticlines may represent novel traps for hydrocarbon exploration on the Namibian margin.The Megaslide Complex is unconformably overlain by few hundreds of metres of highly contorted strata which define an upper Slump Complex. Combined seismic attributes and detailed seismic facies analysis allowed mapping of headscarps, thrust imbrications and longitudinal shear zones within the Slump Complex that indicate a dominantly downslope movement of a number of coalesced collapse systems. Spatial and stratal relationships between these shallow failures and the underlying megaslides suggest that the Slump Complex was likely triggered by the development of topography created by the activation of the main structural elements of the lower Megaslide Complex.This study reveals that gravity-driven linked systems undergo lateral segmentation during their evolution, and that their upper section can become unstable, favouring the initiation of a number of shallow failures that produce widespread degradation of the underlying megaslide structures. Gravity-driven linked systems along other margins are likely to share similar processes of segmentation and degradation, implying that the megaslide-related, hydrocarbon trapping structures discovered in the Namibian margin may be common elsewhere, making megaslides an attractive element of deep-water exploration along other gravitationally unstable margins.

Quality control and risk assessment of seismic profiles using area-depth-strain analysis

Area-depth-strain (ADS) analysis is a method for quantifying the structural style, balance, boundary displacement, detachment locations, and subseismic strain from a seismic profile, information that is particularly useful for quality control and risk analysis. The method is based on measurements of excess area, width, depth, and bed length of multiple horizons in a structure. A balanced structure is indicated by a well-defined line or lines on an area-depth graph. The structural uncertainty or risk of the interpretation is quantified using the fit of the data to the least-squares line(s), the match between the ADS detachment values and the interpreted geometry, and the magnitudes of the calculated strains. The method also clearly separates syntectonic (growth) units from nontectonic (pregrowth and no-growth) units. Different area-depth graph styles represent (1) classic detachment folds, fault-propagation folds, and ramp anticlines; (2) buckle-style detachment folds; and (3) fault-bend folds. The focus here is on the first two. Numerical models of the detachment fold styles show the similarities and differences between their geometries and ADS interpretations. Both styles are evaluated with seismic profiles across oilfield-scale structures. The Alpha/Bobo field, Nigeria, is a classic detachment fold, and an Angolan anticline is of the buckle style. Profiles across the Alpha/Bobo field before and after the first well was drilled demonstrate the improvement in the ADS interpretation of the lower detachment location and the reduction of layer-parallel strains in the revised profile. The Alpha/Bobo field and the Angola fold illustrate the distinction between growth and no-growth intervals and the interpretation of growth strata. Both fields show the use of predicted versus observed ADS results to suggest possible improvements to the interpretation.

Controls on timing of exhumation and deformation in the northern Peruvian eastern Andean wedge as inferred from low-temperature thermochronology and balanced cross section

In northern Peru, a 500 km long regional balanced section has been constructed across the eastern Andean wedge, using fieldwork, industrial seismic sections, and wells. The structure is characterized by a thin-skinned thrust system involving the Eastern Cordillera (EC), the sub-Andean zone (SAZ), and the Maranon foredeep. In the SAZ and the easternmost foredeep the development of the thrust system has been driven by the combination of two structural events. Permian thrust faults had been reactivated to form a basement duplex underlying the SAZ and the foredeep. At the same time a Triassic-Jurassic extensional basin has been transported as a crustal ramp anticline on to the duplex roof fault, giving rise to the EC. The impingement of the EC was responsible for the deformation of the SAZ and the propagation of the thrust wedge. The minimum shortening calculated is 142 km, representing a shortening strain of ~ −28%. A sequential restoration calibrated by (U-Th)/He and Fission Track dating on apatites and vitrinite reflectance values shows that shortening rates vary from 7.1 mm yr−1 between 17 and 8 Ma to 3.6 mm yr−1 between 8 Ma and today and suggests that the thrust wedge commenced propagation between 30 and 24 Ma. When compared with other Andean thrust wedges, we suggest that the timing of the thrust wedge propagation is not a simple function of the distance to the hinge of the Bolivian orocline and the propagation is not controlled by the precipitation regime. We rather suggest that reactivated basement faults favored thrust wedge propagation.

The origin and internal structures of submarine-slide deposits in a lower Pleistocene outer-fan succession in the Kazusa forearc basin on the Boso Peninsula of Japan

This study investigates the internal geometry and formation processes of submarine-slide deposits in a lower Pleistocene outer-fan succession in the Kazusa forearc basin on the Boso Peninsula of Japan. The submarine-slide deposits are ~40m thick, with a minimum length of ~900m and a width of ~700m. Both the submarine-slide deposits and host deposits comprise siltstones intercalated with very thin- to medium-bedded, sheet-like turbidites and volcanic ash beds. Based on the sequence-stratigraphic framework of the submarine-fan succession, we conclude that the submarine-slide deposits formed during a glacioeustatic sea-level lowstand at about 1.16Ma.The submarine-slide deposits are characterized by thrust sequences with a ramp anticline in the frontal part. A basal slide plane in the lower part of the deposits is developed at a horizon located 2–4cm below the base of a coarse volcanic ash bed and is associated with sheared deposits. Slide planes are sealed in the upper part of the submarine-slide deposits in association with drag folds and chaotic deposits. Finally, the submarine-slide deposits are transitionally overlain by ~3-m-thick chaotic muddy deposits, and are finally overlain by siltstones intercalated with very thin- to medium-bedded, sheet-like turbidites and volcanic ash beds, which show lithofacies features similar to those of the submarine-slide deposits. The variations in the deformation styles indicate that sliding occurred as a synsedimentary process in the outer-fan environment, and the basal slide plane formed when the porosity of the muddy deposits was reduced to ~55% or less.Based on the empirical relationship between the submarine-fan length and lower-fan slopes from modern examples, the gradient of the outer-fan is estimated at 0.31°–0.46°, which is lower than the threshold gradient of 1.2° for a 40-m-thick submarine slide with the estimated basal porosity. Based on the distribution of deformed deposits within the lower-fan host deposits, the minimum volume and runout distance of the submarine-slide deposits are estimated to be ~3×107m3 and ~5km, respectively. Furthermore, based on data from modern submarine slides, the elevation difference between the failed mass in both the starting and depositional areas is estimated to be 80–170m. Thus, the initiation of a submarine slide in the outer-fan environment was likely influenced by a combination of external forces, such as seismic shaking and increased pore-water pressure in association with the seepage of gas and water from underlying deposits in the Kazusa forearc basin. This study suggests that submarine slides occur in distal and low-gradient deep-water environments in active-margin basins.

Structural overprinting of Mesozoic thrust systems in eastern California and its importance to reconstruction of Neogene extension in the southern Basin and Range

Geologic observations from the Resting Spring and Nopah Ranges (California, USA) together with a synthesis of regional data indicate that previous reconstructions of the Death Valley extensional terrane need to be revised because they do not account for three-dimensional, preextensional structures that complicate structural markers used in these reconstructions. This conclusion arises from detailed mapping that indicates structural overprinting of early northeast-trending fold-thrust systems of the Sevier orogenic belt by younger northwest-trending structures. The northwest-trending fold-thrust system includes a series of folds and a major thrust that includes the Nopah Peak and the Gerstley faults. This thrust fully involved crystalline basement, and there are extensive basement exposures in the hanging wall of the thrust in the southern Nopah Range. Basement involvement presumably produced a large ramp anticline to the southwest of the Nopah Range in what is now the Death Valley extensional terrane. The existence of this ramp anticline is supported by occurrences of successively older rocks to the southwest, beneath the Tertiary unconformity. The northwest-trending fold-thrust system is also recognizable to the north and east of the Gerstley–Nopah Peak fault as a series of west-northwest– to northwest-trending folds that have been known for more than 30 years, but were overlooked in earlier reconstructions. We use our observations together with recently published data on the State Line fault and extensional structure in Pahrump Valley to consider two preliminary map-view restorations of the Resting Spring and Nopah Ranges relative to the Spring Mountains, a relatively unextended block to the east. These restorations place the northwest-trending thrust system along strike from the southern Spring Mountains, where a similar overprinting is observed, supporting the basic restoration. One restoration that strictly adheres to published estimates for motion across the Pahrump Valley suggests, however, that the widely cited correlation of the Wheeler Pass and Chicago Pass thrust system is highly suspect. Our preferred interpretation concludes that the Chicago Pass–Shaw thrust system of the Resting Spring∼Nopah Ranges is an along-strike equivalent of the Lee Canyon thrust; the Wheeler Pass thrust correlates to the Montgomery Mountains thrust; and both thrust systems have an uncertain continuation into the Death Valley extensional terrane. The Resting Spring–Nopah–Spring Mountains restoration provides a template for future restorations. We emphasize that the three-dimensional preextensional geometry, together with other markers like Mesozoic magmatic belts and older sedimentary facies trends, provides an opportunity for using modern visualization and database systems to develop high-precision reconstructions using the abundance of crosscutting markers. Thus, although this study, along with other recent studies, indicates that previous reconstructions are not workable, future studies that include the full three-dimensional data could lead to a nearly unique solution to the preextensional paleogeography.

Quantifying the geometry, displacements, and subresolution deformation in thrust-ramp anticlines with growth and erosion: From models to seismic-reflection profile

We quantify the evolution of thrust-ramp anticlines exhibiting growth and/or erosion using the area-depth-strain (ADS) method. We illustrate, validate, and explore the limitations of the ADS method for a series of kinematic models and experimental sand (analog) models. We then apply the method to a seismically imaged thrust-ramp anticline from the deep-water Niger Delta. The ADS method allows determination of the boundary displacements, displacement rates, and layer-parallel strains using the excess areas (areas of marker beds above their regional level) of multiple horizons. The kinematic and experimental models demonstrate that excess areas for pre-deformation units lie on a straight line on an area–depth graph, the inverse slope of which is the boundary displacement and the intercept is the elevation of the basal detachment. For growth strata, the line connecting an area-depth value to the value representing the basal detachment gives the displacement after deposition of that horizon. For partially eroded markers, the calculated displacements are less than the actual displacements; however, originally planar unconformities yield the displacements that accrued after the formation of the unconformities. The experimental sand models show substantial layer-parallel shortening in the anticlines, displacements on the main thrust ramp that are only half the displacement on the basal detachment, and small tectonic dilations. The seismic example is an area-balanced ramp anticline with clearly defined pre-deformation and growth sequences. Fault separation at the center of the ramp in the pre-deformation interval is approximately equal to the total boundary displacement but decreases both upward and downward. The layer-parallel strains have the opposite pattern, with the lowest values at the center of the ramp and increasing values both upward and downward.

The Confusion Range, west-central Utah: Fold-thrust deformation and a western Utah thrust belt in the Sevier hinterland

The Confusion Range in west-central Utah has been considered a broad structural trough or synclinorium with little overall shortening. However, new structural studies indicate that the Confusion Range is more accurately characterized as an east-vergent, fold-thrust system with ∼10 km of horizontal shortening during Late Jurassic to Eocene Cordilleran contractional deformation. For this study, four balanced and retrodeformable cross sections across the Confusion Range and adjacent Tule Valley were constructed using existing mapping and new field data, and these were tied with a fifth strike-parallel section. Ramp anticlines and anticlinal duplexes characteristic of strong Lower Paleozoic carbonate units are balanced by faulted and rotated detachment folds in more ductile Upper Paleozoic strata. The apparently synclinal aspect results from two different sets of thrust structures that uplift and expose Lower Paleozoic rocks on the flanks of the range. Fold-thrust structures are continuous southward for more than 130 km, forming a thrust belt of regional extent herein named the western Utah thrust belt. This thrust belt is comparable in length and magnitude of shortening to the central Nevada thrust belt, and both merge southward with the Sevier frontal thrust belt. Together, these and related fold-thrust zones in eastern Nevada indicate significant, broadly distributed Mesozoic shortening in the Sevier hinterland. The Cordilleran thrust belt in eastern Nevada and western Utah thus consists of a frontal zone—the Sevier frontal thrust belt—where major thrust faults with 50–100 km of displacement breach the surface, and a hinterland zone, characterized by a number of distributed fold-thrust belts, each accommodating on the order of 10 km of shortening.

Geomorphology in relation to tectonics: A case study from the eastern Himalayan foothills of West Bengal, India

The effects of neotectonic activity on geomorphic features have been studied in a large alluvial fan in the foothills area of the eastern Himalayas. The interfluve area between the rivers Mal and Murti is an alluvial fan composed of Quaternary sediments characterized by clay, sand, pebble, and boulder beds. Most of the river valleys in this area show well developed terraces. There are four major terrace surfaces, named as T1, T2, T3 and T4 according to increasing height from the river bed. Two E–W scarps named as Matiali and Chalsa scarps that cut across the fan represent traces of the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT) respectively. There are two other NNW–SSE and NNE–SSW lineaments which have partially guided the courses of the Neora and Murti rivers. These are interpreted as conjugate sets of normal faults transverse to the orogenic trend. The E–W scarps are the manifestation of the frontal limbs of the ramp anticlines over two blind thrusts. Fault propagation folding has affected the fan surface. Recurrent movements on the thrusts and consequent downcutting of the rivers have led to the formation of the raised terraces on the banks of these rivers. The terraces are formed by cut-and-fill process. Later transverse normal faulting has given rise to a horst of the Neora–Mal interfluve block.

Tectonic control on the drainage system in a piedmont region in tectonically active eastern Himalayas

The impact of neotectonic activity on drainage system has been studied in a large alluvial fan in the eastern Himalayan piedmont area between the Mal River and the Murti River. Two distinct E-Wlineaments passing through this area had been identified by Nakata (1972, 1989) as active faults. The northern lineament manifested as Matiali scarp and the southern one manifested as Chalsa scarp represent the ramp anticlines over two blind faults, probably the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), respectively. The fan surface is folded into two antiforms with a synform in between. These folds are interpreted as fault propagation folds over the two north dipping blind thrusts. Two lineaments trending NNE-SSW and nearly N-S, respectively, are identified, and parts of present day courses of the Murti and Neora Rivers follow them. These lineaments are named as Murti and Neora lineaments and are interpreted to represent a conjugate set of normal faults. The rivers have changed their courses by the influence of these normal faults along the Murti and Neora lineaments and their profiles show knick points where they cross E-W thrusts. The overall drainage pattern is changed from radial pattern in north of the Matiali scarp to a subparallel one in south due to these conjugate normal faults. The interfluve area between these two rivers is uplifted as a result of vertical movements on the above mentioned faults. Four major terraces and some minor terraces are present along the major river valleys and these are formed due to episodic upliftment of the ground and subsequent down-cutting of the rivers. The uppermost terrace shows a northerly slope north of the Chalsa scarp as a result of folding mentioned above. But rivers on this terrace form incised channels keeping their flow southerly suggesting that they are antecedent to the folding and their downcutting kept pace with the tectonism.

Structural evolution of the frontal fold–thrust belt, NW Himalayas from sequential restoration of balanced cross-sections and its hydrocarbon potential

Abstract Two sedimentary lithotectonic zones are traditionally recognized in the northwestern Himalayan frontal fold–thrust belt in the Nahan salient: the Lesser Himalaya Zone (LHZ) and the Sub-Himalaya Zone (SHZ). The LHZ is made up of a sequence of Proterozoic to Early Cambrian rocks and the SHZ is made up of Cenozoic rock sequences, which were deposited subsequent to the India–Asia collision. Serial balanced cross-sections show that the structural geometries become increasingly complex from independent ramp anticlines near the foreland through imbricate fan/duplex to stacked-up horses towards the hinterland. Sequential restoration suggests a structural evolution in which a foreland propagating, in-sequence thrusting event was followed by out-of-sequence thrusting in an approximately break-back style. During the out-of-sequence movement, some of the ramps formed during in-sequence thrusting were repeatedly reactivated, leading to very complex structural geometries, particularly in the LHZ. In such a complexly deformed terrain, a rigorous structural modelling approach, combined with a robust geochemical and geochronological database, should be used to carry out calibrated petroleum system modelling, and thus reduce exploration risk.

Structural styles in the Papuan Fold Belt, Papua New Guinea: constraints from analogue modelling

Abstract Cross sections, seismic data and centrifuge analogue modelling reveal the structural styles in the oil-producing areas of the Papuan Fold Belt. They include inverted basement faults, detachment faults in the Jurassic section 1–2 km beneath the Neocomian Toro Sandstone reservoir, and tight, overturned folds in the reservoir sequence with stretched and boudinaged forelimbs, cut by break-thrusts. Additional features include highly variable thicknesses in the Cretaceous Ieru Formation, the regional seal sequence, including through-going detachments that isolate the overlying thick Miocene Darai Limestone. Centrifuge analogue modelling of intact, plane-layered strata determined that the mechanical stratigraphy and the thickness of weak beds above the lower décollement horizon exert the greatest control on the structural style. Large-offset thrust faults were only produced in models with pre-cut faults, generating early inversion and then large ramp anticlines, similar to those in the Kutubu Oilfield, which has reserves of >350 million barrels. It is suggested that the Kutubu Oilfield trend was underlain by a large normal fault and that, by analogy with the Vulcan Sub-basin, oil-rich source rocks may be confined to the hanging wall or north side of this fault. Oil would have been generated and expelled during thin-skinned deformation.

Messinian-Early Pliocene crustal shortening along the Tyrrhenian margin of Tuscany, Italy

This paper illustrates the results of structural studies carried out in the western margin of Tuscany along a major crustal structure. Surface deformation of sediments filling different basins aligned on top of this major structure (from north to south: the Fine Basin, the Sassa-Guardistallo basin, the Rio Guardigiano area in the Lustignano basin) allow us to date its tectonic activity to the Messinian-Early Pliocene. In these areas, structures such as reverse and strike-slip faults and mesoscopic folds are widely developed. Structural analysis determined a compressive stress field with the σ1 oriented from E-W to NE-SW active from Messinian to Early Pliocene. At the southern end of this crustal structure, the Gavorrano antiform and the granitic pluton (radiometric age of granite ∼4.4 Ma) coring this fold correlate with a thrust ramp anticline at depth, and thus constrain thrust activity to the Early Pliocene. These data document a Messinian-Early Pliocene compressive activity that contrasts with models invoking continuous extensional tectonics affecting the hinterland since the Late Oligocene-Middle Miocene in the frame of a back-arc-slab retreating process. The results presented therefore raise the question of which geodynamical model could account for such a complex structural evolution of Northern Apennines hinterland.