Oblique Ramp Research Articles

Structural inheritance through 3D reservoir modeling and ant-tracking attribute implications for structural configuration of Upper Cretaceous Pab Sandstone, Kirthar Fold Belt, Pakistan

This research presents a comprehensive investigation to enhance understanding of subsurface structural complexities within the Upper Cretaceous Pab Sandstone reservoir level in a gas-producing field within the Kirthar Fold Belt of the Southern Indus Basin, Pakistan. The seismic interpretation for accurate structural delineation and characterization of intricate fault systems is often challenging in Fold and Thrust Belt settings. The 3D structural maps of Upper Cretaceous Pab Sandstone at the reservoir level and the application of the ant-tracking attribute for fault extraction improve the structural understanding of the gas-producing field in the Kirthar Fold Belt in the Southern Indus Basin, Pakistan. Attribute-assisted 3D seismic interpretation has revealed several subsurface structure elements, including a large thrusted anticline extended towards the north-south, the pattern of north-south oblique ramp thrusts on the southeastern flank, and a combination of easterly vergent thrusts with a counter-back thrust creating a local pop-up structure in the area. A 3D structural model at the reservoir level was generated using horizon and fault framework volume-based modeling approach, serving as a structural model for Pab sandstone. The structural smoothing, variance and ant-tracking attribute were applied iteratively to get the best results and correlated with manual interpretation. 3D seismic interpretation and application of ant-tracking for fault extraction identifies discrete structural styles and revealed that all thrusting occurred due to compressional tectonics of the Plio-Pleistocene period. The apparent different styles result from the reactivation of earlier extensional fault systems and probably during phased periods of compression. The resulting 3D reservoir model can be used as an input to populate petrophysical properties and results can be extended for future field development plans. Integration of well and 3D seismic data interpretation and the application of automatic fault extraction techniques are highly recommended in structurally complex areas and have an equal implication for worldwide basins with similar conditions for reliable structural interpretation in Fold and Thrust Belts with structural complexity.

Tectonostratigraphic model of the Fila Costeña in southern Central America based on new biostratigraphic data: implications of a two-thrust-fault hypothesis

Biostratigraphic determinations in the surroundings of the Térraba River, Fila Costeña, Costa Rica, lead to a two-thrust-fault model to explain the distribution of sedimentary rocks. Eocene limestones of the Fila de Cal Formation appear as a single succession with a NE-SW predominant strike, and a thrust fault is required to explain their presence above other Oligocene rocks. Another thrust fault explains the outcrop of Oligocene rocks above presumable Miocene strata. A structural model based on surface data suggests a shallower basement to the southwest and deeper to the northeast; both parts are connected by a frontal thrust ramp, an oblique ramp (close to frontal) and a lateral ramp. In that sense, the superficial trend of thrust faults is interpreted as the result of slip along these thrust ramps. Based on these assumptions, minimum NE-SW shortening estimations vary between 8.3 and 22.2 km in the study area. This underlines the importance of geologic fieldwork and biostratigraphic determinations for structural modeling and minimum shortening estimations of the Fila Costeña and other sedimentary successions for which these approaches are possible.

The Geometry and Kinematics of the Latest Paleozoic Allatoona Fault, One of the Youngest Thrusts in the Southernmost Appalachian Hinterland, Alabama and Georgia, U.S.A.

The Allatoona thrust fault in the southernmost hinterland of the Appalachian Blue Ridge-Piedmont megathrust sheet is among the latest structures in the kinematic sequence of events along the west flank of the orogen. It is an out-of-sequence, craton-directed thrust fault that cuts metamorphic isograds and earlier thrusts, and it has a nearly linear trace of ≥280 km, making it one of the major thrust faults in the orogen. On the northwest, the fault cuts Pennsylvanian or younger(?) regional cross antiforms that cause significant orogenic curvature of older underlying thrust sheets and is likely Permian in age. To the southeast, however, units within the fault hanging wall maintain a nearly constant width resulting in a significant change in the regional structural architecture of the orogen. In the central segment of the fault, where it marks the western/eastern Blue Ridge domain boundary, a ~20 km-long eyelid window (Mulberry Rock window) framed by three amphibolite facies thrust sheets overlying the greenschist facies Talladega belt allochthon, allows a 3-D view into the structural architecture, kinematics, and trajectories of the regional thrusts. Two earlier thrusts within the window (Mulberry Rock and Burnt Hickory Ridge thrusts, with a combined minimum horizontal net slip component of 27 km) are cut by the Allatoona fault, which is a ~15 m-wide high strain zone with top-to-the-northwest displacement, and a >17.2 km horizontal net slip vector. Structural branch points between the Allatoona and Mulberry Rock thrusts indicate that the Mulberry Rock allochthon is a large north-trending horse beneath the Allatoona fault, centered on the Mulberry Rock window, which is likely the result of oblique ramp thrusting over the massive Mulberry Rock Gneiss. The Allatoona fault cuts down obliquely into the tectonostratigraphy progressively deeper both to the northeast and northwest, locally approaching underlying foreland thrust sheets, and cutting older regional structures. To the northeast, the Allatoona fault lies at the base of the Dahlonega gold belt, becoming an internal eastern Blue Ridge thrust at Dawsonville, Georgia. Although that sequence extends another 120 km into North Carolina, continuation of the Allatoona fault that additional distance is in debate. Regardless, the Allatoona is one of the kinematically latest and longest faults in the southern Appalachian orogen.

Megathrust Heterogeneity, Crustal Accretion, and a Topographic Embayment in the Western Nepal Himalaya: Insights From the Inversion of Thermochronological Data

AbstractBetween 81°30ʹE and 83°E, the Himalayan range's “perfect” arcuate shape is interrupted by an embayment. We hypothesize that thrust geometry and duplexing along the megathrust at midlower‐crustal depths play a leading role in growth of the embayment as well the southern margin of the Tibetan plateau. To test this hypothesis, we conducted thermokinematic modeling of published thermochronologic data from the topographic and structural embayment in the western Nepal Himalaya to investigate the three‐dimensional geometry and kinematics of the megathrust at midlower‐crustal depths. Models that can best reproduce observed cooling ages suggest that the megathrust in the western Nepal Himalaya is best described as two ramps connected by a long flat that extends further north than in segments to the east and west. These models suggest that the high‐slope zone along the embayment lies above the foreland limb of an antiformal crustal accretion zone on the megathrust with lateral and oblique ramps at midlower‐crustal depths. The lateral and oblique ramps may have initiated by ca. 10 Ma. This process may have controlled along‐strike variation in Himalayan‐plateau growth and therefore development of the topographic embayment. Finally, we analyze geological and morphologic features and propose an evolution model in which landscape and drainage systems across the central‐western Himalaya evolve in response to crustal accretion at depth and the three‐dimensional geometry of the megathrust. Our work highlights the importance of crustal accretion at different depths in orogenic‐wedge growth and that the midlower crustal accretion determines the location of plateau edge.

Horizontal/vertical motion and wedge geometry of the Sinú Fold Belt, South-Caribbean accretionary prism, NW Colombia: Implications for a morphostructural zoning

The Sinú Fold Belt, located in the southwestern part of the South Caribbean Deformed Belt, northwest Colombia, has been interpreted as the outer portion of an accretionary wedge resulting from the subduction of the Caribbean Plate under the South American Plate. Several geological sections, across and along the offshore portion of the Sinú Fold Belt, have been constructed through geological interpretation of 2D seismic sections constrained by a few exploration wells. Their analysis has allowed us to quantify the amount of horizontal motion (shortening responsible for the structures and basal detachment displacement) and vertical motion (tectonic subsidence/uplift and wedge vertical thickening) undergone by the belt over time; unravel its recent evolution consisting of almost no active structures in the south edge and a large amount of active structures northwards; quantify its pore pressure, coefficient of friction and strength using its wedge geometry consisting of high taper angles in the lower continental slope near the deformation front and lower taper angles in the upper continental slope away from it; and understand the aseismic and trench-lacking character of the belt. These observations have been crucial to identify the main features of the morphostructural zones across the belt (Colombian Basin, lower continental slope, upper continental slope and continental shelf) and establish a new along-strike zoning (north region influenced by the Magdalena Fan/Canoas Oblique Ramp, Sinú Fold Belt s.s., and south salient influenced by the Uramita Fault).

Mechanism of fuel mixing of hydrogen multi-jet in presence of upstream convergent/divergent ramp at supersonic flow

The enactment of the fuel mixing structure is crucial for the advance of supersonic vehicles. All-inclusive efforts have been done to disclose the impacts of different parameters on instrument of the fuel combination with air within the combustion chamber. In the present work, comprehensive computational investigations have been done to explore the importance of oblique ramp upstream on the fuel mixing process of hydrogen multi-jets at supersonic cross airflow. The primary attention of the current study is to compare the role of interaction of air and fuel by the existence of an oblique ramp upstream of four cross jets. Flow analysis is also done to unveil the main difference of convergent and divergence ramps located upstream of each injector. For simulation of the proposed models, Computational Fluid Dynamics (CFD) is employed to resolve RANS equations with the SST turbulence model in high-speed free stream. The main significant factors i.e. mixing efficiency and circulation factor are also compared in our work for comparison of the flow parameters and mixing concepts. According to our investigations, the presence of the upstream oblique jet meaningfully enhances the fuel mixing as flow moves downstream of injectors. The outcomes also showed that productivity of the divergent ramp is higher than that of the convergent one due to high jet diffusion in the depth of the domain by the creation of a strong horseshoe vortex.

Modes of Oblique Inversion: A Case Study From the Cretaceous Fold and Thrust Belt of the Western Transdanubian Range (TR), West Hungary

AbstractInversion of pre‐existing normal faults is a frequent phenomenon in orogenic belts. However, their impact on the evolution of the Cretaceous Eoalpine orogeny is poorly studied, despite the widespread presence of Triassic and Jurassic normal faults. Our study area is the SW part of the Transdanubian Range (TR), which represents the uppermost thick‐skinned nappe of the Eoalpine orogeny in the Alpine realm. The variable trend of Cretaceous contractional structures within the Transdanubian Range has been traditionally explained by poly‐phase shortening. However, based on the combined analyses of field observations and 3D seismic data, we found that the complex geometry of Cretaceous contractional structures can be explained rather by the oblique inversion of segmented Late Triassic grabens. In general, the SW part of the TR can be characterized by N‐S trending contractional structures. NW‐SE striking oblique ramps are nucleated above Late Triassic normal faults, whereas NE‐SW trending folds and thrust with limited strike‐parallel length evolved due to the inversion of pre‐existing breached relay ramps, which connected the segments of Late Triassic normal faults. We determined different types of oblique ramps, incuding normal faults, which suffered transpressional reactivation, oblique hanging wall breakthrough thrusts, oblique footwall short‐cut and hanging wall by‐pass thrusts. Only very steep normal faults, or those that strike at low angle to the regional E‐W shortening direction, were not reactivated.

Influence of ramp geometry and orientation on fault propagation folding: Insights from the 3D finite element method

Structural geologists have extensively studied fault-related folds because of their diversity, economic significance, and relationship to seismicity. In this study, we present five series of 3D finite-element models (A, B, C, D, and E) to investigate the effect of ramp geometry and orientation, as well as the friction coefficient, on the evolution of the geometry of fault-propagation folds (FPFs) in a convergent system. Series A, B, C, D, and E examine the effect of ramp dip, listric ramp, concave ramp, oblique ramp, and friction coefficient on FPFs. The results of the models show that 1) although the fold amplitude depends on the slip and dip of the fault, increasing them does not always increase the amplitude, 2) The slip and amplitude gradients become larger in the case of listric faults compared to angular faults of similar dip, 3) The amplitude uplift contours for series A, B, and E are parallel to each other, hence these series produce inclined folds while for series C and D these contours are not parallel, so these series create double-plunging and plunging folds, respectively, 4) The influence of variation in along-strike (Series C and D) and parameters that affect the entire FPFs (Series E) on the structural evolution of the FPFs are greater than variation in the cross-section (Series A and B) so that the concave ramps (Series C) and variation in frictions (Series E) change the folding styles. Hence, these parameters are two key factors in the style of fault-related folding.

Paleoseismological evidence for historical ruptures along the Meduno Thrust (eastern Southern Alps, NE Italy).

We carried out new geological, morphotectonic, geophysical and paleoseismological investigations on the Meduno Thrust that belongs to the Pliocene-Quaternary front of the eastern Southern Alps in Friuli (NE Italy). The study area is located in the Carnic Prealps, where a series of alluvial terraces, linked to both climatic and tectonic pulses characterises the lower reach of the Meduna Valley. In correspondence of the oblique ramp of the Meduno Thrust, the Late Pleistocene Rivalunga terrace shows a set of scarps perpendicular to the Meduno valley, often modified by human activity. In order to reconstruct the tectonic setting of the area and identify the location for digging paleoseismological trenches, integrated geophysical investigations including electrical resistivity tomography, seismic refraction and reflection, ground penetrating radar and surface wave analyses (HVSR, ReMi and MASW), were carried out across the scarps of the Rivalunga terrace. Geophysical surveys pinpointed that in correspondence of the oblique ramp, stress is accommodated by a transpressive thrust system involving all the seismo-stratigraphic horizons apart from the ploughed soil. Trenching illustrated the Meduno Thrust movements during Late Pleistocene-Holocene. Trenches exhibited both shear planes and extrados fracturing, showing deformed alluvial and colluvial units. 14C datings of the colluvial units show that the most recent fault movements occurred after 1360 CE and 1670 CE. The age of the deformed stratigraphic units compared with the earthquakes listed in current catalogues, suggests that the 1776 earthquake (Mw 5.8, Io = 8–9 MCS) could represent the last seismic event linked to the Meduno thrust activity. This study provided new quantitative constraints improving seismic hazard assessment for Carnic prealpine area.

Analogue model of the Bala Rud Fault, Zagros: an oblique basement ramp in a fold-and-thrust belt

The role of pre-existing structures during the building of the Zagros fold-and-thrust belt is a matter of debate. The orientation of the pre-existing basement faults varies in different parts of the Zagros belt. Major directions of the basement faulting in the Arabian plate and Zagros are N–S, NE–SW, NW–SE and E–W. Reactivation of these inherited pre-existing faults occurred at the Cretaceous. In this study, analogue modelling was constructed to test the effect of various orientations of the pre-existing basement faults on the deformation style in the northern boundary of Dezful Embayment. The model consisted of sand layers that partly overlay a viscous layer of silicon above wooden plates. Model results explain the development of deflection in the deformation front and the incompatibility location of faults in the basement and surface. The presence of the basal decollement horizon between the basement and overlying sedimentary sequences separated faulting geometry in the basement from sedimentary cover. Model results suggest that the flexure of the Zagros Mountain Front Faults (ZMFF) in the north part of Dezful Embayment is resulted by reactivation of pre-existing Bala Rud fault (BRF) which is an oblique ramp. The close similarity between the major structures produced during the analogue experiments and those observed in nature, suggests that BRF as a pre-existing basement fault has a major role in the deformation of the structures in the study area.

Magnetic fabric in carbonatic rocks from thrust shear zones: A study from the Northern Apennines (Italy)

We present the results of an integrated structural and anisotropy of magnetic susceptibility (AMS) investigation in calcareous and marly rocks from thrust shear zones. A total of 17 sites from six localities along the frontal and oblique ramp of the Olevano-Antrodoco-Sibillini thrust and back thrust (Northern Apennines, Italy) were studied to investigate both magnetic fabric and structural characteristics of Cretaceous to Neogene calcareous and marly rocks. In most of the sites AMS is controlled by the paramagnetic minerals (prevailingly phyllosilicates), with minor contribution of ferromagnetic (sensu latu) ones. Structural analysis shows the presence of SC- and S-tectonites associated to predominant simple and pure shear, respectively. The combination of density diagrams and cluster analysis allowed discriminating the contributions of sedimentary and tectonic processes to the bulk magnetic fabric. Six different subfabrics were distinguished, related to the structural data and associated to deformation stages and regimes: A. sedimentary fabric; B. intersection lineation; C. magnetic foliation parallel to S planes with k1 parallel to the S/C intersection; D. k1 moving toward the slip vector; E. magnetic foliation parallel either to the S or the C planes, with k1 parallel to the slip vector or the transport direction; F. k1 parallel to extensional planes intersection. The magnetic foliation has a double tendency to parallelize to pressure solution cleavage (S) and shear planes (C), while the magnetic lineation tends to progressively align with the slip vector, except for pure-shear-dominated sites at less than 15–20 cm from the thrust, where it aligns with the transport direction. The magnetic fabric is dominated by simple shear deformation. The protocol applied for AMS analysis shows a great potential to unravel mixed sedimentary and/or tectonic features in magnetic fabrics. AMS can be considered as a useful tool in unravelling the variation of simple-pure shear deformation regime in shear zones.

Fluvial terraces as quantitative markers of late Quaternary detachment folding and creeping thrust faulting in the Peruvian Huallaga basin

In the western Huallaga basin, located in the Peruvian Subandes, a series of nine fluvial terraces with elevations of up to 160 m above current floodplain level (+FP) were formed against the Biabo anticline. The terraces are progressively more folded with age and terrace elevation and present growth strata, suggesting ongoing folding of the anticline. Optically and InfraRed Stimulated Luminescence (OSL and IRSL) dating of fluvial terrace sediments yielded ages of 10.8 ± 0.7 ka for the T2, 24.2 ± 2 ka for the T4 and 46 ± 9 ka for the T7 terrace. Uniform uplift rates of 2 mm a−1 for the past 46 ka were calculated. Terrace profiles and two new seismic profiles that cross the Biabo anticline longitudinally and transversally were used to calculate shortening rates of 2.6 mm a−1 for the anticline over the past 46 ka. Our calculations took into account potential contributions of detachment thrusting parallel to the Biabo fold and reverse tear faulting perpendicular to the fold tip as visible on the seismic sections. A maximum uplift rate of 0.62 mm a−1 due to reverse thrusting of the Bellavista tear fault for the past 46 ka could have contributed to the overall uplift of the Bellavista terraces. Based on trigonometric relationships, a reverse fault slip rate of 0.86 mm a−1 and an age of 137 ka were calculated for this fault.In the eastern part of the Huallaga basin, a series of two fluvial terraces were formed with elevations of 13–15 m and 30 m + FP and with OSL ages of 6.9 ± 0.7 ka for T1 and 30.2 ± 3.1 ka for T2. The interpretation of a third new seismic profile crossing the study area in combination with field observations and trigonometric relationships shows that the terraces were formed by a complex system of oblique ramp and strike-slip faulting resulting in uplift and shortening rates of 1 mm a−1 and 1.96 mm a−1 between 30.2 ka and 6.9 ka respectively; and uplift and shortening rates of 2 mm a−1 and 3.70 mm a−1 from 6.9 ka until the present. Reverse slip of the Shapaja oblique ramp, that was responsible for uplift of the terraces, was calculated at 2.19 mm a−1 for the period 30.2 to 6.9 ka and 4.15 mm a−1 from 6.9 ka until the present. Measured total slip in combination with the calculated slip rates allowed an age calculation of 181 ka for the Shapaja thrust ramp. Our inferences showed that the studied tectonic structures absorb most of the present-day deformation in the Huallaga basin. The calculated uplift and shortening rates are compatible with published, present-day crustal movement velocities, and Miocene to present shortening rates for various, studied sites of the tropical Subandes and suggest that it is deforming at a relatively uniform rate.The timing of sedimentation of the terrace sediments may have corresponded to periods of increased precipitation which occurred coeval with the Heinrich events of the Northern Hemisphere. Terrace records similar as ours have been described for other, tectonically highly active areas in the tropical Subandes, but not in the tectonically less active Amazonian lowlands. It is suggested that uplift rates of at least 1 mm a−1 are needed to preserve suborbital climate cycles as individual terraces. The tropical Subandes could therefore be a new key area to investigate the interactions between tectonics, suborbital climate forcing and fluvial response during the late Quaternary.

Three-dimensional strain accumulation and partitioning in an arcuate orogenic wedge: An example from the Himalaya

AbstractIn this study, we use published geologic maps and cross-sections to construct a three-dimensional geologic model of major shear zones that make up the Himalayan orogenic wedge. The model incorporates microseismicity, megathrust coupling, and various derivatives of the topography to address several questions regarding observed crustal strain patterns and how they are expressed in the landscape. These questions include: (1) How does vertical thickening vary along strike of the orogen? (2) What is the role of oblique convergence in contributing to along-strike thickness variations and the style of deformation? (3) How do variations in the coupling along the megathrust affect the overlying structural style? (4) Do lateral ramps exist along the megathrust? (5) What structural styles underlie and are possibly responsible for the generation of high-elevation, low-relief landscapes? Our model shows that the orogenic core of the western and central Himalaya displays significant along-strike variation in its thickness, from ∼25–26 km in the western Himalaya to ∼34–42 km in the central Himalaya. The thickness of the orogenic core changes abruptly across the western bounding shear zone of the Gurla Mandhata metamorphic core complex, demonstrating a change in the style of strain there. Pressure-temperature-time results indicate that the thickness of the orogenic core at 37 Ma is 17 km. Assuming this is constant along strike from 81°E to 85°E indicates that, the western and central Nepal Himalaya have been thickened by 0.5 and 1–1.5 times, respectively. West of Gurla Mandhata the orogenic core is significantly thinner and underlies a large 11,000 km2 Neogene basin (Zhada). A broad, thick orogenic core associated with thrust duplexing is collocated with an 8500 km2 high-elevation, low-relief surface in the Mugu-Dolpa region of west Nepal. We propose that these results can be explained by oblique convergence along a megathrust with an along-strike and down-dip heterogeneous coupling pattern influenced by frontal and oblique ramps along the megathrust.

Mesoscopic, magnetic and petrofabric study of the High Himalayan gneisses and leucogranite along oblique and frontal ramps of the Vaikrita Thrust in Satluj and Bhagirathi valleys: Thrust locking and superposed folding

Field, microstructural and anisotropy of magnetic susceptibility (AMS) studies were performed along the oblique thrust ramp of the Satluj river valley and frontal thrust ramp of the Bhagirathi river valley. The presence of asymmetric and sheath folds indicates intense shearing near the thrust. The simple shear gradually decreases with an increase in distance from the thrust and pure shear with upright folds becomes prominent. The AMS studies suggest slightly higher pure shear in the frontal ramp region as compared to the oblique ramp region where oblique slip was predominant. The sense of shear criteria and orientation of superposed folds indicate locking of the thrust. Normal faults are the youngest structure.

Evolution of thrust system deformation in an intracratonic chain: Study of the Southern-Central Tunisian Atlas

The development of belt structures in intracratonic chains is guided by the convergence system. In the Southern-Central Tunisian Atlas, several parameters control the evolution of thrust folds during different tectonic phases. One of these phases is tectonic inheritance, which leads to the reactivation of pre-existing normal faults during compressive phases. The angle between the direction of these faults and the shortening axis (NNW-SSE) is the most important parameter for interpreting the mode of the evolution of thrust folds. Jebel Elkebar is an example of a structure developed on NW-SE-oriented faults that is perpendicular to the shortening axis. Based on the geometry of its folds, Jebel Elkebar is interpreted as a 'Fault Related Fold'. The E-W-oriented Orbata structure is oblique to the direction of the shortening axis and is interpreted through the model 'Fault Propagation Fold' with 'Breakthrough'. The Gafsa Fault, which is parallel to the shortening axis, is a transpressional fault interpreted through the 'Strain Partitioning' mode, which is associated with the oblique ramp fold. The development of various thrust folds requires the presence of a basal decollement level during the Triassic succession. In the Southern-Central Tunisian Atlas, the deformation is variable (geometry of fold closure) and is correlated with the depth of the decollement level; indeed, the intensity of deformation is proportional to the depth of the decollement level. Consequently, the most important deformation is in the higher successions and is a vertical migration of the decollement level associated with thin-skinned deformation.

Multiple movements recorded in a crustal weakness zone in NE Iberia: The Vallès-Penedès Fault revisited

Structural analysis and Anisotropy of Magnetic Susceptibility are here applied to the fault zone (several hundreds of meters thick) associated with an E-W striking, 20 km-long segment of the Vallès-Penedès Fault (Catalan Coastal Ranges). The results obtained indicate a complex structure, mainly resulting from the imprint of Paleogene compressional events on a fabric inherited from the Variscan orogeny, and its Late-Variscan and Mesozoic extensional history. The prevailing foliation on the fault gouge and microbreccia is oblique (striking NW-SE) to the main E-W trace of the fault. S-C structures affecting the foliation show both sinistral and dextral shear components on NE-SW and NW-SE planes, respectively, consistent with horizontal striations on quartz rods and planar bodies. Pure reverse and normal dip-slip shear components are also inferred from S-C structures at the meso- and micro-scale, indicating a NW to N (reverse), and S-SSE (normal) transport directions. The magnetic foliation averages the orientation of the observed foliation at the outcrop scale and the magnetic lineation shows two maxima. The first maximum has an overall horizontal plunge, parallel to the strike-slip transport direction inferred from S-C structures. The second maximum is parallel to the dip direction of foliation planes and also parallelizes the reverse transport direction and occasionally the intersection lineation between S and C planes, thus revealing the strong heterogeneity and partitioning of deformation. Cataclasis and pressure-solution are the main deformation mechanisms, in agreement with clay composition and low paleotemperatures determined for the Paleogene. The presented results are consistent with Paleogene transpression occurred through strain partitioning between the main thrust separating the Catalan Coastal Range from the Ebro basin (reverse slip) and the Vallès-Penedès fault zone (sinistral strike-slip), with an important influence of dextral shear associated to the secondary, WNW-ESE, segment of the fault (either an oblique ramp or resulting from the re-activation of the Llobregat Fault). A subsequent, Neogene extensional imprint slightly modified the compressional fabrics in the fault zone.

Controls of structural inheritance on orogenic curvature and foreland basin sedimentation: Insights from the Przemyśl area, Western Carpathians

Orogenic curvatures can have various origins and are widely debated worldwide. In the Poland-Ukraine border area, the Outer Western Carpathians are characterized by a marked curvature. The origin of this curvature was analysed by integrating stratigraphic information with structural constraints and anisotropy of the magnetic susceptibility (AMS) data. Hangingwall frontal ramp domains are characterized by a relatively simple deformation dominated by layer-parallel shortening and folding around a regional NW-SE trending axis, recorded by an AMS lineation with a similar trend. On the other hand, the N-S trending hangingwall oblique ramp domain is characterized by maximum AMS axes recording transpressional strain either dominated by simple shear (sub-horizontal AMS lineation) or pure shear (steeply plunging AMS lineation) components. Early Miocene basin inversion with two distinct depocentres created a number of different detachment surfaces and thickness variations for the sedimentary successions involved in thrusting. The main depocentre of the Lower-Middle Miocene foredeep was originally located in the recess area of the curved Carpathian front. On the other hand, the occurrence of a salient to the west resulted in the axial zone of the foreland flexure being filled with allochthonous units, thereby dramatically reducing the accommodation space for foredeep sediments in this area. Our results suggest that thrust-belt geometry was controlled by the inherited Mesozoic extensional basin architecture.

The Jurassic structural evolution of the western Daqingshan area, eastern Yinshan belt, North China

ABSTRACTThe western Daqingshan area, located in the eastern Yinshan belt, is dominated by the southern Daqingshan fold-and-thrust system and the northern Shiguai basin. Based on detailed structural investigations, stratigraphic controls, and geochronology, a three-stage tectonic evolution is proposed for the western Daqingshan area during the Jurassic. The discovery of syndepositional normal faults in the Early–Middle Jurassic sequences suggests that an N–S extensional regime (ca. 200–170 Ma) characterized the first deformational stage, which controlled the initial formation of the Shiguai basin. Subsequently, the relatively expansive rift basin was dissected by the initial development of the Daqingshan fold-and-thrust system that was associated with a N–S compressional regime (ca. 170–160 Ma). This phase of deformation involved the Lower–Middle Jurassic synrift sediments into a series of E–W-trending compressional structures, and controlled the deposition of Late–Middle Jurassic Changhangou growth strata ahead of the deformation front. Finally, the progression of Daqingshan fold-and-thrust system was dominated by NW–SE compression (ca. 160–145 Ma), which converted the previous E–W-trending compressional structures into a stepped geometry marked by several NE-trending oblique footwall ramps, and resulted in the depocentre of the Late Jurassic Daqingshan synorogenic conglomerate migrating markedly northeastwards. The driving mechanisms for these three palaeostress fields are considered as asthenosphere upwelling following Permian–Triassic collisional orogenesis, closure of the Mongol–Okhotsk Ocean, and NW-directed subduction of the Palaeo-Pacific plate, respectively.

Structural control of orogenic gold deposits of the Verkhoyansk-Kolyma folded region, northeast Russia

The Verkhoyansk-Kolyma folded region (VKFR) contains numerous Late Jurassic – Early Cretaceous orogenic gold deposits which are the source for large alluvial gold placers. This study is focused on determination of the relationship between ore-controlling structures, tectonic evolution of the region and analyzing different-aged deformations and structure of gold deposits in the region. Tectonic structures of mesothermal orogenic gold deposits were formed as a result of Late Jurassic-Barremian frontal accretion, generally prior to the onset of transpressional deformation. The accretionary events were different in different parts of VKFR. In the Allakh-Yun metallogenic zone this was the accretion of the Okhotsk terrane to the eastern margin of the North Asian craton, and in the Verkhoyansk and Yana-Kolyma metallogenic belts the accretion of the Kolyma-Omolon superterrane to the same craton. Regional-scale thrust faults are main tectonic structures controlling distribution of gold deposits in the area. Ore-controlling structures for the early orogenic gold-sulfide disseminated and gold-quartz lode deposits, which were formed prior to the emplacement of orogenic granitoid plutons, are shear zones. They form systems of listric-type imbricate fans closely associated with a detachment of the Verkhoyansk fold-and-thrust belt. Late orogenic gold-quartz deposits in fault zones, which were formed after the emplacement of orogenic granitoid plutons, are controlled by intersections of thrusts and lateral and oblique ramps and by the related folds. Formation of the ore-hosting structures of the late orogenic intrusion (granitoid)-related deposits is due to the dynamic effect of intruding magma on the host rocks.

Structural analysis of Nalagarh lobe, NW Himalaya: implication of thrusting across tectonic edge of NW limb of Nahan salient, Himachal Pradesh, India

The Main Boundary Fault (MBF), convex towards southwest, forms the leading edge of the Nahan salient. Near the southern end of an oblique ramp, a lobe-shaped physiographic front, named in this work as Nalagarh lobe, has developed across NW limb of salient. The lobe has formed across the MBF that separates the hanging wall Lower Tertiary Dharmsala rocks from the footwall Upper Tertiary Siwalik rocks and overlying Quaternaries. In front of lobe, thrust fault splays (Splay-1 and Splay-2) and associated tectonic fabrics have developed within the Late Pleistocene fan deposit. Structural elements developed across the front of Nalagarh lobe are analysed with reference to evolution of lobe. An unweathered ~15-m-high hanging wall or wedge top forms the uplifted and rejuvenated bedrock fault scarp of the MBF. Below the MBF, the fan deposit has underthrust along Splay-1. Later the Splay-2 formed within fan deposit near south of Splay-1. Geometry of the overturned limb of tight to isoclinal fault propagation fold, formed on Splay-2 plane, suggests that the fold formed by normal drag, produced by intermittent fault-slips along Splay-2. The displacement along Splay-2 offset the marker bed to ~1 m by which some clasts rotated parallel to the traces of brittle axial planes of fold. The variable fold geometry and style of deformation are analysed along length of thrust splays for 5 km. It is revealed that the lobe is bounded by transverse thrust faults along its NW and SE margins. The geometry of salient and oblique ramp suggests that the transverse thrust faults and associated transverse folds formed by right-lateral displacement along the NW limb of the salient. Marking the northern margin of the intermontane piggyback basin of Pinjaur dun, the MBF is interpreted to be an out-of-sequence thrust that has brought up the Lower Tertiary Dharmsala rocks over the Late Pleistocene fan deposit. The geometry of lobe and its bounding transverse faults suggest that faults are intimately associated with the kinematics of the transition between the Nahan salient and Kangra recess. The transition is a transfer zone forming a long pre-Himalayan lineament across which the stratigraphic set of the Tethys and Lesser Himalaya is different. The study suggests that the lateral ramp on the Main Himalayan Thrust does not exist beneath the apex and also beneath the SE limb of the salient in the Sub-Himalayan region. This ramp should be present only beneath near end point of SW limb of the Nahan salient.