Fold Thrust Research Articles

Variable magnetic fabrics under heterogeneous deformation across a shallow fault zone in the Iberian Chain (Monroyo thrust, N Spain)

A Cenozoic, N100E-striking thrust fault located in the Iberian Chain, provides an opportunity for testing the applicability of anisotropy of magnetic susceptibility (AMS) to the study of fault rocks deformation in shallow fault zones. The Monroyo thrust, which is probably a splay of the Castellote–Herbers fault, involves Cretaceous and Paleogene rocks in its hanging wall and footwall, respectively, shows a history of frontal thrusting without a strike-slip component. Fault kinematics are inferred from macro- (mapped trace of the thrust surface and folds in the Cretaceous series of the hanging wall) and meso-structures (pressure-solution foliation in Paleocene conglomerates of its footwall). The studied segment of the Monroyo thrust shows a 20 m thick fault zone consisting of brecciated limestones, marls and clays, showing pressure-solution foliation and oriented phyllosilicates (muscovite and chlorite), that can be identified in X-ray diffraction analysis. Analysis of AMS in fault rocks indicates that magnetic fabrics are dependent on the deformation degree, which is interpreted to vary across this relatively narrow fault zone. With increasing deformation, the magnetic lineation changes from parallel to the intersection lineation between foliation and shear planes in the area closest to the hanging wall, to a scattered pattern within the foliation plane in intermediate areas, and finally it gets parallel to the projection of the transport direction onto the magnetic foliation plane near the contact with the footwall (pitch of 90o within the foliation plane). The results obtained also give clues on the tectonic history of the Iberian Chain, in which frontal, NNE-directed thrusting seems to be the main mechanism for E–W trending basement-involved thrusts, in the linking zone between the Iberian Chain and the Catalonian Range.

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.

Sackung and enigmatic mass movement folds on a structurally-controlled mountain ridge

The recently emerged concept of “slope tectonics” suggests that mass movement processes can produce structures similar in morphological expression to that of extensional, strike-slip and compressional tectonic deformations. Further, there has long been debate about the (tectonic or gravitational) origin of lineaments found on hillslopes. Here we present cases of where geological structure and inherited tectonic structures have preconditioned the development of mass movements and creation of slope tectonic features on a monoclinal ridge (Smrk Mt.) in the Outer Western Carpathians, Czechia. Geomorphic mapping from LiDAR-derived topography, structural measurements, electrical resistivity tomography and ground penetrating radar, were used to map synthetic and antithetic scarps, thrusts, and folds, and assess their relation to geological and tectonic structures. The scarps are found to be strongly related to transverse (NNW to NNE trending) inherited normal and strike-slip faults and mainly cross-cut the topography of the mountain ridge. Some of the folds are thought to have originated from buckling or compression in the distal parts of landslides. In other places, transpressional folds have developed oblique to major lateral shear surfaces interpreted to be sackung (mass movement scarps) that have been inherited from tectonic faults. The transpressional folds developed due to localized contraction along bends in the lateral shear surface as mass movement proceeded. Ramp-flat thrust folds developed in the compressional parts of landslides, where the landslide intersects with slope parallel sackung and/or inherited fault planes inclined to the slope. Altogether, this study demonstrates a connection between kinematics of deep-seated gravitational slope deformations (DSGSDs), shallower landslides and the origin of transpressive, ramp-flat thrust, and detachment structures in rock slopes.

Paleomagnetic age and tectonic constraints on the genesis of the giant Jinding Zn-Pb deposit, Yunnan, China

The Jinding Zn-Pb deposit is located in the Lanping-Simao basin, Yunnan, China. Paleomagnetic results are reported from the Beichang, Jiayashan, and Nanchang mineralization zones that are hosted in the Lower Cretaceous Jinxing and Upper Paleocene Yunlong Formations. The timing of formation of the Zn-Pb mineralization has not been previously defined but is geologically constrained to be younger than Upper Paleocene. Paleomagnetic analyses of 645 specimens from 42 sites using mostly thermal and then alternating field step demagnetization isolated a stable characteristic remanent magnetization (ChRM) for 29 mineralized sites. Step demagnetization and rock magnetic analyses of Zn and Pb concentrates and tailings show that the main remanence carriers are pyrrhotite and magnetite or low Ti titanomagnetite inclusions in sphalerite and galena. Randomly oriented ChRM directions for 18 clast specimens collected from site 40 provide a conglomerate test that shows that the clasts have not been remagnetized by a metamorphic event. A paleomagnetic fold test on the mineralized specimens is negative, showing that the mineralization is epigenetic because its ChRM substantially postdates the major thrust folding event that formed the dome structure of the Jinding deposit. Using extensive regional geologic and paleomagnetic data from other studies with the Jinding paleomagnetic data yields a mean age of 23 ± 3 Ma for the mineralization.

Shear wave velocity imaging of landslide debris deposited on an erodible bed and possible movement mechanism for a loess landslide in Jingyang, Xi’an, China

The South Jingyang Plateau, with a total area of 70 km2, is located in Shaanxi Province, China. Since 1976, more than 50 landslides of different types have occurred repeatedly on the edge slopes of the plateau due to the start of diversion irrigation on the plateau, resulting in great loss of lives and property. To better understand the initiation and movement mechanisms of these loess landslides, we surveyed them and carried out a detailed investigation of a large landslide in the Xihetan area. Our field survey results revealed that although most of these landslides had a long runout with high mobility, most of the landslide materials originating from the edge slopes may have been in an unsaturated state when the landslide occurred. This suggests that the materials at the toe of the edge slope as well as on the travel path along the river terrace might have played a key role in landslide movement. To examine how the materials on the travel path were involved in the landsliding, we used a multichannel surface wave technique and surveyed shear wave velocity (V s ) profiles of the landslide deposits. We also examined the internal geometry of the deposits that outcropped on the right-side slope of the landslide foot. The longitudinal profile of V s along the direction of movement showed that terrace deposits near the toe of the edge slope may have been sheared upward, indicating that at the toe, the surface of rupture might be located inside the terrace deposits. The V s contours showed an A-shaped fold within the landslide deposits in the middle part of the travel path and became greater in the most distal toe part. The V s profile across the deposits showed a U-shaped belt, in which the soil layers have smaller V s . This belt may be the boundary between the sliding landslide debris and terrace deposits. The observed internal geometry of the landslide deposits indicates that a sliding surface developed within the sandy layer underlying the gravel layer. Therefore, we inferred that after failure, the displaced landslide materials overrode and sheared the terrace deposits along its main sliding direction, resulting in the formation of thrust folds within the terrace deposits, and greater V s on the distal toe part of the landslide.

Analogue modeling of the role of multi-level decollement layers on the geometry of orogenic wedge: an application to the Zagros Fold–Thrust Belt, SW Iran

The presence of evaporate and incompetent formations (i.e., decollement horizons) within the sedimentary sequence of fold-thrust belts can control their structural style and deformation evolution. In the present study, the influence of the decollement layers (e.g., basal and internal decollement layers) on the deformation style of several segments of the Zagros Fold-Thrust Belt (ZFTB), SW Iran (e.g., Fars Arc, Dezful Embayment, and Izeh Zone) was investigated using a series of analogue models of accretionary wedges. The study of seismic profiles to understand the structural evolution of these segments of the belt, where several decollement intervals acted as basal and internal decollements, is complemented by the analogue model results. The experimental results reveal that the thickness of the internal decollement layers influences the creation of fold-dominated or thrust-dominated deformations, respectively. Experimental models and seismic data highlight that incompetent layers act as barrier units against fault propagation (in-sequence and/or out-of-sequence faults) into overlying strata towards southwest by fore-deformation and control the rate of deformation propagation in the ZFTB. The presence of both the basal and internal decollement layers located at different stratigraphic levels is required to form disharmonic decollement folds in the foreland of the ZFTB. In addition, the geometry, spacing, activity, and propagation of faults as well as the topographic height of the critical wedges are directly controlled by low-frictional decollements (Geophys J Int, 165(1):336–356 2006; Geochem Geophys Geosyst, 14:1131–1155 2013). The seismic profiles of the ZFTB showed that in addition to lithological contrasts, the existence and activity of deep-seated and basement faults had a big impact on the structural styles of the region.

A geometric model of faulted detachment folding with pure shear and its application in the Tarim Basin, NW China

We present an improved geometric model of faulted detachment folding with pure shear that is characterized by core thickening and a ramp-discordant backlimb. The model includes a two-stage evolution: 1) detachment folding involving pure shear with fixed hinges, and 2) faulted detachment folding, in which the core of anticline thrusts above a break-through fault in forelimb by limb rotation. The growth strata patterns of the model are also discussed with respect to factors such as limb rotation, tectonic uplift rate, and sedimentation rate. A thrust-related fold, called a TBE thrust fold, in the Tarim Basin in NW China, is analyzed as an example of the theoretical model. The result indicates that the TBE thrust fold has undergone a two-stage evolution with shortening of a few hundred meters. Both the theoretical model and the actual example indicate that the shortening in the detachment folding stage takes up a large proportion of the total shortening. The structural restoration of the TBE thrust fold also provides new evidence that the formation of a series of thin-skinned structures in the SE Tarim Basin initiated in the Late Ordovician. The model may be applicable to low-amplitude faulted detachment folds.

Gas storage and controlling factors in an over-mature marine shale: A case study of the Lower Cambrian Lujiaping shale in the Dabashan arc-like thrust–fold belt, southwestern China

Lower Cambrian units in the upper Yangtze area of China are important strata for shale gas exploration. The Lower Cambrian Lujiaping shale in the Dabashan arc-like thrust–fold belt of the upper Yangtze area is thermally over-mature and has been transformed to slate by epimetamorphism. The Lujiaping shale has also experienced strong structural deformation; its current stratigraphic dip is 35°–80°. Over-maturity and strong deformation are considered unfavorable conditions for gas storage. However, desorption tests using canisters indicate that gas contents in the Lujiaping shale range from 0.05 to 3.39 m3/t (average, 1.12 m3/t); this level of gas presents challenges for conventional theories of shale gas enrichment. To analyze the controls on gas enrichment, we performed petrological, geochemical, and petrophysical experiments on shale samples from the Lujiaping Formation. Results show that the TOC contents are between 1.17% and 6.91%. The porosities of the shales are very low, in the range of 0.68%–3.93%, possibly related to over-maturity. The correlations between petrophysical parameters and the total organic carbon (TOC) content indicate that the organic fraction contribution to micropores and mesopores is highly significant. The Langmuir volumes for the studied shale samples are in the range of 1.07–5.87 m3/t. The gas content and methane adsorption capacity are strongly dependent on the TOC content. Because the studied shales have low porosities and are from shallow depths, their capacity for free gas storage is extremely limited, and the gas is stored mainly in an adsorbed state. The studied shales occur at temperatures of 20–37 °C. The maximum sorption methane capacity occurs at depths of 500–1500 m. Well-developed micropores, high surface area, and strong gas adsorption capacity are important controls on gas enrichment in the Lujiaping shales. The three controls may be related to the inter-particle pores between the organic matter and clay minerals.

Comment on “Cenozoic tectonic deformation and uplift of the South Tian Shan: Implications from magnetostratigraphy and balanced cross-section restoration of the Kuqa depression” by Tao Zhang, Xiaomin Fang, Chunhui Song, Erwin Appel, and Yadong Wang [Tectonophysics, 2014, doi:10.1016/j.tecto.2014.04.044

The recent paper entitled: “Cenozoic tectonic deformation and uplift of the South Tian Shan: Implications from magnetostratigraphy and balanced cross-section restoration of the Kuqa depression” by Zhang et al. [Tectonophysics, 2014, doi:10.1016/j.tecto.2014.04.044] discusses the Cenozoic tectonic deformation and uplift of the South Tianshan Mountains by integrating tectonic investigations, seismostratigraphic analysis and paleomagnetic dating of the thrust–fold belt in the Kuqa Depression at the southern border of the Tianshan. To support their conclusions the authors have reinterpreted a high resolution magnetostratigraphic study of the Kezilenuer Section in a paper entitled “Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range, Western China” by Huang et al. [Earth and Planetary Science Letters Volume 251, pages 346–364 (2006)]. We note here (i) that apparent conflicts in definition of the Xiyu Formation in the Kuche Depression as proposed by Zhang et al. (2014) require amplification and (ii) argue that their age assignment for the Kezilenuer Section is incorrect and yields an anomalously low sedimentation rate for the infilling of this foreland basin.

Tectonic models for the Patagonian orogenic curve (southernmost Andes): An appraisal based on analog experiments from the Fuegian thrust–fold belt

Tectonic models for the evolution of the Patagonian orogenic curve were evaluated using analog experiments that considered either a rotational or a non-rotational orogenic backstop, combined with a basement promontory on the foreland cratonic margin. Five different kinematic configurations were used, aiming to evaluate the influence of the Río Chico Arc as a rigid obstacle on the evolution of the Fuegian thrust–fold belt. Rotations, strains and displacement fields obtained from each analog experiment were compared with the structural geology known from the Fuegian thrust–fold belt, in order to appraise the tectonic models that are more consistent with the natural structure. The push of a counterclockwise rotational backstop, combined with the buttressing effect of a foreland promontory, seem of major importance in controlling the final structure and map-view shape of the thrust wedge.

Positive inversion tectonics in foreland fold-and-thrust belts: A reappraisal of the Umbria–Marche Northern Apennines (Central Italy) by integrating geological and geophysical data

Unraveling the tectonic style in the outer zones of fold-and-thrust belts is generally puzzling because the basement–cover relationships are often hidden in the subsurface as in the outer Northern Apennines of Italy. This study aims to reconstruct the deep setting of the Northern Apennine foreland thrust belt by integrating surface structural–geological and subsurface seismic reflection profile and well data, corroborated by a gravity–magnetic modeling. A remarkable mountain ridge, the Umbria–Marche Apennine Ridge (UMAR), which corresponds to a prominent area of structural and topographic elevation, characterizes the morphology of the Northern Apennines. This mountain ridge is constituted by Meso-Cenozoic carbonates involved in Neogene compressive structures and is surrounded by wide exposures of foredeep deposits. The basement–cover relationships are poorly constrained and both thin- and thick-skinned tectonic styles have been applied. The interpretation of subsurface data allowed recognizing a thick pre-Jurassic sedimentary sequence filling a late Paleozoic(?)–Triassic symmetric fault-bounded extensional basin, lying directly below the UMAR. This deep-rooted basin underwent positive inversion during the Neogene compression and thrust–fold development. The reconstructed thick-skinned inversion tectonic model is consistent with both the modest amount of shortening and the remarkable structural elevation of the UMAR.The outcomes of this study reveal that prominent mountain ridges occurring in foreland thrust belts are most likely related to the deep-rooted basement-involved positive inversion of pre-existing extensional basins.

Geodynamic setting of mineralization of Mississippi Valley-type deposits in world-class Sichuan–Yunnan–Guizhou Zn–Pb triangle, southwest China: Implications from age-dating studies in the past decade and the Sm–Nd age of Jinshachang deposit

The Sichuan–Yunnan–Guizhou (S–Y–G) Zn–Pb triangle is a world-class metallogenic belt in southwestern China that contains hundreds of carbonate-hosted giant-to-small epigenetic Zn–Pb deposits. Here, we provide an overview of the ore geology, geochemistry and ore-forming fluids of the major Zn–Pb deposits in this area. These deposits are most likely Mississippi Valley-type (MVT) deposits that formed as a result of the regional migration of basinal brines along large fault systems and more minor secondary structures. The Sm–Nd age (201±6.2Ma) of ore-stage fluorite from the Jinshachang Zn–Pb deposit, within northeast Yunnan province, China, reveals this deposit formed during the Late Triassic, consistent with the majority of the published isotopic ages for other Zn–Pb deposits in the S–Y–G MVT triangle. These fluorite samples have initial 87Sr/86Sr ratios of 0.711385–0.711463 and εNd values of −8.4 to −8.7, confirming the basin-related nature of the mineralizing fluids. Published geochronological data combined with basic features of MVT deposits (e.g., geology, geochemistry, and ore-forming fluids) and the geological evolution of the study area has enabled us to develop a model for the Zn–Pb mineralization, where this word-class MVT belt has formed as a result of the regional-scale migration of basinal fluids coincident with tectonic activity along ore-controlling structures (e.g., thrust–fold systems). Both the fluid migration and the tectonic activity were probably triggered by the late Indosinian Orogeny, which in turn was a response to the closure of the Paleo-Tethys Ocean.

Provenance of Oligocene–Miocene sediments in the Subei area, eastern Altyn Tagh fault and its geological implications: Evidence from detrital zircons LA-ICP-MS U–Pb chronology

Oligocene–Miocene strata in the Subei and Xiaobiegai basins of the Subei area, located in the eastern Altyn Tagh fault (ATF), northern Tibetan Plateau, record important characteristics of the ATF evolution. Detrital zircons laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb ages from two samples, together with paleocurrent directions and clastic composition in the Xishuigou section demonstrate that sediments in the Subei basin originated from the Danghenanshan range along its southern margin. Detrital zircons U–Pb ages from three samples in the Xiaobiegai basin, together with paleocurrent directions and clastic composition, indicate that sediments in the Xiaobiegai basin may partly originate from terranes along the northeastern margin of the basin in addition to the Danghenanshan range. Our results, combined with regional evolution, suggest that the Xiaobiegai and the Subei basins was a combined basin in Oligocene–early Miocene. This basin was folded, tilted, and dislocated at ca. 8Ma by rapid uplift of the northern Tibetan plateau and rapid strike-slip of the ATF. As a result, the Subei basin became a thrust–fold belt of the Danghenanshan range front, and the Xiaobiegai basin grew into an intermontane basin in the northeastern part of the Danghenanshan range. Thus, the Subei area gradually acquired its present morphotectonic patterns.

Oligocene-Miocene Mammalian Fossils from Hongyazi Basin and Its Bearing on Tectonics of Danghe Nanshan in Northern Tibetan Plateau

A shortage of Cenozoic vertebrate fossils in the Tibetan Plateau has been an obstacle in our understanding of biological evolution in response to changes in tectonism, topography, and environment. This is especially true for Paleogene records, so far known by only two sites along the northern rim of the Plateau. We report a Hongyazi Basin in northern Tibetan Plateau that produces at least three mammalian faunas that span Oligocene through late Miocene. Located at the foothills of the Danghe Nanshan and presently connected to the northern margin of the Suganhu Basin through the Greater Haltang River, the intermountain basin is controlled by the tectonics of the Danghe Nanshan to the north and Chahan’ebotu Mountain to the south, making the basin sediments well suited for inferring the evolutionary history of these two mountain ranges. At the bottom of the local section, the Oligocene Haltang Fauna is best compared to the early Oligocene Desmatolagus-Karakoromys decessus assemblage in the Dingdanggou Fauna in Tabenbuluk Basin. The Middle Miocene Ebotu Fauna from the middle Hongyazi section shares many taxa with the late Middle Miocene Tunggur mammal assemblage in Inner Mongolia, such as Heterosminthus orientalis, Megacricetodon sinensis, Democricetodon lindsayi, and Alloptox gobiensis. Toward the top of the section, the Hongyazi Fauna includes late Miocene elements typical of Hipparion faunas of North China. All three faunas are of typical North China-Central Asian characteristics, suggesting a lack of geographic barriers for faunal differentiation through the late Miocene. Sedimentary packages producing these faunas are arrayed from north to south in progressively younger strata, consistent with a compressive regime to accommodate shortening between Danghe Nanshan and Chahan’ebotu Mountain by thrust faults and folds. With additional constraints from vertebrate fossils along the northern flanks of the Danghe Nanshan, an eastward propagation of the Danghe Nanshan is postulated.

Predicting orogenic wedge styles as a function of analogue erosion law and material softening

The evolution of a compressive frictional wedge on a weak, frictional and planar décollement, subjected to frontal accretion, is predicted with a two step method called sequential limit analysis. The first step consists in finding, with the kinematic approach of limit analysis, the length of the active décollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold. The second step leads to a modification of the geometry, first, because of the thrust fold development due to compression and, second, because of erosion. Erosion consists in removing periodically any material above a fictitious line at a selected slope, as done in analogue experiments. This application of sequential limit analysis generalizes the critical Coulomb wedge theory since it follows the internal deformation development. With constant frictional properties, the deformation is mostly diffuse, a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical, critical value. Frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses. Applying an erosion slope at the critical topographic value leads to exhumation in the frontal, central, or rear region of the wedge depending on the erosion period and the weakening. Erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland, respectively, regardless of the erosion period. Exhumation is associated with duplexes, imbricate fans, antiformal stacks, and major backthrusting. Comparisons with sandbox experiments confirm that the thickness, dips, vergence, and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values.

A preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods

Although Lower Silurian black shales within and around the Sichuan Basin have received renewed attention during the past few years because of their potential prospectivity for shale gas, studies associated with shale gas reservoir evaluation are still rare. In this study, eight black shale core samples were collected from a well drilled recently in the Chuandong Thrust Fold Belt, southwestern China, and their geochemistry and pore structures were investigated using low pressure N2 adsorption analysis and field emission scanning electron microscopy (FE-SEM) observations. The results show that the black shales have total organic carbon (TOC) values ranging from 1.01% to 3.98% and their equivalent vitrinite reflectance values are in the range of 2.84%–3.05%. Both mineral matrix and organic matter pores are well developed with pore sizes ranging from several to several hundred nanometers. The total porosity for the eight samples ranges from 2.60% to 4.74% and the percentages of organic matter pores are estimated to be in the range of 31%–62%. The total surface area ranges from 5.06 m2/g to 19.32 m2/g and the micropore (<2 nm) surface area estimated by the t-plot method ranges from 3.13 m2/g to 9.27 m2/g. The TOC values have positive relationships with the total porosity, total surface area and the micropore (<2 nm) volume and surface area, indicating TOC may be an effective parameter for shale gas reservoir evaluation in the studied area.

Present-day stress of the central Persian Gulf: Implications for drilling and well performance

Abstract The present-day state of stress in the Persian Gulf is poorly understood but has significant impacts on well drilling and performance. The upper Permian to lower Triassic formation of Kangan/Dalan, Persian Gulf, exhibits a complex structural context in the neighborhood of the Oman Mountains and the Zagros orogenies. This formation is divided into four reservoir layers (K1 to K4) where three main lithologies (limestone, dolomite and anhydrite) are alternating. We conduct an analysis of the present-day stress and natural fractures at the wellbore using full-bore FMI logs, leak off test and density logs. For this purpose, borehole breakout and tensile fracture data are used to determine orientation of SH. Furthermore, density log, leak-off test and Kirsch equation for tensile fracture formation in the wellbores are used to calculate the magnitude of Sv, Sh and SH, respectively. Vertical stress (Sv) gradient at 3100 m depth approximates 20 MPa/km (2.9 psi/m), indicating a bulk density of 2.04 g/cm3. A total of 131 drilling induced tensile fractures and 21 breakouts with an overall length of 262 m are observed in two wells, indicating a mean maximum horizontal stress (SH) orientation of N53° (± 18.45°) for drilling-induced tensile fracture (DITF) data and N50° (± 10.79°) for breakout data. The mean orientation of SH rotates counterclockwise with depth from K2 (N70° ± 4.2°) to K4 (N40° ± 5.1°) reservoirs. Noticed correlation between these data and stress orientations from earthquake focal mechanism solution, first of all, indicates that the stresses are linked to the resistance forces generated by the Arabia–Eurasia collision at the Zagros orogeny and secondly confirms the reliability of focal mechanism solution data near continental collision zones. In the Kangan/Dalan Formation, the NW–SE main open fracture direction is found as a common regional direction which is sub-perpendicular to the present-day maximum horizontal stress. Minimum horizontal stress (Sh) gradient in reservoir sections is estimated to be equal to 17 MPa/km (2.5 psi/m). The concluded strike–slip stress regime (SH > Sv > Sh) in the study area is consistent with the compressive regime in the Zagros thrust–fold belt. The present-day stress in the Kangan/Dalan Formation has implications for wellbore stability, lost circulation and well Inflow Performance Relationship (IPR). Wells are more unstable if deviated toward the Sv direction, whereas well productivity and mud loss increase in wells deviated toward SH, which conveys the idea of a strike–slip faulting effect tends to keep the natural fractures open in that direction.

Deformation history and hydrocarbon potential of the Southwestern Gissar Range (Southern Uzbekistan)

The divergent geometry of the Southwestern Gissar Range is related to large-scale underthrusting of the hinterland units of the Tajik depression beneath Southwestern Gissar and the thrusting of orogenic crust toward the northwest over the Beshkent trough. Since the predominant style of deformation in the region consists of folding and faulting, almost all the surficial structures of the orogen are allochthonous. This can significantly hamper understanding of a deeper, sub-thrust layer of the Southwestern Gissar, which hosts major hydrocarbon sources. Regional dynamic models involving ductile layers, as well as thrust faults, folds, and strike-slip faults allowed identification of the most favorable conditions for the localization of structural traps within this highly prospective petroleum basin.

A Review of the Seismicity and Seismotectonics of Delhi and Adjoining Areas

Abstract Understanding of seismicity and seismotectonics of Delhi and adjoining areas is essential as these areas lie in the seismic zone IV and are geologically confined to the Delhi Fold Belt (DFB), juxtaposed to the Himalayan Frontal Thrust Fold Belt. Owing to the set-up, seismicity in this area is ascribed to the Himalayan Thrust System and activation of DFB Fault Systems. Considerably improved instrumental seismic monitoring in this area and data analysis had resolved three regions of pronounced seismicity that lie close to Sonepat, Rohtak and western part of the NCT Delhi, attributed to activation of various portions of the fault systems of the DFB. Based on seismic telemetry network data, the seismicity pattern analysis revealed that the Mahendragarh Dehradun Sub-Surface Fault (MDSSF) and Delhi Sargodha Ridge (DSR) are the two major zones of structural importance for the nucleation of seismicity in this region. These revelations were corroborated with the fault plane solution of the earthquakes. The dominant mechanism in nucleation of seismicity in DFB is the thrust with minor strike slip. The seismicity and seismotectonics of Delhi and adjoining areas endemic to activation of DFB is reviewed and presented in this paper.

Structural evolution of metamorphic rocks in the Talas Alatau, Tien Shan, Central Asia: Implication for early stages of the Talas-Ferghana Fault

The Talas Alatau Ridge consists of three tectonic units separated by thrusts. The central (Talas parautochthon) and northeastern (Kumyshtag sheet) parts consist mainly of unmetamorphosed terrigenous and carbonate deposits. The southwestern part of the Talas Alatau (Usunakhmat sheet) consists of greenschists and is bounded by the Central Talas Thrust (CTT) and Talas-Ferghana Fault (TFF). The latter is a major strike-slip fault with post-Permian dextral displacement estimated at as much as 200km. A structural study shows that elongation axes of strain ellipsoids, stretching lineation and map-scale fold axes, as well as the trend of major faults including the CTT and the TFF, are parallel to each other. The natural deviatoric strain value gradually increases from the CTT to the TFF. The shape of the strain ellipsoid gradually varies within the Usunakhmat sheet from oblate at the frontal thrust to prolate in internal parts of the unit. The magnitude of shortening increases towards the CTT, whereas natural deviatoric strain decreases towards the CTT implying structural thickening of internal parts of the Usunakhmat sheet. Strong parallelism of the TFF, CTF and other map- and outcrop-scale structures and their close connection with strain patterns show that they were formed during the same tectonic event. They were likely formed during Middle to Late Ordovician collision and built up Kyrgyzian microcontinent as a thrust fold belt whereas reactivation of the TFF as a strike-slip fault occurred later, in Late Paleozoic.