Surface Anticlines Research Articles

A Critical Geological Evaluation of the Hydrogen Storage Potential in the Cousland Gas Field, Midland Valley of Scotland

Hydrogen is envisaged to be an important element in the drive to replace hydrocarbons in the energy mix and its geological storage in man-made salt caverns or porous subsurface reservoirs onshore in the United Kingdom is being actively investigated. It has recently been suggested that porous Carboniferous sandstone reservoirs of the partially depleted and abandoned Cousland Field, located c.15 km SE of Edinburgh, could be used for the storage of hydrogen as part of efforts to decarbonise the industrialised and heavily populated central belt of Scotland. The gas field occurs in an onshore surface anticline situated in a culmination along a topographic ridge on the eastern flank of the Midlothian Coalfield. Mapping of outcropping beds led to a well, Cousland-1, being drilled in 1937-39 as part of a national campaign of oil exploration, which discovered natural gas in two thin sands. Despite the initial success, all the subsequent five appraisal wells, drilled between 1939 and 1960, were devoid of commercial hydrocarbons. Despite the disappointing drilling campaign, the field was still developed based on the original Cousland-1 well result and some 0.25 billion standard cubic feet (“scf”) (7.0 million m3) of gas were produced between 1939 and 1965 before the well was plugged and abandoned. This paper draws upon and integrates hitherto unpublished historical records, obtained from the archives of the UK Onshore Geophysical Library, to critically examine the possibility that the Cousland gas field might be re-purposed for hydrogen storage. These studies conclude that it is currently not possible to demonstrate the subsurface configuration, host reservoir distribution or sealing mechanism of the proposed storage container because the information required is not available and may be impossible to obtain. Rather than supporting the case for hydrogen storage, our new evaluation demonstrates that the Cousland field is a poor site that fails to meet the criteria for safe subsurface storage and should not be used for this purpose until and unless the significant challenges can be addressed.

Late Cretaceous mud volcanism in the southwestern Songliao basin records slab rollback of the subducted paleo-Pacific Plate underneath NE China

We present a new seismic reflection dataset and use it to characterize fossil mud volcanoes in the southwestern Songliao graben basin in northeastern China. The results reveal a link between mud volcanism and slab rollback along the eastern Asian margin. This study focuses on the upper 3 km of the Lujiapu sub-basin, which lies in the southwestern Songliao basin. The base of the sequence consists of Lower Cretaceous siliciclastic deposits that are penetrated by mud volcano fluidization pipes. These deposits are overlain by Upper Cretaceous strata that have been deformed by the mud volcanoes. The sequence is capped by undeformed Neogene–Quaternary sediments. These observations constrain the timing of mud volcanism to the Santonian–Campanian (~87–72 Ma). During Late Cretaceous diapirism, normal faulting at the tops of the mud volcanoes formed horsts and grabens. Surface anticlines and inversion structures near the mud volcano source layer indicate that basin inversion occurred during deposition of the Sifangtai and Mingshui formations (Campanian). Similar structures are also identified in the central Songliao basin. Thus, we propose that slab flattening to deepening during paleo-Pacific Plate subduction led to basin inversion. This process triggered the migration of high-pressure fluids and brecciated plastic rocks along pre-existing normal faults, which in turn produced the mud volcano system.

PETROLEUM SYSTEMS MODELLING IN A FOLD‐AND‐THRUST BELT SETTING: THE INVERTED CAMEROS BASIN, NORTH‐CENTRAL SPAIN

The Mesozoic Cameros Basin, northern Spain, was inverted during the Cenozoic Alpine orogeny when the Tithonian – Upper Cretaceous sedimentary fill was uplifted and partially eroded. Tar sandstones outcropping in the southern part of the basin and pyrobitumen particles trapped in potential source rocks suggest that hydrocarbons have been generated in the basin and subsequently migrated. However, no economic accumulations of oil or gas have yet been found. This study reconstructs the evolution of possible petroleum systems in the basin from initial extension through to the inversion phase, and is based on structural, stratigraphic and sedimentological data integrated with petrographic and geochemical observations. Petroleum systems modelling was used to investigate the timing of source rock maturation and hydrocarbon generation, and to reconstruct possible hydrocarbon migration pathways and accumulations.In the northern part of the basin, modelling results indicate that the generation of hydrocarbons began in the Early Berriasian and reached a peak in the Late Barremian – Early Albian. The absence of traps during peak generation prevented the formation of significant hydrocarbon accumulations. Some accumulations formed after the deposition of post‐extensional units (Late Cretaceous in age) which acted as seals. However, during subsequent inversion, these reservoir units were uplifted and eroded.In the southern sector of the basin, hydrocarbon generation did not begin until the Late Cretaceous due to the lower rates of subsidence and burial, and migration and accumulation may have taken place until the initial phases of inversion. Sandstones impregnated with bitumen (tar sandstones) observed at the present day in the crests of surface anticlines in the south of the basin are interpreted to represent the relics of these palaeo‐accumulations.Despite a number of uncertainties which are inherent to modelling the petroleum systems evolution of an inverted and overmature basin, this study demonstrates the importance of integrating multidisciplinary and multi‐scale data to the resource assessment of a complex fold‐and‐thrust belt.

Building a valid structural model in a triangle zone: An example from the Neuquén fold and thrust belt, Argentina

Structural modeling and systematic analysis are necessary in complex areas even before seismic interpretation. Seismic data in these zones usually show poor-quality images of the geologic elements. Therefore, a detailed analysis of the available information (e.g., seismic, previous wells, and surface geology) is needed to build a comprehensive structural model to constrain the seismic interpretation. Although a restorable structural cross section is not necessarily the unique solution, it is a valid interpretation to begin with, which can be tested against additional information. As an example, a structural profile interpretation in the Agrio fold and thrust belt of the Neuquén Basin, northeastern Argentina, is presented and described as invalid on the basis of several unrestorable geometric components. This led to the reevaluation of the structure to understand its possible exploratory potential. Integration of different data such as regional structural styles inferred fault trajectories from seismic reflection termination patterns, synorogenic deposit geometries in seismic, and surface geology, dipmeter data to identify the presence and location of fault position and vergence of the fold axis, and a kinematic forward model, resulted in a new restorable structural model. The new interpretation proposes a “broken triangle zone” framed by two opposite surface anticlines with the same detachment level, later faulted by a basement fault. The interpreted model suggests at least two phases of deformation (1) a thin-skinned phase with the two opposite anticlines sharing the same detachment level that is responsible for about 80% of the shortening and (2) a thick-skinned phase from an east-verging basement fault, transporting the western anticline and crosscutting the eastern anticline, that is responsible for 20% of the shortening and the synorogenic deposits at the frontal region. This new interpretation defined a new exploration prospect drilled by a wildcat well, which validated the interpretation and model predictions.

Structural and Stratigraphic Interpretation of Geophysical Data of the Fenchuganj Gas Field in the Surma Basin, Bangladesh

Abstract: The Fenchuganj structure trending in NNE-SSW direction is a surface anticline (reverse faulted) in the eastern part of the Surma basin, Bangladesh. The eastern flank of the structure has steeper dip than the western one. The present study includes seismic and geological data interpretation which gives details about structure and stratigraphy of the area. Well log interpretation was done to infer lithological pattern and the hydrocarbon potential zones. Correlating the seismic interpretation and well log analysis, four gas sands have been identified at depths 2000m to 3100m. Depending on the present findings, it is concluded that further investigation can give valuable information about real sub-surface structure and hydrocarbon occurrence in this area.

Postseismic afterslip 30 years after the 1978 Tabas-e-Golshan (Iran) earthquake: observations and implications for the geological evolution of thrust belts

This paper presents InSAR observations of postseismic afterslip occurring up to 30 yr after the Mw7.3 1978 Tabas-e-Golshan thrust-faulting earthquake in eastern Iran. Comparison of the surface motion from 1996 to 1999 with that from 2003 to 2010, along with information provided by the Quaternary-averaged slip rates of faults in the region, suggests that the imaged slip is transient and decaying through time. Models of the surface deformation field imply slip on faults dipping at 55 ± 10°, reaching from the surface to depths of 4–5 km, and slipping at 5 ± 1 mm yr-1. These faults outcrop on the margins of low anticlinal hills composed of actively uplifting Neogene deposits. When compared with the previously studied main shock focal parameters (slip on a plane dipping at 16 ± 5° with a centroid depth of \~9 km), and the aftershock distribution (a band at \~6–14 km), the InSAR results imply postseismic slip on a high-angle thrust ramp connecting the surface anticlines to the coseismic low-angle fault plane at depth. In one location, both a thrust ramp and also a backthrust are postseismically active. The InSAR observations demonstrate the role of postseismic afterslip in the growth of these commonly observed thrust belt geometries, and highlight that deformation throughout the seismic cycle can contribute to the geological evolution of regions of active faultin

An inventory of river anomalies in the Po Plain, Northern Italy: evidence for active blind thrust faulting

The Po Plain is a low-relief area characterised by active shortening accommodated by blind thrust faulting. In this almost flat region depositional rates are similar to tectonic rates and deformation is seldom expressed by noticeable surface anticlines. We adopted a geomorphological approach based on the detailed analysis of the drainage network to identify the location of active thrust faults. A total of 36 anomalies represented by sudden river diversions and shifts in channel pattern were accurately mapped. After comparison with the location of subsurface buried anticlines and of historical seismicity, these anomalies could be related to a tectonic origin and included in a database. Their distribution highlights the activity of the buried outer thrust fronts of both the Southern Alps and the Northern Apennines. Among all the anomalies, we identified one related to the seismogenic source responsible for the 12 May 1802 earthquake (Me 5.7), which struck the Oglio River Valley near Soncino (Cremona). We propose that this earthquake was generated by an east-west trending, north-dipping, blind thrust fault that roots into the Alpine system. If this inference is correct, other faults along the Southern Alpine margin are potentially seismogenic.

Latest Pliocene to recent thick-skinned tectonics at the Upper Rhine Graben – Jura Mountains junction

The southernmost Upper Rhine Graben and adjacent Jura experienced basement-rooted shortening that occurred after the deposition of the Pliocene fluvial “Sundgau gravels”. Folds affecting the base of these gravels systematically trend NE to ENE. Combined evidence from reflection seismic lines and contour maps of the base-Tertiary and base-Pliocene levels indicates that these folds probably formed by thick-skinned reactivation of both NNE-SSW and WSW-ENE-striking faults. This thick-skinned shortening is NW-SE oriented, i.e. parallel to the maximum horizontal stresses inferred from seismotectonics. NNE-SSW-striking faults (paralleling the Upper Rhine Graben) have been reactivated in sinistral strike-slip mode. However, dextrally transpressive reactivation of the WSW-ENE-trending faults that belong to the Rhine-Bresse Transfer Zone is interpreted to predominate.Deflections of recent river courses around the crests of en-échelon-aligned surface anticlines suggest that the deformation is ongoing at present. Retro-deformation of the folds affecting the base of the Sundgau gravels indicates horizontal displacement rates of about 0.05 mm/a. This corresponds to a minimum strain rate in the order of 2·10−16 s−1, given the maximum time span of 2.9 Ma for this deformation, i.e. the biostratigraphically determined minimum age of the gravels. A change from thin-skinned tectonics, that prevailed during the main phase of Jura folding, to very probably still ongoing thick-skinned tectonics is inferred to have occurred in the Late Pliocene. We speculate that this change might be linked to the incipient inversion of Permo-Carboniferous troughs within the Alpine foreland in general. This inversion in dextrally transpressive or purely compressive mode along a WNW-ESE-trending basement fault, that is part of the Rhine-Bresse Transfer Zone, which in turn was prestructured during the formation of the Permo-Carboniferous troughs, could have triggered the 1356 Basel earthquake.

Seismically active wedge structure beneath the Coalinga anticline, San Joaquin basin, California

We define the subsurface geometry, kinematics, and seismotectonics of the Coalinga anticline in the San Joaquin basin, central California. Using seismic reflection data and quantitative fault‐related folding techniques, we present a model of the Coalinga anticline that demonstrates that the structure is composed of a stack of imbricated structural wedges, related to two major fault ramps at depth, the deepest of which ruptured during the 1983 Coalinga (Mw = 6.5) earthquake. Because of the lack of basinward deformation and the observed fold shapes, these ramps are interpreted to sole to a common upper detachment, which acts as a back thrust, forming a structural wedge. This back‐thrust system generates the surface expression of the Coalinga anticline and extends to the surface as the Waltham Canyon fault and a series of related east dipping thrusts. This structural analysis helps reconcile the longstanding conflict between the southwest dipping preferred nodal plane of the 1983 main shock and the western vergence of the surface anticline. Furthermore, the seismic reflection data and our model suggest that two potentially seismogenic ramps and a major back thrust underlie the fold, rather than the single fault which has been inferred in previous studies. Using a relocated earthquake catalog, we document the three‐dimensional distribution of earthquakes over a 22 year period relative to both the main fault which ruptured in the 1983 event and within the structural wedge. This analysis indicates that the majority of moment release following the 1983 event occurred within the wedge itself, compatible with a model of wedge emplacement.

Kinematic interpretation and structural evolution of North Oman, Block 6, since the Late Cretaceous and implications for timing of hydrocarbon migration into Cretaceous reservoirs

ABSTRACTOn the basis of structural style and differences in Late Cretaceous evolution, the carbonate platform in northern Oman and the allochthonous wedge comprising deepwater sediments and oceanic crust in the Oman Mountains form distinct structural domains. Imbrication associated with the emplacement of the Semail Ophiolite and predominantly SW-verging thrusting of the Arabian Platform margin culminated in the late early Campanian. The structural grain of NW-trending thrust faults and contractional folds contrasts markedly with the style and grain of the region immediately south of the Oman Mountains (our study area) and implies strong strain partitioning. Kinematic indicators from subsurface data, combined with the age of growth faulting, provide the basis for the interpretation that maximum horizontal stress was oriented NW-SE in this foreland region rather than NE-SW during the Campanian. The dominant tectonic control on the formation of faults is believed to have been an oblique “collision” of the Indian Continent with the Arabian Plate during the Santonian-Campanian. Deformation in this domain was dominated by distributed strike-slip and normal faulting. This period of faulting was significant for two reasons: (1) The faults both enhanced existing structures and formed new traps. They also allowed vertical migration of hydrocarbons from Palaeozoic reservoirs (e.g. Haushi clastic accumulations) into Shu’aiba and Natih carbonates above. Until that time, some 75 Ma ago, oil was retained in Late Palaeozoic and older traps. This period of deformation is a “Critical Event” within the context of Oman’s hydrocarbon distribution.(2) Faults with NNW and WNW orientations that developed at that time appear to be directly associated with important fracture systems that affect the productivity of several giant fields comprising Natih and Shu’aiba carbonate reservoirs (e.g. Lekhwair, Saih Rawl).Following this tectonic event, late Maastrichtian to Palaeocene uplift and erosion in excess of 1,000 m, is recorded by truncation of the Aruma Group and Natih Formation, as well as part of the Shu’aiba Formation below the base Cenozoic unconformity. Seismic velocity and porosity anomalies from Lekhwair field in the northwest to the Huqf-Haushi High in the southeast, provide additional support for the areal distribution of this event. Around the Lekhwair and Dhulaima fields, the circular to elliptical subcrop pattern below this unconformity does not support the notion of a peripheral bulge related to the emplacement of the allochthon.The stress field changed during the late Cenozoic with the opening of the Red Sea and Gulf of Aden, and the collision of the Arabian Plate with the Iranian Plate. NE-SW-oriented maximum horizontal stress during the late Cenozoic led to the formation of major folds resulting in, for example, the surface anticlines over the Natih and Fahud fields as well as causing inversion along the Maradi Fault Zone. This may also have led to the uplift of the Oman Mountains. The regional northerly subsidence caused by crustal loading of the Arabian Plate gently tilted traps during the Pliocene-Pleistocene from Lekhwair to Fahud.

The structural geology of the High Central Zagros revisited (Iran)

The main purpose of this study in the High Central Zagros area near the townships of Lordegan and Yasuj was to investigate the nature of the High Zagros Fault (HZF) and the structural styles in the adjacent foreland. A second task was to determine whether the prolific petroleum plays productive further out in the Zagros Foreland would extend into the study area and if new petroleum plays could be recognized. Two selected cross-sections across and one just behind the HZF provide evidence for its overthrust nature. In some places the HZF exhibits a low-angle thrust plane, locally with a substantial amount of thrusting over the foreland. The interpretation is based on surface geological mapping, available well data, remote sensing from satellite data, aeromagnetic and high resolution gravimetry data, magnetotelluric, WARRP (wide angle reflection refraction profiling) and reflection seismic data. The integration of this unique set of newly acquired geophysical data complements recently published papers, which were based more on surface geology and rather limited subsurface data. The data were integrated to construct a regional balanced cross-section and to develop structural petroleum play concepts for the area investigated. Play potential may exist both in the overthrust terrain of the High Zagros as well as in the subthrust. Analysis of the width of the surface anticlines (or more precisely: their frequency) gives a first indication of the depth of the décollement plane. Within the adjacent Zagros Foreland evidence was found for the presence of multiple detachment planes above the ubiquitous deep Hormuz level, i.e. at the level of the Kazhdumi and the Pabdeh–Gurpi Formations. These intermediate décollement planes are thought to be responsible for the generation of deeper structural plays unrelated to the structuring that can be observed at the surface.

Two conceptual models of displacement transfer and examples

This paper presents two conceptual models of displacement transfer, reverse symmetry model and infinitely equal division model, based on the fault-bend folding theory. If the fault shape is held constant in the trend, then the distribution of slip magnitude, geometry of imbricate structures and its axial surface map all display reverse symmetry on the process of displacement transfer, as called reverse symmetry model in this paper. However, if the ramp height of thrust fault decreases gradually along its strike, the displacement is postulated to be equally and infinitely divided to every thrust that is formed subsequently, this kinematic process is described using infinitely equal division model. In both models, displacement transfer is characterized by the regular changes of imbricate thrusting in the trend. Geometric analysis indicates that the displacement transfer grads can be estimated using the tangent of deflective angle of hinterland structures. Displacement transfer is often responsible for the distortion and branching of the surface anticlines, especially in the region where the multi-level detachment structures is developed. We also present some examples from the frontal structures of the Southern Tianshan fold-and-thrust belt, Xinjiang, China. Displacement transfer between deep imbricate thrusts in the middle segment of Qiulitag anticline zone causes the Kuqatawu and Southern Qiulitag deep an-ticlines left-lateral echelon. The region, where these two deep anticlines overlap, is characterized by duplex structures, and extends about 18 km. The shallow anticline is migrated southward displaying obvious “S” form in this area.

A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran

SUMMARY The Zagros mountains of SW Iran are one of the most seismically active intra-continental fold-and-thrust belts on Earth, and an important element in the active tectonics of the Middle East. Surface faulting associated with earthquakes is extremely rare, and so most information about the active faulting comes from earthquakes. We use long-period teleseismic P and SH body waves to determine the orientation and depth of faulting in 16 new earthquakes, and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the Zagros. We use this information to investigate the style and distribution of active faulting in the Zagros, and how it contributes to the N–S shortening of the Arabia–Eurasia collision. When the data are ranked in quality and carefully evaluated, simple patterns are seen that are not apparent when routine catalogue data are taken at face value. An important change in the fault configuration occurs along strike of the belt. In the NW, overall convergence is oblique to the trend of the belt and the surface anticlines, and is achieved by a spatial separation (‘partitioning’) of the orthogonal strike-slip and shortening components on separate parallel fault systems. By contrast, in the SE, overall convergence is orthogonal to the regional strike and achieved purely by thrusting. In the central Zagros, between these two structural regimes, deformation involves parallel strike-slip faults that rotate about vertical axes, allowing extension along the strike of the belt. The overall configuration is similar to that seen in other curved shortening belts, such as the Himalaya and the Java–Sumatra trench. All the Zagros earthquakes we have been able to check have centroids shallower than ∼20 km and are confined to the upper crust. Many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover, which is active even beneath areas of known shallow structural decollement such as the Dezful embayment. The dominant style of shortening is high-angle reverse faulting with dips >30° though some lower-angle thrusting occurs in places. Active thrust and reverse faulting is relatively confined to the lower topography on the SW edge of the belt today, and only strike-slip faulting affects the higher topography. Profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the Simple Folded Belt, but has progressively migrated SW over the last 5 Ma. There is no evidence for a seismically active structural decollement, such as a low-angle thrust, beneath the Zagros, nor is there any seismic evidence for active subduction, either beneath the Zagros or beneath central Iran. Instead the Arabian margin seems to have shortened by distributed thickening of the basement. Only in the syntaxis of the Oman Line, at the SE end of the Zagros, is there any evidence for a low-angle thrust of regional extent. Here, earthquakes continue 50 km north of the Zagros Thrust Line (the geological suture between the Arabian margin and central Iran) reaching depths of ∼30 km, and may represent thrusting of Arabian basement beneath central Iran to this extent.

Quaternary deformation of the Bajo Segura blind fault (eastern Betic Cordillera, Spain) revealed by high-resolution reflection profiling

The blind reverse Bajo Segura Fault is located at the eastern extreme of the Trans-Alboran shear zone (Betic Cordillera, southeast Iberian Peninsula). The surface expression of recent activity of this blind ENE–WSW fault is represented by coseismic surface anticlines and growth synclines on both sides of the anticlines. In the synclines, the deformation of the most recent Quaternary materials is obscured by a sedimentary unit more than 30 m thick which was deposited during the later part of the Late Pleistocene and the Holocene. The present study reports three high-resolution seismic profiles made in the northern growth syncline, which was the one developed most by the Bajo Segura Fault. In these seismic profiles we recognize the boundary between pre-growth strata and growth strata. This marker, Early Pliocene in age, dates the start of the activity of this blind reverse fault. The geometry observed in the seismic profiles of the syntectonic strata, dating from the Late Pliocene and Quaternary, indicates a limb rotation folding mechanism. On seismic profile 2, the complex geometry of the Benejúzar anticline forelimb can be attributed to several splay faults close to the surface of Bajo Segura Fault.

Reinterpreted Oriskany structure at the North Summit field, Chestnut Ridge anticline, Pennsylvania

A widely accepted structural model for folds in the outer central Appalachian foreland is partially based on the geologic structure of the North Summit field. The model includes a simple surface anti cline that is detached in Silurian Salina Group salts and cored by imbricated Devonian Tully-Helderberg rocks thrust inward toward a depressed axial low. New data from wells at North Summit show that the core of the Chestnut Ridge anticline is not filled with imbricated reservoir rocks but that the reservoir is deformed into a series of faulted folds. Gas was trapped by a combination of closure and sealing faults. Space problems within collapsing synclines above competent reservoir rocks, the Huntersville-Helderberg lithostructural unit, are resolved by distortion and evacuation of overlying, incompetent Hamilton rocks. Huntersville-Helderberg rocks deformed into a variety of structural shapes, not solely the imbricated model that traditionally has been applied to Plateau folds of the central Appalachian foreland. Disparity in relief between pre- and post-Salina rocks indicates that primary detachment occurs in evaporite beds of the Salina Group. Erratic dips in presalt units and unresolved differences in structural relief between synclines flanking the Chestnut Ridge anticline, however, suggest that the basal detachment lies within the Martinsburg-Reedsville shales under the Allegheny Mountains. Regional decollement in Martinsburg shales ends in a triangle zone at the Intra-Plateau Front, at the Chestnut Ridge anticline, whereas the Salina decollement continues westward under the Pittsburgh Plateau.

The Dead Sea Fault and related subsurface structures, Gaziantep Basin, southeast Turkey

Late Cretaceous and Miocene collisions of the Arabian, Anatolian and Eurasian plates, as shown by widespread ophiolitic exposures along the suture, created favourable geological conditions for the formation of the surface and subsurface structures in the Gaziantep Basin, southeast Turkey. The late Cretaceous (Maastrichtian) emplacement of the Kocali–Karadut ophiolite complex induced subsidence in the northwestern zone of the Kastel Basin during the early Alpine Orogeny and influenced the structural evolution of the foreland area. The Dead Sea Fault, which originated in the Red Sea in Miocene time, propagated towards the northwest in the Suez Gulf and the north-northeast in southeast Turkey, and influenced the structural evolution of the Gaziantep Basin. These two major tectonic events produced many thrusts, thrust-related subsurface and surface anticlines, faults, fractures, flower structures and basaltic flows in the area. Geological and geophysical investigations indicate the existence of two important structural phases. The older structures were formed during the late Cretaceous movements, but they have been reactivated by latest Miocene tectonic activities with appearance of the Strands of the Dead Sea Fault in the sedimentary basin. The geothermal studies show also that, as a result of the Tertiary transgressions and volcanic activity, the northern and southern sectors of the Gaziantep Basin underwent differing subsidence and structural histories.

Structure and shortening of the Kangra and Dehra Dun reentrants, Sub-Himalaya, India

Research Article| August 01, 1998 Structure and shortening of the Kangra and Dehra Dun reentrants, Sub-Himalaya, India Peter M. Powers; Peter M. Powers 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331 Search for other works by this author on: GSW Google Scholar Robert J. Lillie; Robert J. Lillie 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331 Search for other works by this author on: GSW Google Scholar Robert S. Yeats Robert S. Yeats 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (8): 1010–1027. https://doi.org/10.1130/0016-7606(1998)110<1010:SASOTK>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter M. Powers, Robert J. Lillie, Robert S. Yeats; Structure and shortening of the Kangra and Dehra Dun reentrants, Sub-Himalaya, India. GSA Bulletin 1998;; 110 (8): 1010–1027. doi: https://doi.org/10.1130/0016-7606(1998)110<1010:SASOTK>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Surface geology, oil-well, seismic-reflection, and magnetostratigraphic data are integrated to evaluate the structural style and the shortening rate at the Himalayan front (Sub-Himalaya) of northwest India. The Sub-Himalaya, between the Main Boundary thrust and the Himalayan Frontal fault, is the primary surface expression of shortening between the Himalaya and the Indian plate. At certain locations, the Himalayan Frontal fault is a blind thrust beneath anticlines of Siwalik (Tertiary) molasse, parallel to the Himalayan orogen. The Main Boundary thrust is sinuous, so the width of the Sub-Himalaya ranges from 30 to 80 km. Where the Sub-Himalaya is narrow (Nahan salient), Tertiary rocks are exposed in imbricate thrust sheets; where the Sub-Himalaya is broad (Kangra and Dehra Dun reentrants), alluvium fills wide synclinal valleys (duns).Seismic-reflection data reveal that surface anticlines form in association with south-vergent thrusts that root in a decollement at the base of the Tertiary section. Reflection profiles and well data also indicate that the basement lithology changes northward from Precambrian crystalline rocks beneath the Indo-Gangetic plains to Precambrian and Cambrian metasedimentary rocks beneath the Sub-Himalaya. The Sub-Himalayan decollement dips 2.5° northward beneath the Kangra reentrant, but it is steeper, 6°, beneath the Dehra Dun reentrant.The Kangra and Dehra Dun reentrants display fault-propagation folds having steep limbs in the north, and fault-propagation and fault-bend folds that have gently north-dipping limbs in the south. A balanced cross section of the Kangra reentrant shows that a minimum of 23 km shortening has occurred since 1.9–1.5 Ma, yielding a shortening rate of 14 ± 2 mm/yr. Shortening has occurred at a rate of 6–16 mm/yr across the Dehra Dun reentrant. These data are similar to other published shortening rates and indicate that approximately 25% of the total India-Eurasia convergence at this longitude is accommodated within the Sub-Himalaya. Given continued convergence and the presence of overpressured wells in the Kangra reentrant, the region is likely at risk from moderate and/or great earthquakes in the future. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A breakthrough in the quality of seismic data from the fold belt of Papua New Guinea

BP and other operators have been exploring for hydrocarbons in the Papuan Fold Belt for 70 years. From the earliest ventures it was evident that the broad, elevated surface anticlines were indicative of similar structures at depth. Geologic field‐work confirmed the surface structures and allowed cross‐sections of the subsurface to be generated by extrapolation to depth. This approach has become the paradigm for exploration in the fold belt. It has allowed exploratory drilling to proceed with some significant success.

Structural Development of Jebel Abd Al Aziz, Northeast Syria

ABSTRACT Jebel Abd Al Aziz, one of the most prominent topographic features in northeastern Syria, is a large surface anticline. An integrated structural and stratigraphic study conducted by Unocal from 1988 to 1995 resulted in recognition that Jebel Abd Al Aziz originated from inversion of a pre-existing graben. Understanding the complex structural and stratigraphic history of the Jebel Abd Al Aziz is important to hydrocarbon exploration and development in the northern Arabian tectonic plate. This importance is demonstrated by the strong correlation between hydrocarbon productive areas and the areas of Plio-Pleistocene structural inversion. Our study illustrates how the evolution of this structure is recorded in its local stratigraphy. Prior to the development of the Jebel Abd Al Aziz structure, Senonian shelf carbonates prograded southward from Turkey into the Palmyride-Sinjar Trough that extended from west central to northeastern Syria. The shelf edge of this carbonate system was south of and subparallel to the Syrian border. In the Jebel Abd Al Aziz area, fine-grained basinal mudstones were deposited on a thin, transgressive, rudistid, bioclastic unit. In Early Maastrichtian, an east-west-trending graben developed at the present site of Jebel Abd Al Aziz. Reactivated northwest- and northeast-striking faults bound structural blocks within the graben. Seismic data indicate that the edges of the rift basin were deeply eroded. Valleys, cut into the sides of the basin along the trend of the older cross-cutting regional faults, exposed Carboniferous and possibly older strata. Olistostromes formed along the basin-bounding fault scarps and small turbidite fans developed at the channel mouths. Paleocurrent direction data from the turbidite sand bodies corresponds well with the trends of the valleys mapped on seismic data. Maastrichtian-age sediments are largely confined to the graben proper. Early Tertiary sediments filled a wider basin, but there is evidence that minor episodic inversion on some northeast and northwest trending faults occurred during the Eocene and early Miocene. The main inversion of the Jebel Abd Al Aziz structure occurred in the Late Pliocene and Pleistocene. Inversion produced a large fault-propagation fold above east-west trending faults near the northern margin of the graben. Smaller folds developed above other graben-bounding faults and the northeast- and northwest-striking faults within the graben underwent oblique slip during the deformation.

Quantitative Evaluation of Fractures on Monkshood Anticline, a Detachment Fold in the Foothills of&amp;nbsp; Western Canada

Monkshood anticline is a well-exposed surface anticline located in the Foothills in northeastern British Columbia. Extension fractures are well developed in the Prophet Formation (a carbonate-shale-chert sequence) throughout this fold. Virtually all of these fractures are filled with some combination of calcite and quartz cements. The fractures formed and the mineral cements were deposited during the fold development. Most fractures on Monkshood anticline have formed at high angles to bedding, and they commonly fall into two to three distinct orientation sets. The dominant fracture trend is aligned with the fold axis through the backlimb of the fold, but there is considerable variance in the dominant orientation in the forelimb. An order of magnitude variation is found in fracture densities across this fold, and over two orders of magnitude variation occur in mean fracture aperture. These variations in density and aperture do not correlate with particular structural positions. Fracture trace lengths exhibit power-law distribution patterns, suggesting fractal character. Fracture aperture displays a roughly linear correlation to volumetric fracture strain, but shows no consistent association with either density or trace length. Stochastic modeling of the fracture networks on Monkshood anticline suggests density is the primary factor affecting fracture network connectivity, but fracture size also can play an important supporting role. The occurrence of a dominant fracture orientation set can impart a significant directional bias to connectivity. Measurement stations on Monkshood anticline that have both high fracture densities (for connectivity) and substantial fracture apertures (for conductance) occur primarily in midlimb positions.