Half-graben Research Articles

Petroleum systems of Chinese nonmarine basins

AbstractThe petroleum systems of Chinese nonmarine rifted and depression basins, dominated by lacustrine strata, have distinctive source rocks, reservoir types and trap characteristics. The rifted basins are characterized by numerous faults and multiple structural salients and sags (half grabens). Sags include many subdivisions and smaller isolated sags. Most sags constitute relatively independent petroleum systems that have efficiently generated and expelled hydrocarbons, have excellent reservoir properties in a variety of sand‐body types, and have multiple traps. These attributes impart a petroliferous character to the entire sag. Depression basins (intracratonic flexural basins) developed on large cratons and hosted large lacustrine systems. They feature very gentle structure, large deltaic sand‐bodies, source rocks in extensive contact with sand‐bodies, heterogeneous low‐porosity‐low‐permeability reservoirs and large, widespread lithology‐controlled pools. In recent years, large oil and gas reserves have been discovered in these two types of lacustrine‐dominated basins, contributing significantly to the growth of reserves in onshore China, and stratigraphic oil and gas pools have become the major type of accumulation in nonmarine lacustrine basins.

The role of deep seismic reflection data in understanding the architecture and petroleum potential of Australia's onshore sedimentary basins

The onshore energy security program, funded by the Australian Government and conducted by Geoscience Australia, has acquired deep seismic reflection data across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia. Detailed interpretation of deep seismic reflection profiles from four onshore basins, focussing on overall basin geometry and internal sequence stratigraphy, will be presented here, with the aim of assessing the petroleum potential of the basins. At the southern end of the exposed part of the Mt Isa Province, northwest Queensland, a deep seismic line (06GA–M6) crosses the Burke River structural zone of the Georgina Basin. The basin here is >50 km wide, with a half graben geometry, and bounded in the west by a rift border fault. Given the overall architecture, this basin will be of interest for petroleum exploration. The Millungera Basin in northwest Queensland is completely covered by the thin Eromanga Basin and was unknown prior to being detected on two seismic lines (06GA–M4 and 06GA–M5) acquired in 2006. Following this, seismic line 07GA–IG1 imaged a 65 km wide section of the basin. The geometry of internal stratigraphic sequences and a post-depositional thrust margin indicate that the original succession was much thicker than preserved today and may have potential for a petroleum system. The Yathong Trough, in the southeast part of the Darling Basin in NSW, has been imaged in seismic line 08GA–RS2 and interpreted in detail using sequence stratigraphic principles, with several sequences being mapped. Previous studies indicate that the upper part of this basin consists of Devonian sedimentary rocks, with potential source rocks at depth. In eastern South Australia, seismic line 08GA–A1 crossed the Cambrian Arrowie Basin, which is underlain by a Neoproterozoic succession of the Adelaide Rift System. Stratigraphic sequences have been mapped and can be tied to recent drilling for mineral and geothermal exploration. Shallow drill holes from past petroleum exploration have aided the assessment of the petroleum potential of the Cambrian Hawker Group, which contains bitumen in the core, indicating the presence of source rocks in the basin system.

Recent paleoseismic investigations in Northern and Western Thailand

Recent paleoseismic investigations have identified a number of active faults in Northern and Western Thailand. Northern Thailand is an intraplate basin and range province, comprised of north-south-trending Cenozoic intermontane grabens and half grabens, bounded by north- to northwest-striking normal to normal-oblique faults and northeast-striking left-lateral strike-slip faults. The basin-bounding normal faults are marked by steep, linear range fronts with triangular facets and wineglass canyons and have slip rates of 0.1 to 0.8 mm/yr. Based on limited data, the average vertical displacement-per-event is about 1.0 to 1.5 m. These faults are characterized by recurrence intervals of thousands to tens of thousands of years and are capable of generating earthquakes up to moment magnitude (M) 7, and larger. The northeast-striking strike-slip faults are marked by shutter ridges, and deflected drainages. Slip rates are 3 mm/yr or less. Western Thailand is dissected by a number of northwest- and north-northwest-striking, right-lateral strike-slip faults related to the Sagaing Fault in Myanmar. Although showing much less activity than the faults in neighboring Myanmar, these faults display abundant evidence for late Quaternary movement, including shutter ridges, sag ponds, and laterally offset streams. The slip rate on these faults is estimated to be 0.5 to 2.0 mm/yr. These faults are considered capable of generating maximum earthquakes of up to M 71/2.

J. B. Dawson 2008. The Gregory Rift Valley and Neogene–Recent Volcanoes of Northern Tanzania. Geological Society Memoir No. 33. viii + 102 pp. London, Bath: Geological Society of London. Price £65.00, US $130.00; GSL members’ price £32.50, US $65.00; AAPG/SEPM/GSA/RAS/EFG/PESGB members’ price £39.00, US $78.00 (paperback). ISBN 978 1 86239 267 0.

J. B. Dawson 2008. The Gregory Rift Valley and Neogene–Recent Volcanoes of Northern Tanzania. Geological Society Memoir No. 33. viii + 102 pp. London, Bath: Geological Society of London. Price £65.00, US $78.00 (paperback). ISBN 978 1 86239 267 0. - Volume 147 Issue 2

Interpretation of Late Ordovician glaciogenic reservoirs from 3-D seismic data: an example from the Murzuq Basin, Libya

AbstractUnderstanding the recessional behaviour of ancient pre-Cenozoic ice sheets based on seismic reflection studies is generally difficult through scarcity of data. In North Africa, however, hydrocarbon exploration has produced high quality seismic reflection datasets that permit an analysis of the morphology and internal sedimentary architecture of incisions of Late Ordovician age related to the Hirnantian glaciation. Analysis of a high-resolution 3-D seismic dataset covering a small area in western Libya (the N Murzuq Basin) reveals a sharply defined, WNW–ESE-oriented palaeo-escarpment, with a higher (cliff-forming) western margin and a lower (basin-forming) eastern margin. The palaeo-escarpment defines the western flank of a sub-basin extending up to 60 km in width, known as the Awbari Trough. The escarpment and the trough are interpreted as the morphological expression of a major unconformity dividing pre-glacial sediments below from Late Ordovician (?Hirnantian) glacially related sediments above. Two hypotheses are considered for the origins of both the escarpment and the Awbari Trough: (1) as a tectonic feature such as a half graben that was active during sedimentation and (2) a glacially related palaeotopography, with the latter interpretation preferred, owing to the lack of evidence for syn-sedimentary fault activity. The width of the Awbari Trough compares to the large-scale cross-shelf troughs in modern high latitude settings, such as the Barents Shelf, produced by ice streams. The Awbari Trough was progressively filled in by gravity flow deposits throughout the course of the glaciation, until the initial incision became filled in with sediments during an overall glacial retreat phase and ceased to influence sedimentation patterns. Glacial re-advance across the basin produced a second unconformity observed in seismic data. Above this unconformity, meltwater processes incised a shallow (~ 20 m) and wide (~ 5 km) subglacial tunnel valley. Stabilization of the ice front prior to its ultimate retreat resulted in the deposition of a delta complex prior to the Early Silurian transgression.

No Moho uplift below the Baikal Rift Zone: Evidence from a seismic refraction profile across southern Lake Baikal

The late Cenozoic Baikal Rift Zone (BRZ) in southern Siberia is composed of several individual topographic depressions and half grabens with the deep Lake Baikal at its center. We have modeled the seismic velocity structure of the crust and uppermost mantle along a 360 km long profile of the Baikal Explosion Seismic Transects (BEST) project across the rift zone in the southern part of Lake Baikal. The seismic velocity structure along the profile is determined by tomographic inversion of first arrival times and 2‐D ray tracing of first arrivals and reflections. The velocity model shows a gently deepening Moho from the Siberian Platform (41 km depth) into the Sayan‐Baikal Fold Belt (46 km depth). We can exclude the presence of any Moho uplift around the ∼10 km deep sedimentary graben structure of southern Lake Baikal. The lower crust includes a distinct 50–80 km wide high‐velocity anomaly (7.4–7.6 ± 0.2 km/s), slightly offset to the northeast from the rift axis. We interpret this feature as resulting from mafic intrusions. Their presence may explain the flat Moho by compensation of lower crustal thinning by intrusion of mafic melts. The Pn wave velocities (8.15–8.2 km/s) are normal for the area and do not show any sign of decompression melting in the sub‐Moho mantle, such that possible lithosphere thinning has not reached the base of the crust. On the basis of the results of the BEST project, we suggest that the BRZ is formed by passive rifting in the rheologically weak suture between the Siberian Platform and the Amurian plate.

Crustal structure of the NW Moroccan margin from deep seismic data (SISMAR Cruise)

During the SISMAR cruise (spring 2001), new deep seismic data were acquired on the Atlantic Moroccan margin. The purpose of the present work is to analyse and to interpret a part of the data consisting of four wide-angle seismic profiles (4, 5, 8 and 10) recorded offshore El Jadida. Lines 4 and 5, perpendicular to the margin, show a crustal thickness of 35 km under the western Moroccan Meseta which thins progressively seaward where it reaches a minimum thickness of 10 km. The calculated velocity models show that this crust consists of two crustal layers overlain by a sedimentary cover corresponding to the post-Paleozoic sediments, characterised by a low velocity. The detailed shallow structure is obtained primarily from multichannel seismic data. Half grabens are imaged and display deeper basins located at the foot of the continental slope where the sedimentary cover reaches a maximum thickness of 6 km. The sedimentary strata are disrupted by salt migration along faults and forming diapirs. The first crustal unit corresponds to the upper crust, with p-wave velocities ranging from 6 km/s at the top to 6.5 km/s at the base. The lower crust represented by the second unit is characterized by velocities increasing from 6.7 to 7.5 km/s at the base of the lower crust.

Patterns and dynamics of rifting on passive continental margin from shelf to slope of the northern South China Sea: Evidence from 3D analogue modeling

Affected by thermal perturbation due to mantle uprising, the rheological structure of the lithosphere could be modified, which could lead to different rifting patterns from shelf to slope in a passive continental margin. From the observed deformation style on the northern South China Sea and analogue modeling experiments, we find that the rift zone located on the shelf is characterized by half grabens or simple grabens controlled mainly by long faults with large vertical offset, supposed to be formed with normal lithosphere extension. On the slope, where the lithosphere is very hot due to mantle upwelling and heating, composite grabens composed of symmetric grabens developed. The boundary and inner faults are all short with small vertical offset. Between the zones with very hot and normal lithosphere, composite half grabens composed of half grabens or asymmetric grabens formed, whose boundary faults are long with large vertical offset, while the inner faults are relatively short. Along with the thickness decrease of the brittle upper crust due to high temperature, the deformation becomes more sensitive to the shape of a pre-existing weakness zone and shows orientation variation along strike. When there was a bend in the pre-existing weakness zone, and the basal plate was pulled by a clockwise rotating stress, the strongest deformation always occurs along the middle segment and at the transition area from the middle to the eastern segments, which contributes to a hotter lithosphere in the middle segment, where the Baiyun (白云) sag formed.

Evolution of the Miocene sedimentary fill of the Gediz Graben, SW Turkey

The Gediz Graben is a continental extensional basin filled with Miocene to Recent sediments. The evolution of the graben can be expressed in terms of two phases: (i) Miocene half graben phase; and (ii) post-Miocene graben phase. The Miocene half graben phase was governed by the southern margin structure evolution of which significantly controlled the stratigraphic variability in the basin. Alluvial, fluvial and lacustrine systems developed in the graben as a function of distance to the southern margin. Thus, alluvial deposits with coarse-grained facies dominate the southern margin and grade into finer fractions of fluvial and/or lacustrine deposits to the north. This depositional architecture repeated three times during the Miocene and deposited Alaşehir, Çaltılık and Gediz formations. These formations depict distinct thickness and grain size decrease from south to north to emphasize the half-graben configuration for the favour of the southern margin. The post-Miocene graben phase started with the faulting of the northern margin. This resulted in relatively balanced partitioning of the subsidence between the two margins although the preceding asymmetry was inherited by the graben. Sediment influx became bipolar and alluvial fan systems developed along the both margins.

Seismic stratigraphy of the western South Korea Plateau, East Sea: implications for tectonic history and sequence development during back-arc evolution

The western South Korea Plateau in the East Sea (Sea of Japan) is occupied by rifted continental fragments formed in association with the early phase of back-arc opening. The present study focuses on the seismic stratigraphy of the sedimentary succession and the underlying acoustic basement in this region, based on closely spaced multichannel seismic reflection profiles. The sedimentary succession occurs mainly within a series of subparallel basement troughs (grabens or half grabens) bounded by faulted continental blocks (horsts) or volcanic ridges, and commonly floored by extrusive volcanic rocks showing hyperbolic reflectors. These features are strongly suggestive of continental rifting accompanied by normal faulting, volcanic activity and high rates of basin subsidence. The sedimentary succession can be subdivided into four seismic units. Unit 1 is characterized by short and irregular high-amplitude reflectors and interpreted as a syn-rift deposit consisting of a non-marine volcanics/sediment complex in topographic lows. Units 2 and 3 formed in an open marine environment during the Middle Miocene to Early Pliocene, characterized by an onlap-fill and later draping marine sedimentary succession dominantly composed of hemipelagic sediments and turbidites with frequent intercalation of mass-flow deposits. Along the western margin of the plateau, these units were deformed under a compressional regime in the Early Pliocene, associated with the back-arc closing phase. Unit 4 (deposited since the Early Pliocene) comprises hemipelagic sediments and turbidites with evidence of sporadic slides/slumps.

Phanerozoic tectonics of south China block: New insights from the polyphase deformation in the Yunkai massif

The Yunkai massif is a key region to decipher the tectonic evolution of the south China block since the contained high‐grade metamorphic rocks experienced a polyphase deformation. The earliest event (D1) corresponds to a top‐to‐the‐northwest ductile shearing, coeval with amphibolite facies metamorphism, probably developed during a postorogenic synmigmatization extensional event occurred in early Paleozoic time. The main ductile deformation (D2) is top‐to‐the‐northeast shearing, occurred in early Mesozoic time, coeval with amphibolite to greenschist facies metamorphism and associated with the development of northeast verging recumbent folds within the Neoproterozoic‐Paleozoic sedimentary cover of the south China block. The tectonic significance of D2 is possibly linked with the subduction of Indochina beneath south China along the Song Ma belt of North Vietnam. Another compressional event (D3) of Late Jurassic to Cretaceous age is indicated by SE verging recumbent folds with NE–SW trending axes developed both in the sedimentary cover and gneissic basement. The northwestward subduction of the Pacific plate or the SE thrusting of the Songpan‐Ganzi fold belt upon the Yangtze craton can be considered as the geodynamic causes of this late Mesozoic event. Last, NE–SW trending brittle sinistral strike‐slip and normal faulting (D4) control the massif uplifting and the subsiding of continental half grabens. This structure, which is widespread in the eastern margin of the Eurasia continent, can be related either to the westward subduction of the Paleo‐Pacific plate or to lithosphere removal during the late Mesozoic.

Neocomian to early Aptian syn-rift evolution of the normal to oblique-rifted North Gabon Margin (Interior and N'Komi Basins)

The North Gabon coastal rift basins consist of a set of 130–150 long-segment asymmetrically tilted half grabens (Interior Basin) and 000–020 short-segment en échelon half grabens (N'Komi Basin) separated by 040–060 major transverse faults. Tectono-sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism over continental sedimentation. During Berriasian to early Barremian times, uniform uniaxial 040–060 extension was responsible for the stretching of the brittle upper crust over a 100-km wide domain. During late Barremian–early Aptian times, the main locus of extension stepped westward resulting in severe end-rift uplift and erosion of the failed Interior and N'Komi rift basins. Early Cretaceous coastal rifts in North Gabon display a wide range of styles from oblique rifting (N'Komi Basin), normal rifting (Interior Basin) to transform rifting. The pre-existing Precambrian tectonic fabric exerts a strong control over the mode and over the 100–300 km-scale segmentation of the rifting.

A seismic stratigraphic analysis of Mariana forearc basin evolution

New seismic data collected in the 14.5°–18.5°N Mariana segment of the Izu‐Bonin‐Mariana island arc system image six seismic stratigraphic sequences that can be mapped throughout the inner forearc. These sediments were most likely deposited from 35 Ma to the present. The oldest stratigraphic Units 1, 2, and 3 are syn‐rift volcaniclastic deposits. Unit 4 deposits accumulated during a period of mild structural inversion, which resulted in several isolated reverse‐faulted anticlines within the forearc sedimentary basin. A late period of extensional deformation began near the end of Unit 5 deposition and continued through Unit 6 sedimentation to the present. Seismic lines show that the basement of the forearc is composed of large rotated fault blocks and half grabens with NE, NW, and NNE trends. Fault offset calculations show that basement faults with dips between 45° and 50° account for only ∼4% total extension in the forearc. South of 16.3°N, normal growth faults initiated during basement extension offset the frontal arc high from a deep forearc basin. From correlations with the known geologic history, we hypothesize that extension during deposition of Units 1 through 3 corresponds to rifting of the Eo‐Oligocene Arc, between ∼35 Ma and 29 Ma, older deposits being too thin to be seismically resolvable. Localized compression during Unit 4 accumulation occurred some time after Eo‐Oligocene rifting in the early Miocene. Late‐stage normal faulting near the end of deposition of Unit 5 and throughout Unit 6 accumulation may be associated with the opening of the Mariana Trough backarc basin from ∼8 Ma years to the present. There is a higher density of these later faults in the inner forearc between 15.5° and 17°N and in the outer forearc between 14°N and 18°N. Recent extension is at least partially accommodated by reactivation of older basement faults with the same NE, NW, and NNE‐ trends. Stratigraphic relationships indicate that the inner forearc south of 16.3°N has differentially subsided and tilted trenchward, possibly as a result of a recent change in subducting slab geometry or subducted relief under the forearc.

The Timmins-Porcupine Gold Camp, Ontario: Anatomy of an Archean Greenstone Belt and Ontogeny of Gold Mineralization

New structural and geochronological data and interpretations from the Timmins-Porcupine gold camp, Abitibi subprovince, more precisely define the stratigraphy and the generation, timing, and effects of deformation. Tisdale (basalts) and Porcupine (volcaniclastic and turbiditic sedimentary rocks) assemblage rocks exhibit three major foliations (S2, S3, S4, plus a late constrictional fabric), of which S3 and S4 are also present in younger Timiskaming assemblage sedimentary rocks. Pre-Timiskaming folding has long been recognized in Tisdale assemblage rocks, but no foliation had previously been attributed to such an event. D1 regional uplift and extension produced a low-angle unconformity between the Tisdale and Porcupine assemblages, with partial excision of upper Tisdale stratigraphy and perhaps of the Blake River assemblage. D2 involved imbrication of a set of south-over-north thrust sheets rooted in the Porcupine-Destor deformation zone. D3 en echelon folds along the northern flank of the Porcupine-Destor deformation zone are a result of left-lateral strike-slip along this zone. The Timiskaming basin, lying along the curvilinear trace of the Porcupine-Destor deformation zone, formed as a D3 transtensional half graben, and the Dome fault formed its faulted margin. S4 foliation formed during subsequent right-lateral strike-slip movement and folding of S3 foliation. Intense D5 constrictional strain deformed earlier lineations, as the previously dilatational jog that hosts Timiskaming assemblage rocks acted as a compressional jog during right-lateral movement along the Porcupine-Destor deformation zone. These D2 to D5 phases represent an overall thrusting–strike-slip transpressional regime. There were several phases of gold mineralization in the Timmins-Porcupine gold camp. Clasts of ankerite veins occur in Timiskaming assemblage conglomerate at the Dome mine, and D3 foliation and folds crosscut early ankerite veining and alteration and some quartz veining. Thus, low-grade mineralization and possibly the quartz-fuchsite veins predate Timiskaming sedimentation. However, the bulk of gold postdates Timiskaming sedimentation. Quartz-carbonate-tourmaline-gold veins (Hollinger-McIntyre, Hoyle Pond, Dome, Aunor–Delnite mines) formed within Timiskaming assemblage and Tisdale assemblage rocks. These gold deposits developed largely as oblique slip and extensional vein arrays formed during north-south shortening and local strike-slip faulting. Younger deposits (Pamour) are associated with D4 dip-slip faulting and were weakly deformed during late D4. Thus, the largest known Archean lode gold camp formed throughout protracted orogenesis consisting of a broadly transpressional regime with unusually well developed early thrusting.

Modern Flood Deposition, Erosion, and Fan-Channel Avulsion on the Semiarid Red Canyon and Palomas Canyon Alluvial Fans in the Southern Rio Grande Rift, New Mexico, U.S.A.

Abstract In the summer of 2006, large floods occurred on the Red Canyon (footwall-derived) and Palomas Canyon (hanging-wall-derived) alluvial fans, which are located on opposite sides of the Rio Grande floodplain in the Palomas half graben of the southern Rio Grande rift. The initial phase of the Red Canyon flood took place on the active central fan lobe, but subsequent avulsion of the main channel shifted the flood to the south, where it eroded all but two small relicts of the former inactive, toe-cut lobe before depositing up to 2 m of coarse gravel. Deposition of 0.5 m of gravel-size detritus by the Palomas Canyon flood was largely confined to the active, incised channel and to gravel bars that built into the river, although floodwater spilled over the southern bank of the channel near its mouth, depositing a thin (< 15 cm) veneer of fine sand on the floodplain. Measurements of mean and maximum grain size of the flood deposits, channel-bed slopes, and flood depth based on vegetative high-water marks allow estimates of mean flood velocity (Red Canyon = 8.6 m s−1; Palomas Canyon = 7.4 m s−1), floodwater discharge (Red Canyon = 971 m3 s−1; Palomas Canyon = 179 m3 s−1), and volume of flood deposits (Red Canyon = 3.19 × 105 m3; Palomas Canyon = 1.72 × 104 m3). Mapping of the modern floodplain and outcrop analysis of the Palomas Formation (Pliocene–lower Pleistocene) in the Palomas basin provide insight into facies architecture in half grabens. Footwall–derived alluvial-fan detritus consists principally of shallow, multistory gravel–conglomerate channels produced by channel avulsion across the surface of the small fans. Asymmetrical basin tilting causes toe-cutting of the footwall-derived fans by the axial river and incision of hanging-wall-derived alluvial-fan channels, the latter resulting in ribbon-shaped gravel–conglomerate channels interbedded with finer-grained, distal-fan sediment. The Red Canyon and Palomas Canyon floods changed the location, width, and depth of the Rio Grande, causing flooding of the western floodplain for ~ 1 km upstream of the constriction. Because the Rio Grande has been dammed since 1916, it no longer has the stream power to effectively redistribute large volumes of fan detritus and must be dredged in order to avoid upstream flooding by the river.

Three-dimensional inversion modelling of a Neoproterozoic basin in the southern Prince Charles Mountains, East Antarctica

Three-dimensional inversion modelling of airborne magnetic data acquired in the Prince Charles Mountains, East Antarctica, provides an insight into the sub-ice distribution, and three-dimensional geometry of a Neoproterozoic sedimentary basin. A three-dimensional starting model was created from two two-dimensional GM-SYS modelling and our current geological understanding of the study area. Three-dimensional VPmg inversion modelling was performed on the aeromagnetic data to obtain an acceptable fit between the observed response collected in the field, and the calculated response of the three-dimensional model. Modelling suggests that the base of the basin undulates and is relatively unstructured, however the margins of the basin thicken from north (~ 4 km) to south (~ 11 km). The volume of the Sodruzhestvo Group that in-fills the sedimentary basin has been calculated at approximately 35 000 km 3. Modelling of Banded Iron Formations at the southern margin of the basin and a sharp magnetic contrast in the north, reveal that both contacts dip toward the south. We interpret this asymmetric geometry of the sedimentary basin as having began as a set of half grabens, bounded by an underlying listric fault that flattened at depth below the sedimentary basin. Subsequent Early Palaeozoic inversion of this structure resulted in reactivation along the low-angle basal detachment, but rather than taking its original course underneath the sedimentary basin, the fault ramped up along the southern margin. This process caused exhumation of the underlying Banded Iron Formations, which are now juxtaposed at similar crustal levels to the Neoproterozoic cover rocks.

Magnetostratigraphy of the Zanda basin in southwest Tibet Plateau and its tectonic implications

The Zanda basin is one of the very important basins at the north slope of the Himalaya Range. Thus the study of the basin strata will provide critical information about the tectonic evolution of the Himalayan Orogenic Belt. 268 oriented block samples were collected in the 750-m-thick sections of the Zanda basin. The characteristic remanent magnetization (ChRM) was isolated that decays linearly to the origin between 500°C and 690°C for most studied samples. An age range of 9.5–2.6 Ma was estimated from the correlation between our observed polarity column and the Geomagnetic Polarity Time Scale (GPTS). The age of the Zanda basin does not support the models that the South Tibetan Detach system (STDS) is one of the basin controlling faults. Given the sedimentological features in the basin and the tectonic features at the north edge of the basin, the Zanda basin was a half graben that was possibly controlled by the Karakorum fault on the northeast.

Neogene subsidence and stratigraphy of the Simeulue forearc basin, Northwest Sumatra

Interpretation of new multi-channel seismic data reveals the Neogene subsidence history of the Simeulue forearc basin, located off northwestern Sumatra. The trench-parallel elongated basin extends over 260 km in NW–SE direction and 100 km in SW–NE direction. Maximum water depth is about 1300 m and the Neogene sedimentary fill shows a thickness of up to 5 s two-way traveltime. We identified three stages of subsidence evolution after the formation of a regional basal Neogene unconformity which are characterized by changing accumulation space, sedimentation rates, source areas and tectonic phases. An Early/Mid Miocene stage is marked by occurrence of subsidence in several half grabens forming along the western border of the basin. During the Late Miocene/Pliocene this small scale subsidence changed to a steadily subsiding trench-parallel trough. The present setup of the forearc region under the influence of strike-slip faults due to oblique subduction is active at least since this time as evidenced by wrench faulting. At the end of this stage subsidence expanded significantly eastwards drowning a large carbonate platform that evolved in the then shallow southern and eastern parts of the basin. Presently, the central part of the Simeulue basin is subject to inversion, probably related to reactivation of Early/Mid Miocene half grabens.

Sedimentology and geochemistry of the middle miocene playa lake evaporites in the Gürün basin (S of Sıvas), central Anatolia, Turkey

The Gurun basin is a half graben filled by alluvial, fluvial and playa-lake deposits of the Gurun Formation accumulated under the N−S direction extensional tectonic regime and volcanic rocks.

Preserved extensional structures in an inverted Cretaceous rift basin, northwestern Argentina: Outcrop examples and implications for fault reactivation

During the Cretaceous‐Eocene interval a system of intracontinental rift basins, the Salta group rift, evolved in northwestern Argentina. Individual segments of the rift later suffered different degrees of inversion during Cenozoic shortening. The Tres Cruces subbasin, on the west side of the Eastern Cordillera, was strongly deformed, being now part of a thick‐skinned thrust belt with a predominantly N‐S structural trend. On its eastern border, tilting due to folding and thrusting and subsequent erosion have produced exceptional outcrops of preserved east‐trending extensional structures including half grabens, rollover anticlines, and extensional fault‐propagation folds. Farther west, the synrift succession is only intermittently exposed, although the interference of north‐ and east‐trending structures as well as peculiar, dome‐shaped anticlines with spur‐like extensions suggest that north‐ and east‐trending Cretaceous faults were reactivated, particularly near their intersections. Compilation of published data and analysis of our new data focused on the Salta rift indicates three main factors favoring the contractional reactivation of normal faults: dip angles lower than approximately 60°, especially for faults striking roughly normal to contraction; strikes no closer to the contraction direction than approximately 30°; and low downdip fault curvatures. Occasional dip‐slip reactivation of east‐trending faults does not match the present and long‐term Andean stress regimes and presents an unresolved problem.