Half-graben Research Articles

Controls on synrift turbidite deposition on the hanging wall of the South Viking Graben, North Sea rift system, offshore Norway

Three-dimensional seismic, wireline-log, core, and biostratigraphic data from the South Viking Graben, North Sea rift system, are integrated to investigate the controls on the temporal and spatial development of an Upper Jurassic synrift turbidite system deposited on the hanging-wall dipslope of a salt-influenced half graben. Turbidite deposition was coeval with the initiation and upslope (paleo-landward) migration of activity across a gravity-driven normal fault array. Three main synrift stratal units are identified, and these are mapped using seismic and well data. The lowermost unit (upper Oxfordian) comprises thick amalgamated turbidites, which are restricted to the hanging wall of the earliest, most basinward, growth fault. The middle unit (Kimmeridgian) is more areally extensive than the underlying system, draping the now inactive basinward growth fault and extending upslope into the hanging wall of a newly activated landward growth fault. The uppermost unit (lower to middle Volgian) is more sheetlike and was deposited when activity across all growth faults had mostly ceased and slope topography had been almost fully healed. This study demonstrates that hanging-wall dipslopes within rifts can be characterized by volumetrically significant, sand-rich, gravity flow-dominated depositional systems, and that the reservoir architecture of such deposits can be strongly controlled by syndepositional growth faulting. In addition, this study provides insights into the response of turbidites to tectonically driven changes in bathymetry, which may be applicable in a range of basin settings.

Modelling of deformation around magmatic intrusions with application to gold-related structures in the Yilgarn Craton, Western Australia

This study simulates rock deformation around high temperature granite intrusions and explores how gold bearing shear zones near intrusions were developed in the Yilgarn, using a new continuum damage mechanics algorithm that considers the temperature and time dependent elastic-visco-plastic constitutive behaviour of crustal materials. The results demonstrate that strain rates have the most significant effects on structural patterns for both extensional and compressional cases. Smaller strain rates promote the formation of narrow high-strain shear zones and strong strain localisation along the flank or shoulder areas of the intrusion and cold granite dome. Wider diffuse shear zones are developed under higher strain rates due to strain hardening. The cooling of the intrusion to background temperatures occurred over a much shorter time interval when compared to the duration of deformation and shear zones development. Strong strain localisation near the intrusion and shear zone development in the crust occurred under both extensional and compressional conditions. There is always clear strain localisation around the shoulders of the intrusion and the flanks of the “cold” granitic dome in early deformation stages. In the models containing a pre-existing fault, strain localisation near the intrusion became asymmetric with much stronger localisation and the development of a damage zone at the shoulder adjacent to the reactivated fault. At higher deformation stages, the models produced a range of structural patterns including graben and half graben basin (extension), “pop-up” wedge structures (compression), tilted fault blocks and switch of shear movement from reverse to normal on shear zones. The model explains in part why a number of gold deposits (e.g. Wallaby and Paddington deposits) in the Yilgarn were formed near the flank of granite-cored domes and deep “tapping” faults, and shows that the new modelling approach is capable of realistically simulating high strain localisation and shear zone development.

Cenozoic faulting of the Bohai Bay Basin and its bearing on the destruction of the eastern North China Craton

The Cenozoic Bohai Bay Basin is located at the center of the Eastern Block of the North China Craton. The structural architecture of this basin provides important clues on the deep-seated lithosphere thinning of the North China Craton. The Cenozoic regional stress field is characterized by NW-oriented extension. However, the various Cenozoic structural patterns of normal faulting and related transverse folding in the Bohai Bay Basin are controlled not only by Cenozoic stress field, but also by strain field and Mesozoic basement fault assemblages in this area. Regionally, the Cenozoic tectonic features and the dynamic evolution of the eastern North China Craton are dominated by two lithosphere-penetrating fault systems including the sinistral Tan–Lu Fault System and the dextral Lan–Liao Fault System. To the west of the Lan–Liao Fault System, Cenozoic extensional tectonics includes NNE-trending listric normal faults that controlled half grabens. However, between these two fault systems are WNW-trending half grabens which show basement-involved faulting in the north and overlapping relations between sedimentary cover and basement in the south. To the east of the Tan–Lu fault, the North Yellow Sea Basin is a WNW-trending fault depression with faulting in the south and overlapping relations in the north. These structural features are inherited from the Mesozoic tectonic framework of this area, whose tectonic characteristics were completely controlled by two opposite strike-slipping faults, the trans-extensional or oblique rifting in the Paleogene, followed by extensional faulting and subsequent subsidence. Furthermore, the culmination of the decratonization of the North China Craton was also related to an eastward jump of Cenozoic subduction of the Pacific Plate and the far-field effect of eastward extrusion of Cenozoic subduction of the Indian Plate, and was not essentially restricted to the early Mesozoic processes. Therefore, the Cenozoic, especially ∼25Ma marks the time of cessation of the processes that led to lithosphere thinning and destruction of the Eastern Block of the North China Craton.

The Dayingzi detachment fault system in Liaodong Peninsula and its regional tectonic significance

Large scale lithosphere thinning is an important characteristic of the destruction of the North China Craton (NCC) during the late Mesozoic. A series of extensional structures were developed under extensional setting, among which is the Dayingzi detachment fault system (DFS). The DFS is constituted by three parts, volcano-sedimentary basins at the hanging wall, the Dayingzi-Huanghuadian detachment fault zone, and Paleoproterozoic metamorphic rock series and Mesozoic plutons at the footwall. In the section across the detachment fault zone, there is a sequence of tectonites including fault gouge, microbreccia, cataclastic-mylonites, mylonites, and gneissic biotite monzonite granite. Microstructural characteristics of tectonites and electron backscatter diffraction (EBSD) patterns of quartz indicate that the rocks from the footwall experienced a process from upper greenschist facies to lower greenschist facies. SHRIMP and LA-ICP MS U-Pb dating of zircons from the volcanic rocks in the basins, the tectonic evolution of the DFS is summarized as follows: 1) regional extension started at 135.0±1.2 Ma ago, when the detachment fault cut through the middle crust. Faulting induced the upwelling of magma and eruption of volcanic rocks and deformed a series of medium-acid volcanic rocks; 2) after 135.0±1.2 Ma, a large scale detachment faulting was active cross-cutting the mid-upper crust. The western margin of Jurassic and Triassic granite was ductilly and brittly sheared; besides, the Cretaceous volcano-sedimentary rocks were tilted when the master fault approached the surface; 3) at around 127±1 Ma, the detachment fault stopped its activity and was intruded by the unsheared Cretaceous granite near Chaoyang. Comparison with the Liaonan metamorphic core complex (MCC) and other extensional structures in Liaodong Peninsula led to a general trend of including three zones in the Peninsula: MCC zone, detachment fault systems (DFS) zone, and half graben zone. MCC commonly cuts through the mid-lower crust, DFS through the mid-upper crust, and half graben through the upper crust. Therefore, development of the extensional structures in Liaodong Peninsula indicates that they are the results of crustal extension and thinning at different crustal levels. They may provide a deep insight into the dynamic mechanism, history of destruction and lithosphere thinning of the North China Craton (NCC).

Structural anatomy and dynamics of evolution of the Qikou Sag, Bohai Bay Basin: Implications for the destruction of North China craton

The Qikou Sag is a complex Cenozoic faulted basin at the center of the Bohai Bay Basin. The Paleogene sediments in the basin show a maximum thickness of 11,000m in Paleogene representing intense crustal extension. The sag has a typical dish-like structure in the upper part, with the central domain represented by a deep half graben, and surrounded by different types of gentle slopes and sub-sags linked with the basin-margin uplifts. The inner structure of the sag is jointly controlled by the basin-margin extensional faults, intrabasinal extensional tectonics and transfer tectonics. The sag is divided into four different extensional tectonic domains by the basement fault along the coastline, the Haihe–Xingang Fault and the Qidong Fault. These sub-sags are in turn controlled by different detachment systems. The listric Cangdong and Hangu faults mark the major boundary faults controlling the extensional structure of the basin.The structure and tectonics of the Qikou Sag provide a direct record of the late stages of rifting and destruction of the North China Craton. The Mesozoic basement structures of the sag determine the tectonic styles in different sub-sags. The complete inversion of the structures in Mesozoic profoundly influenced the development and evolution of the basin during Paleogene. We identify three main stages in the tectonic evolution of the Qikou: the Oligocene faulting stage, the Miocene–Pliocene thermal subsidence stage and the Pleistocene–Quaternary depression stage. In the Oligocene, fault activities and the basin architecture were mainly controlled by the listric boundary fault system of the basin and the secondary fault system in the hanging wall under the detachment and extension in the upper crust. Since Miocene, the boundary fault of the basin underwent weak extension. The upwelling of the upper mantle and the formation of corresponding shallow depressions contributed to the evolution and tectonic architecture of the Qikou Sag.

Early cretaceous extensional structures in the Liaodong Peninsula: Structural associations, geochronological constraints and regional tectonic implications

Various extensional structures, e.g., half grabens, detachment faults, and metamorphic core complexes, were formed in the Liaodong Peninsula. There are two metamorphic core complexes (mcc’s) in the western part of the Peninsula, i.e. the Liaonan mcc and the Wanfu mcc. They share the same lower plate and constitute a conjugate mcc pair. The Dayingzi detachment fault system and the three half grabens are exposed in the central and eastern parts, respectively. U-Pb dating of zircons from syntectonic plutons in the lower plates of the detachment faults and volcanic rocks from half graben basins indicates that their formation spans from 135 to 106 Ma, although the individual structure may be formed at a particular stage. Despite the differences in age of formation, in the regional attitudes, and in rooting depths, the extensional structures have great similarities in their kinematics, geometrical asymmetry, and coeval tectono-magmatic activities etc. Macroscopically, the extensional structures constitute conjugate associations, but a particular one generally has asymmetric patterns. Early Cretaceous extensional structures extend from the Liaodong Peninsula to North China, Northeast China, South China, and eastern Mongolia and Transbaikal area in Russia. The extensional structures from different areas share many common features. The Liaodong Peninsula is the miniature of the East Asia with respect to the formation of extensional structures in Early Cretaceous. It is suggested that the interaction of the Izanagi Plate with Eurasia Plate is responsible for the extension of crust. The structural mobility of the lithosphere, partly attributed to the fluid flow at the depth, and detachment faulting in both the crustal and mantle lithosphere provide important constraints on the development of Early Cretaceous extensional structures in the East Asia.

Geometry and evolution of rift-margin, normal-fault–bounded basins from gravity and geology, La Paz–Los Cabos region, Baja California Sur, Mexico

The southern end of the Baja California peninsula is cut by a north-striking, left-stepping, active, normal-fault system—the marginal fault system of the oblique-divergent plate boundary within the Gulf of California. We conducted gravity surveys across the normal-fault–bounded basins, and, along with optically stimulated luminescence dating of offset piedmont surfaces and geologic data, we estimated fault-slip rates and assessed fault patterns across basins, gaining insight into basin evolution to better understand the role of upper-crustal processes during development of an obliquely rifted plate margin. Gravity surveys across the La Paz, San Juan de los Planes, and San José del Cabo basins revealed basin depths ranging from ∼500 to 3000 m. The La Paz basin is a half graben with two smaller basins that reflect the two main east-dipping splays of the Carrizal fault. Within the San Juan de los Planes and San José del Cabo basins, there are buried faults, indicating that during the early stages of basin formation, strain was distributed across these smaller intrabasin faults prior to development of the basin-bounding faults. Slip rates coupled with basin depths suggest that the La Paz and San Juan de los Planes basins began forming ca. 2–5 Ma, overlapping in time with the formation of the main plate boundary at this latitude. The San José del Cabo basin has the greatest depth to bedrock (1.6–2.7 km), signifying that it accommodates a greater slip rate or a longer duration of slip than the other faults within this system.

Paleostress analysis of Cenozoic faulting in the Kraishte area, SW Bulgaria

Paleostress calculations from fault planes in Paleogene sediments and in the underlying basement were used to determine the orientation and the chronology of the principal stresses during the Cenozoic tectonic evolution of the Kraishte area (southwest Bulgaria). The results show that middle Eocene–early Oligocene WSW–ENE extension led to the formation of grabens and half grabens filled with thick continental to marine deposits in the hanging wall of extensional detachments.The Paleogene sediments and their basement were then intruded by subvolcanic bodies and dykes during SW–NE extension between 32 and 29 Ma. These WSW–ENE and SW–NE extensional stages are related to the general rollback of the Hellenic slab.From the late Oligocene to the earliest Miocene, SSE–NNW transtension generated coal-bearing sedimentary basins. The anticlockwise rotation of the main tensile axis by almost 50° with respect to the previous tectonic stage was probably related to a switch from the Aegean back-arc extension to crustal stretching and extrusion of continental fragments around the Moesian platform.Since the middle Miocene extension in the Kraishte area, accommodated by faults with relatively small displacement, led to the formation of restricted basins filled with alluvial to lacustrine deposits.

Fault framework and kinematic evolution of inversion structures: Natural examples from the Neuquén Basin, Argentina

Mesozoic tectonic inversion in the Neuquen Basin of west-central Argentina was investigated by analyzing a three-dimensional seismic and borehole data set over prominent inversion structures related to transpressional tectonics along the Huincul arch. This conspicuous strike-slip deformation has been active since the Early Jurassic to Early Cretaceous and caused the reactivation of normal faults and the inversion of Triassic half grabens, some of which are imaged by the seismic data. Detailed structural and stratigraphic mapping allowed the identification of two main fault systems associated with inversion structures: (1) deep faults that affected basement and synrift strata where preexisting faults were selectively reactivated during inversion based on their length and (2) shallow faults that affected postrift and syninversion strata. Normal faults formed at high angle to the reactivated half-graben bounding fault as a result of hanging-wall expansion and internal deformation as it accommodated the shape of the curved footwall during oblique inversion. Structural restorations suggest that contraction during inversion was initially accommodated by folding and internal deformation of synrift sedimentary wedges, followed by displacement along half-graben bounding faults. We suspect that during late inversion, the weight of the overburden inhibited additional fault displacement and folding became the main shortening-accommodating mechanism. Natural examples described in this study exhibit significant along-strike variation in structural style and high structural complexity associated with relatively small amounts of inversion, which suggest the need for incorporating more complex deformation scenarios into analog models. Fault framework development and its linkage to a specific mechanism (e.g., hanging-wall deformation because of oblique inversion) enables the predictive analysis of subseismic faulting and fracture distribution, which can impact our understanding of the petroleum systems in the Neuquen Basin's northern embayment, especially regarding the significant fault development within the Cuyo Group, which may impact the Los Molles petroleum system.

Early – Middle Miocene Suez Syn-rift-Basin, Egypt: A sequence stratigraphy framework

ABSTRACT The analyses of thirteen planktonic and benthonic biozones, paleobathymetry and electric log data were used to interpret the sequence stratigraphy of the Early to early Middle Miocene syn-rift section in the Gulf of Suez. The study area is located in the central province of the Gulf and includes six boreholes located in two half grabens and the October Field. The new framework proposes the Suez Supersequence and Suez Depositional Sequence DS 50 instead of the five paleontological sequences commonly cited in the literature (S10 to S50). The Supersequence starts above the regional unconformity that separates the pre-and syn-rift rocks, commonly referred to as Terrace T00. The shallow-marine deposits of the Aquitanian Nukhul Formation form the lowstand systems tract. The Burdigalian Mheiherrat Formation starts with the Uvigerina costata flooding event and forms the transgressive systems tract deposited in outer-neritic to upper-bathyal settings. The overlying Langhian Hawara Formation was deposited in upper to middle bathyal settings and represents the maximum flooding interval. The Langhian Asl Formation (early falling stage systems tract, upper bathyal to outer neritic) and overlying Langhian Lagia Member of the Ayun Musa Formation (late falling stage systems tract) closed the Supersequence. Suez Depositional Sequence DS 50 lies unconformably on the Supersequence, and represents a major transgression starting with the Praeorbulina glomerosa s.l. flooding event. DS 50 corresponds to the Ras Budran Member of the Ayun Musa Formation (paleontological sequence S50). Its setting is outer neritic and its upper sequence boundary is an unconformable with the Belayim Formation. The Suez Supersequence is interpreted in terms of 35 genetic parasequences and DS 50 by 10 more. The parasequences are interpreted by the coincidence of quantitative paleontological faunal and paleobathymetric breaks with the electric log shifts. The sequences and parasequences are correlated between the six wells to show the evolution of the half-grabens and October Field at different times.

Tectonostratigraphy of Lower Cretaceous in Southern Iraq

Stratigraphic units during Lower Cretaceous in southern of Iraq were affected largely by tectonic agent. Neo-Tethys ocean occupied the distance between passive margins of Arabian and Iranian plates. South of Iraq represented the continental shelf of passive margin of Arabian plate. This tectonic situation made the sediments in eastern parts of passive margin in south of Iraq effected by marine agents, and the sediments in western parts was effected by continental agents, whereas the middle parts was effected by both agents. As result of that, three sediment regions were formed in south of Iraq. First, marine sediment region that occupy the eastern part of south of Iraq near the Iraq-Iran borders. Second, mixed sediment region that occupy Basra area and extend toward north. Third, continental sediment region that occupy western part of Basra near the western desert. Half graben basins that formed by listric faults in the passive margin caused the local changes in distribution of sediments in south of Basra. These three sediment regions formed all during Lower Cretaceous and maybe extend toward the middle of Iraq and north of Baghdad

The role of backstop shape during inversion tectonics physical models

The style of deformation of rocks from basin-infilling sequences in positively inverted natural basins was discussed upon the results of laboratory experiments carried out in sandboxes with sand packs laid down in the space between two wooden blocks. The space simulated stages of crustal extension leading to (1) a half graben due to extension above a listric extensional detachment, with the blocks simulating the footwall and hanging wall, or (2) a graben, with the blocks simulating the external margins that drifted apart above a horizontal detachment. Combinations of two different angles were used to simulate the dip of curved normal faults along the internal face of the wooden blocks. Backstops in the half graben had a convex up internal face. Backstops in the graben had a concave up internal face. Shortening as partitioned in forward and backward movements within the sand packs, and the kinematics of contraction was largely influenced by the convex or concave internal faces. A buttress effect characterized by rotation of the sand pack close to the footwall was stronger for footwall with steeper-dipping internal faces. The results were compared to other physical experiments and applied to an inverted basin found in nature.

The Latemar: A Middle Triassic polygonal fault-block platform controlled by synsedimentary tectonics

Detailed field mapping of a Middle Triassic carbonate buildup, the Latemar in the western Dolomites, northern Italy, has been carried out. The Latemar is an isolated carbonate buildup that nucleates on a fault-bounded structural high (horst) cut into the underlying late Anisian carbonate bank of the Contrin Fm. This study demonstrates that extensional synsedimentary tectonics is the main factor controlling its geometry and provides an age for this tectonic phase. In an early phase, slopes were mostly composed of well bedded, clinostratified grainstones and rudstones. In a later stage, the deposition of grainstones was accompanied by the emplacement of clinostratified megabreccias. The upper portion of slopes is a microbial boundstone with abundant Tubiphytes and patches or lenses of grainstone. Boundstones may occasionally expand into the platform interior and downward to the base of the slope. The depositional profile was that of a mounded platform. The buildup is dissected by a dense framework of high angle fractures and faults, and by magmatic and sedimentary dikes, exhibiting two principal directions trending NNW–SSE and ENE–WSW. Faults trending WNW–ESE were also observed. Magmatic dikes are related to the emplacement of the nearby Predazzo intrusion and are thus upper Ladinian. Kinematic indicators of strike-slip activity were observed on fault planes trending NNE–SSW and NNW–SSE, that can be attributed to Cenozoic Alpine tectonics. Faults, magmatic dikes and sedimentary dikes show systematic cross-cutting relationships, with strike-slip faults cutting magmatic dikes, and magmatic dikes cutting sedimentary (neptunian) dikes. ENE–WSW and WNW–ESE faults are cut by all other structures, and record the oldest tectonic activity in the region. Structural analysis attributes this tectonic phase to an extensional stress field, with a direction of maximum extension oriented ca. N–S. Several lines of evidence, including sealed faults and growth wedge geometries allow us to recognize this old tectonic phase as syndepositional, and to date it to the latest Anisian ( avisianum and crassus ammonoid biochronozones). The horse-shoe shape of the platform and its margins, rectilinear for long tracts, are explained by the existence of a network of extensional faults, whose activity lasted for part of the growth of the platform. The Latemar is thus a polygonal, fault-block platform controlled by synsedimentary tectonics. The resulting sedimentary architecture is that of a growth wedge, deposited probably on the uplifted side of a submerged half graben. Continuous reactivation of faults at platform margins locally determined anomalous facies transitions, with well layered platform interior directly in contact with the clinostratified slopes.

Tectonic and climatic controls on Holocene channel migration, incision and terrace formation by the Rio Grande in the Palomas half graben, southern Rio Grande rift, USA

In order to evaluate potential effects of tectonics and climate change on the behaviour of the axial Rio Grande in the Rio Grande rift, a 16·5 km stretch of modern floodplain and Holocene terraces were mapped in the tectonically active Palomas half graben, south-central New Mexico, USA. In addition, 51 cores and natural exposures were logged and 20 radiocarbon dates were obtained from charcoal, bulk organic matter, mollusc shells and pedogenic calcite. The Holocene alluvium comprises four terraces above the modern floodplain, each of which formed by a period of river incision followed by stability and renewed floodplain construction to a level below that of the previous terraces. Estimated times of incision between Terraces I and II, II and III, and III and IV are after 12 400, 8040 to 5310, and 760 to 550 yr bp, respectively, whereas the incision between Terrace IV and the modern floodplain occurred within the last 260 years. Although there is some evidence for tectonic control on river behaviour in the southern part of the basin, terrace formation is interpreted as being related to climate change, with periods of incision corresponding to times of increased aridity and low sediment/water discharge ratio in the Rio Grande. This process may have resulted from a reduction in intensity and magnitude of summer storms which supply sediment to the axial river, coupled with an increase in spring discharge peak caused by snowmelt in upstream mountain catchments.

Analog modeling of divergent and convergent transfer zones in listric normal fault systems

Transfer zones are common features in passive margins such as the Gulf of Mexico, where deformation between adjacent listric normal faults is accommodated by the formation of complex secondary fault systems. Two common types of transfer zones are (1) convergent, with the main faults dipping toward each other, and (2) divergent, with the main faults dipping away from each other. Analog clay models are developed for both convergent and divergent transfer zones with the faults approaching each other, offset by 90°, and overlapping each other. During extension, the structures initiate as symmetric grabens and later transform into asymmetric half grabens. The main fault and associated synthetic faults form a narrow zone of deformation, whereas the antithetic faults develop into a broader zone constituting evenly spaced discrete fault segments. Orientations, lengths, densities, and sizes of connected fault clusters vary with the type of transfer zone, structural position relative to the fault offsets, and total extension. The experiments show that the antithetic faults in convergent transfer zones and synthetic faults in divergent transfer zones tend to be more consistent in orientation and connect easily. Synthetic faults in convergent transfer zones and antithetic faults in divergent transfer zones change orientation along strike toward the transfer zone. Fault connectivity in approaching transfer zones is generally higher than in normal offset or overlapping configurations. The analog models provide important insights on the geometry of fault patterns and possible fluid-migration pathways within transfer zones.

STRUCTURAL GEOMETRY OF THE BEIJIAO SAG OF THE QIONGDONGNAN BASIN

As a typical half-graben in the deep water area of the southern Qiongdongnan basin,the Beijiao sag with the leg bend-like deformation of its sequence stratigraphy is consistent with the model of extensional fault-bend fold.Using the theory of extensional fault bend fold and balanced cross-section,we reconstructed the evolutionary process of the Beijiao sag and Beijiao low-uplift in terms of extentional tectonics.After analysing and measuring the structural deformation of the sag,the geometric character and kinematic process of the fault bend fold are established.It is estimated that the sag totally extended 12 km with an average extension rate of 0.44 km/Ma during the period from 50 Ma to 23 Ma,and the sea level of this area rose about 200 m by the end of Eocene.

Geomorphic evidence for tilting at a continental transform: The Karamursel Basin along the North Anatolian Fault, Turkey

Continental transform boundaries are characterised by regional transcurrent faults that locally slip obliquely and spawn rapidly subsiding and tilting basins. A type example is the North Anatolian Fault (NAF) that accounts for the westward motion of the Anatolian “platelet” relative to Asia at about 25 mm/yr. Many of the basins along the NAF are asymmetric half grabens that border a strand of the NAF on the extensional side of a fault bend and tilt obliquely toward the fault. A prominent example is the 17 km-long Karamursel half graben south of the NAF in Izmit Gulf, one of the starved basins flooded by the Marmara Sea. A tilt rate of 3°/10 kyr has been proposed for the submerged part of the basin within 2 km south of the NAF, on the basis of a tilted early Holocene paleo-shoreline. Very rapid tilt and subsidence have been reported for similar basins along the NAF, but the locus of tilt shifts relative to the basin and is short lived at any one place. We find evidence of recent tilting from surface flow patterns on the steep flank of the basin above the southern coastline, up to 6–8 km from the NAF. Drainage divides between 13 rivers mark a northward 8–10° tilted sub-planar surface that extrapolates down into the basement below the progressively tilted sediments of the basin. River profiles are only slightly concave below this surface. We interpret this as the bevelled surface that preceded tilting, and the immature comb-like drainage to be symptomatic of recent tilt. On the north flank of the basin, the drainage is equally steep but markedly different. It suggests a backward eroding fault scarp. Northward drainage into the Black Sea suggests a subtle but regional northward tilt of the Kocaeli Plateau, possibly a flexural response to unloading along the fault. Along the western NAF, rapid progressive tilting of sediments is typical of submarine transform basins. They provide ground truth for investigating the geomorphic effects of rapid tilting on land.

The plant debris beds of the Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight, southern England: their genesis and palaeontological significance

The Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight primarily represents high sinuosity fluvial, but also lacustrine and terrestrial deposition on a low relief floodplain occupying half grabens, with active normal faults immediately to the north. Plant debris beds form a very small proportion of the succession but are the main source of vertebrate fossils, including dinosaurs. These conspicuous, but thin, grey units are distributed randomly throughout the Wessex Formation and are of limited lateral extent. The lower part of most plant debris beds comprises a poorly sorted, matrix-supported conglomerate. The upper part of most consists of structureless mudstone, which usually grades up into colour-mottled but predominantly red and purple floodplain mudstones. The plant debris beds contain an unusual mixture of freshwater molluscs, diverse aquatic and terrestrial vertebrates, and mainly terrestrial plants with a variable quality of preservation. We conclude that each plant debris bed represents a locally generated sheetflood, which was then transformed on the floodplain into a debris flow by the acquisition of surface material. The debris flows surged into, and were deposited in depressions on the alluvial plain, including abandoned chute and cut-off channels, oxbows and ponds. These units are therefore intrabasinal and interfluvial, and the fossils that they contain are locally derived. Similar deposits of palaeontological significance have not been reported elsewhere in the literature. In view of this, their origin and sedimentology, their palaeontological significance, and the taphonomy of the flora and fauna they contain are discussed.

Geophysical issues and challenges in Malay and adjacent basins from an E & P perspective

Malay Basin, a northward-trending pull-apart extensional rift basin, formed during the late Eocene-early Oligocene and then underwent thermal subsidence and sedimentation during the early Miocene. Reorientation of regional stress fields during the mid-Miocene caused structural inversion resulting in east-west anticlines and half grabens.

A complete magnetic-polarity stratigraphy of the Miocene continental deposits of Mae Moh Basin, northern Thailand, and a reassessment of the age of hominoid-bearing localities in northern Thailand

Northern Thailand comprises more than 40 Tertiary intermontane basins. These basins, tectonically interpreted as grabens or half grabens, have yielded hominoid fossils and rich Neogene mammal faunas. Relative ages provided by biochronological studies are controversial and neither absolute ages nor correlations with marine deposits are available. A precise temporal framework provided by magnetic-polarity stratigraphy is therefore crucial to understand the evolution of these Southeast Asian mammalian faunas, to estimate the timing and duration of basin formation, and finally to allow correlations with other Neogene hominoid localities of the Old World. A complete magnetostratigraphic study, which significantly extends previous paleomagnetic investigations, has been conducted in the Miocene sequence of Mae Moh Basin, Lampang Province, northern Thailand. Based on the biostratigraphic constraints, the 15 polarity zones recognized from the composite section have been correlated with Chrons C5ACr–C5r.3r, between 14.1 and 12.0 Ma. The present study provides a high-resolution magnetostratigraphic reference for the continental middle Miocene of all Southeast Asia. By correlation with the Miocene sequence of the nearby basin of Chiang Muan, the large-bodied hominoid ( Khoratpithecus chiangmuanensis )–bearing levels can be precisely dated between 12.4 and 12.2 Ma.