Jurassic Extension Research Articles

Fluid systems and basin evolution of the western Lower Saxony Basin, Germany

AbstractThe western Lower Saxony Basin is characterized by magnetic and gravity anomalies and an unusually strong coalification of surface‐near sedimentary rocks, indicating that high temperatures were reached in the past. The temperature history of the basin has been disputed for many years leading basically to two different hypotheses: one proposes igneous intrusion during Mid Cretaceous time, causing the high maturity in this area, whereas the other theory explained the coalification as a result of deep burial during late Jurassic and early Cretaceous times. Petrographical data are summarized which show the maturity distribution within the basin, reflecting maximum palaeotemperature conditions. This maturity distribution is visualized along three cross sections of up to 50 km length, as well as the palaeotemperature distribution for six time steps. Furthermore, geochemical data on petroleum source rocks are presented which indicate early mature/immature type I/II kerogen in Wealden and Posidonia Shales and mature/overmature type III kerogen in the coal‐bearing Upper Carboniferous. Fluid inclusion compositions in quartz crystals are highly variable ranging from liquid to gaseous, and gas compositions are also variable. These data were interpreted in the context of burial history. This study provides new data and numerical models with the intention to clarify the cause of the high coalification in the basin. 2D structural and basin modelling along three cross sections, combined with new fluid inclusion measurements and results of previous coalification studies, 1D modelling, and fission track dating demonstrate high heat flow during the Late Jurassic extension stage followed by a deep burial of the basin during the Early Cretaceous, associated with only moderate heat flows. The presence of an igneous intrusion of Cretaceous age is not supported. Furthermore, timing of hydrocarbon generation from the three major source rocks is discussed.

The relationships between tectonics and sedimentation in the Late Cretaceous series of the eastern Atlasic Domain (Algeria)

Our study of the Late Cretaceous series in the eastern Atlasic Domain of Algeria is intended to show the geometry of the sedimentary deposits and the sequential palaeogeographic evolution of the region’s basins. The Late Cretaceous series consist of four transgressive–regressive sedimentary megasequences corresponding to 2nd order cycles, noted as I–IV. Each megasequence is made up of three or four sequences that have been correlated with 3rd order cycles. The beginning of megasequences I and II mark a rapid tectonic deepening concomitant with an acceleration of sea-level rise which allowed the deposition of pelagic sediments at their initiation, during the latest Albian and early Turonian times. This tectonism reflects the reactivation of basement faults resulting in extension and block-tilting. The succeeding sequences, showing alternating marls and carbonate beds with benthic facies were deposited in a homoclinal ramp and are indicative of a gradual development of shallow marine conditions. In the megasequences III (Coniacian to Santonian) and IV (Campanian to Maastrichtian) the sequences were wholly deposited on a homoclinal ramp. The major basement faults are NE-SW trending transtensional strike-slip faults, which delimit the margins of the basins, and WNW-ESE and NW-SE trending normal faults that led to block tilting during the Late Cretaceous. Both types of faults strongly influenced palaeogeography and sedimentation. The evolution of the subsidence through the Late Cretaceous shows a progressive sagging of these basins, interrupted by short phases of extension highlighted by accelerations on the subsidence curves. These phases constitute, after the Jurassic extension, a new extensional episode occurring during the latest Albian and Early Turonian, which led to a tilting of basement blocks. The Late Cretaceous extension phases reflect reorientations of the stress in the African plate, during its northeastwardly directed drift, and rifting processes on its northeastern margin. A first compressive phase, which was relatively localized, is registered at the end of the Maastrichtian, leading to palaeogeographic reorganization of the Atlasic basins.

Structural synthesis of the Northern Calcareous Alps, TRANSALP segment

The Northern Calcareous Alps (NCA) are the site of very large top-to-north convergent movements during Cretaceous–Tertiary Alpine mountain building. To determine the amount of shortening, the depth of detachment and the style of deformation, we retro-deformed an approximately 40 × 40 km area comprising the Lechtal and Allgäu Nappes. On the basis of all available geological data and processed sections of the TRANSALP reflection seismic experiment, coherent 3D models were constructed by splining lines from N–S cross-sections. Integration of 3D kinematic modeling and field data shows the following. The structure of the Lechtal Nappe is controlled by the Triassic Hauptdolomit. Four main thrusts link to a detachment at 2–6 km depth below sea level. Shortening estimates vary, from 25% (east) to 42% (west). Additional contraction is accommodated by folding. In the east the subjacent Allgäu Nappe can be traced about 10 km down-plunge, and is shortened by about one third. In the western part the downplunge width is at least 15–20 km, with restorable shortening of one third. The triple (Inntal, Lechtal, Allgäu Nappes) NCA nappe system was moved uniformly N–S to produce laterally heterogeneus shortening of 40–90 km or 50–67%. We suggest that the NCA are underlain by substantial amounts of buried Molasse sediments and/or overthrust units of Helvetic and Rheno-Danubian Flysch, indicating post-Eocene N–S shortening of at least 55 km. Restored to an initial configuration, the basin topography of the NCA reveals strong E–W thickness variations of the Triassic Wettersteinkalk and Hauptdolomit platform carbonates. Such variations may pertain to N–S trending growth faults, which were important precursors to later Jurassic extension of the Austroalpine passive margin. Such structures are unlikely to be seen in the conventional N–S cross-sections, but form an essential geometrical and mechanical element in later, convergent mountain building.

Petrogenesis of Adakitic Porphyries in an Extensional Tectonic Setting, Dexing, South China: Implications for the Genesis of Porphyry Copper Mineralization

The Dexing adakitic porphyries (quartz diorite–granodiorite porphyries), associated with giant porphyry Cu deposits, are located in the interior of a continent (South China). They exhibit relatively high MgO, Cr, Ni and Sr contents, high La/Yb and Sr/Y ratios, but low Yb and Y contents, similar to adakites produced by slab melting associated with subduction. However, they are characterized by bulk Earth-like Nd–Sr isotope compositions (εNd(t) = −1·14 to +1·80 and (87Sr/86Sr)i = 0·7044 – 0·7047), and high Th (12·6–27·2 ppm) contents and Th/Ce (0·19–0·94) ratios, which are different from those of Cenozoic slab-derived adakites. Sensitive High-Resolution Ion Microprobe (SHRIMP) geochronology studies of zircons reveal that the Dexing adakitic porphyries have a crystallization age of 171 ± 3 Ma. This age is contemporaneous with Middle Jurassic extension within the Shi-Han rift zone, and within-plate magmatism elsewhere in South China, indicating that the Dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting. Their high Th contents and Th/Ce ratios, and Middle Jurassic age, argue against an origin from a Neoproterozoic (∼1000 Ma) stalled slab in the mantle. Taking into account available data for the regional metamorphic–magmatic rocks, and the present-day crustal thickness (∼31 km) in the area, we suggest that the Dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust, which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the Shi-Hang rift zone. Moreover, the Dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent, which elevated not only their MgO, Cr and Ni contents, but also the oxygen fugacity (fO2) of the mantle. The high fO2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements, which are required to produce associated Cu mineralization. Therefore, the Cu metallogenesis associated with the Dexing adakitic porphyries is probably related to partial melting of delaminated lower crust, similar to the metallogenesis accompanying slab melting.

Spatio-temporal evolution of strain accumulation derived from multi-scale observations of Late Jurassic rifting in the northern North Sea: A critical test of models for lithospheric extension

We integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern North Sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations. Data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire Viking Graben and its flanking platform areas. These data show how syn-rift accommodation within the basin varied in space and time with sub-kilometer-scale spatial resolution and a temporal resolution of 2–3 Myr. Regional interpretations of 2D seismic reflection, refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust. These image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio-temporal evolution of strain accumulation throughout the lithosphere during the 40 Myr (170–130 Ma) period of Late Jurassic extension across this basin. For the first 25–30 Myr, strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined. As strain rates across the whole basin were consistently very low (< 3 × 10 - 16 s - l ), thermally induced strength loss cannot explain this phenomenon. The strain localisation is manifest in the near-surface by a systematic migration of fault activity. The pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control, especially when estimates of flexural rigidity for this area are considered. The best explanation for what is observed in this time period is a coupling between near-surface strain localisation, driven by brittle (or plastic) failure, and the evolving thermal structure of the lithosphere. We demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate-dependence of frictional strength observed in laboratory studies. During the final 10 Myr of basin formation, strain accumulation was focused within the axis and strain rates declined rapidly. Replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline.

Extensional geometry of the Mid Norwegian Margin before Early Tertiary continental breakup

Interpretation of reflection seismic data integrated with existing crustal models provide an interpretation of the deeply buried Mesozoic extensional geometry of the Vøring and Møre margins in the Norwegian Sea. Whereas Late Paleozoic to Triassic basins are limited by high angle normal faults with NW and SE dips, the Late Jurassic–Early Cretaceous Vøring and Møre basins are characterised by low-angle normal faults that dip towards the NW. Between the Vøring and Møre basins, at the SE end of the Jan Mayen Fracture Zone, a synthetic transfer zone is interpreted. Geometrical models based on basin and fault geometries show that the rift system detached near the upper crust–lower crust transition in the study area. This rift system accommodated between 110 and 140 km of post-Early Jurassic extension according to the geometry of the present-day pre-Cretaceous upper crust. A simple geometrical forward model suggests that a substantial part of this stretching occurred during the Late Cretaceous–Paleocene, thinning the central and NW segments of the Late Jurassic–Early Cretaceous rift.

Occurrence and Dispersal of Magmas in the Jurassic Ferrar Large Igneous Province, Antarctica

Tholeiitic rocks of the Ferrar Large Igneous Province (FLIP) occur in a linear belt from the Theron Mountains to Horn Bluff in the Transantarctic Mountains and extend into southeastern Australasia. The FLIP was emplaced during the initial stages of Gondwana break-up from a source suggested to be in the proto-Weddell Sea region. Magma transport from its source (Weddell triple junction) was controlled by an Early Jurassic zone of extension. The FLIP comprises the Dufek intrusion, Ferrar Dolerite sills and dykes (sheet intrusions), and extrusive rocks consisting of pyroclastic strata overlain by Kirkpatrick Basalt lavas. The Dufek intrusion occurs in deformed supracrustal rocks of the foldbelt along the paleo-Pacific Gondwana margin. A few sills were emplaced in basement rocks, but the majority of the sheet intrusions occur in flat-lying Devonian to Triassic Beacon strata. Only in the central Transantarctic Mountains (CTM) and south and north Victoria Land (SVL, NVL) are extrusive rocks preserved overlying Beacon strata. The greatest cumulative thicknesses of magmatic rocks (ca. 2 km) occur in areas where lavas are preserved (CTM and SVL). Sheet intrusions have complex relationships. Dyke swarms ( sensu stricto) are unknown and dykes cutting basement rocks are uncommon. Nevertheless, these dykes, including a 30-m-wide dyke in SVL, suggest that some magmas locally migrated up through basement rocks. In CTM and NVL the outcrop belt has a width of about 160 km. Sills originally extended farther toward the plate margin but have been cut out by erosion and Cenozoic faulting, most clearly in CTM; geophysical data suggest extension under the East Antarctic ice sheet for at least 100 km. Although Early Jurassic extension is documented in CTM, major rift-bounding faults have not been observed. Models for magma emplacement include transport along the axis of the Transantarctic Mountains and off-axis transport from major rift-bounding faults. Contrasts in geochemistry between lavas of NVL (MgO=67%) and CTM (MgO=24%) and the presence of massive dolerite bodies (CTM, SVL) suggest discrete episodes and locations of magma emplacement, and that there was no long range interconnection along the mountain range in supracrustal rocks.

Salt redistribution during extension and inversion inferred from 3D backstripping

A 3D backstripping approach considering salt flow as a consequence of spatially changing overburden load distribution, isostatic rebound and sedimentary compaction for each backstripping step is used to reconstruct the subsidence history in the Northeast German Basin. The method allows to determine basin subsidence and the salt-related deformation during Late Cretaceous–Early Cenozoic inversion and during Late Triassic–Jurassic extension. In the Northeast German Basin, the deformation is thin-skinned in the basinal part, but thick-skinned at the basin margins. The salt cover is deformed due to Late Triassic–Jurassic extension and Late Cretaceous–Early Cenozoic inversion whereas the salt basement remained largely stable in the basin area. In contrast, the basin margins suffered strong deformation especially during Late Cretaceous–Early Cenozoic inversion. As a main question, we address the role of salt during the thin-skinned extension and inversion of the basin. In our modelling approach, we assume that the salt behaves like a viscous fluid on the geological time-scale, that salt and overburden are in hydrostatical near-equilibrium at all times, and that the volume of salt is constant. Because the basement of the salt is not deformed due to decoupling in the basin area, we consider the base of the salt as a reference surface, where the load pressure must be equilibrated. Our results indicate that major salt movements took place during Late Triassic to Jurassic E–W directed extension and during Late Cretaceous–Early Cenozoic NNE–SSW directed compression. Moreover, the study outcome suggests that horizontal strain propagation in the salt cover could have triggered passive salt movements which balanced the cover deformation by viscous flow. In the Late Triassic, strain transfer from the large graben systems in West Central Europe to the east could have caused the subsidence of the Rheinsberg Trough above the salt layer. In this context, the effective regional stress did not exceed the yield strength of the basement below the Rheinsberg Trough, but was high enough to provoke deformation of the viscous salt layer and its cover. During the Late Cretaceous–Early Cenozoic phase of inversion, horizontal strain propagation from the southern basin margin into the basin can explain the intensive thin-skinned compressive deformation of the salt cover in the basin. The thick-skinned compressive deformation along the southern basin margin may have propagated into the salt cover of the basin where the resulting folding again was balanced by viscous salt flow into the anticlines of folds. The huge vertical offset of the pre-Zechstein basement along the southern basin margin and the amount of shortening in the folded salt cover of the basin indicate that the tectonic forces responsible for this inversion event have been of a considerable magnitude.

Quantitative dynamic modelling of basin development in the central and eastern North Sea region – coaxial stretching and strain localization

A new two-dimensional dynamic lithosphere model is used to simulate the Late Palaeozoic to end Danian evolution of the Norwegian-Danish Basin and the post Permian evolution of the Central North Sea including the Central Graben. The transient heat equation and the equations of motion are solved using the finite element method. The lithosphere deforms by brittle and ductile processes through an elasto-visco-plastic rheology depending on temperature, pressure, strain-rate and material parameters. Strain softening dependent on accumulated strain is incorporated. Deposition, erosion and compaction of sediments are simulated. Results show that it is possible to satisfy observations of crustal structure, sediment thickness and surface heat flow for both basins taking all major tectonic and thermal events into consideration. The evolution of the Norwegian-Danish Basin is modelled using a Late Carboniferous – Early Permian thermal event, main rift phase in Early Permian and a minor extensional phase in Triassic. For the Central North Sea two thermal and three tectonic events are simulated: Late Carboniferous – Early Permian and Middle Jurassic thermal events, Early Triassic and Late Jurassic extension, and Late Cretaceous compression. Results show that strain softening may lead to strain localization during extension and therefore may explain observations of upper mantle dipping reflectors in the North Sea. A pure shear dominated extensional regime may change into a simple shear system.

Geodynamic evolution of the Vulcan Sub‐basin, Timor Sea, northwest Australia: A pre‐compression New Guinea analogue?

The Vulcan Sub‐basin lies immediately inboard of the incipient arc‐continent collision in the Timor Sea and comprises part of the Bonaparte Basin system, the northernmost basin on Australia's North West Shelf. Given the high level of preservation of its extensional fabric, the region can provide important analogues for the likely pre‐orogeny architecture of New Guinea, which enables a better understanding of the onset of, and response to, orogenesis. Structural restoration of regional, depth‐converted 2–D seismic lines shows that although the Late Jurassic Swan Graben is significant and contains a thick source‐rock section, the principal phase of crustal extension took place in the Triassic to Middle Jurassic. Within the Vulcan Sub‐basin, the southern Tilted Fault Block Domain records ∼10% Triassic to Middle Jurassic extension, whereas <5% upper crustal extension has been measured in the northern Hourglass Domain. Similarly, while Jurassic extension in the Tilted Fault Block Domain is both deep and focused, the Hourglass Domain is expressed as a broad sag to the northeast, indicating a strong underlying basement influence on compartmentalisation. The Vulcan Sub‐basin shows four principal stages of evolution: (i) regional, evenly spaced crustal faulting and subsidence in the Triassic ‐ Middle Jurassic; (ii) focused faulting in the Late Jurassic that created grabens with uplift of the shoulders; (iii) regional subsidence from the Middle Valanginian; and (iv) minor extensional and contractional reactivation in the Mio‐Pliocene. The measured brittle extension is much less than that suggested by modelling of lithospheric subsidence, which suggests long wavelength distribution of strain in the ductile lower crust, with upper crustal extension mainly focused along the continent‐ocean boundary. Along the North West Shelf and on a smaller scale within the Vulcan Sub‐basin per se, the obvious, basement‐involved, rectilinear compartments defined by prominent offsetting of both extensional fault systems and abyssal plains have important implications for the development of the New Guinea orogen. Similar scale compartments are recognised in New Guinea and display different structural styles and hydrocarbon prospectivity. The transfer zones separating the compartments are the sites of the major copper‐gold deposits in New Guinea. Using the Vulcan Sub‐basin ‐ Timor area as an analogue, it can be seen that an arc could originally collide with a promontory, such as what is now Timor, and reactivate the lineaments allowing local extension and mineralisation. In addition, interpretation of the structure of the New Guinea Fold Belt may be aided by considering the effects of compression on the geometry of the Vulcan Sub‐basin and of the similar Carnarvon Basin and adjacent extended and broken Exmouth Plateau.

CARNARVON BASIN ARCHITECTURE AND STRUCTURE DEFINED BY THE INTEGRATION OF MINERAL AND PETROLEUM EXPLORATION TOOLS AND TECHNIQUES

The Barrow and Dampier Sub-basins of the Northern Carnarvon Basin developed by repeated reactivation of long-lived basement structures during Palaeozoic and Mesozoic tectonism. Inherited basement fabric specific to the terranes and mobile belts in the region comprise northwest, northeast, and north–south-trending Archaean and Proterozoic structures. Reactivation of these structures controlled the shape of the sub-basin depocentres and basement topography, and determined the orientation and style of structures in the sediments.The Lewis Trough is localised over a reactivated NEtrending former strike-slip zone, the North West Shelf (NWS) Megashear. The inboard Dampier Sub-basin reflects the influence of the fabric of the underlying Pilbara Craton. Proterozoic mobile belts underlie the Barrow Sub-basin where basement fabric is dominated by two structural trends, NE-trending Megashear structures offset sinistrally by NS-trending Pinjarra structures.The present-day geometry and basement topography of the basins is the result of accumulated deformation produced by three main tectonic phases. Regional NESW extension in the Devonian produced sinistral strikeslip on NE-trending Megashear structures. Large Devonian-Carboniferous pull-apart basins were introduced in the Barrow Sub-basin where Megashear structures stepped to the left and are responsible for the major structural differences between the Barrow and Dampier Sub-basins. Northwest extension in the Late Carboniferous to Early Permian marks the main extensional phase with extreme crustal attenuation. The majority of the Northern Carnarvon basin sediments were deposited during this extensional basin phase and the subsequent Triassic sag phase. Jurassic extension reactivated Permian faults during renewed NW extension. A change in extension direction occurred prior to Cretaceous sea floor spreading, manifest in basement block rotation concentrated in the Tithonian. This event changed the shape and size of basin compartments and altered fluid migration pathways.The currently mapped structural trends, compartment size and shape of the Barrow and Dampier Sub-basins of the Northern Carnarvon Basin reflect the “character” of the basement beneath and surrounding each of the subbasins.Basement character is defined by the composition, lithology, structure, grain, fabric, rheology and regolith of each basement terrane beneath or surrounding the target basins. Basement character can be discriminated and mapped with mineral exploration methods that use non-seismic data such as gravity, magnetics and bathymetry, and then calibrated with available seismic and well datasets. A range of remote sensing and geophysical datasets were systematically calibrated, integrated and interpreted starting at a scale of about 1:1.5 million (covering much of Western Australia) and progressing to scales of about 1:250,000 in the sub-basins. The interpretation produced a new view of the basement geology of the region and its influence on basin architecture and fill history. The bottom-up or basement-first interpretation process complements the more traditional top-down seismic and well-driven exploration methods, providing a consistent map-based regional structural model that constrains structural interpretation of seismic data.The combination of non-seismic and seismic data provides a powerful tool for mapping basement architecture (SEEBASE™: Structurally Enhanced view of Economic Basement); basement-involved faults (trap type and size); intra-sedimentary geology (igneous bodies, basement-detached faults, basin floor fans); primary fluid focussing and migration pathways and paleo-river drainage patterns, sediment composition and lithology.

Post-Permian evolution of the Central North Sea: a numerical model

A dynamic thermomechanical model is presented for the post-Permian evolution of the Central North Sea including the Central Graben. Permo-Carboniferous and Jurassic thermal events, as well as Triassic and Jurassic extensional phases and Late Cretaceous shortening, are considered. The equations of force balance for a continuum and the transient heat equation are solved using the finite element method. The lithosphere responds to applied forces by elastic deformation at low deviatoric stress levels, fracture at low temperature and pressure conditions and nonlinear viscous flow at high temperature and pressure. A first-order rheological strain softening is incorporated to simulate a transition from strong to soft material behaviour, such as a change from dislocation creep to diffusion creep. Seismic refraction and reflection data indicate crustal thinning below the Central Graben by a factor of ∼2. While the Triassic rifting appears to have been associated with a more distributed deformation pattern, the Upper Jurassic rifting is more localised. Pure shear extensional models have been advocated to explain this. However, dipping seismic reflectors in the upper mantle indicate that simple shear could be important. The strain softening of the present model allows for modelling of strain localisation during the Upper Jurassic rifting phase and detailed modelling of Moho structures. The present model accounts for the main structural and stratigraphic observations in the Central North Sea. Furthermore, the results indicate that in the part of the Central North Sea investigated in this paper, a pure shear regime in Triassic changed into a simple shear-dominated regime with mantle shear zones developing during Late Jurassic extension.

Early Jurassic extensional basin formation in the Daqing Shan segment of the Yinshan belt, northern North China Block, Inner Mongolia

Structural and stratigraphic studies of the western Daqing Shan segment of the Yinshan belt have recognized an Early Jurassic extensional episode supported by several lines of evidence. First, normal faults cut the lowermost Jurassic sequence and are overlapped by younger Lower Jurassic rocks. Second, Lower Jurassic rocks include growth strata in small-scale graben at the base of the Jurassic basin. Third, rapid lateral facies changes are mapped from boulder conglomerates along the basin-bounding faults to lacustrine and meandering fluvial rocks in the basin center. Fourth, paleodrainage systems provided sediment input from three sides of the basin, two transverse and one axial. Finally, there is a strongly asymmetric distribution of coarse proximal and fine distal facies within the basin. The Early Jurassic extensional episode was responsible for formation of an east-trending half-graben basin in the western Daqing Shan in which at least 1800 m of syn-extensional nonmarine sediments accumulated and are preserved. Previous studies in the Triassic and Jurassic of other parts of northwest and north-central China have concluded that the early Mesozoic was a time of continental amalgamation and contractile deformation. The recognition of an Early Jurassic extensional episode along the northern margin of the North China Block is problematic in this context, at least superficially. We propose two possible explanations for the Early Jurassic extension: transtension associated with strike-slip tectonics, or gravitational collapse of a pre-existing Late Paleozoic–Early Mesozoic contractile orogenic belt. The first possibility, transtensional deformation, is problematic because specific strike-slip faults have not been identified that could control extension in the western Daqing Shan. However, several lines of evidence allow the possibility of such a driving mechanism, including: documented Early Jurassic transtensional systems in the southwestern North China Block, several candidate strike-slip faults along the China–Mongolia border region, structural discontinuity between the Daqing Shan and southern Mongolia, and along strike-changes in structural style within the Yinshan belt. The second possibility, orogenic collapse, is similarly difficult to establish because of the limited amount of data concerning the regional distribution and orientation of pre-Jurassic contractile structures and Early Jurassic extensional structures. However, documented Late Paleozoic–Triassic contractile deformation, as well as the close temporal and spatial association, and parallelism between Early Jurassic extensional structures and older contractile structures in the western Daqing Shan requires consideration of gravitational collapse as a driving mechanism for Early Jurassic extension in the Yinshan belt.

Tectonic evolution of the northeastern part of the African continental margin, Egypt

The area between Manzalah Lake and the southern Galala Plateau in northeast Egypt constitutes the Galalas, Cairo-Suez, southern Nile Delta and northern Nile Delta structural provinces. The northern Galala Fault separates the Galalas Province from the Cairo-Suez Province and is considered to be the westward extension of the Themed Fault in central Sinai. The pre-Eocene rocks are affected by northeast to east-northeast-orientated folds and reverse faults, as well as east-west-orientated oblique-slip faults with dextral and normal components. Some folds and reverse faults are interpreted to have been formed by northwest to north-northwest-orientated compression related to the Syrian Arc movement, whereas the others by the secondary northwest orientated shortening, which accompanied dextral strike-slip component along the planes of the east-west-orientated faults. The east-west-orientated faults were initially formed during the Late Triassic/Early Jurassic extension related to the drifting of the African/Arabian Plate away from the Eurasian Plate as a result of opening of the Neotethyan Sea. The Neotethyan began to close due to convergence between the two plates, leading to the Syrian Arc deformation. This deformation mildly started in Late Cenomanian and followed by a more intensive phase in Conacian/Santonian. It mildly continued in the Maastrichtian, Early Palæocene and Late Palæocene/Early Eocene. The southward thinning of the pre-Eocene rocks controlled the intensity and style of deformation. Two deformational mechanisms are proposed for the Nile Delta hinge zone. The first is related to Late Oligocene—Early Miocene north-northwest-orientated Alpine compression. The second is related to northward gravitational sliding of the post-Oligocene shale and sandstone over Cretaceous-Eocene carbonates.

Hydrocarbon potential of the Kish Bank Basin: integration within a regional model for the Greater Irish Sea Basin

Abstract The Kish Bank Basin lies in the western Irish Sea c. 20 km east of Dublin. It is one of a number of remnants of a larger Permo-Triassic basin system that may have extended across the whole of the Irish Sea. It has a geological history similar to that of the East Irish Sea Basin, initially developing by the reactivation of Caledonian faults that controlled subsequent deposition during Dinantian and Namurian time, with Westphalian deposition in a sag-basin that overstepped the adjacent basement highs. Variscan dextral transpression resulted in the formation of the Codling and Bray faults, and Permian to Jurassic extension formed a set of north-south-trending faults. Liassic outliers are preserved in the hanging walls of the basin margin faults. Early Cretaceous uplift was followed by chalk deposition. Tertiary movements reactivated older faults, isolating the Kish Bank Basin, and producing 9 km of dextral strike-slip along the Codling Fault Zone. The main reservoir in the hydrocarbon play is provided by the Sherwood Sandstone Group, as successfully exploited in the East Irish Sea. Three wells have been drilled to test this reservoir. These encountered high-quality Sherwood Sandstone reservoirs beneath the good potential seal of the Mercia Mudstone Group (which included thick halites). Source rock potential is from either the Westphalian Coal Measures, as penetrated in well 33/22-1, or from inferred Dinantian to Namurian basinal shales. There is good evidence of an active source system, with oil shows in wells 33/17-1 and 33/22-1, data from geochemical analysis of sea-bed cores, a ‘Seepfinder’ survey, sea-bed mounds and seismic evidence of shallow gas. The main risks of the play are the migration pathway and the timing of trap formation with respect to migration. Migration favours the eastern side of the basin, and many of the tilted fault blocks that formed during Permian to Jurassic time have been modified by Early Cretaceous inversion and by Tertiary strike-slip compression. All of the structures that have been drilled to date have been either formed or modified after the time of peak hydrocarbon generation and migration.

Hydrocarbon exploration potential within intraplate shear-related depocenters: Deseado and San Julian basins, southern Argentina

The Deseado and San Julian late Paleozoic-Mesozoic rift basins occupy sizable parts of central Patagonia and offshore southern Argentina, respectively. The basins record a similar (linked) tectonostratigraphic evolution since their inception in the late Paleozoic. Both evolved as continental intracratonic rifts developed above a basement comprising Devonian low-grade metasediments overlying older continental crust, in response to regional tectonic extension that preceded late disaggregation of Gondwana. This Late Permian rifting was followed by continuous filling of the basin during Triassic thermal subsidence. Jurassic extension produced a framework of grabens and half grabens filled with mafic and felsic proximal volcanics and volcaniclastics. The east-west orientation of these Jurassic rift features, in sharp contrast to the north-south trend of the Permian-Triassic rift systems, suggests that the Jurassic depocenters shifted in response to oblique extension: development of a transtensional pull-apart basin. In the upper Valanginian-Aptian, a strong compressive event, marked by a prominent angular unconformity, affected the Deseado and San Julian basins. Wrenching associated with this compressive event produced transpressive structures in both basins. A period of relative tectonic stability was observed during the Upper Cretaceous and Tertiary, the only interruption being a mild transpressive reactivation along major preexisting faults during the Miocene Andean orogeny. The first marine incursion occurred in the Paleocene as the seas transgressed over an extensive peneplained terrain. The Deseado and San Julian basins remain virtually unexplored--only three wells having been drilled. Oil seeps in Neocomian strata in the Deseado basin indicate that this frontier basin has petroleum potential. Source quality intervals were encountered in the Neocomian Bajo Grande Formation of the San Julian Basin; (Begin page 1796) distal lacustrine facies of the Neocomian La Matilde Formation are expected to be potential source rocks in the Deseado basin. The Permian-Triassic section is also thought to contain well-developed source rocks; however, the section has yet to be penetrated. Reservoir-quality rocks occur in clastics of the Upper Cretaceous, Neocomian, Triassic, and Permian formations. Possible play types include Paleozoic reservoirs in positive transpressive features, lateral updip pinch-outs of Neocomian sandstones, and fractured Jurassic volcanics adjacent to fault zones.

The post-Triassic evolution of the Sorgenfrei–Tornquist Zone — results from thermo-mechanical modelling

The Sorgenfrei–Tornquist Zone (STZ) is part of the Fennoscandian Border Zone separating the Danish Basin from the Fennoscandian Shield. The STZ as a structural element is of Palaeozoic origin and represents the north-westerly segment of the Tornquist–Teisseyre Zone, which separates the younger West European crust from the older East European Platform and extends from the Black Sea to the eastern North Sea area. The STZ was reactivated in Triassic–Jurassic extension and Late Cretaceous and Paleogene compression.This paper investigates the regional geological consequences of the reactivations by quantitative modelling along a profile across the STZ in the Danish area. The numerical model invokes elastic, viscous and plastic deformations of the lithosphere as well as surface processes governed by erosion, sedimentation and lateral transport under the influence of eustatic sea level variations and regional isostatic compensation. Surface processes and lithospheric mechanics are coupled through thermal blanketing effects and loading.The results, in general, address the regional geological consequences of the existence of intracontinental zones of structural weakness. More specifically the results show that the Late Cretaceous and Paleogene chalk depocentres in the Danish Basin are a direct consequence of the inversion of the STZ, and that the STZ inversion together with falling sea level in Cenozoic time are amongst the principal controlling factors in the geological evolution in the eastern North Sea area.

Triassic–Jurassic development of the St. George’s Channel basin, offshore Wales, UK

Isopach maps and backstripped tectonic subsidence plots from the Triassic–Jurassic of the St. George’s Channel basin reveal significant departures from current models of extensional basin development: (i) rifting was highly protracted with discrete increments of crustal extension spanning ca. 120 My, (ii) substantial syn-kinematic sediment on the footwalls of major faults records pure shear (sub-seismic scale) deformation to have been important during regional Triassic extension. Forward modelling of depth-converted seismic interpretations is used to test the notion of combined pure and simple shear and to quantify the distribution of crustal extension between large and sub-seismic scale faults. The models indicate that Triassic subsidence was generated by ca. 2.5 km extension on major faults and 10 km extension by sub-seismic scale faulting. In fault-bounded compartments, Jurassic extension was dominated by large-scale faults, whereas in the areas of overlap between major fault segments, Jurassic extension was taken up chiefly by sub-seismic scale faulting. Comparison of measured vitrinite reflectance (VR) and VR modelled from heatflow history yielded by the forward models supports this scheme of episodic, pure shear-dominated extension. This research highlights the inherent difficulty in using seismic interpretation in isolation from quantitative subsidence and/or thermal history data to identify syn- and post-kinematic sedimentary sequences. Furthermore, it provides an opportunity to more fully reconstruct Mesozoic basin development in the West-of-UK area which, in the Cheshire, Central and East Irish Sea basins, is largely precluded by erosion of post-Triassic strata.

Structural styles in the Perth Basin associated with the Mesozoic break-up of Greater India and Australia

The Perth Basin constitutes one of the principal tectonic features along the western continental margin of Australia and formed through Permo-Cretaceous rifting of Greater India and Australia. It is composed of northerly striking sub-basins, troughs and ridges bounded by faults which display evidence for normal dip–slip, and possibly dextral strike–slip movement. Transfer faults divide the basin into compartmentalised regions of similar structural style. Extension and transtension occurred during the Jurassic–Early Cretaceous associated with continental break-up. Stretching was accommodated mainly by the reactivation of pre-existing northerly oriented faults which display both planar and listric geometries. The Darling Fault system along the eastern margin of the basin has a planar profile. The hanging wall succession in the adjoining Perth Basin has undergone block rotation and tilting with syn-sedimentary depositional thickening of rock units towards the fault. Listric normal faults occur in both the onshore and offshore portions of the basin and influence geometry of the sub-basins. Dip on these faults decreases through the basin sequence and probably soles out in the upper level of basement. Rollover anticlines develop within the hangingwall of these structures. Extension oblique to the main structural trends of the basin in the Late Jurassic–the earliest Cretaceous resulted in dextral strike–slip movement along major north-striking faults and the development of en-echelon folds in the basin. Compressive deformation developed during this event at constraining bends of the major fault systems. Two orientations of transfer zones are recognised and significantly influence basin structures. The first are east–west trending transfer faults which formed in the Permian and were reactivated during Jurassic extension but can only be recognised in the northernmost Perth Basin. These transfer faults link basin segments of different characteristics and transfer dip displacement among the northerly striking faults. The transfer zones striking NW only influence Late Jurassic–Early Cretaceous age deformation features. They are characterised by either a swing in a left-lateral sense in the orientation of northerly striking fractures as they cross the transfer zones or a termination of the fracture at the transfer zone. The transfer zones are parallel to, and may be related to, transform belts in the Indian oceanic crust. Basin-wide uplift occurred during the break-up and resulted in basin inversion and erosion of up to thousands of metres of the sedimentary succession. Folding by dextral strike–slip motion along the Darling-Urella fault system also inverted the pre-break-up sequences in major onshore depocentres.

Permo-Triassic and Jurassic extension in the northern North Sea: results from tectonostratigraphic forward modelling

Abstract We have undertaken 2D forward modelling across the northern North Sea, based on reprocessed, interpreted and depth-converted deep reflection seismic lines NSDP84-1 and −2 (15 s twt) and refraction data. Two separate stretching phases, Permo-Triassic and Jurassic, are recognized. The cumulative stretching is consistent with the observed crustal structure and the overall basin configuration, as reproduced by forward modelling. Good agreement between observed and modelled top basement level, and crustal thickness below the platform areas are particularly emphasized. Crustal-scale modelling indicates that crustal thickness varied across the northern North Sea at the onset of the Permo-Triassic rifting, from c . 35 km in the platform areas to less than 30 km in the interior of the basin. This may be ascribed to Devonian(-Carboniferous?) crustal stretching. Thinning of the crust has progressively been narrowed, from post-Caledonian extensional collapse, to less regional Permo-Triassic basins, and finally development of the Viking Graben area in the Jurassic-early Cretaceous time. Most of the Permo-Triassic stretching occurred between the Øygarden Fault Zone to the east and the Shetland Platform (southern transect) and the Hutton Fault alignment to the west. The width of the Permo-Triassic basin was c . 120–125 km, with calculated β mean between 1.38 and 1.40. Permo-Triassic β mean estimates across the present Horda Platform vary between 1.33 and 1.39. The Jurassic β mean estimates for the same area vary between 1.08 and 1.13. Across the Viking Graben, Permo-Triassic β mean varies between 1.28 (southern transect) and 1.41 (northern transect). This is lower than estimates for the Jurassic β mean , which amounts to 1.53 and 1.42. Permo-Triassic and Jurassic β mean estimates across the East Shetland Basin are 1.29 and 1.11, respectively. Lithospheric thermal evolution reflects the general differences between Permo-Triassic and Jurassic stretching, with a much wider thermal perturbation during the former and a focusing and lateral migration towards the east of the peak thermal elevation during the latter. There are still uncertainties related to the degree of (de)coupling between the upper crust and upper mantle during the Permo-Triassic and the Jurassic rift phases. These uncertainties are related to the interplay between age, strain rate, crustal rheology, crustal thickness and long-lived zones of weaknesses.