ALBERT

Description: The arcuate Outer Carpathian accretionary wedge formed in front of the East Alpine-Carpathian-Pannonian (ALCAPA) megablock during the Eocene-Sarmatian. The wedge accreted sediments of the subducting remnants of the Carpathian Flysch Basin, a large oceanic tract left in front of the Alpine orogen. The palaeostress data for the orogenic hinterland (particularly the data related to the Early Miocene extension that was expanding towards the NE), combined with coeval subduction-related volcanism that was expanding towards the NE, indicate that the uneven roll-back of the subduction zone was the main mechanism controlling the development of the northern West Carpathian arc. The palaeostress data for the Tertiary accretionary wedge from the same time period are characterized by outward-fanning {sigma}1 trajectories that changed gradually during the wedge development. In contrast, the palaeostress data for the hinterland are characterized by preferred-directional stress events that changed abruptly during the wedge development. These palaeostress results are in accordance with the behaviour of the wedge and the hinterland, as the wedge behaved as a weak continuum and the hinterland behaved as a block mosaic with weak boundaries. The fault traces of the northern West Carpathian arc converge to both ends of the arc and suggest that the pre-existing basin was the factor that controlled the arc location. These fault trace patterns are asymmetric, indicating a slightly oblique overall convergence in a NE-SW direction. In accordance with this convergence, the palaeostress data for the accretionary wedge indicate that the western part of the wedge, which is characterized by NW-SE-oriented maximum principal compressional stress {sigma}1, was undergoing sinistral transpression. Meanwhile, the eastern part, which is characterized by NE-SW-oriented {sigma}1, was undergoing compression. Apparently, the dynamics of the accretionary wedge was further influenced by the shape of an elongated NE-SW-trending ALCAPA megablock, which was located behind the wedge and advanced in the direction of the general Early Miocene convergence during the most pronounced stages of the wedge development. This megablock served as the local indenter, as its strength surpassed that of the accretionary wedge located to its front. Further dynamic complexities were added because of the complex shape of the Magura Unit, which was located in the most proximal portion of the wedge and was stronger than the units in front of it. Wedge outcrops indicate that the large-scale shortening, which is characterized by the development of detachments and ramps, was preceded by an initial layer-parallel shortening. This is indicated by scaly fabrics and minor reverse faults that rotated into locked positions during the later accretion. Several outcrops with a wedge detachment fault indicate that there was a relatively low amount of friction during its development. The decollement zone is several hundred metres thick and shows evidence of transient fluid flow that was driven by pressure gradients. This is documented by frequent hydrofracturing, sandstone dykes and fibrous veins that opened against the weight of the whole wedge, all of which indicate cycles of higher pore fluid pressures that lowered the basal friction.

Description: The South Atlantic Ocean evolved after rupture of the Sao FranciscoCongoRio de la PlataKalahari cratonic landmass and the Late Proterozoic fold belts. Break-up in the South Atlantic realm developed diachronously: rifting started in the south (Argentina) during the Jurassic and progressed towards the equatorial segment. The central portion was controlled by a rift-resistant cratonic nucleus (the Sao FranciscoCongo craton) and as a result underwent development of narrow basins; parts controlled by Neoproterozoic fold belts developed wide basins. The final break-up of western Gondwana and the onset of plate divergence were marked by thick wedges of seaward-dipping reflectors, located near the incipient ocean-ridge spreading centre that had already been formed by the time Aptian evaporites were deposited. Subsequently, a few episodes of intraplate tectonic and magmatic activity affected the Santos, Campos and Espirito Santo basins. Post-break up development of the offshore basins was affected by gravity gliding over the Aptian evaporites. Continental uplift may be invoked as the main cause of salt mobilization, generating prograding clastic wedges that thickened basin-wards and produced a loading effect on the salt basin. Coupled with onshore erosional unloading and the effects of the gravity gliding, this probably resulted in further flexural uplift of the continental margin.

Description: The study focuses on Equatorial Atlantic margins, and draws from seismic, well, gravimetric and magnetic data combined with thermo-mechanical numerical modelling.Our data and numerical modelling indicates that early drift along strike-slip-originated margins is frequently characterized by up to 10°–20° spreading vector adjustments. In combination with the warm, thinned crust of the continental margin, these adjustments control localized transpression.Our observations indicate that early-drift margin slopes are too steep to hold sedimentary cover, which results in their inability to develop a moderately steep slope undergoing cycles of gravitational instability resulting in cyclic gravity gliding. These slopes either never develop such conditions or gain them at later development stages.Our modelling suggests that the continental margin undergoing strike-slip-controlled break-up experiences warming due to thinning along pull-apart basin systems. Pull-apart basins eventually develop sea-floor spreading ridges. Margins bounded by strike-slip faults located among pull-apart basins with these ridges first undergo cooling. However, spreading ridges leaving the break-up trace along its strike eventually pass by these cooling margins, warming them again before the final cooling proceeds. As a result, the structural highs surrounded by several source rock kitchens witness a sequential expulsion onset in different kitchens along the trajectory of spreading ridges.Supplementary material: Discussion of the methods used, chronostratigraphic results and strike-slip margin characteristics are available at http://www.geolsoc.org.uk/SUP18518

Description: The segmented East Indian continental margin developed after the Early Cretaceous break-up from Antarctica. Its continental crust terminates abruptly without considerable thinning along the Coromondal strike-slip segment and thins considerably before it terminates in the orthogonal rifting segments. The segments have an exhumed continental mantle corridor oceanwards of them. This, proto-oceanic crust, corridor varies in width from segment to segment, indicating a relationship with varying break-up-controlling tectonics of the adjacent margin segments.The top of the proto-oceanic crust is imaged by a higher reflectivity zone, while its base does not have any distinct signature. A contorted system of reflectors represents its internal structure. Its gravity signature is a longer-wavelength anomaly with peak values up to 30 mGal less negative than surrounding values. Its magnetic signature is represented by a positive anomaly with peak values of 0–56 nT. Wide proto-oceanic segments are adjacent to margin segments that are preceded by the orthogonally rifting Cauvery, Krishna–Godavari and Mahanadi rift zones. A narrow proto-oceanic segment is adjacent to the margin segment initiated by the dextral Coromondal transfer zone. A combination of seismic interpretation and gravity/magnetic forward modelling indicates that proto-oceanic crust is most probably composed of lower crust slivers and unroofed hydrated upper mantle, being formed between the late rifting and the organized sea-floor spreading.

Description: Karaha-Telaga Bodas, a vapour-dominated geothermal system located in an active volcano in western Java, is penetrated by more than two dozen deep geothermal wells reaching depths of 3 km. Detailed paragenetic and fluid-inclusion studies from over 1000 natural fractures define the liquid-dominated, transitional and vapour-dominated stages in the evolution of this system. The liquid-dominated stage was initiated by a shallow magma intrusion into the base of the volcanic cone. Lava and pyroclastic flows capped a geothermal system. The uppermost andesite flows were only weakly fractured due to the insulating effect of the intervening altered pyroclastics, which absorbed the deformation. Shear and tensile fractures that developed were filled with carbonates at shallow depths, and by quartz, epidote and actinolite at depths and temperatures over 1 km and 300{degrees}C. The system underwent numerous cycles of overpressuring, documented by subhorizontal tensile fractures, anastomosing tensile fracture patterns and implosion breccias. The development of the liquid system was interrupted by a catastrophic drop in fluid pressures. As the fluids boiled in response to this pressure drop, chalcedony and quartz were selectively deposited in fractures that had the largest apertures and steep dips. The orientations of these fractures indicate that the escaping overpressured fluids used the shortest possible paths to the surface. Vapour-dominated conditions were initiated at this time within a vertical chimney overlying the still hot intrusion. As pressures declined, these conditions spread outward to form the marginal vapour-dominated region encountered in the drill holes. Downward migration of the chimney, accompanied by growth of the marginal vapour-dominated regime, occurred as the intrusion cooled and the brittle-ductile transition migrated to greater depths. As the liquids boiled off, condensate that formed at the top of the vapour-dominated zone percolated downward and low-salinity meteoric water entered the marginal parts of the system. Calcite, anhydrite and fluorite precipitated in fractures on heating. Progressive sealing of the fractures resulted in the downward migration of the cap rock. In response to decreased pore pressure in the expanding vapour zone, walls of the fracture system within the vapour-dominated reservoir progressively collapsed. It left only residual permeability in the remaining fracture volume, with apertures supported only by asperities or propping breccia. In places where normal stresses acting on the fracture walls exceeded the compressive strength of the wall rock, the fractures have completely collapsed. Fractures within the present-day cap rock include strike- and oblique-slip faults, normal faults and tensile fractures, all controlled by a strike-slip stress regime. The reservoir is characterized by normal faults and tensile fractures controlled by a normal-fault stress regime. The fractures show no evidence that the orientation of the stress field has changed since fracture propagation began. Fluid migration in the lava and pyroclastic flows is controlled by fractures. Matrix permeability controls fluid flow in the sedimentary sections of the reservoir. Productive fractures are typically roughly perpendicular to the minimum compressive stress, {sigma}3, and are prone to slip and dilation within the modern stress regime.

Description: Continental break-up mechanisms vary systematically between slow- and fast-extension systems. Slow-extension break-up has been established from studies of the Central Atlantic, European and Adria margins. This study focuses on the intermediate and fast cases from Gabon and East India, and draws from the interpretation of reflection seismic, gravimetric and magnetic data.Interpretation indicates continental break-up via continental mantle unroofing in all systems, with modifications produced by magmatism in faster-extension systems. Break-up of the intermediate-extension Gabon system involves partial upper continental crustal decoupling from continental mantle; whereas, in the fast East Coast India system, decoupled and lower-crustal regimes underwent upwarping in ‘soggy’ zones in the footwalls of major normal faults. Usually, upper-crustal break-up is affected by pre-existing anisotropies, which form systems of constraining ‘rails’ for extending continental crust. This modifies the local stress regimes. They regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle. Different regions attain different amounts of upper-crustal stretching prior to the break-up. The break-up location is then controlled by the upper-crustal energy balance principle of ‘wound linkage’, by which the minimum physical work is performed for linking upper-crustal ‘wounds’, leading to successful upper-crustal break-up.Supplementary material: Supplementary information and figures on the modelling of the mechanisms and architecture is available at http://www.geolsoc.org.uk/SUP18525.

Description: This paper provides an overview of the existing knowledge of transform margins including their dynamic development, kinematic development, structural architecture and thermal regime, together with the factors controlling these. This systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock, seal rock and reservoir rock distribution, expulsion timing, trapping style and timing, and migration patterns. The paper then introduces individual contributions to this volume and their focus. Supplementary material: Location table and map of specific transform examples, structural elements of the Romanche transform margin and glossary of terms used in this article is available at https://doi.org/10.6084/m9.figshare.c.3276407

Description: This paper studies the magma-rich Gop Rift–Laxmi Basin, West India, which underwent the mantle first–crust second break-up mode. It draws from reflection seismic and gravity data from this abandoned system. Seismic images document that the crustal necking was associated with the development of seawards-dipping reflector wedges deformed by landwards-dipping detachment faults. A wide crustal necking zone indicates that the ductile lower crust was still present during necking. Observed uneven detachment fault spacing indicates the effect of upper-crustal anisotropy. Comparison of the seismic images through progressively more mature stages of the rift–drift transition documents that the final stages of thinning represented the time period when the upper-crustal wide and symmetrical rift architecture changed to the asymmetrical one, and the decoupled system to the coupled one. It further indicates that the last crustal layer was broken with a convex-up fault that was associated with an excess magmatic event. The fault propagation represented the first spontaneous deformation unaffected by the pre-existing anisotropy. Subsequent drift of the two plates was associated with melt-assisted spreading and spontaneous faulting. The faulting geometry and sequence controlled which of the conjugate margins ended up with a volcanic outer high, representing the record of the break-up-locating excess magmatism.

Description: A three-dimensional (3D) thermal–kinematic modelling approach based on finite-element techniques is used to study lower-crustal viscosity at transform margins during the continent–ocean transform development stage and after the ridge has passed by. Nine modelling scenarios combining different equilibrium surface heat flows and lower-crustal rheologies are studied. Modelling results indicate that substantial parts of the lower crust at transform margins have the potential to flow at geologically appreciable strain rates, which can lead to uplift/subsidence, as well as lateral variations, in upper- and lower-crustal thicknesses and Moho depth. These low-viscosity zones (i.e. parts of the lower crust with effective viscosities of less than 10 18 Pa s) make up distinct ductility distributions that vary in space and time during margin evolution. Three basic ductility patterns and related thermal processes can be identified: reduced lower-crustal viscosities originating at the continental rift and the continent–ocean boundary (COB), respectively; reduced lower-crustal viscosities along the transform caused by the migrating ridge; and the background distribution of lower-crustal ductility resulting from the equilibrium temperature field. Superposition of all three ductility patterns and the complex interaction of the underlying perturbations of the temperature field result in distinct differences in the potential of lower-crustal flow both in space (parallel and perpendicular to the transform) and with time. Thus, modelling results provide templates for understanding lower-crustal flow at transform margins in general and await further studies comparing model predictions with actual field observations.