Abstract The crustal structure and distribution of crustal types on the northern Angolan rifted continental margin have been the subject of much debate. Hyper-extended continental crust, oceanic crust and exhumed serpentinized mantle have all been proposed to underlie the Aptian salt and the underlying sag sequence. Quantitative analysis of deep seismic reflection and gravity anomaly data, together with reverse post-break-up subsidence modelling, have been used to investigate the ocean–continent transition structure, the location of the continent–ocean boundary, the crustal type and the palaeobathymetry of Aptian salt deposition. Gravity inversion methods (used to give the depth to the Moho and the crustal thickness), residual depth anomaly analysis (used to identify departures from oceanic bathymetry) and subsidence analysis have all shown that the distal Aptian salt is underlain by hyper-extended continental crust rather than exhumed mantle or oceanic crust. We propose that the Aptian salt was deposited c. 0.2 and 0.6 km below global sea-level and that the inner proximal salt subsided by post-rift (post-tectonic) thermal subsidence alone, whereas outer distal salt formation was synrift, prior to break-up, resulting in additional tectonic subsidence. Our analysis argues against Aptian salt deposition on the Angolan margin in a 2–3 km deep isolated ocean basin and supports salt deposition on hyper-extended continental crust formed by diachronous rifting migrating from east to west and culminating in the late Aptian.

Abstract Continental margins and their fossilized analogues are important repositories of natural resources. With better processing techniques and increased availability of high-resolution seismic and potential field data, imaging of present-day continental margins and their embedded sedimentary basins, in which the majority of these resources are located, has reached unprecedented levels of refinement and definition, as illustrated by papers in this volume. This, in turn, has led to greatly improved geological, geodynamic and numerical models for the crustal and mantle processes involved in continental-margin formation from the initial stages of rifting through to continental rupture and break-up, to the eventual development of a new ocean basin. Further informing these models, and contributing to a better understanding of the features imaged in the seismic and potential field data, are observations made on fossilized fragments of exhumed subcontinental mantle lithosphere and ocean–continent transition zones preserved in ophiolites and orogenic belts of both Palaeozoic and Mesozoic age from several different continents, including Europe, South Asia and Australasia.

Abstract The Capel and Faust basins (northern Lord Howe Rise) are located in the SW Pacific between Australia, New Zealand and New Caledonia. New seismic, gravity, magnetic and bathymetry data and rock samples have enabled the construction of a three-dimensional geological model providing insights into the crustal architecture and basin stratigraphy. Multiple large depocentres up to 150 km long and 40 km wide, containing over 6 km of sediment, have been identified. These basins probably evolved through two major Early Cretaceous rifting episodes leading to the final break-up of the eastern Gondwanan margin. Pre-break-up plate restorations and potential field data suggest that pre-rift basement is a collage of several discrete terranes, including a Palaeozoic orogen, pre-rift sedimentary basins and rift-precursor igneous rocks. It is likely that a pre-existing NW-trending basement fabric, inherited from the New England Orogen (onshore eastern Australia), had a strong influence on the evolution of basin architecture. This basement fabric was subjected to oblique rifting along an east–west vector in the ?Early Cretaceous to Cenomanian and NE–SW-oriented orthogonal rifting in the ?Cenomanian to Campanian. This has resulted in three structural provinces in the study area: Eastern Flank, Central Belt and Western Flank.

Abstract Reconnaissance 2D seismic reflection data intended to investigate the petroleum potential of New Zealand’s marine territories have contributed many insights into the geological evolution of the large continental block that surrounds New Zealand. These include: definition of a back-thrust system to the Mesozoic Gondwana subduction margin along the Northland–Reinga Basin and the transition to back-arc rifting; the development of a Mesozoic back-arc rift system through the present New Caledonia and probably the Bounty troughs; the Early Cretaceous cause, at least locally, of the cessation of subduction along the New Zealand sector of the Gondwana margin; evidence for anticlockwise rotation of eastern New Zealand relative to the west in Late Eocene time; an explanation for the development of the Alpine Fault and the South Island compressional strike-slip margin between the Pacific and Australian plates through South Island.

Abstract We investigate the evolution of the Iberia–Newfoundland margin from Permian post-orogenic extension to Early Cretaceous break-up. We used a Quantitative Basin Analysis approach to integrate seismic stratigraphic interpretations and drill-hole data of two representative sections across the Iberia–Newfoundland margin with kinematic models for lithospheric thinning and subsequent flexural readjustment. We model the distribution of extension and thinning, palaeobathymetry, crustal structure, and subsidence and uplift history as functions of space and time. We start our modelling following post-orogenic extension, magmatic underplating and thermal re-equilibration of the Permian lithosphere. During the Late Triassic–Early Jurassic, broadly distributed, depth-independent lithospheric extension evolved into Late Jurassic–Early Cretaceous depth-dependent thinning as crustal extension progressed from distributed to focused deformation. During this time, palaeobathymetries rapidly deepened across the margin. Modelling of the southern and northern profiles highlighted the rapid development of crustal deformation from south to north over a 5–10 myr period, which accounts for the rapid change in Tithonian–Valanginian, deep- to shallow-water sedimentary facies between the Abyssal Plain and the adjacent Galicia Bank, respectively. Late-stage deformation of both margins was characterized by brittle deformation of the remaining continental crust, which led to exhumation of subcontinental mantle and, eventually, continental break-up and seafloor spreading.

Abstract The Cretaceous Hegang Basin is located on the Jiamusi Block, NE China, and separated from the Songliao Basin by the Lesser Xing’an Range (LXR). Seismic interpretation shows that the Chengzihe, Muling and Dongshan formations of the Hegang Basin thicken eastwards with westwards onlap, indicating that the LXR existed as a palaeo-uplift during that period, whereas the Houshigou Formation shows no thickness change, indicating that the LXR was possibly under water at this time. This is supported by results of detrital zircon analysis from the Hegang Basin in which the Chengzihe Formation is dominated by approximately 180 Ma zircons, which can only be provided by the LXR, whereas the Houshigou Formation records no Early Jurassic ages. This view is consistent with previous studies of the Songliao Basin for a provenance change between the Denglouku and Quantou formations. We conclude that the LXR was a highland during deposition of the Chengzihe, Muling and Dongshan formations but that it was under water when the Houshigou Formation was deposited. There was thus a connection between the Hegang and Songliao basins, which marks an eastwards migration of the depositional and extensional centre of the Songliao–Hegang basin system. This eastwards migration implies lithospheric extension driven by palaeo-Pacific roll-back.

Abstract Black shales are integral parts of most foreland-basin deposits and, because they typically reflect maximum basin subsidence, their distributions serve as proxies for the extent of foreland-basin development. In the United States Appalachian area, the distribution of Middle–Upper Ordovician black shales suggests that the Taconian Orogeny proceeded from south to north along the eastern Laurentian margin and that Taconian tectophases were mediated by convergence at continental promontories. In the Late Ordovician Taconic tectophase, changes in the distribution of the Martinsburg and Utica black shales support a reversal of subduction polarity that effected the reactivation of basement structures and basin migration sufficient to yoke the Appalachian foreland basin with adjacent intracratonic basins. Shale distribution suggests that early Chatfieldian (late Sandbian–early Katian), east-verging subduction early in the tectophase generated a cratonic extensional regime with a narrow foreland basin that developed along reactivated Iapetan basement structures. Abruptly, in late Chatfieldian–early Edenian (early Katian) time, westwards migration of basinal Utica black shales and an underlying unconformity suggests change to a compressional regime and westwards subduction vergence. The coincidence of changes in basin shape and migration with the shifts in subduction polarity suggests a causal relationship.

Abstract In the North Atlantic, Laurentia–Eurasia break-up commenced in the Late Carboniferous, largely following the structural grain of the Caledonian Fold Belt. However, in the Arctic region, a 45° offset in the plate boundary between North Greenland and Svalbard was determined by a number of pre-Caledonian fundamental faults in North Greenland. As a result, this segment of the plate boundary experienced significant episodes of combined transtension and transpression, in part controlled by the movement of a temporarily independent Greenland Plate. Late Permian–Mesozoic deposits in the North Greenland Wandel Sea Basin record the plate-boundary history along this offset, in our view in a series of at least 20, variously disturbed, pull-apart basins, most of which can be assigned to four major episodes of pull-apart basin formation. The direction of the pre-existing fundamental faults, in combination with the regional variation in rock properties of both the basin floor and basin fill, explains the marked differences in tectonic style recorded along the plate boundary.

Abstract We use the Bay of Biscay and Western Pyrenees as a natural laboratory to develop and apply an approach to characterize and identify distinctive rifted margin domains in offshore and onshore settings. The Bay of Biscay and Western Pyrenees offer access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift system. Offshore, we use gravity inversion and flexural backstripping techniques combined with seismic interpretation to provide estimates of accommodation space, crustal thickness and lithosphere thinning. Onshore, we focus on key outcrops of the former rift domain to describe the nature of sediment and basement rocks, and of their interface. This qualitative and quantitative characterization provides diagnostic elements for the identification of five distinct structural domains at magma-poor rifted margins and their fossil analogues (proximal, necking, hyperthinned, exhumed mantle and oceanic domains). This new approach can be used to reconcile offshore and onshore observations, and to aid interpretation when only local observations are available. Onshore remnants can be placed in an offshore rifted-margin context, enabling the prediction of first-order crustal architecture. For the interpretation of offshore seismic reflection sections, geological insights into rift structures and basement nature can be suggested based on onshore analogies. Supplementary material: Sensitivity of backstripping results to flexural rigidity is available at http://www.geolsoc.org.uk/SUP18778 .

Abstract We investigate the structural, petrological and compositional features recorded by strongly deformed and melt-percolated Erro–Tobbio peridotites (Voltri Massif, Ligurian Alps, NW Italy), in order to demonstrate that the processes of shear-zone formation and melt percolation are intimately linked by a positive feedback. We focus on spinel and plagioclase peridotites, and extensional shear zones that underwent infiltration by upwelling asthenospheric melts. Shear and porosity bands, which developed during extension prior to melt infiltration, represent important structural and rheological pathways to facilitate and enhance melt infiltration into the extending lithosphere and the ascent of such melts to shallower levels. Our results lend strong support to numerical models addressing the physical processes underlying extensional systems. These show that, in the case of slow–ultraslow continental extension and the subsequent formation of slow–ultraslow spreading oceans, porosity and shear-localization bands may develop in a previously unstructured lithosphere, prior to melt infiltration. Our studies on the Erro–Tobbio peridotites allow a model for the inception of continental extension and rifting to drifting of slow–ultraslow spreading oceans to be proposed. We suggest that integrated studies of on-land peridotites, coupled with geophysical–structural results from modern oceans, may provide clues to the geodynamic processes governing continental extension and passive rifting.

Abstract Based on present knowledge of mantle peridotites from the Ligurian Tethys ophiolites, this paper presents new ideas and a new model for passive rifting to ocean spreading in the slow–ultraslow rifting Europe–Adria realm. Relevant points include: (i) the positive feedback between deformation and melt percolation during passive magmatic rifting; (ii) the positive feedback between natural evidence and experimental data on the behaviour of the mantle lithosphere during passive rifting; (iii) the significance of hidden magmatism and the associated melt thermal advection; (iv) the role of the wedge-shaped weakened and softened axial zone; and (v) the evidence of a transition from passive to active rifting in the Ligurian Tethys. Passive rifting induced passive asthenospheric upwelling and the onset of partial melting. Fractional melts migrated through the mantle lithosphere and stagnated at shallow levels (the hidden magmatism). Melt thermal advection heated the mantle lithosphere to temperatures ( T ) of ≥1200°C and formed a wedge-shaped axial zone of rheological softened/weakened mantle peridotites that served as the future locus of continental break-up. The hotter/deeper asthenosphere ascended within this axial zone, underwent partial melting and formed aggregated mid-ocean ridge basalts (MORBs) that migrated within dunite channels to form olivine gabbro intrusions and basaltic lava flows. Rifting evolved from passive to active, and the actively upwelling asthenosphere established a ridge-type system and thermal regime.

Abstract Mafic and ultramafic rocks intercalated with metamorphosed deep-marine sediments in the Glenelg River Complex of SE Australia comprise variably tectonized fragments of an interpreted late Neoproterozoic–earliest Cambrian hyper-extended continental margin that was dismembered and thrust westwards over the adjacent continental margin during the Cambro-Ordovician Delamerian Orogeny. Ultramafic rocks include serpentinized harzburgite of inferred subcontinental lithospheric origin that had already been exhumed at the seafloor before sedimentation commenced, whereas mafic rocks exhibit mainly enriched- and normal-type mid-ocean ridge basalt (E- and N-MORB) compositions consistent with emplacement in an oceanic setting. These lithologies and their metasedimentary host rocks predate deposition of the Cambrian Kanmantoo Group and are more likely to represent temporal equivalents of the older Normanville Group or underlying Neoproterozoic Adelaide Supergroup. The Kanmantoo Group is host to basaltic rocks with higher degrees of crustal contamination and yields detrital zircon populations dominated by 600–500 Ma ages. Except for quartz greywacke confined to the uppermost part of the sequence, metasedimentary rocks in the Glenelg River Complex are devoid of detrital zircon, and are interstratified with subordinate amounts of metachert and carbonaceous dolomitic slate suggestive of deposition in a deep-marine environment far removed from any continental margin. Seismic reflection data support the idea that the Glenelg River Complex is underlain by mafic and ultramafic rocks, and preclude earlier interpretations based on aeromagnetic data that the continental margin incorporates a thick pile of seawards-dipping basaltic flows analogous to those of volcanic margins in the North Atlantic. Correlative hyper-extended continental rift margins to the Glenelg River Complex occur along strike in formerly contiguous parts of Antarctica. Supplementary material: Geochemical data for mafic and ultramafic rocks in the Glenelg River Complex and correlative terranes, and U–Th–Pb data for western Victoria gabbros are available at http://www.geolsoc.org.uk/SUP18821

Abstract The Indo-Myanmar Ranges (IMR) of NE India are host to various ophiolitic rocks, including metamorphosed Alpine-type harzburgite and lherzolite. Compared to abyssal peridotites of normal oceanic lithosphere, these ultramafic rocks are enriched in trace and rare earth elements. Spilitic pillow lavas along with mafic dykes and sills locally intruded into the serpentinized ultramafic rocks and associated pelagic sediments exhibit alkaline compositional affinities. Ophiolite formation and emplacement were by a process analogous to that described for mantle exhumation in hyper-extended continental margin settings and ophiolites in parts of the European Alps, involving very slow passive continental margin rifting accompanied by slow upwelling or extensional unroofing of the subcontinental upper mantle up to the seafloor. Preliminary palaeomagnetic measurements conducted on ultramafic rocks within the IMR ophiolite belt give a virtual geomagnetic pole (VGP) at 47° N, 045° E for thermal demagnetization (TDM) measurements and 33° N, 013° E for the alternating field demagnetization (AfD) measurements, requiring an anticlockwise rotation of the ultramafic bodies by 14° during the subduction process. The original trend of the spreading axis of the ophiolites was probably NE–SW, with spreading directed NW–SE. Computation of palaeolatitude of the ultramafic rocks gives an average value of 24.67°. Comparison between the palaeolatitude and the present latitude of the sample sites provides a mere latitudinal shift of less than 1°. Field studies, combined with an analysis of structural and tectonic features in the IMR, suggest a generalized WNW–ESE (east–west) compression and NNE–SSW (north–south) extension contradictory to the NNE–SSW contraction indicated by seismic data. Area balancing techniques employed along sections orientated perpendicular to regional tectonic strike in the IMR reveal systematic variations in the amount of crustal shortening, with a maximum of approximately 60% recorded in the Nagaland–Manipur segment along 25.644° N, 93.826° E–25.076° N, 95.897° E. The amount of shortening gradually decreases away from the axis of maximum shortening and on both sides. Calculations of relative plate motion based on rotation vectors given by different workers for various plate pairs represented in the region reveal that the interaction between the Indian and Myanmar plates can ideally produce the structural and tectonic features of this range. Dextral shear coupled to oblique subduction of the Indian Plate below the Myanmar Plate can best explain all of the structural and tectonic features present in the IMR.

Abstract Continental margins and their fossilized analogues are important repositories of natural resources. With better processing techniques and increased availability of high-resolution seismic and potential field data, imaging of present-day continental margins and their embedded sedimentary basins has reached unprecedented levels of refinement and definition, as illustrated by examples described in this volume. This, in turn, has led to greatly improved geological, geodynamic and numerical models for the crustal and mantle processes involved in continental margin formation from the initial stages of rifting through continental rupture and break-up to development of a new ocean basin. Further informing these models, and contributing to a better understanding of the features imaged in the seismic and potential field data, are observations made on fossilized fragments of exhumed subcontinental mantle lithosphere and ocean–continent transition zones preserved in ophiolites and orogenic belts of both Palaeozoic and Mesozoic age from several different continents, including Europe, South Asia and Australasia.