Abstract Drilling, 3D seismic and deep seismic studies in recent years have significantly progressed our knowledge of continental margins. Some of the key studies are brought together in this volume, focusing on the evolution and architecture of transform faults and transform margins, giving special attention to the relationship, and differences, between transform and rift margins. Papers from a wide range of global settings consider thermal history, depositional regime, hydrocarbon prospectivity and (in a continental setting) geothermal potential. This scene-setting paper examines our current state of knowledge on transform and rift settings, and provides a brief introduction to the subject matter and context of the papers in the volume.

Abstract Compared segments of the East and West Indian passive margins have different evolutions and crustal architecture. The East Indian margin is less magmatic. It results from a crust first–mantle second break-up scenario of a continent experiencing two rift events. The West Indian margin is more magmatic. It results from a mantle first–crust second break-up scenario of a continent experiencing four rift events. The architecture across both margins can be divided into stretching, thinning and hyperextension zones. The East Indian margin is characterized by oceanward-dipping listric normal faults that accommodate thinning in the thinning and hyperextension zones, and a zone of exhumed mantle separating continental and oceanic crusts. The West Indian margin in contrast is characterized by landward-dipping listric faults that accommodate magma-assisted thinning in the thinning and hyperextension zones, and no exhumed mantle. The final break-up affects the lithospheric mantle layer in the East Indian case and the crustal layer in the West Indian case. Although the temperature-dependent rheologies of these two last unbroken layers are somewhat different, seismic interpretation suggests that they are both broken by upward-convex normal faults, which succeeded the development of listric faults. They appear to be the first spontaneously formed faults in the break-up-delivering process, although their nucleation may be magma-assisted. The main difference between the controlling factors of the aforementioned break-up scenarios affecting similar lithospheres at similar extension rates is the cumulative length of time of the pre-break-up rift events, which is 62 and 115 myr for the East and West Indian margins, respectively.

Abstract This paper presents the time and space evolution of crustal deformation and their respective sedimentary infill of the 600 km wide, asymmetric conjugate rifted margin of the Santos–Benguela basins. Based on a geoseismic transect obtained with interpretation of long-offset seismic reflection and tied by wells, we interpret six main synrift unconformities, corresponding to different deformation phases processed from the Valanganian to Early Albian. Confined by these unconformities, sedimentary growths with progressively young relative ages towards the boundary with the oceanic crust are interpreted as evidence of oceanward rift migration. The combination of this information with crustal structure derived from long-offset seismic reflection illuminating the deep crust of the Santos–Benguela conjugate margins, resulted in a complete view of sedimentary infill, internal compartments, and crustal structure. These data were used to guide a dynamic model of rifting resulting in a simulated lithospheric section. We show that the margin architecture can be explained by the combination of an early, protracted phase of distributed deformation, followed by basinward rift migration. Distributed deformation lasted from the Valanginian to Early Aptian (135–117 Ma), initiating with isolated lakes that later coalesced into a wide basin-scale lake (>450 km). From the Mid Aptian to Early Albian (117–110 Ma), rift migration formed the main structural compartments and unconformities, as well as the distal hinge zones we observe today in the seismic lines. During this time, the inner proximal margins were left behind to thermally subside, whereas outer proximal and distal margins were tectonically active. Coexistence of these two processes explains the enigmatic simultaneous formation of proximal sag-like geometries, with late synrift accumulation of a salt layer up to 3 km thick, with tectonically active faults in the distal margin, promoting crestal block uplift that could explain the deposition of Late Aptian, shallow water, pre-salt carbonate rocks.

Abstract The Cauvery Basin in East India represents a failed rift zone in the west and transform-related termination in the east. The deformation associated with a westward-propagating rift zone involves stretching and necking-related deformation from west to east. The rift axis and the flanks exhibit maximum and minimum deformation. In this study, we document the increasing role of buoyancy-driven processes and the development of the rift asymmetry during the advanced stages of rifting in a magma-poor setting. We use a series of reflection seismic profiles intersecting the failed rift zone maturing eastward. The onset of buoyancy-controlled extension correlates with the localized extension. It creates a relatively symmetrical axial dome, and exhibits rift flank rotations and central up-warping. This permanent uplift is associated with lower crustal ductile flow. Notably, the deep-seated syn-rift buoyancy forces progressively operate eastward. We deduce the associated transient dome uplift and its subsequent dissipation using a seismic flattening technique. The axial dome formation is associated with an upwelling asthenosphere and lower lithospheric mantle. This correlates with localized contraction within flat-lying fault blocks at its flanks, concurrently forming the typical hanging-wall and footwall geometry. The multiple shallow- to deep-seated mechanisms promote strain acceleration in the uplifted regions along the rift zone.

Abstract Reactivation of continental transform faults (hereafter transforms) is identified herein as a significant factor in continental break-up, based on a global review of divergent margins and numerical modelling. Divergent margins that have reactivated transforms are characterized by linear and abrupt terminations of thick continental crust. Transforms represent some of the largest structures on Earth, and these megastructures represent major lithospheric weaknesses and are therefore prone to reactivation upon changes in the stress field, which typically occur during plate break-up. The blunt termination of the margins is consistent with observations of very limited pre-break-up lithospheric thinning of such margins. This mode of break-up appears to occur abruptly, and contrasts notably with highly tapered and slowly extended divergent margins. Magma leakage along transforms is well-known worldwide where divergence occurs across such features. This leakage may evolve to dyke injections, further reducing the plate strength. We observe that many of the blunt margins we attribute to transform reactivation have been prone to above-normal magmatism and are marked by seaward-dipping reflectors underlain by high-velocity lower crustal intrusions. The magmatism may be directly related to the separation of abruptly terminated margins, whereby the large resulting lateral thermal gradients trigger edge-driven convection and melt addition.

Abstract 1D/2D data-based studies of active spreading centres brought the knowledge of extension rate-dependent stretching-dominated v. buoyancy-dominated spreading. 3D reflection seismic data from the extinct centre of an initial oceanic corridor in the Caribbean allow us to see an along-strike transition between stretching- and buoyancy-dominated spreading where the spreading through detachment faulting is a precursor to the magma-assisted spreading. Studying progressively more evolved portions of the spreading centre, going from its end towards its centre, we see a progressively higher ascent of the asthenosphere, which heats the developing core complex in the exhuming footwall of the initial stretching-dominated system. The asthenospheric ascent is associated with thermal weakening of the core complex, which eventually results in ductile deformation reaching the upper portion of the complex. Subsequently, the core complex is penetrated by the dyke located at the top of the asthenospheric body. The dyke, which subsequently evolves to a diapir-shaped body, reaches the sea floor and establishes a magma-assisted steady-state seafloor spreading. These observations lead to a model explaining the initiation of the magma-assisted spreading in the initial oceanic corridor. Furthermore, they also improve our knowledge of multiple interacting mechanisms involved in the breakup of the last continental lithospheric layer, subsequent disorganized spreading and younger organized spreading.

Abstract The Sagaing Fault (SF), one of the world's most active strike-slip faults, defines a plate boundary on the eastern West Burma Block margin, from the Andaman Spreading centre, northwards for >1600 km to the eastern Himalayas. In the Northern Andaman Sea the SF traverses the Late Miocene–Recent Moattama Basin. There, 2D and 3D seismic reflection data show highly unusual fault patterns, that overall resemble a giant (area >33 000 km 2 ) horsetail structure. A horsetail pattern typically implies loss of displacement at a fault tip, which is potentially incompatible with the SF forming a transform margin. In the thinner, northern part of the Late Miocene–Recent basin three branches of the SF can be identified. These become lost in the thickest (>7 km), central part of the basin, and two branches emerge to the south where the basin thins. The fault patterns are interpreted to represent a previously unknown interaction of thin- and thick-skinned styles, where relatively shallow detached structures and sediment loading have interacted with basement-involved strike-slip faults that form a releasing bend geometry at the basement level. The Moattama Basin demonstrates how very thick sedimentary basins can produce fault patterns that differ from classic structural models.

Abstract Transform and passive margins developed during the continental rifting and opening of oceanic basins are fundamental elements of plate tectonics. It has been suggested that inherited structures, plate divergence velocities and surface processes exert a first-order control on the topographic and bathymetric evolution and thermal history of these margins and related sedimentary basins. Their complex spatial-temporal dynamics have remained controversial. Here, we conducted 3D magmatic-thermo-mechanical numerical experiments coupled with surface processes modelling to simulate the dynamics of continental rifting, continental transform fault zone formation as well as persistent oceanic transform faulting and zero-offset oceanic fracture zones development. Numerical modelling results allow to explain the first-order observations from passive and transform margins, such as diachronous rifting, heat flow rise and cooling in individual depocentres as well as contrasting basin tectonics of extensional and transtensional origin. In addition, the models reproduce the rise of both marginal ridges and transform marginal plateaus, and their interaction with erosion and sedimentation. Comparison of model results with observations from natural examples yields new insights into the tectono-sedimentary and thermal evolution of several key passive and transform continental margins worldwide.

Abstract A comparison of transform margins that started their evolution as continental transforms shows differences in their tectonic style, which can be attributed to the variable kinematic adjustments they underwent during the post-breakup continental-oceanic stage of their development. Three end-member examples are presented in detail. The Cape Range transform fault zone (Western Australia) retained its strike-slip character during its entire continental-oceanic stage, as documented by the transform-perpendicular system of spreading-related magnetic stripe anomalies. The Coromandal transform fault zone (Eastern India) adjusted its kinematics to a transtensional one during its continental-oceanic stage, as indicated by the transform-oblique system of magnetic stripe anomalies and extensional component of movement indicated by a narrow zone of crustal thinning. The Romanche transform fault zone (Equatorial Africa) adjusted its kinematics to transpressional, as documented by the changing geometries of magnetic stripe anomalies and transpressional folding during its continental-oceanic development stage. Based on the recognition of the aforementioned adjustments, we suggest a new categorization of transforms into (1) those that experience transpressional adjustment, (2) those that experience transtensional adjustment and (3) those that do not experience any adjustment during their continental-oceanic development stage.

Abstract Transform margins are a function of the pre-existing crustal architecture (pre-transform) and the interplay of syn- and post-transform geodynamic processes. We use a suite of geospatial databases to investigate four transform margins: East Africa (Davie Deformational Zone, DDZ), Equatorial Africa, and the South African and Falkland (Malvinas) margins (Agulhas–Falkland Fracture Zone, AFFZ). The East African margin is the most complex of the four. This is a consequence of Late Jurassic–Early Cretaceous transform motion affecting highly heterogeneous crust, and post-transform deformation that varies along the margin. Equatorial Africa most closely adheres to traditional definitions of ‘transform margins’, but actually comprises two principal transform systems – the Romanche and St Pauls, dictated by the pre-transform distribution of mobile belts and West African craton. All four margins are spatially associated with volcanism, and each exhibits narrow uplifts associated with transpression or transtension. But the causal relationship of these features with transform processes differ. Volcanism along the East African margin is pre- and post-transform. Syn-transform volcanism on the AFFZ is spatially limited, with the AFFZ possibly acting as a conduit for magmatism rather than as a causal driver. Transform margins are varied and complex and require an understanding of pre-, syn- and post-transform geodynamics.

Abstract Gondwana started to split up during the Early Jurassic ( c. 180 Ma) with the separation of Antarctica and Madagascar from Africa, followed by the separation of South America and Africa during the Early Cretaceous. Thanks to recent seismic profiles, the architecture of rifted margins and the transform fault zones, which developed as a result of the relative motion between tectonic plates, have been recently evidenced and studied along the whole eastern and southeastern Africa margins (i.e. in the Western Somali Basin, the Mozambique Basin, the Natal Basin and the Outeniqua Basin). But, the structure and overall kinematic evolution of the three major transform fault zones – such as the Agulhas, the Davie and the Limpopo fracture zones (FZ) – that together control the opening of major oceanic basins (Antarctic Ocean, Weddell Sea and Austral South Atlantic) remain poorly studied. The interpretation of an extensive regional multi-channel seismic dataset coupled with recent studies allows us to propose a detailed regional synthesis of the crustal domains and major structural elements of the rifted margins along the whole eastern and southeastern Africa. We provide new constraints on the structure and evolution of these three transform systems. Although our findings indicate common features in transform style (e.g. a right-lateral transform system, a wide sheared corridor), the deformation and magmatism along these systems appear quite different. In particular, our results show that the Davie and Agulhas transform faults postdate the development of the rift zone-controlling faults, whereas the Limpopo margin seems to be a simple intra-continental transform. Moreover, the Davie and Agulhas FZ recorded spectacular inversions during the transform stage, whereas transtensional deformation is developed along the Limpopo FZ. This different style of deformation may be explained by two main forcing parameters: (i) the far-field forces that may induce a rapid change of regional tectonic stress and (ii) the magmatic additions that modify mainly the crustal rheology. In the post-drift history, several reactivations of transform fault zones are recorded, implying that some transform margins are excellent recorders of large plate kinematic changes. Such reactivations can serve also as drains for magmatic fluids in the vicinity of hotspots emplacement.

Abstract The Diaz Marginal Ridge (DMR), on the southern transform margin of South Africa, is a bathymetric feature parallel to the Agulhas Falkland Fracture Zone (AFFZ) that has long been considered an archetype marginal ridge; and yet its origin and evolution remains unconstrained. Using recently acquired seismic data we present a new structural interpretation of the DMR and its association with the evolution of both the AFFZ and the Southern Outeniqua Basin. In contrast to previous scenarios invoking thermo-mechanical explanations for its evolution, we observe a more straightforward structural model in which the genesis of the DMR results from the structural inversion of a Jurassic rift basin. This inversion resulted in the progressive onlap of latest Valanginian–Hauterivian-aged stratigraphic units, important for the formation of stratigraphic plays of the recent Brulpadda discovery. Paradoxically, this contraction is contemporaneous with renewed extension observed in the inboard normal faults. The orientation of the DMR and inboard structures have been demonstrated to be controlled by the underlying Cape Fold Belt (CFB) fabric. The onset of motion across the AFFZ shear system led to east–west-orientated maximum stress and north–south-orientated minimum stress. We propose this stress re-orientation resulted in strain partitioning across existing structures whereby in addition to strike-slip on the AFFZ there was coeval extension and contraction, the nature of which was determined by fault orientation. The fault orientation in turn was controlled by a change in orientation of the underlying CFB. Our model provides new insights into the interplay of changes in regional stress orientation with basement fabric and localized magmatism along an evolving transform. The application of horizontal strain partitioning can provide an explanation of similar features observed on other transform margins.

Abstract During the end of the lower Cretaceous, the connection between the South and the Central Atlantic accretionary axis led to the oblique opening of the Equatorial Atlantic, and to the separation of Africa and South America by alternating transform and rift margins. At the western end of the Equatorial Atlantic, we investigate the structure of the Cretaceous margins surrounding the Demerara plateau, north of French Guiana and Suriname. These margins were previously described as transform northward and divergent eastward. From the bathymetry and deep structures, we propose to divide the northern transform into three margin segments, with two transform segments separated by a divergent one. These two transform margins are very different, the northwestern one being linear and associated with a steep and erosive continental slope, the northeastern one consisting of several faults and ridges en echelon disposed. In between, the divergent margin appears to be a pull-apart basin localized by structures inherited from the previous Jurassic rifting. Additionally, the eastern divergent margin may have been localized by a thermal anomaly tentatively related to a hotspot. It is proposed that the deformation was first localized in divergent (rift) basins, subsequently connected by transform faults. The structure of the transform fault varies with the offset between adjacent rift basins: a large offset forms a linear transform and a short (less than 200 km) offset forms en echelon structures.

Abstract Transform marginal plateaus (TMPs) are large and flat structures commonly found in deep oceanic domains, but their origin and relationship to adjacent oceanic lithosphere remain poorly understood. This paper focuses on two conjugate TMPs, the Demerara Plateau off Suriname and French Guiana and the Guinea Plateau, located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic oceans. The study helps to understand (1) the tectonic history of both Demerara and Guinea plateaus and (2) the relationship between the Demerara Plateau and the adjacent oceanic domains, and finally, (3) throws light on the formation of TMPs. We analyse two existing wide-angle seismic-derived velocity models from the MARGATS seismic experiment (Demerara Plateau), and adjacent composite industrial seismic lines covering the Demerara and Guinea plateaus. The Demerara Plateau displays a 30 km thick crust, subdivided into three layers, including a high-velocity lower crust. The velocities and velocity gradients do not fit the values of typical continental crust but instead correspond to volcanic margin- or large igneous province-type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is made from extrusive volcanic rocks of the same magmatic origin, forming thick seaward (westward)-dipping reflectors (SDRs) sequences. This SDR complex extends to the Guinea Plateau as well and was emplaced during hotspot (Sierra Leone)-related volcanic rifting preceding the Jurassic opening of the Central Atlantic and forming the western margin of the plateau. North–south composite lines linking the Demerara and Guinea plateaus reveal the spatial extent of the SDR complex but also a pre-existing basement ridge separating the two plateaus. The entire Demerara–Guinea margin would therefore be an inherited Jurassic volcanic margin bordering the Central Atlantic Ocean to the east, with a possible conjugate being the Bahamas Plateau on the other side of the ocean. This margin was then reworked during a non-coaxial Cretaceous second phase of rifting potentially accompanied by a magmatic event. Opening of the northern margin occurred in a transform mode splitting the Jurassic volcanic margin into two parts (the Guinea and Demerara TMPs), conceivably along a pre-existing basement ridge. Rifting of the eastern part of the Demerara Plateau occurred surprisingly along the eastern limit of the Jurassic SDR complex, forming the present-day eastern divergent margin of the Demerara Plateau. After that stage, the Demerara and Guinea plateaus are individualized on each side of the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies related to hotspots and superposed tectonic phases in the case of other TMPs that share numerous characteristics with the Demerara Plateau.

Abstract The Guyana Basin formed during the Jurassic opening of the North Atlantic. The basin margins vary in tectonic origin and include the passive extensional volcanic margin of the Demerara Plateau in Suriname, an oblique extensional margin inboard at the Guyana–Suriname border, a transform margin parallel to the shelf in NW Guyana, and an ocean–ocean margin to the NE, which morphed from transform to oblique extension. Plate reconstructions suggest rifting and early seafloor spreading began with NNW/SSE extension ( c. 190–160 Ma) but relative plate motion later changed to NW/SE. The fraction of magmatic basin floor decreases westwards and the transition from continental to oceanic crust narrows from 200 km in Suriname to less than 50 km in Guyana. The geometry and position of the onshore Takutu Graben suggest it formed a failed arm of a Jurassic triple junction that likely captured the Berbice river during post-rift subsidence and funneled sediment into the Guyana Basin. Berriasian to Aptian shortening caused crustal-scale folds and thrusts in the NE margin of the basin along with minor inversions of basin margin and basin-segmenting faults. Stratigraphically trapped Liza trend hydrocarbon discoveries are located outboard of inverted basement faults, suggesting a link between transform margin structure and their formation.

Abstract Shelf to basin floor clastic-sediment-routing models for individual structural parts of a transform margin are presented. Using 3D and 2D seismic, gravity and well data from the Guyana, Coromandal, Romanche, Saint Paul and Zenith–Wallaby Perth margins as examples, and drawing from a wide range of analogues, four discrete tectonic segments are recognized as characteristic of all transform margins: (1) the transform margin sensu stricto ; (2) the local pull-apart segment on the transform margin; (3) the margin segment developed from the narrow horsetail splaying off from the transform; and (4) the margin segment developed from the extensional end of the horsetail structure, although there are other transitional variants specific to each margin. The routing of clastic sediments varies markedly across these segments. The shelfal staging area varies in width between the segments, increasing in width between segments 1 and 4, with wider shelves (10–25 km) characterized by significant sediment sorting through longshore drift, and narrower segments (0–10 km) often fed directly from focused fluvio-deltaic systems. The overall deep-water slope gradient decreases from segments (1) to (4), with sediment runout distance from shelf to basin floor increasing. The seafloor topography along the margin is highly variable, with significant sand sequestration on slopes with intra-slope ramps and terraces found in segments (2)–(4), with rapid downslope transitions from tributary slope gullies to slope channel complexes and then ponded intra-slope frontal splay complexes on the intra-slope terraces. Mass-transport deposits (MTDs) are ubiquitous and found as either regional slope MTDs or local collapses of slope channel complex margins. Stratigraphic and/or combination traps are associated with the ramp–terrace transitions. Local sectors show pinned long-lived intra-slope channel complex and canyon development, with prominent long-lived intra-slope stacking of intra-slope sandbodies. The outboard basin-floor- or trough-confined deep-water frontal splays (lobes), and their head area erosion by slope-originated MTDs, are directly controlled by the deposystems of the inboard slope systems. The lateral and vertical evolution of deep-water slope architecture is directly controlled by the inherited seafloor topography, set up by the underpinning crustal fabric across the four segments.

Transform margins form a significant portion of Earth's continent–ocean transition and are integral to continental break-up, yet compared to other margins are poorly understood. This volume brings together new multidisciplinary research to document the structural, sedimentological and thermal evolution of transform margins, highlighting their relationship to continental structure, neighbouring oceanic segments, pull-apart basins and marginal plateaus. Special emphasis is given to the comparison of transform and rifted margins, and to the economic implications of transform margin structure and evolution. Transform case studies include the Agulhas–Falkland transform, Coromandal transform (East India), Davie margin and Limpopo transform (East Africa), Guyana transform margin, Demerara transform margin (Suriname), Romanche and St Paul transforms (equatorial Africa), Sagaing transform (Andaman Sea) and Zenith–Wallaby–Perth transform (West Australia). The broad-scale interplay between transform and rifted margin segments in the North and Central Atlantic, and Caribbean, is also examined.