This contribution explores the formation and evolution of hyper-extended basins, associated with the early stage of core complex formation, controlled by low-angle normal faults active at <30 degrees. Based on a high-resolution industrial 3D seismic reflection survey along the southern margin of the South China Sea (SCS) (Dangerous Grounds), we mapped and analyzed the 3D geometry of low-angle normal fault systems and the related stratigraphy. Two main hyper-extended basins were documented, filled by up to 6 km of sediments including pre- to post-rift sequences. The observed normal faults on depth migrated seismic sections show an average dip angle of <30 degrees and appear planar, characterized by continuous reflections with no clear steepening at depth and sole-out on distinct decollement levels. Detailed fault surface mapping reveals the occurrence of km-scale corrugations together with large wavelength undulation. The formation of these hyper-extended basins is associated with polyphased syn-rift infill during the development of the low-angle normal faults. The first syn-rift sequence appears as chaotic and discontinuous packages that has been dismembered and fragmented during the activity of low-angle normal faults. The second syn-rift package shows unexpected sedimentary wedges developing successively toward the footwall and the hangingwall. This geometry results from the interplay between the main low-angle normal fault and antithetic faults defining a so-called extensional fishtail. The deep structure of these basins shows nascent domes with limited evidence of magmatism. Eventually, these basins likely capture the earliest stage of core complex development in the proximal margin of the southern SCS.

The geodynamic evolution of the East Mediterranean Sea (EMS) in the Mesozoic remains a “Gordian knot” tying together the Atlantic and Tethys oceanic systems. A key to unravel its complex evolution resides in determining the structure and formation age of its passive margins. Here, we investigate the Mesozoic architecture of different segments of the southern passive margin of the EMS using 2D and 3D seismic data combined with stratigraphic observations and gravity anomaly maps. Based on these offshore datasets, a new structural rift domain map of the eastern EMS is proposed showing the distribution of different crustal domains. Regional cross sections from the Western Desert to the northern Levant passive margins systematically show a sharp crustal neck of the EMS passive margins passing laterally in less than 30 km from moderately thinned continental crust (20–25 km thick) overlain by post-rift carbonate platform to distal deep basin (top basement > 8 s TWT). This sharp crustal thinning observed on both the southern and eastern margins of the EMS hampers the unambiguous identification of transform passive margin segments from orthogonal ones. The age of rifting and spreading of the different EMS basins are only partially constrained by boreholes and seismic correlation. To reduce uncertainties, we analyze and integrate the stratigraphic rift record from onshore former conjugate margins, which are now integrated in the Alps, Balkanides, Hellenides and Taurides belts on the northern side of the EMS. With this offshore/onshore approach, we identified the spatial and temporal evolution of rifting and spreading activity around EMS, highlighting poly-phased and poly-directional events from the Permian to the Cretaceous. We show that two competing geodynamic engines controlled extensional processes in the EMS: the opening of the Central Atlantic since the Late Triassic to the southwest and the evolution of multiple subduction zones in the Tethys domain to the northeast.

The Gulf of Mexico opened as a Late Triassic-Mid Jurassic continental rift that was first largely covered by the Mid-Jurassic Louann Salt and later split apart by a triangular-shaped oceanic crust. Salt in the Gulf of Mexico largely hampers the imaging and interpretation of underlying pre-salt and crustal geometries, which are fundamental for assessing the early kinematic evolution of the margin. To better define these deep geometries and their lateral variations, we built three seismic-based crustal-scale cross-sections across the Florida-Yucatan conjugate margins, in the areas where the Louann Salt is thinner. They were used, together with magnetic and gravity anomalies data, to build a crustal domains map for the study area. Cross-sections show a meaningful along-strike variation: the South Florida-East Yucatan area is characterized by a narrower rifted continental crust that evolves sharply to oceanic crust whereas in the North Florida and central-western Yucatan areas, the rifted continental crust is wider and the transition to the oceanic crust corresponds to a narrow magmatic or exhumed mantle domain. Bulk continental crust extension was determined using the area balancing method. Estimated horizontal extension values vary from a minimum of -120 km in the South Florida-East Yucatan conjugate to a minimum of -240 km in the North Florida-Central Yucatan conjugate, being systematically larger in the northern margin. Based on our observations and considering previous models, we propose that the study area evolved from an early rift involving magmatism, to a magma-poor margin, with continental break-up (OCT formation) being characterized by mantle exhumation and associated magmatism along the North Florida and central-western Yucatan areas. As in other analogues such as the Gulf of Guinea, the thick evaporite deposited during the late rift, while mantle was being exhumed.

Rifted margins are the result of the successful process of thinning and breakup of the continental lithosphere leading to the formation of new oceanic lithosphere. Observations on rifted margins are now integrating an increasing amount of multi-channel seismic data and drilling of several Continent-Ocean Transitions. Based on large scale geometries and domains observed on high-quality multi-channel seismic data, this article proposes a classification reflecting the mechanical behavior of the crust from localized to diffuse deformation (strong/coupled to weak/decoupled mechanical behaviors) and magmatic intensity leading to breakup from magma-rich to magma-poor margins. We illustrate a simple classification based on mechanical behavior and magmatic production with examples of rifted margins. We propose a non-exhaustive list of forcing parameters that can control the initial rifting conditions but also their evolution through time. Therefore, rifted margins are not divided into opposing types, but described as a combination and continuum that can evolve through time and space.

Our understanding of continent-ocean transition structures and magmatism in the absence of excessive magmatic additions has been guided by the observations and models developed at the magma-poor Iberia-Newfoundland conjugate margins. Recently these models have been challenged in the South China Sea in light of new IODP Expeditions 367-368-368X. We have used an integrated analysis of high quality seismic reflection and gravity anomaly data, calibrated against recent deep sea drilling results, to investigate margin structure and tectono-magmatic interplay during continental breakup and early seafloor spreading between the SE China-NW Palawan conjugate margins.The Eocene-Oligocene South China Sea rifting initiates in a heterogeneous and likely thermally un-equilibrated lithosphere formed by the Mesozoic Yenshanian orogeny. Gravity and joint inversion methods confirm lateral variation of basement densities across the conjugate margins. Lithospheric and basement heterogeneities induced a rifting style characterized by a series of highly thinned rift basins associated with extensional faulting soling out at various crustal levels. Final rifting in late Eocene triggered decompression melting forming mid-ocean ridge type magmatism, which emplaced within thinned continental crust as deep intrusions and shallow extrusive rocks. This initial magmatic activity was concomitant with continued deformation of continental crust by extensional faulting. Integrated analysis of seismic reflection profiles and gravity anomaly data combined with deep-sea boreholes accurately locate the continent-ocean boundary. We show that the initial igneous crust, continentward of oceanic magnetic anomaly C10n, is asymmetric in width and in morphology for the conjugate margins. The wider and faulted newly accreted domain on the SE China side indicates that magmatic accretion is associated with tectonic faulting during the formation of initial oceanic lithosphere. We suggest that deformation was not symmetrically distributed between the conjugate margins during the initiation of seafloor spreading but evolved asymmetrically until the stabilisation of the spreading ocean ridge around C10n. The analysis of the South China Sea breakup reveals a transient interplay between faulting, magmatic budget and extension rates during the formation of the continent-ocean transition and initial emplacement of igneous crust.

The Demerara Plateau (offshore Suriname and French Guiana) is located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic Oceans. The study of its crustal structure is fundamental to understanding its tectonic history, its relationship with the adjacent oceanic domains and to enlightening the formation of Transform Marginal Plateaus (TMPs). This study presents two wide-angle seismic velocity models from the MARGATS cruise seismic experiment, and adjacent composite seismic reflection lines. The plateau itself is characterized by a 30 km thick crust, subdivided into three layers, including a high velocity lower crust (HVLC). The velocities and velocity gradients do not fit values of typical continental crust but could fit with volcanic margin or Large Igneous Province (LIP) type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is likely composed of extrusive volcanic rocks of the same magmatic origin, forming thick seaward dipping reflector sequences tilted to the west. This SDR complex was emplaced during hotspot related volcanic rifting preceding the Jurassic opening of the Central North Atlantic and forming the present-day western margin of the plateau. The internal limit of the SDR complex corresponds to the future limit of the eastern margin. The Demerara Plateau would therefore be an inherited Jurassic volcanic margin boarding the Central Atlantic. This margin was reworked during the Cretaceous at the eastern limit of the Jurassic SDR complex, creating the present-day northern transform margin and the eastern divergent margin along the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies such as hotspots and superposed tectonic phases in the history of TMPs, which share a great number of characteristics with Demerara.

We present a new interpretation of conjugated ION GXT seismic profiles through the South Atlantic volcanic passive margins (VPMs). The detailed analyses of the depth isopach maps of the continental crust thickness combined with the seismic interpretation syn-rift volcanic successions and post-rift sequences allow to detail their 3D structural architecture. We evaluate the impact of the Early Cretaceous magmatism on the continental rifting and on the post-breakup subsidence from the Rio Grande/Walvis conjugated ridges to the Salado Fracture Zone N of Argentina.

We present the first regional balanced and restored sections across the northwestern part of the Zagros Fold-and-Thrust Belt in Kurdistan Region of Iraq and a 2D kinematic model that illustrates the evolution of the belt since Late Cretaceous time. The balanced cross-section, based on surface and sub-surface data, is characterized by multi-detachment folds detached above a Lower Triassic basal ductile level, with intermediate detachment levels that induced internal complexities like accommodation thrusting and/or disharmonic folding. Our work suggests that the two main structural steps in the detachment level in the High Folded Zone may be related to low-angle thrusts rooted at the brittle/ductile transition. Growth strata of Late Cretaceous and Paleocene times have been recognized for the first time in the Kurdistan Fold-and-Thrust Belt. This allows us to constrain timing of deformation and to estimate the evolution of the shortening and the advance of the deformation front since Late Cretaceous. Deformation of the Zagros belt is characterized by a combination of thin- and thick-skinned tectonics that reactivated the Late Cretaceous-Paleogene obduction belt.

Summary The petroleum exploration in foothills domains remains challenging in terms of positioning a well in an optimal manner to target different objectives in places where there are several mechanical discontinuities in the stratigraphy separating them and inducing often shifted structural tops. In consequence, seismic imaging, as well as a good geological knowledge, is mandatory to predict a well trajectory suited for all the targeted objectives. In Kurdistan Region of Iraq, the mechanical stratigraphy is rather simple but the tightening of the structures often leads to the activation of internal mechanical discontinuities and a shift of the main objectives tops. In consequence, and as in any foothills environment, side-tracked wells have to be carefully planned. After drilling the first leg of a well, and finding out that we were clearly out of our predictions in terms of structural positioning and in the evidence that the deepest objective cannot be drilled with the first leg, the decision to drill a side-track came quickly. Meanwhile, we performed a fast-track processing on the closest 2D line in order to image the high dips of the drilled formations. The output allowed us to help optimizing the side track trajectory and to ahead with better constraints.

The discoveries of Tupi in 2007 and of Jubilee Fields in 2006 have triggered a black gold rush on the world’s passive margins. Evolving extensional models in the distal area of margins in the Academy together with an increasing number of high quality and deeper seismic data (3D datasets, long offset acquisition, SPAN 2D, Broadseis, etc.) lead to develop new ideas and concepts for the deformation and subsidence history of Passive Margin in general and sheared margin (hyper-oblique or transform margins) in particular.

We study the evolution of the Eocene-Recent Phu Khanh Basin opened during the rifting of the South China Sea (SCS). This sub-basin formed when continental crust ruptured along the East-Vietnam Boundary Fault (EVBF) at the western edge of the SCS. Using high quality long-streamer seismic lines we interpret structures that highlight the different phases of the SCS rifting and processes related to crustal boudinage.Extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the Moho discontinuity mark the central part of the basin. The mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling. There, a low-angle detachment fault separates several crustal blocks from the Moho. The cylindrical axis of the Moho rise is roughly parallel to the trend of the South China Sea propagator. Above the mantle, the upper and lower crusts form large crustal boudins. The network of normal faults is dense in the upper crust and occasionally propagates into the lower crust. However, the lower crust is missing at some places. The seismic facies above the Moho rise is poorly stratified and might have been affected by a certain degree of metamorphism. At the apex of mantle uplift, there are frequent indications of fluid circulations, including volcanic edifices and gas escapes features. Three stages of extension are clearly identifiable, with ages of the two youngest constrained by well calibration: the first and oldest rift sequence is situated between the tilted pre-rift basement and the Oligocene horizons (32 Ma); the second is delimited by the Oligocene to the Mid Miocene (15.5 Ma) horizons, and the third is bound by the Mid Miocene and the Upper Miocene (before 10.5 Ma) horizons. These three rift episodes formed in at least two extension directions, the first N-S and the second NW-SE. The distinct Mid Miocene (15.5 Ma) horizon is tilted and the above layers show a diverging reflection. These are in turn sealed by an erosional unconformity before 10.5 Ma. Although tectonic activity appears diachronous from north to south, we suggest that cessation of rifting did not occur before 12-10.5 Ma. This differs from models derived from magnetic anomalies observed in the South China Sea (15.5 to 20 Ma). (C) 2013 Published by Elsevier B.V.

The Lengguru fold-and-thrust belt in West Papua (Indonesia) has all the characteristics of a young orogen involved in a rapidly changing tectonic setting. The analysis of the young wedge shows however that its internal shortening has ceased recently, and that it is nowadays suffering severe extension. Recent topographic data, marine industrial seismic lines and drilling, were used with field observations and measurements to create detailed cross-sections and a new structural map. The study allows us to distinguish two superimposed prisms composed of stacked Mesozoic marine sediments of the Australian margin against a crustal buttress. The construction of these two wedges is younger than 11Myr. The structures of the Lengguru belt external zones are sealed by an unconformable clastic series, indicating that the construction of the Lengguru prism had aborted suddenly due to a change in the way the Australian and Pacific plate convergence was accommodated. At that time, the internal zones probably started to exhume and the tectonic regime became extensional. Nowadays the internal part of the Lengguru fold-and-thrust belt is undergoing an active east–west extension. We believe that the extension observed in the Lengguru wedge is coeval with a transition from a compressive to a transtensional regime illustrated in the Central Range of Papua, and the onset of the Tarera-Aiduna and Paniai left-lateral faults. The structure of the Lengguru belt therefore results from events occurring over a very short time span; a previous Late Miocene northeast–southwest compression linked to the subduction process, a second from Middle Miocene to Early Pliocene and a Late Pliocene-Quaternary global extension in the whole range. The evolution is compared with that of the Seram wedge and the Misool–Onin–Kumawa continental ridge to the west; where deformation is accommodated at a localized zone which jumps as convergence between Australian and Pacific plate proceeds. This evolution of the belt reflects rapid changes in the accommodation oblique shortening, with the isolated orogenic wedge of Lengguru fold-and-thrust belt left to collapse.This example illustrates the way a long-lasting subduction terminates. At the lithospheric scale, the deformation remains rooted at the suture zone. However at the surface, the shortening is suddenly widespread over a large area during a very short time span (formation of the Lengguru belt) prior to being transferred to another plate boundary.

The Misool-Onin-Kumawa Ridge (Eastern Indonesia) is a broad anticline in the lower plate of the Seram subduction system. In the south it lies between the Seram accretionary wedge and the young Lengguru fold-and-thrust belt (< 8 My). A large seismic dataset from recent petroleum exploration in the area allows the ridge to be interpreted as the result of a dual system of thin-skinned and thick-skinned tectonics. A forebulge effect may be superimposed on the emergent sections of the ridge (Onin and Kumawa Domes), where the morphology has been reactivated.The evolution results from what appears to be a continuum of deformation through three major stages: (1) formation of a Messinian thin-skinned fold-and-thrust belt over a shaly-silty Permian-Paleocene unit; (2) a Pliocene thick-skinned event responsible for the uplift of the ridge, possibly induced by the onset of continental subduction; and (3) recent Pleistocene deformation when thin-skinned tectonics resumed in the Seram Trough. Currently, the Seram wedge abuts the ridge, transferring compression northward into the Salawati Basin.The jumps of active detachment levels may be a response to changes in subduction parameters (velocity, rugosity, etc.) during the transition between oceanic and continental subduction, or at least from thinned crust to thicker continental crust. (C) 2009 Elsevier Ltd. All rights reserved.

The hydrocarbon rich NW Borneo Margin is a complex structural domain where deformation style is a mix ofcompression and extension features associated with transverse structures. Regional work results in a new geodynamic modelintegrating structure interactions giving geometrical and kinematics coherency.