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We investigate, using observations from seismic reflection data, the lateral variability in breakup extrusive magmatic addition along the strike of the Pelotas segment of the austral South Atlantic rifted margin and its control on post-rift accommodation space and sediment deposition. Our analysis of regional seismic reflection profiles shows that magmatic addition on the Pelotas margin varies substantially along strike from extremely magma-rich to magma-normal within a distance of ∼300 km. Using 2D flexural back-stripping, we determine the post-rift accommodation space above top volcanics. In the north, where volcanic seaward-dipping reflectors (SDRs) are thickest, the Torres High shows SDRs up to ∼20 km thick, and post-breakup water-loaded accommodation space is much less than in the south, where magmatic addition is normal and SDRs are thinner. We show that post-breakup accommodation space correlates inversely with SDR thickness, being less for magma-rich margins and more for magma-normal/intermediate margins. The Rio Grande Cone, with large sediment thickness, is underlain by small SDR thicknesses allowing large post-breakup accommodation space. A relationship is observed between the amount of volcanic material and the two-way travel time (TWTT) of first volcanics: first volcanics are observed between 1.2 and 2.2 s TWTT for the highly magmatic Torres High profile, while, in contrast, for the normally magmatic profiles in the south, first volcanics are observed between 4.2 and 6.5 s TWTT. The observed inverse relationship between post-breakup accommodation space and SDR thickness is consistent with predictions by a simple isostatic model of continental lithosphere thinning and magmatic addition melting during breakup. The methodology that we use in this paper provides a new approach for investigating the complex magmatic and sedimentary evolution of rifted continental margins. [ABSTRACT FROM AUTHOR]

We investigate the crustal structure of the Northeastern (NE) South China Sea (SCS) rifted margin to constrain its crustal thickness and basement nature with implications for the Mesozoic and Cenozoic evolution of the SCS. First‐order interfaces interpreted from seismic reflection data were integrated into a 3D gravity inversion scheme to determine Moho depth and crustal thickness variations. A joint inversion of seismic and gravity data allowed us to determine crustal density variations along 2D profiles. The distal margin of the NE SCS is divided into two distinct crustal domains: the Southern Rift System (SRS), and the Southern High (SH). The SRS shows an extremely thinned crust on top of which thick Cenozoic sequences are observed. It is separated from the oceanic crust (∼6–8 km thick) by the SH, a comparatively thicker crustal domain (∼10–15 km thick) with significant magmatic additions. The distal NE SCS margin formed during the Cenozoic rifting of the SCS. The SH likely corresponds to a polygenic piece of crust, recording polyphase magmatic activity since the Mesozoic, with potentially significant activity during Cenozoic post‐rift time. The NE SCS margin is conjugate to Palawan whose basement is considered to be part of the exotic Luconia microcontinent that collided with Eurasia during the Late Cretaceous. Basement similarities between Palawan and the SH are highlighted, suggesting that the latter might also be part of Luconia. Our results suggest that the docking/suture zone between Eurasia and Luconia might have acted as a preferred zone for the Cenozoic rift development. Key Points: The Northeastern South China Sea rifted margin shows a contrasted crustal structure constrained by seismic and gravity dataPart of the distal margin corresponds to a dense polygenic continental crust that underwent polyphase magmatic activityThe former Mesozoic suture zone between Eurasia and Luconia acted as a weakness zone for the Cenozoic rifting [ABSTRACT FROM AUTHOR]

Seismic reflection interpretation at magma-poor rifted margins shows that crustal thinning within the hyper-extended domain occurs by in-sequence oceanward extensional faulting which terminates in a sub-horizontal reflector in the topmost mantle immediately beneath tilted crustal fault blocks. This sub-horizontal reflector is interpreted to be a detachment surface that develops sequentially with oceanward in-sequence crustal faulting. We investigate the geometry and evolution of active and inactive extensional faulting due to flexural isostatic rotation during magma-poor margin hyper-extension using a recursive adaptation of the rolling-hinge model of Buck (1988) and compare modelling results with published seismic interpretation. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on published seismic reflection profiles can be generated by the flexural isostatic rotation of faults with initially high-angle geometry. Our modelling supports the hypothesis of Lymer et al. (2019) that the S reflector on the Galician margin is a sub-horizontal detachment generated by the in-sequence incremental addition of the isostatically rotated soles of block-bounding extensional faults. Flexural isostatic rotation produces shallowing of emergent fault angles, fault locking, and the development of new high-angle shortcut fault segments within the hanging wall. This results in the transfer and isostatic rotation of triangular pieces of hanging wall onto exhumed fault footwall, forming extensional allochthons which our modelling predicts are typically limited to a few kilometres in lateral extent and thickness. The initial geometry of basement extensional faults is a long-standing question. Our modelling results show that a sequence of extensional listric or planar faults with otherwise identical tectonic parameters produce very similar seabed bathymetric relief but distinct Moho and allochthon shapes. Our preferred interpretation of our modelling results and seismic observations is that faults are initially planar in geometry but are isostatically rotated and coalesce at depth to form the seismically observed sub-horizontal detachment in the topmost mantle. In-sequence extensional faulting of hyper-extended continental crust results in a smooth bathymetric transition from thinned continental crust to exhumed mantle. In contrast, out-of-sequence faulting results in a transition to exhumed mantle with bathymetric relief. [ABSTRACT FROM AUTHOR]

Determining the volume and timing of magmatism during rifting and breakup is challenging due to the similar density and seismic velocity of inherited continental crust, magmatic additions and serpentinized mantle; and the difficulty of dating magmatic additions. Here rules of thumb to estimate these are proposed based on the characteristics of the top basement and Moho on seismically imaged margins. A simple kinematic model is used to generate first‐order crustal shapes of margins as a function of magma volume and timing of emplacement, which are successfully compared to a representative number of rifted margins. It appears that 'magma‐rich margins' require melt emplacement in advance of crustal thinning but not necessarily enhanced melt volume, while margins with exhumed mantle require a delay in melt emplacement but not necessarily a low magmatic volume. An alternative classification for the magma‐poor/magma‐rich dichotomy is proposed to better represent the crustal shape variability of rifted margins. [ABSTRACT FROM AUTHOR]

We investigate the lateral variability of breakup volcanic addition along-strike of the Pelotas segment of the Austral South Atlantic rifted margin and its control on post-rift accommodation space and sediment deposition. Our analysis of regional seismic reflection profiles shows that magmatic addition on the Pelotas margin varies substantially along strike from extremely magma-rich to magma-normal within a distance of ~300 km. Using 2D flexural backstripping we determine the post-rift accommodation space above top volcanics. In the north, where SDRs are thickest, the Torres High shows SDRs up to ~ 20 km thick and post-breakup water-loaded accommodation space is ~2 km. In contrast, in the south where magmatic addition is normal and SDRs are thinner, post-breakup water-loaded accommodation space is ~ 3 - 4 km. We show that post-breakup accommodation space correlates inversely with SDR thickness, being less for magma-rich margins and more for magma normal/intermediate margins. The Rio Grande Cone, with large sediment thickness, is underlain by small SDR thicknesses allowing large post-breakup accommodation space. A relationship is observed between the amount of volcanic material and the TWTT of first volcanics; first volcanics are observed at 1.25s TWTT for the highly magmatic Torres High profile while, in contrast, for the normally magmatic profiles in the south, first volcanics are observed at 4.2s TWTT or deeper. The observed inverse relationship between post-breakup accommodation space and SDR thickness is consistent with predictions by a simple isostatic model of continental lithosphere thinning and decompression melting during breakup. [ABSTRACT FROM AUTHOR]

We investigate the lateral variability of breakup volcanic addition along-strike of the Pelotas segment of the Austral South Atlantic rifted margin and its control on post-rift accommodation space and sediment deposition. Our analysis of regional seismic reflection profiles shows that magmatic addition on the Pelotas margin varies substantially along strike from extremely magma-rich to magma-normal within a distance of ~300 km. Using 2D flexural backstripping we determine the post-rift accommodation space above top volcanics. In the north, where SDRs are thickest, the Torres High shows SDRs up to ~ 20 km thick and post-breakup water-loaded accommodation space is ~2 km. In contrast, in the south where magmatic addition is normal and SDRs are thinner, post-breakup water-loaded accommodation space is ~ 3–4 km. We show that post-breakup accommodation space correlates inversely with SDR thickness, being less for magma-rich margins and more for magma normal/intermediate margins. The Rio Grande Cone, with large sediment thickness, is underlain by small SDR thicknesses allowing large post-breakup accommodation space. A relationship is observed between the amount of volcanic material and the TWTT of first volcanics; first volcanics are observed at 1.25s TWTT for the highly magmatic Torres High profile while, in contrast, for the normally magmatic profiles in the south, first volcanics are observed at 4.2s TWTT or deeper. The observed inverse relationship between post-breakup accommodation space and SDR thickness is consistent with predictions by a simple isostatic model of continental lithosphere thinning and decompression melting during breakup. [ABSTRACT FROM AUTHOR]

Seismic reflection interpretation at magma-poor rifted margins shows that crustal thinning within the hyper-extended domain occurs by in-sequence oceanward extensional faulting which terminates in a sub-horizontal reflector in the top-most mantle immediately beneath tilted crustal fault blocks. This sub-horizontal reflector is interpreted to be a detachment surface which develops sequentially with oceanward in-sequence crustal faulting. We investigate the geometry and evolution of active and inactive extensional faulting due to flexural isostatic rotation during magma-poor margin hyper-extension using a recursive adaptation of the rolling hinge model of Buck (1988) and compare modelling results with the seismic interpretation. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on seismic reflection data can be generated by the flexural isostatic rotation of faults with initially high-angle geometry. Flexural isostatic rotation produces shallowing of emergent fault angles, fault locking and the development of new high-angle short-cut fault segments within the hanging-wall. This results in the transfer and isostatic rotation of triangular pieces of hanging wall onto exhumed fault footwall, forming extensional allochthons which our modelling predicts are typically limited to a few km in lateral extent and thickness. While earthquake seismology favours a planar fault geometry with in the brittle seismogenic crust, seismic reflection imaging suggests a more listric geometry. Our modelling results show that a sequence of extensional listric or planar faults with identical parameters (i.e. location, heave, surface dip, Te) produce very similar sea-bed bathymetric relief. Listric and planar fault geometries do however produce distinct Moho and allochthon shapes. We propose that the initial fault geometry, prior to flexural isostatic rotation, is planar in the seismogenic crust becoming listric at depth as the brittle plastic transition is approached. Extensional faulting and thinning of hyper-extended continental crust may eventually lead to mantle exhumation. Where extensional faulting is in-sequence, this results in a smooth bathymetric transition from thinned continental crust to exhumed mantle. In contrast out-of-sequence faulting results in a transition to exhumed mantle with bathymetric relief. [ABSTRACT FROM AUTHOR]

Geomechanical models based on linear elasticity have been used to predict the mode and distribution of subseismic fractures around larger faults. These models can be tested against field observations of surface breaks (fractures) formed in the aftermath of large earthquakes. This paper presents forward models based on elastic dislocation theory of the deformation due to the [M.sub.s] 7.3 earthquake at El Asnam, Algeria in 1980. Using fault parameters from previous geodetic studies to define a set of larger faults, our models calculate the deformation field in the surrounding rock volume. We compare predicted strain and stress fields with the surface deformation measured in the area following the 1980 earthquake. Using a combination of the redistributed elastic dislocation stress due to slip on the major faults and a small component of overburden stress, the models successfully predict normal faults and tensile fractures in the hanging wall of the reverse fault system. Orientations of the predicted faults vary along strike of the fault system, being parallel to the main reverse fault in the south but oblique to it along the central segment, agreeing with observed surface breaks. The results of our forward modeling are not sensitive to the magnitude or direction of a regional tectonic stress. The predicted fractures are controlled by the near-field, short-term redistributed stresses due to coseismic slip. The agreement between modeled and measured deformation patterns adds confidence in the use of elastic dislocation theory to accurately predict small faults generated by coseismic slip on large faults. INDEX TERMS. 8010 Structural Geology: Fractures and faults; 8020 Structural Geology: Mechanics; 8107 Tectonophysics: Continental neotectonics; 8123 Tectonophysics: Dynamics, seismotectonics; 8168 Tectonophysics: Stresses--general; KEYWORDS: earthquake, surface break, elastic dislocation, fracture prediction, geodetic, neotectonic.

Atlantis Bank is an anomalously uplifted oceanic core complex adjacent to the Atlantis II transform, on the southwest Indian Ridge, that rises >3 km above normal seafloor of the same age. Models of flexural uplift due to detachment faulting can account for ~1 km of this uplift. Postdetachment normal faults have been observed during submersible dives and on swath bathymetry. Two transform-parallel, large-offset (hundreds of meters) normal faults are identified on the eastern flank of Atlantis Bank, with numerous smaller faults (tens of meters) on the western flank. Flexural uplift associated with this transform-parallel normal faulting is consistent with gravity data and can account for the remaining anomalous uplift of Atlantis Bank. Extension normal to the Atlantis II transform may have occurred during a 12 m.y. period of transtension initiated by a 10[degrees] change in spreading direction ca. 19.5 Ma. This extension may have produced the 120-km-long transverse ridge of which Atlantis Bank is a part, and is consistent with stress reorientation about a weak transform fault. Keywords: oceanic core complex, southwest Indian Ridge, transtension, Atlantis II.

A study on central Baikal rift flank uplift and topography questioned the applicability of the flexural cantilever model to the rift and other rift basins. The analysis, however, failed to consider many observed faults and employ a model that would represent the real upper crustal geological feature and process. The study committed an error in readily discounting the flexural cantilever model in rift basin modeling and failed to indicate the model's capacity to incorporate upper crustal fault geometries.

The post-Cretaceous subsidence history of the Voring Basin, part of the Atlantic passive margin offshore mid-Norway, has been investigated. Extension and [Beta]-factors related to rifting and continental break-up during the Palaeocene have been quantified using both forward and reverse basin-modelling techniques. In the preferred geological model it is assumed that rifting occurred in the Voring Basin during the Palaeocene (prior to break-up), following an earlier rift event during the Late Jurassic. During Palaeocene rifting the basin may have been dynamically uplifted by the Iceland mantle plume. In the east of the basin there was no Palaeocene extension. Subsidence analysis shows that in the centre of the basin forward and reverse models converge to predict a modest Palaeocene stretching factor ([Beta]) of c. 1.15. In the west of the basin, closest to the Atlantic margin, forward models of upper-crustal faulting also predict a [Beta] of c. 1.15, but reverse (backstripped) models of subsidence predict a [Beta] of up to 1.75. We suggest that lower-crustal and mantle-lithosphere thinning close to the margin were greater than the extension accommodated by upper-crustal faulting and that some lower-crustal/mantle-lithosphere stretching associated with continental separation was partitioned below the Voring Basin, up to 150 km landwards of the margin. Keywords: Norway, Atlantic Ocean, Palaeocene, models, rifting.

South Atlantic rifting has caused the Reconcavo-Tucano-Jatoba (RTJ) Rift and other smaller rifts in northeast Brazil. The rifting also resulted in the uplifting of Albian marine sediments. Magmatic underplating and upliftment is caused by local erosion in the football uplift. The magmatic underplating has also removed the sediments of the thermal sag phase from the RTJ Rift.

The post-rift history of the North Viking Graben has been backstripped in 3D, producing a sequence of palaeobathymetric maps that culminates at the Late Jurassic synrift stage. The backstripping takes into account the three main processes which drive post-rift basin development: thermal subsidence, flexural-isostatic loading and sediment compaction. Before backstripping was performed, the Norwegian Trench, a bathymetric feature within the present-day seabed, was smoothed in order to remove associated decompaction artefacts within the backstripping results. Palaeobathymetric restorations at the top and base of the Paleocene take into account regional transient dynamic uplift, probably related to the Iceland Plume. 350 m of uplift is incorporated at the Base Tertiary (65 Ma) and 300 m at the Top Balder Formation (54 Ma), followed by rapid collapse of this same uplift. At the top of the Lower Cretaceous (98.9 Ma), very localized fault-block topography, inherited from the Jurassic rift, is predicted to have remained emergent within the basin. At the Base Cretaceous (140 Ma), the fault-block topography is much more prominent and numerous isolated footwall islands are shown to have been present. At the Late Jurassic synrift stage (155 Ma), these islands are linked to form emergent island chains along the footwalls of all of the major faults. This is the Jurassic archipelago, the islands of which were the products of synrift footwall uplift. The predicted magnitude and distribution of footwall emergence calibrates well against available well data and published stratigraphic information, providing important constraints on the reliability of the results. [ABSTRACT FROM AUTHOR]

Rifted margins are often classified as magma-poor or magma-rich based on a magmatic budget interpretation from seismic reflection data. The southern segment of the East Indian rifted margin is often regarded as a type-example of a magma-poor margin displaying exhumed mantle. However, in its southern segment, 9 km thick transitional crust, previously interpreted as magmatic crust, separates the exhumed mantle from thin oceanic crust. Such thick transitional crust is atypical for a magma-poor margin, so we investigate its likely formation and potential implications for the evolution of magma-poor margins. Using an integrated set of geophysical techniques alongside seismic reflection data, we test the existence of exhumed mantle and the composition of the transitional crust. These geophysical techniques consist of gravity inversion, residual depth anomaly analysis, flexural subsidence analysis and joint inversion of gravity and seismic data. We apply these methods to high-quality seismic reflection data (ION line INE1-1000) on the southern segment of the East Indian rifted margin and test a series of geological scenarios for the margin structure using our integrated quantitative analysis. Of these, our quantitative analysis, seismic observations and the regional plate kinematic history support a structure consisting of thinned continental crust inboard of exhumed, serpentinized mantle followed by thick (∼9 km) magmatic crust transitioning into thin oceanic crust (∼5 km). The juxtaposition of exhumed mantle and thick magmatic crust is explained by the occurrence of a jump in seafloor spreading during the Early Cretaceous formation of the south-east Indian Ocean. The final rifted margin structure contains characteristics of both magma-poor and magma-rich rifted margins resulting from two distinct rift events with different magmatic budgets. The investigation of the East Indian rifted margin structure and evolution shows the importance of incorporating the plate kinematic history and quantitative validation of seismic interpretation into the analysis. Classifying the East Indian margin as a typical magma-poor rifted margin is misleading causing us to question the use of end-member terminology to describe rifted margins. [ABSTRACT FROM AUTHOR]

Seismic data made available by the UK OGA (Oil & Gas Authority) has been used to constrain a model of crustal structure and heat-flow history for the UK Rockall Basin. Top basement/base sediment has been interpreted around the full extent of the seismic dataset. This has produced a model for the thickness of the sediment fill within the basin which is thicker than previous published estimates. The new sediment-thickness model has been incorporated into a 3D backstripping study, producing maps of subsidence and thinning factor. Analysis of backstripped subsidence shows the thinning factor reaching peak values of c. 0.8-0.85 (β factor >5) in the south-central axial area, reducing in magnitude northwards to c. 0.7. The new sediment-thickness model has also been incorporated into a 3D gravity-inversion study, mapping Moho depth, crustal thickness and thinning/stretching factor. The results show crustal-basement thickness reduced to c. 6 km, thinning factor c. 0.8, in the south-central area, while it spans the range c. 6-10 km further north. The results are compatible with previous seismic refraction work in both the UK and Irish sectors of the Rockall Basin.We believe that the extension which created the basin was non-magmatic and that the axial region is underlain by highly-thinned continental crust. The results from the gravity inversion have been used to make predictions about the top-basement heat-flow history. Heat flow in the basin centre is predicted to have been initially high, reducing with time, associated with cooling of the transient synrift heat-flow anomaly. On the basin flanks heat flow was less variable over time, its magnitude controlled primarily by constant radiogenic heat input from the basement, rather than by the transient geotherm anomaly. There remain considerable uncertainties associated with our interpretation and analysis. These uncertainties have been addressed with sensitivity analyses. A regional gravity-inversion model, using the new sediment-thickness data spliced into regional public-domain information, shows that structural and stretching continuity can be mapped at the crustal scale along the full length of the UK/Irish Rockall Basin, contrary to conclusions from some previous studies. [ABSTRACT FROM AUTHOR]

Inversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean-continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean-continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE-WSW trend while the later seafloor-spreading axis had a NE-SW trend. [ABSTRACT FROM AUTHOR]

The nature and origin of the J-magnetic anomaly along the Iberia-Newfoundland margins are controversial and its validity for plate kinematic reconstructions questioned. At present, it is interpreted as either an oceanic isochron or an edge effect of oceanic crust corresponding to lithosphere breakup. Both interpretations result in restorations that are in conflict with the current knowledge from Pyrenean and North Atlantic geology. We combine seismic interpretations and dating of magmatic additions with magnetic data to examine the nature and formation process of this anomaly and discuss its value for plate restorations. We show that the J-anomaly is the result of polygenic and multiple magmatic events occurring during and after the formation of the first oceanic crust. Therefore, we conclude that the J-anomaly cannot be used for plate kinematic studies and, more generally, we question the validity of using ill-defined magnetic anomalies outside unequivocal oceanic domains for plate reconstructions. [ABSTRACT FROM AUTHOR]

A kinematic model of lithosphere and asthenosphere deformation has been used to investigate lithosphere stretching and thinning modes during continental rifting leading to breakup and seafloor spreading. The model has been applied to two conjugate profiles across the Iberia–Newfoundland rifted margins and quantitatively calibrated using observed present-day water loaded subsidence and crustal thickness, together with observed mantle exhumation, subsidence and melting generation histories. The kinematic model uses an evolving prescribed flow-field to deform the lithosphere and asthenosphere leading to lithospheric breakup from which continental crustal thinning, lithosphere thermal evolution, decompression melt initiation and subsidence are predicted. We explore the sensitivity of model predictions to extension rate history, deformation migration and buoyancy induced upwelling. The best fit calibrated models of lithosphere deformation evolution for the Iberia–Newfoundland conjugate margins require; (1) an initial broad region of lithosphere deformation with passive upwelling, (2) lateral migration of deformation, (3) an increase in extension rate with time, (4) focussing of the deformation and (5) buoyancy induced upwelling. The model prediction of exhumed mantle at the Iberia–Newfoundland margins, as observed, requires a critical threshold of melting to be exceeded before melt extraction. The preferred calibrated models predict faster extension rates and earlier continental crustal separation and mantle exhumation for the Iberia Abyssal Plain–Flemish Pass conjugate margin profile than for the Galicia Bank–Flemish Cap profile to the north. The predicted N–S differences in the deformation evolution give insights into the 3D evolution of Iberia–Newfoundland margin crustal separation. [ABSTRACT FROM AUTHOR]

Knowledge and understanding of the ocean-continent transition (OCT) structure, continent- ocean boundary (COB) location and crustal type are of critical importance in evaluating rifted continental margin formation and evolution. OCT structure, COB location and magmatic type also have important implications for the understanding of the geodynamics of continental breakup and in the evaluation of petroleum systems in deep-water frontier oil and gas exploration at rifted continental margins. Mapping the distribution of thinned continental crust and lithosphere, its distal extent and the start of unequivocal oceanic crust and hence determining the OCT structure and COB location at rifted continental margins is therefore a generic global problem. In order to assist in the determination of the OCT structure and COB location, we present methodologies using gravity anomaly inversion, residual depth anomaly (RDA) analysis and subsidence analysis, which we apply to the west Iberian rifted continental margin. The west Iberian margin has one of the most complete data sets available for deep magma-poor rifted margins, so there is abundant data to which the results can be calibrated. Gravity anomaly inversion has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning; subsidence analysis has been used to determine the distribution of continental lithosphere thinning; and RDAs have been used to investigate the OCT bathymetric anomalies with respect to expected oceanic bathymetries at rifted continental margins. These quantitative analytical techniques have been applied to the west Iberian rifted continental margin along profiles IAM9, Lusigal 12 (with the TGS-extension) and ISE-01. Our predictions of OCT structure, COB location and magmatic type (i.e. the volume of magmatic addition, whether the margin is 'normal' magmatic, magma-starved or magma-rich) have been tested and validated using ODP wells (Legs 103, 149 and 173), which provide observational constraints on the west Iberian margin. [ABSTRACT FROM AUTHOR]

[1] The Maturin Basin in eastern Venezuela is considered a good example of a peripheral foreland basin. Earthquake and tomographic data indicate that eastern Venezuela is affected by the oblique subduction of the South American Plate underneath the Caribbean Plate. New forward flexural isostatic modeling of eastern Venezuela has been carried out in order to determine whether the Maturin Basin was generated purely by thrust sheet loading from the Serrania and Monagas Foreland Thrust Belts. A sequence of forward models from middle Miocene to Present was generated for 3 profiles across the Serrania del Interior Thrust Belt, the Monagas Foreland Thrust Belt, and the Maturin Foreland Basin. The predictions of these models are constrained using seismic reflection and well data. The flexural isostatic modeling shows that thrust sheet loading associated with the Serrania del Interior and Monagas Foreland thrust belts is insufficient to generate the observed subsidence within the Maturin Basin. Dynamic fluid flow modeling of subduction related dynamic topography of eastern Venezuela has been used to investigate the influence of South American Plate subduction on the generation of the accommodation space observed in the Maturin Basin. Fluid flow modeling of subduction related dynamic topography suggests that the subduction of the South American lithospheric mantle caused downward deflection of the South American crust affecting the Maturin Basin and the Serrania Thrust Belt. This modeling suggests that the Maturin Basin subsidence has two components: 55% related to thrust sheet loading and 45% driven by continental subduction. INDEX TERMS: 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 9360 Information Related to Geographic Region: South America; KEYWORDS: flexural isostatic modeling, thrust sheet loading, dynamic topography, continental subduction, subsidence, basin formation. Citation: Jacome, M. I., N. Kusznir, F. Audemard, and S. Flint, Formation of the Maturin Foreland Basin, eastern Venezuela: Thrust sheet loading or subduction dynamic topography, Tectonics, 22(5), 1046, doi:10.1029/ 2002TC001381, 2003.

The papers presented in this thematic collection all derive from a meeting on geological processes at passive margins, held at Burlington House in February, 1996. The meeting was co-hosted by the Geological Society, The Norwegian Geological Society and The Norwegian Petroleum Society. We would like to thank our colleagues Bjorn Larsen and Olay Eldholm for ensuring the strong Norwegian participation at the meeting and Mike Daly for organizing contributions from the UK hydrocarbon industry.

Ultra-large rift basins, which may represent palaeo-propagating rift tips ahead of continental rupture, provide an opportunity to study the processes that cause continental lithosphere thinning and rupture at an intermediate stage. One such rift basin is the Faroe-Shetland Basin ( FSB) on the north-east Atlantic margin. To determine the mode and timing of thinning of the FSB, we have quantified apparent upper crustal β-factors (stretching factors) from fault heaves and apparent whole-lithosphere β-factors by flexural backstripping and decompaction. These observations are compared with models of rift basin formation to determine the mode and timing of thinning of the FSB. We find that the Late Jurassic to Late Palaeocene (pre-Atlantic) history of the FSB can be explained by a Jurassic to Cretaceous depth-uniform lithosphere thinning event with a β-factor of ~1.3 followed by a Late Palaeocene transient regional uplift of 450-550 m. However, post-Palaeocene subsidence in the FSB of more than 1.9 km indicates that a Palaeocene rift with a β-factor of more than 1.4 occurred, but there is only minor Palaeocene or post-Palaeocene faulting (upper crustal β-factors of less than 1.1). The subsidence is too localized within the FSB to be caused by a regional mantle anomaly. To resolve the β-factor discrepancy, we propose that the lithospheric mantle and lower crust experienced a greater degree of thinning than the upper crust. Syn-breakup volcanism within the FSB suggests that depth-dependent thinning was synchronous with continental breakup at the adjacent Faroes and Møre margins. We suggest that depth-dependent continental lithospheric thinning can result from small-scale convection that thins the lithosphere along multiple offset axes prior to continental rupture, leaving a failed breakup basin once seafloor spreading begins. This study provides insight into the structure and formation of a generic global class of ultra-large rift basins formed by failed continental breakup. [ABSTRACT FROM AUTHOR]

The Laccadive-Chagos Ridge and Southern Mascarene Plateau in the north-central and western Indian Ocean, respectively, are thought to be volcanic chains formed above the Réunion mantle plume over the past 65.5 million years. Here we use U-Pb dating to analyse the ages of zircon xenocrysts found within young lavas on the island of Mauritius, part of the Southern Mascarene Plateau. We find that the zircons are either Palaeoproterozoic (more than 1,971 million years old) or Neoproterozoic (between 660 and 840 million years old). We propose that the zircons were assimilated from ancient fragments of continental lithosphere beneath Mauritius, and were brought to the surface by plume-related lavas. We use gravity data inversion to map crustal thickness and find that Mauritius forms part of a contiguous block of anomalously thick crust that extends in an arc northwards to the Seychelles. Using plate tectonic reconstructions, we show that Mauritius and the adjacent Mascarene Plateau may overlie a Precambrian microcontinent that we call Mauritia. On the basis of reinterpretation of marine geophysical data, we propose that Mauritia was separated from Madagascar and fragmented into a ribbon-like configuration by a series of mid-ocean ridge jumps during the opening of the Mascarene ocean basin between 83.5 and 61 million years ago. We suggest that the plume-related magmatic deposits have since covered Mauritia and potentially other continental fragments. [ABSTRACT FROM AUTHOR]

We derive wavenumber domain expressions to calculate the gravity anomaly of a body with irregular bounding surfaces and an exponential density-depth relationship. We apply the method to sedimentary basins, which commonly have this type of geometry and density distribution. The mathematical formulation also allows the exponential density-depth relationship to be measured from an arbitrary irregular surface rather than the top surface. Using this arrangement, the gravity anomaly of exhumed sedimentary basins can be predicted if the amount of eroded section can be estimated. The corresponding inverse algorithms are also derived. Examples of the use of the forward algorithms, from the Galicia Interior Basin and the Central Irish Sea Basin, are used to illustrate these methods. [ABSTRACT FROM AUTHOR]

Examines profiles of crustal thickness across rifted continental margins to understand the key observations and controlling parameters. Determination of correlations between strain-rate and stretching factor; Association of narrow margins with rapid strain-rates and consistency with thermal strain-softening predicted by thermorheological models.

Opinion. Comments on the work of Peter van der Beek related to the use of quantitative basin-modeling technique in the study of the central Baikal Rift in Russia. Arguments of the authors; Objectivity and soundness of van der Beek's findings; Response of van der Beek to the criticism.

The recent report of the discovery from submersible sampling of continental material of Proterozoic age on the Rio Grande Rise suggests that the formation processes of the Rio Grande Rise and Walvis Ridge in the southern S. Atlantic need to be re-examined. Existing proposed formation processes involve ocean ridge – mantle plume interaction or simply excess 'on-ridge' magmatism. We use gravity anomaly inversion to map crustal thickness for the central South Atlantic area encompassing the Rio Grande Rise, Walvis Ridge and adjacent South American and African rifted continental margins. We show that the Rio Grande Rise consists of three distinct bodies of anomalously thick crust (Western, Central and Eastern) separated by normal thickness oceanic crust. The Central Rio Grande Rise forms a large elliptical body with maximum crustal thickness of 25 km and Walvis Ridge also has a maximum crustal thickness of 25 km but has a narrower and more linear geometry. We use plate reconstructions to restore maps of crustal thickness and magnetic anomaly to Early Cretaceous times to examine the development of the Rio Grande Rise and Walvis Ridge. These restorations together with ages of magmatic addition suggest that the Central Rio Grande Rise and Walvis Ridge formed a single body between 90 and 80 Ma located on the ocean ridge plate boundary similar to Iceland today. On the basis of crustal thickness mapping, the plate restorations and the magmatic ages, we propose that the Rio Grande Rise was fragmented into its 3 parts and separated from Walvis Ridge by at least 4 ocean ridge jumps during the opening of the South Atlantic Ocean between approximately 90 and 50 Ma. Plate reconstructions of crustal thickness showing rotated structural lineaments imply that the separation of Eastern Rio Grande Rise and Walvis Ridge was highly complex involving simultaneous crustal extension and magmatic addition. We propose that the continental material reported on the Rio Grande Rise, if not drop-stones, was isolated from the main continental land-mass and transported into the ocean by these ridge jumps during the Cretaceous formation of the South Atlantic. Many important questions remain. These include: when and where did ridges jumps take place between the components of the Rio Grande Rise and Walvis Ridge; when was sea-floor spreading between them simultaneously active; and what is the generic relationship between ridge jumps, magmatism and continental crust in the oceanic realm? In addition improved plate reconstructions of the central S. Atlantic with tighter fit are required and could be obtained by using crustal thinning from gravity inversion to restore COB location. Improved plate reconstructions of the 3D rift and breakup propagation of the South Atlantic Ocean would enhance our understanding of: the origin and evolution of the Rio Grande Rise and Walvis Ridge; the deep-water connectivity between the northern and southern South Atlantic; and the formation and evolution of the São Paulo Plateau and the southern Santos Basin. [ABSTRACT FROM AUTHOR]

Rifted margins have been extensively studied, however, there are still key unknowns related to their formation process. An important remaining question is how much extension is required to produce continental crustal breakup. The answer to this question may vary depending on several parameters such as the crustal rheology, obliquity and width of the margin, crustal inheritances, magmatism and extension rate. In this study, we use the Iberia-Newfoundland conjugate rifted margins as a natural laboratory to explore how much extension the continental crust and lithosphere underwent to generate continental crustal breakup and thus form this pair of conjugate rifted margins. To achieve our aim we use; (i) gravity inversion to measure continental crustal thinning, (ii) subsidence analysis to measure continental lithosphere thinning and (iii) seismic interpretation to measure brittle extension from fault heave summation. Integration of thinning from gravity inversion and subsidence analysis is used to determine continental crust extension and continental lithosphere extension respectively. We identify the distal end of continental crust on the Lusigal-12/TGS and Screech-2 conjugate seismic profiles. We then calculate continental crustal extension and continental lithosphere extension from the proximal edge of continental crust to their distal end. Results are 177 km continental crustal extension while 192 km for continental lithosphere extension. In contrast, brittle extension from fault heave summation from seismic interpretation is 135 km. These measurements show an apparent discrepancy between observed brittle crustal extension (i.e. faults heaves) and total calculated continental crustal extension at the scale of the whole conjugate Iberia-Newfoundland margins system.Fault population analysis has been carried out to examine whether sub-seismic resolution faulting significantly contributes to an under-estimate of brittle crustal extension. In addition, we explore the observed apparent extension discrepancy by analysing the relationship between the fault extension in the brittle crust and the total measured crustal thinning using a kinematic model (RIFTER) which is tested against quantitative target data.In conclusion, kinematic modelling and fault population analysis suggest that this discrepancy may be caused by sub-seismic resolution faulting and/or non-brittle deformation. This extension discrepancy should not be confused with depth-dependent lithosphere stretching and thinning. [ABSTRACT FROM AUTHOR]

The conjugate rifted margins of south-east Greenland and Hatton Bank in the North Atlantic, which formed synchronous with the North Atlantic Igneous Province, are classified as magma-rich rifted margins. Existing interpretations propose that the SE Greenland margin has a broad 150 km wide region of anomalously thick igneous crust and that it is asymmetric compared with that of the Hatton Bank margin. The margin asymmetry has been explained by asymmetric magma-rich sea-floor spreading. An alternative hypothesis is that the SE Greenland margin consists of a wide region of hyper-extended continental crust sandwiched between extrusive and intrusive magmatic material, which is absent on the conjugate Hatton Bank margin. Each hypothesis for the structure of the SE Greenland margin has implications for the location of the continent-ocean boundary, spreading rates and magmatic addition affecting the overall evolution of the SE Greenland and Hatton Bank margins. Using data from the SIGMA seismic survey, we investigate the OCT structure on the SE Greenland margin to determine if continental or igneous crust is present. We integrate gravity inversion, residual depth anomaly analysis, velocity analysis and joint inversion of seismic and gravity Moho data, to determine crustal thickness, basement seismic velocity and basement density across the margin. Gravity inversion and RDA analysis both suggest thick crust ranging from 10-15 km. Velocity analysis and joint inversion of seismic and gravity data suggest combined SDR and basement seismic velocities and densities in excess of 6.4 km s-1 and 2900 kg m-3 respectively. Lateral changes in seismic velocity and basement density suggest a change in composition oceanwards to less dense material. Overall, our results favour the presence of thick igneous crust on the SE Greenland margin rather than thinned hyper-extended continental crust sandwiched between extrusive and magmatic material. We interpret this crust, with up to 15 km thickness, as very thick oceanic crust. This interpretation of the SE Greenland margin pushes the location of the continent-ocean boundary inboard. Rather than invoking asymmetric sea-floor spreading to explain the asymmetry of the SE Greenland and Hatton Bank margins, we suggest that the asymmetry results from a two stage process consisting of first distributed magma rich plate divergence and oceanic crustal accretion followed by plate boundary localization and more normal sea-floor spreading. [ABSTRACT FROM AUTHOR]