Hyper-extended Continental Crust Research Articles

The remarkable parallels between the North East Atlantic and Arctic regions

Geological understanding of the NE Atlantic and Arctic regions has increased greatly over the last two decades, revealing remarkable similarities. Continental extension in both regions onset during pausing or cessation of adjacent orogenies – the Verkhoyansk-Chukotka and Alpine orogenies – which relaxed compressional stresses and permitted extension. Severe extension accompanied by high-volume magmatism did not onset abruptly but was the culmination of long-term, regional tectonic and magmatic unrest. It was complex and piecemeal, with extension and volcanism occurring along multiple axes simultaneously, a process still ongoing in Iceland today. Large Igneous Province emplacement did not drive breakup but occurred as a consequence of continental extension. The High Arctic Large Igneous Province did not progress to breakup. As a result, much of the Arctic Ocean is floored by hyper-extended continental crust overlain by seaward-dipping reflectors. This crust resembles the passive margins of the NE Atlantic. The Alpha-Mendeleev Rise in the Arctic Ocean is a double-sided passive margin underlain by hyper-extended continental crust draped with seaward-dipping reflectors. A similar structure has been proposed for the Greenland-Iceland-Faroe Ridge. If correct, present-day volcanism in Iceland is building SDRs today. Up to ∼50% of the NE Atlantic Ocean oceanward of the continental shelves is likely underlain by at least some continental crust. For the Arctic Ocean this figure is ∼80%. Similar structures are found elsewhere, including the Madagascar Channel, the NW Indian Ocean, and the South Atlantic. These new ideas suggest promising directions of future study including assessing the true extent of continental crust in the oceans globally.

Extensional fault geometry and evolution within rifted margin hyper-extended continental crust leading to mantle exhumation and allochthon formation

Abstract. 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.

Mass-Transfer and Fluid Flow along Extensional Detachment Faults in Hyperextended Rift Systems: The Examples of Tasna in the Alps, Mauléon in the Pyrenees, and Hobby High Offshore Iberia

Hyperextended rift systems are characterized by extreme crustal thinning and mantle exhumation associated with extensional detachment faults. These faults cut through thinned continental crust, reaching the underlying mantle and allowing for seawater to infiltrate and react with the crustal and mantle rocks. Hydrothermal fluid systems linked to detachment faults result in fluid–rock reactions occurring along the detachments, resulting in the breakdown and alteration of minerals, loss of elements and strain weakening in both mantle and crustal rocks. We present new geological observations and geochemical data from the modern Iberia and fossil Alpine Tethys Ocean Continent Transition and the West Pyrenean Mauléon hyperextended rift basin. We show evidence for a km-scale fluid flow along detachment faults and discuss the conditions under which fluid flow and mass transfer occurred. Convective fluid systems are of major importance for mass transfer between the mantle, crustal and marine reservoirs. We identified gains in Si, Mg, Fe, Mn, Ca, Ni, Cr and V along extensional detachment faults that we relate to channelized, hydrothermal crust- and mantle-reacted fluid systems migrating along detachments in the hyperextended continental crust. The observation that fault rocks of extensional detachment and syn-extensional sedimentary rocks are enriched in mantle-derived elements such as Cr, Ni and V enables us to define the pathways of fluids, as well as to estimate their age relative to detachment faulting and sedimentation. Because all three examples show a similar mass transport of elements along detachment systems at km-scale, we conclude that these examples are linked to convective fluid systems that may affect the thermal state of the lithosphere, as well as the rheology and chemistry of rocks in hyperextended systems, and may have implications for ore mineral exploration in hyperextended rift systems.

A new tomographic-petrological model for the Ligurian-Provence back-arc basin (North-Western Mediterranean Sea)

The nature of the crystalline basement of the Ligurian-Provence back-arc (LPB) basin is a matter debate as it remains unexplored by direct drilling methods. Several models have been proposed for the lower crustal structure comprising hyperextended continental crust or serpentinized mantle. In this paper, a new Vp-Vs geophysical dataset and corresponding tomography are used to propose a new petrological model for the LPB basin and for the formation of the crust of this back-arc domain. By crossing values of Vp, Vs and Vp/Vs ratios, the Messinian salt layer can be clearly identified down to 5 km depth, which highlights salt diapir structures into its overlying sedimentary cover. The 7.5 km depth corresponds to the transition with a heterogeneous basaltic oceanic crust about 4.5–5 km thick, intruded by rounded felsic gabbro plutons and underplated by a more mafic/ultramafic gabbro. This latter results from fractional crystallization of a hydrous magma inherited from the melting of a supra-subduction mantle which interacted with fluids originating from the subducting Adria slab. These magmas can be traced at the surface by magnetic anomalies punctuating the studied profile. Those new data and observations lead to conclude that the crust of the LPB basin resulted from a fast oceanic accretion during the opening of the back-arc. Its nature remains comparable to an immature oceanic crust with an overall basaltic and gabbroic composition and appears devoid of any serpentinized exhumed mantle.

New insights into the crustal architecture and tectonic evolution of the Eastern Gulf of Mexico

AbstractThe Gulf of Mexico is an intraplate oceanic basin where rifting commenced in the Late Triassic, leading to drifting and ensuing oceanic accretion by Middle‐Late Jurassic, which ceased by the Early Cretaceous. Its tectonic evolution encompasses multiple rifting phases dominated by orthogonal extension, variable magmatism and salt deposition. This complex tectonic history is recorded within the rifted margins of the Gulf of Mexico, including along the eastern part of the basin, where considerable uncertainty remains regarding the tectonic evolution and resulting crustal configuration. This study presents new insights into the crustal types and an updated tectonic framework for the Florida margin. An integrated analysis of seismic and potential field data allows us to characterize the nature of the crust, which shows wide zones of hyperextended continental crust, seaward dipping reflection (SDR) packages, exhumed mantle and magmatic crust. Our results propose elements that could improve the plate model of the Gulf of Mexico, by accounting for the polyphase nature of rifting, the counter‐clockwise rotation of the Yucatan block and the observed increase in magmatic supply.

Architectural elements, geometry, and magmatism of the Sub-Saharan Western African passive margin basins: Seismic and gravity data integration

The Western African Sub-Saharan continental margin includes a large region from Cameroon to South Africa comprising distinct margin types – magma-rich, magma-poor and transform margins. The integrated study of geological, seismic and potential field data allows the mapping of a structural framework subdivided in proximal, necking, distal, outer and oceanic domains, characterized by a set of comparable architectural elements and magmatism. These key domains display a strong and consistent continuity along the continental margin. Longitudinal transitional regions occur in a gradual way, but interactions between structural domains along transversal section may also occur, displaying spatial and temporal overprints. Because of this complexity, the outer domain that is the key feature in the magma-rich margin, tends to disappear northwards where the distal and proximal domains of the magma-poor segments become the most important domains of the margin. In the north of Gabon Basin, it is observed both the reappearance of the outer domain and the reworking of the distal domain by fracture zones. The basement of the distal domain within the magma-poor margin is interpreted as made by a hyperextended continental crust but with magmatic additions. Finally, the new proposed continent-ocean boundary (COB) in the region of the Namibe Basin (transitional zone between magma-rich and magma-poor margins) achieves a best fit in a reconstruction model with the limits recognized at the South America side. The results will imply in a drastic reduction on the gap proposed by other recent reconstruction models and also minimize the intraplate deformation required by some other models.

Crustal structure and subsidence history of the Mannar basin through potential field modelling and backstripping analysis: Implications on basin evolution and hydrocarbon exploration

The Mannar basin is a hydrocarbon-bearing rift basin located between the southeast Indian and the western Sri Lanka margins. In the present study, an integrated analysis of potential field data and multi-channel seismic profiles along with the biostratigraphic information from the exploratory wells is undertaken to delineate the crustal structure, long-term strength of the lithosphere (Te), and subsidence history of the Mannar basin. The constrained potential field modelling and 2-D flexural backstripping analysis across the Mannar basin revealed the presence of a hyperextended continental crust defined by the Te of 25 km and an elevated Moho geometry than the adjacent continental regions. The tectonic subsidence data derived from the 1-D backstripping of wells revealed that the present-day Mannar basin was developed in two stages of rifting; the first rifting occurred contemporaneous with the separation between the East Indian margin and East Antarctica (130-115 Ma), and the second phase of rifting took place during ~100-65 Ma. Both these rifting events gave rise to minor crustal stretching with stretching factors of β1 = 1.07–1.23 and β2 = 1.09–1.15 respectively. We noticed that the total crustal stretching factor (β) and width of the extended crust increase from north to south along the Mannar basin, indicating the anti-clockwise rotation of Sri Lanka during the rifting. The higher β values in the southern part of the basin might have led to greater accommodation space and more sediment influx. The peak subsidence rates (~0.016–0.037 km/Ma) at ~60 Ma indicate that late Cretaceous intrusive volcanism locally enhances geothermal gradient and hence influence the maturation and hydrocarbon generation of sources.

On the kinematics and timing of Rodinia breakup: a possible rift–transform junction of Cryogenian age at the southwest cape of Congo Craton (northwest Namibia)

AbstractAfter tilt correction for Ediacaran thick-skinned folding, a pair of Cryogenian half grabens at the autochthonous southwest cape of Congo Craton (CC) in northwest Namibia restore to different orientations. Toekoms sub-basin trended east-northeast, parallel to Northern Zone (NZ) of Damara belt, and was bounded by a normal-sense growth fault (2 290 m throw) dipping 57° toward CC. Soutput sub-basin trended northwest, oblique to NZ and to north-northwest-trending Kaoko Belt. It was bounded by a growth fault (750 m down-dip throw) dipping steeply (~75°) toward CC. Soutput growth fault could be an oblique (splay) fault connecting a Cryogenian rift zone in NZ with a sinistral transform zone in Kaoko Belt. A transform origin for the Kaoko margin accords with its magma-poor abrupt shelf-to-basin change implying mechanical strength, unlike the magma-rich southern margin where a gradual shelf-to-basin change implies a mechanically weak extended margin. A rift−transform junction is kinematically compatible with observed north-northwest−south-southeast Cryogenian crustal stretching within CC. Post-rift subsidence of the CC carbonate platform varies strongly across the south-facing but not the west-facing shelf. A sheared western CC margin differs from existing Kaoko Belt models that posit orthogonal opening with hyper-extended continental crust. Carbonate-dominated sedimentation over southwest CC implies palaeolatitudes ≤35° between 770 and 600 Ma.

Magnetic amplitude inversion for depth-to-basement and apparent magnetization-intensity estimates

We have developed an inversion method to recover the depth and the total magnetization intensity of the basement under a sedimentary basin using the amplitude of the magnetic anomaly vector (amplitude data). Because the amplitude data are weakly dependent on the magnetization direction, our method is suitable for interpreting areas with remanent magnetization. Our method assumes constant magnetized basement rocks overlain by nonmagnetic sediments. To overcome the inherent ambiguity of potential field data, we assume knowledge of the average depth of the basement and use it as a constraint to regularize the inversion. A sensitivity analysis with synthetic data shows the weak dependency of the magnetic amplitude inversion on the magnetization direction. Different combinations of magnetization directions recover the interface separating sediments from basement rocks. Test on field data over the Foz do Amazonas Basin, Brazil, recovers the shape of the basement relief without any knowledge about the magnetization intensity and direction. The estimated basement relief reveals a smooth basement framework with basement highs in the central part of the area. In a regional-scale perspective, the deeper and constant estimated basement relief at the northernmost limit of the area may suggest changing in crustal domains from a hyperextended continental crust to homogeneous oceanic crust.

Border of the Continental Margin of the Central Arctic Uplifts in Amundsen Basin in Siberia

The seismic structure of the complex underlying the stratified sedimentary cover in the Amundsen Basin in Siberia indicates that the formation of a large-scale rift-related basin lying on a hyper-extended continental crust preceded the spreading process in the Eurasian Basin.

Broadband seismic imaging around the Banda Arc: changes in the anatomy of offshore fold-and-thrust belts

Abstract The complicated geology of the Banda region results from complex collision between the Eurasia, Australia and Pacific plates, ongoing in the region since the Late Oligocene but particularly in the study area since the Middle Miocene (from 15 Ma). Regional 2D broadband seismic data have provided improved imaging of Mesozoic and Cenozoic sedimentary successions in the region. The region comprises the deep and ultra-deep Banda Sea enclosed by a magmatic inner arc and an outer deformed zone, comprising a series of orogens. This outer orogenic zone comprises islands with extended and sometimes hyperextended continental crust, a series of marginal foredeeps and intervening fold-and thrust belts. This paper illustrates how the offshore fold-and-thrust belts that bound the fore-deeps change in size, shape and degree of basement reactivation in a clockwise sense around the Banda Arc.

Crustal structure of the central sector of the NE Brazilian equatorial margin

Abstract The central equatorial Brazilian margin is divided into the Amazon and Barreirinhas divergent segments separated by the Pará-Maranhão transform segment. Analysis of regional 2D seismic lines allowed the definition of the crustal architecture of the margin. In the study area, the Barreirinhas segment has a proximal domain with a 30–35 km-thick continental crust, a 20–40 km-wide necked domain where the crust thins to 10 km, and an outboard domain with hyperextended continental crust. The Pará-Maranhão and Amazon segments consist of exhumation domains and their transition to ocean crust. Their structural styles indicate that this is a magma-poor passive margin with oceanic crust formed in a slow spreading centre. The Pará-Maranhão segment is bounded by two branches of the Saint Paul Fracture Zone that displace crustal domains with structures that document the transition from the distal part of a transform margin to an oceanic fracture zone. Two groups of post-rift volcanic complexes have been identified in the exhumation and oceanic domains, and whose distribution is controlled by the fracture zones. Late Cretaceous–Recent gravitationally-driven slide systems and mass-transport deposits indicate long-lived margin collapse and sediment redistribution fundamentally controlled by the underlying crustal structure of this part of the northeastern Brasilian passive margin.

Seismic characteristics and evolution of post-rift igneous complexes and hydrothermal vents in the Lingshui sag (Qiongdongnan basin), northwestern South China Sea

The study of morphology, distribution, and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes, geodynamics, and tectonic evolution of continental margins. Previous studies concentrated partly on the magma-rich rifted basins, where the lateral magma transport mainly affects the igneous complexes' connection and distribution. However, due to seismic wave shielding effects of the large shallow magmatic bodies, the underlying igneous complexes and their corresponding magma plumbing systems in the magma-poor rifted margins are still in debate. In this study, 2D/3D seismic data and well data are utilized to describe the morphology, architecture, and spatial-temporal distribution of igneous complexes in the Lingshui sag of the Qiongdongnan basin, northwestern South China Sea margin. The identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills. Twenty-six cone-shaped mounds that overlie intruded sills through internal disturbed conduits were also described. Drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents. A uniform Bottom Mounds Horizon of these vents suggests that they probably formed at the same time. Constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers, the magma emplacement was dated to the middle Miocene (ca. 14.6 Ma). Most of the hydrothermal vents are distributed along the F2 fault zone and have direct linkage with the underlying sills, while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust, where the crust thins to 6–10 km. The sills intruded into different layers, from the lower Oligocene to the lower Miocene and the emplaced depth of sills is 1.2–6.3 km, whether or not they feed any vents above. Unlike most of the large volume and laterally linked sills found in the magma-rich rifted margins, the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport, which might coexist with numerous polygonal or small faults and interference reflections. This work highlights the critical role of basin structures in controlling the distribution of post-rift igneous complexes in magma-poor margins, including thinned continental crust, sedimentary thickness, and faults.

The syn-rift stratigraphic record across a fossil hyper-extended rifted margin: the example of the northwestern Adriatic margin exposed in the Central Alps

To understand the overall spatial and temporal evolution and resulting sedimentary stacking patterns within an evolving hyper-extended rift system, we investigated the Austroalpine and Upper Penninic nappes in the Central Alps. These nappes preserve pristine remnants of the northwestern Adriatic rifted margin showing the transition from stretched and hyper-extended continental crust to exhumed mantle of a sediment-starved, magma-poor rifted margin. Our paper reviews sedimentological and structural data that enable us to determine a general chrono-tectono-stratigraphic framework for the syn-rift succession of the margin. The detailed study of the facies distribution within the different rift domains, including proximal, necking, hyper-extended, and exhumed mantle domains, allow us to identify syn- and post-tectonic sedimentary packages. The correlation of these sedimentary packages between the rift domains is based on the identification of basin-wide correlative stratigraphic intervals linked to global or basin-wide Jurassic events: (1) the Early Toarcian Oceanic Anoxic Event; (2) an Early Bajocian bio-siliceous event; and (3) the onset of Tithonian carbonate-dominated sedimentation. Using these timelines, we could recognize the oceanward propagation of the syn-tectonic package from the proximal to the exhumed mantle domains as a function of time. A key observation is that syn-tectonic packages can be compartmentalized into four tectonic system tracts across the margin. Each system tract and their associated bounding surfaces record the stratigraphic evolution and change in deformation mode related to distinct and successive syn-rift phases that correspond to the stretching (~ 10My), necking (~ 6My), hyper-extension (~ 15My), and mantle exhumation (~ 18My) phases.

Different expressions of the crustal structure across the Dongsha Rise along the northeastern margin of the South China Sea

This study presents two long-cable, deep-penetration, multi-channel seismic reflection images and associated gravity modeling to delineate structures and magmatic features that occur along the margins of the eastern and western Dongsha Rise in the northeast South China Sea (SCS). These data show approximately symmetrical sags imaged on the shelf, but the structures begin to vary in multiple ways along the eastern and western Dongsha Rise after crossing the continental shelf. At the slope break, an anticline with two thrust faults and a Mesozoic unconformity are imaged on the eastern Dongsha Rise, whereas negative flower structures are detected on the western Dongsha Rise. Most large-scale normal faults sole into a mid- or lower-crustal detachment below the lower slope on the eastern Dongsha Rise whereas some uplifts with small fault offsets were imaged at the lower slope and seaward-dipping tilted fault blocks were imaged between the lower slope and oceanic basin on the western Dongsha Rise. Magmatism, expressed by different types, such as intrusions, volcanoes and sills, was active since at least 5.5 Ma; and magmatic deposits appear to be well-developed on the eastern Dongsha Rise of the northeastern SCS margin.Gravity modeling showed that the crust below the lower slope/oceanic basin in the eastern Dongsha Rise is hyper-extended continental crust underplated by a high velocity layer with a maximum thickness of 9.4 km below the slope break, rather than thickened oceanic crust. The continent-ocean transition zone (COT) has a broader extent on the eastern Dongsha Rise compared to the western Dongsha Rise. Pure-shear deformation and lower crustal flow have led to the formation of the crustal structural contrasts and the wide northeastern continental margin of the SCS.

Crustal architecture of the Laptev Rift System in the East Siberian Arctic based on 2D long-offset seismic profiles and gravity modelling

The Laptev Shelf in the East Siberian Arctic represents a rare tectonic setting where an active oceanic spreading centre, the Gakkel Ridge, intersects a continental margin. The North America–Eurasia plate boundary follows the Gakkel Ridge and passes into a continental shelf; this has resulted in the development of a wide rift system that has been active since the Late Cretaceous. The new long-offset seismic profiles provide a reliable basis for deciphering the structural characteristics of this rift system. We use two new seismic profiles, along with one acquired in the 1990s, to examine the crustal architecture of the rift system. Our approach combines seismic interpretation, time to depth conversion of seismic profiles and 2D gravity forward modelling. The obtained results indicate the presence of hyperextended continental crust beneath the Ust' Lena Rift Basin and exhumed continental mantle at the base of the syn-rift succession along the rift axis. The upper crust was removed by brittle stretching, while the lower crust experienced extreme ductile thinning. Our results show that continental crust can be eliminated in the course of rifting without a considerable heat input from asthenospheric mantle.

Chapter 4 Cenozoic rifting, passive margin development and strike-slip faulting in the Andaman Sea: a discussion of established v. new tectonic models

Abstract The Andaman Sea evolved from near-pure extension (WNW–ESE) during the Late Palaeogene, to highly oblique extension (NNW–SSE) during the Neogene, to strike-slip-dominated deformation (Late Miocene–Recent). These changes in extension direction and deformation style probably reflect the switch from slab rollback-driven extension to India coupling with Myanmar and driving oblique extension/dextral strike-slip. The East Andaman, Mergui–North Sumatra and Martaban Shelf basins, along with the Alcock and Sewell rises and Central Andaman Basin (CAB), were all involved with this deformation which became increasingly focused on the CAB and the rises with time. Possible revisions to traditional models for the Andaman Sea include: (1) the Alcock and Sewell rises are hyper-extended continental or island arc crust, not Miocene oceanic crust; (2) the East Andaman Basin is predominantly underlain by strongly necked to hyper-extended continental crust, not oceanic crust; or (3) CAB oceanic crust is of Miocene, not Pliocene–Recent age. At present a number of major issues can be addressed but not fully resolved, including: (1) the distribution, timing, volume and origin of magmatism in the basins; (2) the causes and significance of strong crustal reflections imaged on 2D and 3D seismic data; (3) implications for crustal thinning geometries, upper crustal extensional patterns and distribution of igneous intrusions for current models of passive margin development (i.e. volcanic v. non-volcanic margins), and how the back-arc setting modifies these models. Elements of both volcanic and non-volcanic margins are present in the East Andaman Sea, with well-developed necking of continental crust (perhaps due to dry mafic, granulite facies lower crust) and extensive igneous intrusions in the lower and middle crust.

Post-rift volcanic structures of the Pernambuco Plateau, northeastern Brazil

The Pernambuco marginal basin is located on the eastern continental margin of northeastern Brazil, covers an area of 20,800 km2, and represents one of the most prominent frontiers for deep water oil and gas exploration off the Brazilian coast. The onshore region of this basin was highly affected by extrusive and intrusive magmatism during the Upper Albian, and the relation of that event with the volcanic structures observed in the offshore sector has not been thoroughly characterized to date. This study aims to characterize the major extrusive and intrusive volcanic structures of the offshore portion of this basin, which is dominated by the Pernambuco Plateau, and its stratigraphic relations. A set of 143 2D multichannel seismic sections that cover the Pernambuco Plateau region are used to interpret the major tectono-stratigraphic sequences and describe the distribution of volcanoes, sills, vent complexes and related volcaniclastic sequences. The interpretations are supported by aeromagnetic and gravimetric geophysical surveys. Volcanoes are classified into two groups that differ in terms of their morphology: shield-like structures and cone-shaped volcanic structures. Sill intrusions are mainly identified beneath the volcanic structures and are characterized by high-amplitude reflectors with short extensions and abrupt terminations. Volcaniclastic sequences are found adjacent to the volcanoes and are characterized by high-amplitude, disrupted reflections with local chaotic configurations. Vent complexes are classified on the basis of their morphologies as either eye-shaped or crater-shaped. The volcanic features identified within the available seismic dataset are concentrated in two main areas: in the centre of the plateau and near its northeastern border. These two regions are host basement outer highs and are surrounded by hyper-extended continental crust, which forms the plateau itself. The extrusive and intrusive features described in the offshore region were formed during the post rift Cretaceous and Cenozoic intervals and point to the continuation of magmatic events after the rifting process. The findings presented in this report provide a better understanding of the magmatism on the northeastern passive margin of Brazil and can also be useful for future modelling of the Pernambuco Basin petroleum system.

Rift-to-collision transition recorded by tectonothermal evolution of the northern Pyrenees

The impact of rift-related processes on tectonic and thermal evolution of collisional orogens is poorly documented. Here, we study the northern Pyrenees, a region that has preserved a geological record of the transition from rifting to collision. Using modeling of new low-temperature thermochronological data, including fission track and (U-Th)/He on apatite and zircon, we propose a temporal reconstruction of the inversion of the European rifted margin. Our data confirm that rifting and related cooling started in the Late Paleozoic-Triassic. Throughout the Jurassic and Early Cretaceous the European margin recorded slow heating during postrift subdsidence. Modeling of thermochronological data allows distinguishing subsidence and denudation controlled by south dipping normal faults in granitic massifs that reflect a second episode of crustal thinning at 130–110 Ma. Following onset of convergence at 83 Ma, shortening accumulated into the weak and hot Albian-Cenomanian rift basins floored by both hyperextended continental crust and exhumed subcontinental mantle. The lack of cooling during this initial stage of convergence is explained by the persistence of a high geothermal gradient. The onset of exhumation-related cooling is recognized in the whole Pyrenean region at 50–35 Ma. This timing reveals that the main phase of mountain building started when hyperextended rift basins closed and collision between proximal domains of the rifted margin occurred.

Application of the critical Coulomb wedge theory to hyper-extended, magma-poor rifted margins

The Critical Coulomb Wedge Theory (CCWT) has been extensively used in compressional tectonics to resolve the shape of orogenic or accretionary prisms, while it is less applied to extensional and gravitational wedges despite the fact that it can be described by the same equation. In particular, the hyper-extended domain at magma-poor rifted margins, forming the oceanward termination of extended continental crust, satisfies the three main requirements of the CCWT: 1) it presents a wedge shape, 2) the rocks forming the wedge are completely brittle (frictional), and 3) the base of the wedge corresponds to a low friction decollement. However hyper-extended margins present a fully frictional behaviour only for a very thin crust; therefore this study is limited to the termination of hyper-extended continental crust which deforms in the latest stage of continental rifting. In this paper we define a method to measure the surface slope and the basal deep of this wedge that we apply to 17 hyper-extended, magma-poor rifted margins in order to compare the results to the values predicted by the CCWT. Because conjugate pairs of hyper-extended, magma-poor rifted margins are commonly asymmetric, due to detachment faulting, the wedges in the upper and lower plate margins corresponding respectively to the hanging wall and footwall of the detachment system are different. While the stress field in the upper plate wedge corresponds to a tectonic extensional wedge, the one in the lower plate matches that of a gravity extensional wedge. Using typical frictional properties of phyllosilicates (e.g. clays and serpentine), the shape of the hyper-extended wedges can be resolved by the CCWT using consistent fluid overpressures. Our results show that all lower plate margins are gravitationally stable and therefore have a close to critical shape whereas the tectonic extensional wedges at upper plate margins are critical, sub or sup critical due to the detachment initial angle and the duration of the tectonic activity. In this paper we discuss the geometry and structural evolution of the most distal parts of hyper-extended continental margins and the formation of extensional allochthons during hyper-extension using the CCWT theory.