Cretaceous Rift System Research Articles

Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution.

The fundamental role of the Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa during the development of the South Atlantic Cretaceous Rift system

The Borborema and Benin–Nigeria Provinces are composed of a complex network of Neoproterozoic mobile belts, and large-scale shear zones. The evolution of the South Atlantic Cretaceous Rift System was controlled by the structural inheritance of the Borborema and Benin–Nigeria Provinces. The process of lithospheric rupture began and ended around the Borborema and Benin–Nigeria Provinces, which behaved as a “lithospheric relay ramp” and act as the main obstacle to the opening of the South Atlantic. The onset of rifting, and the onset of sea floor spreading are diachronic complex, and spatiotemporal. At the spatial domain, the basins can be described by groups of marginal basins with genetic links to eleven oceanic transform faults, distributed along three rift branches and twelve rift zones, grouped at six structural segments and aborted rift arms. In the time domain, five chronological rift stages are here identified as the result of sorting 33 “tectonic-related events”, that affect the tectono-stratigraphic record of each one of these basins. Oceanic crust initiation was diachronic with emplacement initiating at three times from south to north: (i) at 130 Ma, seafloor spreading begin south of Florianópolis fracture zone; (ii) after salt deposition (113 Ma), between Florianópolis and Bode Verde fracture zone; and (iii) at the end of Albian (~110 Ma), between Bode Verde and Chain fracture zone, and at the entire Equatorial Branch. Not until the end of Albian (~110 Ma) a full, continuous, and stable mid-ocean ridge was established. The time and space distribution of LIP's and the St. Helena and Tristan da Cunha hotspots trails on the Brazilian side allowed us to corroborate these timings within our model. Upper crustal structures did not play a decisive role in the rift compartmentation at the Equatorial and Orthogonal branches, where lithospheric processes controlled the final plate boundaries.

The non-cylindrical crustal architecture of the Pyrenees

We exploit the data from five seismic transects deployed across the Pyrenees to characterize the deep architecture of this collisional orogen. We map the main seismic interfaces beneath each transect by depth migration of P-to-S converted phases. The migrated sections, combined with the results of recent tomographic studies and with maps of Bouguer and isostatic anomalies, provide a coherent crustal-scale picture of the belt. In the Western Pyrenees, beneath the North Pyrenean Zone, a continuous band of high density/velocity material is found at a very shallow level (~10 km) beneath the Mauleon basin and near Saint-Gaudens. In the Western Pyrenees, we also find evidence for northward continental subduction of Iberian crust, down to 50–70 km depth. In the Eastern Pyrenees, these main structural features are not observed. The boundary between these two domains is near longitude 1.3 °E, where geological field studies document a major change in the structure of the Cretaceous rift system, and possibly a shift of its polarity, suggesting that the deep orogenic architecture of the Pyrenees is largely controlled by structural inheritance.

Mapping of sound scattering objects in the northern part of the Barents Sea and their geological interpretation

On cruises 25 (2007) and 28 (2011) of the R/V Akademik Nikolai Strakhov in the northern part of the Barents Sea, the Geological Institute, Russian Academy of Sciences, conducted comprehensive research on the bottom relief and upper part of the sedimentary cover profile under the auspices of the International Polar Year program. One of the instrument components was the SeaBat 8111 shallow-water multibeam echo sounder, which can map the acoustic field similarly to a side scan sonar, which records the response both from the bottom and from the water column. In the operations area, intense sound scattering objects produced by the discharge of deep fluid flows are detected in the water column. The sound scattering objects and pockmarks in the bottom relief are related to anomalies in hydrocarbon gas concentrations in bottom sediments. The sound scattering objects are localized over Triassic sequences outcropping from the bottom. The most intense degassing processes manifest themselves near the contact of the Triassic sequences and Jurassic clay deposits, as well as over deep depressions in a field of Bouguer anomalies related to the basement of the Jurassic–Cretaceous rift system

Influence of rift basin geometry on the subsequent postrift sedimentation and basin inversion: The Organyà Basin and the Bóixols thrust sheet (south central Pyrenees)

The extensional period in the Bay of Biscay-Pyrenean domain during the Early Cretaceous influenced subsequent Upper Cretaceous contractional Alpine structures of the Pyrenean orogen. In the Pyrenees, the Lower Cretaceous rift system shows en echelon geometries for different basins, with the Organya Basin being one of the most important. This basin is located in the southern central Pyrenees, and the inversion of its W-E southern margin has been related to the development of the Boixols thrust sheet, the northernmost of the south Pyrenean thrust sheets. Detailed interpretation of outcrop and seismic reflection data has revealed the presence of a NNW-SSE trending relay area, which corresponds to the western boundary of the Organya Basin. The postrift facies belts parallel this boundary. The geometry of the contractional structures shows related variations such as changes in the structural relief and the plunge and the wavelength across this relay area. The synorogenic sediments also show characteristic thickness variations and onlap geometries perpendicular to thrust-transport direction. These evidences highlight the presence of this extensional margin and corroborate its influence in the subsequent stages of the evolution of the area. Seismic, well, and field data have been incorporated into a 3-D structural model in order to better understand the 3-D geometry of the study area.

Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees

The Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domains.

The late Cretaceous lithospheric mantle beneath the Central Andes: Evidence from phase equilibria and composition of mantle xenoliths

Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high- T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high- T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m 3 for the estimated P– T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high- T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. 143Nd/ 144Nd i ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). 87Sr/ 86Sr i ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pb i isotope ratios of the enriched samples are rather radiogenic ( 206Pb/ 204Pb i 18.8–20.6, 207Pb/ 204Pb i 15.6–15.7, 208Pb/ 204Pb i 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of 208Pb/ 204Pb i ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.

Isotopic composition of Late Mesozoic basic and ultrabasic rocks from the Andes (23–32°S) – implications for the Andean mantle

Mesozoic mantle-derived magmatic rocks from the Jurassic–Early Cretaceous active continental margin of N Chile near 24°S, and from the Cretaceous rift system of NW Argentina near 26°S (central rift) and 32°S (southern rift) are used to constrain the isotopic composition of mantle reservoirs before the onset of the Cenozoic Andean orogeny. They are largely uncontaminated by assimilation of the local Palaeozoic basement as seen in the major and trace element patterns and Sr and Nd isotopic composition of most samples, but small amounts of crustal contamination are detected in the Pb isotope composition. Two different types of mantle sources have been identified. (1) For rocks from the Jurassic arc, now at the continental margin at 24°S, and for rocks from the central rift at 26°S, a depleted mantle source with 143Nd/144Nd(t) 0.5127–0.5128, 87Sr/86Sr(t) ca. 0.7033 may be inferred. Pb isotopic ratios of most samples from the magmatic arc are overall less radiogenic than those from the rift (206Pb/204Pb(t) ca. 18.0–18.4 vs. 18.1–19.1, 207Pb/204Pb(t) 15.55–15.59 vs 15.58–15.64 and 208Pb/204Pb(t) 37.8–38.6 vs. 38.4–38.9). (2) In the southern rift at ca. 30–32°S [Sierras Los Condores region (LC) and Sierras Chajan region (Ch)], an enriched mantle source has been identified, which is similar to the composition of the mantle lithosphere beneath the Brazilian Shield. Samples from LC and Ch show some isotopic differences because of varying degrees of crustal contamination in the LC samples. The LC samples have a higher Th/U ratio than the Ch samples and different Pb isotopic ratios 206Pb/204Pb(t) 18.0–18.6 (LC) vs. 17.7 and 17.8 (Ch), 207Pb/204Pb(t) 15.50–15.66 (LC) vs. 15.50 (Ch) and 208Pb/204Pb(t) 38.6–39.2 (LC) vs. ca. 37.5 (Ch). The Nd isotopic ratios of LC and Ch samples are similar 143Nd/144Nd(t) 0.5123–0.5122 (LC) vs. 0.5123 (Ch), whereas their Sr isotopic ratios differ 87Sr/86Sr(t) 0.7054–0.7079 (LC) vs. ca. 0.704 (Ch). Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00410-001-0344-3.

Geological environments of dinosaur footprints in the intracratonic basins of northeast Brazil during the Early Cretaceous opening of the South Atlantic

The intracratonic basins of northeast Brazil are part of a Cretaceous rift system developed along pre-existing structural trends in the basement during the opening of the South Atlantic Ocean. Sedimentary basins such as those of Sousa, Uiraúna-Brejo das Freiras, Araripe, Cedro, Iguatu, Malhada Vermelha, Lima Campos and Icó contain a wide variety of vertebrate and invertebrate ichnofossils, and invertebrate, vertebrate and plant fossils, including palynomorphs. Among the clastic continental sediments of these basins, dinosaur footprints are the most abundant vertebrate remains. The dinosaurian ichnofaunas all have the same stratigraphic-time-palaeogeographical context, and represent parts of a widespread megatracksite. Similarities in the lithofacies of the deposits where the footprints occur reflect the same tectonic, climatic and sedimentary processes. The environmental setting was influenced by the initial development of the equatorial Atlantic seaway, with an endemic biota living nearby in ephemeral rivers and shallow lakes in a hot climate.

North Sea hydrocarbon plays and their resources

Exploration in the North Sea since 1964 has involved the drilling of 2617 new field wildcat wells and resulted in 722 discoveries. These discoveries, plus the 77 accompanying onshore finds, have originally recoverable hydrocarbon resources totalling 100 billion barrels oil equivalent. There are two main families of hydrocarbon plays when grouped according to their source rocks. In the centre and north of the North Sea, oil and gas finds are associated with a late Jurassic to early Cretaceous rift system buried beneath a Cretaceous and Tertiary cover.

Basement control in the development of the early cretaceous West and Central African rift system

Abstract The structural framework of the Precambrian basement of the West and Central African Rift System (WCARS) is described in order to examine the role of ancient structures in the development of this Early Cretaceous rift system. Basement structures are represented in the region by large Pan-African mobile belts (built at ca. 600 Ma) surrounding the > 2 Ga West African, Congo and Sao Francisco cratons. Except for the small Gao trough (eastern Mali) located near the contact nappe of the Pan-African Iforas suture zone along the edge of the West African craton, the entire WCARS is located within the internal domains of the Pan-African mobile belts. Within these domains, two main structural features occur as the main basement control of the WCARS: (1) an extensive network of near vertical shear zones which trend north-south through the Congo, Brazil, Nigeria, Niger and Algeria, and roughly east-west through northeastern Brazil and Central Africa. The shear zones correspond to intra-continental strike-slip faults which accompanied the oblique collision between the West African, Congo, and Sao Francisco cratons during the Late Proterozoic; (2) a steep metamorphic NW-SE-trending belt which corresponds to a pre-Pan-African (ca. 730 Ma) ophiolitic suture zone along the eastern edge of the Trans-Saharian mobile belt. The post-Pan-African magmatic and tectonic evolution of the basement is also described in order to examine the state of the lithosphere prior to the break-up which occurred in the earliest Cretaceous. After the Pan-African thermo-tectonic event, the basement of the WCARS experienced a long period of intra-plate magmatic activity. This widespread magmatism in part relates to the activity of intra-plate hotspots which have controlled relative uplift, subsidence and occasionally block faulting. During the Paleozoic and the early Mesozoic, this tectonic activity was restricted to west of the Hoggar, west of Air and northern Cameroon. During the Late Jurassic-Early Cretaceous the St. Helena mantle plume may have had an important effect in weakening the lithosphere across a broad zone from Brazil to Sudan. However, causes of extension and stretching are likely to be found in a plate tectonic framework. At that time, the opening of both the Indian and Atlantic oceans were the dominant forces which favoured the development of the WCARS and its propagation by strike-slip movements and extensional displacements along pre-existing Pan-African lithospheric zones of weakness.

Mesozoic plate tectonic reconstructions of the central South Atlantic Ocean: The role of the West and Central African rift system

The Mesozoic opening history of the Central, Equatorial and South Atlantic oceans is closely linked in time and space to the development of the Cretaceous rift system in West and Central Africa. Geological and geophysical studies of the West and Central African rift system indicate it to be dominated by a set of sinistral and dextral strike-slip zones diverging from the Gulf of Guinea and terminating as extensional rift basins in Niger and Sudan. Plate tectonic studies of the opening history of the Atlantic basins indicate four stages of development: stage 1, the Jurassic opening of the Central Atlantic, stage 2, the Early Cretaceous (≈ 130-119 Ma) opening of the South Atlantic with rifting propagating deep into Africa via the Benue Trough, stage 3. the opening of the Equatorial Atlantic (119-105 Ma), and stage 4, the linkage of these oceanic basins and development of a single opening mid-oceanic rift system as seen today. The West and Central African rift system continued to develop until the end of the Cretaceous (i.e., into stage 4) when rifting was terminated by a compressional-shear deformation event causing the folding of sediments in the Benue Trough and reactivation of the Central African shear zone and extensional tectonics in the basins of southern Sudan. This event correlates with an important change in the relative opening of the Atlantic basins. During Tertiary-Recent times the West African rift system has not been active. However, during this period domal uplift and volcanism have occurred in central Cameroon and western Sudan and a very active phase of volcanism has occurred within the last 10 Ma along the present-day Cameroon volcanic line. Although these uplift and volcanic events indicate thermal processes in the upper mantle, there is, as yet, no clear link between their development and plate tectonic processes.

Contrasting modes of rifting: The Benue Trough and Cameroon Volcanic Line, West Africa

The Benue trough of west Africa is commonly believed to be a rift feature that originated in the Cretaceous at about the time that Africa and South America began to separate. Bouguer gravity and available geological data in the trough indicate that its formation was probably the result of regional horizontal stresses in the lithosphere, causing crustal extension and surface subsidence. By contrast, the data for the adjoining Cameroon volcanic line suggests that the associated tensional stresses relate to mantle upwarp causing thinning of the lithosphere and regional crustal uplift similar to that associated with the Kenya rift. Thus the association of passive and active rifts seen in the Afro‐Arabia rift system is also a feature of the Cretaceous rift system in west Africa.

The cretaceous opening of the Northeast Atlantic

A continuous rift system from the Rockall Trough through the Faeroe-Shetland Channel, and the Møre and Vøring basins up to the Tromsø and Bear Island basins is inferred to have developed during the Cretaceous. Oceanic crust was generated in its southern part but its width presumably decreases toward the northeast and probably ends in the Møre Basin. This rift/spreading system requires a clockwise rotation of Greenland and the Rockall Plateau during the Cretaceous around a rotation pole at its northern end at about 74°N 21°E. North of the pole of rotation crustal shortening must have occurred. This compression is interpreted to have taken place on West Spitsbergen. The orogeny there is reinterpreted as 1. (1) a Cretaceous folding phase, 2. (2) a phase of overthrusting at the Paleocene-Eocene boundary, and 3. (3) an extensional faulting phase during the Oligocene. This plate-tectonic model can resolve a series of problems in the Northeast Atlantic region: the connection of the Caledonian fronts of Scotland and Greenland can be easily established. The northern ends of the West Shetland Basin and the North Sea graben; the unusual depth of the Møre and Vøring basins; the marked obliqueness of structural trends between the Northeast Greenland shelf and the Norwegian shelf; the fact that the western Hammerfest Basin was intersected by a younger north-south trending graben which formed the deep Tromsø Basin—all these enigmatic observations can be explained by the Cretaceous rift system.