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Lower Cretaceous to Eocene sedimentary transverse ridge at the Romanche Fracture Zone and the opening of the equatorial Atlantic

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Abstract

A transform-parallel (transverse) ridge runs for several hundred kilometers along the northern side of the Romanche Fracture Zone (RFZ), being most prominent opposite to the eastern Mid Atlantic Ridge/transform intersection (RTI). Seismic reflection profiles and rock sampling indicate that the western part of the transverse ridge is made of uplifted slivers of oceanic lithosphere. In contrast, the eastern part (east of the RTI) consists of a thick (>4 km) sedimentary succession affected by faulting and thrusting, We call it the Romanche Sedimentary Sequence (RSS). If this sedimentary sequence were underlain by oceanic crust, its predicted age should be not more than similar to 60 Ma. However, dolomitized Maiolica-type pelagic limestones containing calpionellids of early Cretaceous age (similar to 140 Ma) were sampled from the RSS, in addition to Paleocene/Eocene quartzs and stones, siltstones, claystones, diatomites, and limestones. We present a description of the rock samples and a reconstruction of their environment of deposition and of their age. The RSS may represent material deposited during initial continent/continent transform motion in a narrow, deep, E-W elongated basin that communicated with the central Atlantic, The RSS was later subjected to transpression and uplift due to transform-related tectonics, Gabbros, but not basalts, were sampled from the igneous basement underlying the RSS. It is not clear whether the RSS overlies oceanic crust or some sort of pre-oceanic, northern Red Sea-type crust. The presence of lower Cretaceous pelagic deposits near the Romanche eastern RTI implies an age for the initial stage of the opening of the equatorial Atlantic older than the generally assumed Aptian/Albian. (C) 2001 Elsevier Science B.V. All rights reserved.

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... Ocean (Kodaira et al. 1998;Müller et al. 2001;Rey et al. 2003;Gaina et al. 2009); † a set of microcontinents along the Romanche oceanic fracture zone in the Equatorial Atlantic (Honnorez et al. 1994;Gasperini et al. 1997Gasperini et al. , 2001Nemčok et al. 2004); † the Wallaby and Zenith plateaus in the Indian Ocean (Planke et al. 2002;Sayers et al. 2002;Nelson et al. 2009;Gibbons et al. 2012;Hall et al. 2013); † the Lomonosov Ridge (e.g. Jokat et al. 1992), an incompletely released microcontinent, narrowly abutting the Russian and Canada margins; † the Hovgaard Ridge in the Norwegian-Greenland Sea (e.g. ...
... The first of these is an ENE -WSW-trending ridge inside the Romanche oceanic fracture zone between longitudes 15830 ′ W and 118W. It is more than 220 km long and about 24 km wide, with dredge hauls constraining a minimum length and width of 155 km and 12 km, respectively (Gasperini et al. 2001). Although the available dredges and seismic images do not allow an exact determination of its geometry, they permitted a discovery of pelagic sediments as old as 147-144 Ma, based on zonal fossil Calpionella alpina (see Michalík & Reháková 2011 for the age definition). ...
... COT/COB, continent -ocean transition/ continent -ocean boundary. environment of these sediments is interpreted to be a narrow, deep elongated basin along the transform juxtaposing the African and South American continents (Gasperini et al. 2001). The age of the strata is much older than the known Aptian age of the basins elsewhere along the offshore transform margins, suggesting that an older localized rift may have developed along the large Precambrian shear zones which have been mapped in this area (Fig. 2). ...
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The study focuses on the role of wrenching-involved continental break-up in microcontinent release, drawing from a review of examples. It indicates that the main groups of release mechanisms in this setting are associated with ‘competing wrench faults’, ‘competing horsetail structure elements’, ‘competing rift zones’ and ‘multiple consecutive tectonic events’ controlled by different stress regimes capable of release. Competing-wrench-fault-related blocks are small, up to a maximum 220 km in length. They are more-or-less parallel to oceanic transforms. The competing horsetail-structure-element-related blocks are larger (up to 610 km in length) and are located at an acute angle to the transform. Competing-rift-zone-related blocks are large (up to 815 km) and are either parallel or perpendicular to the transform. The multiple-consecutive-tectonic-event-related blocks have variable size and are generally very elongate, ranging up to 1100 km in length. The role of strike-slip faults in release of continental blocks resides in: linking the extensional zones, where the blocks are already isolated, by their propagation through the remaining continental bridges and subsequent displacement; facilitating rapid crustal thinning across a narrow zone of strike-slip-dominated faults; and slicing the margin into potentially detachable fault blocks.
... In the case of the Lower Cretaceous pelagic limestone dolomitization appears to have occurred in connection with late Oligocene–early Miocene uplift and exhumation, whereas dolomite cementation of the Tertiary diatomite occurred shortly after deposition. Both events are in line with the tectonic evolution of the Romanche transverse ridge and independently confirm previous paleotectonic reconstructions (Bonatti et al. 1994;Bonatti et al. 1996;Gasperini et al. 2001). ...
... These youngest sediments occur along the apparent dip slope on the north flank of the ridge (Fig. 2, Seismic Line ROM-20). Further details of lithology and biostratigraphy are given inGasperini et al. (2001); in this paper we focus on the aspects of dolomite formation in pelagic chalks or limestones of Early Cretaceous age and in biosiliceous oozes (Station S16-60) that most probably correspond to the diatom oozes of late Eocene age of Station S16-59. The dolomitized limestone of Early Cretaceous age was dredged at Station AT-163 from 2470 to 2950 m water depth (Fig. 1). ...
... The dolomitized limestone of Early Cretaceous age was dredged at Station AT-163 from 2470 to 2950 m water depth (Fig. 1). Its Early Cretaceous (middle Berriasian–early Valanginian) age is documented by rare calpionellids (Gasperini et al. 2001). Its sedimentary facies corresponds to the Maiolica facies, a facies of generally light-colored coccolith chalks and limestones found widely in the central Atlantic and the western part of Tethys and its continental margins (Bernoulli 1972;Bernoulli and Jenkyns 1974;Jansa et al. 1979;Fourcade et al. 1990). ...
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Two dredge samples from the Romanche Fracture Zone in the equatorial Atlantic document dolomite formation in pelagic sediments in a deep-sea environment. The samples come from a highly deformed sedimentary succession, the Romanche Sedimentary Sequence, constituting the transverse ridge accompanying the transverse valley of the fracture zone to the north. These sediments were folded and uplifted to or near the seafloor prior to the growth of an early Miocene carbonate platform which unconformably overlies them. Dolomitized pelagic limestones of Early Cretaceous age preserve solution molds and unaltered tests of radiolaria (opal-A) and ghosts of calcareous nannoplankton in a dolomicritic groundmass. Carbon isotope data indicate a normal marine source of carbon, and oxygen isotope compositions are consistent with precipitation from cool, marine pore waters at or near the sediment-water interface. The 87Sr/ 86Sr ratio suggests dolomitization at around 25 Ma, presumably when the sediments were exhumed to or near to the seafloor. In contrast, a dolomite-cemented diatomite of presumably late Eocene age shows no relics of a carbonate precursor. Dolomite crystals grew freely in the originally highly porous rock, cementing it into a tight fabric. The absence of compaction suggests that cementation by dolomite took place soon after deposition and before significant burial of the sediment. Carbon and oxygen isotope compositions of this dolomite suggest that its formation also occurred from cool, marine waters. The 87Sr/ 86Sr ratio shows the value of seawater around 35 Ma. Under the assumption that dolomite cementation was by seawater, it occurred shortly after deposition at or near the seafloor.
... A more detailed analysis, accounting for latitudinal variation in sediment thickness and integrating direct drilling data, would be a more complete approach to correct for sedimentation effects, but we do not attempt such precise reconstructions here. Another significant source of error is the vertical tectonic movements in fracture zones and aseismic ridges (Bonatti 1978;Barker 1983;Gasperini et al. 2001), in which subsidence rates are faster than predicted by the depth vs. age curves. Given that our paleobathymetric reconstructions are based only on paleobasement depths, and considering that in fracture zones in the South Atlantic subsidence is generally faster than predicted by the depth vs. age models used in our analyses (e.g., Bonatti 1978;Barker 1983;Gasperini et al. 2001), both effects (i.e., sediment accumulation and vertical tectonic movements) would reduce depth. ...
... Another significant source of error is the vertical tectonic movements in fracture zones and aseismic ridges (Bonatti 1978;Barker 1983;Gasperini et al. 2001), in which subsidence rates are faster than predicted by the depth vs. age curves. Given that our paleobathymetric reconstructions are based only on paleobasement depths, and considering that in fracture zones in the South Atlantic subsidence is generally faster than predicted by the depth vs. age models used in our analyses (e.g., Bonatti 1978;Barker 1983;Gasperini et al. 2001), both effects (i.e., sediment accumulation and vertical tectonic movements) would reduce depth. Therefore, the reconstructions presented here should be considered a "maximum depth", which is a conservative approach to estimate paleobathymetry, and any exposed land resulting from such reconstructions could be potentially larger due to these unaccounted factors. ...
... A more accurate approach should use more complicated models, considering mantle plume temperature, magma supply rate and lithosphere loading by the extra volcanics of the hotspot. Similarly, fracture zones seem to have particular subsidence histories (Bonatti 1978;Gasperini et al. 2001). Nonetheless, regarding the Walvis Ridge and Rio Grande Rise, it seems reasonable to suppose that these features had a faster subsidence than predicted by the age vs. depth curves used here (Barker 1983). ...
... (2) along-strike oceanward promontories or peninsulas along the orogen creating local and regional diachronous collisions (Zagorevski & van Staal 2011), but with lateral continental sediment transport to adjacent trenches and subduction zones fronting pre-collisional oceanic arcs (Callec et al. 2010); (3) long-distance oceanward transport of zircon-bearing continental-derived turbidites funnelled for great distances offshore along fracture zones as observed in the present-day Atlantic; for example, the Pleistocene turbidites of the equatorial Vema Fracture Zone (Emelyanov & Trimonis 1977;Perch-Nielsen et al. 1977); (4) pre-collisional trench-line controlled zircon-bearing, coarse to fine pelagic siliciclastic continental sediments deposited onto the downgoing subducting oceanic plate entering the subduction factory (Vroon et al. 1995(Vroon et al. , 2001Ingersoll et al. 2003;Carpentier et al. 2008Carpentier et al. , 2009Callec et al. 2010); (5) undiluted continental pelagic sediments (e.g. Marini et al. 2005;Plank & Langmuir 1998); that is, deposited wind-borne pelagic distal deposits from non-vegetated early Palaeozoic continents that are made available to the subduction factory; (6) isolation of continentally derived blocks of early stable margin rift-drift sediments and, potentially, basement within long transforms, as observed in the Romanche fracture zone in the Atlantic, 900 km off the margin (Gasperini et al. 2001). Thus, longdistance sedimentary or tectonic transport of siliciclastic sediments, some of which are zircon-bearing, from margins and isolation of continental blocks from the early stable margin along transforms can influence arc chemistry. ...
... Thus, longdistance sedimentary or tectonic transport of siliciclastic sediments, some of which are zircon-bearing, from margins and isolation of continental blocks from the early stable margin along transforms can influence arc chemistry. The Romanche example results in Cretaceous to Eocene margin sediments, including contourites, and perhaps crustal basement fragments, that have undergone early thrust faulting during rift-related continent-continent transpression, but are now located close to the present-day Mid-Atlantic Ridge (Gasperini et al. 2001). Subduction-accretion of such an oceanward block that had experienced transpression to beneath the Notre Dame arc could duplicate the so-called Dashwood metasediments-Cape Ray Pluton relationship in a distal location from the Laurentian margin. ...
... Because zircon-bearing siliciclastic turbidites can occur 500-950 km from stable continental margin sources along fracture zones and trenches (e.g. in the modern Atlantic; Emelyanov & Trimonis 1977;Gasperini et al. 2001;Carpentier et al. 2008Carpentier et al. , 2009) and because undiluted pelagic lithogenous brown and red clays occupied exceptionally broader distributions, and were primarily of terrigenous or volcanic origins if preserved from Palaeozoic oceans (e.g. Hüneke & Mulder 2011) or offshore intra-fracture zone or margin detached basement blocks, they are alternatives to a Dashwood ribbon as an input to the subduction factory. ...
... This produced an approximately 500 km long (eastwest) and 70 km wide (north-south) matching pair of fold and thrust belts on the African and South American margins (Figs 2 & 3 (2010) and Sandwell et al. (2013). Black rectangles show the transpressional fold belts in Figure 2. The inset map shows the seabed topography from GEBCO (2008) and the location of the dredge hauls by Gasperini et al. (2001). The black outline indicates the maximum possible size of the exotic continental block defined by the topography. ...
... Two exotic continental blocks are shown: 'A' was described by Gasperini et al. (2001) and 'B' is from Honnorez et al. (1994). OCB, ocean -continent boundary. ...
... The earliest sediment found along the Romanche Fracture zone lies trapped in a highly deformed zone that has been folded and uplifted at location A in Figure 1. This folded sequence contains late Tithonian-early Berriasian (147 -144 Ma) deep marine shales that contain Calpionella alpina, single-celled plankton (Gasperini et al. 2001) recovered from dredge hauls. The dredge hauls indicate that the minimum proven length (east -west) of the block is 156 km, with a 12 km minimum width (the inset map in Fig. 1 shows the location of the dredge hauls). ...
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The Romanche Fracture Zone was originally a corridor of Aptian-age dextral transtensional rifting along the Equatorial Atlantic margins. Late Albian plate tectonic compression occurred due to a change in plate vectors, when the African and South American continents were still in contact across a 500 km-long section of the Romanche Fracture Zone. This dextral compression produced reactivation of the rift faults to produce asymmetric landward-vergent anticlines and thrusts that trend ENE to NE. Fold-axial planes dip seaward, parallel to the rift faults. Minor asymmetric anticlines were developed on the long seaward-dipping fold limbs and these have subvertical axial planes. The asymmetry of the minor folds is due to the southward stratal dip having been oblique to the horizontal maximum principal stress during the Albian inversion. The folds on the African margin were subsequently tightened by compression in Santonian and Oligo-Miocene times. Aptian-age ENE strike-slip faults were reactivated during the compression phases to produce broad positive flower structures up to 30 km wide that formed topographical ridges along the original strike-slip faults. The intervening and broader flat-bottomed synclines do not appear to be associated with rift faults. The folding and thrust faulting created seabed relief of 1–2 km at the end of the Albian; evidenced by the amount of subsequent erosion that removed the better-quality reservoirs in the upper Albian sequence from the major fold crests. Consequently, there has been a significant number of failed oil exploration wells drilled along the fold crests. The fold ridges would have diverted turbidite channels in the onlapping Cenomanian–Campanian sequence and these will be preferentially located on the landward side of the anticlinal crests. Late Cretaceous stratigraphic and structural traps located between the major anticlines have not yet been explored for hydrocarbons along the Romanche Fracture Zone margins.
... However, none of these resembles the Shackleton transverse ridge, either in their morphology or their geometry with respect to plate boundaries. Although continental material may be incorporated into fracture zones (e.g., Gasperini et al., 2001), transverse ridges are formed by uplift of oceanic lithosphere in response to plate interactions at fracture zones and are not primary features. Proposed uplift mechanisms (Bonatti, 1978; Kastens et al., 1998) include mantle hydration, excess volcanism, and compressive stress. ...
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The opening of Southern Ocean gateways was critical to the formation of the Antarctic Circumpolar Current and may have led to Cenozoic global cooling and Antarctic glaciation. Drake Passage was probably the final barrier to deep circumpolar ocean currents, but the timing of opening is unclear, because the Shackleton Fracture Zone could have blocked the gateway until the early Miocene. Geophysical and geochemical evidence presented here suggests that the Shackleton Fracture Zone is an oceanic transverse ridge, formed by uplift related to compression across the fracture zone since ca. 8 Ma. Hence, there was formerly (i.e., in the Miocene) no barrier to deep circulation through Drake Passage, and a deep-water connection between the Pacific and Atlantic Oceans was probably established soon after spreading began in Drake Passage during the early Oligocene.
... However, sampling and analyses of oldest sediments and basement rocks from the Agulhas Ridge segments and its plateau are needed and may provide proof for this speculative idea. Bonatti (1990), Bonatti et al. (1996) and Gasperini et al. (2001) showed that older oceanic and even continental lithosphere may become trapped within younger oceanic lithosphere along transform faults of the equatorial Atlantic in a scenario in which ridge jumping and transform migration provide the mechanism. The series of parallel basement ridges along the AFFZ indicate that the transform fault axis may have migrated within a range of 50–100 km and, therefore, would allow the entrapment of older continental lithosphere. ...
Article
Transform faults constitute conservative plate boundaries, where adjacent plates are in tangential contact. Transform faults in the ocean are marked by fracture zones, which are long, linear, bathymetric depressions. One of the largest transform offsets on Earth can be found in the South Atlantic. The 1200 km long Agulhas Falkland Fracture Zone (AFFZ), form by this, developed during the Early Cretaceous break-up of West Gondwana. Between approx. 41°S, 16°E and 43°S, 9°E the Agulhas Falkland Fracture Zone is characterised by a pronounced topographic anomaly, the Agulhas Ridge. The Agulhas Ridge rises more than 2 km above the surrounding seafloor. The only equivalent to this kind of topographic high, as part of the AFFZ, is found in form of marginal ridges along the continental parts of the fracture zone, namely the Falkland Escarpment at the South American continent and the Diaz Ridge adjacent to South Africa. But the Agulhas Ridge differs from both the Falkland Escarpment and the Diaz Ridge in the facts (1) that it was not formed during the early rift-drift phase, and (2) that it separates oceanic crust of different age and not continental from oceanic crust. A set of high-resolution seismic reflection data (total length 2000 km) and a seismic refraction line across the Agulhas Ridge give new information on the crustal and basement structure of this tectonic feature. We have observed that within the Cape Basin, to the North, the basement and sedimentary layers are in parts strongly deformed. We observe basement highs, which point towards intrusions. Both the basement and the sedimentary sequence show strong faulting. This points towards a combined tectono-magmatic activity, which led to the formation of basement ridges parallel to the Agulhas Ridge. Since at least the pre-Oligocene parts and, locally, the whole sedimentary column are affected we infer that the renewed activity began in the Middle Oligocene and may have lasted into the Quaternary. As an origin of the renewed tectono-magmatic activity we suggest modifications in spreading rate and direction as a result of the Discovery hotspot chain activity starting ~ 25 Ma (Kempe and Schilling, 1974) and the significant deceleration of the African plat since at least 19 Ma (O'Connor et al., 1999). Kempe, D., Schilling, J.G. (1974), Discovery Tablemount basalt:Petrology and geochemistry. Contrb. Mineral. Petrol., 44, 101-115. O'Connor, J.M., Stoffers, P., van den Bogaard, P., McWilliams, M. (1999), First seamount age evidence for significant slower African plate motion since 19 to 30 Ma. Earth Planet. Scie. Letts., 171, 575-589.
... Given a half spreading rate of 16 mm/a [Cande et al., 1988], the age of this portion of the transverse ridge basaltic crust is over 50 Ma, with some uncertainty due to temporal variations of spreading rate and offset length, as well as to possible migrations of the transform boundary. Transpressive deformation may also have played a role in shaping the Romanche transverse ridge, as suggested by folding and thrusting observed in lower Cretaceous to Eocene deposits immediately E of the eastern ridge-transform intersection [Bonatti et al.,1996;Gasperini et al., 2001]. ...
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Most oceanic islands are due to excess volcanism caused by thermal and/or compositional mantle melting anomalies. We call attention here to another class of oceanic islands, due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics. Sunken tectonic islands capped by carbonate platforms have been previously identified along the Vema and Romanche transforms in the equatorial Atlantic. We reprocessed seismic reflection lines, did new facies analyses and 87Sr/86Sr dating of carbonate samples from the carbonate platforms. A 50 km long narrow paleoisland flanking the Vema transform, underwent subsidence, erosion, and truncation at sea level; it was then capped by a 500 m thick carbonate platform dated by 87Sr/86Sr at ∼11–10 Ma. Three former islands on the crest of the Romanche transverse ridge are now at ∼900 m bsl; they show horizontal truncated surfaces of oceanic crust capped by ∼300 m thick carbonate platforms, with 10–6 Ma Sr isotopic ages. These sunken islands formed due to vertical tectonics related to transtension/transpression along long-offset slow-slip transforms. Another tectonic sunken island is Atlantis Bank, an uplifted gabbroic block along the Atlantis II transform (SW Indian Ridge) ∼700 m bsl. A modern tectonic island is St. Peter and St. Paul Rocks, a rising slab of upper mantle located at the St. Paul transform (equatorial Atlantic). “Cold” tectonic islands contrast with “hot” volcanic islands related to mantle thermal and/or compositional anomalies along accretionary boundaries and within oceanic plates, or to supra-subduction mantle melting that gives rise to islands arcs.
... Of particular relevance are the following factors: (1) recognition of Neocomian (ca. 140 Ma) marine sediments on the Romanche Fracture Zone, between northeastern Brazil and West Africa (Gasperini et al. 2001); and (2) recognition of granitic basement in the Rio Grande Rise (Carvalho 2013, Lisboa 2013a. The former point shows that the Tethyan waters reached the Brazilian equatorial margin as early as the beginning of the Cretaceous; the latter indicates that the physical barrier constituted by the "Microcontinent Rio Grande" and the Walvis Ridge effectively restrained free interchange of marine waters between the Pelotas and Santos basins. ...
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Paleontological data obtained in recent years reinforce the hypothesis that Aptian marine sedimentation in the sedimentary basins of the Brazilian continental margin - except the Pelotas basin, the southernmost Brazilian basin - Took place under the domain of waters coming from the north through the Tethys Sea (Central Atlantic). Tethyan waters could reach the basins of the Brazilian conti-nental margin via the seaway then existing in the present-day region of northeastern Brazil. Here there are records in several basins, notably in the São Luís (Codó Formation), Parnaíba (Codó Formation), Araripe (Santana Formation), Tucano (Marizal Formation), Sergipe (Riachuelo Formation) and Camamu (Algodões Formation) basins. Despite irrefutable marine evidence - e.g., dinoflagellates, echinoids, foraminifera, molluscs and fishes, conspicuously present in the Araripe Basin - There are very few paleogeographic reconstructions that include the seaway which is totally ignored in the international literature. The skepticism is even greater in relation to the Tethyan affinity although the evidence has been well documented by molluscs and dinoflagellates, together with ammonoids in the Sergipe Basin. That skepticism may be due to the fact that, in tectonic and geodynamic terms, the opening of the South Atlantic indeed proceeded from south to north, at least in the part that extends from Argentina to the northeastern Brazilian state of Paraíba.
... Transpressional deformation occurred during the migration time from one site to the other with the rise of a ~300-km transverse ridge which emerged above sea-level at ~5 Ma, then it subsided until present (Bonatti et al., 1994). Note the presence of relics of continental crust, ripped from the African margin, along the transverse ridge as revealed by some dredgings (Gasperini et al., 2001;Davison et al., 2016). The same transverse ridge is observed along the Saint-Paul system about 1000 km west of the Romanche ridges. ...
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... CFZ : Chain Fracture Zone. (Bonatti et al., 1994;Gasperini et al., 1997;2001), the Shackleton Transform (Livermore et al., 2004), or the Barracuda Ridge (Pichot et al., 2012) in the Atlantic Ocean) or relatively modest reliefs (in the order of tens or hundreds meters) commonly observed at fracture zones. ...
Article
The location of the India-Arabia plate boundary prior to the formation of the Sheba ridge in the Gulf of Aden is a matter of debate. A seismic dataset crossing the Owen Fracture Zone, the Owen Basin, and the Oman Margin was acquired to track the past locations of the India-Arabia plate boundary. We highlight the composite age of the Owen Basin basement, made of Paleocene oceanic crust drilled on its eastern part, and composed of pre-Maastrichtian continental and oceanic crust overlaid by ophiolites emplaced in Early Paleocene on its western side. A major fossil transform fault system crossing the Owen Basin juxtaposed these two slivers of lithosphere of different ages, and controlled the uplift of marginal ridges along the Oman Margin. This transform system deactivated ∼40 Myrs ago, coeval with the onset of ultra-slow spreading at the Carlsberg Ridge. The transform boundary then jumped to the edge of the present-day Owen Ridge during the Late Eocene-Oligocene period, before seafloor spreading began at the Sheba Ridge. This migration of the plate boundary involved the transfer of a part of the Indian oceanic lithosphere formed at the Carlsberg Ridge to Arabia. This Late Eocene-Oligocene tectonic episode at the India-Arabia plate boundary is synchronous with a global plate reorganization event corresponding to geological events at the Zagros and Himalaya belts. The Owen Ridge uplifted later, in Late Miocene times, and is unrelated to any major migration of the India-Arabia boundary. Keywords India-Arabia plate boundary; Arabian Sea; Transform margins; Oman; Owen Basin; Indus
... On the other hand, oceanic dispersal may also rely on the presence of volcanic islands, floating island, rafting on buoyant vegetation and island hopping. In the Atlantic Ocean, several islands of considerable size (more than 200 km in length) persisted along the present-day submerged Rio Grande Rise and Walvis Ridge at 50 Ma and the long set of islands (at least 800 km in length) had stretched from the Brazilian coast at 20 °S (at the present-day Martin Van Archipelago) at 50-40 Ma [71]. Likewise, there were also many islands in the Pacific Ocean such as Fijian Islands, Borneo, West Sulawesi, and Hawaii Islands, formed by volcanic eruptions in the Late Cretaceous (100-65 Ma) [72]. ...
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Background Social wasps Polistes , Ropalidia , and Parapolybia , belonging to the subfamily Polistinae, have obviously different distribution patterns, yet the factors leading to this difference remain unknown. Results The 17 newly sequenced mitogenomes of Polistes , Ropalidia , and Parapolybia contain 37 genes, and there are obvious differences among the compositions of the three genera. The monophyly of the genus Polistes and a monophyletic Ropalidiini: ( Ropalidia + Parapolybia ) are concordant with previous morphological analysis of the subfamily Polistinae. Our inferred divergence time demonstrates Polistes (at around 69 Ma) was diverged earlier than Ropalidia and Parapolybia (at around 61 Ma). The rearrangement of both trnY and trnL1 are shared by all the Polistinae. In addition, the unique rearrangement of TDRL derived at 69 Ma is detected in Polistes , and Ropalidia contains a Reversal which may derive at 61 Ma. Hereafter, the possibility is elaborated that Polistes originated in Aisa and then dispersed from Africa to South America, and Polistes and Ropalidia spread from Southeast Asia to Australia. At last, continental drift and Quaternary Ice Ages are inferred to be two main limiting factors in the current distributions of the three genera. Conclusions Obvious differences occur in the mitochondrial composition of Polistes , Ropalidia , and Parapolybia . According to the reconstructed time-calibrated framework, it is inquired that the continental drifts and the climate are mainly diffusion limiting factors of the three genera.
... processes include the occurrence of vertical and horizontal tectonic stresses due to changes in the ridge/transform geometry, which could affect the kinematics at the transform boundary causing transtension or transpression (Menard and Atwater, 1969;Bonatti, 1978;Bonatti et al., 1994b;Pockalny et al., 1988;Gasperini et al., 2001;Palmiotto et al., 2013). (Bonatti et al., 1994b). ...
Article
Transverse ridges are large topographic anomalies running adjacent to slow-slip oceanic transforms. They form due to different processes, including thermal stresses, hydration-dehydration of peridotites, non-linear viscoelastic rheology of the oceanic crust and vertical tectonic motions of lithospheric slivers induced by changes in ridge/transform geometry, causing transpression and/or transtension along the transform boundary. A prominent transverse ridge on the southern side of the Vema transform (Central Atlantic) rose probably between 12 and 10 Ma along the entire length (≈ 320 km) of the transform, exposing a relatively undisturbed section of oceanic lithosphere. We used pelagic limestones encrusting serpentinized peridotites sampled from the lower slopes of the uplifted lithospheric section to date this uplift and define mechanisms of its emplacement. Ages were obtained both by micropaleontology (foraminifera and nannofossils) and by ⁸⁷Sr/⁸⁶Sr isotope ratios. No ages older than ≈ 12 Ma were obtained, even in samples recovered at sites with crustal ages (determined by magnetic anomalies) well over 12 Ma; on the other side, ages as young as 5.6–8.3 Ma were found in clusters of samples collected from the eastern part of the transverse ridge, probably due to mass-wasting episodes that rejuvenated the substratum. These results support the hypothesis that the Vema Transverse Ridge rose between 12 and 10 Ma due to flexural uplift related to transtension along the transform, in line with a general model whereby transverse ridges rise during discrete events as a consequence of changes in ridge-transform geometry.
... The Red Sea northward-propagating oceanic rift impacts against the Zabargad Shear Zone (Bonatti 1985), a major morphotectonic feature striking almost N-S that intersects and offsets the Red Sea axis northward by *100 km, and marks the southern limit of the northern Red Sea (Fig. 1). We suggest the Zabargad Shear Zone (SZ) may be a "proto-transform fault" that, if the Red Sea were to continue opening, might develop into an "initial" major oceanic transform, similar to those offsetting today the equatorial Mid Atlantic Ridge (Bonatti et al. 1996;Gasperini et al. 2001). Zabargad Island lies along the southern end of this feature, while a probably extensional (pull apart) basin (Mabahiss Deep) lies at its NE end (Figs. 1 and 2). ...
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We present here 3D seismic reflection and gravity data obtained from an off-axis area of the NW Red Sea, as well as results of a study of gabbroic rocks recovered in the same area both from an oil well below a thick evaporitic-sedimentary sequence, and from a layered mafic complex exposed on the Brothers Islets. These new data provide constraints on the composition, depth of emplacement and age of early syn-rift magma intrusions into the deep crust. The Brothers are part of a series of sub-parallel NW-striking topographic highs associated with SW-dipping extensional fault blocks with significant footwall uplift during rifting that brought early syn-rift deep crustal rocks up to the seafloor. Assuming an important role played by magmatism in the evolution of narrow rifts helps to solve the controversy on the nature of the crust in the northern/central Red Sea (i.e., the crust outside the axial oceanic cells is either oceanic or it consists of melt-intruded extended continental crust). Gabbros show petrologic and geochemical signatures similar to those of MORB-type gabbroic cumulates and are compatible with their having been emplaced either in a continental or in an oceanic context. We explored the different hypotheses proposed to explain the lack of magnetic anomalies in the presence of oceanic crust in the northern Red Sea. Our results, combined with a review of all the geophysical and geological data in the area, suggest a stretched and thinned continental crust with few isolated sites of basaltic injections, in line with a model whereby asthenospheric melt intrusions contribute to weaken the lower crust enabling some decoupling between upper and lower crust, protracting upper crust extension and delaying crustal breakup. Our findings show that continental rupture in the northern Red Sea is preceded by intrusion of basaltic melts with MORB-type elemental and isotopic signature, that cooled forming gabbros at progressively shallower crustal depths as rifting progressed toward continental separation.
... Eocene deposits immediately E of the eastern ridge-transform intersection [Bonatti et al.,1996;Gasperini et al., 2001]. ...
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Oceanic islands can be divided, according to their origin, in volcanic and tectonic. Volcanic islands are due to excess volcanism caused by mantle melting anomalies. Non-volcanic islands, or "tectonic", are formed due to vertical tectonic motions of blocks of oceanic lithosphere along transverse ridges flanking transform faults at slow and ultraslow mid-ocean ridges. Vertical tectonic motions are due to a reorganization of the geometry of the transform plate boundary, with the transition from a transcurrent tectonics to a transtensive and / or transpressive tectonics. The formation of a positive topographic anomaly called "transverse ridge", often strongly asymmetric, may result from the establishment of either a transpressive and transtensive regime. Transverse ridges are formed by uplifted lower oceanic crust and/or upper mantle rocks. When they are at sea level, they form an oceanic non-volcanic island. Tectonic islands can be located also at the ridge – transform intersection, being the “inner corner high”. In this case the uplift is due by the movement of the long-lived detachment faults located along the flanks of the mid-ocean ridges. A modern example of inner corner high near the sea level is the “Anna De Koningh” seamount, located at the intersection between the Southwest Indian Ridge and the Dutoit transform fault (Indian Ocean). Bathymetry data and multichannel seismic reflection profiles have identified four tectonic sunken islands in the equatorial Atlantic. The "Vema" sunken island is at the summit of the transverse ridge adjacent to the Vema transform fault; it is now about 450 m below sea level. It is capped by a carbonate platform about 500 m-thick, 50 km-long and only 5 km-wide. Samples of Vema’s carbonates dated by 87 Sr/ 86 Sr indicate that the formation of the island occurred about 10 Ma. The same age corresponds to a kinematic change of the ridge – transform geometry and the establishment of transtensive tectonics, with flexure of the oceanic lithosphere and uplift of the Vema transverse ridge. However, the discovery of "Miogypsina" in samples dredged at the non-conformity boundary between the basement and the carbonate platform suggest a stage of emergence of the island during Early Miocene, when the island was an inner corner high at the ridge – transform intersection. Three tectonic sunken islands, "Romanche A, B and C", are on the summit of the eastern transverse ridge flanking the Romanche megatrasform; they are now about 1,000 m below sea level. Multichannel seismic reflection profiles show a strong horizontal reflector at a depth of about 1200 m. Above this reflectors we observed stratified seismic units about 250-300 m-thick representing carbonate platforms consisting of shallow-water carbonates dated by 87 Sr/ 86 Sr, between 11 and 6 Ma. A sunken tectonic island, i. e., “Atlantis Bank," today about 700 m below sea level, is located in the South-Western Indian Ridge, along the Atlantis II transform fault, there is the. This island does not have a carbonate platform; it was at sea level when it was located at the ridge-transform intersection. The only modern example of oceanic tectonics island is the Saint Peter - Paul Archipelago (equatorial Atlantic), located along the active zone of the St. Paul transform fault. This archipelago is the top of a peridotitic massif that extends in the direction of the active transform fault and it is now a left overstep undergoing transpression. Markers of sea level dated by 14 C estimate a rate of uplift of the St. Paul Massif of about 1.5 mm/a for the last 6000 years. During my PhD, a multidisciplinary study led to a model to explain the origin and evolution of oceanic tectonic islands: oceanic volcanic islands are characterized by rapid growth and subsequent thermal subsidence and drowning; in contrast, oceanic tectonic islands may have one or more stages of emersion related to vertical tectonic events along the large oceanic fracture zones.
... The extensive dredging in the northern portion of the Doldrums MTS evidenced large regions of exposure of the oceanic basement along the Doldrums transform valley. This is typical of large fracture zone systems along the MAR, such as the 15°20'N (Kelemen et al., 2004), the Vema and the Romanche FZs (Gasperini et al., 2001). One distinctive feature of the ITR-1 is, however, the occurrence of large portions of basement rocks including peridotites and deformed gabbros attributed to tectonic denudation through detachment faulting (Skolotnev et al., 2006). ...
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The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transform domains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs). Among these megatransforms, the Doldrums system (7-8ºN) is arguably the less studied, although it may be considered the most magmatically active. New geophysical data and rock samples were recently collected during the 45 th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allow to reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled with extensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km 2 . The new data clearly indicate that the Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward the central part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminary interpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regime or the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections. We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphere by tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, and the exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys.
... The extensive dredging in the northern portion of the Doldrums MTS evidenced large regions of exposure of the oceanic basement along the Doldrums transform valley. This is typical of large fracture zone systems along the MAR, such as the 15°20'N (Kelemen et al., 2004), the Vema and the Romanche FZs (Gasperini et al., 2001). One distinctive feature of the ITR-1 is, however, the occurrence of large portions of basement rocks including peridotites and deformed gabbros attributed to tectonic denudation through detachment faulting (Skolotnev et al., 2006). ...
Article
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The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transform domains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs). Among these megatransforms, the Doldrums system (7-8ºN) is arguably the less studied, although it may be considered the most magmatically active. New geophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allow to reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled with extensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate that the Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward the central part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminary interpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regime or the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections. We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphere by tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, and the exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys.
... The study area is centered around the Chain FZ; in the north-south direction it extends from the south of the Romanche TF to just north of the Charcot FZ ( Figure 1b). Regionally, it belongs to the equatorial Atlantic Zone, which started opening in the Early Cretaceous, ∼100-140 Ma (Bonatti, Ligi, Borsetti, et al., 1996;Gasperini et al., 2001;Granot & Dyment, 2015;Larson & Ladd, 1979;Moulin et al., 2010) and developed some of the most prominent transform discontinuity systems on our planet (Figure 1a). It was only recently that a high-resolution bathymetry and potential field (magnetic and gravity) data sets were obtained around (Bird, 2003) is for illustration purposes. ...
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Oceanic transform faults and fracture zones represent major bathymetric features that keep the records of past and present strike‐slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain Fracture Zone (FZ), on the South American Plate. The crustal structure across the Chain FZ, at the contact between ~10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ~4.6‐5.9 km, which compares with the observations reported for slow‐slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within fracture zones to the mechanism of lateral dike propagation, previously considered to be valid only in fast‐slipping environments. Furthermore, the combination of our results with other datasets enabled us to extend the observations to morpho‐tectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower‐order Mid‐Atlantic Ridge segmentation around the equator.
... Adjacent to this, on the northern margin of the Demerara Rise, compressional structural features are observed below a peneplaned Albian-aged angular unconformity (Gouyet 1988;Reuber et al. 2016). Transgression and opening of the Equatorial Atlantic gateway between the Central and South Atlantic followed (Gasperini et al. 2001;Friedrich and Erbacher 2006). Cenomanian-Coniacian organic-rich sediments were deposited along the Guyanas margin (Canje Formation) and in adjacent basins (i.e. ...
Article
Segmentation of the Guyanas continental margin of South America is inherited from the dual-phase Mesozoic rifting history controlling the first-order post-rift sedimentary architecture. The margin is divided into two segments by a transform marginal plateau (TMP), the Demerara Rise, into the Central and Equatorial Atlantic domains. This paper investigates the heterogeneities in the post-rift sedimentary systems at a mega-regional scale (>1000 km). Re-sampling seven key exploration wells and scientific boreholes provides new data (189 analysed samples) that have been used to build a high-resolution stratigraphic framework using multiple biostratigraphic techniques integrated with organic geochemistry to refine the timing of 10 key stratigraphic surfaces and three megasequences. The results have been used to calibrate the interpretation of a margin-scale two-dimensional seismic reflection dataset and build megasequence isochore maps, structural restorations and gross depositional environment maps at key time intervals of the margin evolution. Our findings revise the dating of the basal succession drilled by the A2-1 well, indicating that the oldest post-rift sequence penetrated along the margin is late Tithonian age (previously Callovian). Early Central Atlantic carbonate platform sediments passively infilled subcircular-shaped basement topography controlled by underlying basement structure of thinned continental crust. Barremian-Aptian rifting in the Equatorial Atlantic folding and thrusting the Demerara Rise resulting in major uplift, gravitational margin collapse, transpressional structures, and peneplanation of up to 1 km of sediment capped by the regional angular base Albian unconformity. Equatorial Atlantic rifting led to margin segmentation and the formation of the TMP, where two major unconformities developed during the intra Late Albian and base Cenomanian. These two unconformities are time synchronous with oceanic crust accretion offshore French Guiana and in the Demerara-Guinea transform, respectively. A marine connection between the Central and Equatorial Atlantic is demonstrated by middle Late Albian times, coinciding with deposition of the organic-rich source rock of the Canje Formation) (average TOC 4.21 %). The succession is variably truncated by the middle Campanian unconformity. Refining the stratigraphic framework within the context of the structural evolution and segmentation of the Guyanas margin impacts the understanding of key petroleum system elements. Supplementary material: https://doi.org/10.6084/m9.figshare.c.5280490
... Polistes may spread to South America via the Atlantic Ocean due to the signi cantly smaller distance.On the other hand, trans-ocean migrations may also rely on the presence of volcanic islands and rafting on buoyant vegetation. In the Atlantic Ocean, several islands of considerable size (more than 200 km in length) persisted along the present-day submerged Rio Grande Rise and Walvis Ridge at 50 Ma and the long set of islands (at least 800 km in length) had stretched from the Brazilian coast at 20° S (at the present-day Martin Van Archipelago) at 50 − 40 Ma[73]. Likewise, there were also many islands in the Paci c Ocean such as Fijian Islands, Borneo, West Sulawesi, and Hawaii Islands, formed by volcanic eruptions in the Late Cretaceous (100 − 65 Ma) ...
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Background Social wasps Polistes, Ropalidia, and Parapolybia, belonging to the subfamily Polistinae, have obviously different distribution patterns, yet the factors leading to this difference remain unknown. Results In this study, mitochondrial genomes (mitogenomes) of 21 species of these three wasp genera were used to phylogenetic analyses, including 17 newly sequenced ones. It is revealed that both evolutionary selection pressure of protein-coding genes (PCGs) and gene rearrangement events are related to the corresponding distribution patterns. In addition, our fossil-calibrated divergence time estimation suggests the diversification of Polistes was in the Late Cretaceous (~ 69 million years ago, Ma), and that of Ropalidia and Parapolybia occurred in the Tertiary (~ 61 Ma). In view of the divergence time and the history of continental drifts, we speculate that Polistes may spread from Africa to South America via the Atlantic Ocean rather than from Asia to South America. On the other hand, combining divergence time and climate changes of both past and the present-day, it is inferred that Quaternary Ice Ages and temperature could be limitation factors in their present distribution patterns. Conclusions There are obvious differences in the mitochondrial composition of Polistes, Ropalidia, and Parapolybia with different distribution ranges. According to the reconstructed time-calibrated framework, we found that the climate and the continental drifts are diffusion limiting factors of the three genera.
... However, sampling and analyses of oldest sediments and basement rocks from the Agulhas Ridge segments and its plateau are needed and may provide proof for this speculative idea. Bonatti (1990), Bonatti et al. (1996) and Gasperini et al. (2001) showed that older oceanic and even continental lithosphere may become trapped within younger oceanic lithosphere along transform faults of the equatorial Atlantic in a scenario in which ridge jumping and transform migration provide the mechanism. The series of parallel basement ridges along the AFFZ indicate that the transform fault axis may have migrated within a range of 50-100 km and, therefore, would allow the entrapment of older continental lithosphere. ...
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The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the stress field across the fracture zone. Moreover, passing the Agulhas Ridge’s location between 80 and 69 Ma, the Bouvet and Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridge’s structure. The low crustal velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South Atlantic. In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which distributes plume material southwards across the AAFZ, as a source for the magmatic material.
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Along the Rio Muni transform margin, the transition from continental to oceanic crust occurs across a region of approximately 75-km width. The crust in this transition region, termed proto-oceanic crust (POC), is neither purely oceanic nor continental in composition and structure. Improved seismic reflection images from the PROBE deep-imaging dataset, combined with gravity modelling, have shed new light on the structural architecture of the margin and the composition of the POC. On these newly migrated seismic reflection sections, four fracture zones associated with large steps in the Moho are identified, splitting the POC into three segments. Models in which these segments are composed of oceanic or stretched continental crust do not provide satisfactory predictions of the gravity anomaly. A model of serpentinized peridotite for two segments of POC, with the third segment composed of oceanic crust in between, does satisfy the observed gravity anomaly. Three alternative geological scenarios are proposed to explain the segmentation and composition of the POC: (a) serpentinized upper mantle becoming unroofed and emplaced at basement surface level along detachment surfaces confined to discrete segments by the fracture zones, (b) oblique-slip on transform faults that allow the circulation of water into the mantle and emplacement of serpentinized upper mantle material; or (c) intense faulting of anomalous oceanic crust as a result of magma depletion allowing hydrothermal circulation and the emplacement of serpentinized peridotites.
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Sedimentary basin inversion, the shortening of formerly extensional basins, is accommodated mainly by compressional reactivation of extant faults and fractures across a wide range of scales. As such, inversion is a large-scale manifestation of Byerlee friction, the dynamic criterion for fault reactivation governing the effective shear strength of the shallow crust. Basin inversion generates distinctive deformational architecture, and it is implicated strongly in sedimentary basin exhumation. As a principal source of horizontal stress, inversion drives significant sedimentary porosity reduction and resultant fluid flow. Upper crustal deformation is critically dependent on fluid overpressure (i.e., pore fluid pressures greater than would be calculated from a hydrostatic gradient), and, perhaps more than in any other tectonic setting, overpressures are potentially large during sedimentary basin inversion. This review therefore includes discussion of the role of fluid overpressure in inversion and the evidence for it. Collectively, inversion has profound implications—good and bad—for the prospectivity of many petroliferous sedimentary basins. Thus, recognizing the evidence for basin inversion, quantifying its magnitude and understanding the mechanisms that accommodate inversion and other phenomena affected by it, have become essential components of the basin analyst's remit.
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Zagorevski & van Staal (2014) question our model (Dewey & Casey 2013) in which we invoked forearc suprasubduction zone (SSZ) spreading to form the Bay of Islands Ophiolite Complex (BOIC) as part of the Notre Dame arc of the Appalachian–Caledonian Orogen. We interpret the formation of the BOIC to have occurred by spreading proximal to a ridge–trench–trench (R–TR–TR) triple junction. Our model explains (1) the formation of the BOIC within 15–20 myr of obduction, (2) the associated subcretion of the metamorphic sole shortly after formation during subduction of the ocean lithosphere bordering Laurentia, (3) subsequent oblique ophiolite encroachment to the Laurentian passive margin above a west-facing subduction zone as part of the Notre Dame forearc–arc system, and (4) the final obduction with the high-angle palaeo-ridge geometry relative to the Laurentian margin, with the emplacement initiated in the Late Arenig (Dapingian) during the Taconic Orogeny. We respond to the statements and questions posed by Zagorevski and van Staal that contest our model, which we further clarify and develop. Zagorevski and van Staal conclude, mistakenly, that we stated that the formation of the c . 489–484 Ma BOIC was associated with ‘intra-oceanic subduction initiation’ by the conversion of a ‘ridge–transform boundary’ in the Early Ordovician. They imply that we proposed a typical mid-ocean ‘intra-oceanic’ spreading centre environment for the origin of the BOIC that is linked to subduction initiation on a transform. On the contrary, in our fig. 1, p. 716, we proposed that the BOIC formed by sea-floor spreading adjacent to a pre-existing older lithosphere (Karson & Dewey 1978; Casey & Dewey 1984; Cawood & Suhr 1992; Kurth-Velz et al . 2004), represented by the earlier Middle–Late Cambrian Coastal Complex (CC) and Lush’s Bight (LB) oceanic tract, that we interpret as SSZ forearc. The CC–LB …
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Integration of seismic, potential field, and borehole data from conjugate basins along the South Atlantic continental margin, particularly the northeastern Brazilian and northwestern African segments, indicates that the rift architecture is controlled by fracture zones that extend from the oceanic crust and penetrate through the continental crust, locally corresponding to Precambrian structures in cratonic regions. The fracture zones may divide the continental margin into several compartments with independent sedimentary depocentres, separate crustal domains along oceanic transforms, and affect the rift architecture by shearing. Oceanic transform zones may leak igneous rocks originated from the mantle. This work discusses conjugate sedimentary basins in the South Atlantic salt basins, particulary from Jacuípe to Sergipe-Alagoas on the Brazilian side, and from Gabon to Rio Muni on the African side. The following aspects are emphasized: (1) rift depocentres are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) the southernmost segment of the South Atlantic continental margins is characterized by Early Cretaceous volcanic rocks that underlie continental lacustrine Neocomian to Barremian syn-rift sediments; (3) the pre-rift sequences (Mesozoic and Palaeozoic sediments) that underlie the syn-rift depocentres in Gabon and Sergipe/Alagoas are mainly devoid of volcanics; (4) there is seismic evidence of magmatic underplating in the deeper portions of the continental crust, which are expressed by antiformal features locally aligned with transform fractures; (5) basement-involved extensional faults and volcanic activity along leaking transform faults are imaged along several conjugate segments of the margin, particularly along the equatorial margin (Romanche fracture zone); (6) in some segments of the divergent margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with emplacement of oceanic ridges; (7) locally, the outermost rift blocks near the continental-oceanic crust boundary seem to be highly eroded by post-rift uplift caused by transform fault shearing or by magmatic underplating: (8) tectonomagmatic episodes climaxed in the Late Cretaceous/Early Tertiary in northeastern Brazil and extended to the Late Tertiary on the West African margin, forming large volcanic complexes along transverse lineaments that affect both oceanic and continental crust.
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Oblique-shear margins are divergent continental terrains whose breakup and early drift evolution are characterized by significant obliquity in the plate divergence vector relative to the strike of the margin. We focus on the Rio Muni margin, equatorial West Africa, where the ca. 70-km-wide Ascension Fracture Zone (AFZ) exhibits oblique–slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole lithosphere failure and continental breakup (ca. 117 Ma). Oblique extension is recorded also by strike–slip and oblique–slip fault geometry within the AFZ, and buckling of Aptian synrift rocks in response to block rotation and local transpression. Rio Muni shares basic characteristics of both rifted and transform margins, the end members of a spectrum of continental margin kinematics. At transform margins, continental breakup and the onset of oceanic spreading (drifting) are separate episodes recorded by discrete breakup and drift unconformities. Oceanic opening will proceed immediately following breakup on a rifted margin, whereas transform and oblique-shear margins may experience several tens of millennia between breakup and drift. Noncoeval breakup and drift have important consequences for the fit of the equatorial South American and African margins because, in reconstructing the configuration of conjugate continental margins at the time of their breakup, it cannot be assumed that highly segmented margins like the South Atlantic will match each other at their ocean–continent boundaries (OCBs). Well known ‘misfits’ in reconstructions of South Atlantic continental margins may be accounted for by differential timing of breakup and drifting between oblique-shear margins and their adjacent rifted segments.
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L’ipotesi di Wilson di una singola faglia trasforme con una ridottissima zona principale di scivolamento trova eccezione in alcune trasformi oceaniche a grande dislocazione e a bassa velocita’ di scivolamento (dislocazione in termini di età > 30 Myr) che possono essere incluse in una nuova classe di limiti trasformi oceanici caratterizzati da un ampia e complessa zona di deformazione che presenta analogie con i grandi sistemi trascorrenti continentali.
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This paper presents a reconstruction of the palaeodrainage evolution of the Niger River in West Africa in order to contribute to the understanding of sediment supply to the Niger Delta. It has been covered extensively in literature that the Niger River has undergone changes along its course in the Holocene, as implied by the large bend it makes in Mali. However, other enigmatic bends further downstream are indicative of an older and more complicated history that has yet to be understood, and is the focus of this paper. Until now, sediment supply from the Niger River has been considered as being negligible compared to that of the Benue River. The results of this study imply that the contribution from the Niger River was more important than previously thought. The Niger River obtained its present-day geometry in three phases: a Bida Basin phase (Maastrichtian-Miocene); a Iullemmeden Basin phase (Miocene-Pleistocene); and a presentday Niger River phase (Holocene). In the Miocene, an important capture event occurred, increasing the incipient drainage basin by 10(6) km(2), thereby changing the provenance of the sediment supplied to the Niger Delta from mainly crystalline basement to mixed lithologies including sandstone, shale, limestone and volcanic outcrops.
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The southern South Atlantic Ocean contains several features believed to document the traces of hotspot volcanism during the early formation of the ocean basin, namely the Agulhas Ridge and the Cape Rise seamounts located in the southeast Atlantic between 36°S and 50°S. The Agulhas Ridge parallels the Agulhas-Falkland Fracture Zone, one of the major transform zones of the world. The morphology of the ridge changes dramatically from two parallel segments in the southwest, to the broad plateau-like Agulhas Ridge in the northeast. Because the crustal fabric of the ridge is unknown relating its evolution to hotspots in the southeast Atlantic is an open question. During the RV Polarstern cruise ANT-XXIII-5 seismic reflection and refraction data were collected along a 370 km long profile with 8 Ocean Bottom Stations to investigate its crustal fabric. The profile extends in NNE direction from the Agulhas Basin, 60 km south of the Agulhas Ridge, and continues into the Cape Basin crossing the southernmost of the Cape Rise seamounts. In the Cape Basin we found a crustal thickness of 5.5–7.5 km, and a velocity distribution typical for oceanic crust. The Cape Rise seamounts, however, shows a higher velocity in comparison to the surrounding oceanic crust and the Agulhas Ridge. Underplated material is evident below the southernmost of the Cape Rise seamounts. It also has a 5–8% higher density compared to the Agulhas Plateau. The seismic velocities of the Agulhas Ridge are lower, the crustal thickness is approximately 14 km, and age dating of dredge samples from its top provide clear evidence of rejuvenated volcanism at around 26 Ma. Seismic data indicate that although the Cape Rise seamounts formed above a mantle thermal anomaly it had a limited areal extent, whereas the hotspot material that formed the Agulhas Ridge likely erupted along a fracture zone.
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The Cote d'Ivoire-Ghana Marginal Ridge (CIGMR) and the Benue Trough were formed during the Early Cretaceous under similar tectonic conditions and continental sedimentary environments. Early stages of sedimentary infilling are characterized by the deposition of subaerial, lacustrine, deltaic, and transitional marine sediments, successively, from the Late Jurassic to the Albian. Intracontinental transcurrent faulting, occurring in the Aptian-Albian, was superimposed on earlier rifting structures, resulting in pull-apart sub-basins. the extensional-transtensional tectonics caused unstable conditions during the sedimentation, resulting in syn-sedimentary deformations such as water-escape structures, listric normal faults, slumps, and intraformational conglomerates. From the Albian to the end of the Cretaceous, the two domains were under marine conditions, and there was deposition of shallow-water carbonates of fine terrigenous sediments. Compressional movements occurred in the late Albian-Cenomanian along the CIGMR and during the Santonian in the Lower Benue Trough resulting in intense deformations in both domains, with folding, cleavage, and shearing. Thermal events, partly related to the compressional phase, were recorded in the sediments of the two basins, including hydrothermal activity and low-grade metamorphic crystallizations. After the compressional event, the two basins have different evolutions; the CIGMR became a passive margin, with open marine sedimentation, while the Benue Trough was almost completely uplifted when a second compressional phase occurred in the Upper Benue Trough at the Cretaceous/Tertiary boundary.
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The main Alpine-Mediterranean Mesozoic lithofacies, excluding flysch, are outlined and placed in their paleogeographic setting within the broader context of an evolving ocean basin. In the external (ophiolite-free) zones of the Alpine-Mediterranean orogeny, Mesozoic pelagic facies almost invariably overlie kilometers-thick successions of Bahamian-type platform carhonates. Wherever the basement of these platform carbonates is exposed, it is continental, comprising low- to high-grade metamorphic rocks and granites. We suggest that the pelagic sediments of these zones were deposited on a deeply submerged continental margin of the Atlantic type. Palinspastic reconstructions of the central Alpine- Mediterranean area place the depositional setting of most of these pelagic facies on the southern continental margin of the Tethys. In this area supply of clastics and organic matter was minimal, thus encouraging pelagic conditions. The earliest pelagic sediments of the Alpine-Mediterranean region are of Triassic age and comprise gray and red limestones or cherts that commonly are associated with volcanics. These sediments were deposited in embayments and basins between extensive carbonate platforms and reefs. During the Liassic Epoch, a phase of block faulting, probably related to rifting in the oceanic Tethys, destroyed many of these shallow-water sites, and pelagic conditions became more widespread. During the Jurassic Period a basin-swell morphology was produced by irrcgular suhsidence of the different blocks. On submarine highs, or seamounts, the following stratigraphically condensed facies were developed: pisolitic ironstones, red biomicrites containing ferromanganese nodules and crusts, crinoid-pelagic bivalve- gastropod-ammonite biosparites, pelagic pelmicrites and micro-oncolitic sparites and certain red, fine-grained nodular limestones. In the neighboring basins more expanded successions containing slumped blocks and turbidites accumulated; the basinal facies were developed as red, more clay-rich, nodular limestones, gray limestone-marl interbeds, radiolarian cherts, and white nannofossil limestones. The Cretaceous Period saw a smoothing of submarine topography and a general deepening of the water as the continental margin continued to founder. Deposition of varicolored marls and red and white coccolith limestones was widespread. True ocean-floor lithofacies are represented by those rocks associated with or lying upon ophiolites. In the central Mediterranean area they comprise ophicalcites, umbers, radiolarites, white nannofossil limestones and black shales. Their age is Jurassic and Cretaceous. In the western central Atlantic pelagic facies of Late Jurassic and Cretaceous ages occur. These facies resemble both the continental margin and ocean-crust lithologies of the Alpine Mediterranean Tethys. A section through the Mesozoic portion of this undeformed continental margin and ocean-basin complex comprising the Bahamas, the inter-platform straits, and oceanic realm illustrates a paleogeographic arrangement that strongly resembles the reconstructed section for the Alpine-Mediterranean Tethys. This resemblance illustrates the parallel evolution of these two now widely separated areas, so that they both can be considered as representatives of an east-west Mesozoic seaway or Tethyan realm that stretched from the Caribbean to Indonesia.
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The geometrical fit of the continents now separated by oceans has long been discussed in relation to continental drift. This paper describes fits made by numerical methods, with a `least squares' criterion of fit, for the continents around the Atlantic ocean. The best fit is found to be at the 500 fm. contour which lies on the steep part of the continental edge. The root-mean-square errors for fitting Africa to South America, Greenland to Europe and North America to Greenland and Europe are 30 to 90 km. These fits are thought not to be due to chance, though no reliable statistical criteria are available. The fit of the block assembled from South America and Africa to that formed from Europe, North America and Greenland is much poorer. The root-mean-square misfit is about 130 km. These geometrical fits are regarded as a preliminary to a comparison of the stratigraphy, structures, ages and palaeomagnetic results across the joins.
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A Permo-Triassic reconstruction of western Pangea (North America, South America, Africa) is proposed that is characterized by (1) definition of the North Atlantic fit by matching of marginal offsets (fracture zones) along the opposing margins, (2) a South Atlantic fit that is tighter than the BuIlard fit and that is achieved by treating Africa as two plates astride the Benue Trough and related structures during the Cretaceous, (3) complete closure of the Proto-Atlantic Ocean between North and South America, accomplished by placing the Yucatan block between the Ouachita Mountains and Venezuela, (4) a proposed Hercynian suture zone that separates zones of foreland thrusting from zones of arc-related magmatic activity; to the northwest of this suture lie the Chortis block and Mexico and most of North America, and to the southeast lie South America, the Yucatan Block, Florida and Africa, and (5) satisfaction of paleomagmatic data from North America, South America, and Africa. Beginning with the proposed reconstruction, the relative motion history of South America with respect of North America is defined by using the finite difference method. Within the framework provided by the proposed relative motion history, an evolutionary model for the development of the Gulf of Mexico and Caribbean region is outlined in a series of 13 plate boundary reconstructions at time intervals from the Jurassic to the present. The model includes (1) formation of the Gulf of Mexico by 140 Ma, (2) Pacific provenance of the Caribbean plate through the North America-South America gap during Cretaceous time, (3) Paleocene-Early Eocene back arc spreading origin for the Yucatan Basin, whereby Cuba is the frontal arc and the Nicaragua Rise-Jamaica-Southern Hispaniola is the remnant arc, and (4) 1200 km of post-Eocene cumulative offset along both the Northern and Southern Caribbean Plate Boundary Zones, allowing large-scale eastward migration of the Caribbean plate with respect to the North and South American Plates.
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A simplified model of continental extension including lower crustal flow was developed, using the thin-sheet approximation in the estimation of lithospheric yield strength and gravitational buoyancy forces arising from lateral variations in crustal thickness and temperature. The model predicts three distinct modes of extension as a function of temperature and strain rate: (1) core complex mode (concentrated upper-crustal extension with lower crustal thinning over a broad area and any mantle lithosphere extending in a local area), (2) wide rift mode (uniform crustal and mantle lithospheric thinning over a width greater than the lithospheric thickness), and (3) narrow rift mode (concentrated crustal and mantle lithospheric extension).
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The paper reports on the major element composition of basaltic glasses from the Mid-Atlantic Ridge transecting the equatorial mega-fracture zones from 7°S to 5°N (65 stations, 10-20 km sampling intervals, 3.5-5 km water depth range. Many of the basaltic glasses are Na2O, SiO2, and MgO rich, similar to other basalt glasses erupted along the deepest regions of the mid-ocean ridge system, suggesting melt generation by relatively low degrees of partial melting at rather shallow depth in the upper mantle. Along the ridge axis, the compositional variations show regular and systematic long-wavelength trends with a major discontinuity at the complex St. Paul transform fault. A corresponding long-wavelength trend in upper mantle potential temperature, mean pressure, and degree of melting and crustal thickness variation is inferred using parameterized petrologic decompression melting models. -from Authors
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The northeast Brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the South Atlantic. These basins represent the locus of intersection of the Southern and Equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases. Similar data are not found in its counterpart in Africa, especially for the Neocomian. These Early Cretaceous rift basins of northeast Brazil illustrate key three-dimensional geometries of intracontinental rift systems, mainly controlled by the basement structural framework. During the main rift phase (Syn-rift II, Neocomian-early Barremian) extensional deformation was distributed over three main rift axes: (1) the Gabon-Sergipe Alagoas (GSA) trend, (2) the Recôncavo-Tucano-Jatobá (RTJ) trend and (3) the Cariri-Potiguar (CP) trend. During this phase, extensional deformation jumped west from the easternmost basins (GSA trend) to a series of NE trending intracratonic basins (RTJ and CP trends), characterized by a set of asymmetric half grabens separated by basement highs, transfer faults, and/or accommodation zones. These basins are typically a few tens of kilometers wide and trend NE-SW, roughly perpendicular to the main extension direction during the Neocomian. Preexisting upper crustal weakness zones, like the dominantly NE-SW trending shear zones of the Brazilian/Pan-African orogeny, controlled the development of intracrustal listric normal faults. Internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework. The megashear zones of Pernambuco (Brazil) and Ngaundere (Africa) seem to have behaved like a huge accommodation zone, accommodating extensional deformation along the RTJ/GSA trends with simultaneous extension along the CP trend. During the late Barremian (Syn-rift phase III), a significant change in rifting kinematics occurred, when the CP trend was aborted and major rifting initiated at the Equatorial branch. During the Aptian, while the Equatorial branch and Benue trough (Africa) experienced the main rift phase, the RTJ trend was aborted and the GSA trend developed a transitional phase between the rift and drift stage. The GSA trend and the offshore Potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode.
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A high-resolution seafloor spreading history of the South Atlantic since chron C34 is constrained by a combination of Seasat altimeter data and underway marine geophysical data. A set of 45 finite rotation poles defines the relative position of Africa and South America at roughly 2-m.y. intervals. A set of 12 stage poles constrain the relative motion of these two plates at 5- to 10-m.y. intervals. The position of the stage poles continuously migrates, reflecting the continuously changing azimuths of the fracture zones. Major changes in spreading direction are observed in the Late Cretaceous and early Cenozoic as the fracture zones sweep out broad S-shaped curves similar to the pattern seen on the Kane fracture zone in the central Atlantic. Small offset fracture zones were found to be the most accurate recorders of changes in plate motion; large offset fracture zones, such as the Agulhas-Falkland fracture zone, were the least reliable recorders. At 30°S, spreading rates decrease throughout the Late Cretaceous from a high of 75 mm/yr at the end of chron C34 to a low of 30 mm/yr around chron C27. A period of slow spreading between chron C30 and chron C20 corresponds to a zone of fracture zone proliferation, an increase in the amplitude of geoid anomalies over fracture zones, greater relief on topographic profiles, and locally, evidence of intraplate crustal deformation. Spreading rates increase at chron C20 to about 50 mm/yr and then gradually decrease during the late Paleogene and Neogene. A comparison of synthetic fracture zones based on the South Atlantic stage poles to the observed trends of fracture zones in the equatorial Atlantic indicates that the Vema and Marathon fracture zones were generated by South Atlantic spreading, as opposed to central Atlantic spreading, at least as far back as 35 m.y. ago and possibly 50 m.y. ago.
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The study of the exotic blocks of the Hawasina Nappes (Sultanate of Oman) leads to give apposit data that allow us to propose a new paleogeographic evolution of the Oman margin in time and space. A revised classification of exotic blocks into different paleogeographical units is presented. Two newly introduced stratigraphic groups, the Ramaq Group (Ordovician to Triassic) and the Al Buda’ah Group (upper Permian to Jurassic) are interpreted as tilted blocks related to the Oman continental margin. The Kawr Group (middle Triassic to Cretaceous) is redefined and interpreted as an atoll-type seamount. The paleogeography and paleoenvironments of these units are integrated into a new scheme of the Neotethyan rifting history. Brecciae and olistoliths of the Hawasina series are interpreted to have originated from tectonic movements affecting the Oman margin and the Neotethyan ocean floor. The breccias of late Permian age were generated by the extension processes affecting the margin, and by the creation of the Neotethyan oceanic floor. The breccias of mid-late Triassic age coincide in time with the collision of the Cimmerian continents with Eurasia. In constrast, the breccias of late Jurassic and Cretaceous age are interpreted as resulting to the creation of a new oceanic crust (Semail) off the Oman margin.
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An oceanographic expedition was carried out in May and June 1996 with the R/V Gelendzhik to the Romanche Fracture Zone (F.Z.), in the frame of an Italian-Russian joint program for the geological study of the equatorial Atlantic (PRIMAR). We present here a cruise report, with some preliminary data and scientific results. The choice of the ship was determined mostly by the availability of a new, state of the art oceanic multibeam system (SIMRAD EM-120-S). The ’96 expedition focused on the Romanche F.Z. western ridge/transform intersection (RTI) and complemented a 1994 expedition that covered the eastern RTI. Multichannel seismic reflection, gravimetric and magnetometric profiles were collected, as well as bottom rock samples. Moreover, we acquired over 10,000 nautical miles of high resolution multibeam bathymetry. With these new data the entire active part of the Romanche transform (>900 km) has been covered by multibeam morphobathymetry.
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An improved tectonic database for the South Atlantic has been compiled by combining magnetic anomaly, Geosat altimetry and onshore geologic data. We used this database to obtain a revised plate-kinematic model. Starting with a new fit-reconstruction for the continents around the South Atlantic, we present a high-resolution isochron map from Chron M4 to present.Fit reconstructions of South America and Africa that require rigid continental plates result in substantial misfits either in the southern South Atlantic or in the equatorial Atlantic. To achieve a fit without gaps, we assume a combination of complex rift and strike-slip movements: 1.(1) along the South American Parana-Chacos Basin deformation zone,2.(2) within marginal basins in South America (Salado, Colorado Basin) and3.(3) along the Benue Trough/Niger Rift system in Africa. These faults are presumed to have been active before or during the breakup of the continents.Our model describes a successive “unzipping” of rift zones starting in the southern South Atlantic. Between 150 Ma (Tithonian) and approximately 130 Ma (Hauterivian), rifting propagated to 38 °S, causing tectonic movements within the Colorado and Salado basins. Subsequently, between 130 Ma and Chron M4 (126.5 Ma), the tip of the South Atlantic rift moved to 28 °S, resulting in intracontinental deformation along the Parana-Chacos Basin deformation zone. Between Chron M4 and Chron MO (118.7 Ma) rifting propagated into the Benue Trough and Niger Rift, inducing rift and strike-slip motion. After Chron MO, the equatorial Atlantic began to open, while rifting and strike-slip motion still occurred in the Benue Trough and Niger Rift. Since Chron 34 (84 Ma), the opening of the South Atlantic is characterized by simple divergence of two rigid continental plates.
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According to palaeogeographic reconstructions, the equatorial Atlantic Ocean basin started to open in the Cretaceous period, not earlier than 120 Myr ago1–3. From this and a conventional description of the process of sea-floor spreading, one would expect the oldest oceanic crust in this region to be 120 Myr old, and to be found at the edges of the ocean basin. Contrary to these expectations, we report here the recovery of 140-Myr-old 'Maiolica'-type pelagic limestones from the centre of the basin, near the intersection of the Mid-Atlantic Ridge and the Romanche transform fault. The limestones occur within a tecto-nically deformed and uplifted sedimentary sequence, > 4km thick and > 200 km long, which includes continent-derived quart-zitic siltstones of Palaeocene and Eocene age. This sequence probably accumulated in a proto-Romanche transform valley that was connected to a palaeo-central-Atlantic basin, indicating that continental separation had already started here 140 Myr ago. The entrapment of these old deposits within younger ocean crust can be explained by repeated ridge jumping and transform migration during the evolution of the equatorial Atlantic.
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The Romanche transform offsets the Mid-Atlantic Ridge (MAR) axis by about 950 km in the equatorial Atlantic. Morphobathymetric data show the existence on the northern side of the transform of a major 800-km-long aseismic valley oriented 10° to 15° from the active valley; it disappears about 150 km from the western MAR segment. The aseismic valley marks probably the former location of the Romanche transform that was active up to roughly 8-10 Ma, when the transform boundary migrated to its present position. A temporary microplate developed during the migration and reorientation of the transform. This microplate changed its sense of motion as it was transferred from the South American to the African plate. A prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side. Vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge/transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence. -from Authors
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A comparison of geographically extreme occurrences of calpionellids (southern Mexico, state of Oaxaca, and northeastern Caucasus) with Mediterranean faunas shows that faunal successions are practically identical. There are minor differences concerning the relative frequencies of individual species, but the same species are present throughout, no mutually exclusive endemic species could be observed. Detailed chronostratigraphic correlations over great distances, down to the level of standard subzones, are thus possible. Important palaeobiogeographic limitations do nevertheless exist, but these correspond rather to the total absence of calpionellids in strata of an age where they should be present. Calpionellids appeared first at the beginning of the late Tithonian in the Mediterranean realm and Cuba. From there they extended in successive spreads, reaching eastern Mexico in the early Calpionella Standard Zone (basal Berriasian) and southern Mexico and the Caucasus at the transition to the Calpionellopsis Standard Zone (mid Berriasian). The maximal spread is higher up in the Calpionellopsis Standard Zone (late Berriasian).
Chapter
New stratigraphical, sedimentological and structural data supported by geophysical interpretations allow an update of the geology of the Benue trough. An axial high, running NE – SW flanked by two parallel depressions in which several subbasins can be recognized, characterizes the deep structure of the basin. Continental to marine paleoenvironments prevailed during Cretaceous times and magmatic activity, with both tholeiitic and alkaline affinities, was almost continuous during the Late Cretaceous. The Cretaceous sediments have undergone deformation during a short diachronous compressional tectonic phase turning the basin into a folded chain. The Cretaceous tectonic history of the trough, starting with the opening of the Gulf of Guinea, is marked by the predominance of wrench movements along its axis resulting in the formation of several ‘en échelon’ subbasins.
Chapter
Mesozoic deep water sedimentary rocks uplifted and exposed in basement complexes on the islands of Fuerteventura (Canary Islands) and Maio (Cape Verdes) help document the early Atlantic ocean and the volcanic history of these islands. Comparisons with DSDP sites, oil wells and on-land geology confirm that the lower Cretaceous Fuerteventura quartzose clastics formed part of a deep water fan complex located close to the Atlantic continent-ocean boundary. Comparable flysch is known in the Moroccan basin, in the Betic-Maghrebide system and the feeding Tan-Tan-Cape Bojador delta system. The inferred disconformity between Cenomanian and Senonian is part of a widespread depositional hiatus associated with slumping on the adjacent west African continental margin. On Maio pelagic limestones comparable with the Tethyan Maiolica facies were deposited on a subsiding ?middle/late Jurassic ocean ridge before uplift related to onset of volcanism probably in the late Cretaceous.-from Authors
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Remnants of the Liguria-Piemont Ocean with its Jurassic ophiolitic basement are preserved in the South Pennine thrust nappes of eastern Switzerland. Analysis of South Pennine stratigraphy and comparison with sequences from the adjacent continental margin units suggest that South Pennine nappes are relics of a transform fault system. This interpretation is based on three arguments: (1) In the highly dismembered ophiolite suite preserved, Middle to Late Jurassic pelagic sediments are found in stratigraphic contact not only with pillow basalts but also with serpentinites indicating the occurrence of serpentinite protrusions along fracture zones. (2) Ophiolite breccias (»ophicalcites«) occurring along distinct zones within peridotite-serpentinite host rocks are comparable with breccias from present-day oceanic fracture zones. They originated from a combination of tectonic and sedimentary processes: i.e. the fragmentation of oceanic basement on the seafloor and the filling of a network of neptunian dikes by pelagic sediment with locally superimposed hydrothermal activity and gravitational collapse. (3) The overlying Middle to Late Jurassic radiolarian chert contains repeated intercalations of massflow conglomerates mainly comprising components of oceanic basement but clasts of acidic basement rocks and oolitic limestone also exist. This indicates a close proximity between continental and oceanic basement. The rugged morphology manifested in the mass-flow deposits intercalated with the radiolarites is draped by pelagic sediments of Early Cretaceous age.
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Folds commonly form along transform fault systems. In the San Andreas strike-slip fault, two mechanisms of folding have been described that are regarded as incompatible: (1) wrench tectonics, or distributed shear, and (2) strain partitioning, shortening being perpendicular to the fault. The compatibility and relative importance of these two mechanisms are not well understood, in part because folds are the result of lengthy deformation. This paper synthesizes paleomagnetic data, sedimentary thickness maps, fault patterns, and seismic reflection profiling data along the San Andreas transform fault system that indicate a 20° 30° clockwise (map view) reorientation of Pliocene en echelon folds to their present position, subparallel to the San Andreas fault. The folds continue to develop today as a result of shortening perpendicular to the fault. Folds apparently rotated with continuing strain in the wrench zone and with increasing shortening across the shear zone. Rotation likely occurred when changing Pacific plate motions increased the component of convergence across the fault system. This field example documents deformation by these two different mechanisms in the same fold belt, which has broad implications for interpreting other fold belts formed in similar settings. The geologic record suggests that the opposing theories of wrench deformation and strain-partitioned shortening perpendicular to the fault are both important and compatible processes that act at different times and places.
Article
We present a plate kinematic evolution of the South Atlantic which is based largely on the determination of the equatorial fracture zone trends between the African and South American continental margins. Four main opening phases are dated by oceanic magnetic anomalies, notably MO, A34, and A13, and are correlated with volcanism and tectonic events on land around the South Atlantic Ocean. The Cera and Sierra Leone rises are probably of oceanic origin and were created 80 m.y. ago or later in their present-day positions with respect to South America and Africa.
Article
The discovery of calpionellids and radiolarians in thin sedimentary layers intercalated in the upper part of MORB tholeiitic pillow basalts allows us to date the oceanic crust of Maio Island (Capo Verde Islands) as early Valanginian instead of, as was hitherto believed Late Jurassic. This new dating fits better with reconstructions of the geological history of the Central Atlantic based on magnetic anomalies. The overlying light-coloured pelagic limestones (Maiolica facies) with radiolarians, aptychi and ammonites are Valanginian-Barremian in age. These limestones are lithologically similar to the white limestones drilled at several DSDP sites in the Central Atlantic.RésuméLa découverte de calpionelles et de radiolaires, dans plusieurs minces intercalations sédimentaires situées à la partie supérieure des basaltes tholéiitiques en coussins de type MORB, permet de dater la croûte océanique affleurant sur l'île de Maio, archipel du Cap Vert, du Valanginien inférieur et non pas du Jurassique supérieur comme on l'admettait jusqu'ici. Cette nouvelle datation s'accorde avec les reconstitutions proposées de l'Atlantique central basées sur les anomalies magnétiques. Cette étude a également permis d'attribuer au Valanginien-Barrémien les calcaires pélagiques clairs à radiolaires, Aptychus et ammonites de Maio, antérieurement rapportés au Jurassique supérieur-Aptien. Ces calcaires à cherts, de faciès identique à celui de la Maiolica téthysienne, sont comparables aux “White Limestones” rencontrés dans de nombreux forages océaniques de l'Atlantique central.
Article
Detailed study of microfaults and associated striated fault planes affecting the Early Cretaceous infilling of Rio do Peixe basins (northeast Brazil) allows us to determine a regional stress field with a NNW-SSE oriented extensional component. In the basement, reactivated E-W and NE-SW ductile shear zones of Pan-African age both controlled the architecture and elongation of the basins. These en-échelon half-grabens illustrate the first stages of intracontinental rifting of a pre-fractured crust. During the Early Cretaceous, the northeast Brazilian province was subjected to NW-SE extension related to the eastward opening of the South Atlantic ocean.
Article
The Mesozoic and Cenozoic rocks and sediments penetrated at Deep Sea Drilling Project sites in the North American basin show similar sequences of lithology, age, faunal assemblages, and petrographic composition, thus permitting recognition of six formations. These formations were mapped using a combination of reflection seismic data and drilling results. Pelagic limestones dominate the Late Jurassic-Early Cretaceous; the oldest rocks are Oxfordian. These sediments were deposited in a deep bathyal environment above the carbonate compensation depth (CCD). The CCD shoaled abruptly in the Barremian; this shoaling was accompanied by development of euxinic conditions and formation of carbonaceous shales which continued through the Cenomanian. Starved-basin conditions and a shallow CCD resulted in deposition of pelagic multi-colored clays in the Late Cretaceous. The presence of Meastrichtian chalks above carbonate-poor Late Cretaceous clayey deposits indicates abrupt but temporary deepening of the CCD in the North American basin in the latest Cretaceous. Deposition dominated by clayey sediments continued into the Paleocene on the Bermuda Rise, the only locality where this interval is represented. Siliceous deposits accumulated during early and middle Eocene time, probably below the CCD; the resultant cherty unit is a prominent seismic reflector (horizon Ac over much of the North American basin. The late Eocene and Oligocene are represented by clays, siliceous clays, and mass-flow deposits in the Bermuda Rise area. Along the continental margin, a major unconformity dated as late Eocene to Oligocene bevels Eocene to Lower Cretaceous rocks. Hemipelagic deposition of gray-green mud was dominant in the North American basin throughout the Neogene and continues to the present. End_of_Article - Last_Page 475------------
Article
Gravity and magnetic anomalies bordering the continental margins of the southern South Atlantic Ocean are compared, in detail, on conjugate sides of the ridge crest, and a model for the boundary between oceanic and continental basement is given. The area of study includes the predominantly sheared margins of the Agulhas-Falkland fracture zone and the rifted margins of Argentina and southern Africa south of the Rio Grande Rise and Walvis Ridge, respectely. These margins have associated with them, for the most part, linear magnetic anomalies that can be modeled as edge effect anomalies separating oceanic from continental basement. Coincident with the magnetic anomalies are gradients in the isostatic gravity anomaly. We have taken the location of these geophysical lineaments on the African margin and rotated them clockwise to fit the anomalies on the Argentine margin. This fit, which gives us a new pole of total closing for the South Atlantic Ocean, obviates, for the most part, the gaps and overlaps observed in other reconstructions. The improved fit thereby suggests rigid plate behavior and minimum stretching of continental crust during the early opening of the southern South Atlantic Ocean.
Article
The pattern of seismogenic faulting in the San Gorgonio Pass-San Bernardino basin area of the San Andreas fault (SAF) zone is mapped from about 7000 quality-selected focal mechanisms (1981-Nov '92). They are derived from phase data of the southern California network by a relocation procedure based on location-dependent station-corrections and by a grid-search procedure. About 3/4 of the mechanisms have been interpreted as planes of rupture, or 'slip planes', that delineate many distinct faults. One of these faults is right-lateral, dips steeply northeast and is continuous through the San Gorgonio Pass area; we interpret it as the main branch of the SAF. A very large earthquake seems possible on this portion of the SAF because of its continuity and because of the unusually deep reach of the seismicity (23 km). Southwest of this fault, a volume of diffuse and persistemt seismicity has a sharp downward cut-off which may reflect a basal detachment dipping 20 deg northeast and intersecting the SAF along the deepest seismicity. Northeast of the SAF, the floor of the seismicity is shallower by as much as 10 km. Stress characteristics are derived from slip planes in 10 selected subregions. The regime is transpressional near the constricting bend of the SAF at San Gorgonio Pass; it is dominated by horizontal extension in the San Bernardino basin area; and it is again transpressional in the eastern San Gorgonio Mountains north of the Cucamonga thrust. The broad features of this zonation may be related to the intersection of the SAF with the Pinto Mountain fault and with the San Jacinto fault. A change of fault kinematics in the Yucaipa cluster coincides with the 1992 Landers and Big Bear main shocks and may be a manifestation of static stress change.
Article
The misfit problems raised by the pre-drift reconstructions of the South and Equatorial Atlantic compel us to resort to intraplate deformations. It is shown that acceptable deformations of the African plate in line with the Benue Trough and the Transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the South Atlantic. These problems can instead be alleviated by considering a degree of deformation of the South American plate along an intraplate boundary in line with the Parana Province, the Rio Grande Rise and the Walvis Ridge. In spite of the scarcity of field observations in this area of South America, we show that such a deformation cannot be ruled out, but it will have to be associated with deformations in the Equatorial Atlantic and in other areas of the African and South American plates.
Article
Rocks significantly older than their theoretically predicted age have been recovered in the vicinity of some Atlantic Transform Zones. Rocks of Palaeocene (55–58 Myr) and possibly even of Cretaceous age are found near the Vema Transform, on tectonically uplifted crust which should not be older than 30 Myr. We present here a model which might explain the presence of uplifted, anomalously old crust near the Vema Transform Zone (TZ). The proposed model involves axial rift propagation and reorientation and migration of the transform as a result of a change in the regional stress field. During transform migration a crustal wedge, formerly part of the African plate, is transferred to the South American plate. The ensuing oscillatory spreading can explain the anomalous age of the uplifted crustal block. Compression resulting from rift propagation and transform migration provides a mechanism for crustal uplift.
Article
Since the publication of our previous time scale (Berggren and others, 1985c = BKFV85) a large amount of new magneto- and biostratigraphic data and radioisotopic ages have become available. An evaluation of some of the key magnetobiostratigraphic calibration points used in BKFV85, as suggested by high precision 40Ar/39Ar dating has served as a catalyst for us in developing a revised Cenozoic time scale. This paper presents a revised (integrated magnetobiochronologic) Cenozoic time scale (IMBTS) based on an assessment and integration of data from several sources. Biostratigraphic events are correlated to the recently revised global polarity time scale (CK95). The construction of the new GPTS is outlined with emphasis on methodology and newly developed polarity history nomenclature. The radioisotopic calibration points (as well as other relevant data) used to constrain the GPTS are reviewed in their (bio)stratigraphic context. An updated magnetobiostratigraphic (re)assessment of about 150 pre-Pliocene planktonic foraminiferal datum events (including recently available high southern (austral) latitude data) and a new/modified zonal biostratigraphy provides an essentially global biostratigraphic correlation framework. Finally, the current status of Cenozoic chronostratigraphy is reassessed and estimates of the chronology of lower (stage) and higher (system) level units are presented. -from Authors
Article
The structure and evolution of Early Cretaceous rift basins in Western and Central Africa are described. Two stages of rift development and fracturing have been identified: (1) from Neocomian to Early Aptian roughly E-W and NW trending troughs (Upper Benue, N Cameroon, S Chad, Sudan etc.) opened in response to a submeridian extensional regime in Central Africa while in Western Africa the N-S trending transsaharian fault zone acted as a sinistral wrench; (2) from Middle Aptian to Late Albian large northwest trending troughs (E Niger, Sudan, Sirte, etc.) opened in response to a northeast extensional regime while the Central African fault zone (from Benue to Sudan) exhibited strike-slip movements, generating pull-apart basins.These rift and fracture systems delimit three large blocks within the African plate: a Western block, an Arabian-Nubian block and an Austral block. The Arabian-Nubian block tends to separate from the two other blocks, migrating towards the north during the first stage of basin development and then towards the NE during the second stage. The opening of the Atlantic Ocean was the dominant driving force for the Western and Austral blocks while the Arabian-Nubian block probably moved in response to the opening of the Indian Ocean and to the evolution of the Tethyan margin.
Article
The geology of the Equatorial Atlantic is dominated by a broad east-west megashear belt where a cluster of large fracture zones offsets anomalously deep segments of the Mid-Atlantic Ridge (MAR). The origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum. The notion of a mantle thermal minimum in the Equatorial Atlantic is supported by an equatorial minimum of zero-age topography, a maximum in mantle shear waves seismic velocity and a minimum in the degree of melting, indicated by the chemistry of MAR basalts and peridotites. This thermal minimum has probably been a stable feature since before the Cretaceous separation of Africa from South America; it caused a pre-opening equatorial continental lithosphere thicker and colder than normal. The Cretaceous Benue Trough in western Africa and the Amazon depression in South America are interpreted as morphostructural depressions created or rejuvenated by strike-slip, transpressional and transtensional tectonics ducing extension of the cold/thick equatorial lithosphere. The oceanic rift propagating northward from the South Atlantic impinged against the equatorial thicker, colder and, therefore, stronger than normal continental, lithosphere that consequently acted as a ‘locked zone’. This, and a low magmatic budget due to the cold upper mantle, caused a lower than normal rate of propagation of the oceanic rift into the equatorial belt, with diffuse deformation during mostly amagmatic extension. The thick/cold lithosphere prevented major Cretaceous igneous activity from the St. Helena plume. Eventually initial ‘weak’ isolated nuclei oceanic lithosphere were emplaced, separated by E-W continent/continent transforms. Opening occurred largely by strike-slip motion along these initial transforms. The consequences were that the Equatorial Atlantic opened prevalently along an E-W direction, in contrast to the N-S opening of the North and South Atlantic, and that sheared continental margins are particularly well developed in the Equatorial Atlantic. After further continental separation the cold equatorial mantle caused a low degree of melting (with Na-rich MORB and alkali basalt rather than normal MORB and with undepleted mantle peridotities), thin crust, depressed ridge segments and a prevalence of amagmatic extension. Similar conditions still exist today. Long transforms offsetting short ridge segments kept sea floor spreading unstable and dominated by transform tectonics, with transform migration, transpression, and transtension causing strong vertical motion, emersion and subsidence of lithospheric blocks, development of deep pull-apart basins, and preservation of relict slivers of old lithosphere (occasionally even of continental lithosphere) within younger crust. The equatorial transforms are caused ultimately by a long lived thermal minimum in the upper mantle and not vice versa; however, they then create second-order ‘rebound’ thermal effects that help maintain the thermal minimum in the upper mantle. It can be speculated that mantle thermal minima at the Earth's equator might be related to true polar wander triggered by subduction of dense masses into the mantle.
Article
Mesozoic-Cenozoic magmatic activity in West and Central Africa is reviewed, with particular emphasis on the relationship between Mesozoic magmatism, major phases of continental rifting and the opening of the Equatorial Atlantic. It is suggested that during the initial stages of rifting, the activity of a mantle plume, the St. Helena hotspot, may have been important in weakening the lithosphere across the region. Evidence for magmatism concurrent with the onset of rifting in some basins supports such an active rifting model.Magma compositions range from alkali to tholeiitic basalts and their differentiates. Transitional to tholeiitic basalts are comparatively rare and are generated by greater degrees of partial melting, at probably shallower mantle depths, than associated alkali basalts. In some instances their occurrence may be correlated with higher amounts of lithospheric extension. However, in other instances they appear early in the rift sequence when overall amounts of extension were small. These tholeiitic basalts often have geochemical characteristics dominated by an ancient sub-continental lithosphere isotopic signature, which may have been introduced by crustal contamination.Cenozoic magmatism of alkaline affinity is widespread in West and Central Africa. In many instances, sites of activity appear to be structurally controlled by pre-existing basement fractures/lineaments of Mesozoic-Precambrian age. Most of the volcanic fields lie outside the boundaries of the Cretaceous rifts and many are associated with broad basement uplifts. However, there has also been a rejuvenation of tectono-magmatic activity within several of the Cretaceous rift basins during the Neogene. The parental magmas are considered to be generated mainly by partial melting of a zone at the base of the sub-continental lithosphere, which was variably metasomatised by the activity of mantle plumes beneath the African plate during the Mesozoic.
Article
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.
Article
A new reassembly of the continents around the North Atlantic Ocean is presented. The first criterion used for this reassembly is the identification of the structural framework related to the opening which consists of marginal fracture zones generated by offsets of the Rift. The Africa—North America, Eurasia—Greenland, Greenland—North America and Eurasia—North America adjustments are successively discussed. It is argued that the adjustments are best made at the 3000-meters isobath between Africa and North America and at the 2000-meters isobath for the younger rifts. The difference is attributed to subsidence and modification of continental margins with time. The importance of the Late Paleozoic tectonic phase in determining the subsequent pattern of Mesozoic rifting is emphasized.
Article
The last known acoustic “basement” outcrop along the eastern fossil extension of the Romanche Fracture Zone is located at about 06°30′W and 02°00′N, and lies 250 km west of the southwestern tip of the Ivory Coast-Ghana Ridge, between the Sierra Leone Abyssal Plain and the Guinea Basin, i.e., in the oceanic domain. The easternmost “basement” outcrop was surveyed by Seabeam during the 1988 equamarge II cruise of the R/V Jean Charcot, and rock samples were collected both by one dredge haul during the latter cruise, and by the submersible Nautile during the 1992 equanaute cruise. This paper presents the morphostructural interpretation of the Seabeam survey, the observations from the submersible, and the results of the petrological study of the rock samples. The latter appear to be detrital sediments which were subjected to a low-grade metamorphism in the greenschist facies conditions and a strong multiphase cataclasis. The rock samples derived from observed taluses and breccias are clasts of mainly meta-arkoses, with a few quartzites and meta-siltstones.
Article
Africa, by virtue of its central position within Gondwana, has recorded much of the complex history of plate interactions which have progressively fragmented this supercontinent since the Early Mesozoic. Continental reconstructions reveal both a temporal and spatial relationship between the development of the continental margins of Africa and the formation of rifted sedimentary basins deep within the African continent. The multi-stage opening of the Atlantic Ocean and associated rifting in West and Central Africa provide one of the most impressive examples of ocean-continent tectonic interactions. During the Early Cretaceous the onset of rifting along the future margins of the South Atlantic is contemporaneous with intra-continental rifting generating both strike-slip and extensional basins within West and Central Africa. The syn-rift phase of intra-continental basin development continued until the Santonian, by which time Africa and South America had been physically separated for approximately 10 Ma. Except for minor rifting during the Senonian and Tertiary, the short-lived phase of deformation at about 80 Ma marks the transition into the post-rift or “sag” phase of basin development. This deformational event can be correlated with a period of plate motion changes, recognised from fracture zone geometries, seen in both the Central and South Atlantic oceans. Using present day stress analogies, Cretaceous rifting in Africa can provide a means of indicating the regional palaeostress directions within Africa at this time.
Article
A metamorphic unit, consisting of amphibolites interlayered with felsic gneisses, is exposed on Zabargad Island, in tectonic contact with mantle-derived peridotites. The amphibolites contain relicts of igneous pyroxenites and gabbros which have recrystallized into mafic granulites. The chemistry of the gabbros-pyroxenites' primary assemblage (Al-augite ± Al-orthopyroxene + kaersutitic pargasite ± plagioclase) suggests that the gabbro-pyroxenites are remnants of a basic layered complex which crystallized at pressures not lower than 9–10 kbar and probably not higher than 12–13 kbar. The felsic gneisses were originally made of quartz + plagioclase + orthopyroxene ± garnet ± clinopyroxene ± magnesiohastingsite, an assemblage characteristic of high pressures (− 5–12 kbar) garnet granulite facies. The high-P tonalitic-trondhjemitic composition of some of the gneisses suggests they were originally part of the lower continental crust, an interpretation supported by the presence of characteristic zircon crystals with ovoidal morphology. The association of granulitic gneisses with high-P gabbroic rocks suggests that the lower continental crust was intruded by basaltic magmas. The pyroxenite-gabbro complex underwent deformation when still hot, followed by recrystallization at P of about 5–6 kbar. During this event, breakdown of the Al-pyroxenes under hydrous conditions resulted in granulitic Al-poor pyroxenes + pargasitic hornblende + plagioclase assemblages. Both the mafic and sialic granulites subsequently underwent further decompression under increasing fluid partial pressure and oxygen fugacity, which resulted in widespread amphibolization of the rocks in the presence of relatively high-T, chlorine-rich, metasomatic fluids. The resulting assemblage Mg-hornblende + plagioclase + ilmeno-hematite ± sphene ± quartz suggests T of 600–700°C, Ptot < 2 kbar and high . Finally, rising fluid partial pressure leads to breakdown of Mg-hornblende and crystallization of metasomatic phases such as alkali-richer, alumina-poorer Mg-hornblende, actinolite. Cl-biotite, Cl-scapolite and Cl-apatite. The Zabargad metamorphic unit probably results from underplating of the lower continental crust by basic magma intrusions before and during the early stages of rifting in the central Red Sea. Subsequently, the lower crustal gabbro-gneiss complex was subjected to a number of retrometamorphic events under decreasing P-T conditions, probably related to progressive uplift during rifting and crustal thinning. The last, low-P metasomatic event is probably related to the final stages of the emplacement of Zabargad Island.
Article
The Romanche transverse ridge (equatorial Atlantic) is located in the northern flank of the fracture zone, opposite the eastern ridge-transform intersection (RTI). It constitutes a major positive topographic anomaly that reaches a height of over 4 km above the level predicted by the thermal subsidence curve. A series of E-W aligned peaks, located on the crest of the transverse ridge, were at sea level during early and middle Miocene times; they are presently capped by a similar to 300 m thick, shallow-water carbonate platform that grew on a subsiding oceanic crust basement surface flattened by erosion at sea level. These structures are now about 1 km below sea level. High resolution seismic reflection surveys and multibeam morphobathymetry as well as study of samples recovered from the carbonate platform allowed a reconstruction of the paleoenvironment and of the vertical movements of the peaks starting from the lower Miocene. Ages derived from microfossils suggest that the base of the carbonate platform dates from 17-25 m.y. ago and the sinking of the platform started between 18 and 13 m.y. ago. These data were included in a numerical simulation model that takes into account thermal and tectonic subsidence, growth potential of the carbonates, subaerial and submarine erosion rates and Mio-Pliocene absolute sea level fluctuations. The results outline the subsidence history of the Romanche transverse ridge and suggest post-Miocene subsidence faster than that predicted by the thermal cooling model.
Paleocene and Eocene planktic foraminifera
  • M Toumarkine
  • H P Luterbacher
  • H M Bolli
  • J B Saunders
  • L Gasperini
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Ricerche micro-biostratigra®che sulla Maiolica della regione umbro-marchigi-ana Distributed shear, rotation, and partitioned strain along the San Andreas fault, central California The tectonic evolution of the South Atlantic from Late Jurassic to present Planktonic Stratigraphy
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  • M Poretti
  • M Chiocchini
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