Opening Of Basin Research Articles

Possible Spatial Distribution of the Mesozoic Volcanic Arc in the Present‐Day South China Sea Continental Margin and Its Tectonic Implications

AbstractThe distribution of the Late Mesozoic volcanic arc in the South China Sea (SCS) continental margin has long been a controversial topic due to its significance in understanding the transition mechanism of a margin from subduction to extension. Here a comprehensive analysis was conducted in the margin using reprocessed magnetic data, newly collected drilling/dredging samples, depositional environment, and deformation style inferred from multichannel seismic profiles to jointly constrain the possible distribution of the Mesozoic volcanic arc. From the map of reduced‐to‐the‐pole magnetic anomaly, several high‐positive magnetic anomaly belts can be discriminated, which cross the central Pearl River Mouth Basin, extend southwestward to the Zhongsha (Macclesfield Bank) and Xisha Islands (Paracel Islands), and distribute discontinuously around the southwest subbasin. Geophysical analysis shows that these belts have similar amplitudes and magnetic depths to known volcanic arcs, such as the Luzon, Cagayan, and Sulu arcs. Furthermore, the high‐amplitude positive magnetic anomaly belts coincide with the distribution of Late Mesozoic arc‐like granites, intermediate rocks, and agglomerates, suggesting that the belts possibly originated from the existence of Late Mesozoic volcanic arc. Accretion and compression environment located in front of the inferred arc provides independent supports to our interpretation. Results indicate that the southwest part of the arc is distributed on both sides of the southwest SCS subbasin, whereas the northeast part remains nearly in its original location, further suggesting that the breakup locations for the SCS margin might be the volcanic front/forearc in the northeast and the arc in the southwest during the opening of the SCS basin.

Age and setting of Permian Slide Mountain terrane ophiolitic ultramafic-mafic complexes in the Yukon: Implications for late Paleozoic-early Mesozoic tectonic models in the northern Canadian Cordillera

The Yukon Tanana (YTT) and Slide Mountain terranes (SMT) of the Cordillera in Canada and Alaska were interpreted in terms of opening and closing of a Late Devonian-Permian Japan Sea-style backarc basin behind a continental arc built upon YTT, which rifted from Laurentia during the Famennian-early Mississippian. Formation of Famennian transitional oceanic lithosphere supports rifting, but a combination of existing and new data on the setting and age of SMT ophiolites do not support the Japan Sea model.The studied Clinton Creek and Midnight Dome complexes represent suprasubduction zone ophiolites formed at ca. 265 Ma, consistent with analyses of other SMT ophiolites. Ultramafic rocks dominate most ophiolites. They lack sheeted dikes and contain relatively minor volumes of mafic plutonic and volcanic rocks, suggesting they formed in oceanic core complexes characterised by slow spreading and low magma productivity. The Permian ophiolites formed during or immediately after eclogite formation in YTT, coeval with or immediately preceding emplacement of orogenic peridotites into YTT due to hyperextension. Several tectonic scenarios are discussed. We propose that YTT is a composite terrane comprising a continental block and an oceanic arc-backarc complex with the latter obducted onto the former during the middle Permian-early Triassic Klondike orogeny. Obduction may have come from the west or east, but east-directed obduction is most consistent with geological constraints. Obduction was followed by initiation of west-dipping subduction east of the composite YTT; slab roll back causing extension in the composite upper plate, leading to exhumation of orogenic peridotites. Tectonic relationships show many analogies to the collision between Australia and the New Britain arc, in which collision in the Huon Peninsula of New Guinea is contemporaneous with extension in Australian crust in the adjacent Woodlark basin. Syn-orogenic Permian Klondike calc-alkaline magmatism is attributed to extension in a Woodlark basin-like setting rather than a representing a continental arc.

Tectonic vicariance versus Messinian dispersal in western Mediterranean ground beetles

AbstractThe complex geological history of the western Mediterranean region conceals the interpretation of the evolutionary history of its current fauna, as similar distribution patterns may have very different temporal and geographic origins. Particularly intriguing are some subterranean species in islands, which origin is usually difficult to interpret as their strongly modified morphologies obscure their relationships. We studied subterranean taxa and their likely relatives of two groups of ground beetles in the western Mediterranean: the Duvalius lineage (“isotopic” Trechini) and Molopina (Pterostichini). We included specimens from the islands of Mallorca, Sardinia and Sicily, plus mainland Europe and North Africa. Phylogenetic relationships were reconstructed with a combination of mitochondrial and nuclear data, and divergence dates were estimated with Bayesian methods using the same a priori molecular evolutionary rates for the same gene fragments in the two groups. In the Duvalius lineage, the subgenus Trechopsis, including all the highly modified cave or nivicolous species, was found to be polyphyletic: the species from Mallorca was found to be of Pleistocene origin and sister to the less modified species of subgenus Duvalius from the same island, whereas the Algerian species of Trechopsis were, on the contrary, related to the Sicilian Duvalius, indicating a northern colonisation route during the late Pliocene. Molopina was divided into three main lineages: the genera Abax, Percus, and the Molops groups of genera. The basal diversification of the latter was dated within a temporal window (35–25 Ma) fully congruent with the tectonic opening of the western Mediterranean basin and included six main lineages with uncertain relationships among them: the epigean genera (a) Molops and (b) Tanythrix; and the subterranean (c) Typhlochoromus (Eastern Alps), (d) Speomolops (Sardinia), (e) Henrotius (Mallorca) and (f) a strongly supported clade including the Pyrenean genera Zariquieya, Oscadytes and Molopidius. Despite the similar distribution of some of their subterranean taxa, the two studied groups show a strongly contrasting origin and mode of diversification. While the Duvalius lineage had a recent origin, with complex colonisation patterns and widespread morphological convergence among the subterranean species, the subterranean Molopina had an ancient vicariant origin resulting from the tectonic opening of the western Mediterranean basin.

Miocene basin opening in relation to the north-eastward tectonic extrusion of the ALCAPA Mega-Unit

AbstractThe opening and evolution of the Western Carpathians Miocene basins was closely related to the north-eastward tectonic extrusion of the ALCAPA Mega-Unit lithosphere caused by the final stage of collision of the Eastern Alpine–Western Carpathian orogenic system with the European Platform and Alpine convergence with the Adria plate. The roll back effect of the oceanic or thinned continental crust of the Magura–Krosno realms, subduction below the front of the Carpathians in the north-east, east and relative plate velocities led to gradual stretching of the overriding micro-plates (defined as the ALCAPA and Tisza Dacia Mega-Unit). Diverse movement trajectories of the ALCAPA crustal wedge individual segments (Eastern Alps, Western Carpathians, and Northern Pannonian domain) were accompanied by several counter-clockwise rotational phases. Beside the interpreted Early Miocene “en-block” counter-clockwise rotation, most of the rotations in the Central Western Carpathians were caused by “domino-effect tectonics” inside strike-slip zones and took part in the basin opening, which was in most cases followed by rapid subsidence.

Late Miocene Extensional Collapse of Northern Tunisia

AbstractTwo orthogonal extensional systems produced the extensional collapse of the Tell and Atlas thrust belts in northern Tunisia during the Late Miocene to Pliocene in a context of NW‐SE plate convergence between Africa and Eurasia. The older extensional system shows several low‐angle normal faults (LANFs) and associated high‐angle faults with ENE‐directed transport that produced half‐grabens and hanging‐wall syncline basins during the late Tortonian to Messinian. The direction of extension swinged towards the SE during the Messinian, cutting into, and tilting the previous detachments. Extension was accompanied by the extrusion of 8‐Ma rhyodacites and Messinian basalts, together with the development of mineralized fault breccias. Plio‐Quaternary NW‐SE directed shortening formed inversion arrowhead structures, reverse faults, refolded extensional rollover anticlines and folded the LANFs. ENE‐directed extension is concomitant with the opening of the Tyrrhenian basin. We consequently think that both processes are related and that tearing of the Calabrian slab along the northern Tunisia coast drove the ENE‐directed extension. Meanwhile, the SE‐directed extension that followed was probably related to SE‐directed peeling back of the Tunisian continental lithospheric mantle during NW subduction of the Maghrebian margin. This extension propagated eastwards from the late Tortonian until the Pliocene following the SE migrating subduction front and favored by lateral slab tearing along the Tunisian Atlas dextral Subduction Transfer Edge Propagator boundary. This new hypothesis for the tectonic evolution of northern Tunisia shows for the first time the importance of crustal extension in the denudation of the Tunisian Atlas and Tell foreland thrust belts and its relation to deep mantle tectonic mechanisms.

The Variscan evolution in the basement cobbles of the Permian Ponteranica Formation by microstructural and petrologic analysis

This work reconstructs the metamorphic evolution recorded in the cobbles of the Ponteranica Formation to be compared with the evolutions of the basements of the central Southern Alps and of the clasts of Aga and Vedello and Dosso dei Galli conglomerates. The Ponteranica Formation is part of the lower volcano-sedimentary cycle of the Permian sequences of the western Orobic basin. These sequences cap the Variscan basement in which different tectonometamorphic units were recognized and contoured. We sampled all the crystalline rock types preserved as clasts such as different types of metapelites, meta-intrusives, granitoids, and tourmalinites and reconstructed the relative chronology of the superposed equilibrium assemblages. Only for metapelitic rocks the PT paths were reconstructed on the ground of mineral chemical analyses and geothermobarometry. The cobbles of the Pontereanica Formation record two types of metamorphic evolution. In Type 1 cobbles, the M1 metamorphic assemblage is characterised by white mica, biotite, garnet, staurolite, ± kyanite and is interpreted as the result of the record of the Variscan collision. This assemblage is followed by M2 assemblage characterised by white mica, chlorite, ± biotite that represents the Variscan exhumation. In type 2 cobbles, relict andalusite and possible cordierite characterise the pre-M2 metamorphic re-equilibration that developed under a high thermal regime. The PT paths recorded in cobbles are consistent with the tectono-metamorphic evolutions recorded in the basement units suggesting that during the deposition of conglomerates, the type of tectono-metamorphic units exposed to erosion were equivalent to that presently exposed in the Orobic Alps.The evolution of type 1 cobbles is similar to the evolution recorded in the clasts of Aga and Vedello conglomerates, whereas the clasts of Dosso dei Galli Conglomerate recorded only the effects of the Variscan tectonic burial. The occurrence of tourmalinite cobbles may imply an age up to Upper Permian / Lower Triassic for the Ponteranica Formation. In this view, the age of the Permian conglomerates of central Southalpine decreases westward with the increase of the thermal maturity of the metamorphic imprints recorded in clasts. These observations are consistent with a westward migration of the extensional tectonics responsible for the basin opening.

Regional strike-slip and initial subsidence of Korea Plateau, East Sea: tectonic implications for the opening of back-arc basins

This study focuses on the initial subsidence mechanism of Korea Plateau in the East Sea. New interpretation of digitized multichannel seismic profiles (vertically unexaggerated) in the southern part of the South Korea Plateau delineates that acoustic basement comprises a number of normal-fault blocks, forming intra-plateau basins and troughs. In the western part of the plateau, the normal faults are oblique to the continental margin, whereas in the eastern part, they are bounded by an orthogonal transfer fault. The entire faults are suggestive of a large-scale dextral strike-slip fault system. The dextral fault system in the plateau was accompanied by a regional strike-slip fault along the North Korean continental margin, which offsets the prominent NE-SW granite belts on land and under the sea. The regional dextral strike-slip fault system activated during the late Oligocene/early Miocene to middle Miocene. The regional dextral strike-slip along the western continental margin of the sea was coupled with the complex dextral fault on the eastern margin from Sakhalin to Honshu. The regional strike-slip deformation was due to the changes in plate motion, i.e., southward drift (late Oligocene/early Miocene) and clockwise rotation (middle Miocene) of the Japanese Arc away from the Eurasian Plate.

Quantifying the erosional impact of a continental-scale drainage capture in the Duero Basin, northwest Iberia

AbstractFormerly closed drainage basins provide exceptional settings for quantifying fluvial incision and landscape dissection at different time scales. Endorheic basins trap all the sediment eroded within the watershed, which allows estimates of post–basin opening erosion patterns. The Duero Basin was a former closed basin and is presently drained by the Duero River into the Atlantic Ocean. During the Cenozoic, the basin experienced a long endorheic period, marked by the formation of continental carbonates and evaporites. The retrogressive erosion of the Atlantic drainage coming from the Portuguese coast subsequently captured the internal drainage, and significant fluvial dissection occurred. Presently, the basin contains a relatively well-preserved sedimentary fill. Gridding and surface fitting in this paper provide the first attempt to reconstruct the surface of the top of the former endorheic sedimentary sequence to quantify the erosional impact of the basin opening. At least 2251±524 km3of sediment was removed from the formerly closed basin following the start of exorheism. This volume represents a mean basin-surface lowering of 65±13 m. Erosion estimates and landscape dissection patterns are consistent with geologic evidence of progressive establishment of an outward drainage system.

Assembly and breakup of the Nuna supercontinent: Geodynamic constraints from 1800 to 1600 Ma sedimentary basins and basaltic magmatism in northern Australia

Basin formation from 1800 to 1640 Ma in northern Australia took place in a backarc extensional upper plate setting accompanied by lithospheric thinning and voluminous but episodic basaltic magmatism. West-dipping subduction combined with oceanward retreat of an associated magmatic arc drove these processes and commenced after the separation of Laurentia, precluding any connection with eastern Australia from 1800 Ma until basin closure and re-accretion of the arc ca. 1650–1640 Ma. Early rift-related basaltic magmatism generated km-thick piles of interstratified 1785–1775 Ma shallow-water sediment and tholeiitic lava whose low- and high-Ti compositions bear some resemblance to basalts erupted during the earlier stages of Gondwana breakup. Later basalts evolved in a western Pacific-style marginal marine basin and exhibit compositions consistent with melting of a subcontinental mantle source that had either been metasomatically altered during flat-slab subduction or contaminated with crustal material during an earlier subduction-related event. From 1700 to 1670 Ma, the crust and underlying lithospheric mantle rapidly thinned, and basaltic magmatism increasingly occurred in a turbidite-dominated deep marine environment with concomitant changes towards more subdued subduction-related signatures in the east and less enrichment in incompatible elements. Coincidently, granites emplaced from 1780 to 1670 Ma at deeper crustal levels were extensionally unroofed along with their country rocks. Backarc extension concluded no later than 1655 Ma by which time MORB-like magmas were being sourced directly from the asthenosphere and northern Australia had transitioned from an active to passive continental margin lying along the inboard side of the marginal sea. With ongoing subduction and consumption of oceanic crust to the east, Laurentia began to encroach upon both the arc and Australian margin, resulting initially in collapse of the backarc basin and then continent–continent collision by ca. 1620 Ma. These events represent an important milestone in assembly of the Nuna supercontinent and mark the end of one complete Wilson cycle from the time that the two continents first separated until their re-amalgamation 150 Myr later. Timeframes for the opening and closure of ocean basins in southern Laurentia are similar, raising the possibility that the same convergent margin and backarc extensional processes may once have extended along strike into ancestral North America.

The effect of rheological approximations in 3-D numerical simulations of subduction and collision

Subduction and collision zones evolve differently from one another due to different rheological properties, different amounts of regional isostatic compensation, and the different mechanisms by which forces are applied to the convergent plates. The rheology of mantle and lithosphere is known to have the largest influence on the dynamics of subduction and continental collision. However, previous 3-D geodynamic models of subduction/collision processes have used various rheological approximations, making their results difficult to compare, since there is no clear understanding on the extent of these approximations on the dynamics. Here, we test the effect of rheological approximations in a geometrically simplified 3-D model in which the dynamics of subduction and collision is entirely driven by slab-pull (i.e., “free subduction”). The results exhibit a wide range of behaviors depending on the rheological law employed: from linear viscous (LV) to temperature-dependent visco-elasto-plastic (VEP) rheology that takes into account both diffusion and dislocation creep. Rheological approximations exert a strong effect on slab morphology, trench migration rates and topography in the upper plate. For example, we find that slab dynamics varies drastically between end member models. In LV approximations, slabs do not break-off, but instead buckle at the transition zone. The dynamics is dominated by low strain rates and diffuse stresses, and topography is controlled by lateral extrusion of material which homogenizes amplitudes in the upper plate. In simulations with non-linear VEP rheology, slab break-off occurs at the corners of the indenting continent, opening of a back-arc basin is fast and topography has a laterally heterogenous distribution of amplitudes. Opening of a back-arc basin is observed only in simulations with non-linear VEP rheology. When the rheology is approximated with multiple discrete layers, the dynamics also resembles more the non-linear VEP case.

The Upper Oligocene of Montgat (Catalan Coastal Ranges, Spain) : new age constrains to the western Mediterranean Basin opening.

The Oligocene deposits of Montgat are integrated in a small outcrop made up of Cenozoic and Mesozoic rocks located in the Garraf-Montnegre horst, close to the major Barcelona fault. The Oligocene of Montgat consists of detrital sediments of continental origin mainly deposited in alluvial fan environments; these deposits are folded and affected by thrusts and strike-slip faults. They can be divided in two lithostratigraphic units separated by a minor southwest-directed thrust: (i) the Turo de Montgat Unit composed of litharenites and lithorudites with high contents of quartz, feldspar, plutonic and limestone rock fragments; and (ii) the Pla de la Concordia Unit composed of calcilitharenites and calcilithorudites with high contents of dolosparite and dolomicrite rock fragments. The petrological composition of both units indicates that sediments were derived from the erosion of Triassic (Buntsandstein, Muschelkalk and Keuper facies), Jurassic and Lower Cretaceous rocks (Barremian to Aptian in age). Stratigraphic and petrological data suggest that these units correspond to two coalescent alluvial fans with a source area located northwestwards in the adjoining Collserola and Montnegre inner areas. Micromammal fossils (Archaeomys sp.) found in a mudstone layer of the Pla de la Concordia Unit assign a Chattian age (Late Oligocene) to the studied materials. Thus, the Montgat deposits are the youngest dated deposits affected by the contractional deformation that led to the development of the Catalan Intraplate Chain. Taking into account that the oldest syn-rift deposits in the Catalan Coastal Ranges are Aquitanian in age, this allows to precise that the change from a compressive to an extensional regime in this area took place during latest Oligocene-earliest Aquitanian times. This age indicates that the onset of crustal extension related to the opening of the western Mediterranean Basin started in southern France during latest Eocene-early Oligocene and propagated southwestward, affecting the Catalan Coastal Ranges and the northeastern part of the Valencia trough during the latest Chattian-earliest Aquitanian times.

Structures and Their Tectonic Implications of the Southern Part of Cuddapah Basin, Andhra Pradesh, India

Systematic structural observations in field outcrops can give clues to interpret orogenic events. Deformational structures studied in basement complexes as well as Cuddapah basinal sedimentary rocks indicate at least three events. The formation of N–S trending greenstone belts on gneissic basement complex is related to the oldest Kenoran orogeny. The later developed Hudsonian orogeny is related to opening of basin and numerous younger pluton as well as mafic dike emplacements. The Greenville or Eastern Ghats Orogeny is most intensively imprinted in the Cuddapah basin sedimentary rocks. The crescent shape of the Nallamalai fold belt in the east is due to the E–W compression related to thrusting followed by NNE–SSW compression. The E–W maximum compression (σ1) is related to the Eastern Ghats Orogeny, which is responsible for the development of several sympathetic E–W faults due to N–S extension (σ3). The formation of the Palaeo–Mesoproterozoic supercontinent Columbia is related to the E–W compression and the Napier Complex of east Antarctica connected with India occupying the indentation of the E part of the Cuddapah basin. Palaeostress study from the southern part of Cuddapah basin reveals that the initial E–W compression and associated fracturing produced E–W extensional regime, and subsequent NNE–SSW shear developed conjugate fracture set. However, the initial E–W compression was associated with the N–S extension component to develop broadly E–W parallel normal faults such as Vempalle fault and later strike-slip faults related to changes in the stress field. The proposed model explains the development and evolution of the Cuddapah basin as a new insight into the regional geodynamics.

Tectonic significance of the Late Carboniferous Zhunmubutai ophiolitic mélange from Xi‐Ujimqin, Inner Mongolia

The Zhunmubutai ophiolitic mélange is a newly discovered ophiolite suite located between the Solonker‐Linxi ophiolite belt and the Erenhot‐Hegenshan‐Xi‐Ujimqin ophiolite belt, the central part of Xilinhot Microcontinent (XLMC). It was composed of plagiogranite, pyroxene peridotite, pillow basaltic andesite, and associated mafic rocks and surrounded by a quartz diorite pluton and mélange zone containing sandstones and slates of Early Permian Shoushangou Formation. In this study, zircon grains from a quartz diorite and a plagiogranite yield 206Pb/238U ages of 313 ± 1 Ma (MSWD = 1.60, n = 20) and 328 ± 1 Ma (MSWD = 0.9, n = 93), respectively. Sixty‐four zircons from the slate yielded ages ranging from 307 to 2,313 Ma, and with a peak at 340 Ma. These data indicate the Zhunmubutai ophiolitic mélange formed 300 to 340 Ma. The Zhunmubutai plagiogranites that have low contents of K2O (0.07–0.33 wt.%), K2O/Na2O (0.02–0.09), and Rb/Sr (0.01–0.05) show an oceanic plagiogranites affinity. They are also characterized by low contents of TiO2, Nd, Ta, and significant LREE enrichment. Furthermore, the εHf (328 Ma) values range from 14.23 to 18.87. These features suggest that the plagiogranite was originated by the anatexis of amphibolites and with crustal contamination. Combined with previous studies, suggest that the Zhunmubutai ophiolitic mélange may have formed in an extensional environment during the early stages of opening of an ocean basin.

Age and geodynamic evolution of the Black Sea Basin: Tectonic evidences of rifting in Crimea

The timing and direction of opening of the Black Sea Basin are debated. However, parts of its margins were inverted during Cenozoic and can be studied onshore. The Crimean Mountains are located in the middle of the northern margin of the basin, and at the onshore prolongation of the mid-Black Sea High. We present the first detailed mapping of large striated normal faults in Crimea. These faults define graben structures that trend parallel to the continental margin. Kinematic analysis of the faults combined with new biostratigraphic data show that the syn-rift sequence is Valanginian to Late Albian in age. It consists of siliciclastic deposits with limestone olistoliths. In contrast, the post-rift Late Cretaceous carbonaceous sequence of Crimea is devoid of normal faults or olistoliths. It unconformably overlies the graben structures. The onset ages, and the trends of extension are quite similar in the northern (Crimea) and the southern (Turkey) inverted margins of the basin. The Early Cretaceous extension directions are normal to the mid-Black Sea High and the Black Sea margins. We conclude that rifting of Black Sea Basin occurred from the Valanginian to the Late Albian (∼39 Ma) and drifting during the Late Cretaceous. Based on the directions of rifting, on the lack of evidence of strike slip motions near the mid-Black Sea High, and on published paleomagnetic data, we propose that the Black Sea opened with rotations accommodated by transform faults at its western and eastern margins, as a response to asymmetric slab rollbacks of the Neo-Tethys plate. The inversion of the Crimean margin results from two successive shortening events: Early Eocene NE-SW compression, Eocene to Present SE-NW compression. Their timing support the idea that compressional stresses generated by continental collisions in Turkey were transmitted through the strong Black Sea lithosphere up to Crimea. • Early Cretaceous graben structures are mapped along the northern margin of the black Sea basin, in Crimea. • Fault kinematic analysis and biostratigraphic data indicate that the rifting of Black Sea Basin occurred from the Valanginian to the Albian (∼35 Ma). • We propose that Black Sea opened with rotations accommodated by transform faults as a response to asymmetric slab rollbacks of the Neo-Tethys plate. • Compressional stresses generated by continental collisions in Turkey were transmitted through the Black Sea lithosphere as soon as the Early Eocene.

New constraints on the age of the opening of the South Atlantic basin

Abstract Detailed, high quality, marine total field magnetic data recently acquired over parts of the South Atlantic Ocean off the southwestern margin of South Africa display a pattern of well-defined, NW-SE striking linear magnetic anomalies that can be traced with confidence over distances >150 km. The magnetic anomalies are interpreted to be M-series seafloor spreading anomalies M9 to M11, which are consistent with the initiation of seafloor spreading at approximately 135 Ma (Late Valanginian/Early Hauterivian). Seafloor spreading models indicate a more rapid (44 mm/yr) initial spreading phase between M11 and M4/M5 followed by slower (29 mm/yr) spreading from M4/M5 to M0. This two rate spreading model also matches M-series anomalies previously reported over the conjugate South American margin offshore Argentina where the rates are slightly ( Breaks in the continuity of the linear anomaly pattern, observed in map view, are oriented approximately NE-SW and are considered sites of possible fracture zones. One such discontinuity, which we have termed the “Cape Lineament” (CL), marks a significant change in crustal character and Cretaceous depositional history, as revealed by gravity data and seismic reflection data, respectively. Crust NW of the CL, in the Orange Basin, is characterized by greater thickness and the presence of seismically-imaged seaward dipping reflectors (SDRs) whereas SE of the CL the crust has a more “normal” oceanic thickness and SDRs are either absent or more limited in areal extent. Magnetic, gravity and seismic data were combined to develop a crustal model of the South African margin north of the CL that includes: (a) a region of rifted/attenuated continental crust landward of magnetic anomaly G, (b) a 60–80 km wide zone of substantially intruded, underplated thin continental crust overlain by SDRs between anomalies G and M11, and (c) a ∼90 km wide zone of thick oceanic crust associated with a wide intrusion zone and smoother seafloor spreading magnetic anomalies that thins progressively to normal oceanic crustal thickness at the seaward edge of the overlying SDRs. South of the CL, the change in crustal character from attenuated continental crust to normal thickness oceanic crust occurs over a much shorter distance of approximately 35–40 km, possibly indicating the diminished influence of magmatic material. Although linear magnetic anomalies are observed both NW and SE of CL, anomalies to the SE display a better correlation with those predicted by our seafloor spreading model. We have successfully reconstructed the positions of the African and South American margins south of 32°S at M11 time (i.e., ∼135 Ma) using a new rotation pole at 38.86°N, 31.46°W. This pole applies only to the southernmost margins of the Austral segment where M11 is observed. Further north at this time the margins are undergoing non-rigid deformation that includes both crustal extension and magmatic underplating.

Geochemistry of Early Paleozoic boninites from the Central Qilian block, Northwest China: Constraints on petrogenesis and back-arc basin development

Early Paleozoic boninites occur in the Central Qilian orogenic belt, Northwest China. Their petrogenesis provides insights into lithosphere process and tectonic evolution of the Qilian block. In this paper, we carry out a study of geochronological, geochemical and Sr-Nd isotopic compositions for the Early Paleozoic boninites in the Lajishan area of the Central Qilian block. The Lajishan boninites (∼483 Ma) have high Al2O3/TiO2 (36.7–64.7) and CaO/TiO2 (31.1–49.6) ratios, and high MgO (7.86–10.47 wt%), Cr (439–599 ppm) and Ni (104–130 ppm) contents, indicating that the boninites result from a refractory mantle source. They are depleted in high field-strength elements (HFSE) and enriched in large ion lithophile elements (LILE), coupled with slightly high initial 87Sr/86Sr values of 0.7059–0.7074 and low εNd(t) values of −1.05 to +2.66, indicating that the mantle source was metasomatized by subducted slab-derived components. We found that an assemblage of low-Ca group and high-Ca group boninites occurred in the Lajishan belt. The high-Ca group boninites were derived from relatively fertile mantle with slightly higher melting degree, whereas the low-Ca group boninites were generated by partial melting of more refractory mantle wedge peridotites with slightly lower melting degree. The assemblage of low-Ca group and high-Ca group boninites reveals that the low-Ca group boninites were generated by the further melting of the more refractory mantle source after the segregation of the high-Ca group boninitic magmas in response to the back-arc basin opening. In the light of reported boninites worldwide, a diagram of Zr/Y vs. CaO/Al2O3 is used to identify boninites in fore-arc and back-arc regions. We suggest that the Lajishan boninites represent the products of back-arc basin development in response to the northward subduction of the Qaidam-West Qinling ocean slab.

Deep structure of the continental margin and basin off Greater Kabylia, Algeria – New insights from wide-angle seismic data modeling and multichannel seismic interpretation

During the Algerian-French SPIRAL survey aimed at investigating the deep structure of the Algerian margin and basin, two coincident wide-angle and reflection seismic profiles were acquired in central Algeria, offshore Greater Kabylia, together with gravimetric, bathymetric and magnetic data. This ~260 km-long offshore-onshore profile spans the Balearic basin, the central Algerian margin and the Greater Kabylia block up to the southward limit of the internal zones onshore. Results are obtained from modeling and interpretation of the combined data sets.The Algerian basin offshore Greater Kabylia is floored by a thin oceanic crust (~4 km) with P-wave velocities ranging between 5.2 and 6.8 km/s. In the northern Hannibal High region, the atypical 3-layer crustal structure is interpreted as volcanic products stacked over a thin crust similar to that bordering the margin and related to Miocene post-accretion volcanism. These results support a two-step back-arc opening of the west-Algerian basin, comprising oceanic crust accretion during the first southward stage, and a magmatic and probably tectonic reworking of this young oceanic basement during the second, westward, opening phase. The structure of the central Algerian margin is that of a narrow (~70 km), magma-poor rifted margin, with a wider zone of distal thinned continental crust than on the other margin segments. There is no evidence for mantle exhumation in the sharp ocean-continent transition, but transcurrent movements during the second opening phase may have changed its initial geometry. The Plio-Quaternary inversion of the margin related to ongoing convergence between Africa and Eurasia is expressed by a blind thrust system under the margin rising toward the surface at the slope toe, and by an isostatic disequilibrium resulting from opposite flexures of two plates decoupled at the continental slope. This disequilibrium is likely responsible for the peculiar asymmetrical shape of the crustal neck that may thus be a characteristic feature of inverted rifted margins.

The classic Wilson cycle revisited

Abstract In the first application of the developing plate tectonic theory to the pre-Pangaea world 50 years ago, attempting to explain the origin of the Paleozoic Appalachian–Caledonian orogen, J. Tuzo Wilson asked the question: ‘Did the Atlantic close and then reopen?’. This question formed the basis of the concept of the Wilson cycle: ocean basins opening and closing to form a collisional mountain chain. The accordion-like motion of the continents bordering the Atlantic envisioned by Wilson in the 1960s, with proto-Appalachian Laurentia separating from Europe and Africa during the early Paleozoic in almost exactly the same position that it subsequently returned during the late Paleozoic amalgamation of Pangaea, now seems an unlikely scenario. We integrate the Paleozoic history of the continents bordering the present day basin of the North Atlantic Ocean with that of the southern continents to develop a radically revised picture of the classic Wilson cycle The concept of ocean basins opening and closing is retained, but the process we envisage also involves thousands of kilometres of mainly dextral motion parallel with the margins of the opposing Laurentia and Gondwanaland continents, as well as complex and prolonged tectonic interaction across an often narrow ocean basin, rather than the single collision suggested by Wilson.

Crustal characteristic variation in the central Yamato Basin, Japan Sea back-arc basin, deduced from seismic survey results

The crustal structure of the Yamato Bank, the central Yamato Basin, and the continental shelf in the southern Japan Sea back-arc basin is obtained based on a seismic survey using ocean bottom seismographs and seismic shot to elucidate the back-arc basin formation processes. The central Yamato Basin can be divided into three domains based on the crustal structure: the deep basin, the seamount, and the transition domains. In the deep basin domain, the crust without the sedimentary layer is about 12–13 km thick. Very few units have P-wave velocity of 5.4–6.0 km/s, which corresponds to the continental upper crust. In the seamount and transition domains, the crust without the sedimentary layer is about 12–16 km thick. The P-wave velocities of the upper and lower crusts differs among the deep basin, the seamount, and the transition domains. These results indicate that the central Yamato Basin displays crustal variability in different domains. The crust of the deep basin domain is oceanic in nature and suggests advanced back-arc basin development. The seamount domain might have been affected by volcanic activity after basin opening. In the transition domain, the crust comprises mixed characters of continental and oceanic crust. This crustal variation might represent the influence of different processes in the central Yamato Basin, suggesting that crustal development was influenced not only by back-arc opening processes but also by later volcanic activity. In the Yamato Bank and continental shelf, the upper crust has thickness of about 17–18 km and P-wave velocities of 3.3–4.1 to 6.6 km/s. The Yamato Bank and the continental shelf suggest a continental crustal character.

Source and petrogenesis of Paleoproterozoic meta-mafic rocks intruding into the North Liaohe Group: Implications for back-arc extension prior to the formation of the Jiao-Liao-Ji Belt, North China Craton

Meta-mafic rocks as sills, dykes and veins are widely distributed within the North Liaohe Group, Liaodong Peninsula, which consist mainly of meta-diabase/gabbro and (garnet) amphibolite with minor felsic clastic rocks as enclaves. Although a long-standing debate existed about the petrogenesis for the meta-mafic rocks, it is a critical judgement for using geochemistry and geochronology of these rocks to reveal the complicated tectonic evolution of the Paleoproterozoic Jiao-Liao-Ji Belt (JLJB). In this paper, a combined study of zircon U-Pb isotopic data, whole-rock geochemistry and Sm-Nd isotope analyses are reported for these rocks to investigate their sources, petrogenesis and tectonic settings. Abundant zircon U-Pb LA-ICP-MS age data reveal that the meta-mafic rocks have a magmatic emplacement age of ca. 2130 Ma with a metamorphic age of ca. 1878 Ma. These meta-mafic rocks have characteristic geochemical compositions with moderate SiO2 (46.68–53.90 wt%), low TiO2 (0.56–2.33 wt%) and variable total Fe2O3 (9.65–17.58 wt%), MgO (2.77–11.49 wt%), Cr (1.44–1580 ppm) and Ni (6.4–168 ppm) similar to evolved tholeiitic basalt. They exhibit enriched mid-ocean ridge basalt (E-MORB)-like geochemical features (e.g., trace element patterns) with highly variable εNd(t) values from −1.5 to +3.1, relative enrichment of light rare earth elements (LREEs) and depletion of high field strength elements (HFSEs; e.g., Nb, Ta, Zr, Hf, and Ti). The geochemical trends of major and trace elements and Nd isotopes suggest that the primary magma for these meta-mafic rocks were probably derived from partial melting of a depleted asthenospheric mantle in the spinel stability field, with extensive fractional crystallization of a three-phase assemblage of olivine, clinopyroxene and plagioclase, crustal assimilation (e.g., continental detritus), and limited metasomatism from subduction-related fluids and/or melts. In view of the widespread distribution of Paleoproterozoic magmatism, including the Liaoji granites, and the voluminous sedimentary rocks in the Liaodong Peninsula, a newly tectonic model has been proposed that a ca. 2.1 Ga oceanic plate subduction event induced a back-arc basin opening, which closed to form the JLJB in the period of ca. 1.9 Ga.