A Biogeochemical Imprint of the Panama Basin in the North Andean Arc
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Abstract Oceans and continents mingle at convergent margins. However, the effects of this interaction in the construction and evolution of the continental crust remain poorly understood. Here we use geochemical data from the Panama Basin and the Northern Volcanic Province of Colombia to reveal that the oceanological and biogeochemical processes of a subducted ocean basin are imprinted in the compositions of continental arc volcanoes. The Panama Basin is a biologically highly productive area of the Eastern Equatorial Pacific in which the strongly biogenic sedimentation is reclassified and preserved differently depending on tectonically controlled depositional environments. Due to a shallow lysocline, sediments deposited on newly formed spreading centers are carbonate‐rich, whereas those accumulated on older subsiding seafloor become gradually richer in terrigenous components, organic carbon and authigenic U. Volcanoes of the North Volcanic Province of Colombia erupt high‐Mg# andesites that are common in some arcs, but display unusually high U contents and a symmetrical or “parabolic‐shaped” along‐arc trace element and isotopic variations that appear unrelated to differentiation or the pre‐existent crustal architecture. Instead, the parabolic‐shaped elemental trends mirror the reconstructed compositional variations of sediments deposited across axis on the currently subducted Sandra and Buenaventura ocean ridges. We interpret that subduction of these ocean ridges delivered a compositionally variable sediment influx that influenced the compositions of arc magmas. These findings demonstrate a strong connectivity between oceans and continents, and further imply that arc volcanoes can be reliable records of the oceanological and biogeochemical conditions of long subducted ocean basins.Keywords:
Volcanic arc
Convergent boundary
Terrigenous sediment
Andesites
Seafloor Spreading
Oceanic basin
Biogeochemical Cycle
Continental Margin
Abstract A broad continuum exists between two distinct end‐member types of mountain building. Alpine‐type orogenic belts develop during subduction of an ocean basin between two continental blocks, resulting in collision. They are characterized by an imbricate sequence of oceanward verging nappes; some Alpine belts exhibit superimposed late‐stage backthrusting. Sediments are chiefly platform carbonates and siliciclastics, in some cases associated with minor amounts of bimodal volcanics; pre‐existing granitic gneisses and related continental rocks constitute an autochthonous–parautochthonous basement. Metamorphism of deeply subducted portions of the orogen ranges from relatively high‐pressure (HP) to ultrahigh‐pressure (UHP). Calcalkaline volcanic–plutonic rocks are rare, and have peraluminous, S‐type bulk compositions. In contrast, Pacific‐type orogens develop within and landward from long‐sustained oceanic subduction zones. They consist of an outboard oceanic trench–accretionary prism, and an inboard continental margin–island arc. The oceanic assemblage consists of first‐cycle, in‐part mélanged volcaniclastics, and minor but widespread cherts ± deep‐water carbonates, intimately mixed with disaggregated ophiolites. The section recrystallized under HP conditions. Recumbent fold vergence is oceanward. A massive, slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable, non‐subducted plate. It consists of abundant, dominantly intermediate, metaluminous, I‐type volcanics resting on old crust; both assemblages are thrown into open folds, intruded by comagmatic I‐type granitoids, and metamorphosed locally to regionally under high‐ T , low‐ P conditions. In the subduction channel of collisional and outboard Circumpacific terranes, combined extension above and subduction below allows buoyancy‐driven ascent of ductile, thin‐aspect ratio slices of HP–UHP complexes to midcrustal levels, where most closely approached neutral buoyancy; exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume. At massive sialic buildups associated with convergent plate cuSPS (syntaxes), tectonic aneurysms may help transport HP–UHP complexes from mid‐ to upper‐crustal levels. The closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts, so volcanic–plutonic belts are poorly developed in Alpine orogens compared with Circumpacific convergent plate junctions. Generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation. Phase equilibrium studies show that, under typical subduction‐zone P–T trajectories, clinoamphibole ± Ca–Al hydrous silicates constitute the major hydroxyl‐bearing phases in deep‐seated metamorphic rocks of MORB composition; other hydrous minerals are of minor abundance. Ca and Na clinoamphiboles dehydrate at pressures of above approximately 2 GPa, but low‐temperature devolatilization may be delayed by pressure overstepping; thus metabasaltic blueschists and amphibolites expel H 2 O at melt‐generation depths, and commonly achieve stable eclogitic assemblages. Partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions. For reasonable subduction‐zone geothermal gradients however, white micas ± biotites remain stable to pressures >3 GPa. Accordingly, attending descent to depths of >100 km, mica‐rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of H 2 O, and transform incompletely to stable eclogite‐facies assemblages. Underflow of amphibolitized oceanic lithosphere thus generates most of the deep‐seated volatile flux, and the consequent partial melting to produce the calcalkaline suite, along and above a subduction zone; where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths, volatile flux severely diminishes. Thus, continental collision in general does not produce a volcanic–plutonic arc whereas in contrast, the long‐continued contemporaneous underflow of oceanic lithosphere does.
Accretionary wedge
Continental Margin
Adakite
Volcanic arc
Convergent boundary
Obduction
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The studies of continental deep subduction and ultrahigh-pressure metamorphism have not only promoted the development of solid earth science in China, but also provided an excellent opportunity to advance the plate tectonics theory. In view of the nature of subducted crust, two types of subduction and collision have been respectively recognized in nature. On one hand, the crustal subduction occurs due to underflow of either oceanic crust (Pacific type) or continental crust (Alpine type). On the other hand, the continental collision proceeds by arc-continent collision (Himalaya-Tibet type) or continent-continent collision (Dabie-Sulu type). The key issues in the future study of continental dynamics are the chemical changes and differential exhumation in continental deep subduction zones, and the temporal-spatial transition from oceanic subduction to continental subduction.
continental collision
Convergent boundary
Collision zone
Continental Margin
Eclogitization
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Continental Margin
Isostasy
Seafloor Spreading
Convergent boundary
Oceanic basin
Lithospheric flexure
Passive margin
Ridge push
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Convergent boundary
Continental Margin
Volcanic arc
Passive margin
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Seafloor Spreading
Continental Margin
Oceanic basin
Trough (economics)
Basement
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Abstract Oceans and continents mingle at convergent margins. However, the effects of this interaction in the construction and evolution of the continental crust remain poorly understood. Here we use geochemical data from the Panama Basin and the Northern Volcanic Province of Colombia to reveal that the oceanological and biogeochemical processes of a subducted ocean basin are imprinted in the compositions of continental arc volcanoes. The Panama Basin is a biologically highly productive area of the Eastern Equatorial Pacific in which the strongly biogenic sedimentation is reclassified and preserved differently depending on tectonically controlled depositional environments. Due to a shallow lysocline, sediments deposited on newly formed spreading centers are carbonate‐rich, whereas those accumulated on older subsiding seafloor become gradually richer in terrigenous components, organic carbon and authigenic U. Volcanoes of the North Volcanic Province of Colombia erupt high‐Mg# andesites that are common in some arcs, but display unusually high U contents and a symmetrical or “parabolic‐shaped” along‐arc trace element and isotopic variations that appear unrelated to differentiation or the pre‐existent crustal architecture. Instead, the parabolic‐shaped elemental trends mirror the reconstructed compositional variations of sediments deposited across axis on the currently subducted Sandra and Buenaventura ocean ridges. We interpret that subduction of these ocean ridges delivered a compositionally variable sediment influx that influenced the compositions of arc magmas. These findings demonstrate a strong connectivity between oceans and continents, and further imply that arc volcanoes can be reliable records of the oceanological and biogeochemical conditions of long subducted ocean basins.
Volcanic arc
Convergent boundary
Terrigenous sediment
Andesites
Seafloor Spreading
Oceanic basin
Biogeochemical Cycle
Continental Margin
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Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent-ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental-crust domain is characterized by tilted fault blocks generally overlain by thick syn-rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8–10 s twtt. The thickness of the continental crust changes greatly across the basin, from ~20 to 25 km under the shelf and uppermost slope, to ~9–6 km under the lower slope. The oceanic-crust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a ~5–10 km wide zone where oceanic-type features directly abut or lap on continental-type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along-strike variation in the tectonic structure of the continental margin. The NE-most lines display ~20–40 km wide segments of intense faulting under the slope and associated continental-crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across a ~100–110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.
Continental Margin
Seafloor Spreading
Oceanic basin
Basement
Convergent boundary
Passive margin
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Continental deep subduction after the closure of large oceanic basins is commonly ascribed to the gravitational pull of the subducting oceanic slab. However, it is not clear how continental lithosphere adjacent to small oceanic basins was subducted to mantle depths. The Sesia Zone in the Western Alps provides an excellent target for exploration of subduction dynamics in such a tectonic setting. Here we report the first finding of coesite in a jadeite-bearing orthogneiss from the Sesia Zone, providing the first evidence for deep subduction of the continental crust to mantle depths for ultrahigh-pressure (UHP) metamorphism in this zone. Three coesite inclusions were identified by laser Raman spectroscopy in two garnet grains. Based on zircon U-Pb dating and trace element analysis, the UHP metamorphic age was constrained to be 76.0 ± 1.0 Ma. The phase equilibrium modeling yields peak metamorphic pressures of 2.8-3.3 GPa, demonstrating the continental deep subduction to mantle depths of >80 km. The subducted continental crust was a rifted hyperextended continental margin, which was converted to the passive continental margin during seafloor spreading and then deeply subducted during the oblique convergence between the Adria microplate and Eurasian plate in the Late Cretaceous. Because the slab pull could only play a limited role in closing small oceanic basins for continental collision, the distal push of either continental breakup or seafloor spreading is suggested as the major driving force for the deep subduction of continental crust in the Western Alps. Therefore, deep subduction of the continental crust bordering small oceanic basins would have been induced by the far-field stress of compression, whereas that bordering large oceanic basins was spontaneous due to the oceanic slab pull. This provides a new insight into the geodynamic mechanism of continental deep subduction.
Continental Margin
Seafloor Spreading
continental collision
Convergent boundary
Eclogitization
Collision zone
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