Fate of sediments on the descending plate at convergent margins

1981 ◽  
Vol 301 (1461) ◽  
pp. 233-251 ◽  
Keyword(s):  
Continental Crust ◽  
Sedimentary Cover ◽  
Indirect Evidence ◽  
Accretionary Prism ◽  
Oceanic Lithosphere ◽  
Silica Content ◽  
Isotopic Ratios ◽  
Convergent Margins ◽  
Arc Magmas ◽  
Sediment Cover

The evolution of the continents and of continental crust is strongly dependent on the trajectory of the sedimentary cover on the descending oceanic lithosphere at arc systems. Although direct calculations of accretion are not reliable, indirect evidence strongly suggests that most of the sediment cover is either accreted or underplated (subcreted) to the upper plate. This evidence includes the increased thickening of accretionary prism beneath parts of the inner trench slope that cannot be explained by deformation within the prism and by protolith composition both in subophiolitic metamorphics and in blueschist terrains. That a small fraction of this sediment cover is transported to depths of at least 100 km is demonstrated in several ways. Flux calculations of mass and selected elements through arc systems require addition of a few tens of metres of sediment to the arc magmas. Global correlations of variations between arc magma characteristics and regional geological parameters show: (1) a strong correlation between silica content of average arc magmas and thickness or maturity of crust in the upper plate, attributed to upper plate contamination; (2) regional variations in 87 Sr/ 86 Sr and Pb isotopic ratios of arc basalts that correlate spatially with isotopic ratios in the non-calcareous components of pelagic sediments. This correlation is argued to reflect the variation of terrigenous material in the basal pelagics that are involved in magma production. Subduction of continental crust to depths of 100 km, either as part of the descending plate or by tectonic erosion of the upper plate is not supported by these correlations. Recycling rates of continental crust by accretion and subcretion and of mantle by subduction of oceanic lithosphere in contemporary arcs are very large compared with the growth rate of continental crust, which appears similar to the magma production rates in arc systems. This present production rate of continental crust is very much smaller than that during early Earth history, and is compatible with Phanerozoic ocean freeboard changes. In addition, average SiO 2 and K 2 O contents of contemporary arc magmas are much lower than those estimated for mean continental crust, leading to the conclusion that the magmas being produced at active arcs cannot be used as a model for the development of most of the Earth’s continental areas.

scholarly journals SUBDUCTION EROSION AT PACIFIC-TYPE CONVERGENT MARGINS

2021 ◽  
Vol 40 (6) ◽  
pp. 3-19
Author(s):  
I.Yu. Safonova ◽  
◽  
А.I. Khanchuk ◽  
Keyword(s):  
Continental Crust ◽  
Driving Forces ◽  
Accretionary Prism ◽  
Convergent Margins ◽  
Island Arcs ◽  
The Pacific ◽  
Tectonic Erosion ◽  
Definition Of ◽  
Orogenic Belts ◽  
Oceanic Slab

The paper presents a review of processes of subduction or tectonic erosion at the Pacific-type convergent margins (PTCM) including definition of “tectonic erosion”, its triggers, driving forces and consequences. We review examples of tectonic erosion at the Circum-Pacific PTCMs and at the fossil PTCMs of the Paleo-Asian Ocean (PAO) currently hosted by the Central-Asian Orogenic Belt (CAOB). Recent geological and stratigraphic studies have shown two types of PTCMs: accreting and eroding. Accreting PTCMs consist of older deposits of accretionary and frontal prisms and grow oceanward, i.e. the trench retreats. Eroding PTCMs are characterized by the destruction of the prism, approaching arc and trench and typically form during shallow-angle and fast subduction of an oceanic slab with oceanic floor topographic highs. The mechanism of tectonic erosion includes destruction of oceanic slab, island arcs, accretionary prism, fore-arc and related prism. Tectonic erosion is a common phenomenon at many Circum-Pacific PTCMs, e.g., in South America, Tonga and Nankai troughs, Alaska. Accretion and subduction of oceanic rises contributes greatly to the processes of formation, transformation and destruction of continental crust at PTCM. The episodes of tectonic erosion can be also reconstructed for an ancient ocean, for example, for the PAO, which evolution and suturing formed the CAOB. Many CAOB foldbelts (Altai, Tienshan, eastern Kazakhstan, Transbaikalia, Mongolia) carry signs of disap-pearance of big volumes of continental crust (arcs). Studying processes responsible not only for the formation of continental crust, but also for the disappearance of big volumes of crustal mate-rial is important for correct evaluation of the nature of intra-continental orogenic belts, e.g., CAOB, and development of reliable tectonic models.

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

2020 ◽  
Vol 158 (1) ◽  
pp. 1-12
Author(s):  
Yildirim Dilek ◽  
Yujiro Ogawa
Keyword(s):  
East Asia ◽  
Subduction Zone ◽  
Accretionary Prism ◽  
Oceanic Lithosphere ◽  
Pacific Rim ◽  
Convergent Margins ◽  
Lithospheric Extension ◽  
Subduction Zone Processes ◽  
Crustal Exhumation ◽  
Se China

AbstractContinents grow mainly through magmatism, relamination, accretionary prism development, sediment underplating, tectonic accretion of seamounts, oceanic plateaus and oceanic lithosphere, and collisions of island arcs at convergent margins. The modern Pacific–Rim subduction zone environments present a natural laboratory to examine the nature of these processes. The papers in this special issue focus on the: (1) modern and ancient accretionary margins of Japan; (2) arc–continent collision zone in the Taiwan orogenic belt; (3) accreting versus non-accreting convergent margins of the Americas; and (4) several examples of ancient convergent margins of East Asia. Subduction erosion and sediment underplating are important processes, affecting the melt evolution of arc magmas by giving them special crustal isotopic characteristics. Oblique arc–continent collisions cause strong deformation partitioning that results in orogen-parallel extension, crustal exhumation and wrench faulting in the hinterland, and thrust faulting–folding in the foreland. Trench-parallel widths of subducting slabs exert major control on slab geometries, the degree of coupling–decoupling between the lower and upper plates, and subduction velocity partitioning. An initially large width of the subducting Palaeo-Pacific Plate against East Asia caused flat subduction and resistance to slab rollback during the Triassic Period. These conditions resulted in shortening across SE China. Foundering and delamination of the flat slab during the Early Jurassic Epoch led to slab segmentation and reduced slab widths, followed by slab steepening and rollback. This pull-away tectonics induced lithospheric extension and magmatism in SE China during Late Jurassic – Cretaceous time. Melting of subducted carbonaceous sediments commonly produces networks of silicate veins in CLM that may subsequently undergo partial melting, producing ultrapotassic magmas.

Relative roles of source composition, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks

1984 ◽  
Vol 310 (1514) ◽  
pp. 675-692 ◽  
Keyword(s):  
Subduction Zone ◽  
Continental Crust ◽  
Fractional Crystallization ◽  
Magma Mixing ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Oceanic Lithosphere ◽  
Alkaline Basalt ◽  
Complex Process ◽  
Heterogeneous Mantle

There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt—andesite-dacite-rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt-trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of ‘within-plate’ character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust.

Geodynamics of the France Southeast Basin

2010 ◽  
Vol 181 (6) ◽  
pp. 477-501 ◽  
Author(s):  
Xavier Le Pichon ◽  
Claude Rangin ◽  
Youri Hamon ◽  
Nicolas Loget ◽  
Jin Ying Lin ◽  
...  
Keyword(s):  
Seismic Activity ◽  
Sedimentary Cover ◽  
Central Basin ◽  
The South ◽  
Western Basin ◽  
Active Deformation ◽  
The North ◽  
The Alps ◽  
Sediment Cover ◽  
Surrounding Areas

AbstractWe investigate the geodynamics of the Southeast Basin with the help of maps of the basement and of major sedimentary horizons based on available seismic reflection profiles and drill holes. We also present a study of the seismicity along the Middle Durance fault. The present seismic activity of the SE Basin cannot be attributed to the Africa/Eurasia shortening since spatial geodesy demonstrates that there is no significant motion of Corsica-Sardinia with respect to Eurasia and since gravitational collapse of the Alps has characterized the last few millions years. Our study demonstrates that the basement of this 140 by 200 km Triassic basin has been essentially undeformed since its formation, most probably because of the hardening of the cooling lithosphere after its 50% thinning during the Triassic distension. The regional geodynamics are thus dominated by the interaction of this rigid unit with the surrounding zones of active deformation. The 12 km thick Mesozoic sediment cover includes at its base an up to 4 km thick mostly evaporitic Triassic layer that is hot and consequently highly fluid. The sedimentary cover is thus decoupled from the basement. As a result, the sedimentary cover does not have enough strength to produce reliefs exceeding about 500 to 750 m. That the deformation and seismicity affecting the basin are the results of cover tectonics is confirmed by the fact that seismic activity in the basin only affects the sedimentary cover. Based on our mapping of the structure of the basin, we propose a simple mechanism accounting for the Neogene deformation of the sedimentary cover. The formation of the higher Alps has first resulted to the north in the shortening of the Diois-Baronnies sedimentary cover that elevated the top of Jurassic horizons by about 4 km with respect to surrounding areas to the south and west. There was thus passage from a brittle-ductile basement decollement within the higher Alps to an evaporitic decollement within the Diois-Baronnies. This shortening and consequent elevation finally induced the southward motion of the basin cover south of the Lure mountain during and after the Middle Miocene. This southward motion was absorbed by the formation of the Luberon and Trévaresse mountains to the south. To the east of the Durance fault, there is no large sediment cover. The seismicity there, is related to the absorption of the Alps collapse within the basement itself. To the west of the Salon-Cavaillon fault, on the other hand, gravity induces a NNE motion of the sedimentary cover with extension to the south and shortening to the north near Mont Ventoux. When considering the seismicity of this area, it is thus important to distinguish between the western Basin panel, west of the Salon-Cavaillon fault affected by very slow NNE gliding of the sedimentary cover, with extension to the south and shortening to the north; the central Basin panel west of the Durance fault with S gliding of the sedimentary cover and increasing shortening to the south; and finally the basement panel east of the Durance fault with intrabasement absorption of the Alps collapse through strike-slip and thrust faults.

Rift inheritance controls the switch from thin- to thick-skinned thrusting and basal décollement re-localization at the subduction-to-collision transition

2021 ◽  
Author(s):  
Stefano Tavani ◽  
Pablo Granado ◽  
Amerigo Corradetti ◽  
Giovanni Camanni ◽  
Gianluca Vignaroli ◽  
...  
Keyword(s):  
Sedimentary Cover ◽  
Complete Removal ◽  
Lower Plate ◽  
Accretionary Wedge ◽  
Convergent Margins ◽  
Middle Crust ◽  
Rifted Margins ◽  
Thrust System ◽  
Structural Levels ◽  
Rifted Margin

In accretionary convergent margins, the subduction interface is formed by a lower plate décollement above which sediments are scraped off and incorporated into the accretionary wedge. During subduction, the basal décollement is typically located within or at the base of the sedimentary pile. However, the transition to collision implies the accretion of the lower plate continental crust and deformation of its inherited rifted margin architecture. During this stage, the basal décollement may remain confined to shallow structural levels as during subduction or re-localize into the lower plate middle-lower crust. Modes and timing of such re-localization are still poorly understood. We present cases from the Zagros, Apennines, Oman, and Taiwan belts, all of which involve a former rifted margin and point to a marked influence of inherited rift-related structures on the décollement re-localization. A deep décollement level occurs in the outer sectors of all of these belts, i.e., in the zone involving the proximal domain of pre-orogenic rift systems. Older—and shallower—décollement levels are preserved in the upper and inner zones of the tectonic pile, which include the base of the sedimentary cover of the distal portions of the former rifted margins. We propose that thinning of the ductile middle crust in the necking domains during rifting, and its complete removal in the hyperextended domains, hampered the development of deep-seated décollements during the inception of shortening. Progressive orogenic involvement of the proximal rift domains, where the ductile middle crust was preserved upon rifting, favors its reactivation as a décollement in the frontal portion of the thrust system. Such décollement eventually links to the main subduction interface, favoring underplating and the upward motion of internal metamorphic units, leading to their final emplacement onto the previously developed tectonic stack.

scholarly journals Constraining crustal silica on ancient Earth

2020 ◽  
Vol 117 (35) ◽  
pp. 21101-21107 ◽  
Author(s):  
C. Brenhin Keller ◽  
T. Mark Harrison
Keyword(s):  
Continental Crust ◽  
Plate Tectonics ◽  
Incompatible Element ◽  
Silica Content ◽  
Efficient Mechanism ◽  
Element Abundances ◽  
Terrigenous Sediments ◽  
Earth History ◽  
Crustal Composition ◽  
Earth’S Mantle

Accurately quantifying the composition of continental crust on Hadean and Archean Earth is critical to our understanding of the physiography, tectonics, and climate of our planet at the dawn of life. One longstanding paradigm involves the growth of a relatively mafic planetary crust over the first 1 to 2 billion years of Earth history, implying a lack of modern plate tectonics and a paucity of subaerial crust, and consequently lacking an efficient mechanism to regulate climate. Others have proposed a more uniformitarian view in which Archean and Hadean continents were only slightly more mafic than at present. Apart from complications in assessing early crustal composition introduced by crustal preservation and sampling biases, effects such as the secular cooling of Earth’s mantle and the biologically driven oxidation of Earth’s atmosphere have not been fully investigated. We find that the former complicates efforts to infer crustal silica from compatible or incompatible element abundances, while the latter undermines estimates of crustal silica content inferred from terrigenous sediments. Accounting for these complications, we find that the data are most parsimoniously explained by a model with nearly constant crustal silica since at least the early Archean.

scholarly journals The evolution of the Triassic-Jurassic Maliac oceanic lithosphere: insights from the supra-ophiolitic series of Othris (continental Greece)

2015 ◽  
Vol 186 (6) ◽  
pp. 399-411 ◽  
Author(s):  
Jacky Ferrière ◽  
Frank Chanier ◽  
Peter O. Baumgartner ◽  
Paulian Dumitrica ◽  
Martial Caridroit ◽  
...  
Keyword(s):  
Continental Crust ◽  
Late Jurassic ◽  
Structural Data ◽  
Oceanic Lithosphere ◽  
Late Triassic ◽  
Major Result ◽  
Data Set ◽  
Thrust Sheets ◽  
Sedimentary Succession ◽  
Major Reference

AbstractMajor ophiolitic thrust sheets are widespread within the internal Hellenides, particularly in the Pelagonian domain (Greece and Albania). The ophiolitic sheets are notably well exposed in western Othris mountains of continental Greece. In that area, the structural stacking of oceanic nappes obducted in the Jurassic is particularly well constrained. New sedimentological and structural data from recently studied outcrops, together with new micro-paleontological data, allow to reconsider the architecture of the ophiolitic nappes and their evolution in the Othris mountains. Our new data set includes notably the description of a Mid-Late Jurassic sedimentary succession, from basal litharenites and radiolarites to syn-obduction mélange, on top of the uppermost Mega Isoma ophiolitic Unit. These results are crucial in the perspective of constraining the Jurassic contractional evolution of the Maliac Ocean from the beginning of the subduction and intra-oceanic obduction to the final obduction on the Pelagonian continental crust. Another major result concerns the dating of primary conformable series of Middle and Late Triassic age on top of the pillow-lavas of the Fourka unit. Since this lava unit, with MORB affinities, is one of the syn-obduction Jurassic nappes, we propose that this very large Fourka nappe represents the major reference unit of the initial (Triassic) Maliac oceanic crust.

Geochronological and geochemical evidence of continental crust ‘relamination’ in the origin of intermediate arc magmas

Lithos ◽  
2018 ◽  
Vol 322 ◽  
pp. 52-66 ◽  
Author(s):  
Arturo Gómez-Tuena ◽  
José G. Cavazos-Tovar ◽  
Mattia Parolari ◽  
Susanne M. Straub ◽  
Ramón Espinasa-Pereña
Keyword(s):  
Continental Crust ◽  
Geochemical Evidence ◽  
Arc Magmas

Solubility of magnetite-hematite assemblages in slab-derived saline fluids

2020 ◽  
Author(s):  
Carla Tiraboschi ◽  
Carmen Sanchez-Valle
Keyword(s):  
Subduction Zones ◽  
Microprobe Analysis ◽  
Mantle Wedge ◽  
Sedimentary Cover ◽  
Low Pressure ◽  
Oceanic Lithosphere ◽  
Major Elements ◽  
Piston Cylinder ◽  
Water Saturated ◽  
Subducting Slab

<p>In subduction zones, aqueous fluids derived from devolatilization processes of the oceanic lithosphere and its sedimentary cover, are major vectors of mass transfer from the slab to the mantle wedge and contribute to the recycling of elements and to their geochemical cycles. In this setting, assessing the mobility of redox sensitive elements, such as iron, can provide useful insights on the oxygen fugacity conditions of slab-derived fluid. However, the amount of iron mobilized by deep aqueous fluids and melts, is still poorly constrained.</p><p>We experimentally investigate the solubility of magnetite-hematite assemblages in water-saturated haplogranitic liquids, which represent the felsic melt produced by subducted eclogites. Experiments were conducted at 1 GPa and temperature ranging from 700 to 900 °C employing a piston cylinder apparatus. Single gold capsules were loaded with natural hematite, magnetite and synthetic haplogranite (Na<sub>0.56</sub>K<sub>0.38</sub>Al<sub>0.95</sub>Si<sub>5.19</sub>O<sub>12.2</sub>). Two sets of experiments were conducted: one with H<sub>2</sub>O-only fluids and the second one adding a 1.5 m H<sub>2</sub>O–NaCl solution. The capsule was kept frozen during welding to ensure no water loss. After quench, the presence of H<sub>2</sub>O in the quenched haplogranite glass was checked by Raman spectroscopy, while major elements were determined by microprobe analysis.</p><p>Preliminary results indicate that a significant amount of Fe is released from magnetite and hematite in hydrous melts, even at relatively low-pressure conditions. At 1 GPa the FeO<sub>tot</sub> quenched in the haplogranite glass ranges from 0.60 wt% at 700 °C, to 1.87 wt% at 900 °C. In the presence of NaCl, we observed an increase in the amount of iron quenched in the glass (e.g., at 800 °C from 1.04 wt% to 1.56 wt% of FeO<sub>tot</sub>). Our results suggest that hydrous melts can effectively mobilize iron even at low-pressure conditions and represent a valid agent for the cycling of iron from the subducting slab to the mantle wedge.</p>

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