Subduction Of Oceanic Plateaus Research Articles

A Steep Slab at the Yap Trench Resulted from Subducting Oceanic Plateaus

Abstract Interaction of oceanic plateaus with trenches plays a vital role in subduction activities and tectonic evolutions. The Yap trench is a rare case of an oceanic plateau subduction system. However, the knowledge of the impacts of plateau-trench interaction on subduction activity is still insufficient, due to a lack of seismological observations. Using ocean-bottom seismometer data near the Yap trench from April 2016 to May 2017, we conduct seismicity analyses in the Yap subduction zone by utilizing a machine-learning algorithm and matched-filter detections. The pattern of seismicity in the Yap trench exhibits characteristics similar to typical active subduction zones. The seismicity delineates a steep subducting slab, which may have resulted from the blocking of the buoyant Caroline Plateau. The majority of earthquakes are shallower than 80 km in the event-detectable area in the Yap trench, much shallower than the potential slab depth of 350 km from the previous seismic tomography images.

Late Jurassic oceanic plateau subduction in the Bangong–Nujiang Tethyan Ocean of northern Tibet

Oceanic plateau accretion and subsequent flat-slab subduction in modern convergent settings have profoundly influenced the nature of subduction and mantle dynamics. However, evaluating similar impacts in ancient convergent settings, where oceanic plateaus have been subducted but geological records are limited, remains challenging. In this study, we present geochronological and geochemical data for a suite of ore-associated plutonic rocks from the Gaobaoyue area of northern Tibet. These rocks have zircon U–Pb ages of 152–146 Ma, with high Sr contents and Sr/Y and La/Yb ratios, low MgO, Yb, and Y contents, and depleted Sr–Nd–Hf isotopic compositions, consistent with an adakitic affinity that was generated by the partial melting of subducting oceanic crust. We compare the Late Jurassic adakitic magmatism with the spatiotemporal evolution of magmatism in northern Tibet to infer oceanic plateau subduction and subsequent flat-slab subduction in the Bangong–Nujiang Tethyan Ocean. This tectonic model explains (i) slab-derived adakitic magmatism, (ii) the observed lull in magmatic activity, (iii) intraplate compression and uplift, and (iv) subduction jump and initiation. We also propose that the subduction of heterogeneous oceanic crust (i.e., buoyant oceanic plateau subduction) provided favorable conditions for tectonic exhumation, vertical slab tearing, and the formation of Cu–Au deposits. Our findings not only have implications for establishing the fundamental process of oceanic plateau accretion in ancient subduction zones but also provide an alternative explanation for Late Jurassic complex tectonomagmatic activity in northern Tibet.

The relationship between the nature of margins and the subduction of oceanic plateaus: Insights from variations in the forearc basement and sediments along the New Guinea Trench in the West Pacific

The generation of erosive and accretionary margins has yet to be determined. Here, we explore the variations in the forearc structures of the New Guinea Trench (NGT) based on the latest high-quality multichannel seismic profiles perpendicular or subparallel to the New Guinea Trench axis. Our results reveal that for the domain of the oceanic crust subducted beneath the NGT (O-NGT), the thickness of the sedimentary strata decreases from the forearc sedimentary basin to the forearc slope. The basement is generally characterized by homogeneous intra-basement reflections, and few discontinuous, prominent layered reflections in the basement are identified. On the forearc slope, arcward-dipping fault-plane reflections have been identified within the basement, and an abrupt topographic decrease occurs in the eastern part of the forearc basement uplift. Widespread sedimentary sequences are identified as the present, transparent and horizontal seismic reflections with high continuities and low amplitudes. The sedimentary sequences show a large lateral variation of thickness and present folded features near the seaward-dipping faults due to the development of slump bodies. However, the sedimentary sequences in the forearc slope of the domain of the oceanic plateau subducted beneath the NGT (P-NGT) are obviously thicker than those of the O-NGT and present folded features. The stratigraphic thickness from the forearc sedimentary basin to the forearc slope clearly increases. Our initial results initially indicate that the NGT might constitute an internal reverse polarity subduction zone that changes from an erosive margin in the O-NGT to an accretionary margin in the P-NGT.

Subducted oceanic plateau fed crustal growth: Insights from Amdo dacites in central Tibetan Plateau

The oceanic lithosphere subduction has been attested to exert the first-order control on the formation of the continental crust, but the role of oceanic plateaus on the crustal growth is elusive. It is difficult to identify the component of the oceanic plateaus in the continental crust because of their disappearance after subduction into deep mantle or obduction to be a part of the ophiolite mélanges in the ancient orogenic belts. Amdo microcontinent in the central Tibetan Plateau is an ideal place to assess the role of oceanic plateaus on crustal growth as it preserves the well-exposed arc magmatic rocks and fragments of the Meso-Tethyan oceanic plateaus to its south. Here, we present an integrated investigation of zircon U Pb ages and Hf isotope, as well as whole-rock elements and Sr-Nd-Hf isotopes for a suit of dacites in the Amdo microcontinent. Zircon U Pb dating indicates that the Amdo dacites formed at ca. 117 Ma, postdated the major formation period of the Meso-Tethyan oceanic plateaus. The Amdo dacites have relatively high SiO 2 (63.45–66.82 wt%) and low Mg# values (20.0–36.1), belonging to calc-alkaline series. The zircon thermometer and oxybarometer give an estimated temperature and oxygen fugacity of crystallization as ca. 700–720 °C and ΔFMQ = + 0.98, respectively. They have low Y (5.51–6.61 ppm) and Yb (0.44–0.53 ppm) contents and high La/Yb ratios (28.4–30.3). Despite their low Sr/Y ratios (6.86–13.5) caused by a small degree of fractionation of plagioclase, the Amdo dacites have an adakitic geochemical affinity, which was most likely derived from the thickened continental lower crust. These dacites have more depleted Sr-Nd-Hf isotope compositions (whole-rock 87 Sr/ 86 Sr i = 0.709796–0.710563, ε Nd (t) = −3.27 to −2.82, ε Hf (t) = 4.20–4.83 and zircon ε Hf (t) = 4.2–8.8) than the Neoproterozoic and Cambrian orthogneisses, as well as the Jurassic magmatic rocks. Simple binary mixing modeling reveals that the Amdo dacites could represent the hybrid melts of 50–60% of the subducted Meso-Tethyan oceanic plateau materials with 50–40% of the Amdo ancient continental crustal materials. The decoupled whole-rock Nd Hf isotopes (Δε Hf (t) = 7.55–8.43) of the Amdo dacites could be explained by the addition of zircon-free sediments overlie on the oceanic plateaus. These results indicate that subduction of the Meso-Tethyan oceanic plateaus with/without the overlying sediments made a substantial contribution to continental crustal growth. In conjunction with previous studies, the subduction of the Meso-Tethyan oceanic plateaus may discontinuously occur for about 37 million years, thus highlighting the significance of subduction of the Meso-Tethyan oceanic plateaus during the secular evolution of the central Tibetan Plateau. • ~117 Ma Amdo dacites first reported here formed under arc magmas-like oxidized environment. • Amdo dacites were most likely derived from the thickened continental lower crust. • Amdo dacites represent a hybrid melt of the ancient crustal basement and oceanic plateau materials. • Subduction of the oceanic plateau has an enormous potential for crust growth.

Joint Geodynamic‐Geophysical Inversion Suggests Passive Subduction and Accretion of the Ontong Java Plateau

AbstractIn this study, we for the first time applied a joint geodynamic‐geophysical inversion approach to oceanic plateau subduction models, and compared the subduction style and corresponding topography and Bouguer gravity of two representative subduction scenarios with passive or active collision. We showed that the case of passive collision of the Ontong Java Plateau (OJP) crust better explains the topography, gravity, and seismic data than the active collision scenario. This implies that the OJP did not control the regional dynamics during the collisional process. We conclude that previous studies may have overestimated the role of the OJP in triggering subduction initiation, subduction polarity reversal, and even Pacific Plate rotation.

Genetic Relationship between Subduction of Slab Topographic Anomalies and Porphyry Deposit Formation: Insight from the Source and Evolution of Rio Blanco Magmas

AbstractThe Rio Blanco deposit, which is one of the largest porphyry Cu–Mo deposits in northern Peru, formed coevally with the subduction of the Inca Oceanic Plateau at 12–10 Ma. However, the genetic relationship between the subduction of oceanic plateaus and the porphyry deposit formation remains unclear. Igneous rocks emplaced at 23–12 Ma in northern Peru, including the Portachuela batholith (which hosts the Rio Blanco porphyry complex), are normal calc-alkaline to weakly adakitic. In comparison, the 12–8 Ma igneous rocks, including the ore-related Rio Blanco porphyry complex, have typical adakitic signatures, such as high Sr/Y ratios (up to 180) and LaN/YbN ratios (up to 32). The Rio Blanco igneous rocks (Portachuela batholith and Rio Blanco porphyry complex) have uniform zircon εHf(t) values (+0.3 ± 1.2) and δ18O values (6.5 ± 0.14‰). These geochemical characteristics indicate that the Rio Blanco igneous rocks evolved from mantle-derived parental melts in a long-lived, stable, homogeneous isotopic reservoir at the crust–mantle boundary. However, whereas both the Portachuela batholith and the Rio Blanco porphyry complex formed from hydrous parental magmas (>5 wt %; based on plagioclase hygrometry), the ones of the Rio Blanco porphyry complex seem to be more oxidized, hydrous, and sulfur-rich compared with the older batholitic rocks. Reverse zoning in plagioclase phenocrysts, with a systematic core–mantle–rim variation in An (anorthite) and Fe (total iron) contents, are common in the intermineralization rocks. The An content of the mantles of the plagioclase phenocrysts correlates positively with the Fe content, but in the rims, the An contents significantly decrease while Fe remains constant. The apatite inclusions in the mantles are richer in S (0.24 ± 0.06 wt %) and Cl (1.42 ± 0.32 wt %) than those in the phenocryst cores (S: 0.09 ± 0.07 wt %; Cl: 1.03 ± 0.56 wt %) and rims (S: 0.14 ± 0.09 wt %; Cl: 0.83 ± 0. 35 wt %). These systemic geochemical variations in the plagioclase phenocrysts suggest recharge by S- and Cl-rich melts followed by fluid exsolution. This magma recharge and subsequent fluid exsolution may have triggered porphyry Cu mineralization at Rio Blanco. The coincidence of timing between the geochemical transition and collision (initial subduction) of the Inca Oceanic Plateau with the South American plate may indicate a change in the tectonic regime to a compressional state of stress and a thickening of the crust during the collision. The tectonic transition would have facilitated the fractionation of mantle-derived magma in a deep crustal hot zone, resulting in oxidized, volatile-rich residual melts. Replenishment of the upper-crustal magma chamber by such volatile-rich magmas and the subsequent discharge of fluids are interpreted to be fundamental for porphyry Cu mineralization at Rio Blanco and plausibly for the formation of Late Miocene porphyry ore deposits in northern Peru in general.

Oceanic plateau subduction during closure of the Bangong-Nujiang Tethyan Ocean: Insights from central Tibetan volcanic rocks

Identification of arc magmatic rock associations in a subduction zone has important implications for specifically revealing the geodynamic evolution of the subduction system. The closure time of the Bangong-Nujiang Tethyan Ocean and the detailed subduction processes have been hotly debated, hindering our understanding of the tectonic evolution of central Tibet. Here, we investigated the ca. 110-104 Ma Gerze lavas (basalts, basaltic andesites, andesites, dacites, and rhyolites) in southern Qiangtang. Fusion of slab fluid-metasomatized mantle wedge could yield the basalts, and such basaltic magmas, if contaminated with ancient basement orthogneisses, could have formed the andesites. The basaltic andesites with high Nb and Nb/La are similar to the Nb-enriched arc basalts and probably originated from slab melt-metasomatized mantle. The dacites were generated by fractional crystallization of the subducted melange-derived intermediate magmas. The rhyolites have geochemical characteristics (high SiO2 and La/Yb; low MgO and Sr/Y) similar to those of Jamaican-type adakites and were possibly sourced from the subducted oceanic plateau at low pressures. The Gerze Jamaican-type adakites and Nb-enriched basalt association could imply intense slab-mantle interactions. The Gerze lava suites show clear arc affinities, indicating that oceanic subduction may have lasted until 100 Ma. Based on previous studies and a noticeable ca. 145-125 Ma magmatic lull in southern Qiangtang, we suggest that the Bangong-Nujiang oceanic subduction geodynamics involved normal subduction (170-145 Ma), flat subduction (145-125 Ma), and slab roll-back (125-101 Ma). Moreover, the flat subduction was most likely caused by subduction of the oceanic plateau. Therefore, we propose, for the first time, that Tethyan oceanic plateau subduction during the Early Cretaceous could explain the tectonic evolution of the Bangong-Nujiang Ocean and distinctive magmatism in southern Qiangtang, central Tibet.

Dynamics of terrane accretion during seaward continental drifting and oceanic subduction: Numerical modeling and implications for the Jurassic crustal growth of the Lhasa Terrane, Tibet

The oceanic plateau subduction and resulted terrane accretion have played significant roles in the continental crustal growth; however, the modes and dynamics of plateau accretion during subduction remains poorly understood. The oceanic subduction can be driven by the combined effects of slab pull, ridge push, as well as the possible overriding plate push. The latter one has been rarely studied, but can be the case during the multiple Tethyan slabs subduction and terranes accretion for the formation of Tibetan lithosphere in the Mesozoic. Using 2-D petrological-thermo-mechanical models, we systematically investigated the modes and dynamics of terrane accretion during the dominant seaward continental drifting and oceanic plateau subduction. The model results show that the plateau with continental crustal affinity favors accretion to the overriding plate during the subduction process. Three distinct terrane accretion modes are identified: frontal accretion, bottom accretion, and a transitional mode with both the frontal and bottom accretion. In contrast, the plateau with oceanic crustal affinity generally favors subduction and recycling into the deeper mantle, rather than accretion, which is mainly due to the higher density and possible eclogitization of the oceanic crustal materials during subduction. The numerical simulations further reveal that the controlling factor for the subduction style, i.e. flat versus steep subduction, is the age of the oceanic slab and the seaward overriding plate motion. The rheological properties and the convergence rate play minor roles in affecting the subducting slab angle. The model results are further compared with the Jurassic magmatic rock distribution and geochemical characteristics of the Lhasa Terrane in Tibet, which reveals (1) the transition from flat to steep subduction of the southward Bangong-Nujiang Tethyan slab; and (2) the mechanism of Jurassic crustal growth in the central Lhasa subterrane which may be strongly correlated with the bottom accretion of the subducted oceanic plateau.

Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska

Cenozoic strata from forearc basins in southern Alaska record deposition related to two different types of shallow subduction: Paleocene–Eocene spreading-ridge subduction and Oligocene–Recent oceanic plateau subduction. We use detrital zircon geochronology (n = 1368) and clast composition of conglomerate (n = 1068) to reconstruct the upper plate response to these two subduction events as recorded in forearc basin strata and modern river sediment. Following spreading-ridge subduction, the presence of Precambrian and Paleozoic detrital zircon ages in middle Eocene–lower Miocene arc-margin strata and Early Cretaceous ages in lower Miocene accretionary prism–margin strata indicates that sediment was transported to the basin from older terranes in interior Alaska and from the exhumed eastern part of the Cretaceous forearc system, respectively. By middle-late Miocene time, diminished abundances of these populations reflect shallow subduction of an oceanic plateau and associated exhumation that resulted in an overall contraction of the catchment area for the forearc depositional system. In the southern Alaska forearc basin system, upper plate processes associated with subduction of a spreading ridge resulted in an abrupt increase in the diversity of detrital zircon ages that reflect new sediment sources from far inboard regions. The detrital zircon signatures from strata deposited during oceanic plateau subduction record exhumation of the region above the flat slab, with the youngest detrital zircon population reflecting the last period of major arc activity prior to insertion of the flat slab. This study provides a foundation for new tectonic and provenance models of forearc basins that have been modified by shallow subduction processes, and may help to facilitate the use of U-Pb dating of detrital zircons to better understand basins that formed under changing geodynamic plate boundary conditions.

Rock uplift and exhumation of continental margins by the collision, accretion, and subduction of buoyant and topographically prominent oceanic crust

AbstractUnderstanding the causes of rock and surface uplift is important because they control the location of mountain building, depocenters, and drainage characteristics and can influence climate. Here we combine previous thermochronological data with field observations to determine the amount of exhumation, rock, and surface uplift that occurs in the upper plate of Central and South American subduction zones during the collision, accretion, and subduction of oceanic plateaus and aseismic ridges. The collision of buoyant and topographically prominent oceanic plateaus and ridges can drive at least 5 km of rock uplift within 2 Ma. Uplift appears to be an immediate response to collision and is generally independent of the slab dip. The amount of rock uplift is controlled mainly by excess topography associated with the ridge (ultimately linked to buoyancy) and erosion, while it is also influenced by the strength of the subduction interface related to the presence of volcanic asperities and overpressured sediments in the subduction channel. The quantity of exhumation is strongly dependant on climate‐induced erosion and the lifespan over which the topography is uplifted and supported. Sediment draining into the trench may leave the elevated ridge axis sediment starved, increasing the shear stresses at the ridge subduction interface, leading to positive feedback between ridge subduction, rock uplift, and exhumation. Trench‐parallel variations in exhumation have a direct impact on exploration paradigms for porphyry‐related metalliferous deposits, and it is likely that porphyry systems are completely eroded by the impingement of plateaus and aseismic ridges within temperate and tropical climates.

Influence of geometry and eclogitization on oceanic plateau subduction

Subduction is driven by the negative buoyancy of the slab, which depends on both the temperature and composition of the lithospheric plate. In the case of subduction of an oceanic plateau, larger thickness of the lower-density crust and harzburgite layers can locally decrease the negative buoyancy available to drive subduction. However, the ability of a plateau to have an impact on subduction depends on the total buoyancy of the slab, and therefore also depends on size of the plateau and the basalt to eclogite transition, which substantially increases the density of the crust. Using 2-D numerical finite element models of subduction, we investigate the role of eclogitization and oceanic plateau size (thickness and length) in the process of oceanic plateau subduction. Model results show that eclogitization of the crust substantially increases the chances that a plateau will be subducted because the extra buoyancy of larger plateaus is lost through the phase transition. For kinematically driven models, all plateaus are subducted regardless of the thickness or width. Extrapolation of these results to 3-D geometry shows that sufficient slab buoyancy is available to sustain subduction for narrow plateaus with slab widths 6–7 times the plateau width (depending on plateau thickness and length). In dynamically driven models, subduction of oceanic plateaus of various sizes can substantially slow subduction, and large plateaus (>200km long and >25km thick) can lead to break-off of the slab and underplating of the plateau.

Nazca–South America interactions and the late Eocene–late Oligocene flat‐slab episode in the central Andes

The most prominent features of the Andean range are the Altiplano and Puna plateaus, which were constructed by crustal shortening and uplift over the past ∼45 Myr. The early construction of these plateaus may have controlled subsequent growth of the orogen. Proposed models have suggested that an abrupt acceleration in relative motion between the Nazca plate and the South American plate at ∼30 Ma may have led to compression of the continent. However, the major plate motion change occurred at 25–23 Ma, and paleomagnetic rotations and crustal shortening of the Andean forearc require that the Arica Bend formed prior to about 25 Ma. Inferred history of flat‐slab subduction along the Altiplano section of the Andean margin and the structure of the adjacent South American cratonic shield combine to suggest an alternate scenario, based partly upon geodynamic models of oceanic‐continental plate interactions in subduction zones. We propose that central Andean tectonism may have been controlled by two distinct regimes of subduction: (1) oblique subduction along the central Andean margin during the late Eocene and Oligocene accompanied by downdip alignment with the center of the Amazonian Shield (flat‐slab activity in this phase of orogenesis may have been caused by a combination of cratonic root enhanced tectonics and oceanic plateau subduction) and (2) an abrupt transition to trench‐normal subduction after ∼25 Ma toward the more distal São Francisco Craton was accompanied by a return to normal angle subduction. Similar interactions are hypothesized to have occurred during the Laramide Orogeny in western North America.

Structural architecture of oceanic plateau subduction offshore Eastern Java and the potential implications for geohazards

The region offshore Eastern Java represents one of the few places where the early stage of oceanic plateau subduction is occurring. We study the little investigated Roo Rise oceanic plateau on the Indian plate, subducting beneath Eurasia. The presence of the abnormal bathymetric features entering the trench has a strong effect on the evolution of the subduction system, and causes additional challenges on the assessment of geohazard risks. We present integrated results of a refraction/wide-angle reflection tomography, gravity modelling, and multichannel reflection seismic imaging using data acquired in 2006 south of Java near 113°E. The composite structural model reveals the previously unresolved deep geometry of the oceanic plateau and the subduction zone. The oceanic plateau crust is on average 15 km thick and covers an area of about 100 000 km2. Within our profile the Roo Rise crustal thickness ranges between 18 and 12 km. The upper oceanic crust shows high degree of fracturing, suggesting heavy faulting. The forearc crust has an average thickness of 14 km, with a sharp increase to 33 km towards Java, as revealed by gravity modelling. The complex geometry of the backstop suggests two possible models for the structural formation within this segment of the margin: either accumulation of the Roo Rise crustal fragments above the backstop or alternatively uplift of the backstop caused by basal accumulation of crustal fragments. The subducting plateau is affecting the stress field within the accretionary complex and the backstop edge, which favours the initiation of large, potentially tsunamogenic earthquakes such as the 1994 Mw= 7.8 tsunamogenic event.

Horizontal subduction zones, convergence velocity and the building of the Andes

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene–Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene.

The role of oceanic plateau subduction in the Laramide orogeny

The cause of the Laramide phase of mountain building remains uncertain. Conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant Hess or Shatsky oceanic plateaux. Independent verification of these models has remained elusive, because the putative ridges or plateaux are no longer at the Earth’s surface. Inverse convection models have identified two prominent seismic anomalies on the recovered Farallon plate. Here we combine inverse convection models with reconstructions of plate motions, to show that these seismic anomalies coincide palaeogeographically with the restored positions of the Shatsky and Hess conjugate plateaux as they subducted beneath North America. Specifically, the distribution of Laramide crustal shortening events tracked the passage of the Shatsky conjugate beneath North America, whereas the effects of the Hess conjugate subduction were restricted to the northern Mexico foreland belt. We propose that continued subduction caused the oceanic crust to undergo the basalt–eclogite phase transformation, during which the Shatsky conjugate lost its extra buoyancy and was effectively removed. Increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward, followed by regional-scale surface rebound. We conclude that Laramide uplift resulted from the removal, rather than emplacement, of the Shatsky conjugate.

Cenozoic provenance history of synorogenic conglomerates in western Argentina (Famatina belt): Implications for Central Andean foreland development

In Famatina, at the modern northern flat-slab segment of the Central Andean belt, synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting. Stratigraphically constrained provenance analyses record a multi-event history of the Andean foreland and a complex pattern of exhumed uplands, which, in turn, controlled the depocenter history. Western and eastern sources can be differentiated. The western domain was composed of equigranular granitoids and lower and upper Paleozoic volcano-sedimentary and sedimentary units, whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower Paleozoic clastic rocks. The surrounding regions expose medium- to high-grade metamorphic rocks, which are not recorded in any Cenozoic conglomerate of Famatina. Unroofing sequences can be deduced from the compositional trends. Conglomerate composition and paleocurrent data show input from intraba-sinal sources and important participation of basement since the beginning of the Neogene. We infer an early Miocene broken foreland during deposition of the Del Creston Formation.A significant source inversion, with clasts supplied from the eastern basement domain, is recorded ∼2000 m from the base, within the middle Miocene Del Abra Formation; reactivation of western granitic thrust sheets occurred by the late middle Miocene and late Miocene, as indicated by provenance studies of the Santo Domingo and El Durazno Formations. In contrast to simple asymmetric foreland models, provenance analysis documents early participation of crystalline rocks, suggesting early development of intrafore-land basement highs within the southern Central Andes. At a regional scale, basement thrusting and associated intermontane basin development were diachronous. This interpretation is supported by broad propagation of basement thrusting from the northern Sierras Pampeanas (and Famatina) toward the central and eastern Sierras Pampeanas between early-middle Miocene and late Miocene–Pliocene time. Early Miocene reconstructions of the Juan Fernandez Ridge position on the Central Andean margin suggest that during this interval, shallow subduction was likely unrelated to oceanic plateau subduction under the Sierras Pampeanas.

Mesozoic episodic magmatism in South Korea and its tectonic implication

NE trending granitoid batholiths in South Korea are part of the huge Phanerozoic granitoid belt in east Asia. The crystallization ages of granitoid plutons show three episodes of Mesozoic magmatism in South Korea: Triassic (248–210 Ma), Jurassic (197–158 Ma), and Late Cretaceous–early Tertiary (110–50 Ma), with a Late Jurassic–Early Cretaceous (∼50 m.y.) hiatus. The granitoid ages and kinematic analyses of the Mesozoic structures in South Korea suggest that the Middle Jurassic (180–170 Ma) dextral strike‐slip ductile shear zones formed by the northwest (orthogonal) subduction of the Izanagi plate, whereas the Cretaceous sinistral faults formed by the northward (oblique) subduction of the same plate, which generally agree with proposed reconstruction of plate motions, albeit uncertain, around east Asia. Among the models proposed to explain magmatic lull in a continental arc, shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in South Korea and Japan. The huge width (∼500 km) of the east Asian granitoid belt may be explained by oceanward retreat of subduction zone in the Cretaceous in association with change in the movement direction of the subducting plate.

Slab dip vs. lithosphere age: No direct function

One paradigm of subduction relates the dip of the slab to the buoyancy of the downgoing lithosphere along subduction zones, with the negative buoyancy proportional to the age of the oceanic lithosphere. We measured the dip of the slab down to depths of 250 km along 164 sections crossing 13 subduction zones and compared it with the age of the subducting oceanic lithosphere both at the trench and at depth. We show here that this relationship is far more irregular than previously suggested, and that it is not possible to simply correlate the increase of the slab dip to the increasing age of the downgoing cooler lithosphere. Younger oceanic lithosphere may show steeper dip than older segments of slabs (e.g., Central America vs. South America), in contrast with predictions of models considering only slab pull. The combination of slab age and subduction rate better accounts for slab dip; however the correlation is not satisfactory (correlation coefficient equal to 0.450). These results suggest that supplemental forces or constraints have to be accounted for, such as thickness and shape of the hangingwall plate, absolute plate velocity, presence of lateral density variations in the hosting upper mantle, effects of accretion/erosion, subduction of oceanic plateaus and slab deformation due to the motion of the mantle relative to the subducting plate.

Structural and geochronological constraints of early Ross orogenic deformation in the Pensacola Mountains, Antarctica

During end-Neoproterozoic to early Paleozoic time, the southern margin of Gondwana was affected by widespread subduction-related orogenesis. In East Antarctica this event is known as the Ross orogeny, whose manifestation is exposed along the 3500 km length of the Transantarctic Mountains. In the Pensacola Mountains, the Ross orogeny is characterized by two contractional deformation events, which bracket a brief period of latest Middle Cambrian to early Late Cambrian extension and sedimentation. The oldest and most intense of these deformation events effected a succession of latest Neoproterozoic metagraywackes (Hannah Ridge Formation), producing tight to isoclinal F-1 folds that possess a slaty axial-planar cleavage. Steeply plunging asymmetric F-2 folds and their associated cleavage are superimposed upon the NNE-SSW D-1 structural grain, forming a consistent clockwise oblique angle. Together with several shear-sense indicators, the angular relationship of D-2 and D-1 suggests that the F-2 folds formed as a result of sinistral reactivation of D-1 structures. At Serpan Peak, new meso- and microstructural observations of the Serpan Peak granite suite reveal two phases of emplacement: an early granodioritic phase emplaced prior to the regional D-1 deformation event, and a later phase of syntectonic biotite granites emplaced during D-2. New U-Pb SHRIMP ages for three phases of the Serpan Peak granite suite constrain their emplacement-and, by structural relationship, the early Ross D-1 and D-2 deformation events-as having occurred at 505 +/- 5 Ma. The ages of D-1 and D-2 are identical within error, and we interpret them as being the product of a single progressive sinistral transpressional deformation event. Our new structural and geo-chronological constraints necessitate that deformation of the Hannah Ridge Formation occurred in conjunction with exhumation from lower greenschist conditions prior to deposition of the upper Middle Cambrian Nelson Limestone (< 10 m.y.). Exhumation was closely followed by ca. 500 Ma bimodal volcanism inferred to represent an extensional continental backarc setting. During the latest Cambrian-Early Ordovician, a second, less intense contractional deformation event deformed and inverted the backarc region. Our new data reveal that, along the Pensacola Mountains sector of East Antarctica, the Ross orogeny was manifested as a condensed sequence of rapidly changing tectonic events along a continuously subducting margin, characterized by switches in deformation mode within the continental margin. We interpret such tectonic switching to be the result of the intermittent arrival of buoyant oceanic plateaus at the Ross subduction zone, resulting in short-lived periods of flat-slab subduction and contractional deformation that punctuated the neutral or extensional strain regime generally associated with normal subduction. Given the limited length of plate margin likely to be affected by oceanic plateau subduction, specific correlation of stratigraphic and tectonic events along the entire length of the Ross orogen are unlikely. Nonetheless, similar patterns of punctuated tectonic events associated with sinistral transpressional deformation during the Ross orogeny are observed elsewhere along the orogen.

Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts

The sizes of continental blocks, basaltic oceanic plateaus, and island arcs that would cause collisional orogenesis when they enter a subduction zone are calculated in an analysis based upon the assumption of local isostasy and the assumption that plate subduction is primarily driven by the negative buoyancy of the lithosphere. Buoyancy analysis indicates that the bulk density contrast between 80-m.y.-old oceanic lithosphere capped by a 7-km-thick basaltic crust and the less dense underlying asthenosphere is on the order of 0.04 gm/cm 3 . Oceanic lithosphere that is ∼10 m.y. old is the youngest that is more dense than the asthenosphere and hence inherently susceptible to subduction. Subduction zone metamorphism causes the crustal layer of basalt/gabbro to transform into more dense amphibolite and eclogite. Where eclogite formation is extensive, the descending oceanic lithosphere increases in bulk density by as much as 0.04 gm/cm 3 . Lithosphere that is 100 km thick with a 30-km-thick granitic continental crust resists Subduction because it is ∼0.09 gm/cm 3 less dense than the asthenosphere. Contrasts in lithospheric bulk density (crust + mantle) of 3 are the difference between whether subduction is nearly inevitable (as for normal ocean crust) or greatly resisted (as for thick, ancient continents). Collisional orogenesis is defined as a plate interaction of the sort that causes a rearrangement of plate motions, generally with the initiation of a new subduction zone and the creation of mountains. Buoyancy analysis indicates that only bodies of continental and oceanic island are crust that are > ∼15 km thick make the lithosphere buoyant enough to jam a subduction zone. Oceanic island arc complexes built upon ocean crust typically must be active for more than ∼20 m.y. to attain crustal thicknesses so that their attempted subduction causes collisional orogenesis. Oceanic plateaus where basaltic crust as much as ∼17 km thick caps 100-km-thick lithosphere are inherenty subductable and actually less buoyant than normal oceanic lithosphere following subduction metamorphism. Basaltic plateaus must have crustal thicknesses >∼30 km to typically cause collisional orogenesis during subduction. Short subducting seamounts (