Onset Of Subduction Research Articles

The evolution of the Uralian orogen

Abstract The Uralian orogen is located along the western flank of a huge (>4000 km long) intracontinental Uralo-Mongolian mobile belt. The orogen developed mainly between the Late Devonian and the Late Permian, with a brief resumption of orogenic activity in the Lower Jurassic and Pliocene–Quaternary time. Although its evolution is commonly related to the Variscides of Western Europe, its very distinctive features argue against a simple geodynamic connection. To a first order, the evolution of Uralian orogen shows similarities with the ‘Wilson cycle’, beginning with epi-continental rifting (Late Cambrian–Lower Ordovician) followed by passive margin (since Middle Ordovician) development, onset of subduction and arc-related magmatism (Late Ordovician) followed by arc–continent collision (Late Devonian in the south and Early Carboniferous in the north) and continent–continent collision (beginning in the mid-Carboniferous). In detail, however, the Uralides preserve a number of rare features. Oceanic (Ordovician to Lower Devonian) and island–arc (Ordovician to Lower Carboniferous) complexes are particularly well preserved as is the foreland belt in the Southern Urals, which exhibits very limited shortening of deformed Mesoproterozoic to Permian sediments. Geophysical studies indicate the presence of ‘cold’, isostatically equilibrated root. Other characteristic features include a Silurian platinum-rich belt of subduction-related layered plutons, a simultaneous development of orogenic and rift-related magmatism, a succession of collisions that are both diachronous and oblique, and a single dominant stage of transpressive deformation after the Early Carboniferous. The end result is a pronounced bi-vergent structure. The Uralides are also characterized by Meso-Cenozoic post-orogenic stage and plume-related tectonics in Ordovician, Devonian and especially Triassic time. The evolution of the Uralides is consistent with the development and destruction of a Palaeouralian ocean to form part of a giant Uralo-Mongolian orogen, which involved an interaction of cratonic Baltica and Siberia with a young and rheologically weak Kazakhstanian continent. The Uralides are characterized by protracted and recurrent orogenesis, interrupted in the Triassic by tectonothermal activity associated with the Uralo-Siberian superplume.

Contrasting modes of supercontinent formation and the conundrum of Pangea

Repeated cycles of supercontinent amalgamation and dispersal have occurred since the Late Archean and have had a profound influence on the evolution of the Earth's crust, atmosphere, hydrosphere, and life. When a supercontinent breaks up, two geodynamically distinct tracts of oceanic lithosphere exist: relatively young interior ocean floor that develops between the dispersing continents, and relatively old exterior ocean floor, which surrounded the supercontinent before breakup. The geologic and Sm/Nd isotopic record suggests that supercontinents may form by two end-member mechanisms: introversion (e.g. Pangea), in which interior ocean floor is preferentially subducted, and extroversion (e.g. Pannotia), in which exterior ocean floor is preferentially subducted. The mechanisms responsible remain elusive. Top–down geodynamic models predict that supercontinents form by extroversion, explaining the formation of Pannotia in the latest Neoproterozoic, but not the formation of Pangea. Preliminary analysis indicates that the onset of subduction in the interior (Rheic) ocean in the early Paleozoic, which culminated in the amalgamation of Pangea, was coeval with a major change in the tectonic regime in the exterior (paleo-Pacific) ocean, suggesting a geodynamic linkage between these events. Sea level fall from the Late Ordovician to the Carboniferous suggests that the average elevation of the oceanic crust decreased in this time interval, implying that the average age of the oceanic lithosphere increased as the Rheic Ocean was contracting, and that subduction of relatively new Rheic Ocean lithosphere was favoured over the subduction of relatively old, paleo-Pacific lithosphere. A coeval increase in the rate of sea floor spreading is suggested by the relatively low initial 87Sr/ 86Sr in late Paleozoic ocean waters. We speculate that superplumes, perhaps driven by slab avalanche events, can occasionally overwhelm top–down geodynamics, imposing a geoid high over a pre-existing geoid low and causing the dispersing continents to reverse their directions to produce an introverted supercontinent.

Ductile deformation of passive margins: A new mechanism for subduction initiation

The onset of subduction at passive margins has been extensively investigated and debated. However, the force constellations and mechanisms that enable the development of a subduction system from a passive margin remain unclear. This study presents new insights into the conditions and processes by which lateral density differences between oceanic and continental lithospheres in passive margins may lead to initiation of a low‐angle subduction system. The presented study consists of (1) analytical calculations of flow fields generated in passive margins, and (2) analogue experiments of mature passive margins performed in a centrifuge. The analytical formulation predicts temporal and spatial evolution of an interface between the oceanic and continental lithospheres, and demonstrates that oceanic underthrusting may occur by rotation of this interface. The analogue experiments show that incipient subduction may develop by ductile deformation within the lithosphere, involving no sliding along the ocean‐continent interface, so that the frictional resistance between the plates need not be overcome. The force induced by the negative buoyancy of the oceanic plate with respect to the asthenosphere, is found to be in some cases irrelevant to subduction nucleation. Results of both the analogue experiments and the analytical calculation are compared to the south‐east Australian passive margin, and show an excellent fit to its geometry and stress distribution. The proposed mechanism is also applied to the only two cases of subduction of Atlantic tectonic system, the Lesser‐Antilles and the South Sandwich subduction systems.

Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma

We examine active deformation of the region surrounding the eastern South Caspian Sea using a combination of seismic, geological, geodetic, and geomorphological data. Global positioning system (GPS) velocities indicate a westward component of motion of the South Caspian basin, relative to Eurasia and central Iran. We identify major zones of shear (the Ashkabad and Shahrud fault zones) that accommodate this westward extrusion on either side of the South Caspian block. Estimates of total strike-slip motion could, at present-day slip rates derived from GPS observations, be achieved in ~10 m.y. Therefore, the northwest extrusion of the South Caspian region, which is accommodated by subduction beneath the North Caspian region (stable Eurasia) along the Apsheron-Balkan sill, may also date from that time. This suggests that the onset of subduction may be older than previous estimates (ranging from 1.8 to 5.5 Ma). Our results are summarized in a new kinematic model that significantly clarifies the active tectonics of this complicated region.

Tectonic evolution of the Triassic fold belts of Tibet

The Triassic fold belt of North Tibet is mainly composed, from west to east, of the Bayan Har, Songpan–Garzê, and Yidun (or Litang–Batang) terranes. The Indosinian orogeny results from interactions between the South China, North China and Qiangtang (North Tibet) blocks during the closure of the Palaeotethys. A synthesis of the tectonic and geochronological data available on this belt is presented and a new geodynamic model of its formation is proposed. At the end of the Permian, a synchronous activity along three subduction zones, Kunlun–Anyemaqen to the north, Jinsha to the south and Yushu–Batang to the east, induced the growth of wide accretionary orogens until the end of the Triassic period. The onset of subduction in Tibet is contemporaneous with Indosinian tectonism in Indochina (pre-Norian). However, the main tectonic events that lead to the closure of the Tethysian basin and the subsequent building of the Triassic belts are younger (220–200 Ma).

The dynamics of intra-oceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation

Dispersed blocks of various types of metamorphic rocks in serpentinite mélanges of the northern Dominican Republic (Hispaniola) provide fossil evidence for the dynamics of the subduction zone channel in the intra-oceanic Caribbean subduction zone system between 120 and 55 Ma. Comprehensive petrological and geochronological data on three exemplary samples of eclogite and blueschist are presented that allow a series of different but interrelated pressure–temperature–time paths to be delineated. Eclogites indicate a low P/T gradient during subduction and record conditions in the nascent stages of the subduction zone. Lu–Hf data yield 103.6 ± 2.7 Ma for peak metamorphic conditions of 23 kbar/750 °C. An anticlockwise P–T path is defined. Other blocks record the continuous cooling of the evolving subduction zone and show typical clockwise P–T-paths. Omphacite blueschists reach maximum P–T-conditions of 17–18 kbar/520 °C at 80.3 ± 1.1 Ma (Rb–Sr age data). The mature subduction zone is typified by jadeite blueschists recording very high (“cold”) P/T gradients. A Rb–Sr age of 62.1 ± 1.4 Ma dates peak metamorphic P–T conditions at 16–18 kbar/340–380 °C. The array of P–T–t data allows overall cooling rates of the subduction zone at depths of c. 60 km to be constrained at 9 °C/Ma. Cooling rates and exhumation rates (i.e., vertical component of retrograde trajectories) of the metamorphic blocks are 9–20 °C/Ma and 5–6 mm/a, respectively. The derived P–T–t array is compared with a 2-D numerical subduction-zone model published by Gerya et al. [Gerya, T.V., Stöckhert, B. and Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 142, 6-1-6-19.; 45° slab dip, 40 Ma lithosphere age, convergence rates of 10–40 mm/a], which incorporates weakening of lithospheric mantle of the hanging wall by fluids emanating from the downgoing slab, resulting in an increasingly more funnel-shaped subduction channel system with time. The numerically derived array of simulated P–T–t paths as well as the calculated rates of exhumation and cooling agree well with the P–T–t data derived from the metamorphic blocks of the Rio San Juan serpentinite mélanges when convergence rates of 15 to 25 mm/a are chosen. This value is also in accord with available paleogeographic reconstructions calling for a long-term average of 22 mm/a of orthogonal convergence. On the basis of the comparison, the onset of subduction in the Rio San Juan segment of the Caribbean Great Arc can be constrained to approximately 120 Ma. This segment was thus obviously active for more than 65 Ma. An orthogonal convergence rate of 15–25 mm/a requires that a minimum amount of 975–1625 km of oceanic crust must have been subducted. Both petrological/geochronological data and numerical simulation underscore the broad spectrum of different P–T–t paths and peak conditions recorded by material subducted at different periods of time as the subduction zone evolved and matured.

The lithium isotopic composition of orogenic eclogites and deep subducted slabs

We have modelled the Li budget of subducting oceanic crust during dehydration, using recently established input parameters. The results show that the entire prograde metamorphic process, up to anhydrous eclogite, can account for a decrease in δ 7Li of only ≤ 3‰. Given that altered, oceanic crust entering the subduction zone should have Li isotopic compositions between − 3 and + 14‰, dehydration cannot account for markedly light Li isotopic compositions (δ7Li < 0 to − 10‰) previously measured in some high-pressure metamorphic (HPM) rocks. We have analysed another 41 orogenic HPM rocks from 11 different localities. These samples show a wide range in Li abundances from 1 to 77 μg/g. Li isotopic compositions of the rocks display both very heavy ( δ 7Li > + 6‰) and very light ( δ 7Li < 0‰) compositions, as low as − 21.9‰. Notably some of the samples with highest Li concentrations are also isotopically light, which would not be predicted by isotopic fractionation as a consequence of Li loss during dehydration. Li abundances in excess of 30 μg/g in orogenic HPM rocks of basaltic composition (eclogites) are higher than any value of altered MORB and presumably result from addition of Li after the onset of subduction, most probably during eclogitisation or exhumation. Hence we propose that light- δ 7Li values are generated by kinetic fractionation of the Li isotopes during diffusive influx of Li from the country rocks into the exhuming eclogite bodies. Our conclusions are in stark contrast to the previously accepted model, as we predict the deeply subducted eclogites to have a Li isotopic signature heavier than the mantle.

Middle Miocene volcanism in the vicinity of the Middle Hungarian zone: Evidence for an inherited enriched mantle source

Abstract Middle Miocene igneous rocks in the vicinity of the Middle Hungarian zone (MHZ) show a number of subduction-related geochemical characteristics. Many of these characteristics appear to be time-integrated, showing a decreasing subduction signature with time. In contrast to previous models, which suggest southward-dipping subduction of European lithosphere beneath the Alcapa microplate (along the Western Carpathians) is responsible for the chemical characteristics seen in middle Miocene volcanics, we propose that source enrichment occurred via the subduction of either the Budva-Pindos or Vardar Oceans. Recent seismic studies have revealed that the proposed southward-dipping subduction was not developed beneath the entire Western Carpathians or, even if it had, was overprinted by the collision of the European plate and the Alcapa unit at 16 Ma. This subduction is thought to have started 30 Ma ago, therefore the time between the onset of subduction and collision cannot account for extensive source enrichment in the overlying mantle wedge. It is also pertinent to note that the middle Miocene igneous rocks of the MHZ in their reconstructed positions are not parallel to the supposed suture expected for subduction-related arc volcanoes. Our review suggests an alternative hypothesis, whereby source enrichment is related to the subduction of either the Budva-Pindos or Vardar Ocean during the Mesozoic–Paleogene. In this model the Alcapa microplate was transferred to its present tectonic position via extrusion and rotations. Geophysical modeling and mantle xenoliths provide evidence that this process occurred at the scale of the lithospheric mantle, indicating that the subduction-modified lithospheric mantle was coupled to the crust. Melting in the lithospheric mantle of the Alcapa unit was triggered by the extension during the formation of the Pannonian Basin. The preserved subduction-related geochemical character of volcanics in intra-plate settings that are otherwise directly unaffected by subduction, can be attributed to tectonic transport of metasomatised mantle from a previous subduction-affected setting. This model provides an alternative approach to understanding the geochemical complexity seen among intra-plate calc-alkaline volcanics, where chemical characteristics can be explained without the involvement of plumes.

Turbidite Systems in the Inner Forearc Domain of the Hikurangi Convergent Margin (New Zealand): New Constraints on the Development of Trench-Slope Basins

The development of trench-slope basins on subduction wedges is strongly related to subduction processes. Theseconfined basins form on the lower trench slope, and their edges consist of structurally controlled linear bathymetric highs. In this particular setting, sedimentary processes may be controlled both by tectonism and by sea-level changes. Thus, the study of the sedimentary record may permit evaluation of the spatial and temporal contribution of tectonic activity on the development and stratigraphic evolution of trench-slope basins. The recent evolution of the Hikurangi subduction wedge (North Island of New Zealand) is characterized by the uplift and subaerial emergence of the trench­slope break, which has provided good exposure of the Miocene Akitio trench-slope basin. Identification of the main sedimentary discontinuities, reconstruction of the geometry of the sedimentary units, and paleocurrent studies were undertaken in the Akitio basin in order to propose a stratigraphic and structural scheme for the evolution of this emerged trench­slope break. Deep-sea facies associations are representative of three main fine-grained gravity-driven systems that may develop during various stages of development of mature trench-slope basins. These are, from base to top, large submarine slides (olistostrome deposits), fine-grained sand-rich submarine fans, and low-gradient submarine ramps (sheet-like turbidites). This basin-fill megasequence constitutes a lowstand systems tract, which can be subdivided into basin-floor fan, slope fan, and progradingwedge complex. Slope facies associations are common and highlight the important contribution of slope processes to the basin sediment budget. Rapid shifts of facies and important paleoenvironmental changes, reflecting slope creation or modification, characterize five major discontinuities that are related to regional tectonic events (i.e., onset of subduction at 25 Ma, beginning of regional subsidence at 15 Ma) or local tectonic events (i.e., local uplift of structural edges of the Akitio trench-slope basin at 17.5 Ma and 16.5 Ma, acceleration of subsidence at 13.2 Ma). Moreover, we show that the timing of development of some lowstand system tracts (i.e., of slope fans and of prograding-wedge complexes) may also be controlled largely by changes in style and/or amplitude of tectonic activity.

Paleoecology of an early Miocene, rapidly submerging rocky shore, Motuketekete Island, Hauraki Gulf, New Zealand

More than 70 macrofossil taxa (including 14 bivalves, 6 gastropods, 8 corals, 4 echinoderms, and 10 barnacles) are recorded from early Miocene (Otaian) Kawau Subgroup strata (Cape Rodney Formation and Motuketekete Limestone, lower Waitemata Group) at Motuketekete Island, Hauraki Gulf, north of Auckland City. Both in situ and transported fossils occur in deposits of greywacke boulder conglomerate, cobble to pebble conglomerate/sandstone, bioclastic calcareous grainstone, and an allochthonous breccia debris event unit, which correspond to lithofacies A, C, D, and E, respectively, of Ricketts et al. Greywacke boulders accumulated at the base of a greywacke paleocliff or sea stack that was planed off at its top to form a shore platform during the Miocene. A >2 m long, 16 cm thick coral colony grew atop a mixed substrate of boulders, pebbles, and sand, and exhibits two successional regrowth phases following debris‐influx events. Boulders and cobbles bored by pholadid bivalves (Parapholas aucklandicum Powell) are common in these basal bouldery talus deposits. A diverse suite of macrofossils, including chaliciform corals, occurs in somewhat finer grained deposits that buried the greywacke basement and boulder talus pile, and indicates either slightly deeper or more turbid conditions in the shallow photic zone. The cross‐bedded, bioclastic Motuketekete Limestone overlies these coarse‐grained Cape Rodney Formation units. Its fauna indicates deepening, with replacement of shallow by offshore taxa (e.g., Crenostrea with Bathylasma), as the Waitemata Basin underwent rapid tectonic subsidence and redeposition of sediments with the onset of subduction along the Hikurangi convergent margin. A newly identified lens of “upper breccia” (lithofacies E) in the Motuketekete Limestone contains rounded blocks of colonial coral and Tertiary siltstone. The Motuketekete occurrence of lithofacies E extends the known geographic range of this geologically instantaneous deposit; it records a regional tectonic event that is interpreted as an avalanche/debris flow triggered by faulting. The breccia appears to be a reliable marker for local lithostratigraphic correlation in lower Waitemata strata exposed north of Auckland. Appendices provide a systematic analysis of the barnacle fauna and the description of a new gastropod species, Bolma (Bolma) ballancei. The overall biotic content of the Kawau Subgroup, especially those taxa associated with hard substrate community development, indicate warm subtropical conditions similar to those found elsewhere in northern New Zealand during the early Miocene.

Onset of subduction as the cause of rapid Pliocene-Quaternary subsidence in the South Caspian basin

Backstripping a representative stratigraphic column from the northwest of the South Caspian basin shows that ∼2.4 km of tectonic subsidence has occurred since ca. 5.5 Ma, at ∼1.5–10 times the rate recorded in typical foreland basins. Roughly half the basin's sedimentary thickness of ∼20 km has accumulated in this time, while the upper part of the succession has begun to deform by buckle folding. The South Caspian basement appears to be either thick oceanic crust, or thinned, high-velocity continental crust. Seismicity and gravity data indicate that the basement is in the initial stages of subduction under the middle Caspian region to the north. We propose that basement subduction began ca. 5.5 Ma to create the major Pliocene-Quaternary subsidence. Subduction may have been triggered by a regional reorganization in the Arabia-Eurasia collision at that time, possibly following the construction of the Turkish-Iranian Plateau.

Composition of the crust and upper-mantle in the Central Andes (19°30′S) inferred from P wave velocity and Poisson's ratio

The present-day crustal structure of the Andean chain results from deformation processes and magmatic activity since the onset of subduction. One line of investigation to resolve questions about the mechanism of crustal thickening is to get information about the composition of the crust and upper-mantle by imaging the structures in velocity and Poisson's ratio (or V p/ V s). A high density seismic network was operated during six months in 1994 across the whole Andean chain. The P and S travel times of the best located local earthquakes, mainly situated in the subduction zone, have been inverted for V p and V p/ V s using Thurber's 3D iterative simultaneous inversion method. The resulting models, giving short-scale information about the crustal and upper-mantle composition, provide new limits on the respective involvement of crustal shortening and magmatism in the thickening of the Andean crust. Beneath the Western Cordillera, the seismic properties are well explained by typical active volcanic arc processes, and the thickening of the crust is explained by magmatic addition. Conversely, beneath the Central Altiplano Basin and the Eastern Cordillera, our results preclude significant volume of magmatic addition from the mantle and reinforce crustal shortening as the main thickening process. Moreover, cratonic lower crust and subcontinental lithospheric upper-mantle are observed under the Eastern Cordillera up to 67°W, evidence of the underthrusting of the Brazilian craton, which drives the crustal shortening. Beneath the Central Altiplano Basin, a thin lithospheric mantle is observed above a well marked asthenospheric wedge which extends in depth below the western part of the Eastern Cordillera. Thus partial melt is observed in the shallow mantle west of the Western Cordillera up to the Brazilian craton.

A mantle flow mechanism for the late Paleozoic subsidence of the Paraná Basin

The Carboniferous‐Permian stage in the evolution of the Paraná Basin is characterized by the long‐wavelength deposition of up to ∼2.5 km of sediment in the continental interior. The tectonics of the region during this time were dominated by convergence and active subduction along the nearby Panthalassan margin of Gondwana. While subsidence in the basin is clearly related to the major tectonic events at the plate margin, there is a distinct delay between the onset of subduction and the initiation of basin deposition. This, and the nature of basin stratigraphy, suggests that the large‐scale subsidence may have been the dynamic response of the lithosphere to mantle flow associated with the penetration of accumulated slab material through the phase change at 660 km depth. We present numerical experiments of mantle convection which show that subducting slabs can stagnate at the 660 km depth phase change and intermittently penetrate through. The associated (negative) dynamic topography remains at relatively low amplitudes as the subducting material pools and then increases rapidly to a maximum during the phase of penetrative flow. We find that the predicted delay time between initial descent of the slab and the maximum surface topography corresponds with evidence from the geological record. Several models of sedimentation based on the evolving dynamic topography are presented, and a simulation generated using infill rates independently estimated for the basin provides a close fit to the amplitude and horizontal length scale of material accumulation in the Paraná Basin.

Subduction and obduction processes in the Swiss Alps

The significance of the Briançonnais domain in the Alpine orogen is reviewed in the light of data concerning its collision with the active Adriatic margin and the passive Helvetic margin. The Briançonnais which formerly belonged to the Iberian plate, was located on the northern margin of the Alpine Tethys (Liguro-Piémont ocean) since its opening in the early-Middle Jurassic. Together with the Iberian plate the Briançonnais terrane was separated from the European plate in the Late Jurassic–Early Cretaceous, following the northern Atlantic, Bay of Biscay, Valais ocean opening. This was accompanied by the onset of subduction along the northern margin of Adria and the closure of the Alpine Tethys. Stratigraphic and metamorphic data regarding this subduction and the geohistory of the Briançonnais allows the scenario of subduction–obduction processes during the Late Cretaceous–early Tertiary in the eastern and western Alps to be specified. HP–LT metamorphism record a long-lasting history of oceanic subduction-accretion, followed in the Middle Eocene by the incorporation of the Briançonnais as an exotic terrane into the accretionary prism. Middle to Late Eocene cooling ages of the Briançonnais basement and the presence of pelagic, anorogenic sedimentation lasting until the Middle Eocene on the Briançonnais preclude any sort of collision before that time between this domain and the active Adria margin or the Helvetic margin. This is confirmed by plate reconstructions constrained by magnetic anomalies in the Atlantic domain. Only a small percentage of the former Briançonnais domain was obducted, most of the crust and lithospheric roots were subducted. This applies also to domains formerly belonging to the southern Alpine Tethys margin (Austroalpine–inner Carpathian domain). It is proposed that there was a single Palaeogene subduction zone responsible for the Alpine orogen formation (from northern Spain to the East Carpathians), with the exception of a short-lived Late Cretaceous partial closure of the Valais ocean. Subduction in the western Tethyan domain originated during the closure of the Meliata ocean during the Jurassic incorporating the Austroalpine–Carpathian domain as terranes during the Cretaceous. The subduction zone propagated into the northern margin of Adria and then to the northern margin of the Iberian plate, where it gave birth to the Pyrenean–Provençal orogenic belt. This implies the absence of a separated Cretaceous subduction zone within the Austro-Carpathian Penninic ocean. Collision of Iberia with Europe forced the subduction to jump to the SE margin of Iberia in the Eocene, creating the Apenninic orogenic wedge and inverting the vergence of subduction from south- to north-directed.

Foundering of the lithosphere at the onset of subduction

An analytic theory is presented for the foundering of the lithosphere at the onset of subduction. The two‐dimensional cartesian model calculates the instantaneous response of a viscous half‐space to a surface load that corresponds to the negative buoyancy of a semi‐infinite plate. In the early stages of foundering, just after the mantle floods onto the top of the lithosphere, the edge of the slab sinks into the mantle with a downward velocity uz given by (1/6e) (Δρ/ρ)4 (ρgd²/μ) (d/xn)², where ρ is the density of the mantle, Δρ is the density of the lithosphere minus the density of the mantle, d is the thickness of the lithosphere, g is the acceleration of gravity, μ is the viscosity of the mantle, and xn is the flexural length scale of the lithosphere. Alternatively, uz is given by (3/e) (Δρ/ρ)³ (d/xn)² uz (Stokes), where uz (Stokes) is the Stokes velocity of fall of a sphere of radius d/2 with excess density Δρ through a fluid of viscosity μ and density ρ. The initial subsidence velocity of the edge of a lithospheric slab is much slower than the sinking velocity of a similar‐sized and negatively buoyant sphere. For typical values of the above parameters, the edge of the plate sinks into a mantle of viscosity 1018 P as at a rate of about 7.5 mm yr−1. Lithospheric foundering is facilitated by low mantle viscosity and old, thick plates.

Neoproterozoic sedimentation on the northeast margin of Laurentia and the opening of Iapetus

AbstractVery thick shallow water sedimentary sequences were deposited in Neoproterozoic time along the future margins of Laurentia. On the eastern margin these include the Eleonore Bay and Hecla Hoek sequences of Greenland and Svalbard; these are described and their geotectonic context briefly reviewed. They present both geotectonic and geodynamic problems: why did subsidence continue for some 300 Ma prior to the opening of Iapetus, and how could 15–20 km of sediment be accommodated in an ensialic environment?Prolonged slow stretching appears to have affected the eastern margin of Laurentia while the western (Cordilleran) margin progressed through the rift-drift transition as the Pacific opened. It is proposed that expansion of the Pacific was associated with both the convergence of Proto-Gondwanan continental terranes during the Pan-African orogeny (the extended SWEAT hypothesis) and also the maintenance of very slow extension rates on the future Iapetus margin. The strain-hardening effect of slow stretching may have been inhibited by a continuous basin-fill of juvenile heat-producing Grenville detritus. The onset of subduction in the Pacific freed up this margin; major rifting took place between East Greenland and possibly the Tornquist margin of Baltica in Vendian time, followed by the opening of northern Iapetus.

Subduction of the lithosphere and back arc dynamics: Insights from physical modeling

Physical modeling of the subduction of an elastico‐plastic lithosphere underlained by a low‐viscosity asthenosphere and driven by both horizontal compressional force and gravitational sinking of the subducting plate has shown the subduction regime to be strongly affected by Δρ (Δρ = ρ1 ‐ ρa where ρa and ρ1 are the asthenosphere and lithosphere densities). At Δρ = 0, a steady state process develops which requires a horizontal lithosphere compression of σh = (0.3–0.4)σs ≈ 108 Pa, where σs is the lithosphere yield limit. The central part of the leading wedge of the overriding plate is nonisostatically elevated in this case, especially in the region of the frontal arc which corresponds to a free air gravity anomaly maximum of 200–400 mGal. A decrease in Δρ (Δρ&lt;0) results in an increase of compression, growth of the frontal arc, and flattening of the Benioif zone, and, when compression is very intensive, a failure of the lithosphere and the formation of a new subduction zone either at the back arc zone abutting the volcanic arc, directly at the volcanic arc, or at the area just behind the outer rise, depending on the specific conditions. At Δρ&gt;0, compression of the overriding plate is considerably less from the onset of subduction and then decreases during subduction. At some point, compression changes into tension. Tensional nonhydrostatic stresses grow during the process to a value near σh = (0.1–0.2)σs ≈ 0.4 × 108 Pa. Both subsidence of the overriding wedge below the isostatic equilibrium level and an increase in the Benioif dip angle accompany this process. The gravity anomaly maximum is displaced into the volcanic arc in this subduction regime, where it is normally less than 150 mGal. If some weaknesses are present in the overriding plate, tensional stresses result in a failure of the lithosphere at these locations and the broken lithosphere segment (the arc plate) starts to move counter to the subduction plate. The arc plate slides along the underthrusting plate's surface without loss of contact and causes back arc opening. Hydrostatic suction prevents the plates from being separated. This effect is caused by the peculiarity of a mechanical system whereby a low‐viscosity asthenosphere and a plastic lithosphere which effective yield limit is considerably lower than the hydrostatic stresses in the plate. If the overriding lithosphere is not weakened enough to fail under the existing tension, then the hanging slab breaks down under its own weight when it reaches a specific length. Both interacting plates undergo isostatic recovery in this case, and the whole cycle is repeated again. The hydrostatic suction effect also provides back arc tension under other conditions, such as when asthenospheric flow forces the subducted plate to move away from the island arc.

Some thermomechanical aspects of the subduction of continental lithosphere

On the basis of thermal and mechanical modeling, it is concluded that the subduction of continental lithosphere can lead to its breakup and the formation of a new plate contact within the middle or lower crust. As a result, (part of) the subducting continental crust is transferred to the upper plate. Breakup is caused by the resistive forces acting upon subducting continental crust, due to the buoyancy of crustal material and to friction at the plate contact, as well as the decrease in strength of the subducting crust once it has been subducted to a depth of a few tens of kilometers. The crustal thickness and the thermal and compositional structure of the continental crust just before the onset of subduction have a large influence on the depth to which continental crust can be subducted (prior to its breakup). The depth to which continental crust can be subducted coherently decreases as the surface heat flow or the crustal thickness of the subducting continental plate increases. In many cases, breakup is found to occur at a time when the upper surface of the continental plate has been subducted to a depth of 25–50 km. The subduction of a cold continental shield or of continental lithosphere with a relatively small crustal thickness, on the other hand, may lead to the subduction of both upper and lower crust to mantle depths.

Olistostromes and the Onset of Subduction

Olistostromes found in a number of major fold belts are of an age comparable with the start of underthrusting as indicated by attendant arc-related volcanism. They occur at the inner edge of inferred accretionary wedges, and are commonly associated with contemporaneous basaltic rocks. These olistostromes may result from disruption of the crust during lithosphere fracturing at the commencement of subduction. Components of the olistostromes provide evidence concerning the site of initial underthrusting.

Biostratigraphic and tectonic implications of 40Ar-39Ar dates of ash layers from the northeast Gulf of Alaska

Ash layers from Deep Sea Drilling Project site 178 in the northeast Pacific Ocean have been dated by the 40 Ar- 39 Ar stepwise heating technique to resolve published discrepancies concerning the length of time explosive volcanism has affected the eastern Aleutian arc and Alaskan Peninsula. The results of the investigation indicate that the record of ash-fall deposition at site 178 extends back at least 6.5 m.y. Assuming that 6.5 m.y. ago marks the onset of renewed calc-alkalic volcanism of the volcanic arc, proposed models of continuous and discontinuous motion between the Pacific and North American lithospheric plates can be evaluated. If appreciable time elapsed between the onset of subduction and the onset of arc volcanism, the 6.5-m.y. record of ash-fall deposition in the north-east Pacific is most compatible with models of continuous plate motion throughout late Cenozoic time.