Steep Subduction Research Articles

Multiple episodes of continental subduction during India/Asia convergence: Insight from seismic tomography and tectonic reconstruction

High wavespeed tomographic anomalies shallower than 1100 km beneath the India/Asia collision zone are interpreted as continental slabs subducted during collision. Combining anomaly positions with paleogeographic reconstructions of India, we constrain the spatio-temporal evolution of multiple episodes of continental subduction likely related to these anomalies. This study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone. The evolution of the western part is characterized by two episodes of steep subduction of the northern margin of India. The first episode, involving an area with a lateral extent as large as 1500 km, started at about 40–30 and ended by a slab break-off process at ∼ 15 Ma, The second episode consists on subduction beneath the Hindu Kush mountains since ∼ 8 Ma. To the east of the collision zone, no anomaly related to steep subduction along the northern edge of India is found. We interpret two tomographic anomalies beneath the eastern border of the Indian plate, beneath Burma and beneath the Andaman Sea, as the result of two successive episodes of southeastward extrusion followed by subduction. We suggest that both extruded portions were initially located along the northern margin of India, and that they slid around the eastern syntaxis, then southward along the eastern boundary of Indian plate. Both portions subducted along the eastern border of India, south of the eastern syntaxis. We provide a rough estimate of the amount of Indian lithosphere consumed during these subduction and extrusion episodes. By comparing this amount with the total amount of Indian lithosphere at the onset of collision, we conclude that these processes accommodated most of the India/Asia convergence during collision.

Impact of regional mantle flow on subducting plate geometry and interplate stress: insights from physical modelling

SUMMARY Physical models of subduction investigate the impact of regional mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates. The initial mantle flow direction beneath the overriding plate can be horizontal or vertical, depending on its location with respect to the asthenospheric flow field. Imposed mantle flow produces either over or underpressure on the lower surface of the slab depending on the initial mantle flow pattern (horizontal or vertical, respectively). Overpressure promotes shallow dip subduction while underpressure tends to steepen the slab. Horizontal mantle flow with rates of 1–10 cm yr −1 provides sufficient overpressure on a dense subducting lithosphere to obtain a subduction angle of ∼60 ◦ , while the same lithospheric slab sinks vertically when no flow is imposed. Vertical drag force (due to downward mantle flow) exerted on a slab can result in steep subduction if the slab is neutrally buoyant but fails to produce steep subduction of buoyant oceanic lithosphere. The strain regime in the overriding plate due to the asthenospheric drag force depends largely on slab geometry. When the slab dip is steeper than the interplate zone, the drag force produces negative additional normal stress on the interplate zone and tensile horizontal stress in the overriding plate. When the slab dip is shallower than the interplate zone, an additional positive normal stress is produced on the interplate zone and the overriding plate experiences additional horizontal compressive stress. However, the impact of the mantle drag force on interplate pressure is small compared to the influence of the slab pull force since these stress variations can only be observed when the slab is dense and interplate pressure is low.

Tethyan and Indian subduction viewed from the Himalayan high- to ultrahigh-pressure metamorphic rocks

The Himalayan range is one of the best documented continent-collisional belts and provides a natural laboratory for studying subduction processes. High-pressure and ultrahigh-pressure rocks with origins in a variety of protoliths occur in various settings: accretionary wedge, oceanic subduction zone, subducted continental margin and continental collisional zone. Ages and locations of these high-pressure and ultrahigh-pressure rocks along the Himalayan belt allow us to evaluate the evolution of this major convergent zone. (1) Cretaceous (80–100 Ma) blueschists and possibly amphibolites in the Indus Tsangpo Suture zone represent an accretionary wedge developed during the northward subduction of the Tethys Ocean beneath the Asian margin. Their exhumation occurred during the subduction of the Tethys prior to the collision between the Indian and Asian continents. (2) Eclogitic rocks with unknown age are reported at one location in the Indus Tsangpo Suture zone, east of the Nanga Parbat syntaxis. They may represent subducted Tethyan oceanic lithosphere. (3) Ultrahigh-pressure rocks on both sides of the western syntaxis (Kaghan and Tso Morari massifs) formed during the early stage of subduction/exhumation of the Indian northern margin at the time of the Paleocene–Eocene boundary. (4) Granulitized eclogites in the Lesser Himalaya Sequence in southern Tibet formed during the Paleogene underthrusting of the Indian margin beneath southern Tibet, and were exhumed in the Miocene. These metamorphic rocks provide important constraints on the geometry and evolution of the India–Asia convergent zone during the closure of the Tethys Ocean. The timing of the ultrahigh-pressure metamorphism in the Tso Morari massif indicates that the initial contact between the Indian and Asian continents likely occurred in the western syntaxis at 57 ± 1 Ma. West of the western syntaxis, the Higher Himalayan Crystallines were thinned. Rocks equivalent to the Lesser Himalayan Sequence are present north of the Main Central Thrust. Moreover, the pressure metamorphism in the Kaghan massif in the western part of the syntaxis took place later, 7 m.y. after the metamorphism in the eastern part, suggesting that the geometry of the initial contact between the Indian and Asian continents was not linear. The northern edge of the Indian continent in the western part was 300 to 350 km farther south than the area east of the Nanga Parbat syntaxis. Such “en baionnette” geometry is probably produced by north-trending transform faults that initially formed during the Late Paleozoic to Cretaceous Gondwana rifting. Farther east in the southern Tibet, the collision occurred before 50.6 ± 0.2 Ma. Finally, high-pressure to ultrahigh-pressure rocks in the western Himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data.

Boninites and Adakites from the Northern Termination of the Tonga Trench: Implications for Adakite Petrogenesis

Adakitic rocks were recovered by dredging from the northern termination of the Tonga Trench during the 1984 voyage of the R.V. Natsushima and the 1996 voyage of the R.V. Melville. These contain magmatic zircons that have been dated at 2� 5M a by U^Pb methods, indicating that they are contemporaneous with boninite magmatism previously described from this area. This is the first time adakites and high-Ca boninites have been reported from the same active tectonic setting. The Tonga adakites are classified as high-SiO2 adakites, and are compositionally consistent with an origin as partial melts of subducted Pacific oceanic crust and sediment. Geochemical modelling suggests that the adakites are not involved in the petrogenesis of the Tongan high-Ca boninites. However, the recovery of adakites and boninites from the termination of the northern Tonga Trench suggests that both magma types are related to the unique tectonic setting of this region, where a transition from steep subduction to a transform fault plate boundary has created a slab window with an associated slab edge. Boninites are generated as a result of hot Samoan plume mantle moving through the slab window and subsequently being fluxed by H2O-rich fluids from the subducting Pacific oceanic crust. The Tonga adakites, in contrast, result from the direct melting of the slab edge as a result of the juxtaposition of the subducting slab against hot mantle derived from the Samoan plume.

Initial geometry of western Himalaya and ultrahigh-pressure metamorphic evolution

Ultrahigh-pressure metamorphic rocks on both sides of the western Himalayan syntaxis show different P–T–t paths. The Kaghan unit was metamorphosed under the UHP conditions significantly later (∼46 Ma) than the Tso Morari unit (∼53 Ma), implying that the Tso Morari was subducted earlier (∼57 Ma) than the Kaghan unit (∼52 Ma). The age difference likely reflects the initial shape of Greater India, with the Kaghan unit located greater than 300 km south of the Tso Morari before the collision of two continents. We calculate the dip of the subducting plate using two independent methods. The results show gentle dipping subduction east of the western syntaxis, and steep subduction west of the syntaxis since the time of India–Eurasia collision to the present time. We propose that the steep subduction in the western part is likely related to the proto-Chaman and Karakorum faults along which the Indian plate moved northward. In the eastern part, the overlying Eurasian plate extruded to east, which allowed gentle dipping subduction of the Indian continent. Although the main period of eastward extrusion of the Eurasian continent occurred between 30 and 15 Ma, our results suggest that this was likely taking place since the early India–Asia collision. Using those geometrical constraints, a 3D image of the slab is reconstructed in the western part, showing the sharp bending of the western syntaxis along the proto-Chaman fault. This bending resulted in the warping of the slab surface to form a conical fold with a north-dipping axis located near the western syntaxis.

Geochemistry of Neoproterozoic mafic intrusions in the Panzhihua district (Sichuan Province, SW China): Implications for subduction-related metasomatism in the upper mantle

Metasomatism above subduction zones is an important process that produces heterogeneous mantle and thus a diversity of igneous rocks. In the Panzhihua district, on the western margin of the Yangtze Block (SW China), two Neoproterozoic mafic intrusions, one olivine gabbro and one hornblende gabbro, have identical ages of 746 ± 10 and 738 ± 23 Ma. Both of the gabbros are tholeiitic in composition and have arc-like geochemical compositions. The hornblende gabbros have K 2O concentrations ranging from 0.70 to 1.69 wt.% and show enrichment of Rb, Ba, U, Th and Pb and depletion of Nb,Ta and Ti. They have variable 87Sr/ 86Sr ratios (0.7045–0.7070) with constant ɛNd( t) values (−0.12 to −0.93). The olivine gabbros have relatively low K 2O (0.19–0.43 wt.%), are depleted in Rb and Th relative to Ba and U, and have obvious negative Nb–Ta and Zr–Hf anomalies on primitive mantle-normalized trace element diagrams. Their ɛNd( t) values range from −0.64 to −1.73 and initial 87Sr/ 86Sr ratios from 0.7070 to 0.7075. Both types of gabbro experienced fractional crystallization of clinopyroxene, plagioclase, amphibole and minor Fe–Ti oxide. The parental magmas of the olivine and hornblende gabbros were formed by about 20% partial melting of garnet–spinel lherzolite and spinel lherzolite, respectively. According to trace elemental ratios, the hornblende gabbros were probably derived from a source strongly modified by subducted slab fluids, whereas the olivine gabbros came from a mantle source modified by subducted slab melts. The close association of the olivine gabbros and hornblende gabbros suggests that a steep subduction zone existed along the western margin of the Yangtze Block during Neoproterozoic time. Thus, the giant Neoproterozoic magmatic event in South China was subduction-related.

The age of deep, steep continental subduction in the NW Himalaya: Relating zircon growth to metamorphic history. Comment on: “The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya” by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005)

Leech et al. [Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005) 83–97], present 3 clusters of ages for growth stages in zircon from quartzo-feldspathic gneisses hosting coesite-bearing eclogite from the Tso Morari Complex, NW India. These age clusters, from oldest to youngest, are interpreted to represent the age of ultrahigh-pressure metamorphism, a subsequent eclogite facies overprint and a later amphibolite facies retrogression and require subduction of Indian crust to have started earlier than previously accepted. However, no petrographic evidence, such as inclusions in the zircons relating to particular metamorphic events, is presented to substantiate the proposed sequence of metamorphic stages. Previously published data from eclogites of the same area indicate that coesite–eclogite is not the first but at least the second eclogite facies stage. In addition, the newly proposed time interval between coesite–eclogite and the amphibolite facies overprint is longer than previously indicated by diffusion modelling of natural garnet–garnet couples in eclogite. Neither the age of ultrahigh-pressure metamorphism nor the timing of initiation of subduction is reliably constrained by the presented data.

Reply to comment by P.J. O'Brien on: “The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya” by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth Planetary Science Letters 234 (2005) 83–97

Reply to comment by P.J. O'Brien on: “The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya” by Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth Planetary Science Letters 234 (2005) 83–97

Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics?

Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts. Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to ∼ 350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to ∼ 2.7 Ga: Kenorland or Ur ∼ 2.7–2.4 Ga; Columbia ∼ 1.6–1.4 Ga; Rodinia ∼ 1100–750 Ma; and Pangea ∼ 230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean.

The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya

Ultrahigh-pressure (UHP) rocks in the NW Himalaya are some of the youngest on Earth, and allow testing of critical questions of UHP formation and exhumation and the timing of the India–Asia collision. Initial collision of India with Asia is widely cited as being at 55 ± 1 Ma based on a paleomagnetically determined slowdown of India's plate velocity, and as being at ca. 51 Ma based on the termination of marine carbonate deposition. Even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid India–Asia convergence rate of 134 mm/a at the time of collision. New U–Pb SHRIMP dating of zircon shows that Indian rocks of the Tso Morari Complex reached UHP depths at 53.3 ± 0.7 Ma. Given the high rate of Indian subduction, this dating implies that Indian continental crust arrived at the Asian trench no later than 57 ± 1 Ma, providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates. India's collision with Asia may be compared to modern processes in the Timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition. The Indian UHP rocks must have traveled rapidly along a short, hence steep, path into the mantle. Early continental subduction was at a steep angle, probably vertical, comparable to modern continental subduction in the Hindu Kush, despite evidence for modern-day low-angle subduction of India beneath Tibet. Oceanic slab break-off likely coincided with exhumation of UHP terranes in the western Himalaya and led to the initiation of low-angle subduction and leucogranite generation.

Corinth rift margin uplift: New evidence from Late Quaternary marine shorelines

New evidence for uplift of the southern margin to the Corinth rift, one of the world's most rapidly extending continental regions, defines an area of uniform uplift separating a more rapidly uplifting western rift flank from a slowly backtilting eastern flank. This major tectonic boundary coincides with geophysical evidence for a junction between flat underlying subducted oceanic plate and steep subduction. We propose that trench rollback by the Anatolian plate over the subducting African plate has led to differential uplift and possible migration of active faulting at the southern rift margin in the last few million years.

Magmatism as a probe to the Neogene shallowing of the Nazca plate beneath the modern Chilean flat-slab

Changes in the Neogene (Miocene to Recent) magmatic style of the modern Chilean flat-slab region and its margins (27–34°S) reflect both variations in Nazca–South American convergence parameters and the effects of subduction of the Juan Fernández Ridge hotspot track. These effects are superimposed on an initially thicker arc crust in a more compressional tectonic regime in the north than in the south. Distinctive magmatic periods across the region reflect relatively steep subduction from ∼27 to 20 Ma, frontal arc migration associated with forearc subduction erosion from ∼20 to 16 Ma, compressional deformation and andesitic volcanism from ∼15 to 9 Ma, a peak of slab shallowing associated with frontal arc migration and arc broadening from ∼8 to 4 Ma, and termination of volcanism over the flat-slab by ∼5 Ma. Changes in magmatic style and distinctive chemical signatures in ∼20 to 16 Ma volcanic rocks coincide with a marginwide transition from a more extensional to a more compressional tectonic regime linked to changing Nazca–South American convergence parameters. North to south differences in ∼15 to 9 Ma magmas partially reflect the southward migration of the northeast-trending arm of the subducting Juan Fernández Ridge, whose effect arrived in the flat-slab region at ∼14 Ma. The termination of andesitic arc volcanism across most of the region at ∼9 Ma and extreme shallowing of the slab beneath the central flat-slab coincides with the arrival and eastward propagation of the east–west-trending segment of the Juan Fernández Ridge. Overall, the subduction of the Juan Fernández Ridge is a perturbation that causes extreme shallowing in part of a regionally shallowing subduction zone. Chemical signatures of the erupted magmas reflect the changing mantle and crustal configuration above the shallowing subduction zone. Those with adakitic-like signatures are derived from subduction zone mafic magmas contaminated by garnet granulitic to eclogitic facies crust in the thickened crust or by crust incorporated into the mantle as a result of forearc subduction erosion. There is no evidence that any of these magmas were generated by melting of the subducted slab.

Stresses and strains in a twisted subduction zone-Fiordland, New Zealand

Summary In the Fiordland region of the South Island of New Zealand, the subducted Australian Plate steepens from south to north, becoming near-vertical below 75 km depth between Doubtful and Milford Sounds. This steepening is accompanied by a sharp change in strike of the subduction zone of about 17°. Here we investigate stress and strain in this twisted subduction zone using earthquakes recorded during a three-month deployment of 24 portable digital seismographs. We determine focal mechanisms of 115 earthquakes of ML 2.5–4.5 using both first motion polarity data and amplitudes of seismogram envelopes. We also invert for the stress tensor orientation for regions within both the subducted and overlying plates, using a method which invokes the Coulomb failure criterion and considers all P wave first motions together, regardless of whether or not they are sufficient to define single event focal mechanisms. Focal mechanisms and stress inversions for earthquakes in the overlying plate indicate normal faulting shallower than 16 km and thrust faulting at greater depths. This stress and strain regime is related to the 17° change in strike of the subduction zone, which leads to an arching up of the subducted plate at the bend. Such arching is consistent with the exposure of lower crustal granulite facies rocks near the bend. Earthquakes in the subducted plate show predominantly normal faulting relative to the plate. Arrival time inversions for 3-D seismic velocity structure indicate that a zone of high Vp (>8.5 km s−1) in the uppermost mantle of the overlying plate abuts the vertical portion of the subduction zone. Stresses and strains in the subducted plate are consistent with a model in which the sharp change in strike of the subduction zone is a consequence of the obliquely converging subducted plate having to bend around this zone. The steep dip of the subducted plate to the north appears to be a natural consequence of the minimization of membrane stress in the plate arising from the sharp bend along strike. This mechanism for steepening the plate is quite different to that of trench retreat usually used to explain steep subduction.

Seismotectonic and stress distribution in the central Chile subduction zone

We determined the stress tensor from focal mechanisms and the shape of the downgoing Nazca Plate subducted beneath central Chile (26–35°S), based on accurately located hypocenters of local and teleseismic events, at three different segments defined by the rupture zones of great thrust earthquakes. Between latitudes 26–33°S the slab flattens at intermediate depths and moves upwards probably related to the subduction of buoyant lithosphere associated to the Juan Fernandez Ridge (JFR). To the south the subduction is steeper and the slab penetrates into the mantle with a constant dip of 27° implying a sharp dip change around 33°S. The slab seismicity reaches a maximum depth of 200 km south of 29°S while to the north deep seismicity is observed at depths of 570–630 km, with a gap between 200 km and these depths. A remarkable along-strike variation in the seismic slab length is observed from 1100 km north of 29°S, to 800 km between 30 and 32°S and 350 km south of 33°S. The seismicity suggests a thick oceanic crust related to the JFR subduction, furthermore this intermediate depth seismicity is more intense and more seismic moment is released than in the adjacent zones. The stress tensors show differences between the steep subduction and the flat slab zones, being more important at intermediate depths where the slab flattens. At shallow depths the stress is compressive due to plate convergence. At intermediate depths the slab is under tensional stress regime, related to the slab pull in the steep subduction zone and also to buoyant forces in the flat slab zone. The maximum depth of interplate coupling between Nazca and South American Plates is 60 km, similar to other places along the Chilean subduction zone.

Andean subduction styles and their effect on thermal structure and interplate coupling

The variability in subduction styles along the Andean margin is investigated with regard to its probable causes and likely effects. It is shown that there is no single Andean or Chilean subduction style. The alternation of segments of flat-slab subduction and steep subduction argue against mechanisms involving continental scale boundary conditions. The best candidate for explaining flat subduction style appears to be buoyancy related to thickened oceanic crust of moderate to young age. A high degree of interplate coupling and great subduction zone earthquakes can occur along both flat-slab segments and steeply subducting segments. In general interplate coupling is higher for flat-slab segments as demonstrated by a statistical analysis of deformation in the upper plate. A flat subduction style is shown to have an important thermal impact resulting in a much colder plate interface and may succeed in explaining the apparent increase in coupling.

Can slab melting be caused by flat subduction?

Slab melting has been suggested as a likely source of adakitic arc magmas (i.e., andesitic and dacitic magmas strongly depleted in Y and heavy rare earth elements). Existing numerical and petrologic models, however, restrict partial melting to very young (≤ 5 Ma) oceanic crust (typically at 60–80 km depth). Paradoxically, most of the known Pliocene-Quaternary adakite occurrences are related to subduction of 10–45 Ma lithosphere, which should not be able to melt under normal subduction-zone thermal gradients. We propose an unusual mode of subduction known as flat subduction, occurring in ∼10% of the world's convergent margins, that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust. Of the 10 known flat subduction regions worldwide, eight are linked to present or recent (<6 Ma) occurrences of adakitic magmas. Observations from Chile, Ecuador, and Costa Rica suggest a three-stage evolution: (1) steep subduction produces a narrow calc-alkaline arc, typically ∼300 km from the trench, above the asthenospheric wedge; (2) once flat subduction begins, the lower plate travels several hundred kilometers at nearly the same depth, thus remaining in a pressure-temperature window allowing slab melting over this broad distance; and (3) once flat subduction continues for several million years, the asthenospheric wedge disappears, and a volcanic gap results, as in modern-day central Chile or Peru. The proposed hypothesis, which reconciles thermal models with geochemical observations, has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins.

Geochemistry and geodynamic setting of volcanic and plutonic rocks associated with Early Archaean volcanogenic massive sulphide mineralization, Pilbara Craton

Early Archaean volcanic rocks and syn-volcanic granites at Strelley, east Pilbara Craton, Western Australia, were formed during an extensional tectonic phase associated with an active convergent continental margin. The volcanic sequence comprises a bimodal suite of fractionated sub-alkalic tholeiitic basalts, andesites and dacites, siliceous laminites and volcanogenic massive sulphide (VMS) deposits in the footwall and hangingwall calc-alkaline dacites. The footwall rocks are characterized by Th/La pm>1, variably fractionated LREE and relatively high Y and Yb (>33 and >3 ppm, respectively). Negative normalized anomalies of Nb, P, Ti, are consistent with derivation via slab dehydration fusing the mantle wedge. The co-magmatic and syn-volcanic Strelley Granite is enriched in LILE relative to HFSE, and has distinctively low Al 2O 3 (11–14) and [La/Yb] n (4–10) suggesting a relatively mature and cool subducted oceanic lithosphere as for the majority of Phanerozoic arc granitoids and convergent margin magma compositions. The chemical composition of the footwall basaltic to dacitic rocks and the sub-volcanic Strelley Granite, display signatures highly prospective for VMS mineralization (large -ve Eu anomalies, low Zr/Y ratios, high level magmatic fractionation), comparable with the Noranda FIIIa-type suite. The hangingwall dacites have more pronounced HREE fractionation than footwall counterparts, and lower Y and Yb (<21 and <1.6, respectively), consistent with slab melting. The hangingwall dacitic rocks display non-prospective VMS signatures, and are unmineralized. Accordingly, Archaean magmatic arcs may have varied between slab dehydration and slab melting. This change in variation of melting and dehydration may be related to key temporal stages in a geodynamically evolving convergent margin. The chemistry of the footwall volcanic rocks and granite reflect typical Phanerozoic conditions of steep subduction, hydrous metasomatism in the mantle wedge, and fractionation. Similar characteristics are also evident in plutonic–volcanic suites associated with VMS deposits in the Late Archaean, as at Noranda, Canada. Early extension with nascent back-arc development coupled with steep subduction is considered critical to the development of VMS deposits.

Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision

The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes M w≥7.8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 – from 6°N to 2.5°N with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 – from 2.5°N to 1°S showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 – from 1°S to 2°S with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 – south of 2°S with flat subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local `flat slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench–ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision.

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Crustal evolution and sedimentation history of the Bay of Bengal since the Cretaceous

Multichannel seismic reflection, gravity, magnetic, and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of India to the Andaman shelf and four NW‐SE profiles in the Western Basin off Madras (seismics on three latitudinal profiles) in the Bay of Bengal. The trend of the fracture zones, the locations of the magnetic chron 34, and the Cretaceous Magnetic Quiet Zone suggest that Greater India separated from Antarctica after a period of transform motion in the early Cretaceous, that is, about polarity chron M0 (120 Ma) and drifted northwestward. Negative gravity anomalies are associated with basement rises including the 85°E Ridge between the continental margin of India and the Ninetyeast Ridge. Magnetic reversals and northward trend of the 85°E Ridge support a hotspot origin of the ridge and its emplacement most likely after the Cretaceous quiet period. Juxtaposition of high‐amplitude hyperbolic reflections, down‐faulted continental blocks buried under thick sediments, and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope, about 80 km seaward of the present continental shelf edge. Eight seismic sequences, as thick as 8.5 km, overlie the early Cretaceous oceanic basement and include four unconformities (lower Eocene, upper Oligocene, upper Miocene, and upper Pleistocene) which have been correlated to the major geologic/tectonic events. We also interpret the late Cretaceous/early Tertiary features on the eastern flank of the buried 85°E Ridge as carbonate reefs. The observation of steep subduction of older (cold) Indian plate beneath the Burmese plate near the Andaman Islands suggests the Sunda Arc in this region as low to intermediate stress subduction zone.