Retroarc Research Articles

The Mesoproterozoic event within the Rehoboth Basement Inlier of Namibia: review and new aspects of stratigraphy, geochemistry, structure and plate tectonic setting

The Mesoproterozoic domain of the Rehoboth Basement Inlier (RBI) in central Namibia is composed of sedimentary and volcanic rocks which have been intruded by coeval granites of batholithic dimension as well as numerous quartz-feldspar porphyries and mafic dykes. Review of the geochemical, petrographical and geochronological data in combination with field observations and the analysis of high-resolution geophysical magnetic and radiometric data suggests Mesoproterozoic evolution of this area in three stages. The development of a first basin is recorded by clastic fluvial and limnic sedimentary rocks of the Billstein formation with only minor if any felsic pyroclastic activity. Intrusive porphyry dykes constrain a minimum age of 1210 +/− 7 Ma. The second stage (Nuckopf, Langberg formations, Kartatsaus member), lasting from at least 1.23 Ga to 1.09 Ga is marked by major felsic igneous activity and strong vertical tectonics: ignimbrite deposits as well as rhyolitic flows occur throughout the RBI and suggest several volcanic eruption centres. The calcalkaline Gamsberg Granite Suite represents the plutonic equivalent to this volcanism; geochemically it shows affinity to arc-related igneous rocks. Contemporaneous sediments range from polymict unsorted debris flow deposits to arkoses and cross-bedded feldspathic sandstones deposited in grabens. An intra- or backarc region of an active continental margin is suggested as possible plate-tectonic setting for stage two. The initiation of the third stage (Opdam formation, Kupferberg, Idaberg members) is marked by regional outpouring of basalts deposited subaerial or in shallow water. High Ti-content and flat REE-patterns in the tholeiites suggest their emplacement in an extensional setting, whereas high Th/Ta and La/Nb ratios, low Ce/Pb values and negative anomalies for Nb-Ta require a subduction-style enrichment of the mantle source. These contrasting features are best explained by extensional collapse related to continent-collision further north: Docking of continents led to slab detachment, allowing the interaction between the asthenospheric mantle and the mantle wedge enriched during the subduction process. At regional scale, the mafic rocks are considered to be part of the Umkondo igneous province. Tectono-metamorphism of the Mesoproterozoic domain took place only during the Pan-African Damara orogeny. It is marked by intense south-eastward directed thrusting within an anastomizing network of shear zones which constitute the South Damara shear zone. Open to tight southeasterly verging folds form second order deformation structures. The metamorphism varies from medium grade in the north to very low grade in the south.

Crustal deformation in the south-central Andes backarc terranes as viewed from regional broad-band seismic waveform modelling

SUMMARY The convergence between the Nazca and South America tectonic plates generates a seismically active backarc region near 31 ◦ S. Earthquake locations define the subhorizontal subducted oceanic Nazca plate at depths of 90‐120 km. Another seismic region is located within the continental upper plate with events at depths 1.80, th = 45‐55 km) than those of the eastern Sierras Pampeanas (Vp = 6.0‐6.2 km s −1 , Vp/Vs < 1.70, th = 27‐35 km). In addition, we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes. Finally, we estimated average P and T axes over the CHARGE period. The entire region showed P- and T-axis orientations of 275 ◦ and 90 ◦ , plunging 6 ◦ and 84 ◦ , respectively.

Seismic activity along the central america volcanic arc: is it related to subduction of the Cocos plate?

We determine seismic strain rate of tectonic earthquakes along the Central America Volcanic Arc. We then compare this result to those obtained from earthquakes related to the convergence of the Cocos and Caribbean plates and to earthquakes in the back-arc region of northern Central America. The seismic strain-rate tensor for shallow-focus earthquakes along the Central America volcanic arc since 1700, has a compressive eigenvector with a magnitude of 0.7 × 10 −8 year −1, and oriented in a 357° azimuth. The extensive eigenvector is oriented in a 86° azimuth, with a magnitude of 0.82 × 10 −8 year −1. When only Centroid Moment-tensor solutions (CMT) are considered, the respective eigenvectors are 1.2 × 10 −8 year −1 and 1.0 × 10 −8 year −1. The compressive eigenvector from the seismic strain-rate tensor for earthquakes along the Cocos-Caribbean convergent margin is 2.0 × 10 −8 year −1, plunging at 25°, and oriented in a 29° azimuth. Its magnitude and direction are similar to those of the compressive eigenvector for earthquakes along the volcanic arc. The extensive eigenvector along the convergent margin, on the other hand, has a large vertical component. The compressive and extensive eigevenvectors are 4.9 × 10 −8 year −1 and 4.6 × 10 −8 year −1, using only CMTs as the database. Earthquakes along the grabens of northern Central America yield a seismic strain-rate tensor whose extensive eigenvector has a magnitude of 2.4 × 10 −8 year −1, oriented in a 109° azimuth. Magnitude and direction are similar to those of the extensive eigenvector for earthquakes along the volcanic arc. The compressive eigenvector along the grabens is practically vertical. Similarities in magnitudes and directions for compressive and extensive eigenvectors suggest to us that the strain field along the Central America volcanic arc is the result of compression along the convergent Cocos-Caribbean margin, and extension in the back-arc region, along the grabens of northern Central America. This field is resolved as strike-slip faulting along the arc.

Constraints on mantle circulation around the deforming Calabrian slab

Combining tectonics, with seismological and geochemical data, we reconstruct the deformation history of the presently narrow Calabrian slab and the path of mantle circulation during the last 10 Ma. We show that during the slab deformation the mantle laterally flowed inside the back arc region permitting its retrograde motion and giving a seismological and volcanological record after 1–2 myr.

GEOCHEMICAL VARIATIONS IN THE BACK-ARC, SOUTH PATAGONIAN LITHOSPHERIC MANTLE BENEATH PATAGONIA (SOUTH AMERICA): TOWARDS A GEODYNAMIC MODEL

Samples of the mantle wedge of convergent plate regions are rare and direct evidence of the interaction between slabderived components and wedge is therefore unusual. Abundant occurrences in Patagonia of mantle xenoliths in alkaline volcanic centres located in the backarc region of the Andean subduction zone (Stern et al., 1990; Laurora et al., 2001; Rivalenti et al., 2004a,b), 300 – 600 km east from the Chile trench provide the possibility to study these processes. Research on Patagonia arc and back-arc magmas provides evidence that the mantle wedge, fertilised by slab components, played an important role either as the source of the arc magmas or as a component of the plateau and postplateau back-arc basalts. These latter consist of older (12-5 Ma) tholeiitic sequences that formed voluminous mainplateau sequences and younger (7-2 Ma), less voluminous alkaline lavas with OIB-like characteristics erupted mostly from monogenetic volcanic centres. Stern et al. (1990) recognised an eastward decrease of slab-related geochemical features in back-arc basalts and proposed a supra-slab model for their origin. More recently, a sub-slab model has been envisaged with partial melting of sub-slab sources and migration of magmas to the surface through slab-windows (D’Orazio et al., 2001; Gorring and Kay, 2001). Abundant occurrences of mantle xenoliths in alkaline volcanic centres located in the back-arc South Patagonian provide a general overview of the lithospheric mantle composition and a reconstruction of its depletion and metasomatic enrichment processes. Combined petrological and texture- related geochemical investigations allowed us to address some important and still debated questions: • which is the role of the lithospheric mantle in the determination of the East-West geochemical variability of the lavas? • is the continental lithospheric mantle a potential reservoir for the Southern Patagonia magmas? • is the eastward fading of the slab signatures in the basalts recorded also in the lithospheric mantle? • which is the geodynamic mechanism determining the upwelling of the asthenosphere in the Patagonian back-arc area (extensional tectonic stress, mantle plume, slab window)? • how can the geochemical composition of the mantle ultramafics be used to infer the composition of migrating melts ? In particular, did the mantle wedge interact with carbonatitic or adakitic metasomatic agents as it has been inferred for Gobernador Gregores (Gorring and Kay, 2000) or some westernmost occurrences (Kilian and Stern, 2002), respectively? Xenoliths from ten localities from the Rio Negro to the Santa Cruz provinces were examined to address the important issues. Major results based on the trace element signatures of minerals and glasses determined in-situ by SIMS and LA ICP-MS techniques as well as isotopic signatures of mineral separates from Pali-Aike and Gobernador Gregores xenoliths can be summarised in the following: • the compositional regional variations observed in Patagonia volcanics (transitional to OIB-like basalts) are much weaker in basalt-borne xenoliths; nevertheless, an eastwards fading of possible slab-related geochemical signatures is observed; • only few spinel-bearing xenoliths document the presence of highly depleted lithospheric mantle domains related to old partial melting episodes; • the majority of sp-bearing xenoliths provide textural and chemical evidence of significant re-crystallisation due to interaction with melts; in westernmost and northernmost localities cpx compositions suggest either equilibrium or reaction with arc-derived magmas and transitional basalts; • in southernmost and easternmost occurrences lithospheric mantle has been extensively percolated via reactive porous flow by uprising OIB-like asthenospheric melts, which probably erased at various extent the signatures of previous metasomatic events; • the thermo-chemical erosion of the lithosphere is particularly important at Pali Aike and Praguaniyeu, the only two occurrences of garnet-bearing xenoliths, from northern and southernmost Patagonia, respectively; noticeably, at Pali Aike the basalts are poured out in a rift system located in a back-arc setting; • glass veins and pockets in xenoliths do not represent distinct metasomatic agents; they formed by melting or infiltration of the peridotite assemblage caused by upraising basaltic magmas to the surface and their reaction with ambient lithospheric mantle; carbonates are likely produced by degassing of magmatic fluids; • evidence for plume activity and carbonatitic and/or adakitic metasomatism is generally weak; • compositional variability of xenoliths results from both the primary vertical heterogeneity and the chemical overprint caused by reactive porous-flow reactions (under variable melt/rock volume ratios) operated by the migration of earlier transitional basalts, later on followed by OIB-like melts generated in deeper mantle sources. Proposed geodynamic interpretation includes supra-slab and sub-slab models. Nevertheless, the documented upwelling OIB-type asthenosphere cannot be simply ascribed to slab window opening, but it is supposed to be related to concurrent processes including corner-flow in the asthenospheric wedge, vertical slab tear along subducted fracture zones and local enhanced lithospheric extension.

Chapter 10 Fuel resources: oil and gas

Petroleum systems are controlled by the evolution of sedimentary basins and the provenance of their sedimentary fills. As the result of a favourable combination of these factors Sumatra is rich in petroleum resources. The discovery and exploitation of commercial accumulations of oil and gas has so far been restricted to the backarc region of Sumatra, NE of the Barisan Range and the active volcanic arc, where three major sedimentary basins, the North, Central and South Sumatra Basins are distinguished. Exploration in the Sibolga, Mentawai and Bengkulu basins along the western margin of Sumatra in the forearc region has, so far, not been as successful. Commercial success has also eluded companies which have explored basins or sub-basins, such as the Ombilin Basin, which occur within the Barisan Range. Plate-tectonic mechanisms and the resultant crustal thicknesses control this distribution of the Sumatran petroleum resources. To the east of the Barisan Range, beneath the backarc basins, the crust has been stretched and thinned and thus has a high geothermal gradient, suitable for the generation of hydrocarbons. In the forearc region, to the west of the Barisan Range, the lithosphere is thicker due to the subduction of the Indian Ocean Plate beneath the Sunda Craton in Sumatra. This effective doubling of lithospheric thickness has resulted in lowering of the geothermal gradient, so that sediments in the forearc basinal setting have a lower thermal maturity. Also, clastic sediments in forearc basins, due to their volcanic and metamorphic provenance, tend to be poor in quartz, and

The Bandurrias gabbro: Late Oligocene alkaline magmatism in the Patagonian Cordillera

The authors present petrological and geochemical data for the Late Oligocene (ca. 27 Ma) Bandurrias gabbro, a small (≈4.5×3 km), isolated intrusion near Coyhaique (45°35′ Lat. S), southern Chile, in the present-day backarc region of the Patagonian Cordillera. Its mineral chemistry (olivine, clinopyroxene, feldspars, amphibole, biotite and analcime) is typical of gabbros with alkaline affinity. The development of ophitic and subophitic textures, the extremely elongated acicular apatite crystals, and the low CaO content of olivines (0.17–0.46%), as well as the low Al VI/Al IV ratio (0.00–0.12) of clinopyroxenes, indicate low-pressure (subvolcanic) crystallization conditions. The major, trace, and rare earth element geochemistry also indicate an alkaline affinity ( Σ alkali=4.25–7.20%, high TiO 2 and P 2O 5 contents, presence of normative nepheline, Ti/V>50). Geochemical patterns are very similar to those of an ocean island-type alkaline basalt. (La/Lu) N ratios range from 6.99 to 10.60, and the Yb N and Lu N values (<10) suggest the presence of garnet as residual phase in the mantle source. Low Th/La ratios (0.11–0.15) are similar to those of a primitive mantle, and the Zr/Nb (6.3–7.6), La/Nb (0.76–1.03), Ba/Nb (7.6–17.4), Ba/Th (81–120), Th/Nb (0.09–0.15), K/Nb (370–467), Th/La (0.11–0.15), and Ba/La (10.0–16.9) ratios are typical of an enriched mantle-type reservoir. The magma composition suggests generation by low degree partial melting in an extensional or transtensional geodynamic setting. The emplacement of the Bandurrias gabbro took place during a major reconfiguration in the Nazca-South American plate convergence during 28–26 Ma, in either the latest phase of slow, oblique subduction or the subsequent faster, less oblique period of subduction. The data do not allow the authors to rule out either alternative.

Sedimentation of the Neogene–Recent Alaşehir (Gediz) continental graben system used to test alternative tectonic models for western (Aegean) Turkey

The Alaşehir (Gediz) Graben exemplifies clastic sedimentation in a long-lived continental half-graben in a semi-arid setting, developed within relatively incompetent metamorphic rocks. Early Miocene to Recent rift-related sediments are exhumed on both flanks of the graben, allowing detailed three-dimensional study. During the Early Miocene, small fan-delta lobes were shed northwards from the rugged Menderes Metamorphic Massif into a bordering lacustrine basin. During Early to Mid-Miocene time, large alluvial fans prograded northwards into this basin. Through-drainage to the Aegean Sea was established as the basin widened and filled. Discrete lobes of coarse alluvial fan sediments of latest Miocene(?)–Pliocene age, also shedding northwards, are likely to have been climatically influenced. Quaternary alluvium party infills the modern Alaşehır rift basin. The sedimentary information can be used to test two alternative tectonic models for the Alaşehır Graben. In the first model, an E–W graben bounded by high-angle faults was active during latest Miocene(?)–Recent time, whereas earlier Miocene sedimentation was controlled by N–S faulting related to a N–S compressional stress regime. In the second hypothesis, the Alaşehır Graben was initiated much earlier, in the Early Miocene and was then either continuously or episodically active until Recent. Our results, especially facies and palaeocurrent data from alluvial sediments, indicate that clastic sedimentation was controlled by mainly E–W faulting in a N–S stress regime. Assuming the Early Miocene clastic sediments are correctly dated, this supports the second (long-lived extension) model. However, rather than steady-state extension for ca. 15 Ma, the sedimentary evidence and regional context are consistent with a pulsed extension model, whereby initial Early to Mid-Miocene extension and related clastic sedimentation was followed by a second phase of extension in latest Miocene(?)–Pliocene time. The driving force of initial, Early Miocene extension was probably gravity spreading towards a south-Aegean subduction zone, whereas the inferred second extension pulse is seen as being triggered by westward “tectonic escape” of Anatolia towards the extending Aegean back-arc region.

Variation in noble gas isotopic composition of gas samples from the Aegean arc, Greece

In contrast to most other arcs with oceanic plate subduction, the Aegean arc is characterized by continent–continent subduction. Noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc, 2 islands in the back-arc region and 7 sites in the surrounding areas. The 3He/ 4He ratios of the samples ranged from 0.027 R A to 6.2 R A ( R A denotes the atmospheric 3He/ 4He ratio of 1.4×10 −6), demonstrating that even the maximum 3He/ 4He ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems, which are equal to the MORB value of 8±1 R A. Regional variations in the 3He/ 4He ratio were observed both along and across the arc. The maximum 3He/ 4He ratio was obtained from Nisyros volcano located in the eastern end of the arc, and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano. Across the volcanic arc, the 3He/ 4He ratio decreased with an increasing distance from the arc front, reaching a low ratio of 0.063 R A in Macedonia, which suggested a major contribution of radiogenic helium derived from the continental crust. At Nisyros, a temporal increase in 3He/ 4He ratio due to ascending subsurface magma was observed after the seismic crisis of 1995–1998 and mantle neon was possibly detected. The maximum 3He/ 4He ratio (6.2 R A) in the Aegean region, which is significantly lower than the MORB value, is not probably due to crustal assimilation at shallow depth or addition of slab-derived helium to MORB-like mantle wedge, but inherent characteristics of the subcontinental lithospheric mantle (SCLM) beneath the Aegean arc.

Correlation of seismotectonic variables and GPS strain measurements in western Turkey

Interrelationships between the multifractal properties of epicenter distribution (Dq), the Gutenberg‐Richter b value, and GPS‐derived strain (shear and dilatation) for the time period extending from 1981 to 1998 are examined in the eastern Mediterranean and western Turkey area. This analysis was conducted in three tectonic subdivisions corresponding to regions dominated by shear, extensional, and compressional deformation. The region of shear coincides with the Northern Anatolian Fault Zone (NAFZ); the region of extension, with the Aegean back arc region; and the region of compression, to the Aegean subduction zone between the African and Anatolian plates. The median b value in the region of extension is larger than those observed in the regions of shear and compression and suggests that large magnitude earthquakes are less probable in this region than along the NAFZ and Aegean subduction zone. A significant correlation is observed between D2 and D15 with dilatation over the 10–40 km range in the back arc region of extension. A marginal correlation between GPS‐derived strain and b value is observed only in the region of compression where it correlates negatively with shear and positively with dilatation. A significant positive correlation is observed between D and b along the NAFZ. A clear relationship between D and b does not exist in regions deformed by extension and compression. The relationship between D and b along the NAFZ may also be an indicator of increased seismic hazard since the positive relationship between D and b is a recent characteristic of the NAFZ observed only in the 20‐year period preceding the Izmit earthquake.

Boron and lithium isotopic variations in a hot subduction zone—the southern Washington Cascades

New B and Li isotopic data are presented for well-characterized basalts from a transect across the Cascades volcanic arc, S. Washington, for a sediment core on the incoming Juan de Fuca plate, and for dacites from Mt. St. Helens. The basalts define two compositional groups, one (I) having affinities to those from intraplate settings and the other (II) resembling typical volcanic arc calcalkalic lavas. These groups are inferred to have distinct sources. Group I lavas have low Li/Y (0.25–0.6) and B/Nb (∼0.1–0.45), near-constant Ba/Nb (7–18), and show little cross-arc variation in fluid-mobile/immobile element ratios. In contrast, Group II lavas have slightly elevated Li/Y (∼0.5) and higher Ba/Nb (21–100) regardless of location; B/Nb is enriched (to ∼2.2) in the frontal arc, but toward the backarc drops to lower values (<0.4) overlapping with intraplate basalts. δ 11B values are among the lowest reported for volcanic arcs. In detail, δ 11B exhibits spatial variation, being highest (∼−3‰ to −0.4‰) in frontal arc lavas then decreasing sharply to OIB-like values (−8‰ to −9‰) in the backarc region; no systematic variation with magma types is apparent. δ 7Li values (2.5‰ to 5‰) are MORB-like and show no systematic spatial variation. Combined with Sr–Nd–Pb isotopic and other trace element data, these results are consistent with weak slab contributions beneath the frontal arc ranging to negligible inputs below the backarc. This pattern is attributed to progressive dehydration of the subducting slab in an unusually warm subduction zone (SZ); initial inventories of fluid-mobile elements (FMEs) apparently were reduced extensively before the slab reached subarc depths, and essentially depleted as it moved beneath the backarc. Slab contributions appear to be superimposed on a compositionally layered mantle structure consisting of two domains sampled by the distinct basalt groups. Group I lavas appear to form by decompression melting of convectively upwelling deeper mantle that is little modified from an OIB–MORB source composition, except for the relatively small effect on B composition beneath the frontal arc. Group II lavas may form by melting of enriched domains within shallower lithospheric mantle. Because both groups have similar δ 11B, production of Group II magmas need not involve modern inputs of slab fluid. This interpretation differs from the standard paradigm for arc magmatism in that flux melting is not invoked as the dominant cause for partial melting; efficient dehydration of the slab may remove the impetus to drive this mechanism.

Tethyan vs. Cordilleran ophiolites: a reappraisal of distinctive tectono-magmatic features of supra-subduction complexes in relation to the subduction mode

Supra-subduction zone (SSZ) ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones. Moreover, owing to their structural characteristics and location in the overriding plate, these complexes are far better represented and preserved than Mid-Ocean-Ridge-Basalt (MORB) ophiolites in orogenic belts. In terms of their structure, tectonics, and magmatic features, SSZ ophiolites may be classified in two main types: (1) “Tethyan complexes” (such as those of the Albanide-Hellenide belt), which mostly consist of complete and extensive volcanic, dyke, plutonic, and mantle sections with prevalent Island Arc Tholeiitic (IAT) affinity and the significant presence of Boninites, obducted as relatively intact lithospheric slabs onto collisional continental margins; (2) “Cordilleran complexes” (including the circum-Caribbean ophiolites of Guatemala, Cuba, and Venezuela), mostly represented by dismembered sections of arc volcanic, plutonic, and subordinate mantle sequences with tholeiitic to Calc-Alkaline (IAC) magmatic affinity and acidic differentiates, commonly associated with metamorphic “subduction complexes” and tectonically emplaced onto or juxtaposed against the continental margin within polygenetic terranes. These two types appear to be related to significantly different subduction modes and intraoceanic plate dynamics whereby SSZ ophiolites were generated. The Tethyan complexes can best be compared with west Pacific-type subduction with accentuated steepening and retreat of the subducted slab, accompanied by progressive decoupling of the converging plates, intense mantle diapirism, and tensional events in the upper plate with generation of large IAT-sheeted dike complexes in “open” oceanic spreading systems. Continuous slab sinking and roll back allow increasing asthenospheric diapirism and interarc spreading from the arc axis to the forearc region with generation of boninites and Back Arc Basin Basalts (BABB) with intermediate MORB/IAT composition. By contrast, the genesis of the Cordilleran complexes requires a subduction mode characterized by a steady state regime, with moderate and constant dip of the subducted slab and limited extension in the backarc region. The magmatic evolution of these ophiolites from IAT to IAC and the significant presence of rhyodacite (and tonalite) differentiates coherently indicate a more mature stage of arc magmatism, as well as the occurrence of efficient differentiation processes developing under nearly “closed-system” conditions in independent magma chambers. Tethyan complexes are likely to be obducted as large and relatively intact slabs onto the continental margins through the interposition of metamorphic soles, which represent relics of the MORB lithosphere underplating the SSZ ophiolites. On the other hand, the common emplacement of Cordilleran complexes within polygenetic terranes appears to be controlled by prolonged accretionary mechanisms that trap, against the continental margin, parts of the arc structure, subduction complexes, melanges, and volcanoclastic products.

Lead isotope composition of Central American volcanoes: Influence of the Galapagos plume

Lead isotopic analyses of lavas from Central America, both along and behind the volcanic arc, help to clarify source components in the mantle wedge. Analysis of previous Pb isotopic data had implied that little or no marine sediment lead was added to the Central American source region, as all samples fell within the MORB field, in contrast to other information (e.g., Ba/La, 10Be, 87Sr/86Sr) that indicated a high subduction component. The data presented here include several analyses of local marine sediment, showing it to be exceptionally unradiogenic in Pb and thus permitting high sediment contributions to the mantle source region without significant changes in Pb isotopes. Combined Pb‐Nd and Pb‐Sr isotopic diagrams clearly illustrate the influence of crustal contamination for samples from Guatemala and Honduras, and of subducted sediment for all lavas of the volcanic front. Samples collected behind the volcanic front are derived from mixing between enriched and depleted mantle sources, and in central Costa Rica (extending to the back arc) overlap Pb, Sr and Nd isotope values for both Cocos Island and some components of the Galapagos hot spot. The restricted geographical occurrence of the enriched mantle signature in Central America, coupled with the persistence of the signal well into the back arc region, imply that these lavas are sampling Galapagos plume‐influenced mantle. The presence of this plume component beneath southern Central America and extending to the northeast beneath the Caribbean confirms a Galapagos hot spot origin for this part of Caribbean Plate.

The thermal effects of steady-state slab-driven mantle flow above a subducting plate: the Cascadia subduction zone and backarc

At subduction zones, geophysical and geochemical observations indicate that the arc and backarc regions are hot, in spite of the cooling effects of a subducting plate. At the well-studied Cascadia subduction zone, high mantle temperatures persist for over 500 km into the backarc, with little lateral variation. These high temperatures are even more surprising due to the juxtaposition of the hot Cascadia backarc against the thick, cold North America craton lithosphere. Given that local heat sources appear to be negligible, mantle flow is required to transport heat into the wedge and backarc. We have examined the thermal effects of mantle flow induced by traction along the top of the subducting plate. Through systematic tests of the backarc model boundary, we have shown that the model thermal structure of the wedge is primarily determined by the assumed temperatures along this boundary. To get high temperatures in the wedge, it is necessary for flow to mine heat from depth, either by using a temperature-dependent rheology, or by introducing a deep cold boundary through a thick adjacent lithosphere, consistent with the presence of a craton. Regardless of the thermal conditions along the backarc boundary, flow within an isoviscous wedge is too slow to transport a significant amount of heat into the wedge corner. With a more realistic stress- and temperature-dependent wedge rheology, flow is focused into the wedge corner, resulting in rapid flow upward toward the corner and enhanced temperatures below the arc, compatible with temperatures required for arc magma generation. However, this strong flow focusing produces a nearly stagnant region further landward in the shallow backarc mantle, where model temperatures and heat flow are much lower than observed. Observations of high backarc temperatures, particularly in areas that have not undergone recent extension, provide an important constraint on wedge dynamics. None of the models of simple traction-driven flow were able to simultaneously produce high temperatures beneath the volcanic arc and throughout the backarc. The most likely way to produce hot and isothermal conditions in the backarc is through vigorous small-scale-free convection in a low-viscosity asthenosphere.

Basaltic volcanism in the Bering Sea: geochronology and volcanic evolution of St. Paul Island, Pribilof Islands, Alaska

The evolution of a Pleistocene to Holocene basaltic volcanic field in the back-arc region of the Aleutian subduction system is investigated at St. Paul Island, Alaska, one of the youngest eruptive centers in the Bering Sea basalt province. New 40Ar/ 39Ar and 14C age determinations indicate that subaerial volcanic activity forming the island began as early as 540 ka and has continued nearly to the present; the youngest eruption occurred approximately 3230 years BP. Magmas erupted on St. Paul are basaltic with MgO contents ranging from 14 to 4 wt.% and phenocryst assemblages of olivine+clinopyroxene±plagioclase; all are alkalic. The surface of St. Paul is composed mainly of numerous tephra cones surrounded by coalescing, low-viscosity pahoehoe lava flows. A central highland spans the island from east to west and is constructed of relatively young eruptive centers where rocks show a minimum of weathering and little deformation by faulting. In contrast, older lava flows forming the wave-eroded base of the island are gently to moderately tilted and faulted. Geochronologic, stratigraphic, and geochemical data indicate that eruptive styles on St. Paul evolved from early, mostly effusive eruptions of chemically little evolved lavas that form the base of the island, to more explosive monogenetic scoria cones, to polygenetic centers forming shields by repeated effusive eruptions of evolved low-viscosity lavas. Localization of the monogenetic and polygenetic centers appears to be related to east–west and northeast–southwest trending fault and fissure systems, with polygenetic centers located at intersections of major structures. The combined volcanic and compositional changes on St. Paul Island suggest that the magmatic system as a whole may be trending toward eruption of more evolved magmas related to the progressive development of crustal magma chambers in which crystal fractionation and magmatic differentiation are occurring.

2D numerical modelling of hydrocarbon generation in subducted sediments at the active continental margin of Costa Rica

The Middle America Trench is the topographic expression of the subduction of the Cocos Plate beneath the Caribbean Plate. In this study, the subduction process was simulated in a numerical basin model to determine the fate of the organic matter. During subduction, temperature rises and the organic matter embedded in sediments generates hydrocarbons. However, equilibrium temperatures are not reached due to the fast subduction process, shifting the petroleum generation zone to depths exceeding 10,000 m. The average organic carbon content of the subducted sediments is low but the fast subduction leads to a high rate of hydrocarbon generation. For example, along a 1 km wide section along the subduction zone a methane generation rate of about 9×10 5 m 3/a is calculated. The hydrocarbon generation process occurs at much greater depth than in conventional sedimentary basins. Due to the low migration velocity, oil is transported from the kitchen area to a greater depth and finally transformed to methane. Interestingly, calculated migration velocities of methane are mostly lower than the current subduction velocity. Thus, even methane is further subducted. This modelling result is confirmed by published data on gas composition at the trench, where (almost) no thermogenic gas has been detected. The subduction of large amounts of methane to very great depths could have an effect on volcanism in the back arc region or on metamorphic processes.

Basaltic volcanism in the Bering Sea: geochronology and volcanic evolution of St. Paul Island, Pribilof Islands, Alaska

The evolution of a Pleistocene to Holocene basaltic volcanic field in the back-arc region of the Aleutian subduction system is investigated at St. Paul Island, Alaska, one of the youngest eruptive centers in the Bering Sea basalt province. New 40Ar/39Ar and 14C age determinations indicate that subaerial volcanic activity forming the island began as early as 540 ka and has continued nearly to the present; the youngest eruption occurred approximately 3230 years BP. Magmas erupted on St. Paul are basaltic with MgO contents ranging from 14 to 4 wt.% and phenocryst assemblages of olivine+clinopyroxene±plagioclase; all are alkalic. The surface of St. Paul is composed mainly of numerous tephra cones surrounded by coalescing, low-viscosity pahoehoe lava flows. A central highland spans the island from east to west and is constructed of relatively young eruptive centers where rocks show a minimum of weathering and little deformation by faulting. In contrast, older lava flows forming the wave-eroded base of the island are gently to moderately tilted and faulted. Geochronologic, stratigraphic, and geochemical data indicate that eruptive styles on St. Paul evolved from early, mostly effusive eruptions of chemically little evolved lavas that form the base of the island, to more explosive monogenetic scoria cones, to polygenetic centers forming shields by repeated effusive eruptions of evolved low-viscosity lavas. Localization of the monogenetic and polygenetic centers appears to be related to east–west and northeast–southwest trending fault and fissure systems, with polygenetic centers located at intersections of major structures. The combined volcanic and compositional changes on St. Paul Island suggest that the magmatic system as a whole may be trending toward eruption of more evolved magmas related to the progressive development of crustal magma chambers in which crystal fractionation and magmatic differentiation are occurring.

Tectonic evolution of the orogenic belt in the region located between Kerman and Neyriz

Tectonic evolution of the orogenic belt in the region located between Kerman and Neyriz is explored within the context of two NE-dipping Neotethyan subduction zones. The northern subduction zone was responsible for formation of the Mesozoic Sanandaj-Sirjan magmatic arc and the Late Cretaceous Shahr Babak-Baft ensimatic back-arc extensional basin. The southern subduction zone was responsible for formation of the reverse primitive Neyriz island-arc, the Central Iranian volcanic belt, and its line of porphyry copper deposits, as well as the Rafsanjan back-arc extensional basin (proto-Rafsanjan depression). Due to the collision of the Neyriz island arc with the Sanandaj-Sirjan active continental margin, the Shahr Babak-Baft back-arc basin was tectonically inverted into the Shahr Babak-Baft ophiolite belt. Finally, the continental collision between the Arabian and the Central Iranian plates took place during the Neogene. This led to the tectonic inversion of the Rafsanjan ensialic basin into the Rafsanjan retro arc basin (the present Rafsanjan depression), and emplacement of the syncollision to late- or post-orogenic granitoid rocks (felsic mass) in the Rafsanjan region.

Tectonics and sedimentation in Southern South America during Triassic

Triassic continental sedimentation in Southern South America is related to the Gondwanides orogenesis directly. There were two distinct phases of sedimentation, (1) the wide alluvial basins during Early Triassic and (2) the restricted extensional basins during Middle and Late Triassic. The main Early Triassic deposits are the Sanga do Cabral Supersequence (Southernmost Brazil and Uruguay) and the Talampaya and Tarjados sequences (Western Argentina). These deposits represent poorly channelized braided rivers with source-areas in the Gondwanides units under compressional tectonic setting. The main Middle and Late Triassic strata are rifts in the back-arc setting such as the Ischigualasto and Cuyo basins (Western Argentina) and in the intracontinental setting (Santa Maria Supersequence, Southernmost Brazil). These basins are mainly composed of alluvial, lacustrine and deltaic deposits with a great pyroclastic contribution in the case of the basins of Argentina. Two rift sequences were identified in the Western Argentina basins composed of syn-rift alluvial deposition followed by lake transgression. The sag phase is marked by the progradation of deltaic and high sinuosity rivers. The Southernmost Brazil deposits were divided into three rift sequences composed of low sinuosity rivers followed by lake transgression. The progradation of deltaic and fluvial systems marks the sag phase. The accommodation space generation within the back-arc rifts was high if compared to the intracontinental coeval basin due to higher base level and higher tectonic subsidence rates. The development of deep organic matter-rich lakes suggests a more humid palaeoclimate in the back-arc region than in the intracontinental setting where playa lakes dominated. The back-arc rifts are related to sinistral transtensional stresses in the western branch of Gondwanides region. The intracontinental rifts (the Southernmost Brazil deposits and coeval rift basins of Southern Africa) were generated by transtensional stresses within a dextral shear zone. The shear is related to stress transmission from the southern branch of the Gonwanides where oblique compression occurred.

Silicic volcanism and back-arc extension related to migration of the Late Cainozoic Australian–Pacific plate boundary

The initial phases of large-scale silicic and bimodal volcanism within an active Miocene arc (Coromandel Volcanic Zone, CVZ) are preserved on several small islands in northern New Zealand. Major- and trace-element compositions classify the rocks as high-K, calc-alkaline, silicic lavas which have some of the low-ion lithophile element and high-field-strength element characteristics of convergent margin lavas. The silicic lavas (∼12 Ma) are spatially associated with a relatively primitive basalt of similar age (12.06±0.25 Ma), which is a sub-alkaline tholeiite with an anomalously high TiO 2 content and as such it is not a typical arc basalt. Isotopically, the samples indicate the presence of oceanic sediment in their petrogenesis. An explanation for the geochemical and isotopic compositions of the northern CVZ rocks involves migration of the Australian–Pacific convergent margin orientation from a north–south direction (Northland Volcanic Arc), through a northeast–southwest direction (CVZ), and finally subparallel to the orientation of the presently active Taupo Volcanic Zone. The silicic eruptives thereby preserve a trail of volcanic edifices within a back-arc basin. The edifice-trail exhibits a general northwest to southeast younging trend and lies perpendicular to the axis of the contemporaneous Coromandel Volcanic Arc. Late-Cainozoic migration of the convergent margin and subduction system is proposed as the mechanism for extension across the back-arc region of the CVZ, producing large-scale back-arc silicic volcanism, and minor associated basalts, perpendicular to the orientation of the main arc.