Retroarc Research Articles

3-B10 新生代ユーラシア大陸東縁部背弧域の上部マントル : 北西九州および韓国の玄武岩からの岩石学的制約(マグマの発生と移動2,口頭発表)

3-B10 新生代ユーラシア大陸東縁部背弧域の上部マントル : 北西九州および韓国の玄武岩からの岩石学的制約(マグマの発生と移動2,口頭発表)

Subduction-triggered magmatic pulses: A new class of plumes?

A variety of atypical plume-like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed, and seismological imaging has shown ubiquitous small-scale convection in the uppermost mantle in regions such as the Mediterranean region, the western US, and around the western Pacific. We argue that the three-dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics. The testable surface expressions of this process are the outside-arc alkaline volcanism, topographic swell, and low-velocity seismic anomalies associated with partial melt. Using three-dimensional, simplified numerical subduction models, we show that focused upwellings can be generated both ahead of the slab in the back-arc region (though ~ five times further inward from the trench than arc-volcanism) and around the lateral edges of the slab (in the order of 100 km away from slab edges). Vertical mass transport, and by inference the associated decompression melting, in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities. The upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation. We discuss representative case histories from the Pacific and the Mediterranean where we find possible evidence for such slab-related volcanism.

Possible deep lithospheric roots beneath South-Eastern Carpathians back-arc region

The Vrancea seismic region represents a unique case of well-defined and intense intermediate-depth earthquake activity as a consequence of specific geodynamic processes at the continental contact between East-European, Moesian and Intra-Alpine plates. Apart the subcrustal earthquakes generated in the Vrancea slab, the analysis of seismicity puts into evidence two other clusters of subcrustal earthquakes (h > 50 km) toward the back-arc side of the SE Carpathians bend, one to the west (Sinaia), the other to the north-west (Brasov Depression) relative to the Vrancea seismic source. The hypocenters lay down between 50 to 105 km in Sinaia and between 50 and 136 km in Brasov Depression. The rate of seismic energy release is much lower (Mw ≤ 3.7 in Sinaia and Mw ≤ 3.1 in Brasov Depression) than for Vrancea activity. The seismicity pattern in the SE Carpathians back-arc region upper mantle correlates well with the high-velocity structures depicted by seismic tomography investigations suggesting possible remnant deep lithospheric roots apart from the narrow well-defined slab generating Vrancea major earthquakes. The results outline significant lateral heterogeneities in the mantle and provide new data for incorporating seismological, geotectonic and volcanological data in a unified modeling of the complex processes taking place in the study region.

Reinterpretation of the Ordovician rotations in NW Argentina and Northern Chile: a consequence of the Precordillera collision?

Early Paleozoic paleomagnetic data from NW Argentina and Northern Chile have shown large systematic rotations within two domains: one composed of the Western Puna that yields very large (up to 80°) counter-clockwise rotations, and the other formed by the Famatina Ranges and the Eastern Puna that shows (~40°) clockwise rotations around vertical axes. In several locations, lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the Paleozoic. Previous tectonic models have explained these rotations as indicative of rigid-body rotations of large para-autochthonous crustal blocks or terranes. A different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of Precordillera in the Late Ordovician. This collision should have generated dextral shear zones in the back arc region of the convergent SW Gondwana margin, where systematic domino-like clockwise rotations of small crustal blocks accommodate crustal shortening. The Western Puna block, bordering the Precordillera terrane to the north, might have rotated counterclockwise as an independent microplate due to tectonic escape processes, in a fashion similar to the present-day relationship between the Anatolia block and the Arabian microplate.

Morphology and tectonics of the Andaman Forearc, northeastern Indian Ocean

SUMMARY The Andaman Sea has developed as the result of highly oblique subduction at the western Sunda Trench, leading to partitioning of convergence into trench-perpendicular and trench-parallel components and the formation of a northward-moving sliver plate to accommodate the trench parallel motion. The Andaman forearc contains structures resulting from both components of motion. The main elements of the forearc are the accretionary prism and outerarc ridge, a series of forearc basins and major N–S faults. The accretionary prism is an imbricate stack of fault slices and folds consisting of ophiolites and sediments scrapped off the subducting Indian Plate. The western, outer slope of the accretionary prism is very steep, rising to depths of 1500–2000 m within a distance of 30 km. There is a difference in the short wavelength morphology between the western and eastern portions of the accretionary prism. The outer portion consists of a series of faulted anticlines and synclines with amplitudes of a few 100 to ∼1000 m and widths of 5–15 km resulting from ongoing deformation of the sediments. The inner portion is smoother with lower slopes and forms a strong backstop. The width of the deforming portion of the accretionary prism narrows from 80 to 100 km in the south to about 40 km between 10 ◦ N and 11 ◦ 30 � N. It remains at about 40 km to ∼14 ◦ 40 � N. North of there, the inner trench wall becomes a single steep slope up to the Myanmar shelf. The eastern edge of the outerarc ridge is fault bounded and, north of the Nicobar Islands, a forearc basin is located immediately to the east. A deep gravity low with very steep gradients lies directly over the forearc basin. The West Andaman Fault (WAF) and/or the Seulimeum strand of the Sumatra Fault System form the boundary between the Burma and Sunda plates south of the Andaman spreading centre. The WAF is the most prominent morphologic feature of the Andaman Sea and divides the sea into a shallow forearc and a deeper backarc region. The Diligent Fault runs through the forearc basin east of Little Andaman Island. Although it has the general appearance of a normal fault, multichannel seismic data show that it is a compressional feature that probably resulted from deformation of the hanging wall of the Eastern Margin Fault. This could occur if the forearc basins were formed by subduction erosion of the underlying crust rather than by east–west extension.

秋田県南部，出羽丘陵西麓～沿岸地域における活断層の分布と断層モデル

秋田県南部，出羽丘陵西麓～沿岸地域における活断層の分布と断層モデル

Pliocene intraplate-type volcanism in the Andean foreland at 26°10′S, 64°40′W (NW Argentina): Implications for magmatic and structural evolution of the Central Andes

The Antilla magmatic complex (26°10′S, 64°40′W, NW Argentina) attests to magma eruption at ca. 4.7 Ma in the Central Andes backarc region, 300 km E of the active arc. The Antilla lavas have an alkaline, predominantly mafic composition and record the most primitive isotopic ratios (87Sr/86Sr = 0.704360 and 143Nd/144Nd = 0.512764) of Central Andes Neogene backarc volcanism between 24°S and 27°S. They show trace-element patterns recalling backarc Pliocene-Quaternary intraplate mafic rocks, but they show lower silica and higher alkali contents, and are interpreted to derive from the depleted subcontinental mantle.

Geochemistry and genesis of behind-arc basaltic lavas from eastern Nicaragua

The petrology and chemistry of the Behind the Volcanic Front (BVF) lavas from eastern mainland Nicaragua and the adjacent Great Corn Island in the Caribbean Sea illustrate the complex nature of sources and processes operating in such a tectonic setting. The older, Early Miocene (∼ 17 Ma) group of low-Ti (< 1 wt.%) basalts–andesites is characterized by a strong LILE/HFSE depletion. The low-Ti lavas from El Rama and El Bluff areas are interpreted as relics of Early Miocene volcanic arc, largely analogous to the nowadays extinct Coyol arc further west. However, these rocks differ in some parameters from the modern volcanic front lavas, most notably in having lower δ 7Li values, Ba/Yb ratios and lower U contents. The younger high-Ti (Ti > 1.5%) lavas, rich in other HFSE as well, are represented both by alkaline (Quaternary trachybasalts: Volcán Azul and Kukra Hill) and subalkaline (basalts–basaltic andesites: Late Miocene, ∼ 11 Ma Great Corn Island and Quaternary, Pearl Lagoon) volcanic rocks. The Late Miocene and Quaternary high-Ti BVF lavas probably represent small-volume decompression melts of a source similar to that of the OIB-like magmas, most likely upwelling asthenosphere having a strong Galápagos mantle imprint. The positive Sr–Nd isotopic correlation indicates an interaction between this OIB component and a depleted lithospheric mantle modified by a subduction-related influx of Sr and, to a lesser extent, other hydrous fluid-mobile elements. However, the rocks show no recognizable influence of the modern subduction. The feeble trace-element (e.g., slightly elevated Ba, K, and Sr at some localities) and a more pronounced Sr–Li isotopic subduction-related signal stems most likely from the Miocene convergence episode. Subduction of the Galápagos hot-spot tracks in Costa Rica produces magmas that can be readily recognized by their elevated Sr isotopic ratios due to seafloor alteration; the Nd isotopic signature remains unaffected. Such a component with relatively unradiogenic Nd and radiogenic Sr is required in the source of the modern volcanic front lavas but is not needed to explain the variation in the studied BVF dataset. Terrains with multiepisodic subduction history should be considered with caution, as the lavas generated by decompression melting of the asthenospheric source in the back-arc region may bear a geochemical imprint of the fossil and not the modern subduction component stored in the local lithospheric mantle.

新生代北部九州のテクトニクス史と火山活動史―背弧火山岩区研究における重要性―

Cenozoic western Pacific is characterized by numerous subduction volcanisms, opening of back arc basins, and within-plate volcanisms in backarc regions. These phenomena suggest that thermal disturbances induced by mantle flow caused mantle melting and eruption of magmas in back-arc regions, although diverse modes have been proposed for the mantle condition. To constrain the origin of the magmatism, it is important to clarify both the geodynamics and physicochemical conditions of the mantle in the arc-back arc system. Cenozoic back-arc volcanisms and tectonics in northern Kyushu are key objectives. This paper focuses on the dynamics of the upper mantle in relation to the tectonics of the area. Cenozoic volcanism of the northern Kyushu district is characterized by eruptions of a number of alkaline volcanisms. This volcanic province extends some one hundred kilometers along the Japan Sea coast and includes the Chugoku area. Although the volcanism has been regarded as a large single volcanic province, the northern Kyushu activity is distinguishable from that in Chugoku by geochemical characteristics. Cenozoic basalt volcanism in northern Kyushu occurred mainly in the Tertiary sedimentary basins, which is common to Cenozoic volcanism in the Japan Sea Basin. The sedimentary basin deposits underwent intensive folding with extensive inversion tectonics occurring mainly around the end of Miocene. Studies on dykes and thrust faults also indicate a compressional stress field dominated in northwestern Kyushu during the late Miocene. Initiation of the basaltic activity took place in Kita-Matsuura area and expanded outwards. Basaltic rocks from the Kita-Matsuura are mostly hypersthene-normative and sub-alkalic, whereas rocks from peripheries, such as North Kyushu and Goto Islands, are predominantly less normative-hypersthene and thus are alkalic. From spatial and temporal variations and genetic conditions of basalt magmas, a most plausible source was a small-scale mantle diapir with a potential temperature of more than 1400°C. Combined with an inversion tectonic background after the Japan Sea opening event, the origin of the back-arc magmatism was unrelated to large-scale mantle upwelling. Fluid-fluxed melting of the mantle is not supported by the chemistry. My proposal is active upwelling of a small-scale mantle diapir, probably from the depth of stagnant slab or mantle transition zone.

日本海拡大時の中央および西南日本前弧域テクトニクス

Studies on geologic structures including some deformation microstructures in both areas along the Median Tectonic Line (MTL) in western Shikoku and of the South Fossa Magna indicate that N-S trending horizontal compression tectonics prevailed in these areas during the earliest Middle Miocene (ca. 15 Ma). Also, a review of structural development in Nankai province indicates that similar compression tectonics prevailed during the periods from the Early to earliest Middle Miocene (ca. 20-15 Ma). These periods are nearly coeval with the timing of the Japan Sea opening. Although there is uncertainty about the reactivation of the MTL around this time, south-dipping normal faults were formed in the Sambagawa belt truncating the MTL to produce the Kuma basin (18-16 Ma), which were later reactivated as thrusts (Tobe and Hanayama thrusts), indicating inversion tectonics. In the South Fossa Magna region, not only a large amount of shortening—as much as 40-50%—was caused in the Momonoki Subgroup (earliest Middle Miocence, 16-15 Ma) by folding strata, but also the constituent quartz grains were moderately plastically deformed and microcracked followed by healing (i.e. healed microcracks), indicating deformation under brittle-ductile transition conditions (ca. 300°C) at ca. 15 Ma. Furthermore, slaty cleavage caused by pressure solution developed in the strata constituting the Oligocene-Early Miocene (>20 Ma) Southern Shimanto belt, and forearc sediments (18-15 Ma) as thick as 4000 m overlying these sediments with angular unconformity (e.g. Tanabe and Kumano Groups in the Kii peninsula) were also deformed by folding with a shortening ratio of 10-20%. All these onland geological facts in the forearc region indicate that the forearc region was in a regime of compression, while the back-arc region was in a regime of extension at the same time during the Japan Sea opening. Similar tectonics also occurred in Italy, where the Tyrrhenian Sea (back-arc basin) has opened since the Pliocene (5 Ma), while shortening tectonics shown by the development of fold and thrust belts and deep (>7 km) basins occurred at the same time in the forearc region. Although these concurrent back-arc extension and forearc shortening are difficult to interpret, they may be caused by ridge push force (i.e. gravity tectonics) after back-arc spreading commences so that extensional force is no longer sustained in the arc.

The Early Mesozoic evolution of the Western Greater Caucasus (Russia): Triassic–Jurassic sedimentary and magmatic history

Abstract The Greater Caucasus (GC) forms a high Alpine fold-and-thrust belt on the southern margin of the East European Platform (EEP). The Triassic, and particularly, the Jurassic history of the Western Greater Caucasus region is important for our understanding of the palaeogeographic and tectonic evolution of the western Tethys area. In order to better constrain the nature and relevance of these events in the evolution of the region, which are classically described as the Late Triassic to Late Jurassic Cimmerian events, a field campaign in the Western Greater Caucasus was undertaken. Analysis of structural, sedimentological and petrological data from 41 sites in the Fore-Caucasus (Malaya Laba, Mount Tkhach-Belaya River), the Central Greater Caucasus (Georgievskoye, Otdaleni) and Southern Slope (Krasnaya Poliana) areas of the Western Greater Caucasus revealed that a broad asymmetric basin, with associated emergent volcanic islands, formed in the area in Jurassic times. Incipient back-arc rifting in Pliensbachian times was coeval with similar rifting episodes in the Pontides and South Caspian Sea areas. The synchroneity of these events may have been related to the renewal of the Tethys subduction to the south of the Eo-Cimmerian accretionary belt. Rift reactivation, with significant thinning of the continental lithosphere, occurred in the Aalenian. Despite the strong Alpine tectonic overprinting, some structural data confirms that the extension trend was east–west (almost parallel to the active margin) resulting in the formation of a series of pull-apart basins in the GC and the South Caspian region behind the Eastern Pontides–Lesser Caucasus subduction-related volcanic belt. In Bajocian times, subduction-related volcanic activity largely expanded from the Eastern Pontides–Lesser Caucasus to encompass the Transcaucasus, the southern part of GC and the Crimea region. Such widening of the volcanic arc was probably due to a shallowing of the northward subducting slab. In the back-arc GC region, this signalled the onset of the post-rift stage. The return of the slab to normal steepness resulted in subsidence in the back-arc region and in the GC with extensive accommodation space creation. This was subsequently filled by the Late Jurassic, Cretaceous and Cenozoic sedimentary successions.

Subduction of the South Chile active spreading ridge: A 17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina)

The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/ 39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4–3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.

The consequences of crustal melting in continental subduction

Orogens grow in part by thrusting and burial of continental crust, in some cases, into the mantle by continental subduction. Deeply buried continental crust may partially melt during ascent, adding magma to the overlying continental crust, creating a continental plateau, and accounting for the large melt volumes observed in modern and ancient orogens. Thermal-mechanical models support a link among continental subduction, melting, and crustal flow in the overriding plate and show that partial melting may be a significant process in exhumation of ultrahigh-pressure (UHP) rocks and orogenic evolution in general. In Cordilleran orogens, the overriding plate is hot and thin and includes a back-arc region favorable for crustal-scale thrusting and high-temperature metamorphism. Collisional orogens have thicker lithosphere and experience lithosphere-scale thrusting and deeper burial of continental crust. In either case, the ultimate fate of continental crust is partial melting, the depth of which controls the processes and rates of crustal differentiation and exhumation. This paradigm relates continental subduction and crustal melting to lower-crustal flow and plateau development in the overriding plate and explains the presence of UHP rocks in migmatite and granite and their occurrence in the overriding plate of some orogens.

The Lower Cretaceous King Lear Formation, northwest Nevada: Implications for Mesozoic orogenesis in the western U.S. Cordillera

Cretaceous paleogeography of the U.S. Cordillera west of the Sevier fold-and-thrust belt is poorly known due to the scarcity of Cretaceous supracrustal rocks in the arc and backarc regions. The Lower Cretaceous King Lear Formation, exposed in the Jackson Mountains and the Krum Hills of northwest Nevada, provides a rare and important record of Early Cretaceous tectonism and paleogeography. Our work shows that the King Lear Formation everywhere overlies deformed and metamorphosed Triassic-Jurassic rocks across a major unconformity. The King Lear Formation is dominated by conglomerates and sandstones deposited in subaerial alluvial-fan and gravelly braided river systems, and it can be divided into three new members based on differences in clast provenance: a lower member derived from local arc and backarc rocks; a middle member dominated by externally derived quartzite and chert clasts; and an upper member derived largely from an intrabasinal volcanic complex. Sedimentary features and provenance analysis indicate that clasts in the middle member were derived from nearby exposures of Paleozoic rocks such as the Roberts Mountains and Golconda allochthons, which must have formed a topographically elevated area in central Nevada during the Early Cretaceous. The stratigraphic record provides evidence of deposition in a tectonically active basin. In contrast with some prior studies inferring that deposition was synchronous with contractional deformation, our new field observations, structural data, and a shallow seismic-reflection profile confirm that the King Lear Formation was deposited in an active half-graben in the Jackson Mountains. These results provide new upper age constraints on the timing of shortening deformation in the arc and backarc regions: shortening must have been complete not only before deposition of the King Lear Formation in the middle Early Cretaceous, but also prior to several kilometers of exhumation of its depositional basement. Possible driving forces for extension leading to development of the King Lear basin include relaxation of thick crust in the hinterland of thrust belts to the east, stresses associated with regional tilting and development of dynamic topography, and stresses related to dextral strike-slip deformation to the west.

Mafic Late Miocene–Quaternary volcanic rocks in the Kamchatka back arc region: implications for subduction geometry and slab history at the Pacific–Aleutian junction

New 40Ar/39Ar and published 14C ages constrain voluminous mafic volcanism of the Kamchatka back-arc to Miocene (3–6 Ma) and Late Pleistocene to Holocene (<1 Ma) times. Trace elements and isotopic compositions show that older rocks derived from a depleted mantle through subduction fluid-flux melting (>20%). Younger rocks form in a back arc by lower melting degrees involving enriched mantle components. The arc front and Central Kamchatka Depression are also underlain by plateau lavas and shield volcanoes of Late Pleistocene age. The focus of these voluminous eruptions thus migrated in time and may be the result of a high fluid flux in a setting where the Emperor seamount subducts and the slab steepens during rollback during terrain accretions. The northern termination of Holocene volcanism locates the edge of the subducting Pacific plate below Kamchatka, a “slab-edge-effect” is not observed in the back arc region.

Bivergent thrust wedges surrounding oceanic island arcs: Insight from observations and sandbox models of the northeastern Caribbean plate

At several localities around the world, thrust belts have developed on both sides of oceanic island arcs (e.g., Java-Timor, Panama, Vanuatu, and the northeastern Caribbean). In these localities, the overall vergence of the backarc thrust belt is opposite to that of the forearc thrust belt. For example, in the northeastern Caribbean, a north-verging accretionary prism lies to the north of the Eastern Greater Antilles arc (Hispaniola and Puerto Rico), whereas a south-verging thrust belt called the Muertos thrust belt lies to the south. Researchers have attributed such bivergent geometry to several processes, including: reversal of subduction polarity; subduction-driven mantle fl ow; stress transmission across the arc; gravitational spreading of the arc; and magmatic infl ation within the arc. New observations of deformational features in the Muertos thrust belt and of fault geometries produced in sandbox kinematic models, along with examination of published studies of island arcs, lead to the conclusion that the bivergence of thrusting in island arcs can develop without reversal of subduction polarity, without subarc mantle fl ow, and without magmatic infl ation. We suggest that the Eastern Greater Antilles arc and comparable arcs are simply crustalscale bivergent (or “doubly vergent”) thrust wedges formed during unidirectional subduction. Sandbox kinematic modeling suggests, in addition, that a broad retrowedge containing an imbricate fan of thrusts develops only where the arc behaves relatively rigidly. In such cases, the arc acts as a backstop that transmits compressive stress into the backarc region. Further, modeling shows that when arcs behave as rigid blocks, the strike-slip component of oblique convergence is accommodated entirely within the prowedge and the arc—the retrowedge hosts only dip-slip faulting (“frontal thrusting”). The existence of large retrowedges and the distribution of faulting in an island arc may, therefore, be evidence that the arc is relatively rigid. The rigidity of an island arc may arise from its mafi c composition and has implications for seismic-hazard analysis.

Synchronism between development of Cenozoic volcanic arcs and back-arc basins: Reasons and consequences

A geographic information system (GIS “Volcanic belts”) was used for analyzing the spatial and temporal relationship between tectono-magmatic cycles in the Cenozoic that took place at the convergent plate boundaries, mostly in volcanic arc-back-arc systems. The onset of back-arc basins and subaerial arc volcanism and their main evolutionary stages are shown to have occurred about the same time. These processes are still ongoing, which is indicated by today’s active volcanoes, high heat flows, and high deep-focus seismicity. The crust underlying both tectonic structures undergoes transformation, which results in a significant thinning of the “granite” layer within the volcanic belts, whereas crust within the back-arc basins changes its properties to the transitional (suboceanic) and oceanic type crusts. All processes that occur at the convergent plate boundaries can be described within the arc-back-arc system, the principal dynamic components of which are the asthenospheric plume upwelling above the continent edge and the oceanward-spreading plume head. This was accompanied by a gradual crustal thinning in the back-arc region and the formation of areas with oceanic crust, as well as by involvement of crustal material, together with rocks of the subducting slab, into subduction processes. As a result, the continental crust is removed from the tectonosphere and stored in the “slab cemetery.” Only a minor portion of the crustal materials is returned to the surface as subduction-related magmatism.

Pliocene tourmaline rhyolite dykes from Ikaria Island in the Aegean back-arc region: geodynamic implications

Very rare rhyolite dykes cross-cutting a Miocene I-type biotite-granite were discovered on Ikaria Island in the Aegean back-arc region. Their intrusion postdates exhumation of the granite to brittle crust at about 6.0-3.6 Ma; hence a Pliocene age is inferred. Petrological, geochemical and isotopic arguments indicate an origin through melting of crustal lithologies (tourmaline greywackes/semipelites) with no detectable contribution from asthenospheric sources. Strontium isotope ratios are relatively low unlike values for sediments entering the Hellenic trench but similar to those for certain Miocene Cycladic I-type granites and low-Rb Permo-Carboniferous Cycladic basement acid orthogneisses. Crust-mantle Sr-Nd isotopic mixing modelling also requires a low-Sr crustal end-member. The presence of Pliocene rhyolitic volcanism in the Aegean back-arc region in places distant from any possible subduction zone influence is attributed to slab tear at the eastern end of the Hellenic trench and attendant entrance and lateral displacement of asthenosphere through the thus formed slab window that, in turn, brought about kinematically mismatched continental escape in the subduction hangingwall. Extension along the boundary between the differentially moving Aegean and Anatolian blocks caused the intrusion of basaltic sills at middle crustal levels and the formation of acid anatectic melts from low-Sr, B-bearing quartzofeldspathic metasedimentary protoliths.

Modelling seismic anisotropy variations across the Hikurangi subduction margin, New Zealand

Anisotropy measurements across subduction zones have produced controversial interpretations related to the mechanisms of plate tectonics. Here we use anisotropic teleseismic wave propagation in two-dimensional finite difference models and four types of analytical modelling to explain previously-determined shear-wave splitting along the Hikurangi subduction margin that suggested high anisotropy with rapid lateral changes. Large-scale models of the subduction zone (hundreds of kilometres) incorporating four main anisotropic domains; the subslab, the slab, the mantle wedge and the far backarc region, result in synthetic shear-wave splitting measurements that closely resemble all large-scale features of real teleseismic observations across the central North Island of New Zealand. The preferred model constrains high (15%) anisotropy to the mantle wedge down to about 100 km under the Central Volcanic Region (CVR), bound to the west by an isotropic region under western North Island; the slab is isotropic and the subslab region has average (3.5%) anisotropy, down to 300 km. The unusually high anisotropy in the mantle wedge is attributed to the presence of aligned melt. The influence of melt on seismic anisotropy is examined with different small-scale (tens of kilometres) analytical modelling approaches calculating anisotropy due to melt occurring in aligned inclusions, such as cracks or bands. The models confirm that small amounts (1–2%) of melt can account for the strong anisotropy required for the wedge region in the preferred large-scale model. Additionally, we show that aligned melt inclusions on the order of tens of meters can provide a suitable explanation for the observed weak trend towards decreasing delay times with higher filtering frequency for teleseismic phases and the differences in local and teleseismic fast polarisations, which both sample the highly anisotropic mantle wedge.

From mantle to crust: Stretching the Mediterranean

The origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated. This work is based on a compilation of SKS fast splitting directions that give an image of flow lines in the mantle around the Mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts. We find that : (1) the two sets of directions are parallel in the three main backarc regions, namely the Alboran Sea, the Tyrrhenian Sea and the Aegean Sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle, suggesting that (2) crustal deformation is mainly driven from below by slab retreat, and (3) the lithospheric fabric is reset within a few millions of years in backarc environments.