Subduction Rollback Research Articles

Complex structure of the lithospheric slab beneath the Banda arc, eastern Indonesia depicted by a seismic tomographic model

Seismic tomography with a non-linear approach has been successfully applied to image the P-wave velocity structure beneath the Banda arc in detail. Nearly one million compressional phases including the surfacereflected depth phases pP and pwP from events within the Indonesian region have been used. The depth phases have been incorporated in order to improve the sampling of the uppermantle structure, particularly below the Banda Sea in the back-arc regions. For the model parameterization, we have combined a highresolution regional inversion with a low-resolution global inversion to allow detailed images of slab structures within the study region and to minimize the mapping of distant aspherical mantle structure into the volume under study. In this paper, we focus our discussion on the upper mantle and transition zone structure beneath the curved Banda arc. The tomographic images confirm previous observations of the twisting of the slab in the upper mantle, forming a spoon-shaped structure beneath the Banda arc. A slab lying flat on the 660 km discontinuity beneath the Banda Sea is also well imaged. Further interpretations of the resulting tomograms and seismicity data support the scenario of the Banda arc subduction rollback.

Structural evolution of the Texas Orocline, eastern Australia

The Texas Orocline, with a half wavelength of ~120km, is the largest and most obvious orocline in the southern New England Orogen and is clearly recognised in geological maps and geophysical images. In the area of the orocline, there is a major unconformity between Devonian–Carboniferous metasedimentary rocks (Texas beds) and the overlying Early Permian rift-related basins. Detailed structural mapping shows that units both above and below the unconformity are folded around the orocline, indicating that at least part of the oroclinal deformation has occurred after the deposition of the Early Permian rocks. In addition, Early Permian (298–290Ma) granitoids are aligned parallel to the oroclinal structure, further indicating that deformation partly occurred during or after the Early Permian. Pre-oroclinal isoclinal folds (F1) and related axial plane cleavage (S1) are well developed in the Texas beds and are curved around the oroclinal structure. Syn-oroclinal structures are characterised by minor kink folds and disharmonic folding (F2) in the core of the orocline. A later phase of kink folding corresponding to a ~N–S shortening direction does not seem to be related to the orocline, and is interpreted as post-oroclinal deformation. A secondary penetrative fabric parallel to the axial plane of the orocline was not observed, indicating low contractional shortening across the orocline (<30%). We demonstrate that the observed strain is too low to account for oroclinal bending during dextral transpression, as previously proposed. We suggest an alternative model involving an initial curved structure, probably related to subduction rollback or a pre-existing curvature in the palaeomargin of eastern Australia, which was amplified by dextral transpression and subsequent E–W contraction.

Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains

Received 24 March 2011; revised 25 May 2011; accepted 31 May 2011; published 8 July 2011. [1] We present three‐dimensional laboratory modeling of the evolution of finite strain and compare these to shear wave splitting observations in the Northwest U.S. under the High Lava Plains (HLP). We show that relationships between mantle flow and anisotropy are complicated in subduction zones and factors such as initial orientation of the olivine fast‐axis, style of subduction, and time evolving flow are important. Due to increased horizontal shear, systems with a component of rollback subduction have simple trench‐normal strain alignment within the central region of the backarc mantle wedge while those with more simple longitudinal sinking are often variable and complex. In the HLP, splitting observations are consistent with rollback‐driven laboratory results. Citation: Druken, K. A., M. D. Long, and C. Kincaid (2011), Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains, Geophys. Res. Lett., 38, L13310, doi:10.1029/2011GL047541.

Palaeoproterozoic accretion processes of Australia and comparisons with Laurentia

The Palaeoproterozoic rocks of Australia and Laurentia preserve an excellent record of the accretionary tectonics associated with transitions between the Columbia (Nuna) and Rodinia supercontinents. The geologic records of Australia and Laurentia suggest that the dominant tectonic driver was one or more subduction zones in which several episodes of crustal accretion occurred between ca. 1790 Ma and 1620 Ma. Correlated orogenic events include the ca. 1800–1780 Ma Yapungku–Yambah (Australia)–Medicine Bow (Laurentia) orogenies, ca. 1740–1690 Ma Strangways–Kimban (Australia)–Nimrod (Antarctica)–Yavapai (Laurentia) orogenies, and the ca. 1650–1620 Ma Leibig–Ooldea (Australia)–Mazatzal (Laurentia) orogenies. There are major differences in the style of accretion: Laurentia is characterized by accretion of dominantly juvenile arc terranes, whereas accreted Australian terranes are more evolved and are isotopically similar to the continental nucleus. Adjacent to its plate margin, the Australian continent contained regions of elevated heat production compared with the Laurentian margin. Brace-Goetze lithospheric strength models for ca. 1700 Ma indicate that the Australian plate margin was significantly weaker than that of Laurentia. This variation in lithospheric strength is interpreted to impact the behaviour of the overriding plate during subduction roll back. Attenuation of the Australian lithosphere during ductile extension caused rifting of large continental fragments from the plate margin. Their subsequent accretion resulted in the re-amalgamation of pre-existing continental lithosphere similar to the lithosphere in the overriding plate. Subduction roll back adjacent to the cold and rigid Laurentian margin (e.g. the Wyoming Craton) had relatively little impact on the overriding plate, with oceanic back-arc basins and juvenile arc terranes developing outboard of the plate margin. Inversion of these arc and oceanic back-arc terranes resulted in episodic continental growth.

Segmented African lithosphere beneath the Anatolian region inferred from teleseismic P-wave tomography

SUMMARY Lithospheric deformation throughout Anatolia, a part of the Alpine–Himalayan orogenic belt, is controlled mainly by collision-related tectonic escape of the Anatolian Plate and subduction roll-back along the Aegean Subduction Zone. We study the deeper lithosphere and mantle structure of Anatolia using teleseismic, finite-frequency, P-wave traveltime tomography. We use data from several temporary and permanent seismic networks deployed in the region. Approximately 34 000 P-wave relative traveltime residuals, measured in multiple frequency bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomograms reveal segmented fast seismic anomalies beneath Anatolia that corresponds to the subducted portion of the African lithosphere along the Cyprean and the Aegean trenches. We identify these anomalies as the subducted Aegean and the Cyprus slabs that are separated from each other by a gap as wide as 300 km beneath Western Anatolia. This gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere. The eastern termination of the subducting African lithosphere is located near the transition from central Anatolia to the Eastern Anatolian Plateau or Arabian–Eurasian collision front that is underlain by large volumes of hot, underplating asthenosphere marked by slow velocity perturbations. Our tomograms also show fast velocity perturbations at shallow depths beneath northwestern Anatolia that sharply terminates towards the south at the North Anatolian Fault Zone (NAFZ). The associated velocity contrast across the NAFZ persists down to a depth of 100–150 km. Hence, our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary. Overall, the resolved upper-mantle structure of Anatolia is directly related with the geology and tectonic features observed at the surface of the Anatolian Plate and suggest that the segmented nature of the subducted African lithosphere plays an important role in the evolution of Anatolia and distribution of its tectonic provinces.

Structural and stratigraphic evolution of the Savu Basin, Indonesia

Abstract The Savu Basin is located in the Sunda–Banda fore-arc at the position of change from oceanic subduction to continent–arc collision. It narrows eastward and is bounded to the west by the island of Sumba that obliquely crosses the fore-arc. New seismic data and published geological observations are used to interpret Australia–Sundaland convergence history. We suggest the basin is underlain by continental crust and was close to sea level in the Early Miocene. Normal faulting in the Middle Miocene and rapid subsidence to several kilometres was driven by subduction rollback. Arc-derived volcaniclastic turbidites were transported ESE, parallel to the Sumba Ridge, and then NE. The ridge was elevated as the Australian continental margin arrived at the Banda Trench, causing debris flows and turbidites to flow northwards into the basin which is little deformed except for tilting and slumping. South of the ridge fore-arc sediments and Australian sedimentary cover were incorporated in a large accretionary complex formed as continental crust was thrust beneath the fore-arc. This is bounded to the north by the Savu and Roti Thrusts and to the south by a trough connecting the Java Trench and Timor Trough which formed by south-directed thrusting and loading.

Evolution of the Adria-Europe plate boundary in the northern Dinarides: From continent-continent collision to back-arc extension

The Sava Zone of the northern Dinarides is part of the Cenozoic Adria‐Europe plate boundary. Here Late Cretaceous subduction of remnants of Meliata‐Vardaroceanic lithosphere led to the formation of a suture, across which upper plate European‐derived units of Tisza‐Dacia were juxtaposed with Adria‐derived units of the Dinarides. Late Cretaceous siliciclastic sediments,deposited on the Adriatic plate, were incorporated into an accretionary wedge that evolved during the initial stages of continent‐continent collision. Structurally deeper parts of the exposed accretionary wedge underwent amphibolite‐grade metamorphism. Grt‐Pl‐ Ms‐Bt thermobarometry and multiphase equilibria indicate temperatures between 550°C and 630°C and pressures between 5 and 7 kbar for this event. Peak metamorphic conditions were reached at around 65 Ma. Relatively slow cooling from peak metamorphic conditions throughout most of the Paleogene was possibly induced by hanging wall erosion in conjunction with southwest directed propagation of thrusting in the Dinarides. Accelerated cooling took place in Miocene times, when the Sava Zone underwent substantial extension that led to the exhumation of the metamorphosed units along a low‐angle detachment. Footwall exhumation started under greenschist facies conditions and was associated with top‐to‐the‐north tectonic transport, indicating exhumation from below European plate units. Extension postdates the emplacement of a 27 Ma old granitoid that underwent solid‐state deformation under greenschist facies conditions. The 40Ar/39Ar sericite and zircon and apatite fission track ages from the footwall allow bracketing this extensional unroofing between 25 and 14Ma. This extension is hence linked to Miocene rift‐related subsidence in the Pannonian basin, which represents a back‐arc basin formed due to subduction rollback in the Carpathians.

Tectonic and magmatic responses to the subduction of high bathymetric relief

Subduction of high bathymetric relief, such as aseismic ridges and magmatic plateaus, is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone. For example, the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab, modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism. In addition, the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback, a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing. Here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum-Pacific subduction systems. We demonstrate that flattening of the subduction dip angle is only significant in the eastern Pacific, where the average slab dip angle is relatively shallow. In the western Pacific, in contrast, the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction. Subduction of high bathymetric relief in the circum-Pacific is commonly associated with reduced arc volcanism, and in many cases, the area of ridge subduction coincides with a volcanic gap. In the overriding plate, ridge subduction is associated with pronounced changes in the style of deformation, involving uplift, reactivation of basement thrusts, development of orogen-perpendicular tear faults and block rotations leading to oroclinal bending. The discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes, and could help constraining the geodynamics of ancient subduction systems.

Mantle flow and dynamic topography associated with slab window opening: Insights from laboratory models

We present dynamically self-consistent mantle-scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics, mantle flow and associated dynamic topography over geological time scales. The adopted setup consists of a two-layer linearly viscous system simulating the subduction of a fixed plate of silicone (lithosphere) under negative buoyancy in a viscous layer of glucose syrup (mantle). Our experimental setting is also characterized by a constant-width rectangular window located at the center of a laterally confined slab, modeling the case of the interaction of a trench-parallel spreading ridge with a wide subduction zone. We found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab. On the contrary, slab-induced mantle circulation, quantified both in the vertical and horizontal sections using the Feature Tracking image analysis technique, is strongly modified. In particular, rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components, with the importance of each evolving according to kinematic stages. Mantle coming from the oceanic domain floods through the slab window, indenting the supra-slab mantle zone and producing its deformation without any mixing between mantle portions. The opening of the slab window and the upwelling of sub-slab mantle produce a regional-scale non-isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca. 1 and 5 km in nature. Assuming that our modeling results can be representative of the natural behavior of subduction zones, we compared them to the tectonics and volcanism of the Patagonian subduction zone. We found that the anomalous backarc volcanism that has been developing since the middle Miocene could result from the lateral flow of sub-slab mantle and that part of the Patagonian uplift could be dynamically supported.

Testing oceanic subduction and convective removal models for the Gibraltar arc: Seismological constraints from dispersion and anisotropy

We discuss mantle structure and dynamics under the Gibraltar arc. For that region, a large variety of geodynamic models has been proposed, and there was, up to recently, no consensus on which of these models is the best one to describe the geodynamic situation of the region. The key to distinguishing the different models probably lies in the upper mantle, and we propose two specially adapted seismological techniques that help to distinguish each of the two models that are more frequently invoked. These are, on one hand, the subduction-rollback (SR) model, and on the other, continental delamination or convective removal (CDCR). The first technique that we use is based on observations of dispersed P-wavetrains that are expected for certain ray directions. Observations made on the Gibraltar arc suggest the presence of subducted oceanic lithosphere under the Alboran Sea. The second technique is based on shear-wave splitting of SKS phases, and it allows us to look for the radially-oriented flow that must be present in the mantle beneath the Alboran Sea, for a CDCR mechanism. There are however no indications of any radial flow up to this point. Both approaches therefore suggest that subduction-rollback is the best model to explain the complex geodynamic situation under the region.

P.3.c.008 Correlation between psychotic symptoms and sleep disturbances in schizophrenia

The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models.Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism.In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off.Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (∼60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere.Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.

Surface deformation and slab–mantle interaction during Banda arc subduction rollback

The reason for the spectacular curvature of the Banda subduction zone is debated. Tomographic images and plate reconstructions reveal subduction of a single slab. The ancient geometry of the Australian plate, as well as the interaction between the slab and the mantle, caused the deformation of the slab. The spectacularly curved Banda arc comprises young oceanic crust1,2 enclosed by a volcanic inner arc, outer arc islands and a trough parallel to the Australian continental margin3,4,5. Strong seismic activity in the upper mantle defines a folded surface6,7, for which there are two contrasting explanations: deformation of a single slab5,8 or two separate slabs subducting from the north and south6,9. Here we combine seismic tomography with the plate tectonic evolution of the region to infer that the Banda arc results from subduction of a single slab. Our palaeogeographic reconstruction shows that a Jurassic embayment, which consisted of dense oceanic lithosphere enclosed by continental crust, once existed within the Australian plate. Banda subduction began about 15 million years ago when active Java subduction tore eastwards into the embayment. The present morphology of the subducting slab is only partially controlled by the shape of the embayment. As the Australian plate moved northward at a high speed of about 7 cm yr−1, the Banda oceanic slab rolled back towards the south–southeast accompanied by active delamination separating the crust from the denser mantle. Increasing resistance of the mantle to plate motion progressively folded the slab and caused strong deformation of the crust. The Banda arc represents an outstanding example of large-scale deformation of the Earth’s crust in response to coupling between the crust, slab and surrounding mantle.

New geophysical insight into the origin of the Denali volcanic gap

SUMMARY Volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins. They are regions where the necessary conditions to produce melt may appear favourable, but where volcanoes are surprisingly absent from the surface. In this study, we present a new model that can explain the occurrence of such volcanic gaps. It is based on seismic imaging and geodynamic modelling of the Denali volcanic gap, a ∼400-km-wide region at the eastern end of the Alaska-Aleutian subduction zone. Here, the thick crust of the Pacific Plate and Yakutat terrane subduct at shallow angle beneath North America. A high-resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth, and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge. We interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate. In steadystate subduction models, melt accumulates at the apex of a vaulted mantle wedge, the ‘pinch zone’, from where it may break through the overlying lithosphere to the surface. Beneath the Denali volcanic gap, the pinch zone is absent (or greatly reduced) because shallow subduction of the Yakutat terrane progressively cools the system, and causes the slab to advance and replace the hot core of the mantle wedge. This regime can be seen as the opposite of subduction rollback. It prevents the formation of a pinch zone, reduces the length of the melting column and causes melt to pool at the base of the overriding plate, thus inhibiting magma generation and extraction.

The Miocene-to-Present Kinematic Evolution of the Eastern Mediterranean and Middle East and Its Implications for Dynamics

The present kinematics in the eastern Mediterranean and Middle East is now well constrained by GPS measurements and dominated by a circular counterclockwise motion. While Arabia rotates rigidly, the rotational velocities increase from the Levant to Aegea, causing extension in western Anatolia. We place the present pattern in light of the post–Middle Miocene geological history and propose that in addition to the effects of the Hellenic subduction and Zagros collision-subduction zones on driving the surface motion, an underlying asthenospheric flow is also required. This counterclockwise toroidal flow is expected to exist around the eastern edge of the African slab owing to subduction rollback, and it helps explain the initiation of the North Anatolia Fault, the extrusion and uplift of Anatolia, and the Mid-Miocene to Recent volcanism in eastern Anatolia. The Afar plume had a similar effect on the acceleration of Arabia at 40 Mya.

Implications for Proterozoic plate margin evolution from geophysical analysis and crustal-scale modeling within the western Gawler Craton, Australia

Complexly deformed late Archean to Mesoproterozoic rocks of the western Gawler Craton are poorly exposed, thus reducing the ability to extract meaningful geological, structural and tectonic information. This study focuses on shear zone analysis via a constrained integration of qualitative aeromagnetic and Bouguer gravity data interpretation with quantitative two-dimensional forward modeling. This integration provides a means to interpret the crustal architecture as well as constrain a sequence of events, which aid the interpretation of a tectonic history for the western Gawler Craton. Results indicate a polyphase shearing history dominated by a crustal-scale array of predominantly west-dipping shear zones. The initiation of ∼ east–west trending SZ 1 structures was likely coincident with ca. 1750–1720 Ma crustal extension, high heat flow and deposition on thinned areas of Archean crust with the development of a dense, possibly underplated lower crustal component. SZ 1-R2 dextral transpressional movements reactivated SZ 1 during the ca. 1720–1670 Ma Kimban Orogeny, which is interpreted to record collision between the Gawler Craton and the North Australian Craton. During this event basins marginal to, and in the interior of the Archean crustal block of the western Gawler Craton were inverted and the dense lower crust was vertically offset, causing some of the long wavelength (∼ 30–50 km) Bouguer gravity anomalies presently observed. Shear zone activity is constrained until ca. 1680 Ma during which time subduction rollback established a new arc position at the southern margin of the Archean crust with magmatism persisting until ca. 1670 Ma. SZ 3 structures overprint and offset the interpreted axis of the Kimban Orogeny within the Archean crust. These structures record a major phase of crustal-scale sinistral strike-slip movement associated with ∼ north–south shortening between ca. 1630 and 1540 Ma prior to their reactivation at ca. 1450 Ma. This SZ 3-R4 reactivation is interpreted to cause southwest-directed crustal transport, which was focused as west-side-up movements internal to the Archean continent, whereas dextral strike-slip was focused at a major rheological boundary between thick Archean crust to the south and thick Paleoproterozoic basins to the north.

Structure and geochemistry of Tethyan ophiolites and their petrogenesis in subduction rollback systems

Suprasubduction zone (SSZ) ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Mantle flow and slab rollback result in one or more episodes of arc splitting and basin opening, producing a collage of ‘protoarc and forearc oceanic lithosphere’ in suprasubduction zone settings. The Jurassic–Cretaceous SSZ Tethyan ophiolites in the eastern Mediterranean region (i.e. Mirdita, Pindos, Troodos, Kizildag, Oman) generally have Penrose-type oceanic crust and contain well-developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution. Igneous accretion of these SSZ Tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins, showing a progressive evolution from MORB-like to IAT (island arc tholeiite) to boninitic (extremely refractory) protoarc assemblages. However, there are some distinct differences in the geochemical evolution of these Tethyan ophiolites that appear to have resulted from variations in their subduction zone geodynamics. Whereas a major part of the Kizildag and Troodos lavas shows island arc affinity similar to their counterparts in the Pindos and Mirdita ophiolites, a significant component of the Oman lavas indicates MORB affinity and the majority of the Kizildag and Oman data plot within the mantle array between N-MORB and E-MORB on the Nb/Yb–Th/Yb discriminant diagram. Furthermore, the Troodos and Oman lavas do not show any particular Th-enrichment in their multi-element patterns, suggesting that fluid/melt input from subducted sediments was not that significant in generation of their magmas. Although all ophiolites exhibit geochemical features indicating increased subduction influence during the melt evolution of their younger extrusive sequences and dike intrusions, as evidenced by their negative ɛ Nd values, their overall characteristic trace-element patterns seem to have been strongly affected by the maturity of the subduction systems in which they developed.

From palaeotectonics to neotectonics in the Neotethys realm: The importance of kinematic decoupling and inherited structural grain in SW Anatolia (Turkey)

In order to asses young, i.e. “neotectonic” fault kinematics and the relation with plate-tectonic processes in SW Turkey we focused on the questions: 1) what produced the structural grain, i.e. how and when are faults generated, 2) what was/is the kinematic behaviour of these faults and 3) during which period(s) were these faults active? Firstly, the distribution of faults has been investigated using a lineament analysis performed on satellite imagery and digital elevation models. We define five main tectonic domains in SW Turkey: 1 and 2) the Northern and Southern Western Anatolian extensional provinces; 3) the eastern Hellenic arc (including Rhodes Island and the submarine Anaximander mountains); 4) The Lycian Taurides and 5) the western limb of the Isparta Angle. Lineament analysis reveals dominance of three lineament groups, i.e. NNE-, ENE-, and WNW-trending, in all identified domains. Variations in the relative importance of lineament groups exist and are attributed to the specific tectonic evolution of each domain. A synthesis of recently published field data shows that the onset of activity of related fault groups is neither synchronous, nor uniform. The “structural grain” for the neotectonic deformation in SW Turkey is formed during the last stage of palaeotectonic deformation (stage 1), which is dominated by the onset of the Menderes Massif exhumation along a top-N major detachment, the Datça break-away fault [Seyitoğlu, G., Işık, V., and Çemen, İ., 2004. Complete Tertiary exhumation history of the Menderes massif, western Turkey: an alternative working hypothesis. Terra Nova, 16, 358–364.], and related SE-ward sliding of the Lycian Nappes in the footwall of this fault. This stage is characterized by spatially variable types of deformation that are delineated by, from NW to SE, the Datça Fault around the Menderes–Lycian contact zone and the Lycian frontal thrust zone, respectively. The kinematic decoupling along these structures explains the co-existence of top-N detachment type extension in the Western Anatolian Extensional province and ongoing nappe emplacement and piggy-back development in the Lycian Taurides. The decoupling also explains the lateral complexity of deformation associated with later neotectonic deformation. From late Miocene onward, stage 2 NNE–SSW extension dominated the Western Anatolian Extensional province, influenced by combined Anatolian extrusion and Hellenic subduction roll-back. Contemporaneously, bidirectional ESE–WNW and NNE–SSW extension controls the Lycian Taurides due to the effects of Hellenic arc expansion superposed on extrusion-roll-back extension. The zone of kinematic decoupling is characterized by structural complexity, with local transpressional features. During stage 3 (post-Late Pliocene), the same types of extension prevail in the Western Anatolian Extensional province and the Lycian Taurides, although a third tectonic regime becomes gradually more important. This regime is associated with a tectonic escape in the Hellenic arc that induced forearc slivers to be sheared of the expanding arc along (mostly) sinistrally displacing ENE-trending strike-slip zones. Several of these zones have been identified in the coastal zones of SW Turkey, either through recent seismicity or field observations and are especially focused in the Lycian Taurides domain. Here, they form what is classically referred to as the Fethiye–Burdur Fault Zone, which becomes gradually penetrated from the SW by oblique- or strike-slip zones, exemplifying the time-transgressive nature of the tectonic escape mechanism. Zones of kinematic decoupling remain persistent throughout the entire Early Miocene–Recent tectonic evolution of SW Turkey and explain the existence of differently deforming tectonic domains.

Cenozoic extensional tectonics of the Western Anatolia Extended Terrane, Turkey

The Western Anatolia Extended Terrane in Turkey is located on the eastern side of the Aegean Extended Terrane and contains one of the largest metamorphic core complexes in the world, the Menderes massif. It has experienced a series of continental collisions from the Late Cretaceous to the Eocene during the formation of the Izmir-Ankara-Erzincan suture zone. Based our field work and monazite ages, we suggest that the north-directed postcollisional Cenozoic extension in the region is the product of three consecutive stages, triggered by three different mechanisms. The first stage was initiated about 30 Ma ago, in the Oligocene by the Orogenic Collapse the thermally weakened continental crust along the north-dipping Southwest Anatolian shear zone. The shear zone was formed as an extensional simple-shear zone with listric geometry at depth and exhibits predominantly normal-slip along its southwestern end. But, it becomes a high-angle oblique-slip shear zone along its northeastern termination. Evidence for the presence of the shear zone includes (1) the dominant top to the north-northeast shear sense indicators throughout the Menderes massif, such as stretching lineations trending N10E to N30E; and (2) a series of Oligocene extensional basins located adjacent to the shear zone that contain only carbonate and ophiolitic rock fragments, but no high grade metamorphic rock fragments. During this stage, erosion and extensional unroofing brought high-grade metamorphic rocks of the Central Menderes massif to the surface by the early Miocene. The second stage of the extension was triggered by subduction roll-back and associated back-arc extension in the early Miocene and produced the north-dipping Alaşehir and the south-dipping Büyük Menderes detachments of the central Menderes massif and the north-dipping Simav detachment of the northern Menderes massif. The detachments control the Miocene sedimentation in the Alaşehir, Büyük Menderes, and Simav grabens, containing high-grade metamorphic rock fragments. The third stage of the extension was triggered by the lateral extrusion (tectonic escape) of the Anatolian plate when the North Anatolian fault was initiated at about 5 Ma. This extensional phase produced the high-angle faults in the Alaşehir, Büyük Menderes and Simav grabens and the high-angle faults controlling the Küçük Menderes graben.

The transition from subduction arc to slab tearing: Evidence from Capraia Island, northern Tyrrhenian Sea

The magmatic island of Capraia in the northern Tyrrhenian Sea records late stages in the evolution of a retreating subduction zone following continental accretion. Subduction rollback in the northern Tyrrhenian Sea was accompanied by an eastward migrating magmatic arc that coincided with Capraia at not, vert, similar7.7 Ma. This was followed by segmentation of the subducting slab through eastward propagating tear faults. We present high-precision, step-heating 40Ar/39Ar data that provide robust constraints for the timing of magmatism in Capraia, indicating that the island was subjected to two distinct magmatic episodes. The bulk of the island, characterised by typical calcalkaline, subduction-related volcanic rocks, formed during a relatively short period between 7.7 and 7.2 Ma. This major magmatic event was followed by a period of quiescence between 7.1 and 4.8 Ma. Magmatic activity resumed at 4.8 Ma, with the eruption of shoshonites from a monogenic centre in the southern part of the island. We interpret the latter as a magmatic event triggered by the formation of a tear fault within the subduction slab. The example provides a unique insight into the destruction of a subduction system during the interlinked process of subduction rollback, slab segmentation, and the development of slab tear faults.

Intra-arc extension in Central America: Links between plate motions, tectonics, volcanism, and geochemistry

This study revisits the kinematics and tectonics of Central America subduction, synthesizing observations of marine bathymetry, high-resolution land topography, current plate motions, and the recent seismotectonic and magmatic history in this region. The inferred tectonic history implies that the Guatemala–El Salvador and Nicaraguan segments of this volcanic arc have been a region of significant arc tectonic extension; extension arising from the interplay between subduction roll-back of the Cocos Plate and the ~ 10–15mm/yr slower westward drift of the Caribbean plate relative to the North American Plate. The ages of belts of magmatic rocks paralleling both sides of the current Nicaraguan arc are consistent with long-term arc-normal extension in Nicaragua at the rate of ~ 5–10mm/yr, in agreement with rates predicted by plate kinematics. Significant arc-normal extension can ‘hide’ a very large intrusive arc-magma flux; we suggest that Nicaragua is, in fact, the most magmatically robust section of the Central American arc, and that the volume of intrusive volcanism here has been previously greatly underestimated. Yet, this flux is hidden by the persistent extension and sediment infill of the rifting basin in which the current arc sits. Observed geochemical differences between the Nicaraguan arc and its neighbors which suggest that Nicaragua has a higher rate of arc-magmatism are consistent with this interpretation. Smaller-amplitude, but similar systematic geochemical correlations between arc-chemistry and arc-extension in Guatemala show the same pattern as the even larger variations between the Nicaragua arc and its neighbors.