Slab Window Research Articles

Geochemistry and petrology of late Eocene Cascade Head and Yachats Basalt and alkalic intrusions of the central Oregon Coast Range, U.S.A.

The Oregon Coast Range underwent extension and basaltic magmatism accompanying tectonic rotation during the Tertiary. Basaltic magmatism was expressed initially in submarine eruptions, but mostly in younger subaerial flows. A tholeiitic basalt series, represented by Yachats units, evolved from a depleted, low K, Rb source through a process of fractional crystallization and magma mixing. An alkalic basalt series, represented by Cascade Head units, evolved primarily by fractional crystallization from parental magmas created by lower degrees of partial melting at greater depth than the Yachats series. Camptonite, found in sills and dikes, was formed by even smaller degrees of melting of sources similar to those of Cascade Head. Small nepheline syenite intrusions, emplaced as sills into middle Eocene Tyee Formation, appear to have formed through extensive fractional crystallization of parental basanite, which is sparsely represented in the Yachats and Cascade Head basaltic lava units. Individual syenite intrusions are not directly related to each other, as each intrusion represents a relatively small batch of magma separated at different evolutionary stages from underlying magma chambers. P–T segregation conditions were estimated for primitive Coast Range basalts. These include depths of ~ 120–160 km and temperatures of 1600–1700 °C for the alkalic series represented primarily by Cascade Head units and depths of ~ 60–120 km with temperatures of ~ 1500–1600 °C for tholeiitic units. A few analyses from other Coast Range igneous units (Tillamook and Goble volcanics) suggest T–P segregations at shallower depths (~ 55–33 km) and lower temperatures (~ 1400 °C). Paired Nd and Sr isotopic analyses of four Coast Range samples fall in the enriched MORB field. The depths and high temperatures indicated for the Central Oregon Coast ancestral basalts require magma generation in the garnet peridotite stability field, with the alkalic series probably originating below the subducting Juan de Fuca plate. This would suggest an origin for the magmas in a slab window or mantle plume environment. We further suggest that major extension documented for the Coast Range may have triggered upwelling of mantle asthenosphere giving birth to a mantle plume.

Ridge subduction and crustal growth in the Central Asian Orogenic Belt: Evidence from Late Carboniferous adakites and high-Mg diorites in the western Junggar region, northern Xinjiang (west China)

The Central Asian Orogenic Belt (CAOB) is a natural laboratory for the study of accretionary tectonics and crustal growth owing to its massive generation of juvenile crust in the Paleozoic. There is a debate, however, on the mechanism of this growth. In the Baogutu area of the western Junggar region, northern Xinjiang (west China), diorite–granodiorite porphyry plutons and dikes are widely associated with Cu–Au mineralization. In this study, we present new results of zircon U–Pb geochronology, major and trace elements, and Sr–Nd–Pb–Hf isotope analyses for two diorite–granodiorite porphyry plutons and two dikes from this area. LA-ICP-MS zircon U–Pb analyses of four plutonic and dike samples yield Late Carboniferous ages of 315–310 Ma. The Baogutu diorite–granodiorite porphyries exhibit low-Fe and calc-alkaline compositions. They are also characterized by high Sr (346–841 ppm) contents, low Y (9.18–16.5 ppm) and Yb (0.95–1.60 ppm) contents, and relatively high Sr/Y (31–67) ratios, which are similar to those of typical adakites. In addition, some samples have relatively high MgO (2.35–8.32 wt.%) and Mg # (48–75), and Cr (22.7–291 ppm) and Ni (32.0–132 ppm) values, which are similar to those of high-Mg andesites. All rock samples exhibit mid-oceanic ridge basalt (MORB)-like Nd–Sr–Pb–Hf isotope features: high ε Nd(t) (+ 5.8–+8.3) and ε Hf(t) (+ 13.1–+15.7) values, and relatively low ( 87Sr/ 86Sr) i (0.7033 to 0.7054) and ( 206Pb/ 204Pb) i (17.842–18.055). The Baogutu adakitic rocks also contain reversely zoned clinopyroxene phenocrysts, which have low MgO cores and relatively high MgO rims. Geochemical modeling indicates that the Baogutu adakitic rocks could have been derived by mixing ~ 95% altered oceanic crust-derived melts with ~ 5% sediment-derived melts. Taking into account the regional geology, I- and A-type granitoids and Cu–Au mineralization, and the presence of Carboniferous ophiolite mélanges in northern Xinjiang, we suggest that the Baogutu adakitic rocks were most probably generated by partial melting of a slab edge close to a subducting spreading ridge in the Late Carboniferous. Ridge subduction and the resultant slab window probably caused strong extension in the overlying lithosphere, extensive melting of subducting oceanic crust, mantle and juvenile lower crust, and interaction between slab-derived melts and the mantle. Thus, events associated with ridge subduction are likely to have played an important role in crustal growth in the CAOB in addition to previously recognized accretion of subduction and arc complexes and post-collisional crustal melting.

Transition from shoshonitic to adakitic magmatism in the eastern Pontides, NE Turkey: Implications for slab window melting

The formation of the eastern Pontides orogenic belt has been widely assigned to a northward subduction of the Neotethyan oceanic slab during the late Mesozoic–Cenozoic. Here we provide an alternate model based on new geological, geochemical and isotopic data. The magmatic activity in the far south of the belt started in the early Campanian with shoshonitic trachyandesites and associated pyroclastics. This sequence is covered by the late Campanian–early Maastrichtian reefal limestones and another stage of high-K volcanism represented by analcimized leucite-rich ultrapotassic rocks of the Maastrichtian–early Paleocene (?) ages. The shoshonitic and ultrapotassic rocks, with K 2O contents ranging from 0.26 to 6.95 wt.%, display broadly similar rare earth and multi-element distribution patterns. Both rock types are enriched in LILE and LREE and depleted in HFSE (Nb, Ta and Ti), suggesting a subduction-enriched mantle source for the magma generation. Subsequently, during the late Paleocene, a stage of acidic magmatism (SiO 2 of 53.25–73.61 wt.%) that shows adakitic geochemical characteristics including high Sr/Y (46–416) and La/Yb (11–51) and low Y (2.6–12.2 ppm), is documented characterized by melting of a mafic source such as the MORB crust with garnet in the residue. The adakitic magmatism began at ~ 56 Ma and migrated toward the north through time, culminating with porphyritic andesites (~ 47 Ma) that were emplaced in the Gumushane–Bayburt line and its vicinity. North of this line, coeval magmas show typical calc-alkaline nature and continued to develop toward further north until the middle to late Eocene. Based on the spatial and temporal variations in the magmas generated in the eastern Pontides orogenic belt, we propose a new geodynamic model to explain the tectonomagmatic evolution of these rocks and correlate the adakitic magmatism to ridge subduction and slab window process within a south-dipping subduction zone. Our model is in contrast to the previous proposals which envisage partial melting or delamination of thickened lower continental crust due to the collision in the south during the Paleocene–Eocene.

Source-side shear wave splitting and upper mantle flow in the Chile Ridge subduction region

The actively spreading Chile Ridge has been subducting beneath Patagonian Chile since the Middle Miocene. After subduction, continued separation of the faster Nazca plate from the slow Antarctic plate has opened up a gap—a slab window—between the subducted oceanic lithospheres beneath South America. We examined the form of the asthenospheric mantle flow in the vicinity of this slab window using S waves from six isolated, unusual 2007 earthquakes that occurred in the generally low-seismicity region just north of the ridge subduction region. The S waves from these earthquakes were recorded at distant seismic stations, but were split into fast and slow orthogonally polarized waves at upper mantle depths during their passage through the slab window and environs. We isolated the directions of fast split shear waves near the slab window by correcting for upper mantle seismic anisotropy at the distant stations. The results show that the generally trench-parallel upper mantle flow beneath the Nazca plate rotates to an ENE trend in the neighborhood of the slab gap, consistent with upper mantle flow from west to east through the slab window.

Mount Etna–Iblean volcanism caused by rollback-induced upper mantle upwelling around the Ionian slab edge: An alternative to the plume model

Volcanism in Sicily (Italy) at Mount Etna (0.5 Ma to present) and the Iblean Plateau (ca. 7.0–1.1 Ma) remains enigmatic, because it is located in close proximity to, but is laterally offset from, the Calabrian subduction zone. Previous work suggests that the volcanism results from a plume or from lateral mantle flow from underneath Africa through a slab window across a lithospheric step. Here I propose that Mount Etna and the Iblean volcanics are related to decompression melting of upper mantle material that is flowing around the southern Ionian slab edge to accommodate east-directed rollback of the Ionian slab that resumed in the Late Miocene. New three-dimensional fluid dynamic subduction models demonstrate that rollback-induced mantle return flow occurs in a quasi-toroidal fashion with a component of downwelling directly below and above the slab, and a component of upwelling next to the subduction zone with maximum upwelling observed next to the subslab region. Significant upwelling (≥71% of the maximum) is observed at 90–430 km depth and extends 55–660 km away from the projected distance of inferred arc magmatism above the slab edge. The models can explain the large spatial separation of as much as 430 km between intraplate volcanism in Sicily and arc magmatism during the past ∼7 m.y., and their contemporaneous activity. The maximum upwelling velocity in the models composes 32% of the trench rollback velocity. With rollback velocities up to 10–19 cm/yr during the past ∼8 m.y., the models predict maximum rollback-induced upper mantle upwelling velocities of 3.2–6.1 cm/yr below Sicily.

Tectonic and petrologic evolution of the Kodiak batholith and the trenchward belt, Kodiak Island, AK: Contact fault juxtaposition?

Two fore‐arc magmatic belts exist in Kodiak Island, the granitic Kodiak batholith and the basaltic trenchward belt. Both are part of the 2100 km long Sanak‐Baranof belt, interpreted to result from trench‐ridge‐trench (T‐R‐T) triple‐junction migration. In this hypothesis, trenchward belt basaltic rocks formed as the spreading ridge first entered the accretionary prism, and the larger Kodiak batholith resulted from a slab window at depth. However, radiometric dates between the two belts differ in age by up to 1–3 Ma. Kodiak batholith U/Pb zircon ages range from 59.2 to 58.4 ± 0.2 Ma (SW‐NE), whereas trenchward belt ages range from 62.6 ± 0.6 (K‐Ar) to 60.15 ± 0.86 (Ar/Ar whole rock) SW‐NE. This suggests that both fore‐arc magmatic belts did not form from a single triple‐junction passage, because at 60–62 Ma the triple junction was several hundred kilometers west of Kodiak Island. Multiple explanations exist for the age disparity between the two belts, including oblique ridge subduction, Contact fault tectonic juxtaposition, slab window thermal lag, multiple triple‐junction passages, or off‐axis ridge magmatism. Geochemical data such as rare earth metal (REE) curves and Zr/Nb of >30 indicate the trenchward belt rocks have a mid‐ocean ridge basalt (MORB)–like character. Major and trace element modeling indicate that trenchward belt rocks resulted from assimilation‐fractional‐crystallization of MORB with Ghost Rocks Formation argillite, whereas the Kodiak batholith formed via partial melting and en masse mobilization of Kodiak Formation metasedimentary rocks with a small basaltic component and subsequent equilibrium‐crystallization and restite separation. The petrologic origin of both belts is compatible with spreading ridge subduction. Fault zone structural data suggest the Kodiak batholith/trenchward belt age differential resulted from oblique ridge subduction and Contact fault displacement.

Constraints on upper mantle anisotropy surrounding the Cocos slab from SK(K)S splitting

SKS and SKKS splitting observations are used to constrain the pattern of mantle flow in the Central American subduction zone beneath Costa Rica and Nicaragua. After removing the effects of shallow wedge anisotropy on SK(K)S waveforms, a best fitting model of anisotropy beneath the Cocos Plate and in the deeper mantle wedge is determined. Fast polarization directions and model symmetry axis orientations in both regions (as well as the shallow wedge) are dominated by roughly arc‐parallel azimuths and, therefore, are not consistent with sublithospheric mantle flow entrained by the subducting Cocos Plate or simple two‐dimensional corner flow in the wedge. In conjunction with geochemical data and local S splitting tomography, the SK(K)S splitting observations and anisotropy models are consistent with flow to the WNW within the mantle wedge on the Caribbean side of the Cocos Plate, possibly drawn through a slab window beneath Panama and southern Costa Rica. Anisotropy in the Pacific mantle beneath the Cocos Plate is also best explained by flow with a component that is roughly parallel to the strike of the slab, although the absolute direction of this flow is not uniquely constrained.

A morphotectonic analysis of central Patagonian Cordillera: Negative inversion of the Andean belt over a buried spreading center?

[1] We use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the Central Cordillera of Patagonia between 46°S and 48°S, in a region overlying subducted segments of the Chile Spreading Ridge. This region shows a number of peculiar features, not expected in a normal subduction-related belt located away from a buried spreading center. First, it is characterized by overall short-wavelength topography and highly contrasted reliefs, typical of deeply incised young mountain belts, but surprisingly, also by an average very low elevation of ∼900 m. Second, the Cordillera lacks central high reliefs; instead, there is a central, depressed domain with average low slope gradients including the deepest point of the study area (300 m below sea level in the bottom of Lake General Carrera/Buenos Aires). This depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips. To the east, the central Patagonian Cordillera is now separated from the flat plains of the Pampean domain (representing its former piedmont) and several elevated basaltic plateaus (mesetas) by 160°N–180°N trending morphotectonic corridors and the western border scarps of the mesetas. These scarps correspond to neotectonic features controlled by recent vertical faults, mostly active after 3 Ma. Remnants of a former peneplain, the Avellanos surface, are widespread within the entire Cordilleran region. From geological evidence, this peneplain formed between 15 and 4 Ma and was connected originally with the mesetas domain and now is dissected by these corridors. It is shown that a negative inversion related to extensional and strike-slip faulting occurred in the frontal region of the Cordillera at 3 Ma synchronously with the emplacement of magmatic products. Timing of the extensional deformation in central Patagonia is concomitant with the migration at depth of different segments of the South Chile Ridge, which entered the trench at 6 and 3 Ma, respectively, and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions. Finally, migration of segments of the Chile Spreading Ridge since 6 Ma triggered regional uplift of the Cordillera followed by limited extension in relation with increased thermal weakening of the crust.

Redefining the Waitemata Basin, New Zealand: A new tectonic, magmatic, and basin evolution model at a subduction terminus in the SW Pacific

The early Miocene Waitemata Basin has long been described as an interarc/intra‐arc basin, formed between twin chains of arc volcanoes in Northland, New Zealand. However, deep marine, polymict, volcaniclastic conglomerates within the basin reveal tectonic and magmatic signals that are not evident from neighboring volcanic edifices. The conglomerates were deposited by high‐density turbidity currents and debris flows and include single sediment cycle megaclasts of lava. These basaltic lavas have ocean island basalt (OIB)‐like geochemical affinities and are precisely dated at 20 Ma by 40Ar‐39Ar methods. Their age and the occurrence of subordinate clasts derived from an ophiolitic nappe to the north indicate the basin postdates the initiation of collision in the wider region. Contemporaneous calc‐alkaline volcanism did occur some 250 km NW of the basin. However, the conglomerates lack clasts of calc‐alkaline/arc affinities indicating an absence of arc‐like volcanism in the vicinity of the basin. Recent tectonic models for the SW Pacific region and mantle tomography highlight the importance of wholesale slab detachment in driving early Miocene calc‐alkaline volcanism and basin development. Although such models provide a slab window for the eruption of nonarc OIB‐like magmas, they would not explain their localized occurrence at the proposed leading edge of the tear (Waitemata Basin), rather than progressively along the entire length of the detachment (Northland), as seen in other detachment settings. In addition, OIB‐like volcanism predates the adjacent calc‐alkaline volcanism on the margin of the basin, a transition that is the opposite of that found at other slab detachment settings. The occurrence of OIB‐like volcanism is better explained by a lateral slab termination in the vicinity of the Waitemata Basin that allowed asthenospheric‐derived magmas to erupt. The basin is inferred to have developed in response to asthenospheric upwelling and associated lithospheric extension or deformation associated with a slab termination zone. This study highlights the importance and potential of combined geochemical, geochronological, and sedimentological studies of conglomerates in reconstructing the geodynamic setting of a basin.

Slab window migration and terrane accretion preserved by low‐temperature thermochronology of a magmatic arc, northern Antarctic Peninsula

Existing paleogeographic reconstructions indicate that the northern Antarctic Peninsula was central to several Mesozoic and Cenozoic tectonic events that have implications for ocean circulation and continental margin evolution. To evaluate the exhumational record of these processes, we collected new samples and measured fission track and (U‐Th)/He cooling ages of apatite and zircon from 13 Jurassic and Cretaceous granitoids in western Graham Land between the northern tip of the peninsula and the Antarctic Circle. Apatite He data reveal distinct ages and systematic age patterns north and south of Anvers Island, near the midpoint of the study area: To the south, apatite He ages range from 16 to 8 Ma and young northward, whereas to the north they range between 65 and 24 Ma (with one exception at 11 Ma) and young southward. Thermal histories inferred from the ages and closure temperatures of multiple thermochronometers in single samples indicate distinct histories for northern and southern Graham Land. Northern sites reveal a Late Cretaceous pulse of rapid cooling (>7°C/Myr) followed by very slow cooling (∼1°C/Myr) to the Recent, whereas southern sites record either a pulse of rapid mid‐Miocene cooling (∼8°C/Myr) or steady and moderate cooling (∼3°C/Myr) from the Late Cretaceous to the Recent. We interpret the Late Cretaceous rapid cooling in the northern part of the study area as a possible manifestation of terrane accretion associated with the Palmer Land event. We interpret the systematic spatial trends in apatite He ages and contrasting thermal histories along the peninsula as recording progressive Late Cenozoic northward opening of a slab window south of Anvers Island. This is consistent with a time transgressive pulse of ∼2–3 km of rock uplift and exhumation in the upper plate following ridge‐trench collision, cessation of subduction, and opening of the slab window, presumably caused by increased asthenospheric upwelling beneath the overriding plate.

Subduction of the Chile Ridge: Upper mantle structure and flow

We deployed 39 broadband seismometers in southern Chile from Dec. 2004 to Feb. 2007 to determine lithosphere and upper mantle structure in the vicinity of the subducting Chile Ridge. Body-wave travel-time tomography clearly shows the existence of a long-hypothesized slab window, a gap between the subducted Nazca and Antarctic lithospheres. P-wave velocities in the slab gap are distinctly slow relative to surrounding asthenospheric mantle. Thus, the gap between slabs visible in the imaging appears to be filled by unusually warm asthenosphere, consistent with subduction of the Chile Ridge. Shear wave splitting in the Chile Ridge subduction region is very strong (mean delay time ~3 s) and highly variable. North of the slab windows, splitting fast directions are mostly trench parallel, but, in the region of the slab gap, splitting fast trends appear to fan from NW-SE trends in the north, through ENE-WSW trends toward the middle of the slab window, to NE-SW trends south of the slab window. We interpret these results as indicating flow of asthenospheric upper mantle into the slab window.

Seismicity and state of stress in Guerrero segment of the Mexican subduction zone

We take advantage of a relatively dense network of seismic stations in the Guerrero segment of the Mexican subduction zone to study seismicity and state of stress in the region. We combine our results with recent observations on the geometry of the subducted Cocos plate imaged from receiver function (RF) analysis, an ultraslow velocity layer mapped in the upper crust of the subducted slab, and episodic slow slip events (SSEs) and nonvolcanic tremors (NVTs) reported in the region to obtain a comprehensive view of the subduction process. Seismicity and focal mechanisms confirm subduction of the Cocos plate below Mexico at a shallow angle, reaching a depth of 25 km at a distance of 65 km from the trench. The plate begins to unbend at this distance and becomes horizontal at a distance of ∼120 km at a depth of 40 km. Some of the highlights of the inslab seismicity are as follows: (1) A cluster of earthquakes in the depth range of 25–45 km, immediately downdip from the strongly coupled part of the plate interface, revealing both downdip compressional and extensional events. This seismicity extends from ∼80 to 105 km from the trench and may be attributed to the unbending of the slab. (2) The slab devoid of seismicity in the distance range of ∼105–160 km. (3) Sparse inslab seismicity revealing downdip extension in the distance range of 160–240 km. The NVTs are also confined to this distance range. The episodic SSEs occur on the horizontal segment, in the distance range of ∼105–240 km, where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted SP phases. Thus, in the distance range of ∼105–160 km, SSEs occur but NVTs and inslab earthquakes are absent. This suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of ∼160 km, giving rise to both the inslab earthquakes and NVTs. No inslab earthquake occurs beyond 240 km. The receiver function images and P wave tomography suggest that the slab begins a steep plunge at a distance of ∼310 km, reaching a depth of 500 km around 340 km from the trench. The negative buoyancy of such a slab should give rise to large extensional stress in the slab. Yet inslab seismicity is remarkably low, which may be explained by a slab that is not continuous up to a depth of 500 km, but is broken at a shallower depth. The resulting slab window may permit subslab material to flow through the gap. This may provide an explanation for the recent rift‐related basalts found near Mexico City. The fore‐arc, upper plate seismicity, which during the period of study (1995–2007) consisted of a moderate earthquake (Mw5.8) near the coast (H = 12 km), its numerous aftershocks, and two shallow events farther inland, demonstrates a trench‐normal extension in the upper plate near this convergent margin, a state of stress that may be explained by tectonic erosion and/or seaward retreat of the trench. Seismicity, location of the mantle wedge, and rupture areas of Mexican earthquakes suggest that the downdip limit of rupture during large/great earthquakes in Guerrero may be 105 ± 15 km. Shallow‐dipping, interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region. The magnitudes of inslab downdip compressional and extensional earthquakes that occur within ∼20 km inland from the coast, in the depth range of 25–45 km, may reach 6.5 and 7.5, respectively. In addition, we now identify normal‐faulting earthquakes in the upper plate. These sources need to be taken into account in the hazard estimation.

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.

Dynamics and implications of slab detachment due to ridge‐trench collision

The approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab. Previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism (e.g., in Baja California), but such a scenario has not been tested using fully dynamic numerical models. We use dynamic two‐dimensional models including a non‐Newtonian rheology to study the approach of a trench‐parallel spreading ridge to a subduction zone. We find that before the ridge approaches within 100 km of the trench, detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity. In models exploring effects of subducted slab length, distance of the ridge from the trench, shear zone strength, and lithospheric yield strength, we find the following: (1) a decrease in subduction velocity as the ridge approaches the trench, (2) a shrinking surface plate that maintains a uniform plate‐like subduction velocity, (3) ridge abandonment distances 100–275 km from the trench, and (4) slab gap distances 195–285 km from the trench. These results are consistent with observations in Baja California, where detachment of the Cocos slab may explain abandonment of observed segments of the East Pacific Rise 50–200 km outboard of the trench and the presence of nonarc related magmatism located 100–250 km inboard of the trench. Ridge subduction does not occur within any of these two‐dimensional cases of ridge‐trench collision, indicating that 3‐D geometric effects may be responsible for ridge subduction.

Tectonic control on southern Sierra Nevada topography, California

In this study we integrate the apatite (U‐Th)/He thermochronometric technique with geomorphic, structural, and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late Cenozoic faulting in the southern Sierra Nevada. The geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low‐relief landscape, little affected by internal deformation, to a more complex landscape affected by numerous faults. Regionally, the relict landscape surface is readily resolved by age‐elevation relationships of apatite He ages coupled to geomorphology. These relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting. On the basis of 63 new apatite He ages and stratigraphic data from proximal parts of the San Joaquin basin we resolve two sets of normal faults oriented approximately N–S and approximately NW. Quaternary west‐side‐up normal faulting along the N–S Breckenridge–Kern Canyon zone has resulted in a southwest step over from the Owens Valley system in the controlling structure on the regional west tilt of Sierran basement. This zone has also served as a transfer structure partitioning Neogene‐Quaternary extension resulting from normal displacements on the NW fault set. This fault system for the most part nucleated along Late Cretaceous structures with late Cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region.

Origin of paired high pressure–ultrahigh-temperature orogens: a ridge subduction and slab window model

Terra Nova, 22, 35–42, 2010 Ultrahigh-temperature (UHT) and high pressure (HP) metamorphic rocks generated at different times in Earth history form paired suites within the same tectonic belts in several regions. We evaluate the thermal regimes and fluid circulation patters in different plate tectonic settings and propose a new model involving ridge subduction where the slab window places hot asthenosphere against the base of the overlying plate and provides an ideal setting for the supply of heat and CO2-rich fluids at relatively shallow domains, which can explain the formation and preservation of UHT metamorphic rocks in the roots of the arc and forearc. The plate geometry below the slab window would inhibit the normal cooling induced by the slab, and a temporary deprivation of slab-derived sediments and hydrous fluids. Our analysis provides a unified model for paired UHT and HP metamorphic orogens and presents a key to the associated thermal and fluid regimes.

Detecting the limit of slab break-off in central Turkey: new high-resolutionPntomography results

Inversion of Pn traveltime residuals from a 39-station broad-band array provides a high-resolution image of the velocity structure in the uppermost mantle beneath central Turkey. Individually picked Pn phase arrivals from events recorded by the North Anatolian Fault Passive Seismic Experiment and the Kandilli Observatory were combined with additional events associated with the Eastern Turkey Seismic Experiment. Tomography results show no change in Pn velocity across the North Anatolian Fault, although longitudinal variations are evident. A region of very low Pn velocities (<7.8 km s−1) is imaged east of the Central Anatolian Fault Zone (CAFZ), with a transition to faster velocities (>8.1 km s−1) west of the fault. The sharp transition along the CAFZ, which follows the palaeotectonic Inner-Tauride Suture, may represent the location of the edge of the slab window, created when the oceanic slab broke off along the Bitlis-Zagros Suture around 11 Ma, as the Arabian plate collided with the Eurasian plate.

Seismic response to slab rupture and variation in lithospheric structure beneath the Savu Sea, Indonesia

Variations in seismic moment release and stress state across the transition from subduction of oceanic crust to arc–continent collision in the Banda Arc are constrained by focal mechanism solutions from the CMT earthquake catalogue. In particular the slab under the western Savu Sea is unusual in that intermediate depth (70–300 km) events indicate that at this depth range the slab is largely in down-dip compression. This contrasts with the intermediate depth, down-dip tension that typifies the Sunda slab to the west and the far eastern Banda slab to the east. Down-dip compression beneath the Savu Sea reflects subduction of transitional crust of the Scott Plateau, more buoyant than the Indian Ocean crust subducting further west. In this region, enhanced magma flux is indicated by unusually narrow volcano spacing in the overlying arc, and suggests that down-dip compression reflects not only more buoyant transitional crust but also a reduction in slab–wedge coupling induced by enhanced magma flux. East of the Savu Sea, the near complete absence of intermediate depth seismicity is attributed to a slab window that has opened where Australian continental crust has collided with the arc. Differences in seismic moment release around this slab window indicate asymmetric rupture, propagating to the east at a much faster rate than to the west.

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.

Geochemical, Sr–Nd and zircon U–Pb–Hf isotopic studies of Late Carboniferous magmatism in the West Junggar, Xinjiang: Implications for ridge subduction?

Voluminous granitic intrusions are distributed in the West Junggar, NW China, and they can be classified as the dioritic rocks, charnockite and alkali-feldspar granite groups. The dioritic rocks (SiO 2 = 50.4–63.8 wt.%) are calc-alkaline and Mg enriched (average MgO = 4.54 wt.%, Mg # = 0.39–0.64), with high Sr/Y ratios (average = 21.2), weak negative Eu (average Eu⁎/Eu = 0.80) and pronounced negative Nb–Ta anomalies. Their Sr–Nd and zircon Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.7035–0.7042, ε Nd( t) = 4.5–7.9, ε Hf( t) = 14.1–14.5) show a depleted mantle-like signature. These features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials. The charnockites (SiO 2 = 60.0–65.3 wt.%) show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu (average Eu⁎/Eu = 0.75) but pronounced negative Nb–Ta anomalies. Sr–Nd and zircon Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.7037–0.7039, ε Nd( t) = 5.2–8.0, ε Hf( t) = 13.9–14.7) also indicate a depleted source, suggesting melts from a hot, juvenile lower crust. Alkali-feldspar granites (SiO 2 = 70.0–78.4 wt.%) are alkali and Fe-enriched, and have distinct negative Eu and Nb–Ta anomalies (average Eu⁎/Eu = 0.26), low Sr/Y ratios (average = 2.11), and depleted Sr–Nd and zircon Hf isotopic compositions (( 87Sr/ 86Sr) i = 0.7024–0.7045, ε Nd( t) = 5.1–8.9, ε Hf( t) = 13.7–14.2). These characteristics are also comparable with those of rocks derived from juvenile lower crust. Despite of the differences in petrology, geochemistry and possibly different origins, zircon ages indicate that these three groups of rocks were coevally emplaced at ~ 305 Ma. A ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the “slab window” opened, upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting in the mantle wedge generated mafic rocks in the region. These results suggest that subduction was ongoing in the Late Carboniferous and, thus support that the accretion and collision in the Central Asian Orogenic Belt took place in North Xinjiang after 305 Ma, and possibly in the Permian.