Back-arc Area Research Articles

GNSS networks for geodynamics in Italy

The recent development of GNSS networks in Italy makes it possible to define an increasingly detailed spatial and temporal resolution of the ongoing crus tal deformation and to visualize the complex interplay between different orogens in the Africa-E urasia collision zone. We have analyzed all the available GPS raw data in the time span 1998-2013 obtaining the coordinate time series in a common reference frame. We have finally produced the veloc ity field providing a detailed picture of the kinematics and deformation pattern of the Italian a rea. The horizontal velocities with respect to Eurasia are within 7 mm/yr; vertical rates reach ab out 5 mm/yr along the Apennines and Alpine belts, and -10 mm/yr in the Tyrrhenian backarc area. The deformation rates are greater than 100 nanostrain/yr in volcanic areas and within 80 nanos train/yr along the Apennines.

Genetic waveform modeling for the crustal structure in Northeast Japan

We propose a genetic algorithm (GA) search procedure for waveform modeling of local crustal earthquakes for optimal one-dimensional (1-D) crustal velocity model. Both waveforms and travel-time data are used for the structure determination. The use of travel times in model evaluation improves the waveform modeling performance in the sense of computation speed and accuracy. We applied this method to broadband waveforms of a local crustal earthquake (M 4.2) in Northeast Japan. P-wave velocities of the crustal model are found to be 4.95±0.30, 5.9±0.02, and 6.51±0.20km/s for a surface layer, upper crust and lower crust, respectively. The surface layer thickness and the Conrad and Moho depths are found to be 3.01±0.8, 17.77±0.4 and 34.59±1.0km, respectively. For epicentral distances <200km, our synthetic waveforms match the observed ones generally well. Early arrivals are mainly observed at stations near the Pacific coast in the forearc area having a thinner crust. In contrast, delayed arrivals appear at stations near the volcanic front and back-arc areas where low-velocity anomalies exist due to the effect of the Pacific slab dehydration and the hot upwelling flows in the mantle wedge. In general, our results agree well with the main tectonic setting of the study area, which confirms the reliability of the proposed approach. Despite a 1-D velocity model is too simple to represent the complex crustal structure, it is still required for the conventional routine analysis of seismology, such as earthquake location and source parameter studies. The current approach is considered as a step toward the genetic full waveform modeling for the 3-D velocity model estimation.

Constraints on the Origin and Evolution of Magmas in the Payún Matrú Volcanic Field, Quaternary Andean Back-arc of Western Argentina

The Payún Matrú Volcanic Field (Pleistocene–Holocene) is located in the Andean back-arc of the Southern Volcanic Zone, western Argentina, and is contemporaneous with the Andean volcanic arc at the same latitude. It includes two polygenetic, mostly trachytic volcanoes: Payún Matrú (with a summit caldera 8 km wide) and Payún Liso (a smaller stratovolcano). The volcanic field includes about 200 scoria cones and alkali basaltic and trachybasaltic lava flows, forming two basaltic fields around Payún Matrú. New 40Ar–39Ar ages extend the activity of Payún Matrú up to 700 ka. The major and trace element and Sr–Nd isotopic compositions of the basaltic lavas and Payún Matrú rocks indicate that the trachytes of Payún Matrú are the result of fractional crystallization of basaltic parent magmas without significant upper crustal contamination, and that the basalts have a geochemical similarity to ocean island basalt (La/Nb = 0·8–1·5, La/Ba = 0·05–0·08). The Sr–Nd isotopic compositions of the basaltic to trachytic rocks range between 0·703813 and 0·703841 (87Sr/86Sr) and 0·512743 and 0·512834 (143Nd/144Nd). Mass-balance and Rayleigh fractionation models support the proposed origin of the trachytes, and an assimilation–fractional crystallization model indicates a low degree of upper crustal contamination in the youngest trachytes. Magnesium numbers (45–55) and contents of Ni (<20–90 ppm) and Cr (30–180 ppm) in the lavas in the basaltic fields indicate that these are not primary magmas. The data also suggest that the basaltic lavas originated in the asthenospheric mantle, probably within the spinel stability field and beneath an attenuated continental lithosphere in the back-arc area. The lack of a slab-fluid signature in the Payún Matrú Volcanic Field rocks, along with unpublished and published geophysical results (mantle tomography and electrical conductivity anomalies) suggest that magmas were generated by decompression-induced melting of upwelling mantle.

Late Miocene shortening of the Northern Apennines back-arc

The inner Northern Apennines (western Tuscany and Tyrrhenian basin) is characterized by a relatively thin continental crust (∼20–25km), high heat flow (>100mWm−2), and the presence of relevant tectonic elision of stratigraphic sequences, a setting known as Serie Ridotta. These features are normally ascribed to an extensional deformation that affected the back-arc area above the subducting Adria plate since the Early-Middle Miocene (∼16Ma). However, various geophysical studies image the continental crust to be currently affected by W-dipping thrust faults (and associated basement uplifts) that have not been obliterated by this claimed long-lasting extensional process. These observations raise the question whether the thrusts are older or younger than the continental extension. To address this question we have reprocessed and interpreted the deep seismic reflection profile CROP03/c that crosses the onshore hinterland sector, and investigated the structural setting of some of the Late Miocene-Pliocene hinterland basins (Cinigiano-Baccinello, Siena-Radicofani, Tafone, Albegna and Radicondoli basins) that are situated at the front or in-between the basement uplifts. The analysis of field structures and commercial seismic profiles has allowed the recognition that both substratum and basins’ infill have been intensely shortened. These findings and the architecture of the basins suggest that the latter developed under a contractional regime, which would have started around 8.5Ma with the onset of the continental sedimentation. This compressive stress state followed an earlier phase of continental extension that presumably started at ∼16Ma (with the blocking of the Corsica-Sardinia rotation), and thinned both the continental crust and sedimentary cover producing most of the Serie Ridotta. The main phases of basin shortening are bracketed between 7.5 and 3.5Ma, and thus overlap with the increase in the exhumation rate of the metamorphic cores at ∼6–4Ma determined through thermochronological data. We therefore propose a correlation between the basin deformation and the activity of the nearby basement thrusts, which would have thus shortened a previously thinned continental crust. This chronology of deformation may suggest a geodynamic model in which the back-arc and hinterland sector of the Northern Apennines was recompressed during Late Miocene-Early Pliocene times. This evolution may be explained through different speculative scenarios involving a blockage of the subduction process, which may vary between end members of complete slab detachment and stalled subduction.

Three-dimensional resistivity modelling of a seismogenic area in an oblique subduction zone in the western Kurile arc: Constraints from anomalous magnetotelluric phases

Abstract Wide-band magnetotelluric surveys were performed at 44 sites in the inland earthquake zone on the western margin of the Kurile arc, where strain accumulation has been observed since a high-magnitude interplate earthquake in 2003 (Tokachi-oki earthquake, M 8.0). We estimated the three-dimensional (3-D) resistivity distribution based on magnetotelluric impedance tensors, which contained anomalously large impedance phases. Although anomalous impedance phases have previously been considered a major impediment for magnetotelluric studies, we found that the anomalous phase can be used to constrain the resistivity distribution. The 3-D resistivity model showed that the earthquake zone consists of resistive areas (> 300 Ω-m), whereas the fore-arc and back-arc areas consist of regional conductive layers (

P-wave anisotropy tomography of central Japan: Insight into subduction dynamics

A P-wave anisotropic tomography under central Japan was determined using a large number of arrival-time data from 22,244 earthquakes recorded by 770 seismic stations on the Japan Islands. The results show that the anisotropy exists widely beneath the study area except for the central portion of the mantle wedge. The fast-velocity directions (FVDs) of the mantle wedge are generally trench-parallel under the fore-arc area and trench-normal under the back-arc area, which may be attributed to the lattice-preferred orientation (LPO) of olivine changing from B-type under the fore-arc area to A-type under the back-arc area with the variation of water content and the occurrence of mantle flow. The subducting Philippine Sea plate (PHS) beneath Tokai is revealed as a high-velocity anomaly dipping northward with NEE to NE–SW FVDs which are consistent with the spreading direction of the PHS during its formation and are inferred as the fossil anisotropy of the PHS. The anisotropy of the upper portion of the Pacific slab under central Japan is also revealed in this study. The FVDs of the upper portion of the Pacific slab are sub-parallel to the Japan Trench and Izu-Bonin Trench at ~60–120km depths and become almost perpendicular to the trenches below 100–120km depth. After comparisons with the Mesozoic magnetic anomaly lineations and the principle stresses in the Pacific slab, we propose that the fossil FVDs of the Pacific slab beneath central Japan were already rebuilt by the principle stresses in the slab.

Development of the Khao Khwang Fold and Thrust Belt: Implications for the geodynamic setting of Thailand and Cambodia during the Indosinian Orogeny

The Indosinian Orogeny in Thailand is often viewed as having developed between strongly linear terranes, which today trend approximately N–S. The terranes were subsequently disrupted by later tectonics, particularly NW–SE trending Cenozoic strike-slip faults. The ENE–WSW to NE–SW striking thrusts and folds in the Khao Khwang Platform area of the Saraburi Group on the SW margin of the Indochina Terrane are not easily explained in the context of this traditional view. Reversal of the clockwise rotation shown to have affected the block north of the Mae Ping Fault zone only enhances the E–W orientation of structures in the fold and thrust belt, and moves the belt further east towards Cambodia. One solution for the trend that fits better with regional understanding from hydrocarbon exploration of the Khorat Plateau is that the Indochina Terrane was actually a series of continental blocks, separated by Permian rifting. During the Early Triassic the early stages of collision (South China-Cathaysian Terrane collision with Vietnam Indochina) resulted in the amalgamation of disparate blocks that now form the Indochina Terrane by closure along the rifts. At the same time or following on from the collision there was closure of the back-arc area between Indochina and the Sukhothai zone. The rift basins, were thrusted and inverted during the early stages of the Indosinian orogeny, and only underwent minor reactivated when later Sibumasu collided with Sukhothai Zone-Indochina Terrane margin during the Late Triassic. The scenario described above requires the presence of a (minor) E–W trending suture in NW Cambodia. Evidence for this suture is suggested by the presence of Permo-Triassic calc-alkaline volcanism.

한반도 남동부 경상호의 조구조 배경과 호화산작용

한반도에는 백악기에 북동-남서 방향으로 달리는 여러 화산호들을 형성하였으며 그 중에서 경상분지의 남동부를 지나는 경상호가 가장 우세하다. 백악기 전기 화산작용은 하양층군의 여러 층준에 대체로 알칼리 혹은 칼크알칼리 현무암질 용암류로 구성되는 화산층을 협재시켰다. 백악기 후기 화산작용은 경상호 내에 두꺼운 칼크알칼리 안산암질 및 유문암질 화산층을 형성하였다. 이 화산작용은 (1) 먼저 현무암질 및 안산암질 용암류의 분출로 특징되며, (2) 후에 데사이트질-유문암질 화쇄강하, 대규모 화쇄류의 분출로 이어지고 이에 관련하여 칼데라와 칼데라후 암맥과 돔의 형성으로 특징되어진다. 백악기 전기(약 120Ma)에 북쪽에서 북북서 방향으로 약간 변화됨으로서 사각방향의 이자나기판 섭입작용은 북동내지 북북동 방향의 좌수향 주향이동 단층을 형성하면서 경상호의 배호 지역에 인리형 분지가 열리고 동쪽으로 확장되어 배호분지를 형성하였다. 이때 배호분지에는 암압의 감소와 함께 마그마 생성의 촉진으로 용암류를 분출시켰으며, 하양층군의 여러 층준에 화산성 협재층을 형성하였다. 따라서 경상분지는 대륙 열곡작용에 의해 형성된 배호분지로 해석된다. 배호분지의 확장과 더불어 경상분지에서는 호화산작용이 100Ma 무렵부터 이미 시작되었으며, 백악기 후기로 가면서 격렬하게 지속되었고 화산호를 형성하였다. 백악기 후기(약 90Ma)에 이자나기판의 섭입방향은 북북서 방향에서 북서 방향으로 변화됨으로서 분지발달이 종식되었으며, 대륙연변부에 대해 사각에서 직각방향으로의 변화는 마그마 생성이 용이하게 유도되어 백악기 후기 동안에 화산작용이 지속적으로 진행되게 하였다. 한편 대구-부산 사이에 북서 방향의 폭넓은 화산대는 이자나기-태평양 해령의 빠른 북서향 섭입작용으로 슬랩창의 높은 열원에 의한 화산작용과 무관하지 않다. The Gyeongsang Arc is the most notable of the Korea Arc that is composed of several volcanic arcs trending to NE-SW direction in the Korean peninsula. The Hayang Group has many volcanogenic interbeds of lava flows by alkaline or calc-alkaline basaltic volcanisms during early Cretaceous. Late Cretaceous calc-alkaline andesitic and rhyolitic volcanisms reconstructed the Gyeongsang Arc that consist of thick volcanic strata on the Hayang Group in The Gyeongsang Basin. The volcanisms characterize first eruptions of basaltic and andesitic lavas with small pyroclastics, and continue later eruptions of dacitic and rhyolitic ash-fall and voluminous ash-flow with some calderas and then domes and dykes. During the Early Cretaceous (about 120 Ma), oblique subduction of the Izanagi plate to NNW from N direction results in sinistral strike-slip faults to open a pull-apart basin in back-arc area of the Gyeongsang Arc, in which erupted lava flows from generation of magma by a decrease in lithostatic pressure. Therefore the Gyeongsang Basin is interpreted into back-arc basin reconstructed by a continental rifting. Arc volcanism began in about 100 Ma with exaggeration of the back-arc basin in the Gyeongsang, and then changed violently to construct volcanic arcs. During the Late Cretaceous (about 90 Ma), orthogonal subduction of the Izanagi plate to NW from NNW direction ceased development of the basin to prolong violent volcanisms.

Late Cenozoic tectono‐stratigraphic sequences of theCrotoneBasin: insights on the geodynamic history of theCalabrian arc andTyrrhenian Sea

AbstractTheCrotoneBasin was generated in the lateCenozoic as a forearc basin of theIonian arc‐trench system. New data are gained through detailed field mapping, high‐resolution stratigraphic analysis of a key area and examination of offshore well data and seismic reflection profiles. Major unconformities divide the basin fill into major sequences, which reveal a three‐stage internal organization thought to reflect geodynamic events of theCalabrian arc and backarc area closely. Thefirst stageis characterized by extensional block faulting and uplift followed by rapid drowning during high subsidence and transtension in the basin along a major NNW‐ to NW‐striking fault system. This stage is interpreted to reflect resumption of rollback after an episode of slab tearing triggered by transitory docking of continental lithosphere in the trench. The initial uplift is inferred to reflect decoupling and rebound after the transitory coupling phase. Thesecond stageis characterized by increased subsidence and continued extension/transtension. This trend presumably reflects a decreasing rate of rollback resulting from a tendency towards viscous coupling after acceleration of slab downwelling. Thethird stageis characterized by short‐lived transpression along major shear zones and local inversion of former basins. This is inferred to reflect entrance into the trench of buoyant continental lithosphere, resulting in significant deceleration of slab rollback and consequently a break in, or slowing of, backarc extension, and predominance of the effects of compression related toAfrica–Europe convergence. Overall, the above evolution resulted in the formation of a progressively narrower and rapidly retreating slab, inducing extreme rates of backarc extension, and may have played a critical role in determining the intermittent nature of the backarc rifting.

Constraints on the history and topography of the Northeastern Sierra Nevada from a Neogene sedimentary basin in the Reno-Verdi area, Western Nevada

Neogene (Miocene–Pliocene) sedimentary rocks of the northeastern Sierra Nevada were deposited in small basins that formed in response to volcanic and tectonic activity along the eastern margin of the Sierra. These strata record an early phase (ca. 11–10 Ma) of extension and rapid sedimentation of boulder conglomerates and debrites deposited on alluvial fans, followed by fluvio-lacustrine sedimentation and nearby volcanic arc activity but tectonic quiescence, until ∼ 2.6 Ma. The fossil record in these rocks documents a warmer, wetter climate featuring large mammals and lacking the Sierran orographic rain shadow that dominates climate today on the eastern edge of the Sierra. This record of a general lack of paleo-relief across the eastern margin of the Sierra Nevada is consistent with evidence presented elsewhere that there was not a significant topographic barrier between the Pacific Ocean and the interior of the continent east of the Sierra before ∼ 2.6 Ma. However, these sediments do not record an integrated drainage system either to the east into the Great Basin like the modern Truckee River, or to the west across the Sierra like the ancestral Feather and Yuba rivers. The Neogene Reno-Verdi basin was one of several, scattered endorheic (i.e., internally drained) basins occupying this part of the Cascade intra-arc and back-arc area.

The sources of energy for crustal melting and the geochemistry of heat-producing elements

Crustal melting to produce granite magmas requires a tremendous amount of energy. In principle, there are three main mechanisms of heating that can extensively melt a fertile crust: radiogenic heat caused by the decay of 40K, 230Th, 235U and 238U; increased subcrustal heat flux caused by the upwelling of deeper, therefore hotter, mantle materials; advection of heat caused by the emplacement and crystallization of hot mantle magmas. Two-dimensional finite elements modeling reveals that a fertile crust thickened to 65–70km would produce copious granite magmatism after 30–40M.y. if its average heat-production is A>1.2μWm−3, but it would scarcely melt if A<0.65μWm−3. Increasing the subcrustal heat flow from the normal value of QM≈0.025Wm−2 to QM≥0.04Wm−2 may also lead to extensive crustal melting, especially if the crust does not thin to less than 30–35km. Very high QM (≥0.06Wm−2) affecting the continental crust is unlikely, but the combination of moderately high QM (≈0.04Wm−2) and a thick fertile crust with A<1.2μWm−3, such as often happens in the volcanic and back-arc areas of subduction zones, is ideal to produce copious granite magmatism. Lastly, the emplacement of hot mantle magmas in a fertile crust can produce crustal melts in just a few thousand years, but the volume of these is equal to or less than the volume of the intruding magma. A clue for understanding the relative importance of each of these three mechanisms comes from the radiogenic heat production of granite rocks calculated from the concentration of 40K, 230Th, 235U and 238U at the time of their formation. This parameter estimated on more than 3400 granites samples of different ages and provenance reveals a strongly asymmetric distribution peaking around 2.4μWm−3, a value much higher than the average continental crust (about 1–1.2μWm−3) and certainly much higher than the average lower continental crust (about 0.4–0.8μWm−3). Only those granite rock types that are clearly connected with mantle heat sources such as the Archean TTG, post-Archean subduction-related trondhjemites, and recent adakites have a heat production equal to or smaller than the lower continental crust. Since the bulk melt/solid partition coefficient of the heat-producing elements (HPE: K, Th and U) is k≤1, the elevated HPE contents of granites indicates that most of them have been derived from HPE-rich sources. We conclude that radiogenic heating is often essential, and always advantageous, for generating large volumes of granite magmas, and that granite magmatism is the main cause of the accumulation of HPE in the upper crust.

Preliminary Study of the Strong Ground Motions of the Tohoku, Japan, Earthquake of 11 March 2011: Assessing the Influence of Anelastic Attenuation and Rupture Directivity

Online material : Vs30 values, various distances estimated for recording stations, and corner frequencies of the low-cut filter applied to data for present study. On 11 March 2011 a megathrust earthquake (M 9.0)—the largest recorded earthquake in the history of Japan—occurred off the Pacific coast of Tohoku-oki (Table 1). The earthquake ruptured the interplate boundary offshore of northeastern Honshu with several earthquake rupture models suggesting peak epicentral displacements up to 30–40 m and duration about 150 s. Several focal mechanism estimates ( e.g. , the weighted cumulative moment tensor [WCMT] with slab geometry from U.S. Geological Survey; http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/finite_fault.php, last accessed August 2011) show a thrust fault striking at ∼195° and dipping at ∼10°, with a rupture model of about 250 km × 150 km. The tectonic setting of the broader Japan area corresponds to a subduction area with the Pacific plate subducting beneath the Eurasian plate. In general, attenuation in subduction zones is controlled by the low-velocity low- Q material of the mantle wedge dividing the zone into along-arc and back-arc regions ( e.g. , Ghofrani and Atkinson 2011; Skarlatoudis et al. 2011; Boore et al. 2009). This phenomenon affects ground motions in Japan in general ( e.g. , Hasegawa et al. 1994; Yoshimoto et al. 2006) and the Tohoku-oki earthquake is no exception, resulting in lower levels of ground motions at larger distances, mainly in back-arc areas. Moreover, this is a case of a megathrust event for which an extremely extensive set of local seismic, strong motion, and geodetic data is available, suggesting it may be one of the best recorded earthquakes in the history of seismology. In the present work we examine the peak and spectral characteristics of the strong motion records available from the Kyoshin network (K-NET; http://www.k-net.bosai.go.jp/, last accessed August 2011). Furthermore, we perform …

Deformación y esfuerzos neotectónicos en Costa Rica

Seismological, vulcanological, geomorphological and structural data and criterion are used (...) to define stress trayectories and the neotectonic deformation that occur in the morphotectonic units of Costa Rica. The neotectonic deformation data show that the trench-forearc-arc-backarc region has been subjected to maximum horizontal compressive stresses (ECHM) that correspond to the maximum principal stress. These stresses trend N30°E between the Middle American Trench and the forearc area. The predominant faulting is thrust or reverse faulting. In the arc, the predominant orientation of is NO°E, and the principal faulting is strike-slip. In the backarc area also trends NO°E. The 20°-30° counterclock-wisy ECHM rotation between the interplate margin and the internal arc is aparent and is explained by oblique convergence between the Cocos-Caribbeean plates.

Tectonomagmatic characteristics of the back‐arc portion of the Calama–Olacapato–El Toro Fault Zone, Central Andes

Post–20 Ma magmatism in the Central Andes is either localized in the magmatic arc or distributed east of it, on the Altiplano‐Puna Plateau. Here there is a distinct concentration of magmatic centers on NW–SE trending lineaments, such as the Calama–Olacapato–El Toro (COT), that extends into the Eastern Cordillera to the east of the Puna. Understanding the possible genetic relationship between prominent structures and magmatic centers on these lineaments is important to elucidate the tectonomagmatic evolution of the Central Andes. We investigated the back‐arc area of the COT using remote sensing, geological, structural, and petrochemical data. Our study demonstrates that this portion of the COT consists of NW–SE striking faults, formed under overall left‐lateral transtension that decreases in activity toward the COT termini. Deformation on the COT occurred during and after activity of prominent N–S striking transpressive fault systems and is coeval with magmatism, which is focused on the central COT. The most evolved magmatic rocks, with an upper crustal imprint, are exposed on the central COT, whereas more primitive, mantle‐derived mafic to moderately evolved magmatic rocks, are found toward the COT termini. This points to a genetic relationship between upper crustal deformation and magmatic activity that led to enhanced magma storage in the central COT. COT magmas may result either from slab steepening or episodic delamination of the asthenospheric mantle.

Shear wave anisotropy beneath the Aegean inferred from SKS splitting observations

[1] SKS splitting parameters are measured in the Aegean region using events recorded at a dense temporary network in the south Aegean and the operating permanent networks, especially focusing in the back-arc and the near-trench areas of the Hellenic arc. In general, fast anisotropy directions are trench perpendicular in the back-arc area and trench parallel near the trench. Anisotropy measurements near the volcanic arc mark the transition between these two regions. In the back arc, a gradual increase is observed in delay times from south to north, with a prevailing NE-SW direction. In Cyclades, this pattern is correlated with GPS velocities and stretching lineations of metamorphic core complexes. Our preferred source of anisotropy in the back-arc region is the mantle wedge flow, induced by the retreating descending slab. The westernmost termination of the trench reveals directions parallel with the Kefalonia Transform Fault and perpendicular to the convergence boundary. Beneath Peloponnese, the trench-parallel flow is probably located beneath the shallow-dipping slab, although scattered measurements may also reflect fossil anisotropy from a past NW-SE strike of the trench. In western Crete, which may be entering a stage of continental collision, the anisotropy pattern changes to trench perpendicular, with a possible subslab source. Good nulls in central east Crete indicate a change in the anisotropy origin toward the east. At the easternmost side of the trench, fast directions are trench parallel. This reflects a similar subslab flow that may become toroidal around the slab edge beneath western Turkey. This may also produce a trench-parallel flow within the mantle wedge.

Geodynamic model of the northwestern Caribbean: scaled reconstruction of Late Cretaceous to Late Eocene plate boundary relocation in Cuba

Reviewing the local geological data of Cuba, its structural relations to Hispaniola and Jamaica as well as a comparison of prevalent plate tectonic models, allow for a refined Late Creta- ceous to Miocene tectonic reconstruction. The Cuban orogenic belt records subduction, volcanic arc formation and accretion along the pre-Eocene northwestern leading edge of the Caribbean plate. Geologic evidence points to a two-stage development with change in subduction polarity from a south- and southwest-dipping Cretaceous to a north-dipping Paleocene to Early Eocene subduction zone. During the Late Campanian, the Cretaceous arc collided with the North American continental margin. Ophiolites and parts of the Cretaceous volcanic arc are thrust onto the North American continental margin until the Late Eocene. After the initial Campanian collision, the Caribbean plate continued its relative northward movement. As a consequence, oceanic lithosphere of the back-arc area was emplaced on the top of the southern extension of the inactive arc. During the Danian, a new north-dipping subduction zone was established that consumed oceanic lithosphere of the Caribbean plate until the Middle Eocene. The arrival of thickened oceanic crust of the Caribbean Large Igneous Province stopped the subduction and the relative northward movement of the Caribbean plate. Subsequently, in the Middle Eocene the east-west striking Oriente transform fault system was formed which since then represents the northern boundary of the Caribbean plate.

Seismic heterogeneity and anisotropy of the Honshu arc from the Japan Trench to the Japan Sea

We determined P- and S-wave tomography and P-wave anisotropic structure under the Honshu arc from the Japan Trench to the backarc area under the Japan Sea using 310 749 P- and 150 563 S-wave arrivals from 4655 local earthquakes recorded by 982 seismograph stations. Arrival times from 1451 suboceanic earthquakes relocated with sP depth phases enable us to determine the structures under the Pacific Ocean and Japan Sea, which expand the study region from the land area to the whole arc from the Japan Trench to the Japan Sea with a width of more than 500 km. The results show strong heterogeneities above the subducting Pacific slab under the Pacific Ocean and most large thrust-type earthquakes occurred in the high-velocity areas where the Pacific slab and the overriding continental plate may be strongly coupled. Low-velocity (low-V) zones are imaged in the mantle wedge with significant along-arc variations under the volcanic front. The mantle-wedge low-V zone extends westwards under the Japan Sea and it is connected with the subducting Pacific slab at depths of 150–200 km under the backarc. The results indicate that the H2O and fluids brought downwards by the subducting Pacific slab are released into the mantle wedge by dehydration and are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under the Honshu arc. The predominant fast velocity direction (FVD) is E–W in the mantle wedge while it is N–S in the subducting Pacific slab. The anisotropy in the mantle wedge is the result of deformation caused by the subduction of the Pacific plate and the induced mantle-wedge convection, while the FVD pattern in the middle of the mantle wedge argues for the 3-D mantle flow or the specific alignment of the olivine in the partial-melting mantle. The N–S (trench-parallel) FVD in the subducting Pacific slab represents either the original fossil anisotropy when the Pacific plate formed or the trench-parallel crystallographic and shaped preferred orientation in the subducting slab due to the slab bending. The present results shed new light on the structural heterogeneities and seismic anisotropy under the Honshu arc, which may improve our understanding of the dynamic processes of subduction zones.

Shear wave anisotropy in the crust, mantle wedge, and subducting Pacific slab under northeast Japan

To study the anisotropic structure beneath northeast (NE) Japan, we made 4366 shear wave splitting measurements using high-quality seismograms of many earthquakes occurring in the crust and the subducting Pacific slab. Our results provide important new information on the S wave anisotropy in the upper crust, lower crust, mantle wedge, and subducting Pacific slab. In the upper crust, the anisotropy is mainly caused by the stress-aligned fluid-saturated microcracks. The measured delay times (DTs) increase to 0.10 s at 10–11 km depth; the fast velocity directions (FVDs) are parallel to either the tectonic stress or the strike of active faults. The maximum DTs for the low-frequency earthquakes near the Moho are 0.15–0.17 s, suggesting strong anisotropy at the base of the crust or in the uppermost mantle. The measurements for the intermediate-depth earthquakes in the Pacific slab show dominant E-W (trench-normal) FVDs in the back-arc area and N-S (trench-parallel) FVDs in the fore-arc area. The trench-normal FVDs in the back-arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the Pacific plate. The maximum DTs for the slab earthquakes reach 0.30–0.32 s at 100 km depth, but only half of the total DTs are produced in the mantle wedge. The small DTs in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge. In the fore arc, the dominant trench-parallel FVDs for the slab earthquakes are consistent with those for the upper crust earthquakes, and ∼80% of the total DTs can be accounted for by the anisotropy in the crust. In the subducting Pacific slab, the trench-parallel FVDs may reflect either the original fossil anisotropy in the Pacific plate when the plate was produced in the mid-ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab.

Deep structure and origin of active volcanoes in China

We synthesize significant recent results on the deep structure and origin of the active volcanoes in mainland China. Magmatism in the western Pacific arc and back-arc areas is caused by dehydration of the subducting slab and by corner flow in the mantle wedge, whereas the intraplate magmatism in China has different origins. The active volcanoes in Northeast China (such as the Changbai and Wudalianchi) are caused by hot upwelling in the big mantle wedge (BMW) above the stagnant slab in the mantle transition zone and deep slab dehydration as well. The Tengchong volcano in Southwest China is caused by a similar process in the BMW above the subducting Burma microplate (or Indian plate). The Hainan volcano in southernmost China is a hotspot fed by a lower-mantle plume which may be associated with the Pacific and Philippine Sea slabs’ deep subduction in the east and the Indian slab’s deep subduction in the west down to the lower mantle. The stagnant slab finally collapses down to the bottom of the mantle, which can trigger the upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and may cause the slab–plume interactions.

Complex basin development in a wrench-dominated back-arc area: Tectonic evolution of the Crati Basin, Calabria, Italy

Abstract Field data and seismic reflection profiles of various resolutions, calibrated by deep well logs, have been used to unravel the tectonic evolution of the Crati Basin (southern Italy). The study area is located in the northern portion of the Calabrian Arc, a well-developed arc-shaped feature of the circum-Mediterranean belts, consisting of a series of ophiolite-bearing tectonic units and overlying basement nappes. NW–SE oriented left-lateral strike-slip faults exerted a major control on the tectonic evolution of northern-central Calabria, from Middle Miocene to Lower Pleistocene times. Such faults, arranged in an en-echelon geometry and dissecting the pre-existing Late Oligocene–Early Miocene orogenic belt, led to a structural setting including major N-S striking synforms – as the offshore Paola Basin and the Crati Basin are interpreted based on our results – separated by a broad antiformal ridge. Since the Middle Pleistocene, both E- and W-dipping normal faults developed in the southernmost sector of the Crati Basin, probably as a consequence of both uplift of the orogenic edifice and Tyrrhenian back-arc extension. The pre-existing regional strike-slip faults became inactive in this sector of the belt. However, working as persistent barriers, it is envisaged here that they inhibited the southern propagation of the newly formed normal faults, which therefore propagated towards the north. A minimum value of cumulative displacement of ca. 600 m has been unraveled for the central sector of the Crati Basin since Middle Pleistocene times. This yields a vertical strain rate of ca. 0.9 mm/y during the last 700 ka.