Oblique Subduction Research Articles

Geochemistry, petrogenesis and tectonic setting of the Samothraki mafic suite, NE Greece: Trace-element, isotopic and zircon age constraints

The Samothraki mafic suite in the north-eastern Aegean Sea, Greece, is an ‘in situ’ magmatic complex comprising gabbros, sparse dykes and basalt flows and pillows cut by late dolerite dykes. We have determined the age of the complex by SHRIMP zircon geochronology of a gabbro as 159.9 ± 4.5 Ma (i.e. Oxfordian; early Late Jurassic), which precludes any correlation with the so-called Lesvos ophiolite further south (253.1 ± 5.6 Ma; Latest Permian). Six distinct, hitherto unrecognised, geochemical groups have been identified among the basalts and dolerites of the Samothraki mafic suite on the basis of trace-element and Nd–Sr isotopic characteristics. All groups show the presence of an enriched component in their source region, be that a long-term enriched mantle reservoir or an enriched melt. They also invariably show evidence of the addition of subduction-related hydrous fluids. Some groups testify to recent depletion events of long-term enriched mantle, some to the involvement of strongly depleted, recently refertilised mantle, and others to significant addition of melts either from subducted sediments or continental crust. None of the groups bear resemblance to mid-ocean ridge or back-arc basin basalts thus suggesting that the Samothraki mafic suite cannot represent mature back-arc basin crust. In view of recent advances in back-arc basin geology and the data collected here, the Samothraki mafic suite could represent a slab-edge segment of an evolved back-arc basin near the trench, a situation that satisfies the inflow of fertile mantle, the strong signature of sediment melts and the rare occurrence of boninite-like melts. Alternatively, it could represent the embryonic stages of back-arc basin formation either by a propagating rift tip into continental crust or by oblique subduction causing back-arc rifting of continental crust and creation of nascent transtensional basins. Restoration of block configuration in NE Greece before extensional collapse of the Hellenic hinterland and exhumation of the Rhodope Metamorphic Core Complex (mid-Eocene to mid-Miocene) results in a continuous ophiolite belt from Guevgueli to Samothraki, thus assigning the latter to the Innermost Hellenic Ophiolite Belt. We propose that the Samothraki mafic suite originated via rift propagation of the Sithonia ophiolite spreading ridge into the Chortiatis calc-alkaline arc (and its basement) which was partly assimilated. Our model does not favour southward intra-oceanic subduction of the Meliata–Maliac Ocean under the Vardar Ocean whereby Samothraki is located in the back-arc spreading centre. The autochthonous rift-related origin, the enriched geochemical characteristics and the lack of common features with known ophiolite types suggests calling the igneous complex of Samothraki Island a “mafic suite” rather than an “ophiolite”.

Quaternary Vertical Displacement along the Liquiñe-Ofqui Fault Zone: Differential Uplift and Coeval Volcanism in the Southern Andes?

The Liquiñe-Ofqui fault zone (LOFZ) is a long-lived, margin-parallel, intra-arc megastructure, which has played a significant role in both orogenic growth and magmatism during the Late Cenozoic. The LOFZ has been regarded as a Late Miocene-Pliocene dextral strike-slip fault zone within a bulk transpressional setting. Nevertheless, little geologic and morphological evidence exists for Quaternary right-lateral offsets. In contrast, vertical separation is the most outstanding feature observable along the master faults. The river network also shows local discontinuities (sinuosity, stream power index) where cut by these first-order faults. In humid climates, topographic equilibrium should be rapidly restored; therefore, at least part of the vertical displacement should be Holocene. Vertical movements can be caused by tectonic forces but are also enhanced by the postglacial isostatic rebound. The latter would have been intense during the Quaternary and especially after the Last Glacial Maximum (i.e., < ca. 14 ka) in the Southern Andes between 38° and 46°S. A consistent Quaternary NE-oriented maximum horizontal stress accompanies both compressional and transpressional tensors along the entire volcanic arc, which supports the argument that part of the vertical displacement could be tectonic in origin. Therefore, competing transpressional and compressional regimes, with both strike-slip and vertical displacement, occur along the entire intra-arc zone, and are spatially and causally related to a wide variety of volcanic outspurs. Compression and transpression can alternate along the intra-arc fault systems (by shifting σ2 and σ3; σ1 remaining constant) because of local or regional factors such as inherited discontinuities and/or variations of the angle between the convergence vector and the continental margin trend. The shifting between σ2 and σ3 should be common in highly oblique subduction zones, which develop "pure shear-dominated" strain regimes. Whatever the driving mechanism of vertical displacements, the most primitive Holocene basaltic magmas were erupted along the LOFZ master faults, where the vertical offsets are more clearly distinguished. At the regional scale, arc volcanism would be sustained by flux melting in the asthenosphere and magma ascent controlled by the bulk transpressional regime. Instead, sporadic vertical adjustments along the LOFZ, either by tectonic and/or climatic processes, would trigger monogenetic volcanism after induced decompressional melting. Thus, volcanic and orogenic processes are coupled and should be considered for understanding the global arc architecture and coeval relief building.

Mesoarchean assembly and stabilization of the eastern Kaapvaal craton: A structural‐thermochronological perspective

Understanding the construction and stabilization of Archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early Earth, as well as for the subsequent evolution of continents. We provide new constraints for the amalgamation and stabilization of the eastern Kaapvaal craton in the vicinity of the Barberton greenstone belt circa 3.3–3.1 Ga. Isotope dilution thermal ionization mass spectrometry U‐Pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high‐grade orthogneiss terranes from low‐grade supracrustal rocks and to relate this movement to the timing of differential exhumation. New and existing data are consistent with accretion at ∼3.23 Ga within an oblique subduction plate boundary manifested in the BGB as an asymmetric flower‐like structural geometry. For &gt;100 Ma following terrane assembly, transform boundary tectonics, manifested partially as transtension on reactivated faults, led to large‐scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths. Stabilization of this portion of Archean lithosphere involved at least a three‐stage process: (1) creation of a thick and rigid mantle lithosphere during crustal growth, subduction, and terrane assembly from circa 3.30 to 3.23 Ga, (2) generation of a rigid crust during strike‐slip and transtensional tectonics coupled with migration of granites and heat‐producing elements into the upper crust between circa 3.2 and 3.1 Ga, and (3) erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2.9 Ga.

Structural control of the emplacement of the Portrerillos porphyry copper, central Andes of Chile

The Potrerillos porphyry is located in the Late Eocene–Oligocene metallogenic belt of porphyry copper deposits of Chile, Cordillera de Domeyko, central Andes. It is formed of a porphyritic biotite–hornblende monzonite that was emplaced at less than 1000 m depth. The intrusion has a cylindrical vertical form at depth, whereas its uppermost section resembles a mushroom whose stem is inclined 20–25° to the northwest. The location of the Potrerillos porphyry coincides with a major facies and east–west thickness change in Jurassic to Late Cretaceous marine sedimentary and Tertiary volcanic host rocks. Structurally, the porphyry was emplaced at the intersection between the northeast-oriented Potrerillos fold-and-thrust belt, and a system of northwest-trending strike-slip faults of the Ciénaga fault system. Both structural elements are related to a sinistral regional transpression that was active during the Middle to Late Eocene. The north–northeast trending Sierra Castillo-Agua Amarga fault system was trench-linked with an oblique subduction along the Peru–Chile trench in the southwestern part of South America at this time. One of the main structural elements of the Potrerillos fold-and-thrust belt is the eastward verging Potrerillos Mine reverse fault. We demonstrate a syntectonic relationship between the Potrerillos porphyry emplacement and the onset of the Potrerillos Mine fault. The control at the shallowest levels of the Potrerillos porphyry emplacement was studied by means of a structural analysis of folds, cleavage and deformed oolites in the country rock. This analysis suggests that the porphyry occupied the core of an anticline. The form of this anticline was modeled as a fault-propagation-fold taking into account a 200 m displacement along the basal fault and a local shortening of 45.5% occurred. This model is compatible with a limited displacement along the Potrerillos Mine fault under hot, ductile conditions. The remaining displacement of 1600 m would have taken place afterwards under cold, brittle conditions. The Potrerillos porphyry was emplaced during a transpressional regime within the Potrerillos fold-and-thrust belt. The fact that the cleavage does not show a concentric distribution around the Potrerillos porphyry, discounts the possibility of a forced emplacement of the porphyry, pushing aside its wall rocks. This is reinforced by the deformed oolites which do not show a strain gradient from outside towards the edge of the body. The clockwise transection of the fold axes by the cleavage and the southeast direction of displacement along the Potrerillos Mine fault are interpreted as related to the regional sinistral transpression related to the Sierra Castillo Fault movement.

Stages of Late Palaeozoic deformation and intrusive activity in the western part of the North Patagonian Massif (southern Argentina) and their geotectonic implications

AbstractAnalyses of structures in the western part of the North Patagonian Massif (southern Argentina) suggest a polyphase evolution, accompanied by continuous intrusive activity. The first two deformations (D1, D2) and metamorphism affected the upper Palaeozoic, partly possibly older Cushamen Formation clastic succession and different intrusive rocks. A second group of intrusions, emplaced after the second deformational episode (D2), in many places contain angular xenoliths of the foliated country rocks, indicating high intrusive levels with brittle fracturing of the crust. Deformation of these magmatic rocks presumably began during (the final stage of) cooling and continued under solid-state conditions. It probably coincided with the third deformational event (D3) in the country rocks. Based on published U–Pb zircon ages of deformed granitoids, the D2-deformation and younger event along with the regional metamorphism are likely to be Permian in age. An onset of the deformational and magmatic history during Carboniferous times, however, cannot be excluded. The estimated ~W–E to NE–SW compression during the D2-deformation, also affecting the first group of intrusive rocks, can be related to subduction beneath the western Patagonia margin or an advanced stage of collisional tectonics within extra-Andean Patagonia. The younger ~N–S to NE–SW compression might have been an effect of oblique subduction in the west and/or continuing collision-related deformation. As a cause for its deviating orientation, younger block rotations during strike-slip faulting cannot be excluded. The previous D2-event presumably also had an effect on compression at the northern Patagonia margin that was interpreted as result of Patagonia's late Palaeozoic collision with the southwestern Gondwana margin. With the recently proposed Carboniferous subduction and collision south of the North Patagonian Massif, the entire scenario might suggest that Patagonia consists of two different pieces that were amalgamated with southwestern Gondwana during Late Palaeozoic times.

The ca. 350 Ma Beja Igneous Complex: A record of transcurrent slab break-off in the Southern Iberia Variscan Belt?

We report the results of an isotopic study of the large gabbro-dioritic Beja Igneous Complex (BIC) in the boundary between the highly contrasting Ossa–Morena (OM) and South Portuguese (SP) Zones of the Southern Iberian Variscan orogen. This boundary is interpreted as a major suture zone materialized by discontinuous, scattered strips of mafic/ultramafic rocks (the so-called Beja–Acebuches ophiolite complex, BAOC), and by mélange deposits of Middle to Late Devonian age in the Pulo do Lobo accretionary prism (PLAP). The Beja gabbro was interpreted either as part of the ophiolite-like units, or as a broadly arc-related massif reflecting the northward subduction of oceanic lithosphere. U–Pb zircon (ID-TIMS) dating of two diorites and a granodiorite points to igneous emplacement ages of 350 ± 2 Ma (Serpa), 352 ± 2 Ma (Torrão), and 353 ± 4 Ma (São Pedro), respectively, whereas a felsic dyke yields a slightly younger age of 345 ± 2 Ma. These results show that published Ar/Ar dates do not represent igneous crystallization ages, but merely reflect regional cooling below ca. 500 °C, at least 10 Ma after the major intrusive event, probably as a result of uplift of the OMZ side of the suture zone relative to the subsiding SPZ. 87Sr/ 86Sr 350 and εNd 350 display a large range of values (from 0.7041 to 0.7093 and from + 4.0 to − 6.1, respectively) which documents a rather complex petrogenetic history, with an important role played by crustal contamination processes. The more primitive Sr and Nd isotope signatures are measured in the mafic cumulates, while radiogenic Sr and unradiogenic Nd isotope compositions occur in the more evolved rock-types. The broad trend of decreasing εNd 350 with decreasing Sm/Nd and increasing SiO 2 concentration is reminiscent of crustal assimilation combined with fractional assimilation (AFC). εNd values of flasergabbros and associated cumulates ascribed to the ophiolite-like unit in the Guadiana valley are close to zero or even slightly negative, irrespective of the age chosen for the correction for in situ radioactive decay of 147Sm in the range 350–500 Ma, assumed to encompass the true geological age. This observation conflicts with a truly oceanic derivation of these rocks, which could rather represent early stage, deformed and metamorphosed equivalents of the Beja gabbro. Most metabasalts of the same area have positive, albeit variable εNd, consistent with mildly depleted mantle sources and/or significant crustal assimilation. Only a few samples reach highly positive values ( εNd 350–500 ca. + 8) similar to those typical for N-MORBs, as do the Acebuches amphibolites near Aracena ( εNd 350–500 ca. + 9). These data suggest that the BAOC is not only disrupted but also composite, since the gabbroic rocks are not cogenetic with the basalts. Based on the Late Devonian age of the collision between the OMZ and SPZ, the emplacement of the Beja gabbro cannot be related to an active subduction of oceanic lithosphere. Instead, the generation in the mantle, and emplacement in the overlying continental crust of copious volumes of mafic magmas with concomitant LP-HT metamorphism is tentatively ascribed to a process of transcurrent slab break-off that followed oblique subduction and collision.

A large latitudinal displacement of a part of Cretaceous forearc basin in Hokkaido, Japan: paleomagnetism of the Yezo Supergroup in the Urakawa area

Paleo-and rock magnetic experiments were carried out on the Cretaceous Yezo Supergroup in the Urakawa area (southern central Hokkaido) in order to detect tectonic episodes of the ancient eastern Eurasian margin. Characteristic remanent magnetization determined for five sites resides in magnetite, and has primary origin as indicated by positive result of tilting test. Bedding-parallel fabric of anisotropy of magnetic susceptibility and anisotropic acquisition of isothermal remanent magnetization (IRM) indicate significant inclination shallowing after deposition and lock-in of remanence. Shallowing ranges from 4° to 11° for the five sites on the basis of IRM anisotropy. The flattening-corrected mean inclination is still anomalously shallow (31°). Considering the correlation between the obtained inclination data and the geomagnetic polarity Chron C33r−C32r, a block containing the Urakawa area was located at a low latitude (16.7°N) during Campanian and then has experienced at largest 3400 km northward transportation. Transcurrent fault motions along the continental margin may be responsible for the transportation, as inferred from extremely large rotational motion (−161°). The Cretaceous Yezo forearc basin contains an exotic landmass that had evolved along a remote continental margin, then been transported as a result of oblique subduction.

Implementation of Routine Regional Moment Tensor Analysis in New Zealand

The active tectonics of New Zealand are dominated by three main features (figure 1). Beneath the North Island there is active subduction of the Hikurangi trough where the Pacific plate is undergoing oblique subduction beneath the Australian plate at ∼ 45 mm/yr ( e.g. , Demets et al. 1990; Demets et al. 1994). The Hikurangi trough extends from the Kermadec trench in the north and terminates in the upper South Island. Large subduction zone earthquakes of around Mw 8.0 in the lower North Island and around Mw 6.9 farther north are expected (Reyners 1998). Slow slip events have also been observed on the Hikurangi subduction interface ( e.g. , Wallace and Beavan 2006) similar to those documented at other Pacific Rim subduction margins ( e.g. , Dragert et al. 2001; Obara et al. 2004; Larson et al. 2004). A number of dextral strike-slip faults are also present throughout the North Island and the upper South Island and have produced a number of M 6.0–7.5 earthquakes in the past century (Doser and Webb 2003). The Wellington region is cut by five active dextral faults that have average recurrence intervals of meter-scale surface rupture that range from ∼ 500 to 5,000 years (Van Dissen and Berryman 1996). The Wairarapa fault is the only one of these to have ruptured in historic times, in 1855, in an event estimated at Mw 8.2 (Darby and Beanland 1992). ▴ Figure 1. Overview of the tectonic setting of New Zealand, which is dominated by the interaction between the Pacific plate and the Australian plate. In the North Island the Pacific plate is subducting beneath the Australian plate at the Hikurangi trough and Kermadec trench. The Alpine fault delineates a dextral strike-slip fault through most of the South Island, and south of the South Island the …

Simple relationship between seismic activity along Philippine Sea slab and geometry of oceanic Moho beneath southwest Japan

SUMMARY Many earthquakes with various types of focal mechanisms occur along the subducting Philippine Sea slab (PHS) beneath southwest Japan, and the hypocentre distribution is quite inhomogeneous. In this region, distinct non-volcanic deep low-frequency tremors are often observed. To clarify the cause of these apparently complicated and peculiar seismic activities, we first construct a detailed configuration of the oceanic Moho (slab Moho) within the PHS based on a receiver function (RF) analysis. To convert the time-domain RFs into their depth-domain equivalents, we adopt a fine-scaled velocity model based on the traveltime tomography. The depth contours of the oceanic Moho exhibit complexities, but the active region of the intraslab earthquakes corresponds well to the oceanic Moho geometry. In the western part of Shikoku, the intraslab seismicity is very high and the focal mechanisms mainly exhibit normal faults with east‐west extension axes. Since the PHS is subducting to the west and the dip angle of the slab becomes steep in the western part of this region, the stress field in this region can be attributed to the slab pull induced by the westwards slab subduction. Similar types of focal mechanisms are observed in the Tokai district with a focal depth greater than 35 km. Since the oceanic Moho exhibits a ridge shape in this region, its stress field can be attributed to the slab pull and the effect of slab bending. Shallow earthquakes in the Tokai and eastern Shikoku regions have strike-slip focal mechanisms and their extension axes are parallel to the strike of the oceanic Moho. These focal mechanisms may be influenced by both the oblique subduction of the slab and low slab dip angle. Earthquakes beneath the Kii Peninsula exhibit strike-slip or downdip compression focal mechanisms with east‐west compression axes. Since the Moho beneath this peninsula shows a valley shape, compressional stress occurs around the Moho. Moreover, the slab convergence compresses this valley. This also induces the east‐west compression field in this region. We have newly confirmed the existence of an aseismic slab beneath the northern part of the Kinki district, central part of Japan. In this aseismic region, the dip direction of the slab is parallel to that of the slab convergence direction. This tendency is observed in the central part of Shikoku, where the slab seismicity is relatively low. These features imply that the intraslab seismicity is constrained by the interaction between the slab geometry and slab motion. We also revealed the possibility that the oceanic Moho beneath not only Ise Bay but also the Kii Channel has a ridge shape. In both these regions, the non-volcanic low-frequency tremor activity becomes low. More than 80 per cent of the tremor epicentres are distributed within contour depths of 38 ± 3 km of the oceanic Moho. We find that the interaction between the Moho geometry and the plate motion of the subducting PHS plays an important role in determining the local stress field and the location of both seismicity and low-frequency tremors.

Neogene subsidence and stratigraphy of the Simeulue forearc basin, Northwest Sumatra

Interpretation of new multi-channel seismic data reveals the Neogene subsidence history of the Simeulue forearc basin, located off northwestern Sumatra. The trench-parallel elongated basin extends over 260 km in NW–SE direction and 100 km in SW–NE direction. Maximum water depth is about 1300 m and the Neogene sedimentary fill shows a thickness of up to 5 s two-way traveltime. We identified three stages of subsidence evolution after the formation of a regional basal Neogene unconformity which are characterized by changing accumulation space, sedimentation rates, source areas and tectonic phases. An Early/Mid Miocene stage is marked by occurrence of subsidence in several half grabens forming along the western border of the basin. During the Late Miocene/Pliocene this small scale subsidence changed to a steadily subsiding trench-parallel trough. The present setup of the forearc region under the influence of strike-slip faults due to oblique subduction is active at least since this time as evidenced by wrench faulting. At the end of this stage subsidence expanded significantly eastwards drowning a large carbonate platform that evolved in the then shallow southern and eastern parts of the basin. Presently, the central part of the Simeulue basin is subject to inversion, probably related to reactivation of Early/Mid Miocene half grabens.

P-wave velocity and gradient images beneath the Okinawa Trough

We determined the high-resolution 3-D P-wave velocity structure under the Okinawa Trough in the region between South Taiwan and North Kyushu (Japan) by inverting 102,511 absolute travel-time data from 5012 local earthquakes and 29,153 relative travel-time residuals calculated from 1873 teleseismic events. For the first time, a combined data set recorded simultaneously by three seismographic networks located in Kyushu, Okinawa Trough, Taiwan and Fujian province of China has been used, which enables us to determine the high-resolution P-wave velocity structure in the crust and upper mantle under the entire arc system. The resulting tomographic and gradient images provide a new insight into the structural heterogeneities related to the opening of the Okinawa Trough and dehydration of the Philippine Sea slab (PHS), and confirm some of the features proposed by previous studies. A high-velocity zone is imaged clearly at depths of 50–300 km along the Ryukyu Arc system from South Kyushu to North Taiwan, showing good agreement with the spatial distribution of seismicity under the subduction zone. The relative residuals from different quadrants also exhibit early relative arrivals at the stations corresponding to the high-Vp regions, and we therefore interpret the high-Vp zone as the subducting PHS slab. Along the volcanic front from South Kyushu to North Taiwan, low-Vp areas are visible, which are mainly caused by the fluids liberated by the extensive dehydration process of the subducting PHS slab. This fluid-related low-velocity zone is located at depths of 20–120 km along the arc with a total length of 1200 km from North Taiwan to South Kyushu. Using the inverted P-wave velocity model we also calculated the gradient images, which are independent of the selection of reference model or color scale, to provide further information and help us to understand and explain the tomoagraphic structures. Three sub-blocks of the Ryukyu Arc system are revealed from the inverted velocity images and the calculated gradient maps. These features of the velocity anomalies together with other parameters determined by the present study suggest that the Okinawa Trough extension, arc magmatism and seismicity are mainly attributed to the dehydration process associated with the PHS slab subduction and the oblique subduction direction of the plate.

Neogene kinematic history of Nazca–Antarctic–Phoenix slab windows beneath Patagonia and the Antarctic Peninsula

The Patagonian slab window is a subsurface tectonic feature resulting from subduction of the Nazca–Antarctic spreading-ridge system (Chile Rise) beneath southern South America. The geometry of the slab window had not been rigorously defined, in part because of the complex nature of the history of ridge subduction in the southeast Pacific region, which includes four interrelated spreading-ridge systems since 20 Ma: first, the Nazca–Phoenix ridge beneath South America, then simultaneous subduction of the Nazca–Antarctic and the northern Phoenix–Antarctic spreading-ridge systems beneath South America, and the southern Phoenix–Antarctic spreading-ridge system beneath Antarctica. Spreading-ridge paleo-geographies and rotation poles for all relevant plate pairs (Nazca, Phoenix, Antarctic, South America) are available from 20 Ma onward, and form the mathematical basis of our kinematic reconstruction of the geometry of the Patagonia and Antarctic slab windows through Neogene time. At approximately 18 Ma, the Nazca–Phoenix–Antarctic oceanic (ridge–ridge–ridge) triple junction enters the South American trench; we recognize this condition as an unstable quadruple junction. Heat flow at this junction and for some distance beneath the forearc would be considerably higher than is generally recognized in cases of ridge subduction. From 16 Ma onward, the geometry of the Patagonia slab window developed from the subduction of the trailing arms of the former oceanic triple junction. The majority of the slab window's areal extent and geometry is controlled by the highly oblique (near-parallel) subduction angle of the Nazca–Antarctic ridge system, and by the high contrast in relative convergence rates between these two plates relative to South America. The very slow convergence rate of the Antarctic slab is manifested by the shallow levels achieved by the slab edge (< 45 km); thus no point on the Antarctic slab is sufficiently deep to generate “normal” mantle-derived arc-type magmas. Upwelling beneath the region may have contributed to uplift and eastward transfer of extension in the Scotia Sea.

The Neogene Veracruz fault: evidences for left-lateral slip along the southern Mexico block

AbstractStructural data combined with analysis of satellite images and seismic profiles show that a major left-lateral strike-slip fault affects the Veracruz basin and post-5 Ma volcanic rocks of the Los Tuxtlas volcanic field (LTVF). The main volcanic alignment of the LTVF is located along this fault. Additional structural data collected in the Trans-Mexican volcanic belt (areas of Xalapa, Teziutlán and Huauchinango) show that the shear zone affects Pliocene Trans-Mexican volcanic rocks. Low seismicity associated to faulted Quaternary markers such as alluvial fans, alluvial terraces and volcanoes argue for active faulting in this area. Plio-Quaternary strike-slip faulting in the Veracruz basin and in the eastern Trans-Mexican volcanic belt is important because it connects two important structural provinces: the left-lateral strike-slip faults province to the south and the left-lateral transtensive faulting that affects the central part of the Trans-Mexican volcanic belt. These three active deformation zones constitute the boundary between the southern Mexico block and the North American plate. It is generally assumed that strike-slip faulting along the Trans-Mexican and Central America volcanic arcs is the result of oblique subduction of the Cocos plate under the North American and Caribbean plates. However slip vectors along the Middle America trench are almost perpendicular to the trench. This Neogene sinistral strike-slip motion could be partially driven by the eastward motion of the Caribbean plate rather than by strain partitioning along the oblique Middle America trench subduction zone.

MESOZOIC BACK-ARC EXTENSION IN THE ACTIVE MARGIN OF THE IRANIAN CONTINENTAL BLOCK: CONSTRAINTS FROM AGE AND GEOCHEMISTRY OF THE MAFIC LAVAS

The Nain-Baft ophiolitic suture, along the active margin of the Central Iranian continental block (the Sanandaj-Sirjan zone) features back-arc extension during the Late Mesozoic. This ophiolitic belt is characterized by occurrence of mafic lavas including pillow lavas, diabasic dikes and layers, massive basaltic lavas and basaltic-andesitic rock fragments in the volcanic breccias. These mafic lavas display both calc-alkaline and island-arc tholeiitic affinities with enrichment in LILE and depletion in HFSE. Conventional K-Ar measurements on amphibole indicate the Middle Cretaceous for the creation and evolution of the Nain-Baft back-arc basins. As a result of oblique subduction of the Tethyan Ocean, a narrow transtensional back-arc basin could start to open along large transcurrent faults in the active margin of the Iranian continental block.

TECTONO-METAMORPHIC HISTORY OF THE DUARTE TERRANE (JARABACOA AREA, HISPANIOLA ISLAND): INSIGHTS ON THE TECTONIC EVOLUTION OF THE NORTHERN RIM OF THE CARIBBEAN OCEANIC PLATEAU

The northern margin of the Caribbean Plate is characterized by a transpressional belt consisting of an assemblage of several terranes derived from Late Cretaceous to Tertiary oblique convergence between the Northern American and Caribbean Plates. Fragments of the oceanic plateau belonging to the Caribbean Plate are incorporated in these belts as strongly deformed and metamorphosed sequences, as recognized in the Jarabacoa area of the Hispaniola Island. In this area the Duarte Terrane consists of two different units showing different metamorphic grades. The lower unit consists of greenschist facies metabasites, whereas the upper one is represented by amphibolite facies metabasites topped by metasedimentary rocks, whose protoliths were clastic deposits supplied by a magmatic arc. Both units suffered four deformation phases, from D1 to D4, that can be strictly correlated. In both units the D1 phase is characterized by a syn-metamorphic foliation associated to a mineral lineation and very rare rootless isoclinal folds. The D2 phase is characterized by a well developed foliation and isoclinal folds acquired under retrograde metamorphic conditions. The D1 and D2 phases developed in the Cenomanian-Turonian time span. The following D3 and D4 phases produced weak deformations without metamorphic imprint. The D3 and D4 phase are probably connected with transpression tectonics of Early Oligocene to Early Miocene age. All these deformations developed in an arc setting connected with southward oblique subduction of the oceanic lithosphere of the North America Plate beneath the Caribbean oceanic plateau.

Effect of three‐dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy

Shear‐wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench‐parallel fast directions. Three‐dimensional flow may give rise to trench‐parallel stretching and provide an explanation for these patterns of seismic anisotropy. Along‐strike variations in slab geometry produce trench‐parallel pressure gradients and are therefore a possible mechanism for three‐dimensional flow. In this study we quantify the effects of variable slab dip, curved slabs, oblique subduction, and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones. Temperature, dynamic pressure, velocity, and strain are calculated with high‐resolution three‐dimensional finite element models. These models include temperature‐ and stress‐dependent rheology and parameterized slab and trench geometry. Thick layers (20–60 km) with strong trench‐parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15° or strong curvature in the slab. In these cases, strong trench‐parallel stretching develops when flow lines have an oblique to trench‐normal orientation. This suggests that trench‐parallel seismically fast directions may not indicate trench‐parallel flow lines in systems with large along‐strike variations. An oblique component of stretching is confined to a 20–30 km layer above the slab in systems with oblique subduction. The effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench‐parallel flow that extends 50–100 km into the core of the mantle wedge.

Tectonic controls on the nature of large silicic calderas in volcanic arcs

Research Article| August 01, 2008 Tectonic controls on the nature of large silicic calderas in volcanic arcs Gwyneth R. Hughes; Gwyneth R. Hughes * 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA *E-mail: gwyneth@stanford.edu. Search for other works by this author on: GSW Google Scholar Gail A. Mahood Gail A. Mahood 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Gwyneth R. Hughes * 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA Gail A. Mahood 1Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA *E-mail: gwyneth@stanford.edu. Publisher: Geological Society of America Received: 20 Jan 2008 Revision Received: 23 Apr 2008 Accepted: 25 Apr 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (8): 627–630. https://doi.org/10.1130/G24796A.1 Article history Received: 20 Jan 2008 Revision Received: 23 Apr 2008 Accepted: 25 Apr 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gwyneth R. Hughes, Gail A. Mahood; Tectonic controls on the nature of large silicic calderas in volcanic arcs. Geology 2008;; 36 (8): 627–630. doi: https://doi.org/10.1130/G24796A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Silicic magma bodies stabilize the continental margin by redistributing low-density material into the upper crust. We examined the tectonic traits of 91 young, arc-related, silicic calderas to test previous assertions that the nature of voluminous silicic arc volcanism depends on specific tectonic characteristics. We find that caldera occurrence positively correlates with convergence rate except in arcs with backarc spreading, which have few or no calderas. Calderas located on oceanic and young continental crust are predominantly dacitic, whereas rhyolitic calderas are mainly located on Mesozoic or older continental crust under extension. Occurrence does not increase with greater local subduction obliquity or duration of present arc activity. Our determination of controls on the formation and composition of silicic calderas in volcanic arcs contributes to the study of continental evolution. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Subduction of a segmented ridge along a curved continental margin: Variations between the western and eastern Sanak–Baranof belt, southern Alaska

Between the western and the eastern parts of the southern Alaskan Sanak–Baranof belt there exist variations in pluton spacing, geochronology, geochemistry and metamorphism. All such characteristics can be explained by increase in heat flux towards the eastern parts of the belt. Despite such differences, the initiation of magmatism propagated eastward at a constant rate of 19 cm/year throughout the entire belt. The average spacing of magmatic centers changes from 165 ± 88 km in the west to 67 ± 41 km in the east. The duration of magmatism as measured by U/Pb zircon and monazite ages increases from 1–2 Ma in the west to 4–5 Ma in the east. The duration of Ar–Ar biotite cooling ages also increases to the east. In agreement with the extended cooling times, large regions of greenschist and amphibolite facies metamorphism are present in the eastern segments, whereas outside of narrow pluton aureoles high-temperature metamorphism is not present in the western Sanak–Baranof belt. Finally, intrusions with adakitic characteristics such as high Sr/Y ratios are present in the east, but not in the west suggesting that some degree of slab melting occurred in the eastern Sanak–Baranof belt. We suggest that the above east–west thermal and magmatic variations can be explained by a decreasing rate of subduction and a persistent slab window in the eastern Sanak–Baranof belt caused by the oblique subduction of a segmented spreading ridge along a curved continental margin, and that this model is compatible with the Resurrection plate hypothesis.

Geophysical characteristics of the Ninetyeast Ridge–Andaman island arc/trench convergent zone

The convergence tectonics of the Ninetyeast Ridge (NER), upon the Andaman island arc–trench system is examined through an analysis of ETOPO2 bathymetry, satellite-derived free air gravity and seismic data. Oblique subduction and the buoyancy forces arising from subduction of the NER render the subduction processes near the Andaman arc highly complex. The bathymetric expression of the NER is visible up to Lat. 10°N but seismic reflection data indicate that it extends up to about Lat. 17°N. The gravity anomalies are strongly positive over the exposed segment of the ridge but are subdued over the buried portion. There is a prominent break in the continuity of the trench gravity low, where the NER seems to impinge upon the island arc. Further, a strong curvilinear belt of negative anomalies just behind and running parallel to the island arc, associated with the forearc basin, is a dominant feature of the gravity map. An offset in the continuity of this strong negative anomaly occurs at about the same latitude where the NER seems to be converging upon the island arc. Seismic reflection data indicate that the NER is very close to the trench. Flexural modeling of the gravity anomalies for the subducting Indian ocean lithosphere, loaded by sediments and the NER, indicates that the NER is at the starting phase of its collision with the island arc and may not have started affecting the subduction process itself. We infer that the en-echelon block structure of the NER in the proximity of the convergent zone is a consequence of complex strike-slip and subduction-related tectonic forces.

Interseismic deformation of the Nankai subduction zone, southwest Japan, inferred from three-dimensional crustal velocity fields

Abstract We have studied crustal deformation in the Nankai subduction zone, southwest Japan, based on three-dimensional GPS velocity fields. Oblique subduction of the Philippine Sea plate has caused two different modes of deformation of the overriding plate: interseismic crustal shortening in the direction of plate convergence, and permanent lateral movement of the forearc. The block boundary dividing the forearc is the Median Tectonic Line (MTL); however, we assumed that its shallower portion is fully or partially locked to a certain depth. The plate boundary and the MTL are represented by many rectangular faults. We carried out inversion analyses with a priori information to estimate simultaneously slip deficit rates at those rectangular faults, together with the rate of lateral movement of the forearc. The results show that the seismogenic subduction faults at a depth of 5–25 km are strongly locked. As for the transition zone at 25–35 km, the inversion analysis results in stronger locking than shown in previous studies, especially when the vertical velocity data are weighted. The rates of lateral forearc movement and slip deficit at the MTL are nearly comparable but in a reverse sense to each other. This shows that the shallower portion of the MTL is strongly locked but that stationary aseismic slip is occurring in the deeper part.