Retroarc Research Articles

Ground-motion prediction equations for interface earthquakes of M7 to M9 based on empirical data from Japan

Ground motion prediction equations (GMPEs) have a major impact on seismic hazard estimates, because they control the predicted amplitudes of ground shaking. The prediction of ground-motion amplitudes due to mega-thrust earthquakes in subduction zones has been hampered by a paucity of empirical ground-motion data for the very large magnitudes (moment magnitude (M) $$>$$ 7) of most interest to hazard analysis. Recent data from Tohoku M9.0 2011 earthquake are important in this regard, as this is the largest well-recorded subduction event, and the only such event with sufficient data to enable a clear separation of the overall source, path and site effects. In this study, we use strong-ground-motion records from the M9 Tohoku event to derive an event-specific GMPE. We then extend this M9 GMPE to represent the shaking from other M $$>$$ 7 interface events in Japan by adjusting the source term. We focus on events in Japan to reduce ambiguity that results when combining data in different regions having different source, path and site effect attributes. Source levels (adjustment factors) for other Japanese events are determined as the average residuals of ground-motions with respect to the Tohoku GMPE, keeping all other coefficients fixed. The mean residuals (source terms) scale most steeply with magnitude at the lower frequencies; this is in accord with expectations based on overall source-scaling concepts. Interpolating source terms over the magnitude range of 7.0–9.0, we produce a GMPE for large interface events of M7–M9, for NEHRP B/C boundary site conditions (time-averaged shear-wave velocity of 760 m/s over the top 30 m) in both fore-arc and back-arc regions of Japan. We show how these equations may be adjusted to account for the deeper soil profiles (for the same value of $$\hbox {V}_\mathrm{S30})$$ in western North America. The proposed GMPE predicts lower motions at very long periods, higher motions at short periods, and similar motions at intermediate periods, relative to the simulation-based GMPE model of Atkinson and Macias (2009) for the Cascadia subduction zone.

Crustal and mantle shear velocity structure of Costa Rica and Nicaragua from ambient noise and teleseismic Rayleigh wave tomography

The Costa Rica–Nicaragua subduction zone shows systematic along strike variation in arc chemistry, geology, tectonics and seismic velocity and attenuation, presenting global extremes within a few hundred kilometres. In this study, we use teleseismic and ambient noise derived surface wave tomography to produce a 3-D shear velocity model of the region. We use the 48 stations of the TUCAN array, and up to 94 events for the teleseismic Rayleigh wave inversion, and 18 months of continuous data for cross correlation to estimate Green's functions from ambient noise. In the shallow crust (0–15 km) we observe low-shear velocities directly beneath the arc volcanoes (<3 km s–1) and higher velocities in the backarc of Nicaragua. The anomalies below the volcanoes are likely caused by heated crust, intruded by magma. We estimate crustal thickness by picking the depth to the 4 km s–1 velocity contour. We infer >40-km-thick crust beneath the Costa Rican arc and the Nicaraguan Highlands, thinned crust (∼20 km) beneath the Nicaraguan Depression, and increasing crustal thickness in the backarc region, consistent with receiver function studies. The region of thinned, seismically slow and likely weakened crust beneath the arc in Nicaragua is not localizing deformation associated with oblique subduction. At mantle depths (55–120 km depth) we observe lower shear velocities (up to 3 per cent) beneath the Nicaraguan arc and backarc than beneath Costa Rica. Our low-shear velocity anomaly beneath Nicaragua is in the same location as a low-shear velocity anomaly and displaced towards the backarc from the high VP/VS anomaly observed in body wave tomography. The lower shear velocity beneath Nicaragua may indicate higher melt content in the mantle perhaps due to higher volatile flux from the slab or higher temperature. Finally, we observe a linear high-velocity region at depths >120 km parallel to the trench, which is consistent with the subducting slab.

Ground-Motion Prediction Equations of Intermediate-Depth Earthquakes in the Hellenic Arc, Southern Aegean Subduction Area

A response-spectra database is compiled of hundreds of seismic records from intermediate-depth earthquakes (earthquakes whose foci are located between 45 to 300 km from the earth's surface) with moment magnitudes of M 4.5-6.7 that occurred in the South Aegean subduction zone. The database consists of high-quality data from both acceleration-sensor and broadband velocity-sensor instruments. The database is much larger than previous databases used in the development of past em- pirical regressions enabling the determination of various parameters of ground-motion attenuation not previously examined. New variables accounting for the highly com- plex propagation of seismic waves in the Greek subduction zone are introduced based on the hypocentral depth and the location of the event, as these factors control the effects of the back-arc low-velocity/low-Q mantle wedge on the seismic-wave propa- gation. The derived results show a strong dependence of the recorded ground motions on both hypocentral depth and distance, which leads to the classification of the dataset into three depth-hypocentral distance categories. Ground motions from in-slab earth- quakes, especially with hypocentral depthsh� > 100 km, are amplified for along-arc stations, an expected effect of channeled waves through the high-velocity slab. The ground motions are also strongly attenuated in the back-arc region, due to the low-Q mantle wedge, which are almost independent of the recording hypocentral distance. In contrast, for shallower in-slab events (60 km <h< 100 km), the corresponding dif- ferentiation of seismic motion for along-arc and back-arc stations is observed beyond a specific critical distance range. Moreover, for longer periods, both along-arc ampli- fication and back-arc anelastic-attenuation factors strongly diminish, suggesting that the longer wavelengths of seismic waves are not affected by the complex geophysical structure, resulting in more similar ground motions for both back-arc and along-arc stations. Finally, results for interface events ( h< 45 km) occurring along the outer Hellenic arc suggest their wave propagation is not affected by the presence of the low-velocity/low-QS mantle wedge, but is mainly controlled by the differences of the anelastic attenuation between the Mediterranean and Aegean lithospheres.

Payenia volcanic province, southern Mendoza, Argentina: OIB mantle upwelling in a backarc environment

The Pleistocene to Holocene Payenia volcanic province is a backarc region of 60,000km2 in Mendoza, Argentina, which is dominated by transitional to alkaline basalts and trachybasalts. We present major and trace element compositions of 139 rocks from this area of which the majority are basaltic rocks with 4 to 12wt.% MgO and 44 to 50wt.% SiO2. The southern Payenia province is dominated by intraplate basalts and the trace element patterns of the Río Colorado and Payún Matrú lavas suggest little or no influence from subducted slab components. The mantle source of these rocks is similar to some EM-1 ocean island basalts. In contrast, the magmas from the northern Payenia province and the Andean retroarc occurrences have received an important input from the subducting slab and their trace element patterns are transitional between intraplate and arc rocks. These magmas are mainly derived from another asthenospheric mantle source which may be similar to normal MORB mantle. The Nevado and Northern Segment basalts have presumably been formed above a shallowly subducting slab and the progression of volcanism from south to north and northwest along the San Rafael block likely marks the downwarping of the slab and the end of the shallow subduction period. The downwarping slab may have generated an enhanced mantle upwelling of both the intraplate and the MORB-like mantle sources.In samples from almost all parts of the Payenia province and in particular many Nevado, Llancanelo and older Payún Matrú basalts, trace element variations suggest a significant contribution from lower crustal melts, possibly up to 70% in the most extreme cases. The contaminating lower crustal rocks must have been depleted mafic rocks with a plagioclase component. The extensive melting of lower crust is probably related to the low thickness of the lithospheric mantle and preheating of the lower crust by earlier Mio-Pliocene volcanism. Rare earth element modelling of mantle melting calls for enriched source compositions and a beginning of melting within the garnet stability field for all Payenia basalts. The Río Colorado and Payún Matrú basalts indicate high solidus pressures around 3–3.1GPa which requires either a thermal or compositional mantle anomaly. The model suggests a thinner lithosphere in the western Payenia region compared to the eastern.

The East Scotia Sea: Mantle to Microbe

The East Scotia Sea is an important geological, oceanographic, and biological gateway between the Pacific and Atlantic realms, containing a particularly good example of back-arc spreading, the East Scotia Ridge. Recent surveys show that the ridge consists of nine segments, separated by nontransform offsets. The five middle segments (E3-E7) have the morphology of slow-intermediate spreading centers and erupt mainly normal mid-ocean ridge basalt lavas. End segments, E2 and E9, are much shallower, have the inflated appearance of faster-spreading ridges, and erupt slightly enriched, plume-influenced rocks with small to moderate subduction components. Spreading rates vary from 62 mm/yr in the north to 70 mm/yr in the south and cannot explain the along-axis variations. Rapid slab retreat is believed to cause flow of Atlantic asthenosphere around the ends of the subducting slab at the South Sandwich Trench, advecting fertile mantle into the back-arc region. Eastward rollback of the slab brings this mantle within 150 km of the volcanic front, where excess melting occurs as a result of fluxing by subduction-related hydrous fluids. This leads to the initiation of magmatically inflated end segments, characterized by thicker crust and elevated topography. Reconnaissance work has detected hydrothermal plumes above the most inflated parts of segments E2 and E9, suggesting a close relationship between mantle inflow, magmatism, and hydrothermal venting. Vent sites at the East Scotia Ridge may be affected by currents from the Weddell Sea and the South Pacific. Hence, the at present unknown vent fauna could be of either Pacific or Atlantic type, or conceivably a combination of both.

Transfer of deformation in back-arc basins with a laterally variable rheology: Constraints from analogue modelling of the Balkanides–Western Black Sea inversion

The balance between extension and contraction in back-arc basins is very sensitive to a number of parameters related to on-going subduction and collision processes. This leads to complex back-arc geometries, where a lateral transition between crustal blocks with contrasting rheologies is often recorded. One good example is the back-arc region of the Balkanides–Pontides orogens, where lateral variations in rheologies are observed between the Balkanides–Moesian block and the Pontides–Western Black Sea Basin. The latter opened during Cretaceous–Eocene, and has been inverted together with the former starting during late Middle Eocene. The inversion generated contrasting geometries along the orogenic strike, with a narrow zone of high deformation in the Balkanides–Moesia region, wide areas of thrusting with low offsets in the Pontides–Western Black Sea Basin and a transitional zone characterized by highly curved geometries. This overall type of inversion is investigated here by the means of analogue modelling testing the role of inherited crustal geometries during inversion. Our modelling suggests that the contrasting architecture of inverted structures observed in the Balkanides–Pontides domain are the result of pre-existing crustal stretching geometries of various blocks inherited from the Cretaceous–Eocene extension. The stretched and weak back-arc basins can transfer contraction deformation at large distances, explaining structures derived by observational studies. The collisional deformation recorded in the Pontides was transmitted at large distances that are in the range of the contraction structures observed in the centre and northern part of the Western Black Sea. In the light of analogue modelling results we argue that the Western Black Sea was a rheologically weaker domain when compared with the adjacent western onshore at the beginning of the inversion, in contrast with previous results derived from numerical modelling studies that argued for a strong West Black Sea domain at the beginning of inversion.

Characterizing the continental basement of the Central Andes: Constraints from Bolivian crustal xenoliths

A critical factor in understanding the development of active continental margins is knowledge of the crustal basement on which magmatic arcs are built. This study reports results from a whole-rock geochemical and zircon U-Pb geochronological study of a suite of crustal xenoliths from the Bolivian Altiplano, Central Andes, that provide new insight into the evolution and composition of the continental basement beneath the region. The xenoliths are hosted in Pliocene–Pleistocene trachyandesitic to dacitic lavas that erupted from monogenetic volcanic centers in the Andean backarc region and comprise both igneous and metamorphic lithologies, including diorites, microgranites, gneisses, garnet–mica schists, granulites, quartzites, and dacites. The xenolith suite exhibits significant Sr-isotopic heterogeneity, with values extending from 0.7105 to 0.7368. Pb isotopic signatures reflect the crustal domains previously constrained from scattered exposures of basement rocks throughout the region. Ion microprobe U-Pb dating of cores and rims from zircon separates from two of the sampled xenoliths reveals predominant early Phanerozoic age peaks (ca. 500 Ma; population 1), late Mesoproterozoic age peaks (1.0–1.2 Ga; population 2), and Paleoproterozoic age peaks (1.7–1.9 Ga; population 3). Populations 1 and 2 are well documented throughout the Andes and correspond to periods of supercontinent formation (e.g., Rodinia at ca. 1.0 Ga) and breakup. Population 3, which is poorly represented in the zircon record of the Andes as a whole, may record geological events during the construction of the Paleoproterozoic Amazonian Craton. The presence of the three age peaks in the detrital zircon population record of a single crustal xenolith demonstrates the important role of crustal recycling in the construction of the modern-day Andean margin. The lithological character of the xenoliths and their detrital zircon ages are inconsistent with current understanding of the eastern extent of the Arequipa-Antofalla basement block beneath the Bolivian Altiplano and instead indicate that it terminates further to the west than previously assumed.

MAGMATISMO E TECTÔNICA DO ORÓGENO ARAÇUAÍ NO EXTREMO LESTE DE MINAS E NORTE DO ESPIRITO SANTO (18°-19°S, 41°-40°30’W)1

This paper focuses on magmatic episodes and their relations to the main regional deformation in thebackarc region of the Araçuaí Orogen (southeastern Brazil). The main tectonic event took placeduring the syncollisional stage (ca. 585-560 Ma) and imprinted the regional, solid-state foliation (Sn)in most rocks of this orogen. In the mapped region (18°-19°S, 41°-40°30’W), the older unit (NovaVenécia Complex) occurs in small areas and consists of migmatitic, sillimanite-garnet-cordieritebiotiteparagneisses with minor calcsilicate intercalations. The widespread garnet-biotite granites(Carlos Chagas, Montanha and Ataléia suites) are correlated to the syncollisional, S-type, G2 supersuite(ca. 585-560 Ma) of the Araçuaí Orogen, because they persistently record Sn, usually associated tostriking mylonitic features. However, we also correlate some massifs of undeformed garnet-biotitegranite to the Carlos Chagas Suite. Outwards from such undeformed pods, this Sn-free, garnet-biotitegranite progressively shows solid-state deformation features superimposed on its igneous fabric, andgives place to the mylonitized granite typical of the Carlos Chagas Suite. In fact, both the undeformedand mylonitized granites of the Carlos Chagas Suite yielded similar magmatic crystallization ages (ca.585-575 Ma; zircon, U-Pb SHRIMP). These G2 suites often show restites and xenoliths of paragneissand calcsilicate rock, and gradational contacts with migmatitic inliers, suggesting a genesis related toanatexis of the Nova Venécia Complex. The usually small, irregular-shaped bodies and veins composedof garnet-cordierite leucogranites, free of Sn, belong to the post-collisional, S-type, G3 suite (ca.540-530 Ma, in the focused region). Most G3 bodies are enveloped by and contain restites of foliatedG2 granites, representing autochthonous to parautochthonous partial melts from the Carlos Chagas,Montanha and Ataléia suites. Sn-free granites and pegmatites, representing post-Sn partial meltsfrom the Nova Venécia paragneisses, also can be correlated to the G3 suite. Very large to relativelysmall intrusions composed of biotite granite, charnockite and/or norite, free of Sn, represent the lastmagmatic episode of the Araçuaí Orogen, in the focused region. These intrusions (Aimorés Suite)belong to the post-collisional, I-type, G5 supersuite (ca. 520-490 Ma). Such a succession of magmaticepisodes suggests that this middle crust sector of the Araçuaí Orogen remained hot (T &gt; 650 °C) fora long time interval (ca. 100 Ma), from ca. 585 Ma to the Cambrian-Ordovician boundary. Thisrequires a long-lasting release of orogenic heat that could be explained by a combination of threesources: i) heat input by convective asthenosphe under the backarc region from the precollisional tothe syncollisional stages; ii) release of radiogenic heat from the crustal pile thickened by thrusts; andiii) ascent of mantle magmas and partial melting of the deep crust during the extensional collapse ofthe Araçuaí Orogen.

Geothermal structure of the eastern Black Sea basin and the eastern Pontides orogenic belt: Implications for subduction polarity of Tethys oceanic lithosphere

The numerical results of thermal modeling studies indicate that the lithosphere is cold and strong beneath the Black Sea basin. The thermal lithospheric thickness increases southward from the eastern Pontides orogenic belt (49.4 km) to Black Sea basin (152.2 km). The Moho temperature increases from 367 °C in the trench to 978 °C in the arc region. The heat flow values for the Moho surface change between 16.4 mW m−2 in the Black Sea basin and 56.9 mW m−2 in the eastern Pontides orogenic belt. Along the southern Black Sea coast, the trench region has a relatively low geothermal potential with respect to the arc and back-arc region. The numerical studies support the existence of southward subduction beneath the Pontides during the late Mesozoic–Cenozoic.

Surface wave tomography and azimuthal anisotropy of the Philippine Sea Plate

Long period and broadband waveform data from 1984 to 2007 extracted mainly from the IRIS Data Center with additional data from 1997 to 2007 extracted from the Ocean Hemisphere Project is used to perform a detailed mapping of Rayleigh wave group velocity and anisotropic structures of the Philippine Sea Plate (PSP). Our results show prominent high Rayleigh wave group velocity anomalies and heterogeneity in three marginal basins of the PSP at period up to 30s. The convergent boundaries around the PSP are associated with low-velocity anomalies. Heterogeneity of velocity distribution at deep depths along the Ryukyu trench is also observed. Our results indicate that the azimuthal anisotropy in the PSP is depth dependent. The fast direction in the West Philippine Basin and the Shikoku Basin is in NE–SW at shorter periods, which is correlated with fossilized fabric produced by final phase of deformation, whereas the fast direction in the Parece Vela Basin is complex at shorter periods and becomes uniformly NE–SW-oriented at longer periods. At the depths near the lithosphere–asthenosphere boundary, the fast directions become parallel to the absolute plate motion for various parts of the PSP. The fast directions along subduction zones around the PSP are predominately trench-normal. The relatively simple patterns of azimuthal anisotropy may imply that the observed anisotropy originates from the A-type LPO of olivine fabric at shallow depths in the back-arc region and with slab-entrained mantle flow at deep depths.

Strain anomalies induced by the 2011 Tohoku Earthquake (M w 9.0) as observed by a dense GPS network in northeastern Japan

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

Timing of formation and geological setting of low-sulphidation epithermal gold deposits in the continental margin of NE China

The margin of NE China, a part of the West Pacific metallogenic belt, contains innumerable low-sulphidation mineral deposits. Gold deposits in this region can be classified into three distinct types based on geology and ore mineral paragenesis: (1) low-sulphidation epithermal silver–gold deposits, (2) low-sulphidation tellurium–gold deposits, and (3) low-sulphidation epithermal tellurium–gold deposits. Ores formed during the late Early Cretaceous and the early Late Cretaceous reflect three distinct metallogenic periods: the Fuxin Stage at 115.98 ± 0.89 Ma, the Quantou Stage at 107.2 ± 0.6 Ma or <103 Ma, and the Qingshankou or Yaojiajie Stage at < 97 Ma and 88.2 ± 1.4 Ma. The Fuxin Stage is dominated by trachyandesitic magmatism, with magmas emplaced at hypabyssal depths. In comparison, the Quantou Stage is characterized by high-K calc-alkaline, calc-alkaline, and sodic andesitic, dacitic, and rhyolitic magmatism of three different suites. The first of these is a high-K calc-alkaline andesitic magmatic suite that was accompanied by the emplacement of a calc-alkaline sodic dacite during the formation of the Ciweigou and Wufeng ore deposits. The second suite is dominated by calc-alkaline sodic rhyolite and high-K calc-alkaline sodic dacite magmatism associated with the formation of the Sipingshan ore deposit. The third suite is typified by high-K calc-alkaline andesitic magmatism associated with the emplacement of calc-alkaline hypabyssal granitoid complexes accompanying the formation of the Dong'an and Tuanjiegou ore deposits. The Qingshankou or Yaojia Stage is characterized by calc-alkaline sodic dacite magmatism associated with the formation of the Wuxing ore deposit. Metallogenesis during the Fuxin Stage characterized by trachytic magmatism is closely related to the formation of a deep-seated fault within a magmatic arc or the back-arc region of an immature continental margin and is associated with the Early Cretaceous subduction of the Pacific plate beneath Eurasia. Ore deposits that formed during the Fuxin Stage were generally related to magmato-hydrothermal fluids associated with mantle-derived magmas. In contrast, metallogenesis during the Quantou and Qingshankou or Yaojiajie stages was closely related to the formation of a mature high-K calc-alkaline magmatic arc within a continental margin setting again associated with the westward subduction of the Pacific plate. This metallogenic event was a product of magmato-hydrothermal systems derived from crust–mantle interaction and mixing of magmas derived from partial melting of different sections of the continental crust.

Peralkaline- and Calc-Alkaline-Hosted Volcanogenic Massive Sulfide Deposits of the Bonnifield District, East-Central Alaska

Volcanogenic massive sulfide (VMS) Zn-Pb-Cu-Ag-Au deposits of the Bonnifield mining district formed during Late Devonian-Early Mississippian magmatism along the western edge of Laurentia. The largest deposits, Dry Creek and WTF, have a combined resource of 5.7 million tonnes at 10% Zn, 4% Pb, 0.3% Cu, 300 grams per tonne (g/t) Ag, and 1.6 g/t Au. These polymetallic deposits are hosted in high field strength element (HFSE)- and rare-earth element (REE)-rich peralkaline (pantelleritic) metarhyolite, and interlayered pyritic argillite and mudstone of the Mystic Creek Member of the Totatlanika Schist Formation. Mystic Creek metarhyolite and alkali basalt (Chute Creek Member) constitute a bimodal pair that formed in an extensional environment. A synvolcanic peralkaline quartz porphyry containing veins of fluorite, sphalerite, pyrite, and quartz intrudes the central footwall at Dry Creek. The Anderson Mountain deposit, located ~32 km to the southwest, occurs within calc-alkaline felsic to intermediate-composition metavolcanic rocks and associated graphitic argillite of the Wood River assemblage. Felsic metavolcanic rocks there have only slightly elevated HFSEs and REEs. The association of abundant graphitic and siliceous argillite with the felsic volcanic rocks together with low Cu contents in the Bonnifield deposits suggests classification as a siliciclastic-felsic type of VMS deposit. Bonnifield massive sulfides and host rocks were metamorphosed and deformed under greenschist-facies conditions in the Mesozoic. Primary depositional textures, generally uncommon, consist of framboids, framboidal aggregates, and spongy masses of pyrite. Sphalerite, the predominant base metal sulfide, encloses early pyrite framboids. Galena and chalcopyrite accompanied early pyrite formation but primarily formed late in the paragenetic sequence. Silver-rich tetrahedrite is a minor late phase at the Dry Creek deposit. Gold and Ag are present in low to moderate amounts in pyrite from all of the deposits; electrum inclusions occur in Dry Creek sphalerite. Contents and ratios of trace elements in graphitic argillite that serve as proxies for the redox state of the bottom waters in the basin indicate that Dry Creek mineralization took place in suboxic to periodically anoxic bottom waters. Trace element data show higher contents of Tl-Mn-As in pyrite from the Anderson Mountain deposit compared to the Dry Creek or WTF deposits and thus suggest that Anderson Mountain may have formed at lower temperatures or under slightly more oxidizing conditions. No exact modern analogue for the tectonic setting of the Bonnifield VMS deposits is known, although the back-arc regions of the Okinawa Trough and Woodlark Basin satisfy the requirement for a submarine, extensional setting adjacent to a continental margin. Limited occurrences of peralkaline volcanic rocks occur in these two potential analogues, but the peralkalinity of those rocks is much less than that of the Mystic Creek Member metarhyolites in the Bonnifield district. The highly elevated trace element (e.g., Zr, Nb) contents of Mystic Creek metarhyolites suggest that a better analogue may be a submarine rifted continental margin. The calc-alkaline composition of the host rocks to the Anderson Mountain deposit suggests that mineralization there formed in a continental margin arc, outboard of the extended continental margin setting of the peralkaline-hosted Dry Creek and WTF deposits.

Geochemistry and Tectonic Significance of Mafic Volcanic Rocks of the Hindoli Belt, Southeastern Rajasthan: Implications for Continent Assembly

Abstract Mafic volcanic rocks that occur within the sedimentary pile of the Hindoli Group were analyzed for major and trace elements (including REE) to establish tectonic setting of volcanism during the early Proterozoic history of the North Indian Craton. The mafic volcanics are sub-alkaline showing compositional variation from picrobasalt to basalt. They are LREE enriched with (La/Yb)N ratio ranging from 4.67-6.19 (avg.5.27) and exhibit slightly concave REE patterns relative to chondrite. The multi-element patterns of these mafic volcanic rocks display relative enrichment in Th and LREE and negative anomalies of Nb and P. These geochemical characteristics are consistent with a subduction related origin. Various variation diagrams, involving immobile trace elements, distinguish the Hindoli lavas as arc basalt. However, their Ti and Nb contents are higher than those of subduction related magmas. Probably the wedge melting, along with mixing of rising asthenosphere might have produced these characteristics. It is suggested that the Hindoli basin originated by rifting of island-arc lithosphere, caused by rising plume in an extensional back arc region. Based on the results of the present geochemical study, it is proposed that in the early Proterozoic the Mewar block had an active-type continental margin on its present eastern side. The continental magmatic arcs and intra-arc basins developed on this margin were subsequently incorporated into the Mewar protocontinent. Possibly, the plate carrying the Bundelkhand block subducted beneath the eastern margin of the Mewar block, resulting in the final amalgamation of the two blocks along Great Boundary Fault zone or Banas Dislocation Zone. The arc related volcanism of north Indian shield at about 1850-1832 Ma, appears to represent the global subduction event, which resulted in the amalgamation and formation of Columbia supercontinent.

The Moho in Australia and New Zealand

Australia and New Zealand share in part a history in the Gondwana supercontinent. Australia has a long and complex tectonic history with the last major accretion in the early Paleozoic, whereas New Zealand is still undergoing major plate boundary processes. The Australian continent is relatively well covered with both active and passive seismic techniques. Multiple sources of information are therefore available for building a model of Moho depth. Results from on-shore and off-shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations. Moho picks from more than 10 500 km of full-crustal reflection profiles provide valuable additional constraints. The composite data set provides good sampling of much of Australia, though coverage remains low in some remote desert areas. The various datasets provide multiple estimates of the depth to Moho in many regions, and the consistency between the different techniques is high. Some of the thinnest crust lies beneath the Archean craton in the Pilbara, and in the neighbourhood of the Simpson desert. Thick crust is encountered beneath parts of the Proterozoic in Central Australia, and beneath the Paleozoic Lachlan fold belt in southeastern Australia. There are a number of zones of sharp contrast in depth to Moho, notably in the southern part of Central Australia. Despite most of the continental material around New Zealand being submerged, Moho data for this region is mainly onshore concentrating on the Australia–Pacific plate boundary. Two major wide-angle reflection transects provide the bulk of the active source data with just a few traditional reflection profiles offshore. The plate boundary provides an abundance of local earthquakes for tomographic imaging and this data is supplemented with receiver functions from both portable and permanent networks. Onshore the combined coverage is as dense as that of Australia, although it could be argued that a higher spatial resolution is required to capture the nature of the Moho of tectonically active New Zealand. Three regions of thickened crust can be identified, one beneath the Southern Alps, another beneath Fiordland, and below the Wanganui Basin between the North and South Island. Thin crust is identified west of the volcanic arc, with extensive underplating below the back-arc region.

Mesozoic arc magmatism along the southern Peruvian margin during Gondwana breakup and dispersal

A high-resolution U–Pb zircon geochronological study of plutonic units along the south Peruvian margin between 17° and 18°S allows the integration of the geochemical, geodynamic and tectonic evolution of this part of the Andean margin. This study focuses on the composite Jurassic–early Cretaceous Ilo Batholith that was emplaced along the southern Peruvian coast during two episodes of intrusive magmatism; a first period between 173 and 152Ma (with a peak in magmatic activity between roughly 168 and 162Ma) and a second period between 110 and 106Ma. Emplacement of the Jurassic part of the composite Ilo Batholith shortly post-dated the accumulation of the volcanosedimentary succession it intruded (Chocolate formation), which allows to estimate a subsidence rate for this unit of ~3.5km/Ma. The emplacement of the main peak of Jurassic plutonism of the Ilo Batholith was also closely coeval with widespread and repeated slumping (during deposition of the Cachíos Formation) in the back-arc region, suggesting a common causal link between these phenomena, which is discussed in the context of an observed 100km trenchward arc migration at ~175Ma, and the relation with extensional tectonics that prevailed along the Central Andean margin during Pangaea break-up.

Submarine volcanoes of the Kolumbo volcanic zone NE of Santorini Caldera, Greece

The seafloor northeast of Santorini volcano in Greece consists of a small, elongated rifted basin that has been the site of recent submarine volcanism. This area lies within the Cyclades back-arc region of the present Hellenic subduction zone where the seafloor of the eastern Mediterranean Sea is descending beneath the Aegean microplate. The Cycladic region and the Aegean Sea as a whole are known to be regions of north–south back-arc extension and thinning of continental crust. Nineteen submarine volcanic cones occur within this small rift zone, the largest of these being Kolumbo which last erupted explosively in 1650AD, causing significant damage and fatalities on the nearby island of Santorini. Previous SEABEAM mapping and seismic studies from HCMR indicate that many of the smaller v'olcanic cones have been built above the present seafloor, while others are partly buried, indicating a range of ages for the activity along this volcanic line. None of the cones to the northeast of Kolumbo had been explored in detail prior to a cruise of the E/V Nautilus (NA007) in August 2010. The ROV Hercules was used to explore the slopes, summits and craters of 17 of the 19 centers identified on multibeam maps of the area. Water depths of the submarine volcano's summits ranged from 18 to 450m. In general, the domes/craters northeast of Kolumbo were sediment covered and showed little evidence of recent volcanic activity. Outcrops of volcanic rock were found in the crater walls and slopes of some of the cones but they typically consisted of volcanic fragments of pumice and lava that have been cemented together by biological activity, indicative of the lack of recent eruptions. Geochemical analysis of samples collected on the northeast cones showed evidence of low temperature hydrothermal circulation on the summit and upper flanks in the form of stream-like manganese precipitates emanating from pits and fractures.

Crustal growth at active continental margins: Numerical modeling

The dynamics and melt sources for crustal growth at active continental margins are analyzed by using a 2D coupled petrological–thermomechanical numerical model of an oceanic-continental subduction zone. This model includes spontaneous slab retreat and bending, dehydration of subducted crust, aqueous fluid transport, partial melting, melt extraction and melt emplacement in form of extrusive volcanics and intrusive plutons. We could identify the following three geodynamic regimes of crustal growth: (i) stable arcs, (ii) compressional arcs with plume development, and (iii) extensional arcs. Crustal growth in a stable subduction setting results in the emplacement of flattened intrusions in the lower crust. At first dacitic melts, extracted from partially molten rocks located atop the slab (gabbros and basalts), intrude into the lower crust followed by mantle-derived (wet peridotite) basaltic melts from the mantle wedge. Thus extending plutons form in the lower crust, characterized by a successively increasing mantle component and low magmatic addition rates (10km3/km/Myrs). Compressional arcs are accomplished by the formation and emplacement of hybrid plumes. In the course of subduction localization and partial melting of basalts and sediments along the slab induces Rayleigh Taylor instabilities. Hence, buoyant plumes are formed, composed of partially molten sediments and basalts of the oceanic crust. Subsequently, these plumes ascend, crosscutting the lithosphere before they finally crystallize within the upper crust in form of silicic intrusions. Additionally, intrusions are formed in the lower crust derived by partial melting of rocks located atop the slab (basalts, gabbros, wet peridotite) and inside the plume (basalts, sediments). Magmatic addition rates are somewhat higher compared to stable arcs (40–70km3/km/Myrs). Subduction in an extensional arc setting results in decompression melting of dry peridotite. The backward motion of the subduction zone relative to the motion of the plate leads to thinning of the overriding plate. Thus, hot and dry asthenosphere rises into the neck as the slab retreats, triggering decompression melting of dry peridotite. Consequently large volumes of mafic (oceanic) crust are formed in the backarc region with total magmatic addition rates being as high as 90–170km3/km/Myrs.

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 …

Change of olivine a-axis alignment induced by water: Origin of seismic anisotropy in subduction zones

The effects of water on the crystallographic preferred orientation (CPO) of olivine aggregates were investigated through simple-shear deformation experiments under asthenospheric upper mantle conditions (pressure=2.1–5.2GPa, temperature=1490–1830K) using a deformation-DIA apparatus. Formation of the A-type olivine fabric (developed by the (010)[100] slip system) was observed under water-depleted conditions (COH<650ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) or a B-type-like fabric (by the (010)[h0l] slip system) were predominantly formed under water-rich conditions (>1000ppm H/Si). In comparison with fabrics of anhydrous olivine (≤111ppm H/Si), those of olivine having higher water contents (≥625ppm H/Si in olivine) showed stronger anisotropic properties (e.g., P-wave anisotropy, S-wave polarization anisotropy, and the ratio of horizontally and vertically polarized shear waves). The water-induced olivine CPO transition from A-type to B-type(-like) fabric accompanies a change in the alignment of the seismic fast a-axes, resulting in flow-parallel and flow-perpendicular shear wave splitting under water-depleted and water-rich conditions, respectively. The rotation of the fast direction of shear-wave splitting across an arc, which is observed in many subduction zones, is well explained by the likely bimodal water distribution along the mantle wedge (i.e., water-rich in fore-arc and water-depleted in back-arc regions) and the developments of two different types of olivine fabrics (i.e., B-type(-like) fabric in fore-arc and A-type fabric in back-arc regions).