Slab Crust Research Articles

Spatial variation in the frequency-magnitude distribution of earthquakes under the tectonic framework in the Middle East

Spatial variations of seismic energy released and b-value over the Middle East region are investigated based on a seismicity catalog from 1995 to 2007. The goal is to use these seismic parameters and based on other geodetic and geophysical observations, such as GPS measurements, strain rate model, fault distribution, focal mechanism, crustal model, Q model, and gravity measurements, etc., to uncover spatial patterns that seem anomalous. Areas of high energy released (cumulative) seem to correspond to the areas of relatively high b-values. Areas of high energy released and high b-values seem to correspond very well with the location of continental collision where earthquake activities are high. The divergent boundary between Arabia and African plates and subduction zone at Makran seem to correspond to low to moderate energy release.Northern Pamir, Azerbaijan-Caucasus, the lower part of Zagros Mountains, eastern Turkey, Owen Fracture Zone, Strait of Bob-el-Mandeb, and south of the Sulaiman Shear Zone seem to correspond to high cumulative energy-released, high strain rate, high b-values, and high fault density. While, the central and eastern Iran, southern Zagros, northern Pakistan, Gulf of Aden, Alborz, southwest of the Caspian Sea, western Caucasus and Kopeh-Dagh seem to correspond with lower b-values.The cross-section map for Hindu-Kush shows general decreasing of the b-values with depth, however, a region of high b-value is observed underneath Pamir at depths from 170 to 230km. This anomaly region can be due to dehydration of Pamir crustal slab at depth.

Lithium isotopic composition and concentration of Himalayan leucogranites and the Indian lower continental crust

The lithium isotopic compositions of adakitic rocks and K-rich volcanic rocks in southern Tibet range from +1.3‰ to +7.5‰ and −4.9‰ to +3.5‰, respectively. The subduction of the Indian Plate beneath the Lhasa Terrane means that traditional lithium isotopic compositions of various reservoirs are unable to explain the aforementioned δ7Li data. Therefore, it is necessary to determine the Li isotopic compositions of the different geological endmembers underneath southern Tibet. Here, we report the lithium isotopic composition and concentration of Indian upper and lower continental crust. On the basis of whole-rock major and trace element data and Sr–Nd–Pb isotope data, leucogranites from Luozha and Longzi are considered as representative of Indian upper crust, whereas two-mica granites from Quedang and Dala and granulites and gneisses from Nyalam are considered as representative of Indian lower crust. The Li concentration of Indian upper crust varies from 23 to 45ppm with a mean of 34ppm, consistent with a weighted mean Li concentration for the upper crust of 35±11ppm. In contrast, the Li abundance of Indian lower crust is estimated to be 33–84ppm with a mean of 58ppm, much higher than the average Li concentration for the lower crust of ~8ppm because of the high modal abundance of biotite and muscovite. The Li isotopic compositions (+0.9‰ to +5.6‰) of Indian upper continental crust are relatively heavy compared with the average Li isotopic composition of upper continental crust. On the other hand, the Li isotopic compositions of Indian lower continental crust (−4.4‰ to −0.1‰) are lighter than those of Indian upper continental crust. The isotopically heavy signature of Indian upper crust is produced by high-δ7Li fluids released from the Indian lower crust slab, whereas the lighter signature of Indian lower crust is generated by the partial melting of residual Indian lower crust slab during metamorphic dehydration of Indian lower crust.

Diamonds Discovered from High–Cr Podiform Chromitites of Bulqiza, Eastern Mirdita Ophiolite, Albania

AbstractVarious combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm−1 and 1333 cm−1, mostly at 1331.51 cm−1 or 1326.96 cm−1. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.

Skarn-mineralized porphyry adakites in the Harlik arc at Kalatage, E. Tianshan (NW China): Slab melting in the Devonian-early Carboniferous in the southern Central Asian Orogenic Belt

The geodynamic control of mineralization in the accretionary evolution of the Central Asian Orogenic Belt (CAOB) has long been controversial. Here we report new field, geochemical and geochronological data on recently defined porphyry and skarn-type ore deposits (Devonian-Early Carboniferous) in the Kalatage area in the middle of the Harlik-Dananhu arc, Eastern Tianshan, NW China in the southern CAOB, with the aim of better understanding the accretionary tectonics and genesis of porphyry and skarn-type mineralization. The Yudai porphyry Cu-(Au) deposits and the Xierqu skarn Cu-Fe-(Au) deposits are closely associated with Middle Devonian adakitic diorite porphyries (382–390Ma), which are calc-alkaline and characterized by high Na2O/K2O ratios and Sr contents (310–1020ppm), strong depletion of HREE (e.g., Yb=0.80–1.44ppm) and Y (7.68–14.50ppm), and all enriched in Rb, Sr, Ba, K and depleted in Nb and Ti. They are characterized by distinctive Eu positive anomalies, high Na2O contents and MORB-like Sr and Nd isotope signatures (high εNd(t)=+6.1to+7.0 and low (87Sr/86Sr)i=0.70412–0.70462). These adakites most likely formed by melting of a young/hot subducted oceanic slab, and adakites in general are important carriers of porphyry Cu±(Au) deposits. Early Carboniferous adakites in the Tuwu area south of Kalatage are known to have similar features. Therefore, skarn-mineralized porphyry adakites get younger from north to south, suggesting southward migration of the Harlik-Dananhu arc from 390Ma to 322Ma. These data indicate that partial melting of hot (and/or young) oceanic crustal slabs were an important mechanism of accretionary crustal growth and mineralization in the southern CAOB.

Diamonds and Other Exotic Minerals Recovered from Peridotites of the Dangqiong Ophiolite, Western Yarlung‐Zangbo Suture Zone, Tibet

AbstractVarious combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish‐orange to colorless. The grains are all 100–200 µm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm−1 and 1333 cm−1, mostly at 1331.51 cm−1 or 1326.96 cm−1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi‐stage formation for this body and similar ophiolites in the Yarlung‐Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones (SSZ), where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.

Exhumation history of the West Kunlun Mountains, northwestern Tibet: Evidence for a long-lived, rejuvenated orogen

How and when the northwestern Tibetan Plateau originated and developed upon pre-existing crustal and topographic features is not well understood. To address this question, we present an integrated analysis of detrital zircon U–Pb and fission-track double dating of Cenozoic synorogenic sediments from the Kekeya and Sanju sections in the southwestern Tarim Basin. These data help establishing a new chronostratigraphic framework for the Sanju section and confirm a recent revision of the chronostratigraphy at Kekeya. Detrital zircon fission-track ages present prominent Triassic–Early Jurassic (∼250–170 Ma) and Early Cretaceous (∼130–100 Ma) static age peaks, and Paleocene–Early Miocene (∼60–21 Ma) to Eocene–Late Miocene (∼39–7 Ma) moving age peaks, representing source exhumation. Triassic–Early Jurassic static peak ages document unroofing of the Kunlun terrane, probably related to the subduction of Paleotethys oceanic lithosphere. In combination with the occurrence of synorogenic sediments on both flanks of the Kunlun terrane, these data suggest that an ancient West Kunlun range had emerged above sea level by Triassic–Early Jurassic times. Early Cretaceous fission-track peak ages are interpreted to document exhumation related to thrusting along the Tam Karaul fault, kinematically correlated to the Main Pamir thrust further west. Widespread Middle–Late Mesozoic crustal shortening and thickening likely enhanced the Early Mesozoic topography. Paleocene–Early Eocene fission-track peak ages are presumably partially reset. Limited regional exhumation indicates that the Early Cenozoic topographic and crustal pattern of the West Kunlun may be largely preserved from the Middle–Late Mesozoic. The Main Pamir–Tam Karaul thrust belt could be a first-order tectonic feature bounding the northwestern margin of the Middle–Late Mesozoic to Early Cenozoic Tibetan Plateau. Toward the Tarim basin, Late Oligocene–Early Miocene steady exhumation at a rate of ∼0.9 km/Myr is likely related to initial thrusting of the Tiklik fault and reactivation of the Tam Karaul thrust. Thrusting together with upper crustal shortening in the mountain front indicates basinward expansion of the West Kunlun orogen at this time. This episode of exhumation and uplift, associated with magmatism across western Tibet, is compatible with a double-sided lithospheric wedge model, primarily driven by breakoff of the Indian crustal slab. Accelerated exhumation of the mountain front at a rate of ∼1.1 km/Myr since ∼15 Ma supports active compressional deformation at the margins of the northwestern Tibetan Plateau. We thus propose that the West Kunlun Mountains are a long-lived topographic unit, dating back to Triassic–Early Jurassic times, and have experienced Middle–Late Mesozoic to Early Cenozoic rejuvenation and Late Oligocene–Miocene expansion.

RATIONAL LENGTH OF SUPPORTED SYSTEM OF NARROW FACES OF CONCAST BILLET FOR PREVENTING ITS CONVEXITY

Urgent task for JSC “Ural Steel” is to determine the length of a rational supporting system on the frame of the mold to prevent the formation of bulges narrow edges of slabs with cross-sectional dimensions of 270Ч1200 mm. The technique is based on a comparison of the strength of the hardened crust narrow workpiece edge at different points in the length of the cast slab and the calculated value Expander pressure of liquid metal on the crust is used to solve this problem. Peel strength of the hardened slab in the possibility of plastic deformation of the shell may have yield strength of conventional cast steel. Experimental determination of the yield strength of cast steel grades 09G2S and 15HSND was conducted on high-temperature setting «Zwick / Roell Z1600H» in the temperature range 900–1400 °C. The value Expander pressure of liquid metal acting on the hardened crust slab was designed by the developed technique, which allows to take into account the specifi c conditions for the considered caster. Comparing the experimental and calculated values of the parameters established that the hardened crust of the narrow side of the slab thickness of 270 mm starts to withstand bursting pressure melt away from the metal mirror is equal to 1610 mm. With regard to the height of the metal mold (800 mm) length of the narrow sides of the support preform should be 810 mm. This can be possible if the number of support rollers with a diameter of 130 mm for each side of the workpiece is increased from four to fi ve. The studies identifi ed a rational extension of the supporting system located on the frame of the mold to prevent the formation of bulges narrow faces of slab blanks with cross-sectional dimensions of 270Ч1200 mm in terms of “Ural Steel”.

Consideraciones geoquímicas y petrogenéticas para establecer la evolución magmática del Complejo Volcánico Nevado del Huila

The Nevado del Huila Volcano Complex is made of rocks that have intermediate to acid composition, calc-alkaline affinity and middle content in K2O. According to SiO2 content, there are three geochemical types: andesites ss, dacitic andesites and dacites. Adakitic tendence in more recent lavas and more dacitic composition. Behaviors of mayor elements, REE, LFSE and HFSE are typical of orogenic andesites of active continental margin. Isotopic composition Sr-Nd resemble to volcanoes of ZVN Andes (Ecuador’s volcanoes, Galeras and Nevado del Ruiz volcanoes from Colombia). Homogeneous isotopic rate Sr-Nd (87Sr/86Sr = 0,7041-0,7042 and 143Nd/144Nd = 0,51279-0,51283) are typical of mantle melts, with limited taking part of derived components of subduced slab or continental crust. Compatible and incompatible trace elements indicate that fractional crystallization is petrogenetic process that has controlled magmatic evolution. Besides to reject others mechanisms: crustal contamination and magmas mixing. These rocks have probably been generated from partial melting of enriched mantle wedge by components of subduced slab. Later fractional crystallization drove magmatic differentiation of this volcano complex.

Structural and thermal control of seismic activity and megathrust rupture dynamics in subduction zones: Lessons from the Mw 9.0, 2011 Tohoku earthquake

The 2011 Tohoku megathrust earthquake ruptured a vast region of the northeast Japan Trench subduction zone in a way that had not been enough anticipated by earthquake and tsunami risk scenarios.We analyzed the Tohoku rupture combining high-frequency back-projection analysis with low frequency kinematic inversion of the co-seismic slip. Results support the to-day well-accepted broadband characteristics of this earthquake. Most of the seismic moment is released during the first 100 s, with large co-seismic slip (up to 55 m) offshore Miyagi in a compact region on the landward side of the trench. Coherent high-frequency radiation areas and relatively low co-seismic slip are a distinctive signature of the slab–mantle interface. The broadband characteristics of the Tohoku rupture are interpreted, integrating the seismic activity and structure information on the NE Japan forearc region, as a signature of along-dip segmentation and segment interactions, that result from thermal structure, plate geometry, material composition and fracture heterogeneities along the plate boundary interface. Deep mantle corner flow and low dehydration rates along the cold subduction slab interface lead to an extended seismogenic slab–mantle interface, with strong bi-material contrast controlling larger propagation distance in the downdip preferred rupture direction. Off Miyagi, plate bending below the mantle wedge, ∼142.3°E at ∼25 km depth, is associated with the eastern limit of the deep M7–8-class thrust-earthquakes, and of the strongest coherent high-frequency generation areas. The region of the slab–crust interface between the mantle wedge limit, ∼142.7°E at ∼20 km depth, and a trenchward plate bending, ∼143.2°E at ∼15 km, acted as an effective barrier resisting for many centuries to stress-loading gradient induced by deep stable sliding and large earthquakes along the slab–mantle interface. The 2011 Tohoku earthquake, whose hypocenter is located on the east side of the mantle wedge limit, released the accumulated stress in this region and succeeded to overcome the plate bending, driving the upper plate boundary interface to slip co-seismically, regardless its frictional property, thanks to a combination of dynamic effects associated with bi-material rupture directivity and stress changes induced by reflection from the surface of waves released by the unstable slip. This conceptual framework provides elements for reappraisal of long-term seismic activity and occurrence of rare and extreme tsunamigenic megathrust in other subduction zones, like those of North-Central Lesser Antilles, Central and Northern Chile.

Origin of podiform chromitite, a new model based on the Luobusa ophiolite, Tibet

Podiform chromitites have been interpreted as the result of melt–rock reaction and related melt mixing in upper mantle sections of ophiolites. However, the discovery of ultrahigh-pressure (UHP) minerals, especially diamond and coesite, in many podiform chromitites and host peridotites, raises fundamental questions about the validity of this model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to disseminated. Chromite grains in both the chromitites and peridotites have variable but relatively high MgO and are classified as magnesiochromite. Many magnesiochromite grains in the massive chromitites contain inclusions of forsterite and pyroxene, as well as diamonds and other unusual minerals. Forsterite inclusions have Fo numbers of 97–99 and NiO contents of 1.11–1.29wt.%. Mg#s (=100∗Mg/(Mg+Fe)) of clinopyroxene inclusions are 96–98 and those of orthopyroxene are 96–97. X-ray studies show that the olivine inclusions have very small unit cells and short cation–oxygen bond distances, suggesting crystallization at high pressure. In contrast, magnesiochromite grains in nodular and disseminated chromitites lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94–96 and lower NiO contents of 0.35–0.58wt.%. In addition, magnesiochromite in massive ores has higher Fe3+/Fetotal (0.42) than that in nodular and disseminated ores, which have ratios of 0.22. Disseminated chromitites also show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt–rock reaction. These observations suggest that the formation of podiform chromitites is a multi-stage process. Magnesiochromite grains and perhaps small bodies of chromitite crystallize deep in the mantle under low ambient ƒO2 from partial melts of peridotite. UHP minerals and highly magnesian olivine and pyroxene inclusions are trapped in these magnesiochromite grains. When oceanic crustal slabs are trapped in suprasubduction zones (SSZ), they are modified by island arc tholeiitic and boninitic magmas, which change the magnesiochromite compositions and deposit chromitite ores in melt channels.

Water in the slab: A trilogy

Abstract The geological and geophysical phenomena associated with water in the slab can be grouped into three different categories: those related to 1) the storage of water in the oceanic lithosphere at the surface, 2) the subduction of a hydrated slab and 3) its dehydration that ultimately leads to mantle regassing. Widespread hydration of the uppermost oceanic lithosphere occurs at mid-ocean ridges and more pervasively at the trench–rise system in response to bending-related fracturing and faulting. The amount of water stored in the upper lithosphere appears to be proportional to the spreading slowness and amount of bending. Hydrothermal alteration of the oceanic plate may also occur more locally along transform margins, fracture zones and volcanic ridges. Subduction of a hydrated slab should be observed from a reduction of seismic velocities, anomalous Vp/Vs ratios and seismic anisotropy in its uppermost portions, and shift of the main phase transformation boundaries characteristic of the mantle. Because of the increased buoyancy and weakness, hydrated or wet slabs should tend to stagnate over the 660 km discontinuity, favoring layered mantle convection patterns. Slab dehydration takes place according to the plate thermal regime function of the slab age and sinking velocity. Hydrous minerals in the oceanic crust and mantle are stable down to a maximum of 300 km and 1200 km depth, respectively, after which minor amounts of water can be retained in nominally anhydrous minerals. There is abundant geophysical evidence for dehydration of the slab crust and sub-Moho mantle, while fragmentary and often indirect evidence supports the presence of water in the lower plane of the Double Seismic Zone and at depths > 300 km.

Structures of dolomite at ultrahigh pressure and their influence on the deep carbon cycle

Carbon-bearing solids, fluids, and melts in the Earth's deep interior may play an important role in the long-term carbon cycle. Here we apply synchrotron X-ray single crystal micro-diffraction techniques to identify and characterize the high-pressure polymorphs of dolomite. Dolomite-II, observed above 17 GPa, is triclinic, and its structure is topologically related to CaCO(3)-II. It transforms above 35 GPa to dolomite-III, also triclinic, which features carbon in [3 + 1] coordination at the highest pressures investigated (60 GPa). The structure is therefore representative of an intermediate between the low-pressure carbonates and the predicted ultra-high pressure carbonates, with carbon in tetrahedral coordination. Dolomite-III does not decompose up to the melting point (2,600 K at 43 GPa) and its thermodynamic stability demonstrates that this complex phase can transport carbon to depths of at least up to 1,700 km. Dolomite-III, therefore, is a likely occurring phase in areas containing recycled crustal slabs, which are more oxidized and Ca-enriched than the primitive lower mantle. Indeed, these phases may play an important role as carbon carriers in the whole mantle carbon cycling. As such, they are expected to participate in the fundamental petrological processes which, through carbon-bearing fluids and carbonate melts, will return carbon back to the Earth's surface.

Amphiboles as indicators of mantle source contamination: Combined evaluation of stable H and O isotope compositions and trace element ratios

Stable isotope and trace element compositions of igneous amphiboles from different tectonic settings (ocean island basalts, intraplate alkaline basalts, subduction-related andesitic complexes) were compiled to help understand the role of fluids and melts in subduction-related mantle metasomatism and to evaluate the use of selected trace element ratios (Pb/Pb*(N)=Pb/(√(Ce·Pr)) and Ba/Nb(N), normalized to primitive mantle) to help detect possible metasomatism. Comparisons of stable H and O isotope compositions and trace element ratios of amphiboles from ocean island basalts (Canary Islands), intraplate basalts, and subduction-related calc-alkaline andesitic series (Carpathian–Pannonian Region, CPR) indicate systematic distributions in δD–δ18O–Pb/Pb*(N)–Ba/Nb(N) diagrams that are related to metasomatic processes in the mantle and the migration of fluids and melts derived from subducted crustal slabs. In order to interpret these data for the amphiboles from the CPR, ophiolites of the Penninic and the Meliata–Vardar complexes as potential sources of subducted crustal melts and fluids in the mantle of the Carpathian–Pannonian Region were also analyzed. On the basis of published fluid/rock partition coefficients the compositions of fluids emanating from subducted ophiolites were calculated. The calculated fluid compositions—especially for blueschists of the Meliata complex—fit the amphibole trends, indicating that such fluids could have been responsible for the mantle metasomatism beneath the CPR.

Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat-North America collision zone

New geochronologic, geochemical, sedimentologic, and compositional data from the central Wrangell volcanic belt (WVB) document basin development and volcanism linked to subduction of overthickened oceanic crust to the northern Pacific plate margin. The Frederika Formation and overlying Wrangell Lavas comprise >3 km of sedimentary and volcanic strata exposed in the Wrangell Mountains of south-central Alaska (United States). Measured stratigraphic sections and lithofacies analyses document lithofacies associations that reflect deposition in alluvial-fluvial-lacustrine environments routinely influenced by volcanic eruptions. Expansion of intrabasinal volcanic centers prompted progradation of vent-proximal volcanic aprons across basinal environments. Coal deposits, lacustrine strata, and vertical juxtaposition of basinal to proximal lithofacies indicate active basin subsidence that is attributable to heat flow associated with intrabasinal volcanic centers and extension along intrabasinal normal faults. The orientation of intrabasinal normal faults is consistent with transtensional deformation along the Totschunda-Fairweather fault system. Paleocurrents, compositional provenance, and detrital geochronologic ages link sediment accumulation to erosion of active intrabasinal volcanoes and to a lesser extent Mesozoic igneous sources. Geochemical compositions of interbedded lavas are dominantly calc-alkaline, range from basaltic andesite to rhyolite in composition, and share geochemical characteristics with Pliocene–Quaternary phases of the western WVB linked to subduction-related magmatism. The U/Pb ages of tuffs and 40 Ar/ 39 Ar ages of lavas indicate that basin development and volcanism commenced by 12.5–11.0 Ma and persisted until at least ca. 5.3 Ma. Eastern sections yield older ages (12.5–9.3 Ma) than western sections (9.6–8.3 Ma). Samples from two western sections yield even younger ages of 5.3 Ma. Integration of new and published stratigraphic, geochronologic, and geochemical data from the entire WVB permits a comprehensive interpretation of basin development and volcanism within a regional tectonic context. We propose a model in which diachronous volcanism and transtensional basin development reflect progressive insertion of a thickened oceanic crustal slab of the Yakutat microplate into the arcuate continental margin of southern Alaska coeval with reported changes in plate motions. Oblique northwestward subduction of a thickened oceanic crustal slab during Oligocene to Middle Miocene time produced transtensional basins and volcanism along the eastern edge of the slab along the Duke River fault in Canada and subduction-related volcanism along the northern edge of the slab near the Yukon-Alaska border. Volcanism and basin development migrated progressively northwestward into eastern Alaska during Middle Miocene through Holocene time, concomitant with a northwestward shift in plate convergence direction and subduction collision of progressively thicker crust against the syntaxial plate margin.

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone, Guatemala: insights from phase equilibria and Raman spectroscopy

AbstractLawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a progradeP–Tpath that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐Tof ∼520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ∼10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived progradeP–Tpath associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐Tsubduction zone.

Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

The variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation.

Scattering of Teleseismic Body Waves by the Lateral Crustal Heterogeneity at the Pacific Trench of Mexico

We examine the scattered field generated at the Pacific trench of Mexico upon the incidence of teleseismic body waves and the waveguide properties associated with the structure of the subduction zone. In this study, we analyze seismic records from a portable array perpendicular to the trench that show a distinctive phase with a phase velocity of ![Graphic][1] following body-wave phases. This signal is better observed following phases with high incident angles such as PS , SP , and SS arriving from the ocean side of the trench and for events at teleseismic distances with magnitudes larger than M w 7.0, although weak scattered signals were detected for events down to M w 6.2 and for direct P and S phases. We found that the lateral transition between the dipping slab and continental crust, with an inclined boundary formed by the subducting slab at the trench, generates a strong discontinuity that scatters seismic waves, generating a secondary field with amplitude comparable to the incident wave. This phase appears for incident body-wave arrivals with characteristic wavelength comparable to the thickness of the continental crust. Analysis of the observed scattered field shows that the trapped waves are vertically polarized surface waves. Numerical modeling suggests that the impedance contrast at the trench and the geometry of the subducting slab form a waveguide-like structure that efficiently generates and transmits trapped seismic waves along the continental margin. [1]: /embed/inline-graphic-1.gif

Indosinian Tectonic Setting of the Southern Yidun Arc: Constraints from SHRIMP Zircon Chronology and Geochemistry of Dioritic Porphyries and Granites

Abstract A mass of granitoid and dioritic intrusions are distributed in the southern Yidun Arc, among which the representative Indosinian intrusions include the Dongco and Maxionggou granitoid intrusions in Daocheng County and hypabyssal intrusions intruding into arc volcanic rocks near the Xiangcheng town. The Dongco and Maxionggou granitoid intrusions consist mainly of porphyraceous monzogranites, megacryst monzogranites and aplite granites. The Xiangcheng hypabyssal intrusions are composed dominantly of dioritic porphyries. SHRIMP zircon ages of 224±3 Ma and 222±3 Ma have been obtained for the Dongco granitoid intrusion and the Xiangcheng dioritic porphyries, respectively. The Xiongcheng dioritic porphyries show a calc-alkaline geochemical feature, and are characterized by higher Sr/Y ratios, depletive Nb, Ta, P and Ti, enriched LILEs, and lower ɛNd (t) (= −3.27), suggesting that they might be derived from mantle source magmas that were obviously contaminated by continent crustal materials. However, the Dongco and Maxionggou granitoids belong to high-potassium calcalkaline series with a per-metaluminous feature, and are characterized by higher CaO/(σFeO+MgO) and Al2O3/(σFeO+ MgO) ratios, lower (La/Yb)n and Sr/Y ratios, depletive Nb, Ta, Sr, P and Ti, enriched LILEs, and very low ɛNd (t) (= −8.10), indicating that the granitoids might be derived from partial melting of continental crust materials mainly of graywacke. Petrogenesis of Dongco and Maxionggou granitoids implies that there was an oceanic crust between the Zongza continental block (ZCB) and western margin of the Yangtze Craton (WMYZC). And the oceanic crust slab subducted westward during the Indosinian Epoch, producing an Andes-type continent marginal arc and a back-arc basin at the WMSCC. Then the oceanic basin closed and a sinistrally lateral collision occurred at ca. 224 Ma-222 Ma between the ZCB and the WMYZC, causing partial melting of sediments in the back-arc basin to generate granitoid magmas of the Dongco and Maxionggou intrusions

Mantle wedge peridotites: Fossil reservoirs of deep subduction zone processes: Inferences from high and ultrahigh-pressure rocks from Bardane (Western Norway) and Ulten (Italian Alps)

The garnet websterites from Bardane (Western Gneiss Region, Norway) derive from cold Archean subcontinental lithosphere involved in Scandian continental subduction to ultrahigh-pressures. Subduction zone metamorphism was promoted by slab fluid infiltration into the cold overlying mantle wedge. The earliest subduction transformation (M3-1) consists of garnet/clinopyroxene exsolution from old pre-subduction orthopyroxene. This stage was likely coeval with fluid input and formation of phlogopite and dolomite rods in the exsolution structures. Magnesite formation after dolomite and entrapment of fluid-related diamond-bearing polyphase inclusions in corona structures around the exsolved orthopyroxenes point to pressure increase to 4.5 GPa (M3-2). Peak pressures of 6.5–7 GPa (c.a. 200 km depth) are witnessed by crystallization of majoritic garnet (M3-3), mostly in veins cutting all the above microstructures. When such veins infiltrate the corona domains, formation of majoritic garnet in coronas is enhanced. This multistage evolution thus envisages episodic fluid influx, favouring rock recrystallization and formation of microdiamond-bearing inclusions and of majoritic garnet veins. These mantle rocks thus record fluid circulation along grain boundaries and microfractures down to 200 km depth in subduction environments. The Ulten Zone peridotites are slices of Variscan mantle wedge. Infiltration of metasomatic subduction fluids favoured transition from spinel-facies to garnet + amphibole ± dolomite parageneses at pressures below 3 GPa. Formation of metasomatized garnet-bearing peridotite mylonites suggest channelled influx of subduction fluids. The incompatible element-enriched signature of all subduction minerals in Bardane indicate that previously depleted websterites have been refertilized by COH subduction fluids. Comparison with the Ulten Zone garnet + amphibole ± dolomite peridotites outlines striking similarities in the metasomatic style and in the COH fluid phase involved. Comparable geochemical fingerprints observed in HP and UHP Ulten and Bardane mantle rocks call for concomitant subduction of the continental crust, that would provide the incompatible element-enriched fluids. For Bardane this implies subduction of crustal slabs to 200 km depth, to release the fluid phases necessary for UHP metasomatism and for majoritic garnet crystallization. Mantle refertilization by crust-derived COH subduction fluids thus operates over a large depth range, down to 200 km depth. The textural features of the Bardane and Ulten Zone mantle rocks indicate that rock recrystallization and refertilization is concomitant with fluid infiltration along channelways, outside which the rocks preserve pre-subduction, long-lasting stories and pristine geochemical characteristics. Comparably with uprising magmas, the subducted continental crust appears to be a quite efficient carrier to the surface of ultradeep mantle tectonic ‘xenoliths’ representing major observatories of diverse metamorphic and geodynamic histories recorded by the Earth's mantle at variable depths.

"Storms of crustal stress" and AE earthquake precursors

Abstract. Acoustic emission (AE) displays violent paroxysms preceding strong earthquakes, observed within some large area (several hundred kilometres wide) around the epicentre. We call them "storms of crustal stress" or, briefly "crustal storms". A few case histories are discussed, all dealing with the Italian peninsula, and with the different behaviour shown by the AE records in the Cephalonia island (Greece), which is characterized by a different tectonic setting. AE is an effective tool for diagnosing the state of some wide slab of the Earth's crust, and for monitoring its evolution, by means of AE of different frequencies. The same effect ought to be detected being time-delayed, when referring to progressively lower frequencies. This results to be an effective check for validating the physical interpretation. Unlike a seismic event, which involves a much limited focal volume and therefore affects a restricted area on the Earth's surface, a "crustal storm" typically involves some large slab of lithosphere and crust. In general, it cannot be easily reckoned to any specific seismic event. An earthquake responds to strictly local rheological features of the crust, which are eventually activated, and become crucial, on the occasion of a "crustal storm". A "crustal storm" lasts typically few years, eventually involving several destructive earthquakes that hit at different times, at different sites, within that given lithospheric slab. Concerning the case histories that are here discussed, the lithospheric slab is identified with the Italian peninsula. During 1996–1997 a "crustal storm" was on, maybe elapsing until 2002 (we lack information for the period 1998–2001). Then, a quiet period occurred from 2002 until 26 May 2008, when a new "crustal storm" started, and by the end of 2009 it is still on. During the 1996–1997 "storm" two strong earthquakes occurred (Potenza and Colfiorito) – and (maybe) in 2002 also the Molise earthquake can be reckoned to this "storm". During the "storm", started in 2008, the l'Aquila earthquake occurred. Additional logical analysis envisages the possibility of distinguishing some kind of "elementary" constituents of a "crustal storm", which can be briefly called "crustal substorms". The concept of "storm" and "substorm" is a common logical aspect, which is shared by several phenomena, depending on their common intrinsic and primary logical properties that can be called lognormality and fractality. Compared to a "crustal storm", a "crustal substorm" is likely to be reckoned to some specific seismic event. Owing to brevity purposes, however, the discussion of "substorms" is given elsewhere. AE is an effective tool for monitoring these phenomena, and other processes that are ongoing within the crust. Eventually they result to be precursors of some more or less violent earthquake. It should be stressed, however, that the target of AE monitoring is diagnosing the Earth's crust. In contrast, earthquake prediction implies a much different perspective, which makes sense only by means of more detailed multiparametric monitoring. An AE array can provide real physical information only about the processes that are objectively ongoing inside different and contiguous large slabs of the crust. The purpose is to monitor the stress propagation that crosses different regions, in order to envisage where and when it can eventually trigger a catastrophe of the system. The conclusion is that continental – or planetary – scale arrays of AE monitoring stations, which record a few different AE frequencies, appear to be the likely first step for diagnosing the evolution of local structures preceding an earthquake. On the other hand, as it is well known, the magnitude of the shock is to be related to the elastic energy stored in the focal volume, rather than to the trigger that starts it.