a recent tectonic reorganization in the south central mediterranean abstract new geodetic data combined with seismicity geologic and geochemical information document a major tectonic reorganization in the central mediterranean around 0 8 0 5 ma when rapid trench migration and consequent tyrrhenian back arc extension which dominated the region s evolution since the tortonian 10 8 ma essentially stopped in response the african convergence in sicily was transferred to a back thrust in the southern tyrrhenian characterized by frequent m5 6 earthquakes in the last 20 years a diffuse transform boundary formed across northeastern sicily to connect the sicilian and calabrian plate boundaries causing opening of the messina straits home to large historical earthquakes and unusual volcanism in the aeolian island arc and the intraplate mount etna a divergence of the ionian region from the main part of africa with northeastward motions that reflect the joint influence of african push and hellenic slab pull and are transmitted to calabria and adria could explain motions observed in the sicily channel calabria and apulia as well as the high level of southeastern sicilian seismic activity

teleseismic tomography of the southern tyrrhenian subduction zone new results from seafloor and land recordings 1 teleseismic traveltime data recorded by temporary ocean bottom seismographs deployed in tyrrhenian sea around the aeolian islands tyrrhenian deep sea experiment tyde have been used for the first time in italy to refine the 3 d model for the deep p wave velocity structure of the southern tyrrhenian subduction zone the arrival times of 35 teleseisms have been combined with those recorded by the italian national network in order to obtain a more complete azimuthal coverage of teleseismic rays 80 events recorded by land stations from 1990 to 2002 have been included in the data set in total 2904 p and 314 pkpdf phases 1300 recorded by ocean bottom instruments have been collected the upper mantle structure is reconstructed down to 500 km by a nonlinear inversion of the relative residuals computed with respect to the reference 1 d velocity model ak135 the obtained tomographic model has a higher resolution than those previously published thanks to the recordings of tyde seafloor stations tomographic results confirm the presence of the tyrrhenian slab imaged as a high velocity body extending from the uppermost mantle down to the bottom velocity model with dip 70 75 nw the model better defines the geometry of the seismogenic part of the slab its lateral extension is about 200 km in the depth interval 150 300 km where most of the deep seismicity is concentrated at uppermost mantle depths the fast structure has smaller lateral dimensions about 100 km the inversion also points out a wide well resolved low velocity zone completely surrounding the steeply dipping fast structure from the lower crust down to about 300 km this feature suggests the presence of a three dimensional circulation of asthenospheric flow around the ionian slab caused by retreat and roll back of the slab our results are in agreement with recent laboratory experiments mantle anisotropy studies geochemical and isotopic analyses and modeling based on residual topography

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

seismic velocity structures of southern italy from tomographic imaging of the ionian slab and petrological inferences summary in this study we have determined detailed vp and vs seismic velocity models of the ionian lithosphere subducting beneath the tyrrhenian basin and of the surrounding mantle by applying a post processing technique to a large sample of local earthquake tomography studies our seismic velocity models permit us to infer the presence of low velocity anomalies within the slab which we interpret as regions that are partially hydrated by fluids released during the subduction process a petrological interpretation of the velocity anomalies gives new details on the magmatism of the volcanic aeolian arc furthermore our velocity models provideamoredetaileddescriptionoftheboundaryoftheslabanditsconnectionwiththelarge seismically active tindari letojanni strike slip system finally these results allow describing in detail some features of the slab as the presence of lateral and vertical tears in conclusion the obtained models provide some constraints for inferences on mantle circulation and on the geodynamical evolution of the central western mediterranean

helium isotopes on the pacific antarctic ridge 52 5 41 5 s 1 the first isotopic data and concentrations of helium are reported for the pacific antarctic ridge between 52 5 s and 41 5 s the 4he 3he ratio is extremely homogeneous over more than 1200 km with a mean ratio of 99 275 r ra 7 29 and a standard deviation of 2719 0 19 which is the lowest dispersion observed for the global mid oceanic ridge system moreover the menard t f is a frontier between two mantles with slightly different helium isotopic ratios 96 595 1520 and 100 347 2330 no difference in the helium concentration between the two ridge segments defined by the menard t f can be observed as well as no significant difference in the u and th contents suggesting that the difference in helium isotopic ratio is old 500 my and may represent a slight difference in degassing or and trace element depletion history

the lithosphere asthenosphere system in the calabrian arc and surrounding seas southern italy a fairly detailed structural model of the lithosphere asthenosphere system thickness s and p wave velocities of the crust and of the uppermost mantle layers has been defined in the calabrian arc region southern tyrrhenian sea calabria and the northwestern part of the ionian sea in southern italy using seismic data from literature as a priori constraints of the nonlinear inversion of surface wave data

sr nd pb o isotopic evidence for decreasing crustal contamination with ongoing magma evolution at alicudi volcano aeolian arc italy implications for style of magma crust interaction and for mantle source compositions abstract new isotopic sr nd pb o data are reported for a suite of basalts mgo up to 10 wt to andesites and associated mineral separates from the island of alicudi aeolian arc southern tyrrhenian sea with the aim of investigating the effects of magma wall rock interaction in continental magmatic systems major and trace elements at alicudi exhibit smooth variations suggesting derivation of the entire suite from a single type of parental magma that underwent fractional crystallisation however 87sr 86sr and 18o values decrease while 143nd 144nd increases with increasing sio2 and decreasing mgo moreover phenocryst phases show isotopic disequilibrium relative to whole rocks and groundmass such an unusual isotope vs major and trace element variation and isotopic disequilibria suggest interaction between magmas and crustal wall rocks but the decrease of 87sr 86sr with increasing differentiation excludes assimilation fractional crystallisation processes we suggest that mafic calcalkaline magmas with primitive sr nd o isotopic signatures were intruded into a deep reservoir where they underwent fractional crystallisation contamination occurred shortly before eruption as the magmas ascended to the surface interactions between magmas and wall rocks were more extensive for the basalts than basaltic andesites and andesites generating a positive correlation between mgo and 87sr 86sr ratios the variable degrees of magma wall rock interaction are suggested to depend on the differing capabilities of magmas to dissolve crustal rocks due to variable temperature and viscosity the present study therefore provides sr nd pb o isotopic evidence on an uncommon modality of magma contamination by upper crust the alicudi andesites have the most primitive isotope signatures over the entire aeolian arc in plots of 206pb 204pb vs 87sr 86sr or 143nd 144nd isotopes the bulk of the aeolian volcanoes fall along a curved trend that connects himu mantle reservoir and the upper crust in contrast the alicudi andesites fall outside this trend and plot on a binary trend connecting himu and em1 mantle reservoirs it is suggested that the alicudi source consisted of a hybrid himu em1 mantle that was modified by fluids or melts coming from an oceanic slab these fluids did not change significantly the isotopic signatures of the wedge but generated high lile hfse ratios which were inherited by the alicudi magma in contrast the compositions of other aeolian volcanoes and of central italian magmatism reflect addition of upper crust to a himu or fozo mantle collectively the data record the complex interplay between different mantle reservoirs and the addition of various amounts and types of subduction related components in the southern tyrrhenian sea

petrology and geochemistry of submarine volcanism in the sicily channel rift abstract submarine magmatism in the sicily channel rift began in the early pliocene and lasted until almost 200 yr ago we present here petrological and geochemical data on volcanic rocks dredged from graham and nameless banks and pantelleria seamounts in the pelagian sector of the sicily channel rift petrological evidence suggests that the ascent of magmas to the surface was relatively rapid probably through channels superimposed over the major tectonic discontinuities of the rift major and trace element data indicate an ocean island basalt affinity for graham and nameless bank alkaline lavas and a depleted tholeiitic signature for one pantelleria seamount which had a shallower mantle source sr nd pb isotopic compositions suggest a heterogeneous mantle source involving both focus zone fozo and high himu components lead isotopic compositions are slightly less radiogenic in the tholeiitic basalt and more radiogenic closer to himu in the alkaline lavas we hypothesize that a decompressional me

on the mesozoic ionian basin summary new seismic reflection profiles of the italian deep crust project crop provide new insights on the structure of the ionian sea in spite of the apennines and hellenides neogene subduction zones two conjugate passive continental margins are preserved at the margins of the ionian sea along the malta escarpment to the southwest and the apulian escarpment to the northeast the ionian sea is likely to be a remnant of the mesozoic tethys ocean confined by these two conjugate passive continental margins the transition from continental to oceanic crust appears sharper to the northeast than to the southwest the basin between southeast sicily and southwest puglia was about 330 km wide and suggests a low spreading rate the inferred oceanic ridge should have been flattened by thermal cooling and buried by later sediments based on stratigraphic and structural constraints to the north in the apennines belt the ocean continued to the northwest this palaeogeography is supported by the seismicity of the apennines slab underneath the southern tyrrhenian sea which implies downgoing oceanic lithosphere the adjacent absence or paucity of deep seismicity does not imply absence of subduction but rather it can be interpreted as due to the more ductile behaviour of the subducted continental lithosphere surprisingly we note that where the oceanic inherited basin is subducting underneath the apennines in the hangingwall of the subduction hinge there are outcropping slices of continental crystalline basement previously deformed by the alpine orogen

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

constraints on mantle circulation around the deforming calabrian slab 1 combining tectonics with seismological and geochemical data we reconstruct the deformation history of the presently narrow calabrian slab and the path of mantle circulation during the last 10 ma we show that during the slab deformation the mantle laterally flowed inside the back arc region permitting its retrograde motion and giving a seismological and volcanological record after 1 2 myr

the southern tyrrhenian subduction zone deep geometry magmatism and plio pleistocene evolution abstract we report on a high resolution vp vp vs and qp model of the southern tyrrhenian subduction zone obtained by the inversion of p and s wave arrival times and t values from intraslab seismicity the arcuate shape of the southern apennines calabrian arc sicilian maghrebides is perfectly mirrored by two rather continuous low and high vp bands lying beneath the belt system at ca 25 and 100 km respectively between 100 and 300 km two independent high vp slabs lie beneath the neapolitan region and the southern tyrrhenian sea separated by unperturbed mantle we suggest that the ca 150 km wide slab window beneath the southern apennines opened after a tear occurring within a composite subduction system formed by the apulian continental lithosphere and the ionian oceanic slab the abrupt slab rupture induced ultrafast southeastward retreat of the ionian slab and the 19 cm yr spreading of the back arc oceanic marsili basin between ca 2 1 and 1 6 ma ago the 25 km low vp zone beneath the arc denotes continental upper crustal rocks below the chain its striking continuity requires a unique orogenic wedge at 25 km depth below the southern apennines the calabrian arc and the sicilian maghrebides the alternative explanation would imply the ubiquitous occurrence of autochthonous lower plate rocks at 25 km depth i e a puzzling autochthonous continental calabria the ionian slab beneath calabria shows high vp high qp and low vp vs anomalies typical of old oceanic lithosphere intermediate depth seismicity is concentrated within its thin oceanic crust suggesting the occurrence of vigorous metamorphism the slab dehydration promotes the melting of the overlying mantle as testified by high vp vs and low qp anomalies between the slab and the aeolian magmatic arc

sks splitting in southern italy anisotropy variations in a fragmented subduction zone abstract in this paper we present a collection of good quality shear wave splitting measurements in southern italy in addition to a large amount of previous splitting measurements we present new data from 15 teleseisms recorded from 2003 to 2006 at the 40 stations of the cat scan temporary network these new measurements provide additional constraints on the anisotropic behaviour of the study region and better define the fast directions in the southern part of the apulian platform for our analysis we have selected well recorded sks phases and we have used the method of silver and chan to obtain the splitting parameters the azimuth of the fast polarized shear wave and delay time t shear wave splitting results reveal the presence of a strong seismic anisotropy in the subduction system below the region three different geological and geodynamic regions are characterized by different anisotropic parameters the calabrian arc domain has fast directions oriented nne ssw and the southern apennines domain has fast directions oriented nnw sse this rotation of fast axes following the arcuate shape of the slab is marked by a lack of resolved measurements which occurs at the transition zone between those two domains the third domain is identified in the apulian platform here fast directions are oriented almost n s in the northern part and nne ssw to ene wsw in the southern one the large number of splitting parameters evaluated for events coming from different back azimuth allows us to hypothesize the presence of a depth dependent anisotropic structure which should be more complicated than a simple 2 layer model below the southern apennines and the calabrian arc domains and to constrain at 50 km depth the upper limit of the anisotropic layer at least at the edge of southern apennines and apulian platform we interpret the variability in fast directions as related to the fragmented subduction system in the mantle of this region the trench parallel observed in calabrian arc and in southern apennines has its main source in the asthenospheric flow below the slab likely due to the pressure induced by the retrograde motion of the slab itself the pattern of in the apulian platform does not appear to be the direct result of the rollback motion of the slab whose influence is limited to about 100 km from the slab the anisotropy in the apulian platform may be related to an asthenospheric flow deflected by the complicated structure of the adriatic microplate or may also be explained as frozen in lithospheric anisotropy

properties of the lithosphere in collisional belts in the mediterranean a review abstract a brief review of the lithosphere asthenosphere properties is given for the mediterranean area the strongest lateral heterogeneities are found in the proximity of tectonically active areas such as collisional belts high s wave velocities below about 70 km denote the presence of deep seated almost aseismic lithospheric roots under the western alps the north central apennines and the cordillera betica an alternative explanation of the data related to mechanisms of intermediate and deep focus earthquakes in the tyrrhenian sea and aegean sea is discussed in the frame of the dynamic subduction model proposed by freund et al 1980 which postulates that the slab is disrupted in blocks and pulled down gravitationally

space variations of stress along the tyrrhenian wadati benioff zone gephart and forsyth s 1984 algorithm for stress inversion of earthquake fault plane solutions has been applied to a set of ninety intermediate and deep events occurring in the southern tyrrhenian region between 1976 and 1995 p and s wave data from local seismic networks in southern italy the italian national network and international bulletins have been used for hypocenter and focal mechanism computations stress inversion runs performed after accurate selection and weighting of fault plane solutions have allowed us to identify stress space variations at a higher level of detail than available from all previous investigations carried out in the study area the maximum compressive stress has been shown to follow the depth decreasing dip of the wadati benioff zone along the entire zone from a depth of 90 km to the depth of the deepest events about 500 km variations to such a stress pattern have been found possibly related to mantle dynamics and the complex composition of the subducting structure the diffused state of down dip compression suggests that the tyrrhenian subduction has already evolved to the point where the lower end of the slab has reached high strength mantle materials the load of the excess mass is entirely supported from below and most of the subducted slab is under compression in agreement with the lack of large shallow thrusting events in the immersion zone the findings of the present study appear to agree well with geodynamic models assuming a passive subduction process with eastward roll back of the ionian lithosphere in the study area in this context the depth decrease of the slab dip may also find a reasonable explanation

new gps constraints on the africa eurasia plate boundary zone in southern italy 1 southern italy is a key area for understanding the tectonic processes in the africa eurasia collision zone we analyze new gps measurements carried out between 1994 and 2001 the results are presented in terms of time series trajectories and velocities of crustal motion as well as a geodetic strain rate field while central italy corsica sardinia and the tyrrhenian sea move like the eurasian plate the overall motion of the sicily rift zone region matches african plate motion unexpected are the north northwest directed motions of northeastern sicily and the eolian islands most striking on the deformation field are a north south oriented compression along the northern sicilian coast compression between apulia and northwestern greece as well as extension in the sicily rift zone and the interior of sicily

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

high resolution subduction zone seismicity and velocity structure beneath ibaraki prefecture japan 1 we use double difference tomography and waveform derived cross correlation differential times to estimate earthquake locations and p and s wave velocity structure in the subduction zone under ibaraki and neighboring prefectures of north central honshu we find evidence in both earthquake hypocenters and the velocity structure that the philippine sea plate or perhaps a microplate fragment may be caught between the subducting pacific plate and overriding okhotsk plate in this region in the southern part of the study area we find a zone of interplate events extending as deep as 60 km forming a distinct lineation in cross section focal mechanisms support the interpretation that these are low angle subduction interface events we infer that these events probably occur on the interface between the pacific and philippine sea plates rather than between the pacific and okhotsk plates in the upper part of the downgoing pacific plate we find a zone of high vp vs ratio 1 95 which may be explained by high pore fluid pressures within the subducting crust at a depth of 60 80 km this region of high vp vs appears to diffuse into the overlying mantle wedge possibly indicating the upward release of fluids from the slab

40ar 39ar geochronological constraints on the formation of the dayingezhuang gold deposit new implications for timing and duration of hydrothermal activity in the jiaodong gold province china abstract china s largest gold resource is located in the highly endowed northwestern part of the jiaodong gold province most gold deposits in this area are associated with the ne to nne trending shear zones on the margins of the 130 126 ma guojialing granite these deposits collectively formed at ca 120 5 ma during rapid uplift of the granite the dayingezhuang deposit is a large 120 t au orogenic gold deposit in the same area but located along the eastern margin of the late jurassic linglong metamorphic core complex new 40ar 39ar geochronology on hydrothermal sericite and muscovite from the dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130 4 ma and is the earliest important gold event that is well documented in the province the dayingezhuang deposit occurs along the linglong detachment fault which defines the eastern edge of the ca 160 150 ma linglong granite granodiorite massif the anatectic rocks of the massif were rapidly uplifted at rates of at least 1 km m y from depths of 25 30 km to form the metamorphic core complex the detachment fault with precambrian metamorphic basement rocks in the hangingwall and the linglong granitoids and migmatites in the footwall is characterized by early mylonitization and a local brittle overprinting in the footwall gold is associated with quartz sericite pyrite k feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault our results indicate that there were two successive yet distinct gold forming tectonic episodes in northwestern jiaodong one event first reactivated the detachment fault along the edge of the linglong massif between 134 and 126 ma and then a second reactivated the shears along the margins of the guojialing granite both events may relate to a component of northwest compression after a middle early cretaceous shift from regional nw se extension to a ne sw extensional regime

seismic anisotropy reveals focused mantle flow around the calabrian slab southern italy 1 sks splitting at the calabrian subduction zone with delay times t up to 3s reveals the presence of a strong anisotropic fabric fast directions are oriented nne ssw in the calabrian arc c a and rotate nnw sse to the north following the arcuate shape of the subducting plate we interpret the trench parallel as local scale mantle flow driven by the retrograde motion of the slab the absence of trench perpendicular below the southern apennines s a excludes the presence of a seismically detectable return flow at its ne edge this may be due to the relative youth and limited width of the s a slab tear a possible return flow is identified farther north at the boundary of the s a and central apennines different and weaker anisotropy is present below the apulian platform a p this implies that the influence of the slab rollback in the sub slab mantle is limited to less then 100 km from the slab

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

residual topography lithospheric structure and sunken slabs in the central mediterranean abstract the central mediterranean comprises subduction collision and backarc extension all in a relatively small area in this paper we analyze the topography of this region examining the influence of subducting slabs on overriding plates we take the observed surface elevation remove the contribution of the crust to it and use the residual topography to identify regions that are unusually low unusually high or normal then we calculate lithospheric thickness where local isostasy applies the results provide information about the structure of the lithosphere in this complex region and about the vertical tectonics in particular we found a difference between the south tyrrhenian subduction zone and the apennines consistent with the processes of slab rollback and slab break off respectively in the south tyrrhenian the edge of the oceanic plate is strongly pulled down whereas the edge of the overriding plate the calabrian peninsula is uplifted this opposite trend indicates weak plate coupling that is the subducting slab which is rapidly rolling back hangs almost entirely on the oceanic plate and the overriding plate is free in contrast in the apennines high residual topography and uplifting are detected on both sides of the mountain belt consistent with complete break off of the subducted lithosphere

high resolution subducting slab structure beneath northern honshu japan revealed by double difference tomography the high resolution seismic velocity structure of the subducting slab beneath northern honshu japan has been obtained by double difference tomography capitalizing on the existence of two planes of seismicity the upper plane lies in the region with average to high vp vs ratios 1 72 1 85 which may correspond to the transformations of metabasalt and metagabbro to blueschist the lower plane is associated with very low vp vs ratios 1 6 1 7 in sharp contrast with high vp vs ratios 1 8 1 85 in the region between the two planes these features may be explained by forsterite enstatite h2o formation from serpentine dehydration in the lower plane and partial hydration of the region between the two planes our results provide direct constraints for petrologic modeling of the subducting slab

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

convergence vs retreat in southern tyrrhenian sea insights from kinematics 1 the tyrrhenian sea is an extensional basin opened by trench retreat and back arc extension during subduction of the calabrian slab in the last 10 12 my subduction is still active beneath the semost part of the tyrrhenian sea as testified by seismicity down to 500 km depth by analyzing seismicity and geodetic data together with recent tomographic images we define the present day situation an evident n s compressional regime prevails in the tyrrhenian region west of the aeolian archipelago while east of them a nnw sse extension is documented by focal mechanisms and gps data with a much smaller strain rate with respect to the past the transition between these two domains is accommodated by a n s discontinuity zone which runs from aeolian islands to mt etna with an extensional to strike slip deformation