seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

volcanic plutonic parity and the differentiation of the continental crust the continental crust is central to the biological and geological history of earth however crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks volcanic and plutonic rocks and the processes by which they differentiate to felsic compositions our analysis of a comprehensive global data set of volcanic and plutonic whole rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics in both tectonic settings our results indicate that fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas emphasizing the role of mafic cumulates as a residue of crustal differentiation a global geochemical data set of volcanic and plutonic rocks indicates that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts brenhin keller and co authors present a global geochemical dataset of the two fundamental building blocks of the continental crust volcanic externally erupted and plutonic internally solidified rocks their results indicate that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics this work indicates that in both tectonic settings fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

stuck in the mud earthquake nucleation and propagation through accretionary forearcs 1 subduction zone earthquakes can propagate to the surface causing large seafloor displacements resulting in tsunamis this requires the earthquake to rupture through clay rich sediments of the accretionary wedge which are largely aseismic as found previously the frictional properties of a range of wet clays at low slip velocity are velocity strengthening thus inhibiting earthquake nucleation however at high slip velocity the same materials weaken almost immediately resulting in a negligible critical slip weakening distance and fracture energy we interpret this behaviour as rapid thermal pressurization of the pore fluid within the clay gouge the lack of fracture energy can explain how a large rupture propagating from depth might not be arrested by clay rich velocity strengthening sediments as is commonly seen the results suggest that generally earthquakes may be difficult to nucleate on mature faults dominated by clay but the propagation of earthquakes through these zones is energetically very favourable

petrogenesis of late mesozoic high ba sr granites in the qiushuwan cumo orefield implications for the distribution of porphyry mo mineralization in the east qinling area of central china abstract the qinling dabie orogenic belt of china is well known for its large mo resources there have been many studies of the genesis of the mo deposits but the reasons for the intensive mineralization during the late mesozoic and the spatially uneven distribution of the resulting ore deposits are not fully understood deposits with reserves of 6 mt of mo occur predominantly in the southern margin of the north china craton ncc and are mostly late mesozoic porphyry deposits in comparison a few relatively insignificant and cu dominated porphyry ore deposits have been identified in the north and south qinling terranes this study presents new whole rock geochemical isotopic and zircon u pb age data for ore bearing granites associated with the qiushuwan cu mo deposit the largest porphyry deposit in the north qinling nq terrane which provide insights into the asymmetric distribution of mo mineralization in this region the rocks display high k calc alkaline to shoshonitic affinities and are metaluminous to weakly peraluminous with high ba and sr contents low rb nb and ta contents and depletion in heavy rare earth elements and y which are features typical of high ba sr granites zircon u pb dating indicates that the granites crystallized at ca 145 ma their narrow ranges of whole rock initial 87sr 86sr ratios 0 7056 0 7069 and end t 7 4 to 9 7 zircon ehf t 2 49 to 5 13 and 18o 5 58 to 6 49 values together with the presence of significant nd hf isotopic decoupling hf 7 61 to 11 3 indicate that the parent magma was derived from partial melting of enriched subcontinental lithospheric mantle with minor assimilation of the lower crust in contrast late mesozoic granitic rocks in the southern margin of the ncc including both mineralized and barren intrusions are geochemically associated with the crust of the northern margin of the yangtze block yb the yb contains mo rich shales that are thought to have been subducted beneath the southern margin of the ncc during the triassic melting of these mo rich black shales would have mobilized mo a process that enabled the formation of the extensive mo mineralization associated with the late mesozoic granites of the southern margin of the ncc this indicates that the extensive mo mineralization within the southern margin of the ncc is genetically related to a mo rich source that was generated by the stagnation of subducted yb continental crust beneath the southern margin of the ncc in comparison the less significant mo mineralization within the east qinling orogenic belt might reflect the variable removal of the subducted yb continental slab during post collisional intracontinental orogenesis

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

the role of frictional strength on plate coupling at the subduction interface 1 at a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc in this paper we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins in numerical models we vary thickness of a serpentinized layer in the mantle wedge 15 to 25 km and the frictional strength of both the sediments and serpentinized mantle friction angle 1 to 15 or static friction coefficient 0 017 to 0 27 to control the amount of frictional coupling between the plates with plastic strain weakening in the lithosphere our numerical models can attain stable subduction geometry over millions of years we find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths in the case of low sediment and serpentinite friction the subduction interface has a shallow dip while the subduction zone develops an accretionary prism a broad forearc high a deep forearc basin and a shallow trench in the high friction case the subduction interface is steep the trench is deeper and the accretionary prism forearc high and basin are all absent the resultant free air gravity and topographic signature of these subduction zone models are consistent with observations we believe that the low friction model produces a geometry and forearc structure similar to that of accretionary margins conversely models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin we find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

construction of a pluton evidence from an exposed cross section of the searchlight pluton eldorado mountains nevada a top to bottom cross section of the searchlight pluton is exposed in a large tilt block in the colorado river extensional corridor of southern nevada hornblende barometry and geologic relations indicate that the pluton was about 10 km thick extending from approximately 3 to 13 km depth the pluton is stratified with about 2 km of fine grained quartz monzonite below the roof 2 km of granite in the center and 6 km of coarser more mafic quartz monzonite at the bottom the lower unit has a pronounced magmatic foliation that was subhorizontal prior to tilting contacts between the units are gradational over a few centimeters to about 20 m geometry field relations and elemental and isotopic data suggest that the three units mark the terminal stages of evolution of magma that filled a very thick magma chamber the upper unit formed as a solidification front that migrated downward from the roof and the middle unit granite and the lower unit represent complementary segregated melt and crystal rich cumulate thus the dominant part of the pluton appears to have solidified in monotonic fashion from more or less uniform magma that was compositionally similar to the upper unit the basal quartz monzonite and the lower part of the granite enclose widely scattered synplutonic hornblende gabbro and diorite pods that range from centimeter to kilometer scale these rocks are commonly fine grained and formed as quenched mafic melts they are isotopically distinct from the main sequence and represent discrete injections into the magma chamber the mafic magmas were contaminated by but did not strongly affect the main sequence magma the nd and sr isotopic compositions of the main sequence magma are intermediate between those of regional crust and enriched mantle derived basalts like those represented by the mafic pods this suggests that the principal magma was a hybrid with about 60 mantle component and thus the pluton represents both addition to and internal reorganization of the crust

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

quantum magmatism magmatic compositional gaps generated by melt crystal dynamics compositional gaps are common in volcanic series worldwide the pervasive generation of compositional gaps influences the mechanical and thermal properties of the crust and holds clues on how our planet differentiates we have explored potential mechanisms to generate these gaps using numerical simulations coupling crystallization kinetics and multiphase fluid dynamics of magma reservoirs we show that gaps are inherent to crystal fractionation for all compositions as crystal liquid separation takes place most efficiently within a crystallinity window of 50 70 vol crystals the probability of melt extraction from a crystal residue in a cooling magma chamber is highest in this crystallinity window due to 1 enhanced melt segregation in the absence of chamber wide convection 2 buffering by latent heat of crystallization and 3 diminished chamber wall thermal gradients this mechanical control of igneous distillation is likely to have played a dominant role in the formation of the compositionally layered earth9s crust by allowing multiple and overlapping intrusive episodes of relatively discrete or quantized composition that become more silicic upward

structure and tectonics of the erosional convergent margin off antofagasta north chile 23 30 s subduction erosion has dominated the evolution of the north chile convergent continental margin since at least the mesozoic we investigate the structure of the antofagasta 23 s sector of this margin along a transect using coincident wide angle and near vertical seismic profiling and gravity data a 2 d velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography a velocity derived density distribution was used to model marine gravity data and substantiate the velocity model the gravity and velocity models imply that the overriding plate is mainly made of arc type igneous basement the upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion the top coincident with a reflection in near vertical seismic images the seismic boundary is interpreted as a detachment separating an upper extended domain with large scale normal faulting from a lower domain apparently undergoing a different type of deformation velocity derived porosity indicates that the front of the margin is probably fluid saturated and disaggregated by fracturation as a consequence of frontal subduction erosion fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate at depths greater than 20 km porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured the change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 antofagasta earthquake m w 8 0 defining the seismogenic zone

mineralogical evidence for fractionation processes in the himalayan leucogranites of the ramba dome southern tibet abstract himalayan leucogranites provide some of the best examples in the world of highly felsic and strongly peraluminous granites and they have been paid special attention with regard to investigating the evolution of continental crust here we describe our mineralogical investigations of a late miocene leucogranite complex within the ramba dome of the tethyan himalaya belt that have helped us decipher the petrogenesis of these high silica peraluminous granites the ramba leucogranites range from two mica granites biotite muscovite to muscovite leucogranites muscovite garnet tourmaline with the two mica granites constituting the main part of the pluton and the muscovite leucogranites commonly occurring as late stage dikes that are located within the pluton or invaded the country rocks variations in the mineral assemblages of the two types of leucogranite are associated with systematic changes in bulk rock geochemistry and the distinctive mineralogical features demonstrate a genetic relationship between the two types of leucogranite that involved fractional crystallization with the muscovite leucogranites being more evolved than the two mica granites chemical data for the minerals in the two mica granites record a long period of differentiation during which a normal granitic melt developed into a highly evolved magmatic system with high flux components e g h2o f b in contrast the mineralogical data for the muscovite leucogranites record the crystallization of a very late residual melt i e a volatile rich highly fluid melt according to recent studies of differentiation processes in silicic magma reservoirs we can interpret the ramba leucogranites as having undergone a high degree of differentiation by in situ fractional crystallization in a crystal mush environment the two mica granites represent a congealed crystal mush that was composed of cumulate crystals and a trapped interstitial liquid and the muscovite leucogranites represent the almost pure liquid that was extracted from the crystal mush modeling using the trace elements sr and ba shows that the extraction probably occurred when the crystallinity of the mush was 59 64 at least for the most evolved muscovite leucogranite sample the two mica granite corresponds to a residual crystal mush that had a terminal porosity of 20 26 filled with a trapped interstitial liquid collectively fractional crystallization can account for the geological mineralogical petrological and geochemical variations between the two types of leucogranite this work highlights the important role of crystal fractionation in the generation of himalayan leucogranites

volcanism and erosion during the past 930 k y at the tatara san pedro complex chilean andes geologic mapping together with 73 new k ar and 40ar 39ar age determinations of 45 samples from 17 different volcanic units plus paleomagnetic orientations geochemical compositions and terrestrial photogrammetry are used to define the chronostratigraphy of the tatara san pedro complex an eruptive center at 36 s on the volcanic front of the andean southern volcanic zone the tatara san pedro complex preserves 55 km3 of lavas that erupted from at least three central vent regions remnant unconformity bound sequences of lavas are separated by lacunae that include significant periods of erosion quaternary volcanism commenced ca 930 ka with eruption of voluminous dacitic magma followed 100 k y later by the only major rhyolitic eruption from 780 ka onward more than 80 of the preserved volume is basaltic andesite 52 57 sio2 but petrographically and geochemically diverse dacitic magmas 63 69 sio2 erupted sporadically throughout this younger dominantly mafic phase of activity a few basaltic lavas 49 52 sio2 are present mainly in portions of the complex older than 230 ka the number of vents the petrologic and geochemical diversity and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the tatara san pedro complex over the past million years nearly two thirds of the preserved volume of the tatara san pedro complex comprises the two youngest volcanoes which were active between ca 188 83 ka and 90 19 ka repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre 200 ka volcanic record particularly on the south flank of the complex dating the inception of a glaciation on the basis of preserved material is difficult but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation on this basis the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca 830 790 610 400 330 230 110 and 17 ka these are broadly consistent with global ice volume peaks predicted by the oxygen isotope based astronomical time scale and with age brackets on north american glacial advances estimated growth rates for the two young volcanoes are 0 2 to 0 3 km3 k y these are three to five times greater than a growth rate estimated from all preserved lavas in the complex 0 06 km3 k y removal of up to 50 95 of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy and it raises the possibility that episodic erosion of mid latitude frontal arc complexes may be extensive and common

crustal structure across the middle ryukyu trench obtained from ocean bottom seismographic data abstract in 1988 an extensive geophysical investigation was performed at the middle ryukyu island arc s japan an aim of the investigation is to obtain a detailed crustal structure at the trench arc back arc system by various geophysical surveys in this paper we present fine crustal models obtained from ocean bottom seismographic obs data along two profiles one profile was acquired from the northwest edge of the philippine sea plate to the okinawa island profile 2 a and the other profile which is perpendicular to profile 2 a was located on the ryukyu arc profile 3 a crustal model of profile 2 a is characterized by a sedimentary wedge and subducting oceanic crust the sedimentary wedge consists of four layers whose velocities are 1 8 2 8 2 9 3 5 and 4 7 5 0 km s at the top of each layer the maximum thickness of the wedge is 9 km at 50 km landward from the trench underlying the sedimentary wedge the model shows subducting oceanic layer 2 and 3 5 3 and 6 8 km s a crustal model of profile 3 indicates three layers of sediment whose velocities are 1 7 2 0 2 4 2 8 and 3 7 km s above a 5 6 6 0 km s layer a low q value layer at the bottom of the sedimentary layer is required to explain an observed amplitude reduction versus offsets three deep crustal reflection phases are observed along profile 3 these phases are interpreted as the reflection phases from the oceanic crust subducting beneath the okinawa island the depth of the subducting oceanic moho is deduced to be 26 km at 120 km landward from the trench axis by the reflection phases

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

effects of smectite to illite transformation on the frictional strength and sliding stability of intact marine mudstones 1 at subduction zones earthquake nucleation and coseismic slip occur only within a limited depth range known as the seismogenic zone one leading hypothesis for the upper aseismic seismic transition is that transformation of smectite to illite at 100 150 c triggers a change from rate strengthening frictional behavior that allows only stable sliding to rate weakening behavior considered a prerequisite for unstable slip previous studies on powdered gouges have shown that changes in clay mineralogy alone are unlikely to control this transition but associated fabric and cementation developed during diagenesis remain possible candidates we conducted shearing experiments designed specifically to evaluate this hypothesis by using intact wafers of mudstone from ocean drilling program site 1174 offshore sw japan which have undergone progressive smectite transformation in situ we sheared specimens along a sawcut in a triaxial configuration oriented parallel to bedding at normal stresses of 20 150 mpa and a pore pressure of 1 mpa during shearing we conducted velocity stepping tests to measure the friction rate parameter a b friction coefficient ranges from 0 28 0 40 and values of a b are uniformly positive both are independent of clay transformation progress our work represents the most direct and comprehensive test of the clay transformation hypothesis to date and suggests that neither illitization nor accompanying fabric development and cementation trigger a transition to unstable frictional behavior we suggest that strain localization in combination with precipitation of calcite and quartz is a viable alternative that is consistent with both field observations and recent conceptual models of a heterogeneous seismogenic zone

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 1 principles procedures and evaluation of the method constraining the pressure of crystallization of magmas is an important but elusive task in this work we present a method to derive crystallization pressures for rocks that preserve glass compositions either glass inclusions or matrix glass representative of equilibration between melt quartz and 1 or 2 feldspars the method relies on the well known shift of the quartz feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure the critical realization for development of the method is the fact that melt quartz and feldspars need to be in equilibrium at the liquidus for the melt composition the method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite melts over a range of pressures and searching for the pressure at which the expected assemblage quartz 1 feldspar or quartz 2 feldspars is found at the liquidus we evaluate errors resulting from uncertainties in glass composition using a series of monte carlo simulations for a quartz hosted glass inclusion composition from the bishop tuff which reveal errors on the order of 20 45 mpa for the quartz 2 feldspars constraint and on the order of 25 100 mpa for the quartz 1 feldspar constraint we suggest actual errors are closer to the lower bounds of these ranges we investigate the effect of fluid saturation in two ways 1 by applying our procedure over a range of water contents for three glass compositions we show that the effect of fluid saturation is more important at higher pressures 300 mpa than at lower pressures 100 mpa but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant h2o concentrations 3 wt and 2 by performing the same type of pressure determinations with a preliminary version of rhyolite melts that includes a h2o co2 mixed fluid phase we use a range of h2o and co2 concentrations for two compositions characteristic of early erupted and late erupted bishop tuff glass inclusions and demonstrate that calculated pressures are largely independent of co2 concentration for co2 1 000 ppm at least for relatively high h2o contents as expected in most natural magmas such that co2 concentration can be effectively neglected for application of our method finally we demonstrate that pressures calculated using the rhyolite melts geobarometer compare well with those resulting from h2o co2 glass inclusion barometry and al in hornblende barometry for an array of natural systems for which data have been compiled from the literature the agreement is best for quartz hosted glass inclusions while matrix glass yields systematically lower rhyolite melts pressures suggestive of melt evolution during eruptive decompression

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

high resolution image of the geometry and thickness of the subducting nazca lithosphere beneath northern chile 1 results obtained from s and p receiver functions produced a clear image of the top and bottom of the subducting nazca lithosphere beneath northern chile using data from the teleseismic events recorded at 15 permanent integrated plate boundary observatory chile ipoc stations we obtained new constraints on the geometry and thickness of the descending nazca lithosphere we observed the subducted crust of the nazca plate at depths ranging from 50 km beneath the coastal cordillera down to 110 km beneath the western cordillera we found significant along strike variations in the geometry of the nazca plate beneath northern chile on closer inspection it appears that the oceanic nazca plate is divided into two distinct segments as it descends beneath the continental south american plate the transition from the relatively steeper 23 and deeper slab to the north of 21 s to the flatter southern segment 19 is shown reasonably clearly by our data this feature could well be associated with variations in the curvature of the plate margin and the geometry of the chile trench which is mainly curved to the north of 21 s we have also mapped the continental moho of the south american plate at depths ranging between 60 and 70 km to the east of the longitudinal valley beneath the coastal cordillera this boundary becomes invisible probably due to the serpentinization of the forearc mantle wedge that reduces the velocity in the uppermost mantle the base of the subducted nazca plate was clearly identified as a sharp boundary in the results obtained from the p and s receiver functions the thickness of the subducted oceanic nazca plate which has an age of 50 my is estimated to be 50 km although this thickness is consistent with that predicted by thermal gradients the explanation of the sharpness of the lithosphere asthenosphere boundary may require another mechanism such as hydration or melting

seismic evidence for widespread serpentinized forearc mantle along the mariana convergence margin 1 seismic imaging of subduction zones can provide constraints on mineral reactions in the slab and surrounding regions we use p to s converted phases from teleseisms recorded at broadband stations in the mariana islands to image the forearc and arc regions of the mariana convergent margin the subducting oceanic crust is observed between 75 and 110 km depth as a thin low velocity zone overlying the subducting moho demonstrating that the basalt to eclogite phase transition must occur at a greater depth a low velocity zone lvz approximately 10 25 km thick whose upper boundary is imaged at about 40 55 km depth is detected in the forearc region of the mantle wedge along the entire margin the anomaly is located too shallow to represent subducted oceanic crust we interpret the lvz as a serpentinized region in the forearc mantle resulting from hydration by slab expelled water the inferred s wave velocity in the lvz of as low as 3 6 km s represents a level of serpentinization of 30 50 corresponding to a chemically bound water content of about 4 6 wt

zircon record of the plutonic volcanic connection and protracted rhyolite melt evolution the potential petrogenetic link between a crystal poor rhyolite the rhyolite canyon tuff and its associated subvolcanic intrusion and crystal rich post caldera lavas from turkey creek arizona usa is examined using zircon chemical abrasion thermal ionization mass spectrometry u pb geochronology and inductively coupled plasma mass spectrometry trace element analyses u pb ages indicate that zircon growth within the rhyolite and the dacite monzonite porphyry magmas was coeval over 300 k y prior to the large eruptive event trends in zircon trace elements hf y dy sm yb eu eu through time in the dacitic monzonitic units and rhyolite reflect melt evolution dominated by crystal fractionation importantly the y dy ratio in zircons in both units remains mostly similar for the first 150 k y of the system s evolution but the dominant population in the rhyolitic unit diverges from that of the dacite monzonite porphyry 150 k y before eruption we interpret this divergence in trace element composition to record the assembly time of the melt rich cap within its intermediate mush zone in the upper crustal reservoir these results are consistent with 1 a connection between plutonic and volcanic realms in the upper crust 2 a protracted time scale for constructing an intermediate mush large enough to hold 500 km 3 of rhyolite and 3 the prolonged extraction of that melt prior to eruption

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

the annual cycle of circulation of the southwest subtropical pacific analyzed in an ocean gcm abstract an ocean gcm interpreted in light of linear models and sparse observations is used to diagnose the dynamics of the annual cycle of circulation in the western boundary current system of the southwest pacific ocean the simple structure of annual wind stress curl over the south pacific produces a large region of uniformly phased stationary thermocline depth anomalies such that the western subtropical gyre spins up and down during the year directing flow anomalies alternately toward and away from the boundary at its northern end near 10 s the response of the western boundary currents is to redistribute these anomalies northward toward the equator and southward to the subtropical gyre a redistribution that is determined principally by linear rossby processes not boundary dynamics when the subtropical gyre and south equatorial current sec are strong in the second half of the year the result is both increased equatorward transport of the new guinea coastal current and poleward transport a

seismic tomography and earthquake locations in the nicaraguan and costa rican upper mantle the central american subduction zone exhibits large variations in geochemistry downgoing plate roughness and dip and volcano locations over a short distance along the arc results from joint inversions for vp vp vs and hypocenters from the tomography under costa rica and nicaragua tucan experiment give insight into its geometry and structure in both costa rica and nicaragua the intermediate depth seismic zone is a single layer no more than 10 to 20 km thick tomographic images show that throughout nicaragua and costa rica the slowest mantle p wave velocities appear below and behind the volcanic front indicating likely zones of highest temperature extending 80 to 120 km depth a sheet of high vp vs thought to be caused by melt is imaged directly beneath the nicaraguan volcanoes whereas a weaker broader anomaly is imaged beneath the costa rican volcanoes potentially indicating a greater extent of melting beneath nicaragua within the downgoing plate anomalously low velocities occur at least 20 30 km below wadati benioff zone seismicity to depths of 140 km beneath nicaragua and to 60 km depth beneath costa rica they indicate 10 20 serpentinized upper mantle of the downgoing plate beneath nicaragua similar to that inferred from refraction seaward of the trench but continuing to subarc depths this unusually hydrated lithosphere may introduce more water into the nicaraguan mantle initiating increased amount of melting and fluid flux to the arc

identifying the crystal graveyards remaining after large silicic eruptions abstract the formation of crystal poor high silica rhyolite via extraction of interstitial melt from an upper crustal mush predicts the complementary formation of large amounts of typically unerupted silicic cumulates however identification of these cumulates remains controversial one hindrance to our ability to identify them is a lack of clear predictions for complementary chemical signatures between extracted melts and their residues to address this discrepancy we present a generalized geochemical model tracking the evolution of trace elements in a magma reservoir concurrently experiencing crystallization and extraction of interstitial melt our method uses a numerical solution rather than analytical thereby allowing for various dependencies between crystallinity partition coefficients for variably compatible and or incompatible elements and melt extraction efficiency results reveal unambiguous fractionation signatures for the extracted melts while those signatures are muted for their cumulate counterparts our model is first applied to a well constrained example searchlight pluton usa and provides a good fit to geochemical data we then extrapolate our results to understanding the relationship between volcanic and plutonic silicic suites on a global scale utilizing the navdat database to identify crystal accumulation or depletion signatures for each suite we suggest that many large granitoids are indeed silicic cumulates although their crystal accumulation signature is expected to be subtle

sediment accretion subduction and dewatering at the base of the trench slope off costa rica a seismic reflection view of the decollement sea beam bathymetric and water gun seismic reflection data off the west coast of costa rica image the base of the trench slope its underlying structure and the behavior of a decollement surface in the presence of relief on the subducting plate a low angle landward dipping reflection marks the master decollement above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope a small horst block at least 9 km long 1 5 km wide and 300 m high is being obliquely subducted beneath the landward trench slope on five seismic lines the decollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level the undulating geometry of the decollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front the dewatering may favor development of bedding parallel zones of low shear strength within the subducting sediments and the master decollement may seek one of these zones

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

relation between subduction megathrust earthquakes trench sediment thickness and upper plate strain 1 giant earthquake moment magnitude mw 8 5 forecasts for subduction zones have been empirically related to both tectonic stresses and geometrical irregularities along the subduction interface both of these controls have been suggested as able to tune the ability of rupture to propagate laterally and in turn exert an important control on giant earthquake generation here we test these hypotheses and their combined influence by compiling a dataset of trench fill thickness a proxy for smoothing of subducting plate relief by sediment input into the subduction channel and upper plate strain a proxy for the tectonic stresses applied to the subduction interface for 44 segments of the global subduction network we statistically compare relationships between upper plate strain trench sediment thickness and maximal earthquake magnitude we find that the combination of both large trench fill 1 km and neutral upper plate strain explains spatial patterns of giant earthquake occurrence to a statistically significant degree in fact the concert of these two factors is more highly correlated with giant earthquake occurrence than either factor on its own less frequent giant earthquakes of lower magnitude are also possible at subduction zones with thinner trench fill and compressive upper plate strain extensional upper plate strain and trench fill 0 5 km appear to be unfavorable conditions as giant earthquakes have not been observed in these geodynamical environments during the last 111 years

magma emplacement differentiation and cooling in the middle crust integrated zircon geochronological geochemical constraints from the bergell intrusion central alps abstract u th pb zircon geochronology is an essential tool for quantifying the emplacement differentiation and thermal evolution of crustal magmatic systems however the power of u pb zircon dates can be enhanced through complementary characterization of mineral texture and geochemistry as this permits more detailed interpretations of geochronological datasets than conventionally achieved our approach to better relating zircon dates and geological processes consists of a multi method analytical workflow including cathodoluminescence imaging cl in situ la icpms epma zircon geochemistry u pb zircon id tims geochronology and solution icpms zircon trace element analysis u pb tims tea these methods are here applied to zircon from the bergell intrusion a composite alpine pluton preserving a 10 km mid crustal transect hand samples of tonalite granodiorite and hybridized granitoid each record 250 700 kyr of autocrystic zircon growth bergell zircons are ubiquitously zoned with ca 10 4 10 6 yr growth histories as evidenced by id tims analysis of microsampled fragments from single crystals u pb tims tea data exhibit compositional trends on multiple spatiotemporal scales including the handsample scale representing in situ differentiation at the emplacement level e g th u lithology scale defining trajectories corresponding to the production of tonalitic versus granodioritic magmas lu hf and pluton scale indicating increasingly evolved melts over 1 6 myr of pluton assembly zr hf these absolute tims tea temporal trends are corroborated by relative la icpms epma core to rim geochemistry we compare records of trace element evolution from tims tea bergell whole rock geochemistry and a global compilation of whole rock geochemical data these findings support zircon compositional evolution as a robust indicator of differentiation at local and crustal scales and provide key empirical constraints on melt differentiation and cooling timescales in the middle crust

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment

evolution of the nankai trough decollement from the trench into the seismogenic zone inferences from three dimensional seismic reflection imaging we mapped the amplitude of the nankai trough subduction thrust seismic reflection from the trench into the seismogenic zone with three dimensional seismic reflection data the decollement thrust forms within the lithologically homogeneous lower shikoku basin facies along an initially nonreflective interface the reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and rapid loading and delayed consolidation of the underthrust section a decollement amplitude map shows a significant decline from high amplitudes at the trench to barely detectable levels 25 30 km landward three other observations coincide with the amplitude decline 1 the decollement initially steps down to deeper stratigraphic levels 2 the wedge taper increases dramatically and 3 the thrust becomes seismogenic the amplitude decline and the coincident decollement and accretionary wedge tectonic and seismogenic behavior are attributed to the loss of fluids and potentially loss of excess fluid pressures downdip along the subduction thrust

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

high silica granites terminal porosity and crystal settling in shallow magma chambers abstract high silica 70 wt sio2 granites hsgs are important carriers of highly incompatible elements thus understanding their origin is relevant to understanding how the composition of the continental crust evolves we examined a large scale geochemical study of plutons in the peninsular ranges batholith in southern california usa to better understand the petrogenetic relationships between hsgs and the batholith using highly incompatible and compatible elements we show that hsgs represent residual liquids within a felsic 69 72 wt sio2 magmatic crystal mush at crystal fractions of 50 60 and residual liquid fractions of 40 50 trace element systematics show that separation of the hsg liquid from the crystal mush is inefficient such that no more than 70 80 of the hsg is fully extracted and the remaining greater than 20 30 remains trapped in cumulate mush we find little evidence of more efficient liquid crystal segregation which suggests that compaction induced segregation may be too slow to be important on a large scale instead the terminal porosity of 20 30 coincides with theoretical maximum packing fraction of unimodal particles settled out of suspension 0 74 which may indicate that crystal settling perhaps in the form of hindered settling drives segregation of viscous silicic melts and crystals unlike compaction settling operates on timescales of 1 10 ky fast enough to generate large volumes of hsg and complementary cumulates with trapped melt before magma chambers freeze many felsic plutons may thus be cumulates but because of trapped melt they are difficult to geochemically distinguish from plutons whose compositions fall along liquid lines of descent the approach here using a combination of highly incompatible and compatible elements provides a way of identifying and quantifying trapped melt fractions finally we show that hsgs appear to form only in the shallow crust

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

experimental constraints on rhyolite melts and the late bishop tuff magma body thermodynamic models are vital tools to evaluate magma crystallization and storage conditions before their results can be used independently however they must be verified with controlled experimental data here we use a set of hydrothermal experiments on the late erupted bishop tuff lbt magma to evaluate the rhyolite melts thermodynamic model a modified calibration of the original melts model optimized for crystallization of silicic magmas experimental results that are well captured by rhyolite melts include a relatively narrow temperature range separating the crystallization of the first felsic mineral and the onset of the ternary minimum quartz plus two feldspars and extensive crystallization over a narrow temperature range once the ternary minimum is reached the model overestimates temperatures by 40 c a known limitation of rhyolite melts at pressures below 110 mpa model and experiments differ in the first felsic phase suggesting that caution should be exercised when applying the model to very low pressures our results indicate that for quartz sanidine plagioclase magnetite and ilmenite to crystallize in equilibrium from lbt magma magma must have been stored at 740 c even when a substantial amount of co2 occurs in the coexisting fluid such temperatures are in conflict with the hotter temperatures retrieved from magnetite ilmenite compositions 785 c for the sample used in the experiments consistent with other recent studies we suggest that the fe ti oxide phases in the late bishop tuff magma body are not in equilibrium with the other minerals and thus the retrieved temperature and oxygen fugacity do not reflect pre eruptive storage conditions

tertiary tectonic history of the southern andes the subvolcanic sequence to the tatara san pedro volcanic complex lat 36 s the tatara san pedro volcanic complex a quaternary to holocene composite volcano in central chile is underlain by late miocene 6 ma plutons and tertiary metavolcanic rocks the plutons were intruded into greenschist facies metavolcanic rocks at a depth of about 4 5 km together these rocks reveal important information about the development of continental arcs in some cases apparent eruptive ages of metavolcanic rocks are younger than ages of intruding plutons indicating that there was 1 pervasive reheating of metavolcanic rocks or 2 that metavolcanic rocks were locally derived and intruded by their own magma chambers plutons we have estimated uplift or denudation rates of nearly 1 mm yr since late miocene time normal faulting currently dominates the brittle behavior of the upper crust therefore uplift is probably related to magmatic addition to the crust rather than to compression and shortening although plutonic and metavolcanic rocks display expected arc geochemical affinities the isotopic similarity of metavolcanic and plutonic rocks to quaternary volcanic rocks is strong this makes it difficult to use subvolcanic rocks as upper crustal contaminants to evaluate the igneous evolution of the quaternary deposits most plutonic metavolcanic and quaternary volcanic rocks exhibit the isotopic characteristics of juvenile crust some samples however exhibit sparse but unmistakable evidence 87 sr 86 sr and 40 ar 39 ar isotopic data for the presence of older probably precambrian through triassic radiogenic crust beneath the volcano crustal structure in this arc can be described by a three layer model in which a central section of precambrian through triassic rock is underplated by juvenile material is intruded by juvenile material or has juvenile material transported through it to be emplaced at or near the surface continued erosion of juvenile material at the surface may be balanced to some degree by continued volcanism and epizonal plutonism

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

structural criteria for identifying granitic cumulates abstractcumulates are igneous rocks that reflect relative concentration of crystals and or loss of melt and that therefore did not crystallize entirely from a magma of their current whole rock composition however this definition does not help to identify cumulates and so the problem is to determine whether the current whole rock composition reflects cumulate processes which is where structural criteria can be useful identification of cumulates in granitoids can be difficult especially in the absence of major whole rock compositional differences between cumulate and host structural and structural mineralogical criteria diagnostic of or at least consistent with granitic cumulates include 1 locally high concentrations of particular minerals relative to concentrations in the bulk of the intrusion 2 abundant euhedral crystals in contact at least with regard to cores that touched before any overgrowths and consequent molding of minerals about each other occurred 3 abundant antecrysts or mantle

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

a review of shallow ore related granites textures volatiles and ore metals shallow texturally complex granites can be associated spatially introduction and temporally with hydrothermal alteration and mineralization granite related ore deposits include porphyry and skarn the formation of a granite related ore deposit represents the confluence type ores and other hydrothermal deposits that are of many chemical and physical geological factors including magma spatially and temporally associated with granitic sensu composition e g concentrations of ore metals chlorine and water lato rocks in this paper i emphasize characteristics magma oxidation state the relative timing of crystallization magma found in porphyry style cu mo au deposits and some ascent magmatic volatile phase exsolution and the depth of em related systems however my treatment here is not meant placement early magmatic crystallization of ore metal rich minerals to cover the full range of granitic rocks and mineralization coupled with late exsolution of the magmatic volatile phase is that make up the granite related ore systems the forpromoted by deeper emplacement levels and lower magmatic water mation of granite related ore deposits by orthomagmatic concentrations and leads to low eyciencies of removal of compatible hydrothermal processes is dependent upon many physical elements from melts into the magmatic volatile phase s water and chemical magmatic factors including but not limited concentrations in the melt and pressures of crystallization can be to the composition and evolution of the associated estimated from phase equilibria and textural relationships of horn magmas the rate of magma ascent the depth and blende in some felsic systems physical factors avect the dynamics geometry of magma emplacement and the local temof fluid magma melt and volatile phase movement which in turn perature and stress fields at the site of magma emavect the size of zones of alteration and mineralization granite placement detailed studies of the igneous rocks textures can record the partial history of some dynamic processes associated with ore deposits can therefore yield critical miarolitic cavities are good evidence for magmatic volatile phase information regarding the source of metals and ligands exsolution both miarolitic cavities and pegmatitic texture exhibit in the ore generative hydrothermal solutions that are evidence for external nucleation of crystals nucleation controlled operative in the formation of granite related ores in this phenomena expressed as variations in granite textures can result review i will present a series of narratives consistent in from a multiplicity of causes including undercooling of the melt my view with prevailing field and experimental data on textural elements such as pegmatite aplite skeletal crystals and the orthomagmatic hydrothermal theory for the genesis graphic textures can form as a result of undercooling interconnected of some granite related deposits volumes of the magmatic volatile phase allow for advection of some high level granites i e those that are emplaced hydrothermal ore fluids through the magma in shallow granitic at depths of 8 km or less represent the crystallized systems miarolitic cavities can be interconnected consistent with remains of magmas that were associated with subvolcanic this hypothesis ore generative hydrothermal systems therefore studies of shallow granites aimed at understanding magmatic evolution and volatile phase exsolution at shallow levels are necessary if we are to understand ore genesis and the deep inputs to present day geothermal systems as well as evaluate hazards associated with the volcanic

coupled stratigraphic and structural evolution of a glaciated orogenic wedge offshore st elias orogen alaska 1 the st elias orogen is the result of 10 myr of oblique convergence and flat slab subduction in the gulf of alaska between north america and the yakutat microplate extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge reflection seismic profiles across the offshore pamplona zone fold thrust belt the frontal st elias orogenic wedge provide constraints for quantifying pleistocene deformation recorded in the glaciomarine yakataga formation the total amount of pleistocene shortening observed varies from 3 to 5 mm yr compared to the current gps derived yakutat north america convergence rate across the st elias orogen of 45 mm yr growth strata and kinematic fold analysis allow comparison of relative timing of fault activity which reveals temporal and spatial shifting of active deformation during the glacial period faulting localized adjacent to the coastline and at the current submarine deformation front the abandoned currently inactive region is colocated with the major glacial depocenter in the region the bering trough these observations imply that glacial processes such as sediment loading and focused erosion during advance retreat cycles has a direct effect on the evolution of individual faults within the pamplona zone and the overall deformation pattern in the offshore st elias margin this information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during pleistocene glacial interglacial periods

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

tracking crystal melt segregation and magma recharge using zircon trace element data abstract the cretaceous yunshan caldera complex in se china consists of an unusual coexisting assemblage of peraluminous and peralkaline rhyolites and a resurgent intra caldera porphyritic quartz monzonite in this study we use zircon trace element data to study the compositional differences of zircons from cogenetic magmas and to track the evolution of the entire magmatic system our results indicate that the zircons from the peraluminous and peralkaline rhyolites formed from highly evolved compositions with high hf concentrations and low ti contents and low th u and zr hf ratios which are distinct from those of the intrusive porphyritic quartz monzonite zircons from the peraluminous and peralkaline rhyolites display overlapping zr hf and hf but the zircons from the peralkaline rhyolites have extremely low eu eu ratios

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

bending stress and dissipation in subducted lithosphere 1 oceanic lithosphere undergoes permanent deformation during subduction once the stresses exceed the elastic limit departures from elastic behavior occur by brittle failure in the shallow lithosphere and by a combination of low and high temperature creep at greater depths we combine laboratory based rheological models with estimates of slab shape from earthquake hypocenters to quantify the bending stress and dissipation in subduction zones the peak stress occurs at the depth of the brittle ductile transition which is controlled mainly by lithospheric age integrals of the stress over the thickness of the plate are used to evaluate the resistive bending force and the bending moment a representative value for the resistive force on old oceanic lithosphere is 3 1012 n m 1 which is comparable in magnitude to ridge push but opposite in direction both the bending force and moment are remarkably insensitive to the rate of subduction in fact the bending moment can be approximated using a simple power law rheology and a stress exponent of n 14 such a large exponent implies that the lithosphere behaves like a perfectly plastic solid for most subduction zones the bending moment saturates along the entire plate as a consequence the bending stress does not influence the development of curvature during subduction this behavior may explain why the curvature of subducted lithosphere is nearly independent of age