observing earthquakes triggered in the near field by dynamic deformations we examine the hypothesis that dynamic deformations associated with seismic waves trigger earthquakes in many tectonic environments our analysis focuses on seismicity at close range within the aftershock zone complementing published studies of long range triggering our results suggest that dynamic triggering is not confined to remote distances or to geothermal and volcanic regions long unilaterally propagating ruptures may focus radiated dynamic deformations in the propagation direction therefore we expect seismicity triggered dynamically by a directive rupture to occur asymmetrically with a majority of triggered earthquakes in the direction of rupture propagation bilaterally propagating ruptures also may be directive and we propose simple criteria for assessing their directivity we compare the inferred rupture direction and observed seismicity rate change following 15 earthquakes m 5 7 to m 8 1 that occurred in california and idaho in the united states the gulf of aqaba syria guatemala china new guinea turkey japan mexico and antarctica nine of these mainshocks had clearly directive unilateral ruptures of these nine seven apparently induced an asymmetric increase in seismicity rate that correlates with the rupture direction the two exceptions include an earthquake preceded by a comparable magnitude event on a conjugate fault and another for which data limitations prohibited conclusive results similar but weaker correlations were found for the bilaterally rupturing earthquakes we studied although the static stress change also may trigger seismicity it and the seismicity it triggers are expected to be similarly asymmetric only if the final slip is skewed toward the rupture terminus for several of the directive earthquakes we suggest that the seismicity rate change correlates better with the dynamic stress field than the static stress change manuscript received 1 march 2002

geochemical evolution of peraluminous plutons in southern nova scotia canada a pegmatite poor suite three southern nova scotia plutons crystallized rapidly at 375 ma from magma containing both mantle and crustal components isotopic and chemical data suggest that the crustal contribution included both lower crustal material and cambro ordovician turbidites of the host meguma group despite local evidence of mixing and mingling of magmas the bulk of the plutons evolved by assimilation and fractional crystallisation evolved portions of the plutons have compositions appropriate for development of rare metal pegmatite fields but pegmatites are relatively rare and little differentiated like parental plutons pegmatites fall into biotite plagioclase and muscovite potassium feldspar assemblages the latter locally contain mn rich garnet biotite giving calculated p t conditions of pegmatite crystallisation of 620 c 0 44 gpa under water saturated conditions host rocks at the time of emplacement experienced p t conditions varying from 500 to 620 c at 0 44 gpa beryl bearing pegmatites occur only where host rocks were below sillimanite grade and pegmatites of any kind are scarce in sillimanite grade host rocks rarity of pegmatites can be ascribed to a combination of insufficient amounts of f li and b in the magma with rapid cooling which prevented extensive fractionation and undercooling our data suggest rather that peraluminous suites produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks and typified by biotite rich cordierite bearing plutons are not favorable locales for major rare metal enriched pegmatite fields such fields appear to require relatively prolonged anatexis resulting from crustal thickening or an areally extensive mafic underplate

effects of variable thermal conductivity on the mineralogy of subducting slabs and implications for mechanisms of deep earthquakes abstract thermal and thermo kinetic models of subduction zones have been based in general on a constant value of thermal conductivity e g 3 138 w m k for solving the conservation of energy equation in order to make such models more realistic we have used variable thermal conductivities k that depend on pressure temperature and mineralogy olivine wadsleyite or ringwoodite as determined recently by xu et al xu y shankland t j linhardt s rubie d c langenhorst f klasinski k 2004 thermal diffusivity and conductivity of olivine wadsleyite and ringwoodite to 20 gpa and 1373 k phys earth planet inter 143 144 321 336 we have used these variable conductivities to model a variety of slabs with thermal parameters vertical subduction rate age of lithosphere at the trench ranging between 3500 and 17 000 km our models show that when radiative transfer is included k could plausibly be increased by as much as 2 w m k relative to a standard constant lithospheric value 3 138 w m k stein c a stein s 1992 a model for the global variation in oceanic depth and heat flow with lithospheric age nature 359 123 129 causing temperatures in the interiors of the slabs to increase by 50 100 c in turn the volume of metastable olivine in the slabs is reduced by as much as 20 30 with the maximum depth of olivine metastability decreasing by 30 50 km in general the maximum depths of olivine metastability are 160 230 km less than the depths of the deepest seismicity even in the extreme case of tonga metastable olivine falls short of the deepest earthquakes by 140 km in our model even taking the various uncertainties into account these results indicate that deep focus earthquakes occur in regions already transformed to wadsleyite or ringwoodite and must therefore be caused by a mechanism other than transformational faulting

source characteristics of large deep earthquakes constraint on the faulting mechanism at great depths 1 source parameters for eight large deep earthquakes are inferred from broadband data of the global seismograph network we completed an unprecedented data set composed of 14 largest deep earthquakes mw ranging from 7 0 to 8 3 from six different subduction zones studied with identical source parameter inversion methods the study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate events in cold slabs show high aftershock activity high rupture velocity and high seismic efficiency in contrast earthquakes in warm slabs generally have low aftershock productivity low rupture velocity and low seismic efficiency however the 23 june 1991 argentina event has source properties that are rather characteristic for earthquakes in cold slabs and that differ from those of other large south american events we suggest that two types of deep earthquakes occur in south america and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes in this model shear failure occurs through a thermal runaway mechanism within a shear zone at great depth and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production the 1991 argentina event located in a pocket of deep seismicity within the slab core may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones whereas the other known south american large deep events such as 1963 peru 1970 colombia and 1994 bolivia are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab

dynamic triggering of deep earthquakes within a fossil slab the 9 november 2009 mw 7 3 fiji deep earthquake is the largest event in a region west of the tonga slab defined by scattered seismicity and velocity anomalies the mainshock rupture was compact but the aftershocks were distributed along a linear feature at distances of up to 126 km the aftershocks and some background seismicity define a sharp northern boundary to the zone of outboard earthquakes extending westward towards the vitiaz deep earthquake cluster the northern earthquake lineament is geometrically similar to tectonic reconstructions of the relict vitiaz subduction zone at 8 10 ma suggesting the earthquakes are occurring in the final portion of the slab subducted at the now inactive vitiaz trench a coulomb stress change calculation suggests many of the aftershocks were dynamically triggered we propose that fossil slabs contain material that is too warm for earthquake nucleation but may be near the critical stress susceptible to dynamic triggering

nature and evolution of the eastern margin of lapetus geochemical and isotopic constraints from siluro devonian granitoid plutons in the new brunswick appalachians in the gander zone ordovician to devonian metaluminous to slightly peraluminous plutons with within plate affinities intrude a lower paleozoic metasedimentary package during the siluro devonian end t and 18o values of the metasedimentary rocks would have been about 7 to 8 and 10 to 14 respectively granitoid end t values range from 0 3 to 4 2 18o values lie between 6 6 and 10 4 and published pb isotopic compositions are radiogenic 207pb 204pb 15 65 end t values of associated silurian gabbros are somewhat positive 1 2 to 2 3 some plutons exhibit evidence for comingling with this mafic component potential source materials include enriched mantle supracrustal and infracrustal sources although some constraints can be put on the first two sources the nature of infracrustal material is difficult to evaluate heterogeneous data obtained from silurian plutons can be explained by crust mantle mixing if some plutons contain a significant mantle component then mesoproterozoic o

remote triggering of deep earthquakes in the 2002 tonga sequences it is well established that an earthquake in the earth s crust can trigger subsequent earthquakes but such triggering has not been documented for deeper earthquakes models for shallow fault interactions suggest that static permanent stress changes can trigger nearby earthquakes within a few fault lengths from the causative earthquake1 2 3 whereas dynamic transient stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances4 5 6 7 8 here we present a detailed analysis of the 19 august 2002 tonga deep earthquake sequences and show evidence for both static and dynamic triggering seven minutes after a magnitude 7 6 earthquake occurred at a depth of 598 km a magnitude 7 7 earthquake 664 km depth occurred 300 km away in a previously aseismic region we found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock but the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible thus suggesting dynamic triggering the origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone we propose that these events are triggered by transient effects in regions near criticality but where earthquakes have difficulty nucleating without external influences

hydrogen and oxygen isotope geochemistry of a type granites in the continental margins of eastern china hydrogen and oxygen isotope studies of both whole rock and mineral separates were carried out for five representative atype granites in eastern china from north to south the whole rock d d and d 18 o values vary from 2145 to 282 and from 21 2 to 5 0 for nianzishan pluton from 2135 to 2110 and from 1 6 to 6 7 for shanhaiguan pluton and from 2145 to 2111 and from 1 5 to 9 1 for laoshan massif respectively the highest whole rock dd and d 18 o values are observed in suzhou pluton and range from 281 to 259 and from 3 5 to 9 2 respectively homogeneous d d and d 18 o values are found in kuiqi massif ranging from 2113 to 299 and from 5 6 to 8 5 respectively the results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution such as magma degassing fractional crystallization as well as post magmatic alteration by local meteoric water under subsolidus conditions the initial d d and d 18 o values of the a type granites in eastern china are constrained to be about 250 5 and 7 5 1 0 respectively this is consistent with mantle like protoliths for granitoid rocks and thus precludes the possibility that the a type granites is derived from the anatexis of the 18 o rich upper crust however recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the a type granitic magmas the variation in the ddepletion of the granites with latitude implies that the paleogeographic localities of the a type granites have not changed significantly since the mesozoic q 2000 elsevier science b v all rights reserved

global large deep focus earthquakes source process and cascading failure of shear instability as a unified physical mechanism abstract we apply a multiple source inversion method to systematically study the source processes of 25 large deep focus depth 400 km earthquakes with mw 7 0 from 1994 to 2012 based on waveform modeling of p pp sh and ssh wave data the earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub events 1 category one with non planar distribution and variable focal mechanisms of sub events represented by the 1994 mw 8 2 bolivia earthquake and the 2013 mw 8 3 okhotsk earthquake 2 category two with planar distribution but focal mechanisms inconsistent with the plane including eighteen earthquakes and 3 category three with planar distribution and focal mechanisms consistent with the plane including six earthquakes we discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture transformational faulting and shear thermal instability we suggest that the inferred source processes of large deep focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre existing weak zones with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub events controlled by orientations of the pre existing weak zones the proposed mechanism can also explain the observed great variability of focal mechanisms the presence of large values of clvd compensated linear vector dipole and the super shear rupture of deep focus earthquakes in the previous studies in addition our studies suggest existence of relationships of seismic moment source duration 3 and moment source dimension 3 in large deep focus earthquakes

gutenberg richter law for deep earthquakes revisited a dual mechanism hypothesis deep earthquake b values appear to vary with slab thermal state and earthquake magnitude the physical reason for the variations and the relation with deep rupture mechanisms are still unclear here i confirm the spatial variations of b value and the dependence on slab temperature using about 40 yr of data from the global centroid moment tensor catalog a new bimodal pattern is observed for the 500 700 km depth range b is close to 1 in the cold tonga slab while in warmer slabs e g south america japan kuril izu bonin mariana b is close to 0 5 for intermediate magnitudes m w 5 3 6 5 and increases to 1 for large magnitudes m w 6 5 to explain these observations i propose a dual mechanism hypothesis in which deep earthquakes nucleate only within the metastable olivine wedge mow but can rupture outside mow by a different mechanism the fractal dimension of earthquake size distribution changes from 2 to 1 as the thermally controlled mow thickness decreases and back to 2 as the mechanism outside mow dominates

geochemical and isotopic characteristics of granitoids of the avalon zone southern new brunswick possible evidence for repeated delamination events nd o and pb isotope data from southern new brunswick suggest that 625 to 360 ma granitoid plutons contain a major juvenile component and small amounts of middle proterozoic or older crustal material plutons fall into four major age groups 600 630 540 560 422 430 and 367 396 ma the first and third groups can be related to subduction in other parts of the appalachian orogen but most of the second and fourth magmatic pulses have lithologic associations and chemistry not readily reconciled to subduction the timing and volume of magmatism presence of mafic end members and juvenile signatures in the plutons suggest that crustal delamination may have been involved little evidence exists for extensive middle proterozoic or older continental crust beneath the avalon zone which consists mainly of late proterozoic continental margin arc material intruded by younger granitoid plutons that include a major mantle component this differs from contiguous gander and meguma zones in which plutons contain a ma

a cascading failure during the 24 may 2013 great okhotsk deep earthquake on 24 may 2013 the largest ever recorded deep earthquake occurred beneath sea of okhotsk a multiple point source inversion procedure is applied to constrain source process of this earthquake based on waveform modeling of both direct p and sh waves and near surface reflected pp and ssh waves our results indicate that the earthquake consists of six major subevents separated in space and time encompassing a horizontal dimension of 64 4 km along n160 e and a downward depth extension of 35 4 km the geographic distribution and focal mechanisms of the inferred subevents and foreshock aftershock locations do not fit into plane rupture we suggest that the earthquake can be best explained by a cascading failure of shear instability within preexisting weak zones in the region with the perturbation of stress generated by a shear instability triggering another

geochemical and geochronological studies of granitoid rocks from the western tianshan orogen implications for continental growth in the southwestern central asian orogenic belt abstract the western tianshan orogen in northwestern china is part of the southwestern margin of the central asian orogenic belt caob which represents the largest tectonic assembly of accretionary complexes and the most significant area of phanerozoic continental growth in the world granitoid rocks have a widespread occurrence and occupy about 30 of the whole exposure of the orogen ages obtained for the granitoids vary from 896 ma to 247 ma while initial 87 sr 86 sr ratios vary from 0 70329 to 0 72070 and e nd values from 14 1 to 7 3 the occurrence of neoproterozoic granitic gneisses implies the presence of precambrian basement in this area during oceanic subduction continental growth occurred as a result of several combined processes i e by addition of oceanic crustal melts adakites the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement however during the post collisional period vertical accretion of underplated juvenile mantle material may have been accomplished during slab breakoff delamination this study suggests a two stage model of continental growth of syn subduction lateral accretion of arc complexes and post collisional vertical accretion of underplated mantle material which may also be relevant for the western segment of the caob in the phanerozoic the reworking and involvement of old crust are more obvious in the western tianshan orogen compared to other parts of the caob comprising the western junggar eastern junggar alataw and altay terranes

global ubiquity of dynamic earthquake triggering surface waves that were generated by 12 out of 15 earthquakes with magnitudes greater than 7 since 1990 led to a global increase in the number of small earthquakes this suggests that dynamic triggering of earthquakes is common and is independent of the tectonic environment earthquakes can be triggered by local changes in the stress field static triggering1 2 3 4 5 6 7 due to nearby earthquakes or by stresses caused by the passage of surface rayleigh and love waves from a remote large earthquake dynamic triggering8 9 10 11 12 13 14 15 16 17 18 however the mechanism frequency controlling factors and the global extent of dynamic triggering are yet to be fully understood because rayleigh waves involve compressional and dilatational particle motion volumetric changes as well as shearing whereas love waves only involve shearing triggering by either wave type implies fundamentally different physical mechanisms here we analyse broadband seismograms from over 500 globally distributed stations and use an automated approach to systematically identify small triggered earthquakes the low amplitude signals of such earthquakes would normally be masked by high amplitude surface waves our analysis reveals that out of 15 earthquakes studied of magnitude m greater than 7 0 that occurred after 1990 12 are associated with significant increases in the detection of smaller earthquakes during the passage of both the love and rayleigh waves we conclude that dynamic triggering is a ubiquitous phenomenon that is independent of the tectonic environment of the main earthquake or the triggered event

the position and nature of the gander avalon boundary southern new brunswick based on geochemical and isotopic data from granitoid rocks in southern new brunswick the gander avalon boundary is obscured by boundary parallel faults and various cover sequences siluro devonian granites which intrude unequivocal gander or avalon rocks display exclusively negative 1 9 1 0 and positive 1 9 0 7 end t signatures respectively such contrasting nd isotopic signatures combined with other geochemical differences between plutons are potentially valuable tools for terrane analysis nine small devonian plutons intruding the boundary zone fall into contrasting geochemical groups with la lu n 4 the former are topaz bearing granites while the latter are volcanic arc type granites except for one pluton with an end t signature of 2 0 end t values range from 0 4 to 0 7 spanning the gap between type avalon and type gander plutons these results suggest the plutons sampled either i stratigraphically overlapping or tectonically interleaved gander and avalon basement rocks or ii a distinct basement source beneath the b

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

the kingston terrane southern new brunswick canada evidence for an early silurian volcanic arc the fault bounded kingston terrane of southern new brunswick consists of felsic and less abundant mafic volcaniclastic rocks and flows and minor interbedded sedimentary rocks of the kingston group intruded by mainly granitic plutons and abundant younger mafic sheets the granitic plutons form about half of the terrane and are characterized by fine grain size and typically granophyric and porphyritic textures indicative of high level emplacement mafic sheets were emplaced in the granitic plutons and parallel to primary layering in the kingston group which is typically steeply dipping and trends northeast to north northeast hence in contrast to some earlier interpretations the kingston terrane does not consist of a bimodal dike complex the kingston group granitic rocks and mafic sheets contain mineral assemblages consistent with metamorphism to upper greenschist lower amphibolite facies new u pb zircon ages of 442 6 ma from a dacitic tuff unit and 437 10 ma from a granitic pluton corroborate previous u pb dates that had suggested that the volcanic and granitic rocks in the kingston terrane are early silurian rather than precambrian as had been earlier assumed the felsic volcanic and granitic rocks are interpreted to be comagmatic on the basis of their field relationships and similar u pb ages and chemical character compositions of both the felsic rocks and less abundant mafic and intermediate and mafic rocks are consistent with calc alkalic affinity and emplacement in a continental margin volcanic arc herein termed the kingston arc a u pb date of 435 5 ma was also obtained from compositionally similar granite that forms a dike in an adjacent fault bounded amphibolite facies metasedimentary unit the similarity in age and composition of the granite dike suggests a link between these metasedimentary rocks and the kingston terrane possibly as part of an accretionary complex if so its location southeast of the kingston terrane indicates that subduction was to the present northwest mafic sheets in the kingston terrane are tholeiitic but have some arc like characteristics consistent with their origin in a former arc setting but the sheets were likely emplaced in a subsequent extensional setting the compositional similarity of the mafic sheets to late silurian mafic volcanic rocks and plutons of the coastal maine magmatic province suggests that they may be related a possible model for evolution of the kingston terrane is proposed wherein the kingston group and related granite formed in an early silurian arc on the margin of ganderia as represented by the new river terrane of late precambrian and early paleozoic age northwestward subduction culminated in collision between the kingston arc and a continental block now represented by the brookville terrane to the southeast subsequent transpressive mo tions in the late silurian may have been responsible for the crustal scale extensional environment in which the coastal maine magmatic province was formed including emplacement of mafic dikes into the deformed former kingston arc transpressive motions related to juxtaposition of the avalon and meguma terranes with the previously accreted more inboard terranes continued through the devonian and into the carboniferous

body wave inversion of the 1970 and 1963 south american large deep focus earthquakes a comprehensive set of teleseismic waveforms from two south american deep focus earthquakes of the predigital era the 1970 colombia mw 8 1 and 1963 peru bolivia mw 7 7 events are inverted for source mechanism seismic moment rupture history and centroid depth the p and sh wave inversion of the colombia event confirms previous work indicating that rupture occurred on a plane that dips steeply west rupture direction paralleled the trend of the wadati benioff zone we decompose the source into subevents based on a source time function which shows two major moment release pulses separated by 20 s the first subevent is located near the initiation point at a depth of 630 km the main moment release was located 70 km to the southeast and 20 km shallower rupture subsequently propagated farther southeast the source time function has an initial subevent accounting for 30 of the moment release of the entire event whereas the long period centroid moment tensor cmt analysis russakoff et al 1997 has the initial subevent yielding 50 the high angle nodal plane rotated 15 clockwise during the rupture explaining the large compensated linear vector dipole clvd component inferred from cmt solutions individual subevents have large clvd and compressive isotropic components a full moment tensor inversion of the colombia and 1994 bolivia events suggests that the initial subevents might contain a large non double couple ndc component for the 1963 peru bolivia event using p waves rupture propagated nnw for a distance of 70 km parallel to the high angle nodal plane and the trend of the wadati benioff zone the focal mechanism changed dramatically after the second subevent causing a very large ndc component both events together with the 1994 bolivia earthquake have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km s between subevents with 2 0 km s on average for the entire event low seismic efficiencies and rupture velocities support a highly dissipative temperature dependent rupture mechanism for large deep focus south american earthquakes compared with events in cold subducting slabs

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

the cadomian orogeny in saxo thuringia germany geochemical and nd sr pb isotopic characterization of marginal basins with constraints to geotectonic setting and provenance abstract the cadomian basement and the cambro ordovician overstep sequence in saxo thuringia is characterized by clastic sedimentation from the late neoproterozoic to the ordovician magmatism in the avalonian cadomian arc preserved in saxo thuringia occurred between ca 570 and 540 ma peri gondwanan basin remnants with cadomian to early palaeozoic rocks are exposed as very low grade metamorphosed rocks in six areas schwarzburg anticline berga anticline doberlug syncline north saxon anticline lausitz anticline and elbe zone a hiatus in sedimentation between 540 and 530 ma cadomian unconformity is related to the cadomian orogeny a second gap in sedimentation occurred during the upper cambrian 500 to 490 ma and is documented by a disconformity between lower to middle cambrian rocks and overlying tremadocian sediments major and trace element signatures of the cadomian sediments reflect an active margin continental arc those of the ordovician sediments a passive margin the cambrian sediments have inherited the arc signature through the input of relatively unaltered cadomian detritus initial nd and pb isotope data from the six saxo thuringian areas demonstrate that there is no change in source area with time for each location but that there are minor contrasts among the locations 1 cadomian sediments from the lausitz anticline the doberlug syncline and the elbe zone have lower 207pb 204pb than all other areas 2 the core of the schwarzburg anticline which is overprinted by greenschist facies conditions and detached is isotopically heterogeneous one part of its metasedimentary units has less radiogenic nd than sediments from other low grade units of similar age in the same area 3 cadomian sediments from the schwarzburg anticline show an input of younger felsic crust 4 the rothstein group shows distinct input of young volcanic material also 5 cadomian sediments from the lausitz anticline the elbe zone and parts of the north saxon anticline are characterized by input from an old mafic crust nd isotope data of the remaining areas yield average crustal residence ages of the sediment source of 1 5 1 9 ga which suggests derivation from an old craton as found for other parts of the iberian armorican terrane collage similarly the pb isotope data of all areas indicate sediment provenance from an old craton the rapid change of lithologies from greywacke to quartzite from the late neoproterozoic cadomian basement to the ordovician does not reflect changes in sediment provenance but is essentially due to increased reworking of older sediments and old weathering crusts that formed during various hiatus of sedimentation this change in sediment maturity takes its chemical expression in lower overall trace element contents in the quartzite dilution effect by quartz and relative enrichment of some trace elements zr mree hree due to detrital zircon and garnet the rb sr systematics of the quartzites and one ordovician tuffite was disturbed most likely during the variscan orogeny which suggests a lithology controlled mobility of alkali and calc alkali elements by comparison with available data it seems unlikely that only nd tdm model ages are useful to distinguish between west african and amazonian provenance nd tdm model ages of 1 5 to 1 9 ga in combination with paleobiogeographic aspects age data from detrital zircon and palaeogeographic constraints especially through tillites of the saharan glaciation in the hirnantian strongly indicate a provenance of saxo thuringia from the west african craton

rare dynamic triggering of remote m 5 5 earthquakes from global catalog analysis probing the effects of a transient stress on the timing of an earthquake occurrence is necessary for understanding the remote interaction of large magnitude events global catalog data containing 35 years of m 5 5 earthquakes allow us to explore for periods of enhanced or suppressed seismic activity we consider 113 m 7 5 main shocks between 1977 and 2012 and focus on seismic activity on time scales from seconds to days following these main shocks we search for evidence of dynamic triggering of large magnitude events similar to the previously observed global increase during the first few days following the 2012 m8 6 indian ocean main shock we restrict the analysis to regions of elevated strain during the passage of surface waves using a threshold of 0 1 microstrain 3 kpa and a temporal window of 1 year we stack daily seismicity rate curves using the exclusion zone declustered m 5 5 catalog events in order to resolve deviations from the background rate our results do not indicate a significant change in activity for at least 10 days when considering the collective set of 113 main shocks and subsets at m8 0 and m8 5 thresholds the results also do not indicate immediate triggering of m 5 5 events we do find two instances of increased seismicity in the elevated strain region within 10 days these increases are subsequent to two main shocks the 1977 m8 3 and 2012 m8 6 both located in the indian ocean we conclude that a global change in m 5 5 earthquake rates following a transient stress from distant earthquakes is a rare occurrence

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

a new statistical test for static stress triggering application to the 1987 superstition hills earthquake sequence over the past several years many investigators have argued that static stress changes caused by large earthquakes influence the spatial and temporal distributions of subsequent regional seismicity with earthquakes occurring preferentially in areas of stress increase and reduced seismicity where stress decreases some workers have developed quantitative methods to test for the existence of such static stress triggering but no firm consensus has yet been reached as to the significance of these effects we have developed a new test for static stress triggering in which we compute the change in coulomb stress on the focal mechanism nodal planes of a set of events spanning the occurrence of a large earthquake we compare the statistical distributions of these stress changes for events before and after the mainshock to decide if we can reject the hypothesis that these distributions are the same such rejection would be evidence for stress triggering we have applied this test to the november 24 1987 elmore ranch superstition hills earthquake sequence and find that those post mainshock events that experienced stress increases of at least 0 01 0 03 mpa 0 1 0 3 bar or that occurred from 1 4 to 2 8 years after the mainshocks are consistent with having been triggered by mainshock generated static stress changes

analysis of peak ground motion in terms of a model of inhomogeneous faulting measurements of peak ground velocity v and acceleration a for earthquakes and mine tremors with local magnitudes ranging from 0 8 to 6 4 have been analyzed in terms of a model of inhomogeneous faulting the fault model involves the failure of a circular asperity of radius ri surrounded by a previously faulted annular region of outer radius r0 the failure of the asperity results in seismic radiation with a characteristic frequency proportional to 1 ri followed by lower frequency radiation proportional to 1 r0 as static equilibrium is regained over the larger region for r0 ri 1 both v and a are associated primarily with the failure of the asperity but also depend on the large scale source parameters specifically v r0 r 0 10 r0 ri 0 15 and a r 0 30 r0 ri 2 0 45 where is the shear wave velocity is the overall stress drop is the modulus of rigidity and is density the terms involving r0 ri correspond to the high frequency radiation associated with the failure of the asperity and the other terms indicate the peak parameters due to the broad scale readjustment observations of peak ground motion for events with seismic moments ranging from 5 1016 to 1026 dyn cm indicate that r0 ri is normally in the range of 1 10 and appears to be independent of earthquake size the inhomogeneous fault model also yields convenient expressions for the small scale displacement and stress drop of the asperity failure as well as for the level of regional stress available to cause slip

the march 9 1994 m w 7 6 deep tonga earthquake rupture outside the seismically active slab we investigate the rupture process of the march 9 1994 m w 7 6 deep tonga earthquake and its relationship to the background seismicity of the subducted tonga slab variations in observed p and s wave pulse duration indicate that the rupture propagated to the nne and extended well beyond the background seismicity we inverted 47 p and sh waveforms including regional broadband waveforms from the southwest pacific seismic experiment using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane the results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10 20 km beyond the bounds of the previous seismic activity a significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region rupture along the near vertical nne striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near horizontal nodal plane this result combined with the planar alignment of aftershocks and the general nne directivity of the waveforms provides strong evidence that the rupture occurred on the near vertical plane thermal modeling of the tonga slab indicates that the rupture terminated in material about 200 c warmer than the temperature that normally limits the occurrence of smaller earthquakes additionally aftershocks seem to be suppressed in the outer regions of the rupture which contain about half of the moment release but only 1 of the 15 well located aftershocks we suggest that slabs may be composed of an inner cold core where seismic rupture initiates and small earthquakes occur and a thermal halo of warmer material which can sustain rupture and only a few aftershocks the mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core nucleation may occur through a process limited to the cold core such as transformational faulting and propagation through the warmer material may occur through ductile faulting or plastic instabilities isolated deep earthquakes in other subduction zones such as the 1994 bolivia event may occur almost completely within the warmer zone accounting for the lack of background seismicity and the dearth of aftershocks

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

tectonic setting of avalonian volcanic and plutonic rocks in the caledonian highlands southern new brunswick canada the caledonian highlands of southern new brunswick consist of late proterozoic to cambrian rocks generally considered typical of the avalon terrane of the northern appalachian orogen mainly tuffaceous volcanic and sedimentary rocks of the broad river group and cogenetic dioritic to granitic plutons with ages ca 620 ma form most of the eastern caledonian highlands they have petrological features indicative of origin in a continental margin subduction zone significantly younger ca 560 550 ma dacitic to rhyolitic lapilli tuffs and flows laminated tuffaceous siltstone basaltic and rhyolitic flows and clastic sedimentary rocks of the coldbrook group form most of the western highlands and occur locally throughout the highlands the mainly tuffaceous lower part of the group has been intruded by gabbroic and syenogranitic plutons that are interpreted to be cogenetic with basaltic and rhyolitic flows in the upper part of the group this voluminous subaerial magmatism may have formed during postorogenic ex

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

rupture complexity of the 1994 bolivia and 2013 sea of okhotsk deep earthquakes the physical mechanism of deep earthquakes depth 300 km remains enigmatic partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations the two largest deep earthquakes the recent great 2013 sea of okhotsk earthquake m 8 3 depth 607 km and the great 1994 bolivia earthquake m 8 3 depth 637 km together provide a unique opportunity to compare their rupture patterns in detail here we extend a travel time sub event location method to perform full teleseismic p waveform inversion this new method allows us to explain the observed broadband records with a set of sub events whose model parameters are robustly constrained without smoothing we find that while the okhotsk event is mostly unilateral rupturing 90 km along strike with a velocity over 4 km s the bolivia earthquake ruptured about half this distance at a slow velocity about 1 5 km s and displayed a major change in rupture direction we explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered

the dog bay liberty line and its significance for silurian tectonics of the northern appalachian orogen1this article is one of a series of papers published in this cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology 2geological survey of canada contribution 20100257 the dog bay line a silurian suture key to deciphering appalachian accretionary history was first recognized in newfoundland it marks where the ordovician tetagouch exploits ensimatic back arc basin teb which had opened within the leading peri gondwanan gander terrane finally closed here we extrapolate this suture into new england placing it between the liberty orrington miramichi inliers lom and the merrimack fredericton trough mft southeastward marine strata of the mft overlie the teb passive margin exposed in the ganderian st croix block and display southeast vergent structures transected by acadian cleavage they structurally underlie southeast vergent thrusts at the base of the lom northwestward the lom central maine matapedia trough cmmt and lower silurian igneous rocks record elements of the upper plate trench arc system respectively a subduction complex forearc basin and arc the cmmt forearc received detritus both from the northwesterly arc region and also from the

550 580 ma magmatism in cape breton island nova scotia canada the product of nw dipping subduction during the final stage of amalgamation of gondwana abstract cape breton island is host to widespread magmatic activity dated between 550 and 580 ma in the south its products include the capelin cove pluton and host coastal volcanic belt both are composed of bimodal tholeiitic calc alkaline suites that were emplaced on a thin sialic crust in a volcanic arc system their trace element and nd isotope variations are consistent with fractional crystallization accompanied by lower crustal contamination in a compositionally zoned magma chamber in contrast the indian brook and kerrs brook plutons and price point volcanic rocks in central cape breton island have higher contents of strongly incompatible trace elements and resemble modern arc related calc alkaline rocks emplaced on a moderately thick continental crust comparison with available data for other 550 580 ma igneous rocks across cape breton island indicates that they were produced above a nw dipping subduction zone this convergence was synchronous with the final stage of globalscale orogenic activity that culminated with the final amalgamation of gondwana available detrital zircon ages in late neoproterozoic 650 545 ma sediments in the antigonish highlands avalon composite terrane suggest derivation from the amazon craton it is proposed that the avalon composite terrane formed off northwestern south america during the late neoproterozoic

regional 18o depletion of neoproterozoic igneous rocks from avalonia cape breton island and southern new brunswick canada neoproterozoic igneous rocks of the avalonian mira terrane cape breton island and caledonia terrane southern new brunswick have experienced regional 18 o depletion the majority of these rocks have 18 o wr values between 1 and 6 markedly different from igneous rocks of the inboard ganderian terranes which have normal high 18 o wr values of 7 to 12 the 18 o depletion of these avalonian terranes resulted from pervasive hydrothermal alteration all of the igneous rocks contain classic propylitic alteration assemblages of turbid feldspar chlorite epidote sericite and calcite and host multiple generations of veins containing the same phases the 18 o values of the rock forming minerals show strong disequilibrium with qtz fs ranging from 2 5 to 11 0 and kfs plg from 0 5 to 1 0 such values are indicative of post magmatic hydrothermal alteration the hydrothermal alteration occurred at ca 560 550 ma during initial transcurrent rifting of avalonia at the gondwanan margin the development of rift wrench basins with associated bimodal volcanism allowed large volumes of hydrothermal fluids to penetrate and circulate within mira and caledonia terrane crust before its submergence in the early cambrian ca 540 530 ma the hydrothermal fluids were predominantly meteoric although the presence of a significant seawater component cannot be ruled out the almost ubiquitous 18 o depletion exhibited by the neoproterozoic rocks in these avalonian terranes is absent in the associated ganderian terranes suggesting that avalonia remained separate from ganderia until at least the cambrian ordovician this 18 o depletion provides an additional geochemical tool for identification of avalonian crust in other areas of the appalachians and europe

dynamic stress variations due to shear faults in a plane layered medium summary a complete set of expressions is presented for the computation of elastic dynamic stress in plane layered media we use a discrete wavenumber reflectivity method to compute the stress field radiated by arbitrary moment tensor sources the expressions derived here represent an interesting tool for both the observational and theoretical analysis of dynamic stress changes associated with earthquake phenomena dynamic stress changes associated with a strike slip fault having unilateral rupture are shown this modelling which is similar to the 1992 landers california earthquake illustrates the effects of distance directivity and depth on transient stress changes

shearing instabilities accompanying high pressure phase transformations and the mechanics of deep earthquakes deep earthquakes have been a paradox since their discovery in the 1920s the combined increase of pressure and temperature with depth precludes brittle failure or frictional sliding beyond a few tens of kilometers yet earthquakes occur continually in subduction zones to 700 km the expected healing effects of pressure and temperature and growing amounts of seismic and experimental data suggest that earthquakes at depth probably represent self organized failure analogous to but different from brittle failure the only high pressure shearing instabilities identified by experiment require generation in situ of a small fraction of very weak material differing significantly in density from the parent material this fluid spontaneously forms mode i microcracks or microanticracks that self organize via the elastic strain fields at their tips leading to shear failure growing evidence suggests that the great majority of subduction zone earthquakes shallower than 400 km are initiated by breakdown of hydrous phases and that deeper ones probably initiate as a shearing instability associated with breakdown of metastable olivine to its higher pressure polymorphs in either case fault propagation could be enhanced by shear heating just as is sometimes the case with frictional sliding in the crust extensive seismological interrogation of the region of the tonga subduction zone in the southwest pacific ocean provides evidence suggesting significant metastable olivine with implication for its presence in other regions of deep seismicity if metastable olivine is confirmed either current thermal models of subducting slabs are too warm or published kinetics of olivine breakdown reactions are too fast

seismic body wave constraint on mechanisms of intermediate depth earthquakes 1 source parameters of six large earthquakes 7 1 mw 7 7 in the depth range between 112 and 277 km from five different subduction zones are derived from teleseismic body waves recorded by global seismic networks except for the 21 october 1995 mexican event which is consistent with the model of simple rupture on a planar surface the investigated earthquakes show complex rupture processes that can be explained by irregularities in the focal region as is the case of shallow events the inferred velocity of the moment release varies between 2 5 and 4 km s corresponding to 40 90 of the shear velocity at the focus four of the six earthquakes show stress drops in the range observed for shallow events 1 10 mpa ruptures extend mainly parallel to the strike of subducting slabs with dimensions reaching up to 50 km the extent of rupture perpendicular to slab dip of generally less than 15 km is comparatively small this suggests that the rupture during large intermediate depth earthquakes propagates predominantly parallel to the strike of the subducting plate and that the seismogenic zone perpendicular to the dip direction is limited to a narrow width the relatively small rupture size perpendicular to slab dip suggests that the rupture process does not extend into regions of significantly different temperatures the spatial extent of the investigated earthquakes is consistent with the orientation of trench parallel faults that are generally observed in near trench outer rise regions suggesting that these events may result from reactivation of such preexisting faults at intermediate depths through dehydration of hydrous minerals concentrated in these zones of weakness depths of most of the earthquakes are consistent with the dehydration embrittlement of serpentine the bolivian earthquake of 23 january 1997 seems to be too deep 277 km for such an explanation

origin and evolution of hydrothermal fluids in the marshoun epithermal pb zn cu ag deposit tarom hashtjin metallogenic belt nw iran abstract the marshoun epithermal pb zn cu ag deposit is located in the tarom hashtjin metallogenic belt thmb northwest iran the epithermal base metal veins are mainly hosted in eocene volcanic and volcaniclastic rocks of the karaj formation and late eocene pyroxene quartz monzodiorite the mineralization process can be divided into five stages 1 quartz chalcopyrite pyrite veins and breccias 2 quartz calcite sphalerite galena chalcopyrite pyrite veins and breccias 3 barren post ore quartz veinlets 4 barren post ore calcite veinlets and 5 supergene mineral assemblages the primary metallic minerals are pyrite chalcopyrite sphalerite and galena gangue minerals are mainly quartz calcite sericite and chlorite the main wall rock alteration is silicification intermediate argillic carbonate and propylitic alteration microthermometric measurements of primary lv fluid inclusion assemblages in quartz and sphalerite indicate that the veins were mainly formed at temperatures between 173 and 285 c from fluids with salinities between 3 4 and 6 3 wt nacl equiv coexisting lv vl and v inclusions in quartz and sphalerite provide evidence for boiling in ore stage breccia and veins additionally the occurrence of bladed calcite as well as plumose and colloform crustiform banded quartz in the ore zones and the presence of hydrothermal breccias are consistent with boiling obtained oxygen and sulfur isotope compositions 18owater 9 2 to 1 5 34sh2s 10 0 to 5 7 indicate there was a major contribution from a magmatic component to metallogenesis the fluid inclusion and stable isotope data indicate that fluid boiling and mixing facilitated hydrothermal alteration and mineralization at marshoun the marshoun deposit is interpreted as an intermediate sulfidation style of epithermal mineralization our data suggest that the epithermal mineralization in the thmb are related to late eocene ca 36 37 ma shallow magmatic hydrothermal activity the altered eocene volcanic subvolcanic rocks particularly at the intersection of subvolcanic rocks with faults were the most favorable locus for exploration targeting of epithermal ores at thmb

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

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

complex and diverse rupture processes of the 2018 mw 8 2 and mw 7 9 tonga fiji deep earthquakes author posting american geophysical union 2019 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geophysical research letters 46 5 2019 2434 2448 doi 10 1029 2018gl080997

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

the role of hydraulic fractures and intermediate depth earthquakes in generating subduction zone magmatism the presence of magmatism and intermediate depth 70 300 km deep seismicity at subduction zones is at first sight surprising magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle the current model for subduction zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge leading to a reduction in melting temperature and magmatism1 2 3 4 but there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge the occurrence of intermediate depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths one proposal put forward to explain intermediate depth seismicity is that high pore pressure might facilitate faulting by decreasing the friction5 6 7 the hypothesis presented here is that non percolating water provides the high pore pressure that the consequent faulting temporarily interconnects the water pores and when a sufficient vertical height of water is interconnected a hydrofracture is produced which transports the water out into the mantle wedge thereby generating subduction zone magmatism

precise u pb zircon ages from alderney channel islands growing evidence for discrete neoproterozoic magmatic episodes in northern cadomia the northernmost exposures of rocks formed during the late neoproterozoic cadomian orogeny in the channel islands northern france region occur on alderney the island mainly comprises foliated quartz diorite once considered to be 2 ga pre cadomian basement and an undeformed basic to intermediate plutonic complex a precise age of 610 2 ma based on u pb analyses of single and small groups of zircons for the foliated fort tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a cadomian magmatic event the age is virtually identical to ages from similar foliated syntectonic quartz diorite bodies on the islands of guernsey and sark and at la hague north normandy indicating that this magmatic and deformational event was regional in extent discordant zircon xenocrysts define an upper intercept age of c 2 ga indicating the presence of palaeoproterozoic basement at depth single zircons from the undeformed bibette head granodiorite give a precise u pb age of 572 1 ma this age is closely similar to that for the emplacement of the northern igneous complex of guernsey the emerging data indicate that cadomian magmatism in the northern channel islands region was not a protracted continuum but occurred during two distinct short lived events separated by c 30 40 my

altering avalonia oxygen isotopes and terrane distinction in the appalachian peri gondwanan realmlaboratory for stable isotope science lsis the university of western ontario contribution 236 distinct 18o depletion is characteristic of a large majority of the 620 550 ma felsic igneous rocks of avalonia in the northern appalachian orogen neoproterozoic rocks in the boston avalon terrane have the lowest 18owr values 3 1 followed by the mira terrane in cape breton island and the caledonia terrane in new brunswick 1 2 the avalon terrane in newfoundland 2 8 and the antigonish highlands in nova scotia 5 3 in contrast this depletion of 18o is observed in very few of the paleozoic felsic igneous rocks from these avalonian terranes and also in very few of the neoproterozoic and paleozoic felsic igneous rocks from the inboard ganderian terranes the low 18o character of the neoproterozoic igneous rocks is related to regional pervasive post magmatic alteration by predominantly meteoric hydrothermal fluids 18oh2o 6 to 4 at 200 450 c the alteration likely occurred during late neoproterozoic transtensional extension of avalonia large scale fluid infiltration and circu

isotopic evidence for multiple contributions to felsic magma chambers gouldsboro granite coastal maine abstract the gouldsboro granite forms part of the coastal maine magmatic province a region characterized by granitic plutons that are intimately linked temporally and petrogenetically with abundant co existing mafic magmas the pluton is complex and preserves a felsic magma chamber underlain by contemporaneous mafic magmas the transition between the two now preserved as a zone of chilled mafic sheets and pillows in granite mafic components have highly variably isotopic compositions as a result of contamination either at depth or following injection into the magma chamber intermediate dikes with identical isotopic compositions to more mafic dikes suggest that closed system fractionation may be occurring in deeper level chambers prior to injection to shallower levels the granitic portion of the pluton has the highest nd isotopic composition e nd 3 0 of plutons in the region whereas the mafic lithologies have nd isotopic compositions e nd 3 5 that are the lowest in the region and similar to the granite and suggestive of prolonged interactions and homogenization of the two components sr and nd isotopic data for felsic enclaves are inconsistent with previously suggested models of diffusional exchange between the contemporaneous mafic magmas and the host granite to explain highly variable alkali contents the felsic enclaves have relatively low nd isotopic compositions e nd 2 1 indicative of the involvement of a third lower e nd melt during granite petrogenesis perhaps represented by pristine granitic dikes contemporaneous with the nearby pleasant bay layered intrusion the dikes at pleasant bay and the felsic enclaves at gouldsboro likely represent remnants of the silicic magmas that originally fed and replenished the overlying granitic magma chambers the large isotopic and chemical contrasts between the enclaves and granitic dikes and granitic magmas may be in part a consequence of extended interactions between the granitic magmas and co existing mafic magmas by mixing mingling and diffusion alternatively the granitic magmas may represent an additional crustal source using granitic rocks such as these with abundant evidence for interactions with mafic magmas complicate their use in constraining crustal sources and tectonic settings fine grained dike rocks may provide more meaningful information but must be used with caution as these may also have experienced compositional changes during mafic felsic interactions

systematic mineralogical diversity in a type granitic intrusions control of magmatic source and geological processes the origin and causes of mineralogical diversity of a type granites are debated the series of a type granite plutons with distinct mineralogical differences emplaced along an upper paleozoic crustal scale shear zone in the cobequid highlands nova scotia provide an opportunity to examine the origin of different a type plutons in a similar tectonic setting based on the ferromagnesian minerals present the plutons are classified into sodic granites with sodic amphibole calcic granites with calcic amphibole and biotite granites sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone whereas plutons in the western shear zone with lesser gabbro are solely biotite granites trace elements and radiogenic isotopes show that the three granite types have different sources intensive parameters including temperature pressure and water in melt contents were estimated from mineralogical and geochemical data modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20 40 partial melting of intracrustal feldspathic rocks whereas the sodic granites are extreme fractionates 90 of coeval mafic magma we propose that supply of upper paleozoic mafic magma probably related to regional extension and decompression melting beneath the magdalen basin created a deep crustal hot zone in the eastern cobequid highlands and extreme fractionation of underplated mafic sills produced the sodic granites heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust creating batches of biotite and calcic granitic melts in different depths fractionated and crustally derived melts segregated along crustal scale faults constructing the complex plutons in the east melting of the crust was further facilitated by the release of water from the crustal rocks upon heating in the eastern shear zone water was released predominantly by magmatic rocks and in lesser amounts compared to the west where neoproterozoic sedimentary and volcaniclastic rocks are more abundant the volatile rich granitic melts in the western part of the shear zone were crystallized rapidly stabilizing only biotite this study demonstrates that the mineralogical variations in a type granites arise from rather similar magma compositions but they are important petrogenetic indicators of varying sources specific magmatic processes and emplacement conditions

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

seismicity in the western great basin apparently triggered by the landers california earthquake 28 june 1992 within 24 hr after the landers earthquake there were three magnitude 3 4 events in western nevada and an unexpected widespread increase in the rate of small events based on combined catalogs for northern nevada southern nevada and southern california and a model that assumes statistical independence of events in these regions the probability of this happening in a 24 hr period by random chance is less than 10 12 per day therefore there is high statistical confidence that the increased seismicity was triggered by the landers event based on the statistical model we develop a list of 227 earthquakes in the first 83 days following the landers earthquake each of which has no more than 10 probability of occurring by random chance the suspect events are broadly distributed in regions that correlate with historical activity in the great basin the events are not uniquely associated with known volcanic activity or with zones that were previously active with microearthquakes or aftershock sequences the magnitudes of the largest triggered events appear to decrease with distance with time the number of suspect events decreases at a rate comparable to the rate of decrease of aftershocks of the landers and big bear earthquakes three of the most significant triggered events that that occurred were as follows mina 500 km from landers m 4 36 min after landers smith valley 590 km from landers m 3 4 56 min after landers and little skull mountain 280 km from landers m 5 6 22 3 hr after landers the evidence for triggering is particularly strong in the case of the little skull mountain event where an increased rate of microseismicity was evident as soon as small events could be identified in the coda of the landers earthquake we evaluate the seismic history for the past 3 yr to understand what was unique about the landers earthquake this includes identifying the previous earthquakes most likely to have caused the strongest shaking in various frequency bands based on a simple screening model the strains from the landers earthquake were uniquely large at low frequencies but at high frequencies they were exceeded frequently by small more local events thus we hypothesize that the cause of triggering is low frequency dynamic strains at periods of about 10 sec or greater and that there is a regional threshold that must be exceeded since previous events with calculated strains that were almost as strong were not causes of widespread triggering this mechanism for triggering satisfies the criterion of being a relatively rare phenomenon since it is likely to occur only when a large long period wave is radiated into an area where strain has been building slowly toward the point where faults are unstable

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1