amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

the messina layered intrusion limpopo belt south africa an example of in situ contamination of an archean anorthosite complex by continental crust the messina layered intrusion was emplaced prior to 3 0 ga into supracrustal rocks presently comprising the central zone of the limpopo belt analyses of relict cumulate plagioclase megacrysts in the intrusion and its tectonic setting show that it was emplaced as a thick body of batholithic dimensions composed of varying proportions of bytownite an77 and hypersthene the magma giving rise to the intrustion was heterogeneous characterized by positive sr values neutral to negative nd values and high 207pb 204pb ratios relative to 206pb 204pb it also was light ree enriched these characteristics probably originated by the interaction of a magma derived from a depleted mantle source with crustal material similar in composition to the rocks of the enclosing beit bridge group and sand river gneisses as a result most of the characteristics of the intrusion are closer to those of proterozoic massif type anorthosite complexes than to those of archean complexes the messina layered intrustion was metamorphosed and deformed prior to 3 0 ga when its composition was changed to quartz hornblende plagioclase gneisses by the introduction of water pb sr and rb were also added to most of these rocks probably derived from the host rocks but the primary isotopic characteristics of relict plagioclase megacrysts and of partially assimilated xenoliths of metapelite were preserved more recently the intrusion was metamorphosed under granulite facies conditions at 2 0 ga prior to the joining of the rocks of the central zone with the zimbabwe and kaapvaal cratons the emplacement metamorphism and deformation of the intrusion had nothing to do with early evolutionary events on those cratons

earthquake potential revealed by tidal influence on earthquake size frequency statistics tidal triggering of earthquakes is debated analysis of global earthquake catalogue data compared with tidal stress histories suggests that the probability of a large earthquake is greater during times of maximum tidal stress amplitude

influence of tidal effects on the periodicity of earthquake activity in diverse geological settings with particular emphasis on submarine hydrothermal systems abstract hydrothermal earthquakes may be defined as earthquakes caused by the hydrofracturing of the oceanic crust by submarine hydrothermal fluids it is suggested that they correspond to the cracking of the ocean crust at the brittle ductile transition that marks the downward limit of seawater circulation into the ocean crust semi diurnal variations in submarine hydrothermal discharge in backarc settings and at the crests of mid ocean ridges have been detected by hydrophone but no influence of tidal effects on the periodicity of microearthquakes around submarine hydrothermal vents has been observed so far although tidal stresses are too small to initiate earthquakes themselves they may trigger earthquake activity when imposed on slowly increasing tectonic stress these effects therefore tend to be observed in geologically active regions where stress levels in the earth s crust are high the influence of tides on the periodicity of microearthquakes tends to be complex because the effects of ocean and earth tides are not uniform at the surface of the earth and because the microearthquakes occur in a wide variety of geological settings earth tides cause compression and dilatation of the oceanic crust which may be the most significant factor in initiating changes in fluid flow rate and microseismicity in submarine hydrothermal systems much of the information now available on the tidal periodicity of microearthquakes is based on relatively short term monitoring which lacks statistical validity the development and installation of deep sea monitoring stations will permit rigorous long term monitoring programmes to be carried out which will lead to a better understanding of the nature and periodicity of microearthquakes around submarine hydrothermal vents

oldest known terrestrial anorthosite at mount narryer western australia abstract fragments of a deformed and metamorphosed layered anorthosite gabbro ultramafic intrusion called the manfred complex occur as inclusions in 3650 and 3400 ma old banded granite gneisses at mount narryer in the western gneiss terrain of the yilgarn block the complex is typical of a world wide group of archaean anorthosites characterized by very large primary grain sizes of equidimensional calcic plagioclase this is the first recorded occurrence of these anorthosites in australia pb pb and sm nd whole rock and u pb zircon geochronology given in the accompanying two papers fletcher i r rosman k j r and libby w g 1988 precambrian res 38 343 354 kinny p d williams i s froude d o ireland t r and compston w 1988 precambrian res 38 325 341 shows that the manfred complex is about 3730 ma old it is therefore the oldest known terrestrial anorthosite complex and one of the oldest known rock units on earth

amphibole plagioclase and clinopyroxene geochemistry of the archean fiskenaesset complex at majorqap qâva southwestern greenland implications for archean petrogenetic and geodynamic processes abstract despite multiple phases of deformation and metamorphism the archean ca 2 97 ga fiskenaesset complex southwestern greenland contains well preserved igneous olivine pyroxene amphibole predominantly hornblende and plagioclase this study presents new major and trace element data for hornblende plagioclase and clinopyroxene from the majorqap q va stratigraphic section of the complex petrographic observations and coherent trace element patterns of hornblende clinopyroxene and plagioclase grains in samples within and between stratigraphic units indicate that trace elements including lile ree and hfse in these minerals were generally immobile during amphibolite to granulite facies metamorphism their mineral chemistry therefore should reflect the trace element composition of the melt from which they grew two distinct petrogenetic suites of rocks are recognized using the trace element composition of hornblende the earlier suite a is characterized by hree depleted hornblende whereas the later suite b contains hree enriched hornblende within suite a hornblendes can be grouped into type a1 type a2 and type a3 hornblendes type a1 hornblendes characterized by low ree nb and ta contents and the absence of eu anomalies most likely have derived from a primitive liquid type a2 hornblendes which have high concentrations of incompatible elements including nb zr and ree and negative eu anomalies appear to have precipitated from a residual liquid following the extensive crystallization of plagioclase type a3 hornblendes are slightly enriched in mree compare to lree and hree displaying negative nb zr and ba anomalies suite a typifies a hydrous crystallization sequence of olivine clinopyroxene orthopyroxene hornblende plagioclase hornblende intercumulus this crystallization sequence played an important role in the formation of anorthosite layers in the fiskenaesset complex within suite b hornblendes are grouped into four types type b1 type b2 type b3 and type b4 hornblendes irrespective of eu anomalies type b1 type b2 and type b3 hornblendes display parallel patterns with coherent hree enrichment over lree in contrast type b4 hornblendes are enriched in lree suite b illustrates a crystallization of sequence of olivine plagioclase hornblende hornblende the presence and chemistry of the igneous hornblendes suggest the fiskenaesset complex formed in an archean convergent plate margin setting

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

kinematics strain pattern and geochronology of the salem attur shear zone tectonic implications for the multiple sheared salem namakkal blocks of the southern granulite terrane india abstract structural mapping strain analysis and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the salem attur shear zone in the southern granulite terrane of south india the salem namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss charnockite and mafic granulite and had undergone multiple phases of magmatism spanning over a period of 3 2 0 5 ga the rocks were deformed by four phases of deformation d1 d4 the d1 deformation was characterized by isoclinal and recumbent ne sw trending f1 fold with a pervasive subhorizontal axial planar granulitic fabric s1 and associated quartzofeldspathic leucosomes granulite metamorphism was dated at ca 2 5 2 3 ga the f1 fold and s1 fabric were coaxially refolded by tight to isoclinal upright to steeply inclined ne sw trending f2 folds during d2 deformation the d2 deformation was associated with f2 axial planar shear zones crenulations and leucosomes s2 fabric large scale d2 shear zones characterized by high temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca 2 5 2 4 ga a p t condition of 7 kb 600 c was inferred for the d2 deformation the d3 deformation was characterized by nw se to e w trending f3 folds and the salem attur shear zone the shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down dip stretching lineation defined by quartz biotite and hornblende minerals and dated at ca 2 0 ga it indicated n nne vergence of thrusting with the mean kinematic vorticity number wm as 0 7 suggesting general simple shear strain with 50 pure shear component the d4 deformation was manifested as nne ssw striking strike slip faults and nw se striking extensional normal faults pseudotachylite veins having an age of 1 9 ga injected during strike slip faulting and granite pegmatite veins showing age of 0 8 0 5 ga intruded during normal faulting the salem namakkal blocks thus recorded a long lived shearing history we suggest that the salem attur shear zone and other shear zones such as palghat cauvery moyar bhavani karur kambam painavu trichur and achankovil shear zones were paleoproterozoic intraterrane shear zones which were overprinted by meso neoproterozoic cambrian ductile and brittle deformations

the detection of low magnitude seismic events using array based waveform correlation summary it has long been accepted that occurrences of a known signal are most effectively detected by cross correlating the incoming data stream with a waveform template such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co located events at lower magnitudes than would be possible using traditional power detectors even greater improvement in signal detectability can be achieved using seismic arrays running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain if two events are co located the time separating the corresponding patterns in the wave train as indicated by the cross correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not we illustrate the power of array based waveform correlation using the 1997 august 16 kara sea event the weak event that occurred 4 hr after the main event was barely detected using an sta lta detector on the spits array but is readily detected by signal matching on a single channel the main event was also recorded by the far more distant norsar array but no conventional detection can be made for the second event a clear detection is however made when the correlation coefficient traces are beamformed over all sensors of the array we estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise we show that for this case waveform correlation using a single channel detects signals of approximately 0 7 orders of magnitude lower than is possible using an sta lta detector on the array beam waveform matching on the full array provides an additional improvement of approximately 0 4 magnitude units we describe a case study in which small seismic events at the barentsburg coal mine on spitsbergen were detected using the signals from a major rockburst as master waveforms many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts we demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency wavenumber analysis upon the set of individual correlation coefficient traces

the chimalpahad anorthosite complex and associated basaltic amphibolites nellore schist belt india magma chamber and roof of a proterozoic island arc abstract new major and trace element data on the proterozoic chimalpahad layered anorthositic complex and associated basaltic amphibolites of the nellore schist belt of south india provide new constraints on their petrogenesis and geodynamic setting the complex consists of layered anorthosites leucogabbros gabbros ultramafic rocks and is spatially associated with basaltic amphibolites despite deformation and metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex whereas the amphibolites display diverse ree systematics the chimalpahad anorthositic gabbroic rocks are characterized by moderately depleted to strongly enriched lree patterns and by flat to depleted hree patterns the field relations major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic gabbroic rocks by fractional crystallization the anorthositic rocks and the basaltic amphibolites share the depletion of nb relative to th and la on primitive mantle normalized diagrams they exhibit signatures of arc magmatic rocks such as high lile and lree relative to the hfse and hree as well as high ba nb ba zr sr y la yb ratios that mimic chondrite normalized ree and primitive mantle normalized trace element patterns of arc magmas similarly on log transformed tectonic discrimination diagrams the chimalpahad rocks plot within the field of phanerozoic magmatic arcs consistent with a subduction zone origin on the basis of field relations and geochemical characteristics the chimalpahad complex is interpreted as a fragment of a magma chamber of an island arc which is tectonically juxtaposed against its original volcanic cover a new preliminary sm nd date of anorthosite from the chimalpahad complex indicates a model age of 1170 ma

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

young cumulate complex beneath veniaminof caldera aleutian arc dated by zircon in erupted plutonic blocks mount veniaminof volcano alaska peninsula provides an opportunity to relate quaternary volcanic rocks to a coeval intrusive complex veniaminof erupted tholeiitic basalt through dacite in the past 260 k y gabbro diorite and miarolitic granodiorite blocks ejected 3700 14 c yr b p in the most recent caldera forming eruption are fragments of a shallow intrusive complex of cumulate mush and segregated vapor saturated residual melts sensitive high resolution ion microprobe shrimp analyses define 238 u 230 th isochron ages of 17 6 2 7 ka 5 11 10 ka and 10 2 4 0 ka 2 for zircon in two granodiorites and a diorite respectively sparse zircons from two gabbros give 238 u 230 th model ages of 36 8 ka and 26 7 ka zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid although historic eruptions have been weakly explosive strombolian fountaining and small lava effusions the young ages of plutonic blocks as well as late holocene dacite pumice are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

absence of earthquake correlation with earth tides an indication of high preseismic fault stress rate because the rate of stress change from the earth tides exceeds that from tectonic stress accumulation tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level we analyze the tidal stresses and stress rates on the fault planes and at the times of 13 042 earthquakes which are so close to the san andreas and calaveras faults in california that we may take the fault plane to be known we find that the stresses and stress rates from earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times while the rate of earthquakes when the tidal stress promotes failure is 2 higher than when the stress does not this difference in rate is not statistically significant this lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0 15 bar h just preceding seismic failure much above the long term tectonic stress rate of 10 4 bar h

chemical assembly of archean anorthosites from amphibolite and granulite facies terranes sw greenland we report whole rock major and trace element compositions obtained by xrf and ina methods for the amphibolite facies buksefjorden and granulite facies nordland anorthosites sw greenland in a previous petrologic study on the same sample suite we documented differences in texture mineralogy and mineral compositions between these two anorthosite bodies chemical analyses confirm differences in composition between the two bodies but these differences cannot be explained by variations in metamorphic conditions and point towards differences in the nature of their protoliths analyzed nordland samples are anorthosites and leucogabbros with 88 98 normative plagioclase whereas those from buksefjorden include anorthosite leucogabbro and gabbro with 55 95 normative plagioclase two or more compositional groupings can be recognized at each site which correspond to differences in color and mineralogy of the hand samples samples from buksefjorden are mainly quartz normative whereas those from nordland are olivine nepheline normative other differences include higher ni co ratio and ree contents in the granulite facies anorthosites from nordland ree pattern shapes are similar however being moderately fractionated at 0 5 10 chondrites with positive eu anomalies calculated equilibrium melt patterns are similar for both anorthosites being relatively flat at 50 150 chondrites suggesting unfractionated but evolved parental magmas olivine must have been present in the protoliths of the nordland rocks compared with buksefjorden otherwise the protoliths contained plagioclase with variable an content an62 an92 and a mafic component with variable fe mg mg 0 3 0 8 this mafic component was either hornblende or a combination of ortho and clinopyroxene in fixed proportions plus a small amount of magnetite mixing calculations demonstrate that some buksefjorden anorthosites contain two varieties of plagioclase a calcic type that may correspond to cumulus crystals and a sodic type that may correspond to a trapped melt component on plots of normative whole rock an versus mg compositions of the buksefjorden and nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions kiglapait skaergaard and from the generally flats trends of plagioclase rich cumulate rocks st urbain and stillwater anorthosites this difference provides further evidence for the distinctive nature of archean calcic anorthosite complexes compared with other types of mafic intrusions moreover this distribution of data points is consistent with the assembly of the protolith of the sw greenland anorthosites mainly as mixtures of plagioclase and hornblende finally the field for the buksefjorden and nordland anorthosites overlaps only slightly with that for the fiskenaesset complex thus extending the known range of compositions for archean anorthosites in west greenland

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

cryogenian 830 ma mafic magmatism and metamorphism in the northern madurai block southern india a magmatic link between sri lanka and madagascar abstract the northern madurai block southern india lies directly south of and partly deformed by the palghat cauvery shear zone system pcss a potential suture of the neoproterozoic mozambique ocean the kadavur gabbro anorthosite complex lies south of the pcss in the northern madurai block and crystallized at 829 14 ma la icpms zircon data in a supra subduction zone setting the complex contains zircon e hf t values of 12 5 to 8 6 that represent palaeoproterozoic t dm model ages 2 3 2 5 ga these broadly agree with a whole rock neodymium t dm model age of 2287 ma oxygen isotope 18 o ratios range from 5 82 and 6 74 the parental magma for the gabbro anorthosites are interpreted to be derived from a juvenile neoproterozoic mantle contaminated by mesoarchaean igneous infra crustal sources the gabbro anorthosites intrude quartzites with dominantly palaeoproterozoic detrital zircons that contain neoarchaean and mesoarchaean hafnium model ages these quartzite zircons contain metamorphic rims that yield an age of 843 23 ma demonstrating the autochthonous nature of the gabbro anorthosite complex later felsic magmatism is recorded by the 766 8 ma crystallisation age of the protolith of a felsic gneiss cryogenian magmatism in the madurai block is interpreted to form part of an extensive arc magmatic province within the southern east african orogen that can be traced from central madagascar through southern india to the wanni complex of sri lanka this province is interpreted to have formed above a south west dipping subduction system as the mozambique ocean was subducted under the neoproterozoic continent azania

the genesis of archaean chromitites from the nuasahi and sukinda massifs in the singhbhum craton india abstract the chromitite deposits of the nuasahi and sukinda massifs are part of layered ultramafic bodies which occur within archaean low grade metamorphic rocks of the iron ore group iog in the singhbhum craton of the indian shield the chromitite seams are interlayered with dunite and associated with orthopyroxenite detailed electron microprobe study reveals very high mg numbers 0 66 0 82 and cr numbers 0 75 0 87 for chromite in massive chromitite from the seams high fo content fo 92 95 for olivine in dunite and high en content en 89 94 for orthopyroxene in orthopyroxenite our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks the primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the mesoarchaean the parental magma from which the massive chromitites crystallized was of siliceous high mg basaltic or boninitic compositions similar to the compositions of spatially associated chromite bearing siliceous high mg basalts of the iron ore group the parental magma may have generated due to the interaction of a depleted mantle with a fluid enriched melt possibly derived in response to the dehydration of a subducting slab monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated h 2 o concentrations in the parent magmas the parental magmas that produced chromitites may have intruded into the volcano sedimentary greenstone belts of a supra subduction zone setting

new age ca 2970 ma mantle source composition and geodynamic constraints on the archean fiskenaesset anorthosite complex sw greenland the archean fiskenaesset complex sw greenland consists of an association of ca 550 meter thick layered anorthosite leucogabbro gabbro and ultramafic rocks peridotite pyroxenite dunite hornblendite the complex was intruded by tonalite trondhjemite and granodiorite ttg sheets now orthogneisses during thrusting that was followed by several phases of isoclinal folding the trace element systematics of the fiskenaesset complex and associated volcanic rocks are consistent with a supra subduction zone geodynamic setting the fiskenaesset anorthosites leucogabbros gabbros and ultramafic rocks collectively yield an sm nd errorchron age of 2973 28 ma mswd 33 with an average initial epsilon nd 3 3 0 7 consistent with a long term depleted mantle source regression of pb isotope data define an age of 2945 36 ma mswd 44 and the regression line intersects the average growth curve at 3036 ma slightly lower pb pb errorchron age is interpreted as reflecting partial disturbance of the u pb system in gabbros leucogabbros and ultramafic rocks during intrusion of ttgs complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca 3200 and 2650 ma zircon ages peak at about 3200 3100 3000 2950 2820 and 2750 ma the 3200 3000 ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks 2950 ma is considered as an approximate intrusion age of sampled ttgs the 29402650 ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the fiskenaesset region on the basis of recently published trace element data and new nd and pb isotope and u pb zircon age data a three stage geodynamic model is proposed to explain the evolution of the fiskenaesset complex stage 1 represents the formation of depleted shallow mantle source 3000 ma epsilon nd 3 3 0 7 for the complex stage 2 corresponds to the development of an intra oceanic island arc between 3000 2950 ma stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950 2940 ma c 2010 elsevier b v all rights reserved less

geochronological evolution of the moyar bhavani and palghat shear zones of southern india implications for east gondwana correlations abstract the amphibolite facies moyar bhavani and palghat shear zones separate the south indian dharwar craton dc from the nilgiri and madurai high grade crustal provinces we present sm nd and rb sr isotopic whole rock mineral and small slab data for remnant granulites mylonitic gneisses and granitoids that reveal a neoproterozoic tectonothermal imprint in these shear zones sm nd garnet crystallisation ages and rb sr mica cooling ages from reworked rocks suggest that pan african structurally controlled retrogression and successive cooling in the moyar shear zone msz garnet 624 591 ma muscovite 594 ma biotite 604 540 ma predates tectonometamorphism in the bhavani shear zone bsz garnet 552 ma biotite 521 491 ma and palghat shear zone psz garnet 521 ma biotite 488 485 ma a rb sr small slab isochron age of 624 ma indicates that pan african retrogression in the msz also led to isotopic reequilibration of the rb sr system on a cm scale a sm nd garnet crystallisation age of 513 5 ma reflects emplacement of a postdeformative pegmatite in the bsz during the late cambrian a sm nd garnet age of 2355 18 ma obtained from a granulite remnant in the msz records early palaeoproterozoic high grade metamorphism which may be related to the amalgamation of the nilgiri block and dc along the palaeo moyar zone a relict charnockite from the southern bsz yielded a reproducible sm nd garnet age of 1705 11 ma presumably recording late palaeoproterozoic metamorphism neoarchaean and palaeoproterozoic nd model ages 3 0 2 2 ga suggest that the shear zone rocks are mainly reworked material from the adjacent nilgiri and madurai crustal domains nd model ages of granites and tonalites which intruded the msz syntectonically provide evidence for 1 7 ga crust generation unsheared metasediments exposed between the bhavani and palghat high strain zones were subjected to pan african high grade metamorphism as constrained by sm nd age data of garnets grown during coaxial deformation at 610 560 ma sm nd whole rock data from these supracrustal rocks indicate average crustal residence ages of 1 9 ga which implies contributions from an unknown proterozoic crustal source and disproves their previously suggested correlation to the archaean sargur group these model ages provide an upper limit for the juxtaposition of dc and madurai block along the bsz and psz

coseismic deformation of the 2010 jiashian taiwan earthquake and implications for fault activities in southwestern taiwan abstract the march 04 2010 jiashian taiwan earthquake mw 6 4 ruptured an unknown fault at depth in southwestern taiwan the main shock initiated near the town of liuquei at 23 km depth and the rupture propagated westward measurements of coseismic displacements from taiwan continuous gps array indicate horizontal displacements of 5 27 mm in the nw sw directions to the west of the epicenter while horizontal movements to the east of the epicenter are absent the gps vertical displacements show an uplift motion of about 5 25 mm near the epicenter in contrast to a small movement of about 5 10 mm observed in the far field gps sites we use coseismic gps displacements and an elastic half space dislocation model to invert for fault geometries and coseismic slip distribution associated with the jiashian earthquake our preferred model exhibits 0 05 0 1 m of reverse slip and 0 04 m of left lateral slip on a n324 trending fault with dip of 40 to ne consistent with the earthquake focal mechanisms from bats usgs neic and global cmt the highest slip of 0 12 m mainly occurs to the west of the epicenter at a depth range of 15 20 km given the rigidity modulus of 60 gpa the geodetic moment is 4 95 1018 n m equivalent to a mw 6 4 earthquake and consistent with the seismic moment estimated from seismic waveform inversion additionally we notice that the mainshock rupture area is surrounded by high seismicity between 1991 and 2007 suggesting that the jiashian earthquake may be triggered by the high stress concentration in the vicinity the calculated coulomb stress changes on nearby fault systems imparted by the coseismic slip suggest that the jiashian earthquake may encourage failures on the chukou fault and inhibit ruptures on the hsinhua fault however the coulomb stress changes are more complicated on the chaochou fault and chishan fault with both positive and negative stress changes

u pb zircon chronology of the pangidi kondapalle layered intrusion eastern ghats belt india constraints on mesoproterozoic arc magmatism in a convergent margin setting abstract the pangidi kondapalle layered intrusion is emplaced within high grade supracrustal rocks in the southern sector of the eastern ghats granulite belt proximal to the south eastern periphery of the indian plate this layered complex consists mainly of anorthosites and gabbronorites with pyroxenite cumulates containing orthopyroxenites and chromitites in a regressive intrusive sequence here we report laser ablation icp ms data and ree geochemistry of zircons from three co spatial anorthositic bodies in this complex our results show mean 207 pb 206 pb ages of 1634 18 ma mswd 0 55 for the anorthosite from pangidi two anorthosite samples from kondapalle yield mean 207 pb 206 pb ages of 1628 36 ma mswd 0 54 and 1693 110 ma mswd 5 5 the zircons from all the anorthosite samples show high ree contents prominent hree enrichment and a conspicuous positive eu anomaly suggesting a common melt source the prominent mesoproterozoic ages revealed from our u pb dating of zircons from these layered complexes suggest subduction related magmatic underplating in a convergent margin setting associated with the amalgamation of the columbia derived fragments within the neoproterozoic supercontinent

the cauvery shear zone southern granulite terrain india a crustal scale flower structure abstract the cauvery shear zone csz is a crustal scale shear system within the southern granulite terrain along the southern margin of the archaean dharwar craton structural interpretation of satellite data and field observations reveal four major shear zones within the csz system they show dextral shear kinematics synchronous with a major neoproterozoic tectono metamorphic event d2 associated with intracrustal melting and migmatisation the disposition geometry and contemporaneity of shear fabrics of the csz system are modelled in terms of a crustal scale flower structure akin to transpressional and collisional orogens in the light of recent seismic evidence for a displaced moho structure and a mid to lower crustal low velocity zone the flower structure across the csz may extend to mantle depths

roots of an archean volcanic arc complex the lac des iles area in ontario canada abstract the lac des iles area is located in the wabigoon subprovince an archean granite greenstone terrain in the superior province in ontario canada the area consists of several unmetamorphosed plutons of biotite and hornblende tonalite and mafic and ultramafic intrusions which are emplaced into an older tonalitic gneiss the mafic ultramafic bodies are concentric in shape and consist of periodotite pyroxenite gabbro and hornblende gabbro cumulates and diorite the largest intrusion the lac des iles complex consists of several mafic and ultramafic centers an internal sm nd isochron from the southern ultramafic complex gives an intrusion age of 2736 28 ma mswd 1 5 nd t 1 2 this age is also assumed to be representative of the time of emplacement of the other mafic and ultramafic bodies as well as the unmetamorphosed tonalite plutons the tonalitic gneiss represents older continental crust and had an nd value of 1 at this time the nd t values of the felsic and mafic ultramafic suites are positive ranging from 0 to 2 this range can be explained if primary magmas derived from a depleted mantle reservoir assimilated continental crust such as the tonalite gneiss during their ascent this implies that the tonalite magmas in the lac des iles area represent mixtures of the tonalite country rock and mantle magmas which formed by partial melting of depleted mantle or of mafic rocks underplated beneath the continental lithosphere the morphology and the rock assemblages of the mafic and ultramafic intrusions in the lac des iles area resemble alaskan type intrusions which are common components of phanerozoic magmatic arcs associated with subduction zones in addition the crystallization sequence in the mafic ultramafic intrusions suggests that they are derived from a parental magma with high al basalt affinities the mafic ultramafic rocks are depleted in high field strength elements but enriched in large ion lithophile elements and the tonalites belong to calc alkaline rock series displaying fractionation trends towards trondhjemitic compositions thus the archean lac des iles area and the root zones of phanerozoic active continental margins share many geological geochemical and petrological features which would suggest that the lac des iles area formed in an andean type continental margin

chromite silicate chemistry of the neoarchean sittampundi complex southern india implications for subduction related arc magmatism the neoarchean layered anorthositic complex at sittampundi in southern india is known for its chromitite layers that are mostly associated with anorthosite an 90 100 the chromitites contain feal rich chromites concentrated in layers between amphibole rich layers with a dominant mineralogy of amphibole spinel plagiocase sapphirine the chromite rich layers contain only amphibole and plagioclase mineral compositions illustrated by x ray composition maps and profiles show subtle chemical differences the chrome spinels are of refractory grade with cr2o3 and al2o3 contents varying between 34 40 wt and 23 28 wt the chromite compositions are noticeably different from those in layered igneous intrusions of the bushveld stillwater type the existence of original highly calcic plagioclase feal rich chromite and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra subduction zone arc setting in terms of mineralogy and field relations the sittampundi chromitites are remarkably similar to anorthosite hosted chromitites in the neoarchean fiskensset anorthositic complex greenland we propose that the sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone this melting process produced hydrous aluminous basalt which fractionated at depth to give rise to a variety of high alumina basalt compositions from which the anorthositic complex with its cumulate chromite rich and amphibole rich layers formed within the magma chamber of a supra subduction zone arc c 2011 elsevier b v all rights reserved

seismicity rate changes in the salton sea geothermal field and the san jacinto fault zone after the 2010 mw 7 2 el mayor cucapah earthquake

foreshock migration preceding the 2016 mw 7 0 kumamoto earthquake japan we investigated the spatiotemporal evolution of an earthquake sequence following a series of large shallow intraplate earthquakes including a mw 6 2 foreshock and mw 7 0 main shock in the kumamoto area of kyushu sw japan to more precisely characterize the evolution of the earthquake sequences we applied a matched filter technique to continuous waveform data using template events obtained via a double difference relocation algorithm migrations of seismicity fronts along the directions of fault strike and dip are clearly seen starting immediately after the mw 6 2 foreshock these migrations are interpreted to result from aseismic slip triggered by the foreshock propagating toward the nucleation point of the subsequent mw 7 0 main shock rupture when combined with static stress changes induced by the mw 6 2 foreshock it is likely that stress transfer from both aseismic and seismic slip during the foreshock sequence loaded stress onto the main shock rupture faults bringing them closer to failure

seismicity around parkfield correlates with static shear stress changes following the 2003 mw6 5 san simeon earthquake 1 earthquakes trigger other earthquakes but the physical mechanism of the triggering is currently debated most studies of earthquake triggering rely on earthquakes listed in catalogs which are known to be incomplete around the origin times of large earthquakes and therefore missing potentially triggered events here we apply a waveform matched filter technique to systematically detect earthquakes along the parkfield section of the san andreas fault from 46 days before to 31 days after the nearby 2003 mw6 5 san simeon earthquake after removing all possible false detections we identify 8 times more earthquakes than in the northern california seismic network catalog the newly identified events along the creeping section of the san andreas fault show a statistically significant decrease following the san simeon main shock which correlates well with the negative static stress changes i e stress shadow cast by the main shock in comparison the seismicity rate around parkfield increased moderately where the static stress changes are positive the seismicity rate changes correlate well with the static shear stress changes induced by the san simeon main shock suggesting a low friction in the seismogenic zone along the parkfield section of the san andreas fault

detection of a collision zone in south indian shield region from magnetotelluric studies magnetotelluric mt investigations were carried out along a profile in the greenschist granulite transition zone within the south indian shield region sisr the profile runs over a length of 110 km from kuppam in the north to bommidi in the south it covers the transition zone with 12 mt stations using a wide band 1 khz 1 ks data acquisition system the mettur shear zone mtsz forms the ne extension of moyar bhavani shear zonethattraverses alongthetransitionzone theregionalgeoelectric strikedirectionofn40 eidentifiedfromthepresent studyisconsistentwiththe strike direction of the mtsz in the center of the profile the 2 d conductivity model derived from the data display distinct high electrical resistivity character 10 000 m below the archaean dharwar craton and less resistive b3000 m under the southern granulite terrain located south of the mtsz the mtsz separating the two regions is characterized by steep anomalous high conductive feature at lower crustal depths the deep seismic sounding dss study carried out along the profile shows dipping signatures on either side of the shear zone the variation of deep electrical resistivity together with the dipping signature of reflectors indicate two distinct terrains namely the archaean dharwar craton in the north and the proterozoic granulite terrain towards south they got accreted along the mtsz which could represent a possible collision boundary 2006 international association for gondwana research published by elsevier b v all rights reserved

tidal triggering of earthquakes in the ning er area of yunnan province china abstract to investigate the potential effect of tidal modulation on the seismicity in the ning er area a seismically and geothermally active zone in yunnan province china we studied the correlation between earth tides and the occurrence of m 6 0 earthquakes dating back to 1970 as well as their aftershock sequences using theoretically calculated tidal stresses and a statistical test the results show a significant correlation between earth tides and the occurrence of earthquakes six of seven main events occurred when the earth tide increased the coulomb failure stress on the source fault four main events occurred in a narrow range of phase angle corresponding to the maximum loading rate of tidal stress furthermore the histories of the aftershock sequence as a function of the tidal phases demonstrate clear tidal modulation with a high significance thus we conclude that earth tides have a clear role in triggering or modulating the rupture of the fault systems in the ning er area

structure and evolution of the cauvery shear zone system southern granulite terrain india evidence from deep seismic and other geophysical studies abstract the southern granulite terrain with exposed archean lower crustal rocks is studied using various geophysical tools the crustal structure derived from seismic reflection and refraction wide angle reflection studies is used to understand the tectonic evolution of the region deep seismic reflection section along the kolattur palani segment shows an oppositely dipping reflection fabric near the moyar bhavani shear zone which is interpreted as a signature of collision between the dharwar craton and another crustal block in the south the thickened crust due to collision was delaminated during the orogenic collapse and modified the central part covering the cauvery shear zone system located between the moyar bhavani and karur oddanchatram shear zones the delaminated lower crust is altered by magmatic underplating as evidenced by the high velocity layer just above the moho the velocity model of the region indicates crustal thickening at the boundary of the dharwar craton and moyar bhavani shear zone and thinning further south back scattered seismic wave field with negative moveout and the moho offset indicate the spatial location and strike slip nature of the shear zones present study suggests that the late archean collision and suturing of the dharwar craton with the southern crustal block at the moyar bhavani shear zone may be responsible for the evolution of late archean granulites late neoproterozoic rifting is observed along the paleo fault zones the seismic studies constrained by gravity magnetic and magnetotelluric data suggest that the moyar bhavani and karur oddanchatram shear zones of the cauvery shear zone system mark terrane boundaries suture zones

trace element systematics of the neoarchean fiskenaesset anorthosite complex and associated meta volcanic rocks sw greenland evidence for a magmatic arc origin abstract new major and high precision icp ms trace element data on the neoarchean ca 2970 ma layered fiskenaesset complex and associated volcanic rocks southern west greenland provide new constraints on the petrogenesis and geodynamic setting of the complex the complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust tholeiitic basalts and gabbros forming an association of ca 550 m thick anorthosite leucogabbro gabbro and peridotite layers the fiskenaesset complex and the associated volcanic rocks were intruded by neoarchean tonalite trondhjemite and granodiorite ttg sheets during thrusting that was followed by several phases of isoclinal folding despite the intense deformation and amphibolite to granulite facies metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex the presence of calcic plagioclase an75 95 and igneous amphibole in anorthosites gabbros and leucogabbros and hornblendite veins in peridotites suggests a hydrous magma source s the major and trace element compositions of tholeiitic basalts amphibolites suggest that they are petrogenetically related to the fiskenaesset complex by fractional crystallization the trace element systematics of the least altered anorthosites gabbros leucogabbros peridotites tholeiitic basalts and calc alkaline high magnesian andesites hma is collectively consistent with a supra subduction zone geodynamic setting on the log transformed tectonic discrimination diagram including la th sm th yb th and nb th ratios tholeiitic basalts display a trend projecting from mid ocean ridge basalt morb field to island arc basalt iab field this trend is interpreted as reflecting a transition from the neoarchean depleted upper mantle to a subarc mantle wedge following the initiation of intra oceanic subduction and arc migration collectively on the basis of field relationships petrographic features and geochemical characteristics the fiskenaesset complex is interpreted as a fragment of a neoarchean oceanic island arc on the basis of ree patterns anorthosites are divisible into four major groups 1 group 1 displays moderately depleted to slightly enriched lree and hree patterns 2 group 2 possesses strongly enriched lree and moderately depleted hree patterns 3 group 3 has strongly enriched lree and depleted hree patterns and 4 group 4 garnet anorthosite exhibits concave upward ree patterns that appear to have resulted from contamination of anorthositic magma by basaltic rocks or magma the different ree patterns in group 1 2 and 3 anorthosites are interpreted to reflect various depths of partial melting and variably enriched in incompatible elements source compositions the majority of anorthosites and leucogabbros were derived from depths above the garnet stability field

re and os concentrations in arc basalts the roles of volatility and source region fo2 variations olivine and spinel compositions major elements including ferric and ferrous iron s re and os contents have been measured for a suite of primitive most 6 wt mgo basalts from the trans mexican volcanic belt tmvb including the western mexican volcanic belt the michoacan guanajuato volcanic field sierra chichinautzin pico de orizaba region palma sola san martin tuxtlas and the eastern alkaline province eap sulfur contents at sulfide saturation were calculated to determine whether the measured s contents are representative of sulfide saturated liquids most of the samples have s contents much lower than expected for sulfide saturation a few have higher contents than calculated perhaps due to the presence of sulfate in the measured total sulfur i e more oxidized samples comparison of the tmvb samples along with previously analyzed morb oib babb and arc samples reveals a continuum of re and os contents that is best explained by variation in oxygen fugacity and thus sulfide stability in the source region high re and os magmatic suites are best explained by derivation by melting of oxidized mantle where sulfide is no longer stable and re and os behave incompatibly whereas low re and os magmatic suites are derived from melting of relatively reduced mantle where sulfide is stable and re and os behave compatibly intermediate examples abound and arc magmas span a wide range of re and os concentrations due to variation of fo2 in the source during genesis of arc magmas low re magmatic suites are furthermore potentially affected by volatility which can lower re by a factor of 3 5

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

detecting low frequency earthquakes within non volcanic tremor in southern taiwan triggered by the 2005 mw8 6 nias earthquake 1 we use a matched filter technique to detect 41 low frequency earthquakes lfes within 700 s of triggered tremor signals in the southern central range in taiwan during the surface waves of the 2005 mw8 6 nias earthquake off the coast of northern sumatra the depth distributions of lfes after double difference relocations concentrate at the depth range of 12 38 km below the background seismicity and above the moho depth inferred from receiver function studies the locations of lfes are close to the downward extension of the steep dipping chaochou lishan fault with only modestly high vp vs ratios 1 75 1 85 our observation indicates that at least portions of triggered tremor consists of many lfes similar to ambient tremor observed at other major plate boundary faults

characteristic mineralogy of arc related cumulate gabbros implications for the tectonic setting of gabbroic plutons and for andesite genesis several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs calcic an85 100 plagioclase and moderately fe rich fo60 80 olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid ocean ridges ocean islands or tholeiitic layered intrusions compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano plutonic complexes in deeply eroded arcs gabbroic cumulates typically occur in zoned plutons that also contain gabbros diorites and quartz diorites of basaltic to andesitic composition in many cases it is clear that the cumulate gabbros are chemically and physically related to the gabbro diorite basalt andesite fractionation trends seen in the zoned plutons zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis

statistical properties of low frequency earthquakes triggered by large earthquakes in southern taiwan abstract the recent discovery of triggered tremors tts and low frequency earthquakes lfes in various tectonic environments provides an opportunity for studying the fundamental properties and physical mechanisms of deep tectonic tremor here we quantify the relationship between tts and lfes beneath the central range in southern taiwan and their statistical properties during the teleseismic waves of six large distant earthquakes using waveforms of 11 lfes triggered by the 2005 m w 8 6 nias earthquake as templates we scan through 12 hours of waveform data around six mainshocks and identify a total of 783 lfes the lfes were mainly located in a compact region between 12 and 36 km in depth near the chaochou lishan fault most of lfes occurred within tt during the passage of large amplitude surface waves and the increase of the lfe rate during the surface waves is statistically significant the lfe rates do not follow an omori s type decay but rather abruptly return to the background rate immediately after the surface wave passage these findings suggest that lfes do not trigger any additional lfes at later times and are primarily driven by an external forcing our observations are consistent with the inference that tts consist of many reoccurring lfes

tidal triggering of earthquakes analysis of the tidal stress tensor at the time of moderate to large earthquakes strongly suggests that shallow 30 km larger magnitude oblique slip and dip slip earthquakes are triggered by tidal stresses no corresponding triggering effect is seen for shallow strike slip earthquakes or for any type of intermediate or deep focus earthquakes which have been studied tidal triggering is also discussed from the viewpoint of the dilatancy diffusion model specifically the model as usually stated excludes the possibility of small earthquakes being tidally triggered

deep low frequency earthquakes in tectonic tremor along the alaska aleutian subduction zone 1 we characterize and locate tremor not associated with volcanoes along the alaska aleutian subduction zone using continuous seismic data recorded by the alaska volcano observatory and the alaska earthquake information center from 2005 to present visual inspection of waveform spectra and time series reveal dozens of 10 to 20 min bursts of tremor along the length of the alaska aleutian subduction zone we use autocorrelation to demonstrate that these tremor signals are composed of hundreds of repeating low frequency earthquakes lfes the tremor activity we characterize is localized in four segments from east to west kodiak island shumagin gap unalaska and andreanof islands although the geometry age thermal structure frictional and other relevant properties of the alaska aleutian subduction zone are poorly known these characteristics are likely to differ systematically from east to west locations near kodiak island are the most reliable because station coverage is more complete lfe hypocenters in this region are located on the plate interface near the down dip limit of the 1964 mw 9 2 alaska earthquake rupture area lfe hypocenters in the remaining areas along the arc are also located down dip of the most recent mw 8 megathrust earthquakes although these locations are less well constrained our results support the hypothesis that tremor activity marks the down dip rupture limit for great megathrust earthquakes in this subduction zone lastly there is no correlation between the presence of tremor and particular aspects of over riding or subducting plate geology or coupling it appears that lfes are a fundamental characteristic of the alaska aleutian subduction zone

back analysis of a seismically induced highway embankment failure during the 1999 duzce earthquake an earthquake of magnitude 7 2 on the richter scale occurred on 12 november 1999 in the duzce bolu region of turkey the region was also hit approximately 3 months before during the devastating kocaeli earthquake of 17 august 1999 besides high casualties and damage to various engineering structures and buildings slope and embankment failures on the highway and the country roads occurred in this study the authors are concerned with the back analysis of an embankment failure that occurred on the four lanes e 5 highway connecting ankara to istanbul at bakacak in the bolu province during the 1999 duzce earthquake both pseudo dynamic and dynamic limiting equilibrium methods are used to back analyse the conditions for the initiation of failure and also displacement responses of the embankment during the earthquake shaking after having given a brief summary of the investigations on the post failure state the geology and geotechnical characteristics of the site and the dynamic limiting equilibrium method developed and used for analyses are described the results of the back analysis based on a pseudo dynamic approach revealed that the maximum ground acceleration to initiate the failure of the embankment was probably about 0 125 g on the other hand the application of a method for computing the displacement and velocity responses of the failed embankment showed that the failure was possible when the embankment was subjected to ground accelerations similar to that recorded at the duzce station in addition the computations also revealed that the time history of accelerations could be very important for the failure of slopes and embankments

temporal variation of mineralogy and petrology in cognate gabbroic enclaves at arenal volcano costa rica gabbroic enclaves ejected during the current eruption phase a 1 and during the latest prehistoric eruption phase a 2 of arenal volcano show systematic variations in texture mineralogy and composition as a function of host rock chemistry and timing of eruption the most differentiated enclaves occur in the more differentiated a 2 lavas enclaves in the a 1 volcanics are consistently less evolved within the current a 1 eruption the most mafic enclaves are amphibole bearing rocks that were erupted during the first 2 3 years of activity 1968 1970 these enclaves occur in the most differentiated a 1 volcanics and are not in equilibrium with their host rocks they crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle we interpret the enclaves as sidewall crystallization products of a melt possibly a high alumina basalt that was immediately parental to the a 1 lavas enclaves that occur in a 1 rocks erupted after 1970 and all of the a 2 enclaves are amphibole free and less mafic than the early a 1 enclaves their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas none of the enclaves contain minerals that might have equilibrated with a primary mantle derived melt geothermometry is consistent with geochemistry with amphibole bearing a 1 enclaves yielding the highest pyroxene temperatures ave 1090 c and a 2 enclaves the lowest ave 1030 c geobarometry suggests mid to upper crustal depths for the crystallization of all enclaves the enclaves are cognate and reflect pre eruptive crystallization of arenal magmas they record evolution from a hydrous basaltic magma to the drier basaltic andesites that characterize the current eruption volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole the a 2 lavas may represent the long term fractionation products of basaltic andesite magmas similar in composition to the a 1 lavas anorthitic plagioclase commonly thought of as a phase stabilized by high ca na and high water pressure continued to crystallize in a system with relatively low ca na and which had dehydrated and or depressurized to the point at which amphibole was no longer stable this suggests that compositional characteristics other than high ca na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at arenal we speculate that the high alumina content of the arenal magmas may be the stabilizing factor

black giants anorthosite new zealand a paleozoic analogue of archean stratiform anorthosites and implications for the formation of archean high grade gneiss terranes the black giants anorthosite a mid paleozoic 349 5 ma u pb zircon age layered anorthosite complex in fiordland new zealand bears striking compositional and lithologic similarities to archean stratiform anorthosites and like many of its archean counterparts occurs within a high grade gneiss terrane preserving a record of metamorphism at mid crustal depths followed by higher pressure metamorphism and burial to lower crustal levels these and other similarities point to formation of the black giants anorthosite and its archean equivalents in comparable tectonic environments most likely a subduction related magmatic arc which in the case of fiordland resulted from plate convergence along the pacific margin of gondwana

an introduction to the sangihe arc volcanism accompanying arc arc collision in the molucca sea indonesia in the molucca sea region of northeastern indonesia the sangihe and halmahera arcs are presently in the process of colliding the earth s only example of a collision between facing volcanic arcs the collision event is more advanced in the northern molucca sea where back arc thrusting occurs along the cotobato and philippine trenches and volcanic centers are inactive and dissected this paper describes the sangihe arc the western arc of the collision the sangihe are is approximately 500 km in length extending from the ne tip of sulawesi to mindanao philippines the arc comprises over 25 quaternary volcanic centers the eight presently active volcanoes being confined to the southern half of the arc the active volcanic front lies 100 110 km above the top of the west dipping benioff zone and volcanoes extend up to 70 km behind the front thereby defining a volcanic arc overlying earthquakes 100 to 180 km deep rock types range from basalt to rhyolite andesites are dominant pyroxene andesites are common on sulawesi and at or near the volcanic front from ruang to buhias pahepa and buang to mamanuk hornblende andesites are common at the volcanic front between mahengetang and sangihe island and in centers located well behind the volcanic front tholeiitic th suites defined on the basis of relative degrees of iron enrichment are confined to the southern volcanic front lack hornblende are low k k55 1 05 have low rb zr ratios 0 10 0 17 low ba y ratios 3 4 4 9 low rb cs ratios 8 14 and have the highest 87sr 86sr ratios 0 7039 0 7042 calcalkaline ca suites occur throughout the arc are low to high k k55 0 6 2 0 have variable rb zr ratios 0 15 0 65 and ba y ratios 4 2 12 4 high rb cs ratios 14 82 and low 87sr 86sr ratios 0 7035 0 7040 zr and y distribution coefficients are lower in th suites dzr 0 0 dy 0 2 0 5 than in ca suites dzr 0 0 0 8 dy 0 3 1 2 ti initially increases with increasing sio2 in some th suites while decreasing with sio2 in all ca suites with the exception of bukide ti zr 138 ti zr ratios of basalts exceed 100 only in th suites 76 130 lil element contents and lil hfs element ratios increase 2 to 3 times across the are away from the volcanic front and 2 to 3 times from south to north along the arc 87sr 86sr ratios decrease away from the volcanic front the south to north increase in lil element contents and lil hfs element ratios without corresponding changes in sr isotopic compositions are interpreted as resulting from decreasing degrees of partial melting northward this reflects the gradual cessation of volcanism in the north due to collision

a neoarchean dismembered ophiolite complex from southern india geochemical and geochronological constraints on its suprasubduction origin abstract ophiolites the remnants of ancient oceanic lithosphere have been described from collisional sutures of various ages with only few examples from archean terranes here we report the discovery of a neoarchean ophiolite suite from the southern margin of the dharwar craton in india tectonically intercalated within a neoproterozoic suture zone the metamorphosed and variably dismembered ophiolite suite exposed around devanur comprises altered ultramafic units websterite gabbros mafic dykes amphibolites trondhjemites and pegmatites associated with ferruginous metachert structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet the major and trace element geochemistry of the mafic dykes indicate derivation from basaltic andesitic magmas with tholeiitic to calc alkaline characteristics the rocks display negative nb anomalies with enrichment of lile k rb ba th and depletion in hfse ti nb hf tb the tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting we present new shrimp zircon u pb data for two trondhjemite samples from this complex which yield 238u 206pb ages of 2528 61 and 2545 56 ma the neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area the suprasubduction zone assemblages and arc magmas suggest a neoarchean ocean closure along the southern margin of the dharwar craton

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

crustal evolution and the temporality of anorthosites abstract types of rock such as komatiite which formed entirely or dominantly during restricted time periods in solar system history are important indicators of how planetary bodies evolved in deep time in this paper we characterize three different types of temporally restricted anorthosites and discuss their significance to the broad scale evolution of planetary processes primordial anorthosites which constitute the bulk of the lunar crust were sampled during apollo and luna missions and have subsequently been identified in our meteorite collections they are characterized by very calcic plagioclase an93 98 and all have magmatic crystallization ages 4 3 ga suggesting that the earliest planetary crust forming processes on the moon and possibly elsewhere involved substantial if not total melting the magma ocean hypothesis which has endured for nearly 50 years argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase which crystallized after extensive precipitation and sinking of olivine and pyroxene another possibility revitalized by critical filtering of age data involves serial magmatism whereby the lunar crust was constructed by younger smaller episodic magmatic events on earth two distinct types of temporally restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics tectonics and magmatic processes archean megacrystic anorthosites formed only between 3 73 and 2 49 ga as small bodies

correlation of tremor activity with tidal stress in the northern cascadia subduction zone 1 we analyze hourly data from five tremor episodes in the northern cascadia subduction zone over the period 2003 2005 provided by the tremor activity monitoring system tams all five tremor episodes correspond to slow slip events observed by gps fourier decomposition is used to separate the hourly tremor counts for each episode into long period 0 2 2 cpd components the tidal component of the observations is compared with theoretical stress variations at depths of 20 30 and 40 km with 40 km being the depth of the interpreted subduction thrust interface the stress variations are predicted by a 2 d ocean tide loading model combined with estimates of stress variations from earth tides we find that the shear stress in the thrust direction and the compressive normal stress on shallow dipping surfaces correlates with the data significantly better than the confining stress over the range of depths investigated the relative amplitudes of tidal shear stress and compressive normal stress result in positive coulomb stress favoring slip peak tremor activity occurs at times of maximum tidal shear stress in the thrust direction which would assist slow slip and would suggest that tidal tremor and slip are colocated the response of the tremor to tidal shear stress is roughly proportional to the mean activity level controlled by tectonic conditions of stress and pore pressure a significant nontidal daily variation in tremor activity of unknown origin is identified

compositions and tectonic settings of chromite deposits through time chromium ore deposits range in age from 3 5 billion year old chromite in the selukwe greenstone belt zimbabwe to the ophiolite hosted miocene deposits of new caledonia they provide indications of the evolving earth9s tectonic processes in oceanic and continental crust and mantle podiform orebodies are hosted in peridotites mostly now regarded as the remains of obducted ophiolite complexes they imply ocean spreading at their sites of original crystallization and plate convergence where subsequently reemplaced by thrusting known ophiolites with chromite deposits date at 800 ma or younger chromite deposits in archean greenstones resemble the ophiolite ores in respect to their peridotite host rocks shapes and intense deformation but are recognizably cumulate ores they are hosted in large ultramafic mafic sill like complexes in simatic volcanic sequences suggesting that the early archean ocean crust may have been more ultramafic in composition than at present stratiform chromite deposits of the bushveld type are hosted in layered mafic complexes within the stable continental shields those of economic grade date between about 2900 and 2000 ma formed at about the time when continental growth rates peaked maximum cr fe ratios diminish with decreasing age suggesting that the younger layered complexes crystallized from more chrome depleted magma this might be an effect of secularly decreasing radiogenic heat production so that continental thermal gradients gradually became too low to retain high levels of chromium in the magma as it ascended some 450 published chrome spinel analyses were compared from five different tectonic settings the settings distinguished are 1 ophiolite lower crustal cumulates 2 podiform chromite in tectonized mantle harzburgite 3 bushveld type layered complexes in continental shields 4 chromitite in serpentinized peridotite metapyroxenite sills in archean greenstone belts and 5 archean anorthositic layered complexes in high grade gneissic terranes conclusions are that chromite deposits seem to categorize four main periods of earth history 1 the early archean approximately 3500 2900 ma greenstone hosted deposits suggest thrusting and obduction processes in an oceanic lithosphere characterized by high thermal gradients and possibly high mg oceanic crust composition layered anorthositic complexes hosted in deeply eroded gneiss terranes are analogues of the bushveld type deposits but were emplaced into unstable crust 2 extensive regions of the continental shields became stable from about 2900 ma onward allowing large bushveld type stratiform deposits to form some were emplaced in regions of crustal extension with thermal gradients higher than at present 3 the period between 2000 and 800 ma marks an apparent proterozoic gap during which cr fe ratios are low in the layered igneous complexes and ophiolites are rare orogenic belts with tectonic features similar to those of mesozoic and tertiary age suggest plate tectonic processes but oceanic crust might have been thicker and difficult to obduct at this time 4 after 800 ma ophiolite hosted podiform chromite deposits predominate indicating plate tectonic systems similar to those of present times

tectonic tremor and lfes on a reverse fault in taiwan we compare low frequency earthquakes lfes from triggered and ambient tremor under the southern central range taiwan we apply the pagerank algorithm used by aguiar and beroza 2014 that exploits the repetitive nature of the lfes to find repeating lfes in both ambient and triggered tremor we use these repeaters to create lfe templates and find that the templates created from both tremor types are very similar to test their similarity we use both interchangeably and find that most of both the ambient and triggered tremor match the lfe templates created from either data set suggesting that lfes for both events have a common origin we locate the lfes by using local earthquake p wave and s wave information and find that lfes from triggered and ambient tremor locate to between 20 and 35 km on what we interpret as the deep extension of the chaochou lishan fault

precambrian continental strain and shear zone patterns south indian case 1 this study addresses the tectonic and mechanical significance of crustal scale strain and shear zone patterns in the south indian precambrian lithosphere it is based on a tectonic map of the dharwar craton derived from landsat imagery existing documentation and our own field observations we document the contrasted responses of the two main parts of the craton to late archean 2 56 2 51 ga shortening the old 3 ga lithosphere of the western dharwar craton stabilized at 2 61 ga underwent moderate shortening and strain localization along spaced shear zones the eastern dharwar craton rejuvenated by late archean juvenile magmatic accretion responded to shortening by flowing laterally against the western dharwar craton shortening operated without significant thickening because of the high buoyancy of the juvenile crust and the very low strength of its mantle lithosphere the late archean crustal scale shear zone network did not accommodate large displacements and only contributed to smoothing out of strain heterogeneities during the latest stages of flow the shear zone pattern only reflects the symmetry of the regional finite strain field of a wide hot orogen it does not result from terrain amalgamation but accompanies crustal flow and does not compare with those of modern active margins which accommodate unidirectional transpression our analysis further suggests that the shear zones bounding the craton did not record large horizontal displacements during final assembly of gondwana but rather transverse shortening and vertical extrusion

magmatic and metamorphic imprints in 2 9 ga chromitites from the sittampundi layered complex tamil nadu india abstract the ca 2 9 ga old sittampundi layered complex slc of south india belongs to a special group of deformed and metamorphosed archaean rocks where chromitites are interlayered with anorthosite detailed field studies have revealed that the slc comprises of a sequence of magmatic rocks consisting of clinopyroxenite anorthosite and mafic rocks of tholeiitic composition minor bif banded iron formations intercalated with the mafic rocks are the only sedimentary rock within the slc chromitite occurs as continuous to discontinuous bands a few mm up to 6 m thick within clinopyroxenite and highly calcic anorthosite an 99 100 but never in the enclosing mafic rocks chromite occurs broadly in three textural types a cumulus grains in chromitite layers b disseminated grains in clinopyroxenite and anorthosite and c small grains included in clinopyroxene and plagioclase a suite of felsic magma intruded the slc at ca 2 51 ga subsequently the entire rock sequence underwent polyphase deformation and metamorphism at ca 2 50 2 45 ga granulite facies and at 0 72 0 50 ga amphibolite facies during the latter event extensive alteration of clinopyroxene to amphibole and chlorite of plagioclase to clinozoisite and of chromite to cr poor green spinel took place chromite compositions show large variations cr 26 60 mg 4 45 fe 3 2 24 depending upon textural type and nature of the associated phases the most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel cr 3 21 mg 45 61 during amphibolite facies metamorphism on a cr vs mg plot pristine magmatic compositions of chromite define an array that suggest increasing cr with concomitant decrease in mg presumably during magmatic fractionation highly calcic plagioclase in anorthosite and fe al rich chromite in chromitite in the slc are consistent with a hydrous parental magma of tholeiitic composition this hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth in terms of mineralogy and field relations chromitite bearing rocks of the slc show remarkable similarity with the archaean anorthosite hosted chromitite deposits in layered magmatic complexes at fiskenaesset greenland and messina south africa but contrasts sharply with the chromitite deposits of proterozoic and phanerozoic age a change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the archaean proterozoic boundary

crustal tomographic imaging and geodynamic implications toward south of southern granulite terrain sgt india abstract the crustal structure toward southern part of sgt is poorly defined leaving an opportunity to understand the tectonic and geodynamic evolution of this high grade granulite terrain surrounded by major shear and tectonically disturbed zones like achankovil shear zone aksz and palghat cauvery shear zone pcsz to develop a geologically plausible crustal tectonic model depicting major structural elements a comprehensive tomographic image was derived using deep seismic sounding data corroborated by bouguer gravity modeling coincident reflection seismic heat flow and available geological geochronological informations along the n s trending vattalkundu kanyakumari geotransect the final tectonic model represents large compositional changes of subsurface rocks accompanied by velocity heterogeneities with crustal thinning 44 36 km and moho upwarping from north to south this study also reveals and successfully imaged anomalous zone of exhumation near aksz having transpression of exhumed rocks at mid to lower crustal level 20 30 km with significant underplating and mantle upwelling forming a complex metamorphic province the presence of shear zones with high grade charnockite massifs in the upper crust exposed in several places reveal large scale exhumation of granulites during the pan african rifting 550 ma and provide important insights of plume continental lithosphere interaction with reconstruction of the gondwanaland

joint inversion of seismic and geodetic data for the source of the 2010 march 4 mw 6 3 jia shian sw taiwan earthquake

spatial temporal evolutions of early aftershocks following the 2013 mw 6 6 lushan earthquake in sichuan china we perform a comprehensive detection of early aftershocks following the 2013 mw 6 6 lushan earthquake which occurred in the southern longmenshan fault zone in sichuan province china about 5 years after the 2008 mw 7 9 wenchuan earthquake we use events in both standard and relocated catalogs as templates to scan through continuous waveforms 2 days before and 3 days after the main shock we successfully reduce the magnitude of completeness mc by more than 1 order and obtain up to 6 times more events than listed in both catalogs aftershocks in the first hour mostly occur around the main shock slip region and aftershocks at later times show systematic expansions in the along strike perpendicular strike and updip directions although postseismic deformation following the lushan main shock has not been clearly identified we suggest that early aftershock expansions are likely driven by afterslip of the lushan main shock this is consistent with the observations that most aftershocks were in the stress shadow of the lushan main shock and that there was significant slip deficit in the top 10 km of the crust we also find that seismicity on the back thrust fault was activated as soon as 20 min after the main shock earlier than previously reported we are unable to detect any clear foreshocks in the last 2 days before the lushan main shock

geochemistry of ultramafic rocks and hornblendite veins in the fiskenaesset layered anorthosite complex sw greenland evidence for hydrous upper mantle in the archean abstract the fiskenaesset complex sw greenland contains the world s best preserved archean 2970 ma layered anorthosite leucogabbro gabbro and ultramafic association the complex was emplaced into archean oceanic crust distal from continental lithosphere and later intruded by tonalites trondhjemites and granodiorites ttg constituting archean continental crust the complex and bordering ttg intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions this study presents new whole rock major and trace element petrographic and sem scanning electron microscope bse backscatter electron image data for a 45 60 m thick ultramafic sill and cross cutting hornblendite veins the ultramafic sill is composed of 75 olivine pyroxene hornblendite 15 pyroxene hornblendite and 10 hornblende pyroxenite despite granulite facies metamorphism and multiple phases of deformation the fiskenaesset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and alaskan type ultramafic complexes hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism formation of orthopyroxene magnetite symplectitic vermicular intergrowths mainly at the expense of olivine is attributed to chemical reactions between late stage residual hydrous melts and olivine field geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion high mgo 16 31 wt contents suggest that the sill was emplaced as a crystal mush rather than as melt only petrographic observations and bse images indicate that olivine orthopyroxene and clinopyroxene were the major crystal phases whereas hornblende represents the main interstitial liquid phase in the crystal mush the sill exhibits three different ree patterns corresponding to three cryptic layers although the origin of these ree patterns is not fully understood they may reflect the intrusion of three batches of magma containing different hornblende forming melt crystal ratios geochemical data indicate that the cross cutting hornblendite veins likely originated from evolved late stage melts but were not related to the melts of the ultramafic sill large negative nb anomalies nb nb 0 04 0 66 suggest that the fiskenaesset hornblendites hornblende pyroxenites and hornblendite veins were derived from a hydrous sub arc mantle peridotite phanerozoic hornblendites are typically associated with supra subduction zone ophiolites and magmatic arcs accordingly it is suggested that water was recycled to the source of the fiskenaesset hornblendites through subduction of altered oceanic crust recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites but also played an important role in the formation of ttg dominated archean continental crust therefore the field petrographic and geochemical data presented in this study suggest that the origin of the mesoarchean to neoarchean terrane in the fiskenaesset region sw greenland is consistent with subduction zone geodynamic processes rather than non uniformitarian density driven vertical crustal overturn and diapiric processes

formation of the neoarchean bad vermilion lake anorthosite complex and spatially associated granitic rocks at a convergent plate margin superior province western ontario canada abstract the bad vermilion lake anorthosite complex henceforth the bvla complex in western ontario is one of the well exposed anorthosite bearing archean layered intrusions in the superior province canada this study presents new whole rock major and trace element data for the various units of the complex oxygen isotope data for the anorthosite and major and trace element data for the spatially associated granitic rocks intruding the bvla complex to constrain their petrogenetic and geodynamic origin zircons from granitic rocks have yielded a 207pb 206pb age of 2716 18 ma constraining the minimum intrusion age of the complex despite deformation and greenschist facies metamorphism primary igneous textures are locally well preserved in the bvla complex its whole rock major and trace elemental compositions and the oxygen isotopic systematics appear not to have been substantially modified by deformation and metamorphism mantle like oxygen isotope signatures and major and trace element compositions are inconsistent with significant crustal contamination of the bvla complex during its emplacement the existence of primary calcic igneous plagioclase coherent negative nb anomalies nb nb 0 08 0 88 and geochemical similarities between gabbros from the bvla complex and gabbros from cenozoic arcs collectively suggest an intra oceanic subduction zone geodynamic setting for the complex near flat ree patterns in the various units of the bvla complex suggest that they were derived from melting of a shallow source beneath a subarc mantle wedge trends in immobile major e g mgo and trace e g zr element data indicate that the mineralogical composition of the complex can be explained by fractional crystallization and accumulation of olivine orthopyroxene clinopyroxene plagioclase and possibly amphibole compositionally the bordering granitic rocks are a2 type and strongly enriched in th and ree 100 times chondrite and depleted of ba sr eu and ti we suggest that they formed in a post collisional extensional tectonic regime following emplacement of the bvla complex in an oceanic arc

an overview of the geochemistry of eoarchean to mesoarchean ultramafic to mafic volcanic rocks sw greenland implications for mantle depletion and petrogenetic processes at subduction zones in the early earth abstract this study reviews the geochemical characteristics of eoarchean to mesoarchean ultramafic to mafic volcanic rocks now amphibolites in sw greenland and compares them with those of cenozoic oceanic island arc basalts to evaluate archean subduction zone petrogenetic processes emphasis is placed on the th ree hfse zr ti and nb systematics of the ca 3800 and ca 3700 ma arc suites in the isua greenstone belt the ca 3075 ma ivisaartoq ujarassuit greenstone belt and amphibolites associated with the ca 2970 ma fiskenaesset layered anorthosite complex on n morb normalized diagrams the isua ivisaartoq ujarassuit and fiskenaesset volcanic rocks are all characterized by depletion of nb relative to th and lree consistent with a supra subduction forearc arc backarc geodynamic setting similarly on the th yb nb yb projection these suites plot within the field of cenozoic oceanic island arc basalts on log transformed immobile trace element ratio nb th la th sm th and yb th diagrams they display a trend projecting from morb mid ocean ridge basalt to iab island arc basalt on the iab crb continental rift basalt oib ocean island basalt morb diagram as for cenozoic oceanic island arc basalts accordingly these trace element compositions are interpreted as reflecting the enrichment of archean depleted upper mantle morb source by subduction derived melts and fluids following the initiation of intra oceanic subduction and arc migration concentrations of mgo and ni in sw greenland archean basalts overlap with but extend to 2 to 4 times higher than those in cenozoic oceanic island arc counterparts in contrast the majority of archean basalts have ree and hfse concentrations 2 to 4 times lower than cenozoic oceanic island arc basalts consistent with more depleted sub arc mantle wedge peridotites in the archean than in cenozoic counterparts such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter archean mantle we infer that less refractory compositions of the present day depleted upper mantle the source of morb and conservative hfse in arc basalts have resulted from re mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post archean times archean basalts in sw greenland share the negative nb and ti anomalies of cenozoic oceanic island arc basalts on average archean basalts however have lower th nb la nb sm tio2 and gd tio2 ratios than cenozoic oceanic island arc counterparts given that rutile controls the nb and ti budgets in arc magmas the lower th nb la nb sm tio2 and gd tio2 ratios in sw greenland basalts are attributed to the mobility of nb and ti in slab derived melts involving rutile fusion in archean subduction zones these elements are less mobile in fluids originating from cenozoic subducting oceanic crust where rutile appears to be generally stable accordingly it is suggested that higher geothermal gradients in the archean may have provided optimized conditions for slab melting and metasomatism of the sub arc mantle wedge by slab derived melts

petrology and geochemistry of the island of sarigan in the mariana arc calc alkaline volcanism in an oceanic setting isotopic studies of rocks from oceanic island arcs such as the marianas indicate that little if any recycling of continental material e g oceanic sediments occurs in these arcs because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs an important question is whether the andesites of continental arcs are produced by a fundamentally different more complex mechanism than the lavas of oceanic arcs an excellent opportunity for study of this question is provided by the island of sarigan in the mariana active arc on which calc alkaline andesites including hornblende bearing types are exposed along with more mafic lavas

spatial temporal evolution of early aftershocks following the 2010 ml 6 4 jiashian earthquake in southern taiwan chi chia tang 1 2 cheng horng lin2 and zhigang peng3 1hubei subsurface multi scale imaging lab institute of geophysics and geomatics china university of geosciences 388 lumo rd wuhan hubei 430074 p r china e mail iori897 gmail com 2institute of earth sciences academia sinica 128 sec 2 academia rd nangang taipei 11529 taiwan 3school of earth and atmospheric sciences georgia institute of technology 311 ferst drive atlanta ga 30332 usa

anticlockwise evolution of ultrahigh temperature granulites within continental collision zone in southern india abstract we report three new localities of corundum and sapphirine bearing hyper aluminous mg rich and silica poor ultrahigh temperature granulites formed during late neoproterozoic cambrian times within the palghat cauvery shear zone system in southern india from petrologic characteristics mineral chemistry and petrogenetic grid considerations the peak metamorphic conditions of these rocks are inferred to lie around 950 1000 c as suggested by al in orthopyroxene thermometer at pressures above 10 kbar as indicated by the equilibrium orthopyroxene sillimanite gedrite quartz assemblage these rocks preserve several remarkable reaction textures the most prominent among which is the triple corona of spinel sapphirine cordierite on corundum with the whole textural assembly embedded within the matrix of gedrite suggesting the reaction ged crn spl spr crd the formation of sapphirine sillimanite assemblage symplectite associated with relict corundum and porphyroblasitc cordierite is explained by the reaction crd crn spr sil the association of sapphirine cordierite symplectite with gedrite sillimanite assemblage as well as with aluminosilicate boundaries indicates the gedrite consuming reaction ged sil spr crd extensive growth of sapphirine cordierite observed on the rim of gedrite porphyroblasts with spinel occurring as relict inclusions within the sapphirine indicates the reaction ged spl spr crd the pressure temperature p t path defined from the observed mineral assemblages and reaction texture is characterized by anticlockwise trajectory with a prograde segment of initial heating and subsequent deep burial followed by retrograde near isothermal decompression such an anticlockwise trajectory is being reported for the first time from southern india and has important tectonic implications since these rocks were developed at the leading edge of the crustal block that was involved in collisional orogeny and subsequent extension during the final phase of assembly of the gondwana supercontinent we propose that the rocks were subjected to deep subduction and rapid exhumation and the extreme thermal conditions were attained either through input from underplated mantle derived magmas or convective thinning or detachment of the lithospheric thermal boundary layer during or after crustal thickening

the fortnightly tide and the tidal triggering of earthquakes in this paper we test the southern california network earthquake catalog and the world wide earthquake catalog for a fortnightly tidal periodicity and find none in studies that test the hypothesis of tidal triggering of earthquakes it is usually necessary to resolve the tidal shear stress onto the plane of the fault and in the direction of the slip vector to determine if the stress is compatible with the fault motion heaton 1975 1982 this analysis requires accurate focal mechanisms and knowledge of the focal plane on which slip occurred for as many earthquakes as possible to improve the statistical sampling of the data set however if we consider the fortnightly tide which is a simple amplitude modulation focal mechanism information is not required our analysis assumes that we have a large data base of earthquakes with varying mechanisms the alignment of the slip vector with the tidal shear stress will vary from earthquake to earthquake however if tidal triggering of earthquakes does occur more earthquakes should be triggered out of the total population when the tidal stress is large than when it is small if the largest semi diurnal tide is normalized to an amplitude of 1 0 the fortnightly tide would have an amplitude of 0 17 on this scale and a period of about 2 weeks munk and macdonald 1960 therefore the effect on the occurrence of earthquakes by the tidal shear stress will be modulated with a 2 week period considering the fortnightly tide allows us to utilize entire catalogs of data to test the tidal triggering hypothesis without knowledge of the focal mechanisms

neoarchean suprasubduction zone arc magmatism in southern india geochemistry zircon u pb geochronology and hf isotopes of the sittampundi anorthosite complex abstract the sittampundi anorthosite complex sac in southern india is one of the well exposed archean layered anorthosite gabbro ultramafic rock associations here we present high precision geochemical data for the various units of sac coupled with zircon u pb geochronology and hf isotopic data for the anorthosite the zircon ages define two populations the older yield a concordia age of 2541 13 ma which is interpreted as the best estimate of the magmatic crystallization age for the sittampundi anorthosite a high grade metamorphic event at 2461 15 ma is suggested by the upper intercept age of the younger zircon population a neoproterozoic event at 715 180 ma resulted in pb loss from some of the metamorphic zircons the magmatic age of the anorthosite correlates well with the timing of crystallization of the arc related 2530 ma magmatic charnockites in the adjacent salem block while the metamorphic age is synchronous with the regional metamorphic event the geochemical data suggest that the rocks were derived from a depleted mantle source sub arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous aluminous basalt magma the magma fractionated at depth to produce a variety of high alumina basalt compositions from which the anorthositic complex with its chromite rich and amphibole rich layers formed as cumulates within the magma chamber of a supra subduction zone arc the coherent initial176hf 177hf ratios and positive ehf values 1 7 4 5 of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source our study further confirms that the southern part of the dharwar craton was an active convergent margin during the neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth

migration of early aftershocks following the 2004 parkfield earthquake a significant number of the early aftershocks following large shallow earthquakes are missing in existing earthquake catalogues an analysis of the waveforms of 3 647 relocated earthquakes along the parkfield section of the san andreas fault reveals 11 times more aftershocks within three days of the 2004 parkfield earthquake than listed in a standard catalogue large shallow earthquakes are immediately followed by numerous aftershocks a significant portion of these events is missing in existing earthquake catalogues mainly because seismicity after the mainshock can be masked by overlapping arrivals of waves from the mainshock and aftershocks1 2 3 4 however recovery of the missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering2 3 4 and for tracking postseismic deformation around the rupture zone associated with the mainshock5 6 7 here we use the waveforms of 3 647 relocated earthquakes8 along the parkfield section of the san andreas fault as templates9 10 to detect missing aftershocks within three days of the 2004 magnitude 6 0 parkfield earthquake we identify 11 times more aftershocks than listed in the standard catalogue of the northern california seismic network we find that the newly detected aftershocks migrate in both along strike and down dip directions with logarithmic time since the mainshock consistent with numerical simulations of the expansion of aftershocks caused by propagating afterslip11 12 the cumulative number of early aftershocks increases linearly with postseismic deformation in the first two days supporting the view that aftershocks are driven primarily by afterslip13 14

geochemistry of anorthositic differentiated sills in the archean 2970 ma fiskenaesset complex sw greenland implications for parental magma compositions geodynamic setting and secular heat flow in arcs abstract the fiskenaesset complex sw greenland is one of the best preserved layered archean intrusions in the world consisting of an association of ca 550 meter thick anorthosite leucogabbro gabbro and ultramafic rocks dunite peridotite pyroxenite and hornblendite despite poly phase deformation and amphibolite to granulite facies metamorphism primary cumulate textures and igneous layering are well preserved in the complex this study reports new major and trace element data for three variably thick 1 to 5 m differentiated dunite through peridotite pyroxenite gabbro leucogabbro to anorthosite sequences sequences 1 2 and 3 in the sinarssuk area of the fiskenaesset region on several variation diagrams samples from these sequences plot along a well defined liquid line of descent consistent with in situ fractional crystallization the average chemical compositions of these sequences are used to constrain their approximate parental magma compositions petrographic observations and geochemical data suggest that sequences 2 and 3 solidified from evolved magmas that underwent olivine fractionation prior to their intrusion in contrast sequence 1 appears to have been derived from a near primary parental magma sio2 43 wt mgo 20 wt al2o3 16 wt cao 9 3 wt ni 840 ppm mg number 80 the trace element patterns of this parental magma are comparable to those of phanerozoic boninites consistent with a supra subduction zone geodynamic setting if the relative thickness of ultramafic layers the sum of dunite peridotite and pyroxenite layers in differentiated sequences is taken as an analog for the original complex emplaced into archean oceanic crust the fiskenaesset complex might have had a minimum thickness of 1000 m with a 500 m thick ultramafic unit at the bottom the thickness of the ultramafic unit in the preserved complex is less than 50 m suggesting that more than 90 of the original ultramafic unit was either delaminated and recycled back into the mantle as a residual cumulate or was destroyed during thrusting and ttg intrusion both the fiskenaesset complex and associated tholeiitic basalts display similar th nb lree patterns and plot along the same differentiation trend on zr versus incompatible trace element diagrams suggesting a possible petrogenetic link between the two suites of rocks however basalts do not display the same differentiation trend as the complex on several major and trace element diagrams in addition basalts and parental magma to the complex do not plot along the same fractionation line on al2o3 tio2 versus incompatible trace element diagrams implying that the fiskenaesset layered intrusive rocks were not derived from tholeiitic basalts through fractional crystallization as previously thought accordingly we infer that the fiskenaesset complex and spatially associated basalts were derived from different mantle sources parental magmas to the fiskenaesset complex originated from a mantle source that was metasomatized by highly aluminous slab derived melts layered anorthosite complexes are mostly restricted to the archean and typically associated with tonalite trondhjemite granodiorite ttg gneisses the petrogenesis of both suites appears to have been controlled mainly by slab melting endorsing independent evidence for a secular change from slab dominated melting to wedge dominated melting in arc magmatism at the end of the archean reflecting higher geothermal gradients in archean arcs than post archean counterparts

tidal modulation of nonvolcanic tremor episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of japan and cascadia in cascadia such episodes typically last a few weeks and differ from normal earthquakes in their source location and moment duration scaling the three most recent episodes in the puget sound southern vancouver island portion of the cascadia subduction zone were exceptionally well recorded in each episode we saw clear pulsing of tremor activity with periods of 12 4 and 24 to 25 hours the same as the principal lunar and lunisolar tides this indicates that the small stresses associated with the solid earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes because the lithostatic stresses are 10 5 times larger than those associated with the tides we argue that tremor occurs on very weak faults

origin of archean anorthosites evidence from the bad vermilion lake anorthosite complex ontario the bad vermilion lake anorthosite complex 2 700 m y is exposed over an area of about 100 km2 near rainy lake ontario as is typical of other archean anorthosites it is composed of coarse 1 30 cm across equidimensional euhedral to subhedral calcic an80 plagioclase in a finer grained mafic matrix the amount of mafic matrix in individual samples ranges from none to about 70 by volume the complex has been variably metamorphosed to greenschist facies zoisite chlorite and hornblende are abundant but primary plagioclase is preserved in many places