tomographic image of melt storage beneath askja volcano iceland using local microseismicity 1 we use p wave and s wave arrivals from microseismic earthquakes to construct 3 d tomographic vp and vs images of the magma storage region beneath askja s central volcano in the northern volcanic zone of iceland a distinctive ellipsoidal low velocity anomaly with both vp and vsvelocities 8 12 below the background is imaged at 6 11 km depth beneath the caldera the presence of a shallow magma chamber is corroborated by geodetic and gravity studies the small vp vs anomaly suggests a lack of pervasive melt we interpret this anomaly as a region of multiple sills some frozen but hot others containing partial melt a second smaller low velocity anomaly beneath the main magma storage region may represent a magma migration pathway this interpretation is supported by the close proximity to the anomaly of clusters of deep magmatically induced earthquakes however the location and shape of this deep anomaly are poorly constrained by the current data set

crustal accretion model for iceland predicts cold crust we present a self consistent crustal accretion model of iceland that includes heat and mass transfer between an upwelling melting mantle and an accreting cooling crust this models predicts sub solidus crustal temperatures that are consistent with seismic estimates of the maximum depth of earthquake hypocenters and mid crustal shear wave quality factors and with geothermal estimates of surface heatflow

the magmatic plumbing system of the askja central volcano iceland as imaged by seismic tomography the magmatic plumbing system beneath askja a volcano in the central icelandic highlands is imaged using local earthquake tomography we use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the p wave velocity vp and the vp vs ratio extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is 4 km in the upper crust depth 8 km below sea level bsl increasing to 10 km in the lower crust at a depth of 20 km bsl the plumbing system of askja is revealed as a series of high vp vs ratio bodies situated at discrete depths throughout the crust to depths of over 20 km we interpret these to be regions of the crust which currently store melt with melt fractions of 10 the lower crustal bodies are all seismically active suggesting that melt is being actively transported in these regions the main melt storage regions lie beneath askja volcano concentrated at depths of 5 km bsl with a smaller region at 9 km bsl their total volume is 100 km3 using the recorded waveforms we show that there is also likely to be a small highly attenuating magmatic body at a shallower depth of about 2 km bsl

crustal extension ag pb zn veins in the xiong ershan district southern north china craton constraints from the shagou deposit the shagou vein type ag pb zn deposit in the xiong ershan district southern margin of the north china craton is hosted within amphibolite facies metamorphic rocks of the late archean to early paleoproterozoic taihua group the ag pb zn veins are localized in ne to nne trending brittle faults and typically display symmetrical zoning consisting of siderite quartz sphalerite galena and quartz calcite from the margin toward the center of each vein ore related hydrothermal alteration is well developed on both sides of the veins dominated by silicification sericitization chloritization and carbonatization sericite separates extracted from a major ag pb zn vein yield a 40 ar 39 ar plateau age of 140 0 1 0 ma 1 and isochron age of 141 1 1 6 ma 1 indicating that mineralization occurred at the beginning of early cretaceous field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz siderite stage i quartz sphalerite ankerite stage ii quartz galena silver minerals ankerite stage iii and quartz calcite stage iv respectively silver minerals are abundant in all veins and are composed of in paragenetic order argentiferous tetrahedrite polybasite jalpaite argentite and native silver these silver minerals commonly occur as replacements of galena chalcopyrite and other sulfides or as fillings of microfractures in sulfides and quartz microthermometric measurements of primary fluid inclusions in quartz carbonates and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures 267 157 c from aqueous carbonic to aqueous fluids with moderate salinities 7 2 15 9 wt nacl equiv coexisting galena sphalerite pair yields sulfur isotope equilibrium temperatures of 205 to 267 c consistent with the overall homogenization temperatures of fluid inclusions the microthermometric data also indicate that both fluid mixing and fluid rock interaction were important mechanisms for ore precipitation carbonate minerals siderite ankerite calcite spanning the entire mineralization history have 13 c v pdb values of 5 2 to 1 4 and 18 o v smow of 10 9 to 15 0 corresponding to calculated values for the ore fluids of 6 5 to 1 8 and 1 4 to 5 4 respectively 34 s v cdt values of sulfide minerals pyrite sphalerite galena range from 1 1 to 5 5 consistent with a deep seated sulfur source galena separates have 206 pb 204 pb ratios of 17 472 to 17 813 207 pb 204 pb ratios of 15 411 to 15 498 and 208 pb 204 pb ratios of 38 178 to 38 506 the isotope data together with geological and geochronological evidence favor a primary metamorphic source for sulfur and other components in the ore fluids a synthesis of available data suggests that the shagou deposit is a typical vein type ag pb zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the north china craton during the late mesozoic a time period that is manifested by pervasive magmatism widespread formation of metamorphic core complexes and development of faulted basins throughout the eastern part of the craton metamorphic devolatilization of the meso neoproterozoic marine sedimentary rocks previously subducted beneath the xiong ershan district facilitated by extensive magmatism and elevated heat flow due to lithospheric extension could have provided large amounts of ore fluids responsible for the ag pb zn mineralization the ne to nne trending faults affiliated with the transcrustal machaoying fault may have acted as pathways for the upward migration of deep seated metamorphic fluids mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the ag pb zn veins in brittle extensional structures

the geochemistry of carbonate replacement pb zn ag mineralization in the lavrion district attica greece fluid inclusion stable isotope and rare earth element studies strata bound carbonate replacement pb zn ag deposits in the lavrion district greece are spatially related to a late miocene granodiorite intrusion 7 10 ma and various sills and dikes of mafic to felsic composition the plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a ca fe skarn carbonate replacement deposits occur predominantly in marbles upper and lower marble of the basal unit kaesariani schists and along a major detachment fault that separates the basal unit from the upper unit orebodies are mainly strata bound carbonate replacement although sulfides also occur in veins the mineralogy of carbonate replacement deposits is dominated by base metal sulfides and sulfosalts of ag bi sn sb as and pb particularly at plaka and kamariza carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite fluid inclusion studies of sphalerite fluorite calcite and quartz in carbonate replacement deposits suggest that they were deposited from 132 to 365 c from co2 poor low to high salinity fluids 1 20 wt nacl equiv carbon and oxygen isotope compositions of calcite 13c 15 6 to 1 5 and 18o 9 2 to 17 3 intergrown with sulfides reflect variable exchange of the ore bearing fluid with the upper and lower marbles and proximity to the plaka granodiorite post archean australian shale paas normalized rare earth and yttrium patterns of the upper and lower marbles and calcite intergrown with sulfides show positive eu and negative ce anomalies as well as y ho ratios between 40 and 80 normalized rare earth and yttrium patterns of fluorite also have positive eu and negative ce anomalies such anomalies for both the carbonates and fluorite reflect the high ph or high f o2 conditions of the late stage hydrothermal fluids and the likely derivation of calcium from marine carbonates precursors of the upper and lower marbles the range of sulfur isotope compositions for sulfides 34s 4 9 to 5 3 with one outlier of 9 4 in carbonate replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate sulfur isotope compositions of barite from carbonate replacement range from 34s 17 2 to 23 7 per mil and reflect miocene seawater sulfate values if a magmatic source of sulfur is assumed along with an average temperature of 250 c for the ore forming fluids as based on fluid inclusion studies sulfides in carbonate replacement deposits were deposited at values of log f o2 41 to 36 and a ph 5 8 to 9 1 however the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source the carbonate replacement deposits resemble manto type sulfide deposits in mexico central colorado south korea nevada and northern greece

intense seismicity during the 2014 2015 barðarbunga holuhraun rifting event iceland reveals the nature of dike induced earthquakes and caldera collapse mechanisms

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

improved 1d velocity model and deep long period earthquakes in kanlaon volcano philippines implications for its magmatic system abstract we determined an improved 1d p wave velocity model for kanlaon volcano one of the most active volcanoes in the philippines for use in precise earthquake relocation the inversion for velocity model was conducted using the program velest which solved the coupled hypocenter velocity problem from dataset of 251 local earthquakes recorded by the kanlaon volcano seismic network from 2014 to 2017 using the improved velocity model we relocated the volcano tectonic vt earthquakes in kanlaon with hypodd resulting in better constrained hypocenter locations based on the decrease in rms and corresponding error assessment tests subsequently we relocated swarms of long period volcanic earthquakes that occurred from september 1 2017 to august 31 2018 and prompted the philippine institute of volcanology and seismology to raise the volcano s alert level these long period earthquakes had emergent arrivals hence we applied a cross correlation procedure using redpy to obtain an improved arrival time picks these improved hypocenters revealed the existence of deep long period dlp earthquakes that occurred at depths of 23 to 36 km beneath the volcano these are the first dlp events documented in kanlaon since the establishment of the kanlaon volcano observatory in 1961 they were found to belong into families based on similarity of waveforms suggesting they were produced by repetitive non destructive sources the occurrences of dlps in kanlaon reflected the pressurization of its deep seated magmatic system their identification is critical in assessing the current state of kanlaon volcano especially since its last magmatic eruption was in 1902

detection of mantle earthquakes beneath the east african rift summary recordings from a temporary seismic network in the rwenzori region of the east african rift show seven earthquakes at a depth range between 53 and 60 km evidence from receiver functions indicates a crustal thickness of about 32 km in this region suggesting that the earthquakes are located well within the mantle beneath the rift the occurrence of faulting at this depth is of relevance for models of continental rift initiation and related deep magmatic processes

lower crustal earthquakes caused by magma movement beneath askja volcano on the north iceland rift the lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity the icelandic mid atlantic rift is characterized by high heat flow abundant magmatism generating up to 25 30 km thick crust and seismicity within the upper 8 km of the crust in a 20 seismometer survey in july august 2006 within the northern rift zone around the askja volcano we recorded 1700 upper crustal earthquakes cutting off at 7 8 km depth marking the brittle ductile boundary unexpectedly we discovered 100 small magnitude ml 1 5 earthquakes occurring in swarms mostly at 14 26 km depth within the otherwise aseismic lower crust and beneath the completely aseismic middle crust a repeat survey during july august 2007 yielded more than twice as many lower crustal events geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath askja we interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding askja they represent bursts of magma motion opening dykes over distances of a few meters facilitated by the extensional setting of the active rift zone

the margaree orthogneiss an ordovician peri gondwanan mafic felsic igneous complex in southwestern newfoundland the igneous protoliths of the margaree orthogneiss intruded the peri gondwanan rocks of the port aux basques complex in the southwestern corner of the hermitage flexure prior to regional deformation and metamorphism field relationships and u pb geochronology indicate that the margaree orthogneiss represents a 20 km long mafic felsic intrusive complex formed by amphibolite dioritic orthogneiss 474 14 4 ma hornblende bearing tonalitic orthogneiss with mafic enclaves 472 2 5 ma and 465 3 ma biotite bearing granitic orthogneiss and minor ultramafic rocks most amphibolite have the chemistry of volcanic arc tholeiite and are interpreted to be coeval with the intrusion of tonalitic and granitic magma with volcanic arc geochemical signatures locally mafic magmatism continued after the intrusion of the 465 ma granite the margaree orthogneiss and its country rock were overprinted by upper amphibolite facies metamorphism and deformation associated with the final closure of the iapetus ocean a late s

geodynamics of iceland and the signatures of plate spreading abstract iceland rising to more than 2 km above sea level originates from the interaction of a mid oceanic ridge and excessive mantle upwelling the plate boundary between the north american and eurasian plates traverses iceland as a series of seismic and volcanic rift zones the building blocks of the volcanic zones are about 32 volcanic systems typically consisting of a central volcano often with a caldera and an associated fissure swarm one or more recent revision of geometry and nature of the volcanic systems include new constraints on the fissure swarms of western volcanic zone and the one active in 2014 2015 associated to the bar arbunga central volcano countrywide seismic network and historical earthquake activity shows that seismicity is most pronounced in transform zones in south and north iceland and at central volcanoes undergoing volcanic unrest in a two year period 2015 2016 the most seismically active volcanoes were katla and bar arbunga measurements of ground deformation using both gps geodesy and interferometric analysis of synthetic aperture radar images acquired by satellites insar reveal well the present day plate spreading process in iceland geodetically measured spreading rate across iceland is consistent with global plate motion models morvel2010 18 19 mm yr in direction n 100 105 e accommodated by a single rift zone in north iceland overlapping rifts in south iceland and a highly oblique rift on the reykjanes peninsula glacial isostatic adjustment gia in response to ice retreat since 1890 is an additional important processes on a regional scale in iceland responsible for rise of central part of iceland of 30 mm year

seismicity of the askja and barðarbunga volcanic systems of iceland 2009 2015 abstract a large seismic network deployed in the icelandic highlands recorded 100 000 earthquakes from 2009 to 2015 we develop a local magnitude scale appropriate for use in central iceland which is similar to the scale used by the iceland meteorological office using this large catalogue of earthquakes we analyze the spatial and temporal changes in seismicity rates and b values in microearthquakes recorded from the usually ductile lower crust we find that b values are high reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt in contrast b values in the upper crust are variable low b values indicative of a high stress environment are observed during seismic swarms such as those around mt her ubrei and around bar arbunga caldera a persistently seismically active area around a geothermal area within askja caldera has a b value around 1 but has a strong annual cycle of seismicity we attribute the annual cycle to varying load from the snow cover modulating the seismicity seismicity driven by the intrusion of a large dyke has a b value well above 1 driven by the high pore fluid pressures and thermal gradients around the dyke

geochronology and geochemistry of the max porphyry mo deposit and its relationship to pb zn ag mineralization kootenay arc southeastern british columbia canada the max porphyry mo deposit is located at the northern end of the kootenay arc in southeastern british columbia canada molybdenum mineralization is genetically related to the variably altered trout lake granodiorite stock which has lithogeochemical affinities to the low fluorine calc alkaline granodiorite hosted porphyry mo deposit type three molybdenite samples from early and late mo bearing veins yield re os dates that overlap within analytical error with a weighted average age of 80 3 0 2 ma these dates are in close agreement with two 206pb 238u weighted average zircon ages for phases of the trout lake stock at 80 3 1 6 and 79 2 1 0 ma indicating that crystallization of the stock and hydrothermal molybdenite formation were coeval and likely cogenetic 40ar 39ar plateau ages for primary biotite in igneous dikes secondary biotite in early potassic alteration and muscovite in late phyllic alteration range from 80 to 76 ma the younger 40ar 39ar plateau ages may reflect unusually slow cooling of the magmatic hydrothermal system or a late thermal disturbance pb zn ag vein deposits are common throughout the kootenay arc and several such veins are located within and proximal to the max porphyry mo deposit the presence of rare molybdenite associated with sphalerite and argentiferous galena in some of these pb zn ag veins suggests that they represent a late stage event related to the max magmatic hydrothermal system fluid inclusions in quartz from mo and pb zn ag veins are similar and can be divided into two compositional groups type 1 aqueous liquid rich fluid inclusions and type 2 aqueous carbonic liquid rich fluid inclusions pressure corrected 1 4 1 7 kbar trapping temperatures of typical type 1 inclusions from mo bearing veins range from 430 to 460 c and for pb zn ag veins from 335 to 365 c the ranges of salinity 0 12 wt nacl equiv in fluid inclusions from both vein types are almost identical and type 2 inclusions from both vein types contain similar amounts of co2 avg xco2 0 08 with traces of ch4 the similarity of chemistry in the two vein types suggests a genetic link between porphyry mo mineralization and adjacent pb zn ag deposits however muscovite from the alteration halo around a pb zn ag vein within the max deposit yielded a 40ar 39ar plateau age of 72 2 0 5 ma this date is significantly younger than the 80 ma age of porphyry magmatism and mo mineralization suggesting either that this event is in fact unrelated to porphyry mo mineralization at 80 ma or that the k ar isotope system has been disturbed by a later thermal event

formation of oskjuvatn caldera at askja north iceland mechanism of caldera collapse and implications for the lateral flow hypothesis the oskjuvatn caldera at askja volcano north iceland was formed as a result of an explosive eruption at askja on 28th 29th march 1875 oskjuvatn is one of the youngest collapse calderas on earth and its initiation and growth were observed and documented by explorers and geologists working in the askja region between 1875 and 1932 it is an example of caldera formation where the volume of the caldera collapse far exceeds the volume of associated erupted products the discrepancy between the collapse volume and associated erupted products has been used by previous workers as a justification for the hypothesis that the injection of lateral dykes from askja fed the february to october 1875 n jahraun fissure eruptions at the sveinagja graben some 60 km north of askja however historical accounts documenting the caldera formation in sufficient detail show clearly that oskjuvatn formed by piecemeal collapse taking over 40 years to reach its current form we use these accounts to undertake a detailed examination of the stages of caldera collapse and to compare them with the known magmatic output of askja in the years following the 1875 eruption arcgis software was used to calculate the volume of oskjuvatn during the various stages of collapse and the volume of material erupted after 1875 while a dyke extending between askja and sveinagja may be implied to account for the missing volume our results demonstrate that the volume of oskjuvatn in july 1875 closely corresponds to the volume of rhyolitic material erupted on 28th 29th march 1875 in addition geochemical evidence indicates that the n jahraun magmas were sourced from a deeper reservoir rather than by lateral injection from askja s shallow crustal magma chamber we therefore suggest that the injection of sills and dykes in a coherent intrusion complex beneath askja central volcano combined with background deflation of the magma storage zone beneath oskjuvatn caldera over 30 to 40 years following 1875 satisfactorily accommodates the volume required to form oskjuvatn caldera

structural evolution of a major fault zone the cape ray fault zone southwestern newfoundland canada the cape ray fault zone is a major paleozoic structure in southwestern newfoundland and occurs at or close to the boundary between two major continental blocks laurentia and avalonia a detailed structural analysis demonstrates that the fault records early reverse sinistral thrusting of the grand bay complex at amphibolite grade d2 followed by a protracted event d3 characterized by reverse dextral thrusting of the grand bay complex rocks on top of the supracrustal rocks of the windsor point group and retrogression to greenschist facies as well as a pre 384 ma orogen parallel dextral transcurrent mylonite d4 during the later stages of the collision regional scale strain partitioning induced heterogeneity of strain both along and across the strike of the cape ray fault zone the east west oriented segment of the cape ray fault zone is a tear fault that accommodated differential displacement along the length of the fault later stages of the deformation include post 384 ma sinistral transcurrent r

deformation in the northern volcanic zone of iceland 2008 2014 an interplay of tectonic magmatic and glacial isostatic deformation gps measurements spanning 2008 to 2014 are used to derive the surface velocity field across the northern volcanic zone nvz of iceland a subaerial part of the divergent boundary between the north american and eurasian plates no volcanic activity nor magmatic intrusions were detected in the zone during this time period we infer an extensional rate of 17 4 0 3 0 2 mm yr in direction 292 0 0 6 0 5 consistent with the results of previous studies and current plate motion models including morvel2010 and geodvel2010 the horizontal velocity field reveals about 50 km wide stretching zone caused by the divergent plate movements glacial isostatic adjustment gia induces uplift of over 20 mm yr at the northern edge of vatnajokull ice cap and 3 4 mm yr horizontal motion directed away from the ice cap deformation in the nvz between 2008 and 2014 can be reproduced by a combination of models relating to several different processes i mogi sources for volcanic and geothermal deformation at the askja and krafla volcanoes ii scaled version of a velocity field derived from a glacial isostatic model and iii simple arctangent based model for secular plate spreading we find the approximate location of the plate boundary spreading axis as well as its locking depth the spreading axis lies through the krafla fremrinamar and askja central volcanoes the most active ones in the nvz it does not appear to follow the general direction of each fissure swarm but rather to change direction at the central volcanoes the locking depth is on average within the 7 9 km range

models of tremor and low frequency earthquake swarms on montserrat recent observations from soufriere hills volcano in montserrat reveal a wide variety of low frequency seismic signals we discuss similarities and differences between hybrid earthquakes and long period events and their role in explosions and rockfall events these events occur usually in swarms and occasionally merge into tremor an observation that can shed further light on the generation and composition of harmonic tremor we use a 2d finite difference method to model major features of low frequency seismic signatures and compare them with the observations a depth dependent velocity model for a fluid filled conduit is introduced which accounts for the varying gas content in the magma and the impact on the seismic signals is discussed we carefully analyse episodes of tremor that show shifting spectral lines and model those in terms of changes in the gas content of the magma as well as in terms of a time dependent triggering mechanism of low frequency resonances in this way we explain the simultaneous occurrence of low frequency events and tremor with a spectral content comprising integer harmonics

source and evolution of ore forming hydrothermal fluids in the northern iberian pyrite belt massive sulphide deposits sw spain evidence from fluid inclusions and stable isotopes a fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits aguas tenidas este concepcion san miguel san telmo and cueva de la mora located in the northern iberian pyrite belt the isotopic analyses were mainly performed on quartz chlorite carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits and also on some fluid inclusion waters homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 c salinity of most fluid inclusions ranges from 2 to 14 wt nacl equiv a few cases with th 80 110 c and salinity of 16 24 wt nacl equiv have been also recognized in addition fluid inclusions from the soloviejo mn fe jaspers 160 190 c and 6 wt nacl equiv and some late to post hercynian quartz veins 130 270 c and 4 wt nacl equiv were also studied isotopic results indicate that fluids in equilibrium with measured quartz 18ofluid 2 to 4 chlorites 18ofluid 8 14 dfluid 45 to 27 whole rocks 18ofluid 4 7 dfluid 15 to 10 and carbonates 18oankerite 14 5 16 13cfluid 11 to 5 evolved isotopically during the lifetime of the hydrothermal systems following a waxing waning cycle at different temperatures and water rock ratios the results fluid inclusions 18o d and 13c values point to a highly evolved seawater along with a variable but significant contribution of other fluid reservoirs such as magmatic and or deep metamorphic waters as the most probable sources for the ore forming fluids these fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust

the source scanning algorithm mapping the distribution of seismic sources in time and space summary we introduce a new method which we call the source scanning algorithm ssa for imaging the distribution of seismic sources in both time and space using trial locations and origin times the method calculates the brightness function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times the spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness the greatest advantages of this method are that 1 it exploits waveform information both arrival times and relative amplitudes without the need to calculate highfrequency synthetic seismograms and 2 it requires neither pre assembled phase picking data nor any a priori assumptions about the source geometry a series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval finally we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip ets sequence in the northern cascadia subduction zone

metamorphic origin of the coeur d alene base and precious metal veins in the belt basin idaho and montana fluid inclusions geochronology and field studies show that the world class base and precious metal veins of the coeur d alene district idaho and montana are the product of deformation coupled with regional metamorphism of the belt basin around 850 ma fluid inclusion data show that the mineralizing solutions were complex co2 ch4 cnhm n2 h2o nacl fluids that changed in composition from ch4 cnhm rich fluids in the older zn rich veins to co2 rich fluids in the younger ag rich veins ore deposition occurred at about 250 to 350 c and at pressures in excess of 1 kbar

implications of fracturing mechanisms and fluid pressure on earthquakes and fault slip data in the east iceland rift zone comparison of the eroded off rift zone left inactive by plate motion with the inner active seismic rift zone allows us to constrain the fracturing mechanisms in eastern off rift zone we measured 423 fault slips including normal and strike slip faults inversion of fault slip data reveals the parallelism of the minimum stress 3 computed for the normal and for the strike slip faulting and consistency with the direction of plate divergence north of the vatnajokull in the active rift zone we analysed 14 250 earthquakes recorded by the icelandic meteorological office imo between 2004 and 2009 especially northeast of the askja volcano two main average focal depths were determined at 5 km her ubrei table mountain and 15 km upptyppingar hyaloclastite ridge the double couple focal mechanisms determined by imo revealed that more than half of the mechanisms are strike slip faulting type both in active and off rift zone reveals an unusual importance of strike slip regime in such an extensional tectonic context this can be explained by stress permutations 1 2 similarities in terms of stress orientations and type of faulting are observed both in the old and present day rift zones we assume that the seismic events may be generated by rapid deep magma intrusion also associated with shallower hydrothermal activities we thus propose that the presence of fluids shear failure double couple focal mechanism and stress permutations in both active and off rift zones are closely linked assuming a drucker prager failure criterion we evaluated analytically that a state near of lithostatic pore pressure is a necessary condition for shear failure at shallow 5 km and deep 15 km depth in a simple context of crustal extension that allows for stress permutations however processes favouring stress permutations cannot be further constrained from our observations since the dynamics of fluid materiel heterogeneity and post glacial rebound can also play a significant role

the early cretaceous yangzhaiyu lode gold deposit north china craton a link between craton reactivation and gold veining the yangzhaiyu gold deposit is one of numerous lode gold deposits in the xiaoqinling district southern margin of the north china craton gold mineralization is hosted in neoarchean to early paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein proximal alteration zone ore related hydrothermal alteration is dominated by sericite quartz sulfide assemblages close to gold veins and biotite quartz pyrite chlorite epidote alteration generally distal from mineralization pyrite is the predominant sulfide mineral locally coexisting with minor amounts of chalcopyrite sphalerite and galena gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with au bearing tellurides mainly petzite and calaverite coexisting with hessite tellurobismuthite and altaite fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures 175 313 c from aqueous or aqueous carbonic fluids with moderate salinity 5 14 wt nacl equiv 34s values of sulfide minerals range mainly from 2 0 to 4 4 whereas auriferous quartz vein samples have 18o values of 12 4 to 9 6 with calculated 18oh2o values of 6 0 to 3 2 gold related pyrite grains yield elevated 3he 4he ratios 1 51 0 32 ra relative to crustal reservoirs and mantle like 20ne 22ne and 21ne 22ne ratios 9 90 9 68 and 0 029 respectively the stable and noble gas isotopes thus suggest deep seated most likely magmatic and mantle derived sources for the ore fluids sulfur and by inference other components in the ore system 40ar 39ar dating of ore related sericite and biotite separates indicates two episodes of gold genesis at 134 5 to 132 3 and 124 3 to 123 7 ma the mineralization ages overlap zircon u pb ages of 141 0 1 6 to 125 8 1 4 ma 2 for the wenyu and niangniangshan monzogranite plutons and a number of mafic to intermediate dikes intruding these plutons all being proximal to the yangzhaiyu gold deposit the synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur the geochronologic data also suggest that gold veining took place billions of years after the stabilization of the north china craton and associated metamorphism in the late archean to early paleoproterozoic this contrasts sharply to lode gold deposits in other precambrian cratons that formed predominantly in late archean to paleoproterozoic temporarily and genetically related to regional high grade metamorphism and compressional or transpressional tectonism available data have demonstrated that the north china craton was reactivated in the late mesozoic as marked by voluminous igneous rocks faulted basin formation high crustal heat flow and widespread metamorphic core complexes in the eastern part of the craton it is thus suggested that the yangzhaiyu gold deposit together with other deposits of similar ages in the xiaoqinling district were products of this craton reactivation event lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy fluid and sulfur required for the formation of the gold deposits

silver lead zinc veins and crustal hydrology during eocene extension southeastern british columbia canada abstract the kokanee range southeastern british columbia canada contains more than 370 ag pb zn vein and replacement deposits hosted by the middle jurassic nelson batholith and surrounding cambrian to triassic metasedimentary rocks all deposits are in the hanging wall of the slocan lake fault an east dipping low angle normal fault that reaches the moho and along which the valhalla metamorphic core complex has been unroofed during the eocene fluid inclusions in sphalerite quartz or siderite indicate a temperature and pressure of trapping of approximately 300 c and 0 17 gpa the fluid inclusions display a large range of salinities from 0 to 18 wt nacl eq assuming lithostatic pressure at the time of mineralization the depth of mineralization was 6 3 km 34ssulfide values display a 20 range and a regional zonation that correlates with the distribution of sulfur isotopic compositions of host rocks in contrast gd13csiderite values are homogeneous 7 1 0 5 over an area of 1200 km2 it is concluded that s was derived from local country rocks whereas c was derived from mantle co2 degassing the 18osiderite and 18orock values display regional zonations revealing fluid flow paths of a large ancient hydrothermal system three different fluids are identified from salinity 18o and d values 1 1 a high salinity deep seated and isotopically crust equilibrated fluid 2 2 a low salinity upper crustal fluid of evolved meteoric origin 3 3 meteoric water mixing between thermally similar fluids having different salinities is proposed to have been the dominant mineralizing process regional isotopic zonations are controlled by the slocan lake fault which channelled deep seated hydrothermal fluids and mantle co2 to higher crustal levels where mixing occurred with upper crustal fluids that leached local sulfur meteoric water invaded the fracture network in the waning stages of the hydrothermal system

geodetic data shed light on ongoing caldera subsidence at askja iceland subsidence within the main caldera of askja volcano in the north of iceland has been in progress since 1983 here we present new ground and satellite based deformation data which we interpret together with new and existing micro gravity data to help understand which processes may be responsible for the unrest from 2003 to 2007 we observe a net micro gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro gravity increase while the subsidence continues we infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary mass movements at active volcanoes can be caused by several processes including water table lake level movements hydrothermal activity and magma movements we suggest that here magma movement and or a steam cap in the geothermal system of askja at depth are responsible for the observed micro gravity variations in this respect we rule out the possibility of a shallow intrusion as an explanation for the observed micro gravity increase but suggest magma may have flowed into the residing shallow magma chamber at askja despite continued subsidence in particular variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation

multiple melt injection along a spreading segment at askja iceland lower crustal earthquakes 12 25 km depth have been detected since august 2005 in the askja volcanic system along the north iceland rift in the normally ductile part of the crust the earthquakes occur in three clusters which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust seismic velocity vp vs ratios are consistent with the presence of partial melt in the lower crust at askja the spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active this pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath iceland compared to the rest of the mid atlantic ridge however the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano askja i e segment center during the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano therefore on long time scales melt supply is probably greater at the segment center with melt redistribution in the upper crust even though there are multiple points of lower crustal injection along the segment

new seismic and tomography data in the southern part of the harghita mountains romania southeastern carpathians connection with recent volcanic activity the southern part of the southeastern carpathians represents the site of the most recent volcanic eruptions of the entire carpathian pannonian region the products of these eruptions range from 42 to 10 ka radiocarbon ages in the south harghita mountains high k calc alkaline rocks with adakite like features and at 1 2 0 6 ma k ar ages in the persani mountains alkali basalts they were emplaced in a post collisional regime ciomadul volcano is located at the southernmost part of the nw se oriented c limani gurghiu harghita range crossing the inner part of the southeastern carpathians and in the rough proximity of the vrancea seismic zone at ca 60 km toward nw its magma generation is attributed to geodynamic events closely related to the seismogenic area a number of particular geophysical and geochemical features located in the study region including 1 the abrupt attenuation of the seismic waves originating from the vrancea intermediate depth foci 2 the most intense heat flow anomaly in romania 3 the most prominent 3he 4he anomaly measured in natural postvolcanic gas emanations are all in favor of the hypothesis of a still existing hot local magma chamber data acquired during recent seismic monitoring of the vrancea zone and its neighborhoods suggest an enhancement of the local seismicity beneath the southern edge of the south harghita mts both at crustal and subcrustal levels at the same time recent tomography images obtained using local earthquake data correlate well with the presence of a vertically extended low velocity zone coming from the upper mantle to the assumed magmatic chambers located in the crust the present data supporting the presence of an active crustal magma chamber beneath ciomadul allow us to consider that future volcanic activity at this volcano cannot be discarded

the fissure swarm of the askja volcanic system along the divergent plate boundary of n iceland divergent plate boundaries such as the one crossing iceland are characterized by a high density of subparallel volcanic fissures and tectonic fractures collectively termed rift zones or fissure swarms when extending from a specific volcano volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events when magma intrudes fractures to form dikes and even feeds fissure eruptions we mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern iceland the study area is 1 800 km2 located within and north of the askja central volcano the style of fractures changes with distance from askja close to askja the swarm is dominated by eruptive fissures the proportion of tectonic fractures gets larger with distance from askja this may indicate that magma pressure is generally higher in dikes close to askja than farther away from it volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3 4 identified calderas in askja the average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity as this deviation decreases gradually northward we suggest that the effect of the triple junction of the north american eurasian and the hreppar microplate is a likely cause for this deviation shallow tectonic earthquakes in the vicinity of askja are often located in a relatively unfractured area between the fissure swarms of askja and kverkfjoll these earthquakes are associated with strike slip faulting according to fault plane solutions we suggest that the latest magma intrusions into either the askja or the kverkfjoll fissure swarms rotated the maximum stress axis from being vertical to horizontal causing the formation of strike slip faults instead of the dilatational fractures related to the fissure swarms the activity in different parts of the askja fissure swarm is uneven in time and switches between subswarms as shown by a fissure swarm that is exposed in an early holocene lava nw of her ubrei but disappears under a younger 3500 4500 bp lava flow we suggest that the location of inflation centres in askja central volcano controls into which part of the askja fissure swarm a dike propagates the size and amount of fractures in the kollottadyngja lava shield decrease with increasing elevation we suggest that this occurred as the depth to the propagating dike s was greater under central kollottadyngja than under its flanks due to topography

isotope geochemistry of the tieluping silver lead deposit henan china a case study of orogenic silver dominated deposits and related tectonic setting the tieluping silver deposit which is sited along ne trending faults within the high grade metamorphic basement of the xiong er terrane in the qinling orogenic belt is part of an important recently discovered mesozoic orogenic type ag pb belt ore formation includes three stages an early barren quartz pyrite stage e an intermediate polymetallic sulfide ore stage m and a late barren carbonate stage l carbon sulfur and lead isotope systematics indicate that the e stage fluids are deeply sourced the l stage fluids are shallow sourced meteoric water whereas the m stage fluids are a mix of deep sourced and shallow sourced fluids sulfur and lead isotope data show that the ore forming fluids must have originated from a source with elevated radiogenic lead and low 34s values that differs significantly from exposed geologic units in the xiong er terrane the lower crust and the mantle this supports the view that the carbonate shale chert sequences of the guandaokou and luanchuan groups south of the machaoying fault might be the favorable sources although little is known about their isotopic compositions a tectonic model that combines collisional orogeny metallogeny and hydrothermal fluid flow is proposed to explain the formation of the tieluping silver deposit during the mesozoic collision between the north china craton and south china block early mid triassic indosinian orogeny crustal slabs containing the carbonate shale chert sequences of the guandaokou and luanchuan groups locally rich in organic matter carbonaceous shale were thrust northwards beneath the xiong er terrane along the machaoying fault metamorphic devolatilisation of this underthrust slab probably provided the ore forming fluids to develop the ag pb ore belt which includes the tieluping silver deposit fluids and magmas were emplaced during extensional stages related to the jurassic cretaceous yanshanian orogeny

seismic activity related to the 2000 eruption of the hekla volcano iceland the 2000 hekla eruption took place from february 26 to march 8 its seismic expressions were a swarm of numerous small earthquakes related to its onset and low frequency volcanic tremor that continued throughout the eruption a swarm of small earthquakes was observed some 80 min before the onset of the eruption and the size of the events increased with time low frequency volcanic tremor with a characteristic frequency band of 0 5 1 5 hz and dominant spectral peak s at 0 7 0 9 hz became visible at 18 19 gmt on february 26 marking the onset of the eruption the tremor amplitude rose quickly and was very high in the beginning of the eruption however it soon began to decrease after about an hour in general the seismic activity related to the 2000 hekla eruption was very similar to what was observed in the previous eruption in 1991 based on knowledge gained from seismicity and strain observations from 1991 this was the first time that a hekla eruption was predicted

seismogenic magma intrusion before the 2010 eruption of eyjafjallajokull volcano iceland we present relatively relocated earthquake hypocentres for 1000 microearthquakes ml 3 that occurred during the 2 weeks immediately prior to the 2010 march 20 fissure eruption at fimmvor uhals on the flank of eyjafjallajokull volcano in iceland our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km2 and approximately 4 km below sea level 5 km below the surface seismic activity in the final 4 d preceding the eruption extended to shallower levels 2 km below sea level and propagated to the surface at the fimmvor uhals eruption site on the day the eruption started we demonstrate using synthetic data that the observed apparent 1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors 0 01 0 02 s in arrival time data where the signal to noise ratio was sufficiently good to make subsample arrival time picks by cross correlation of both p and s wave arrivals the mean depth of 103 events in an individual cluster were constrained to 3 84 0 06 km epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used within clusters of typically 100 recorded earthquakes most of the arrivals exhibit similar waveforms and identical patterns of p wave first motion polarities across the entire monitoring network the clusters of similar events comprise repetitive sources in the same location with the same orientations of failure probably on the same rupture plane the epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex these constrictions may act as a form of valve in the country rock which ruptures when the melt pressure exceeds a critical level then reseals after a pulse of melt has passed through this would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location we infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth with seismogenic pinch points occurring between aseismic compartments of the sills or between adjacent magma lobes as they inflated during the final 4 d preceding the eruption onset between 22 30 and 23 30 utc on 2010 march 20 the seismicity suggests that melt progressed upwards to a depth of 2 km this seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke subsequently on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the fimmvor uhals eruption site

deflation of the askja volcanic system constraints on the deformation source from combined inversion of satellite radar interferograms and gps measurements the askja central volcano in northern iceland has been continuously subsiding at least since 1983 gps and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 in the center of the askja caldera without any eruptive activity we have performed an interferometric analysis of synthetic aperture radar images insar of the area utilizing data from the ers satellites we observe subsidence of the askja caldera and its fissure swarm up to a distance of 25 km from the volcano we evaluate the geometry of the magma chamber at askja from a combined inversion of satellite radar interferograms and gps measurements several models were tested including a mogi point source as well as an ellipsoidal source the use of an ellipsoidal source instead of a mogi source gives an estimate of the dimensions of the magma chamber and its deflating pressure whereas these parameters are not independently resolved if a mogi source is used two source models were also considered in order to explain the additional subsidence observed along the askja fissure swarm we tested a model using two mogi sources at different depths a shallow ellipsoidal cavity with a deeper mogi source and then a shallow mogi source with a deeper elongated ellipsoid oriented along the fissure swarm results indicate that an ellipsoidal source at about 3 km depth can accommodate most of the deflation occurring in the caldera residual subsidence occurs along the askja fissure swarm suggesting the existence of a deeper source of contraction we interpret this signal in terms of subsidence of the plate boundary

experimental stretching of fluid inclusions in calcite implications for diagenetic studies homogenization temperatures of fluid inclusions trapped in calcite crystals may change with increased heating beyond the homogenization temperature fluid inclusions trapped in calcite during basin subsidence will tend to yield homogenization temperatures approaching maximum burial temperatures diagenetic studies that depend on these data will be biased coarse crystalline calcite containing saline fluid inclusions was used to examine the influence of increased temperatures and pressures on measured homogenization temperatures stepped heating experiments were conducted at 1 atm over a temperature range of 90 to 245 c along with hydrothermal experiments at confining pressures of 173 and 691 bar and temperatures of 160 250 and 350 c in all cases significant increases in the homogenization temperatures of the inclusion population were recorded presumably stresses generated during heating of the fluid inclusions exceeded the combined strength of the calcite crystal and the external pore pressure and thus caused fluid inclusion stretching size is a secondary factor that controls fluid inclusion reequilibration

time dependent changes in volcanic inflation rate near three sisters oregon revealed by insar 1 an extensive area near the three sisters volcanic center oregon has been actively uplifting since 1996 in this study we use interferometric synthetic aperture radar insar to assess the three sisters uplift in time and space from 1992 through 2010 we present the first insar line of sight time series of the deformation refine the onset of volcanic intrusive activity assess the changes in deformation through time and determine whether inflation is still occurring we model insar data to determine the source geometry that best describes the uplift and create an inflation time series of the deformation our results reveal an intrusion located at 5 7 km depth with a cumulative volume of magma of 5 7 107 m3 as of fall 2010 the input of magma started gradually around the summer of 1996 increased significantly from 1998 to 2003 and then decreased in rate from 2004 through 2010 we present evidence that a swarm in 2004 was associated with the decrease in the source inflation rate

lower crustal seismic activity in the adana basin eastern mediterranean possible connection to gravitational flexure abstract high quality broadband data together with the application of the double difference relocation technique has been used to study the characteristics of the lower crustal seismicity in the adana basin in southwestern turkey deep events are clearly seen to be restricted only to the adana basin and never extend outside its boundaries furthermore the seismogenic zone is observed to align roughly with the main axis of the basin and plunges steadily in the ssw direction following the basement trend of the adana basin similarities between geometries of the basin evolution and the deep seismic production suggest that both processes are closely related a flexure process is proposed related to the subsidence of the adana basin the seismogenic zone originally at a shallow depth is assumed to have been displaced vertically into the lower crust by flexure the temperature evolution of the crust during the flexure has been studied in detail using finite difference modeling with amplitude and duration parameters taken from earlier studies it has been concluded that the physical conditions for brittle fracturing remained unchanged for an extended period of time after the flexure the brittle layers originally at shallow depths preserved their original thermal properties after the subsidence and will continue to produce earthquakes at considerable depths numerical tests using inferred parameters imply a total vertical shift of 7 8 km for the seismogenic zone discussions for additional processes which may further contribute to the cooling of the crust are also included

fault textures in volcanic conduits evidence for seismic trigger mechanisms during silicic eruptions it is proposed that fault textures in two dissected rhyolitic conduits in iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows detailed field and petrographic analysis of such textures may shed light on the origin of long period and hybrid volcanic earthquakes at active volcanoes there is evidence at each conduit investigated for multiple seismogenic cycles each of which involved four distinct evolutionary phases in phase 1 shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow forming the angular fracture networks that initiated faulting cycles transient pressure gradients were generated as the fractures opened which led to fluidisation and clastic deposition of fine grained particles that were derived from the fracture walls by abrasion fracture networks then progressively coalesced and rotated during subsequent slip phase 2 developing into cataclasite zones with evidence for multiple localised slip events fluidisation and grain size reduction phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction controlled stick slip behaviour increasing cohesion of cataclasites then led to aseismic distributed ductile deformation phase 3 and generated deformed cataclasite zones which are enriched in metallic oxide microlites and resemble glassy pseudotachylite continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite rich flow bands in obsidian phase 4 overall the mixed brittle ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle ductile transition with the rheological response mediated by strain rate variations and frictional heating fault processes in highly viscous magma are compared with those elsewhere in the crust and this comparison is used to appraise existing models of volcano seismic activity based on the textures observed it is suggested that patterns of long period and hybrid earthquakes at silicic lava domes reflect friction controlled stick slip movement and eventual healing of fault zones in magma which are an accelerated and smaller scale analogue of tectonic faults

giant mesozoic gold provinces related to the destruction of the north china craton abstract lode gold deposits in precambrian cratons represent the world s major gold source and were mostly generated during formation and stabilization of the cratons however there is an extraordinary exception in the north china craton ncc where lode gold deposits formed after prolonged stabilization of the craton molybdenite re os and hydrothermal sericite and biotite 40 ar 39 ar dating of major gold deposits from the xiaoqinling district southern ncc bracket their emplacement in the range of 154 1 1 1 to 118 9 1 2 ma n 23 postdating formation of the craton by more than 1 7 billion years fluid inclusions extracted from gold bearing pyrite have elevated 3 he 4 he ratios 1 52 0 22 ra and mantle like ne isotopes 20 ne 22 ne 10 02 9 22 and 21 ne 22 ne 0 033 0 027 indicating presence of mantle derived fluids in the ore system measured 34 s of pyrite and d and 18 o of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic mantle source for sulfur and ore fluids gold deposits of similar ages also widely occur in the eastern and northern margins of the ncc which together with those in the xiaoqinling district have a total reserve of 2500 t gold forming the only known giant late mesozoic gold province in the world s precambrian cratons these deposits formed coevally with extensive felsic to mafic magmatism development of intracontinental rift basins and exhumation of metamorphic core complexes across the eastern ncc events interpreted as indicating thinning and destruction of the lithosphere beneath the craton rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids sulfur and by inference other ore components to form the giant gold provinces

tectonic implications from re os dating of mesozoic molybdenum deposits in the east qinling dabie orogenic belt abstract the east qinling dabie molybdenum belt is part of a larger east west trending metallogenic belt in eastern china most of the molybdenum deposits occur as porphyry or porphyry skarn type but there are also some vein type deposits following systematic re os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits we have recognized that the molybdenum mineralization in the east qinling dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks three pulses of granitoid magmatism and mo mineralization are recognized corresponding to significant tectonic events in the east qinling dabie belt vein type deposits dated at 233 221 ma were formed in detachment fractures indicating localized extension within the collisional setting of the north china and yangtze cratons i type and transitional i and s type granites and related mineralization dated at 148 138 ma may have formed part of a continental magmatic arc with widespread magmatism and back arc extension caused by subduction of the izanagi or paleopacific plate beneath the eurasian continent in a wnw ese direction in the late jurassic early cretaceous both s type and transitional s and i type granite associated porphyry molybdenum deposits dated at 131 112 ma are part of an extensive mineralization event throughout east china that can be ascribed to regional large scale lithospheric thinning delamination and thermal erosion

cooling magma model for deep volcanic long period earthquakes deep long period events dlp events or deep low frequency earthquakes deep lfes are deep earthquakes that radiate low frequency seismic waves while tectonic deep lfes on plate boundaries are thought to be slip events there have only been a limited number of studies on the physical mechanism of volcanic dlp events around the moho crust mantle boundary beneath volcanoes one reasonable mechanism capable of producing their initial fractures is the effect of thermal stresses since ascending magma diapirs tend to stagnate near the moho where the vertical gradient of density is high we suggest that cooling magma may play an important role in volcanic dlp event occurrence assuming an initial thermal perturbation of 400 c within a tabular magma of half width 41 m or a cylindrical magma of 74 m radius thermal strain rates within the intruded magma are higher than tectonic strain rates of 10 14 s 1 and produce a total strain of 2 10 4 shear brittle fractures generated by the thermal strains can produce a compensated linear vector dipole mechanism as observed and potentially also explain the harmonic seismic waveforms from an excited resonance in our model we predict correlation between the particular shape of the cluster and the orientation of focal mechanisms which is partly supported by observations of aso and ide 2014 to assess the generality of our cooling magma model as a cause for volcanic dlp events additional work on relocations and focal mechanisms is essential and would be important to understanding the physical processes causing volcanic dlp events

silver lead zinc veins metamorphic core complexes and hydrologic regimes during crustal extension the kokanee range ag pb zn au vein and replacement deposits formed during eocene crustal extension and unroofing of the valhalla metamorphic core complex in the hanging wall of the transcrustal slocan lake fault the 34ssulfide 18oquartz 18osiderite and galena pb isotopic ratios for vein minerals display regional zonations revealing fluid flow paths of a large fossil hydrothermal system sulfur was derived from local country rocks but carbon was derived from a deep seated reservoir possibly the upper mantle lead is a mixture of local upper crustal pb with lower crustal and depleted upper mantle pb regional isotopic zonations were controlled by deep fracture zones such as the slocan lake fault which channeled lower crustal and mantle pb and mantle co2 to higher crustal levels where mixing occurred with highly evolved meteoric waters that had leached local sulfur and upper crustal ph this is the first ag pb zn vein district for which a genetic link between mineralization and metamorphic core complex unroofing is demonstrated

earthquake distribution patterns in africa their relationship to variations in lithospheric and geological structure and their rheological implications we use teleseismic waveform inversion along with depth phase analysis to constrain the centroid depths and source parameters of large african earthquakes the majority of seismic activity is concentrated along the east african rift system with additional active regions along stretches of the continental margins in north and east africa and in the congo basin we examine variations in the seismogenic thickness across africa based on a total of 227 well determined earthquake depths 112 of which are new to this study seismogenic thickness varies in correspondence with lithospheric thickness as determined from surface wave tomography with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km in regions of thin lithosphere the seismogenic thickness is typically limited to 20 km larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of 1500 ma where the east african rift passes around the archean cratons of africa through the older proterozoic mobile belts these correlations are likely to be related to the production affected by method and age of basement formation and preservation affected by lithospheric thickness of a strong anhydrous lower crust the congo basin contains the only compressional earthquakes in the continental interior simple modelling of the forces induced by convective support of the african plate based on long wavelength free air gravity anomalies indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in africa seismicity along the margins of africa reflects a mixture between oceanic and continental seismogenic characteristics with earthquakes in places extending to 40 km depth

1983 2003 decaying rate of deflation at askja caldera pressure decrease in an extensive magma plumbing system at a spreading plate boundary new deformation data from the askja volcano iceland show that the volcano s caldera has been deflating continuously for over 20 years and confirm that the rate of subsidence is slowing down the decay in subsidence rate can be fitted with a function of the form e t where is 39 years reanalysis of gps data from 1993 1998 show that these data can be fitted with a model calling for two mogi point sources one shallow and another one much deeper 16 2 km depth pressure decrease occurs in both sources the deeper source is responsible for observed horizontal contraction towards askja at distances that cannot be explained by the shallower source plate spreading of 19 mm year distributed evenly over about 100 km wide zone is also favoured by the data

forecasting the path of a laterally propagating dike an important aspect of eruption forecasting is predicting the path of propagating dikes we show how lateral dike propagation can be forecast using the minimum potential energy principle we compare theory to observed propagation paths of dikes originating at the bar arbunga volcano iceland in 2014 and 1996 by developing a probability distribution for the most likely propagation path the observed propagation paths agree well with the model prediction we nd that topography is very important for the model and our preferred forecasting model considers its in uence on the potential energy change of the crust and magma we tested the in uence of topography by running the model assuming no topography and found that the path of the 2014 dike could not be hindcasted the results suggest that lateral dike propagation is governed not only by deviatoric stresses but also by pressure gradients and gravitational potential energy furthermore the model predicts the formation of curved dikes around cone shaped structures without the assumption of a local deviatoric stress eld we suggest that a likely eruption site for a laterally propagating dike is in topographic lows the method presented here is simple and computationally feasible our results indicate that this kind of a model can be applied to mitigate volcanic hazards in regions where the tectonic setting promotes formation of laterally propagating vertical intrusive sheets

motion in the north iceland volcanic rift zone accommodated by bookshelf faulting along mid ocean ridges the extending crust is segmented1 on length scales of 10 1 000 km where rift segments are offset from one another motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis where segments overlap non transform offsets with a variety of geometries2 accommodate shear motions here we use micro seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north iceland between the rift segments we identify a series of faults that are aligned sub parallel to the orientation of the main rift these faults slip through left lateral strike slip motion yet movement between the overlapping rift segments is through right lateral motion together these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf faulting mechanism named after its resemblance to a tilting row of books on a shelf3 the faults probably reactivated existing crustal weaknesses such as dyke intrusions that were originally oriented parallel to the main rift and have since rotated about 15 clockwise reactivation of pre existing rift parallel weaknesses contrasts with typical mid ocean ridge transform faults and is an important illustration of a non transform offset accommodating shear motion between overlapping rift segments

subsidence of askja caldera 2000 2009 modelling of deformation processes at an extensional plate boundary constrained by time series insar analysis abstract many calderas in the world show long term unrest in the form of elevated rates of deformation and seismicity related to pressure changes and magma movements within their magmatic plumbing systems we present new observations of the style of deformation at the askja caldera iceland since 2000 using interferometric analysis of synthetic aperture radar images insar acquired by the canadian radarsat 2 satellite when combined with previously acquired detailed geodetic observations by various techniques we obtain an overview of askja s behaviour through more than four decades the combined dataset reveals that during this non eruptive period askja continuously subsides at a rate of 2 5 3 cm yr in 2000 2009 somewhat lower than the 5 cm yr rate inferred for the 1983 1998 period this behaviour of askja is evaluated and compared to those of other restless calderas the wrapped interferograms show three main features 1 concentric fringes depicting subsidence in the centre of the main askja caldera 2 oval fringes elongated along the rift portraying subsidence and 3 subsidence in an area north of the vatnajokull glacier the average line of sight los velocity from ground to satellite was used as input for inverse modelling of a deflating pressure source beneath the caldera embedded in a homogeneous elastic half space two different source geometries were tested a point pressure source and a horizontal penny shaped crack the concentric fringes centred in the askja caldera are best fit by a point source located at 65 05 n 16 78 w at a depth of 3 2 3 8 km with a volume decrease of 0 0012 0 0017 km3 yr from 2000 to 2009 consistent with previous studies provisional 2d fem models including structural complexities in the crustal layers indicate that the tectonic setting of askja plays an important role in the continuous long term high subsidence rates observed there in order to fully understand the cause and effects of the complicated tectonic setting we encourage the use of a more realistic rheological model of the area which could lead to reinterpretation of previous model results

comparative petrology of gneissic rocks in southwestern newfoundland in southwestern newfoundland pelitic migmatites of the meelpaeg subzone of the gander zone are separated by faults and plutons from metasedimentary rocks of the port aux basques gneiss complex pb

high temperature carbonate hosted ag pb zn cu deposits of northern mexico the high temperature carbonate hosted ag pb zn cu deposits of northern mexico occur in thick carbonate dominant jurassic cretaceous basinal sedimentary sequences floored by paleozoic or older crust the districts lie within or on the margins of a major fold thrust zone structures parallel to the trends of this belt control mineralization in many districts altered and or mineralized volcanic and intrusive rocks of tertiary age are found in all but three of the districts mineralization apparently occurred during a restricted period 47 to 26 m y ago the deposits are characterized by irregular ore lenses that commonly reveal strong structural controls and are stratigraphically discordant orebodies are of three principal morphological types mantos chimneys and pods and are composed of massive sulfides and or calc silicate skarn sulfide mantos and chimneys are enclosed within carbonate rocks and are generally remote from intrusive bodies sulfide mantos commonly grade downward into sulfide chimneys which may in turn grade downward into skarn chimneys cored by dikes podiform skarn and sulfide orebodies principally occur along stock dike or sill contacts but they may also be developed at some distance from an intrusive contact the associated intrusions are not widely mineralized despite the variations between ore types major similarities are seen in structural controls host rock characteristics and sulfide mineralogy structural controls include intrusive contacts faults fold axes fractures fissures and cavern zones intrusive contacts and intrusion related faults are most important in the skarns whereas regional fault fold and fracture systems are dominant controls on mantos and chimneys mineralization commonly occurs below relatively impermeable shaly units sills or volcanic rocks but some extensive orebodies show remarkable restriction to specific horizons within apparently homogeneous carbonate successions studies of mineralized and unmineralized strata have found no consistent links between ore deposition and carbonate composition facies organic content or insoluble components however zones of secondarily enhanced porosity and permeability induced by recrystallization heating deformation or hydrothermal dolomitization are important mineralization controls geochemical data for the deposits are sparse but indicate strongly contrasting conditions among some of the major districts fluid inclusion data indicate temperatures in the range 200 degrees to 500 degrees c and salinities ranging from i to 60 equiv wt percent nacl the hotter more saline solutions are typically from skarn zones evidence for boiling has not been reported the skarn ores of naica concepcion del oro and velardefia show tight clustering of sulfide sulfur isotope values near 0 whereas the skarn and massive sulfide ores of santa eulalia show a wide range of values in most cases there is isotopic disequilibrium between galena and sphalerite pyrrhotite or pyrite calcite veinlet stockworks above mineralization and limestone wall rocks adjacent to mineralization show shifts of carbon and oxygen isotopes to lighter values the tightly clustered sulfur isotopes and high temperature high salinity fluid inclusions have been interpreted as indicating a strong magmatic component in the mineralizing fluids at naica concepcion del oro and velardena but santa eulalia and other districts show evidence suggesting extensive mixing between magmatic and meteoric fluids it appears that the spectrum of mineralization styles shown by these deposits represent differing responses to variations in intrusive associations depth of emplacement host rock characteristics and geochemical evolution of the individual systems

deep magma transport at kilauea volcano hawaii the shallow part of kilauea s magma system is conceptually well understood long period and short period brittle failure earthquake swarms outline a near vertical magma transport path beneath kilauea s summit to 20 km depth a gravity high centered above the magma transport path demonstrates that kilauea s shallow magma system established early in the volcano s history has remained fixed in place low seismicity at 4 7 km outlines a storage region from which magma is supplied for eruptions and intrusions brittle failure earthquake swarms shallower than 5 km beneath the rift zones accompany dike emplacement sparse earthquakes extend to a decollement at 10 12 km along which the south flank of kilauea is sliding seaward this zone below 5 km can sustain aseismic magma transport consistent with recent tomographic studies long period earthquake clusters deeper than 40 km occur parallel to and offshore of kilauea s south coast defining the deepest seismic response to magma transport from the hawaiian hot spot a path connecting the shallow and deep long period earthquakes is defined by mainshock aftershock locations of brittle failure earthquakes unique to kilauea whose hypocenters are deeper than 25 km with magnitudes from 4 4 to 5 2 separation of deep and shallow long period clusters occurs as the shallow plumbing moves with the volcanic edifice while the deep plumbing is centered over the hotspot recent gps data agrees with the volcano propagation vector from kauai to maui suggesting that pacific plate motion azimuth 293 58 and rate of 7 4 cm yr has been constant over kilauea s lifetime however volcano propagation on the island of hawaii azimuth 3258 rate 13 cm yr requires southwesterly migration of the locus of melting within the broad hotspot deep long period earthquakes lie west of the extrapolated position of kilauea backward in time along a plate motion vector requiring southwesterly migration of kilauea s magma source assumed ages of 0 4 my for kilauea and 0 8 my for mauna loa are consistent with this model younger ages would apply if kilauea began its growth south of the locus of maximum melting as is true for loihi seamount we conclude that kilauea is fed from below the eastern end of the zone of deep long period earthquakes magma transport is vertical below 30 km then sub horizontal following the oceanic mantle boundary separating plagioclase and spinel peridotite then near vertical beneath kilauea s summit the migration of the melting region within the hotspot and kilauea s sampling of different sources within the melting region can explain 1 the long term geochemical separation of

mid late ordovician magmatism and metamorphism along the gander margin in central newfoundland fieldwork coupled with u pb dating has clarified the relationships between the exploits and gander derived rocks in central newfoundland during the mid ordovician opening of the exploits back arc basin reinforcing correlation across the peri gondwanan segments of the appalachian caledonian orogen we have separated a new back arc related unit the red cross group from the arc related victoria lake supergroup this group includes 466 3 ma felsic volcanic rock interlayered with alkali basalt enriched mid ocean ridge basalt 457 6 ma gabbro llanvirn caradoc normal mid ocean ridge basalt island arc tholeite limestone black shale and melange locally the red cross group is in parautochthonous contact with low grade gander psammite this contact was overrun by the meelpaeg metamorphic nappe during the devonian the nappe contains gander metapsammite meelpaeg subzone and the coticule bearing howley waters complex exploits which hosts 467 3 ma felsic porphyry the gander and exploits metasediments in the nappe are stitched by arenig llanvirn granite 467 6 ma and 468 3 5 3 ma and caradoc intrusions 458 3 ma and 454 2 ma metamorphic monazite 462 1 ma and titanite 460 3 ma from the nearby mount cormack subzone gander indicate the coeval formation of a low pressure high temperature metamorphic dome the widespread presence of caradoc magmatism suggests that the width of the back arc basin remained relatively narrow

new insights into the appalachian orogen from geology and geochronology along the cape ray fault zone southwest newfoundland reverse oblique overthrusting of high grade port aux basques gneiss over the lower grade supracrustal windsor point group along the cape ray fault zone occurred during late silurian to early devonian time and is attributed to major oblique collision between gondwanan and laurentian continental blocks new u pb zircon 40 ar 39 ar and k ar data limit the timing of this orogenic event the megacrystic granitic phase of the cape ray igneous complex and the cape ray tonalite have been dated at 488 3 ma and 469 2 ma respectively a black rhyolite from the windsor point group yielded an ordovician age 453 5 4 ma a pretectonic gabbro sill within the windsor point group dated at 424 4 3 ma is cut by the windowglass hill granite dated at 424 2 ma synkinematic hornblende and biotite from the grand bay complex mylonites of the port aux basques gneiss have 40 ar 39 ar ages of 407 4 ma and 403 4 ma respectively which is indicative of a late silurian early devonian age for the amphibolite grade deformation these ages are compatible with a previously determined 415 ma monazite age interpreted as the time of peak metamorphism in the port aux basques gneiss and suggests rapid exhumation the east west flexure of the cape ray fault zone represents a tear fault accommodating differential shortening between the cape ray fault zone and the correlative fault system in central newfoundland mid devonian granites 386 3 384 2 ma intervene between the thrusting events and a younger sinistral transcurrent movement this deformation and magmatic history can be correlated throughout the northern appalachians along other major fault boundaries

shrimp and la icp ms zircon geochronology of the xiong er volcanic rocks implications for the paleo mesoproterozoic evolution of the southern margin of the north china craton the paleo mesoproterozoic xiong er volcanic rocks along the southern margin of the north china craton are lithologically and geochemically similar to those formed in subduction related continental margin volcanic arcs the volcanic rocks are primarily composed of basaltic andesites and andesites with minor dacites and dacitic rhyolites traditionally the xiong er volcanic rocks have been divided from lower to upper into the xushan jidanping and majiahe formations but the ages of volcanic rocks in these formations have not been well constrained which has hindered further understanding the tectonic significance of the xiong er volcanic belt at the southern margin of the north china craton shrimp and la icp ms u pb zircon analyses combined with cathodeluminescence cl images have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the xiong er volcanic rocks shrimp and la icp ms u pb analyses on magmatic zircons from two basaltic andesite samples one dacite sample and one rhyolite sample of the xushan formation known as the lowest sequence of the xiong er volcanic rocks indicate that the volcanic eruption of this formation occurred at 1 78 ga but most xenocrystic inherited zircons in these samples yielded 207pb 206pb ages ranging from 2 55 ga to 1 91 ga of three samples collected from the jidanping formation two rhyolite samples 05xe015 and 05xe100 yielded weighted mean 207pb 206pb ages of 1778 5 5 ma and 1751 14 ma respectively similar to the ages of the volcanic rocks in the xushan formation whereas one dacite sample 05xe066 gave a weighted mean 207 pb

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

new mass increase beneath askja volcano iceland a precursor to renewed activity askja is an active central volcano located on the ns trending en echelon rift zone marking the mid atlantic plate boundary in north iceland between 2007 and 2009 we observed a gravity increase at the centre of the caldera this contrasts with net gravity decreases recorded between 1988 and 2007 interpreted previously in terms of magma drainage the recent gravity increase is rapid but similar in terms of lateral extent to the preceding decrease this gravity increase corresponds to a sub surface mass increase of 0 68 1011kg at about 3km depth it is possible that the new gravity increases observed at askja reflect accumulation of magma beneath the caldera and thus may herald a new phase in the activity of this volcano which last erupted in 1961

building icelandic igneous crust by repeated melt injections observations of microseismicity provide a powerful tool for mapping the movement of melt in the crust here we record remarkable sequences of earthquakes 20 km below the surface in the normally ductile crust in the vicinity of askja volcano in northeast iceland the earthquakes occur in swarms consisting of identical waveforms repeating as frequently as every 8 s for up to 3 h we use template waveforms from each swarm to detect and locate earthquakes with an automated cross correlation technique events are located in the lower crust and are inferred to be the result of melt being injected into the crust during melt intrusion high strain rates are produced in conjunction with high pore fluid pressures from the melt or exsolved carbon dioxide these cause brittle failure on high angle fault planes located at the tips of sills moment tensor solutions show that most of the earthquakes are opening cracks accompanied by volumetric increases this is consistent with the failure causing the earthquakes by melt injection opening new tensile cracks analysis of the magnitude distribution of earthquakes within a swarm reveals a complicated relationship between the imposed strain rates and the fluids that cause brittle failure the magnitude of the earthquakes is controlled by the distance fluids can migrate along a fault whereas the frequency of the events is controlled by the strain rate faults at the tips of sills act to focus melt transport between sills and so must be an important method of transporting melt through the lower crust

the shanggong gold deposit eastern qinling orogen china isotope geochemistry and implications for ore genesis abstract the shanggong au deposit in the xiong er terrane east qinling china has resources of about 30 ton au making it one of the largest orogenic mesothermal au deposits hosted in volcanic rocks of the mesoproterozoic xiong er group three stages of hydrothermal activity are recognized early middle and late of which two early and middle were ore producing and characterized by quartz pyrite and polymetallic sulfides respectively the third and late stage is represented by a carbonate quartz assemblage hydrogen oxygen and carbon isotope systematics of the shanggong deposit from a previous work suggest that the early stage fluids were derived from magmatic and or metamorphic devolatilization of sedimentary rocks at depth this is supported by new c s and published sr and pb isotopic data presented in this paper these new data 13c values ranging from 1 5 for early stage ankerite to 2 2 for late stage ankerite negative 34s values for sulfides from the middle stage 19 2 to 6 3 suggest a contribution from organic matter and that the ore fluid evolved from deeply sourced to shallowly sourced with those of the middle stage representing a mixture of these two fluid systems the comparison of the hydrogen oxygen carbon sulfur lead strontium isotope systematics between the shanggong deposit and the main lithologies in the xiong er terrane shows that neither these nor the underlying lower crust and mantle or combinations thereof could be considered as the source of ore fluids for the shanggong au deposit a likely source was a carbonaceous carbonate sandstone shale chert sequence in the underthrusted guandaokou and luanchuan groups exposed south of the xiong er terrane ar ar and rb sr isochron ages for mineral phases of the early middle and late stages together with geological field data constrain the timing of the hydrothermal activity and au metallogenesis at 242 10 167 7 and 112 7 ma respectively this metallogenesis and associated granitic magmatism can be related to the continental collision between the yangtze and north china cratons that resulted in the formation of the qinling orogen led to the different hydrothermal systems that were responsible for the three stages that formed the shanggong au deposit over a period of about 130 myrs

increased capture of magma in the crust promoted by ice cap retreat in iceland retreating ice caps overlying volcanoes are thought to relieve surface pressure causing enhanced magma generation and volcanic activity analysis of crustal stress during a dyke intrusion event associated with retreat of iceland s largest ice cap indicates that ice retreat could instead promote storage of magma in the crust rather than eruption at the surface

seismicity around the hekla and torfajokull volcanoes iceland during a volcanically quiet period 1991 1995 the volcano hekla in south iceland had its latest eruption in january march 1991 the eruption was accompanied and followed by considerable seismic activity this study examines the seismicity in the hekla region 63 42 64 18 n 18 30 20 12 w during a period when the high activity related to the eruption had ceased from july 1991 to october 1995 the aim is to define the level of the normal background seismicity of the area that can be compared to the eruption related activity the hekla volcano proper was generally aseismic during the study period the most prominent earthquake cluster is in the neighbouring torfajokull volcano the epicentres are concentrated in the western part of the caldera and west of it the hypocentres are located at all depths from the surface down to 14 km with highest activity at 5 12 km inside this cluster in the northwest part of the caldera is a spherical volume void of earthquakes approximately 4 km in diameter and centred at 8 km depth this is interpreted as a cooling magma body small low frequency events of volcanic origin were occasionally recorded at torfajokull this activity has mainly occurred in swarms and was most abundant during the first year of the study period presumably reflecting some kind of connection to the 1991 hekla eruption our study area also includes the easternmost section of the south iceland seismic zone a transform zone characterized by bookshelf faulting on transverse faults two lineaments of epicentres were identified roughly corresponding to mapped faults of the south iceland seismic zone the hypocentres are relatively deep mainly at 6 12 km matching the general trend of hypocentral depth increasing toward the east the seismicity is highest in the area of the mapped faults however the epicentres extend beyond them and indicate greater width of the south iceland seismic zone or 20 30 km rather than approximately 10 km as indicated by the length of the surface faults the seismicity in the volcanic systems of hekla and vatnafjoll shows some characteristics of the south iceland seismic zone epicentres are concentrated into two n s lineaments one of which coincides with the location of the 1987 vatnafjoll earthquake mw 5 9 a strike slip event on a n to s trending fault the hypocentres of the hekla vatnafjoll events are mainly at 8 13 km depth which indicates a continuation of the depth trend of the earthquakes of the south iceland seismic zone the events located at hekla proper and immediately north of it are all of low frequency character which can be held as an indication of volcanic origin on the other hand they show clear s arrivals at observing stations like normal high frequency tectonic earthquakes

crustal accretion of thick mafic crust in iceland implications for volcanic rifted margins rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins vrms this mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward seismic velocities crustal drilling and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions the lower crust of these regions is not exposed but is inferred from seismic velocities vp 6 5 km sec and petrological constraints to be gabbroic to ultramafic in composition limited access to crustal sections generated along vrms have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading active processes in iceland provide a glimpse of subaerial spreading with the creati

magma mixing revealed from in situ zircon u pb hf isotope analysis of the muhuguan granitoid pluton eastern qinling orogen china implications for late mesozoic tectonic evolution in this paper we present zircon u pb age and hf isotope data to document the significance of magma mixing in the formation of late jurassic granitoid intrusions in the eastern qinling orogen china the muhuguan granitoid pluton from this orogen consists of monzogranite and lesser biotite granite and granodiorite all containing abundant hornblende rich cumulates dioritic xenoliths and mafic magmatic enclaves mmes the monzogranite and granodiorite are intruded by a number of lamprophyre dykes both a cumulate and a dioritic xenolith samples have concordant zircon u pb ages of ca 161 1 ma but possess contrasting hf isotopic compositions the cumulate has more radiogenic zircon hf isotopes with negative ehf t values 7 9 to 2 5 and tdm1 ages of 0 9 1 1 ga indicating its derivation likely from basaltic rocks of the neoproterozoic to early paleozoic kuanping group in the area the dioritic xenolith has much lower zircon ehf t values of 19 5 to 8 8 and tdm2 ages of 2 4 1 7 ga consistent with a juvenile paleoproterozoic crust source presumably represented by the metabasic rocks of the qinling group in the area individual samples of the monzogranite mme and a lamprophyre dyke have u pb ages of 150 1 152 1 and 152 1 ma respectively demonstrating coeval mafic and felsic magmatism in the late jurassic the lamprophyre dyke has homogeneous highly negative zircon ehf t values 29 8 to 24 8 and archean tdm2 ages 3 0 2 7 ga and its genesis is interpreted as partial melting of an ancient enriched subcontinental mantle source zircons from the fine grained mme show a large range of ehf t between 29 1 and 9 8 overlapping values of the monzogranite and lamprophyre dyke samples zircon u pb age and hf isotopes of the mmes are consistent with their formation by mixing of crustal and enriched mantle derived magmas the main group of zircons from the monzogranite has ehf t values 17 9 to 9 3 and tdm2 ages 2 3 1 8 ga that are compatible with the dioritic xenoliths indicating that the former was produced by partial melting of paleoproterozoic crustal source with involvement of mantle derived magmas mafic magmatism revealed from the muhuguan pluton indicates that the eastern qinling orogen was dominated by lithospheric extension during the late jurassic compilation of existing geological and geochronological data suggests that this extensional event started in late jurassic ca 160 ma and persisted into the early cretaceous until ca 110 ma the jura cretaceous extension may have resulted from the late mesozoic westward subduction of the pacific plate beneath the east asian continental margin

composition and evolution of the archaean crust in central henan china the upper archaean in central henan province is composed chiefly of rocks of the dengfeng complex and the taihua complex rocks of the dengfeng complex show essential features of a granite greenstone terrane they consist of greenstone belts and surrounding granitic and migmatite gneiss terranes when the late superposed structures are removed the archaean conspicuously exhibits alternating gneiss domes and synformal basins which are occupied by granitic rocks and greenstone belts respectively the succession of the greenstone belts is divided into three groups 1 1 the lower group consists of ultramafic complexes which are only locally exposed 2 2 the middle group which comprises most of the greenstone belts contains tholeiitic metabasalts and felsic volcanics which are bimodal in character 3 3 the upper sedimentary group consists of molasse and turbidite formations the metamorphic grade in the dengfeng complex is up to the medium pressure almandine amphibolite facies the mafic volcanics are characterized by flat to slightly enriched ree patterns that resemble the th2 type of condie in most other elements the rocks are also similar to the tholeiitic metabasalts in typical greenstone belts throughout the world however they possess certain unique characteristics as for the granitic rocks the early plutons tend to be na rich and are geochemically similar to tonalitic rocks while the later intrusives tend to be k rich the taihua complex is composed primarily of felsic gneisses and metamorphosed supracrustal rocks interbedded with mafic ultramafic rocks the metamorphic grade in the complex ranges from granulite to upper amphibolite facies geochemical characteristics of the rocks are also similar to those of other high grade terranes throughout the world in general an overall structural grain characterizes the taihua complex which constitutes an elongate fold belt with characteristic features of linear mobile belts that are frequently found in high grade terranes the extensive dengfeng granite greenstone terrane and the linear taihua high grade gneissic terrane are bounded by thrusts or contacts of unknown nature and together make up the late archaean crust in central henan they may have developed more or less contemporaneously on a pre existing sialic crust in this region controlled by shallow convection cells in the upper mantle their development may have occurred in two different tectonic settings the rift type and the subsiding type setting respectively

the st eugene deposit british columbia a metamorphosed ag pb zn vein in proterozoic belt purcell rocks the st eugene deposit is an ag pb zn vein deposit hosted by clastic sedimentary rocks of the purcell supergroup which were metamorphosed during the east kootenay orogeny hydrothermal metasomatism created an alteration zone surrounding the veins that we subdivide into four facies separated by gradational contacts garnet chlorite internal intermediate and external chemical and isotopic exchanges between wall rocks and hydrothermal fluid created a chemical reaction zone that is characterized by gains in feo mno pb zn and sb and by losses in cao na 2 o k 2 o sr rb and cs wall rock 18 o values decrease from the vein selvage outward because of water rock exchange with a metamorphic fluid with a high 18 o value sulfur in vein sulfides is derived from local wall rock mn rich garnet formed near the quartz sulfide vein during metamorphism because the stability temperature of garnet is lowered in rocks hydrothermally enriched in mn the occurrence of manganiferous garnet in belt purcell rocks should be regarded as a metallotect for syngenetic and epigenetic mineralization in those rocks

deep structure of the northeastern japan arc and its relationship to seismic and volcanic activity evidence for possible deep seated magmatic activity beneath northeastern japan can be obtained from seismic observations tomographic inversions of p wave velocity data show low velocity zones distributed in the crust and upper mantle beneath active volcanoes in or around these zones anomalously low frequency micro earthquakes perhaps caused by magmatic activity are found at depths of 25 40 km distinct s wave reflectors corresponding to the upper surfaces of magma bodies are found at depths of 10 18 km large crustal earthquakes also occur around the low velocity zones these observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs

observations of deep long period dlp seismic events beneath aleutian arc volcanoes 1989 2002 abstract between october 12 1989 and december 31 2002 the alaska volcano observatory avo located 162 deep long period dlp events beneath 11 volcanic centers in the aleutian arc these events generally occur at mid to lower crustal depths 10 45 km and are characterized by emergent phases extended codas and a strong spectral peak between 1 0 and 3 0 hz observed wave velocities and particle motions indicate that the dominant phases are p and s waves dlp epicenters often extend over broad areas 5 20 km surrounding the active volcanoes the average reduced displacement of aleutian dlps is 26 5 cm2 and the largest event has a reduced displacement of 589 cm2 or ml 2 5 aleutian dlp events occur both as solitary events and as sequences of events with several occurring over a period of 1 30 min within the sequences individual dlps are often separated by lower amplitude volcanic tremor with a similar spectral character occasionally volcano tectonic earthquakes that locate at similar depths are contained within the dlp sequences at most aleutian volcanoes dlps appear to loosely surround the main volcanic vent and occur as part of background seismicity a likely explanation is that they reflect a relatively steady state process of magma ascent over broad areas in the lower and middle portions of the crust at mount spurr dlp seismicity was initiated by the 1992 eruptions and then slowly declined until 1997 at shishaldin volcano a short lived increase in dlp seismicity occurred about 10 months prior to the april 19 1999 eruption these observations suggest a link between eruptive activity and magma flux in the mid to lower crust and uppermost mantle

olivine friction at the base of oceanic seismogenic zones 1 we investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault triaxial compression tests were conducted on dry olivine powder grain size 60 m at effective confining pressures between 50 and 300 mpa using argon as a pore fluid temperatures between 600 c and 1000 c and axial displacement rates from 0 06 to 60 m s axial strain rates from 3 10 6 to 3 10 3 s 1 yielding shows a negative pressure dependence consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments a combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes sample strengths were pressure dependent and nearly independent of temperature localized shear zones formed in initially homogeneous aggregates early in the experiments the frictional response to changes in loading rate is well described by rate and state constitutive laws with a transition from velocity weakening to velocity strengthening at 1000 c microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600 c consistent with the focal depths of earthquakes in the oceanic lithosphere