recycling oceanic crust for continental crustal growth sr nd hf isotope evidence from granitoids in the western junggar region nw china abstract the juvenile component of accretionary orogenic belts has been declining since the archean as a result there is often controversy regarding the contribution of oceanic basalts to phanerozoic crustal growth as in the case of the central asian orogenic belt caob here we report on three groups of late carboniferous 316 305 ma granitoids in the western junggar region of northern xinjiang nw china which is part of the southwestern caob they consist of adakites and i and a type granites and as a whole have the most depleted isotopic compositions e nd t 6 9 87 sr 86 sr i 0 7030 0 7045 and e hf t 12 16 among the granitoids of the caob these features are nearly identical to those of pre permian ophiolites in northern xinjiang and are clearly different from those of carboniferous basalts in the western junggar region these relationships indicate that the granitoids were mainly derived from recycled oceanic crust by melting of subducted oceanic crust e g adakites and of the middle lower crust of intra oceanic arc that mainly consisted of oceanic crust e g i and a type granites based on evidence from the caob we suggest that recycling of oceanic crust has made a significant contribution to continental crustal growth and evolution during the phanerozoic

response of large scale ionospheric convection to substorm expansion onsets a case study 1 we have studied the response of large scale ionospheric convection to substorm expansion onsets on the basis of two weak substorms of 1 may 2001 during which a large part of the dawn cell of the two cell ionospheric convection pattern was monitored by the superdarn radars ionospheric convection began to enhance first in a localized region of the equatorward part of the dawn cell 2 minutes before the expansion onsets of both substorms and then enhanced in the entire dawn cell successively the enhanced convection persisted throughout their expansion phase possibly even near the footprint of a plasma sheet region without fast flows observed by geotail these observations suggest that ionospheric convection begins to enhance just before substorm expansion onset and then enhances in the entire cell possibly regardless of the presence of fast earthward flows in the corresponding plasma sheet region of the magnetotail the global enhancement of ionospheric convection is consistent with that of magnetotail convection which also begins just before onset

tracing a subducted ridge transform system in a late carboniferous accretionary prism of the southern altaids orthogonal sanukitoid dyke swarms in western junggar nw china abstract two sets of late paleozoic orthogonal dioritic dyke swarms are defined in the eastern part of the western junggar accretionary collage in the southern altaids dioritic dykes hornblende plagioclase are characterized by relatively high sio 2 elevated mgo na 2 o k 2 o mg ni si and cr contents la yb ratios very low y contents and enrichments in large ion lithophile elements lile and light rare earth elements lree which are all comparable with those of sanukitoids these dioritic dykes have low 87 sr 86 sr ratios mostly between 0 70366 and 0 70381 and high positive e nd t 6 6 to 8 4 values suggesting they were sourced from a depleted mantle accordingly these dioritic dykes were probably derived by mixing of depleted mantle components that originated from upwelling of asthenosphere through a slab window with melts that were derived by dehydration of a subducted slab detailed mapping and structural data of the dyke swarms indicate that the nw se trending set was earlier than the ne sw trending dykes paleostress analysis and inversion of the dyke swarms indicate that an early ne sw extension was associated with a nw se trending slab window and was followed by a slightly later nw se extension that was associated with a ne sw trending slab window field relationships together with published age constraints suggest that these two dyke swarms were intruded in the late carboniferous therefore two orthogonal slab windows were opened one ne sw after the other nw se during this time period a subducted ridge transform system could well account for and is consistent with these relationships we propose that in the late carboniferous a nw se trending mid oceanic ridge was subducted beneath the darbut trench generating large scale closely spaced nw se trending dykes that were intruded through a slab window that event was followed by subduction of a ne sw trending transform fault fracture zone that connected with the earlier nw se trending ridge producing widely spaced ne sw trending dykes that were intruded through a ne sw trending vertical slab window this new approach to the relationships between dyke swarms adakitic sanukitoid magmatism ridge subduction and slab windows has broad implications for tectonic reconstruction of ancient accretionary orogens

a tale of amalgamation of three permo triassic collage systems in central asia oroclines sutures and terminal accretion the central asian orogenic belt records the accretion and convergence of three collage systems that were finally rotated into two major oroclines the mongolia collage system was a long n s oriented composite ribbon that was rotated to its current orientation when the mongol okhotsk orocline was formed the components of the kazakhstan collage system were welded together into a long single composite arc that was bent to form the kazakhstan orocline the cratons of tarim and north china were united and sutured by the beishan orogen which terminated with formation of the solonker suture in northern china all components of the three collage systems were generated by the neoproterozoic and were amalgamated in the permo triassic the central asian orogenic belt evolved by multiple convergence and accretion of many orogenic components during multiple phases of amalgamation followed by two phases of orocline rotation

continuing evolution of the pacific juan de fuca north america slab window system a trench ridge transform example from the pacific rim abstract many subduction margins that rim the pacific ocean contain complex records of cenozoic slab window volcanism combined with tectonic disruption of the continental margin the series of slab windows that opened beneath california and mexico starting about 28 5 ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime the timing and areal extent of the resultant slab window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin the initial pioneer slab window thermally weakened the overlying western transverse ranges and california borderlands region starting about 28 5 ma a second thermal pulse occurred in the same region starting about 19 ma during growth of the monterey slab window this additional heating combined with the capture of a partially subducted monterey plate fragment by the cocos plate initiated the pulling apart and rotation of the adjacent continental margin similarly the capture of guadalupe and magdalena plate fragments by the pacific plate and initiation of the guadalupe magdalena slab window about 12 5 ma are coeval with baja california pulling away from the mexico continental margin with the break along the comondu arc in crust already thermally weakened by about 10 my of volcanism in coastal california distributed crustal extension and subsidence accompanied the new transform plate boundary and continued until the slab windows cooled and plate motion coalesced along a through going system of strike slip faults the transform boundary continues to evolve and forward modeling predicts an instability with the current configuration as a result of convergence between the sierra nevada and peninsular ranges batholiths starting about 2 my in the future the instability may be resolved by a shift in the locus of transform motion from the san andreas fault to the eastern california shear zone or by breaking off another fragment of the mojave or southern sierra nevada crustal blocks and translating it northward

near surface transport pathways in the north atlantic ocean looking for throughput from the subtropical to the subpolar gyre abstractmotivated by discrepancies between eulerian transport estimates and the behavior of lagrangian surface drifters near surface transport pathways and processes in the north atlantic are studied using a combination of data altimetric surface heights statistical analysis of trajectories and dynamical systems techniques particular attention is paid to the issue of the subtropical to subpolar intergyre fluid exchange the velocity field used in this study is composed of a steady drifter derived background flow upon which a time dependent altimeter based perturbation is superimposed this analysis suggests that most of the fluid entering the subpolar gyre from the subtropical gyre within two years comes from a narrow region lying inshore of the gulf stream core whereas fluid on the offshore side of the gulf stream is largely prevented from doing so by the gulf stream core which acts as a strong transport barrier in agreement with past studies the transport barrier near the gulf stream core is r

origin of high magnesian andesites in the setouchi volcanic belt southwest japan ii melting phase relations at high pressures melting phase relations of an augite olivine high magnesian andesite and an augite olivine basalt from the miocene setouchi volcanic belt in southwest japan have been studied under water saturated water undersaturated and under anhydrous conditions both the andesite and the basalt are characterized by low feo mgo ratios 0 86 and 0 76 in weight respectively and qualify as primary magmas derived from the upper mantle the andesite melt coexists with olivine orthopyroxene and clinopyroxene at 15 kbar and 1030 c under water saturated conditions and at 10 kbar and 1070 c under water undersaturated conditions 7 wt h2o in the melt the basalt melt also coexists with the above three phases at 11 kbar and 1305 c under anhydrous conditions and at 15 kbar and 1205 c in the presence of 4 wt water present studies indicate that high magnesian andesite magmas may be produced even under water undersaturated conditions by partial melting of mantle peridotite it is suggested that two types of high magnesian andesites in the setouchi volcanic belt augite olivine and bronzite olivine andesites were produced by different degrees of partial melting augite olivine andesite magmas whose mantle residual is lherzolite were formed by lower degrees of partial melting than bronzite olivine andesite magmas which coexist with harzburgite the basalt magmas which were often extruded in close proximity to the high magnesian andesite magmas are not partial melting products of a mantle peridotite which had previously melted to yield high magnesian andesite magmas

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

the cross stream potential vorticity front and its role in meander induced exchange in the gulf stream abstract the potential vorticity pv front observed in the upper thermocline at the northern edge of the gulf stream has been thought of as an inhibitor to lateral motion of parcels on isopycnals the role of the pv front in relation to lateral motion and cross stream exchange was investigated by monitoring the evolving pv field in the vicinity of lagrangian parcels at the northern edge of the stream the observational study involved shipboard acoustic doppler current profiler adcp and conductivity temperature depth surveys to sample the velocity and density fields around water parcels as they flowed through meanders the observations reveal that the pv field in the vicinity of parcels moves laterally across the stream and evolves as parcels negotiate meander crests and troughs allowing parcels to move across the stream without changing their lagrangian pv this lateral motion of the pv front suggests that it is more a response to than an inhibitor of the lateral motion of parcels in the meanders the

plate reconstruction based upon age data of japanese accretionary complexes the ages of the oceanic plates for the japanese accretionary complexes are plotted against time of accretion the plots show that early cretaceous transform faulting modified the arrangement of the pre middle cretaceous accretionary complexes and that three or four mid ocean ridges have collided with the japan arc since permian time the plate configuration for the western north pacific is reconstructed

early paleozoic high mg diorite granodiorite in the eastern kunlun orogen western china response to continental collision and slab break off abstract phanerozoic high mg diorites and granodiorites are known to be related to diverse subduction styles at convergent margin settings but have not been linked to collision processes the santonggou complex in the eastern kunlun orogen is composed of a dominant granodiorite intruded by small stocks and dykes of porphyritic granodiorites and diorites which are all characterized by high mgo and ni but low yb and y contents mafic microgranular enclaves mmes occur in the granodiorite and the porphyritic granodiorites la icpms u pb analyses of zircon from the granodiorite porphyritic granodiorite and porphyritic diorite yield similar ages of 432 2 ma 432 2 ma and 427 3 ma respectively plagioclase in the granodiorite exhibits complex zonations with resorbed and patchy cores plagioclase and hornblende phenocrysts in the porphyritic granodiorites and diorites also show textural and compositional zonations the highest e nd t value of 0 6 in mmes suggests asthenosphere derived mafic magma these lines of mineralogical and geochemical evidence together with results from geochemical modeling suggest that the porphyritic diorites were derived from mantle metasomatized slab melts the porphyritic granodiorites and the granodiorite were generated by simultaneous assimilation and fractional crystallization of the magma approximated by the porphyritic diorites plus mixing of asthenosphere derived mafic and evolved granodioritic magmas pressure estimates from the al in hornblende geobarometer suggest that the granodiorite had been uplifted to near the surface before the emplacement of the porphyritic granodiorites partial melting of the slab upwelling of the asthenosphere and rapid orogenic uplift at 430 ma suggest that the cessation of subduction and the initiation of collision as well as slab break off occurred almost simultaneously in the eastern kunlun orogen

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

early silurian to early carboniferous ridge subduction in nw junggar evidence from geochronological geochemical and sr nd hf isotopic data on alkali granites and adakites abstract the central asian orogenic belt caob evolved through a long lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the paleozoic the northern part of the western junggar region nw junggar offers a window into these processes particularly to constrain the timing of closure of the paleo asian ocean here we report geochemical geochronological and isotopic data from k feldspar granites and adakitic rocks from the nw junggar region zircon u pb ages suggest that the granites were emplaced during early silurian to the early carboniferous 434 328 ma the granites show geochemical characteristics similar to those of a type granites with high sio2 71 13 76 72 wt na2o k2o 8 00 9 59 wt and al2o3 12 28 14 08 wt but depleted sr nb ta and eu they display moderate to high positive end t and ehf t values 4 26 8 21 and 7 69 14 60 respectively and young nd and hf model ages t2dm nd 489 740 ma and t2dm hf 471 845 ma suggesting magma derivation through partial melting of lower crust in the boshchekul chingiz and zharma saur arcs the adakites are characterized by high sr content 406 5 751 6 ppm and low y 13 8 16 4 ppm and yb 1 5 1 8 ppm content yielding relatively high sr y ratios 25 38 49 41 similar to those of modern adakites they have high positive end t and ehf t values 7 85 8 25 and 13 23 15 97 respectively and young nd and hf model ages t2dm nd 429 535 ma and t2dm hf 355 550 ma indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the boshchekul chingiz arc the petrogenesis and distribution of the a type granites and adakites as well as the tectonic architecture of the region suggest that a ridge subduction event might have occurred during the early silurian to early carboniferous in combination with previous studies in the chinese altai we suggest a two sided ridge subduction model for the junggar altai region

from orientation to magnitudes in paleostress determinations using fault slip data abstract determinations of reduced stress tensors using fault slip data yield the orientation of principal stress axes and the ratio of the differences between principal stress magnitudes the use of rupture and friction laws allows determination of the two remaining unknowns that is the reconstruction of the complete stress tensor taking into account the depth of overburden brings an additional constraint the method is applied and discussed in the case of the hoover dam site western u s a where large data sets and rock mechanics information are available differences between intact sample and rock mass properties account for apparent disagreements between paleostress levels determined in similar tectonic environments pore pressure plays an important role where information about pore pressure is absent zero and hydrostatic pore pressure cases should be considered as limits

duskside enhancement of equatorial zonal electric field response to convection electric fields during the st patrick s day storm on 17 march 2015 the equatorial zonal electric field responses to prompt penetration of eastward convection electric fields ppef were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 march 2015 at dusk sector indian longitudes a rapid uplift of equatorial f layer to 550 km and development of intense equatorial plasma bubbles epbs were observed these epbs were found to extend up to 27 13 n and 25 98 s magnetic dip latitudes indicating their altitude development to 1670 km at apex in contrast at few degrees east in the premidnight sector thailand indonesian longitudes no significant height rise and or epb activity has been observed the eastward electric field perturbations due to ppef are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo idd fields whereas they were mostly counter balanced by the idd fields in the premidnight sector in situ observations from swarm a and swarm c and communication navigation outage forecasting system satellites detected a large plasma density depletion near indian equatorial region due to large electrodynamic uplift of f layer to higher than satellite altitudes further this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator this study brings out the significantly enhanced equatorial zonal electric field in response to ppef that is uniquely confined to dusk sector the responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field bz

formation of ionospheric irregularities over southeast asia during the 2015 st patrick s day storm we investigate the geospace response to the 2015 st patrick s day storm leveraging on instruments spread over southeast asia sea covering a wide longitudinal sector of the low latitude ionosphere a regional characterization of the storm is provided identifying the peculiarities of ionospheric irregularity formation the novelties of this work are the characterization in a broad longitudinal range and the methodology relying on the integration of data acquired by global navigation satellite system gnss receivers magnetometers ionosondes and swarm satellites this work is a legacy of the project equatorial ionosphere characterization in asia erica erica aimed to capture the features of both crests of the equatorial ionospheric anomaly eia and trough eit by means of a dedicated measurement campaign the campaign lasted from march to october 2015 and was able to observe the ionospheric variability causing effects on radio systems gnss in particular the multiinstrumental and multiparametric observations of the region enabled an in depth investigation of the response to the largest geomagnetic storm of the current solar cycle in a region scarcely reported in literature our work discusses the comparison between northern and southern crests of the eia in the sea region the observations recorded positive and negative ionospheric storms spread f conditions scintillation enhancement and inhibition and total electron content variability the ancillary information on the local magnetic field highlights the variety of ionospheric perturbations during the different storm phases the combined use of ionospheric bottomside topside and integrated information points out how the storm affects the f layer altitude and the consequent enhancement suppression of scintillations

paleostress reconstructions and geodynamics of the baikal region central asia part 2 cenozoic rifting abstract investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses it appears that changes of stress field with time are an important factor that either controls or results from the rifting process the object of this paper is to study the changes of fault kinematics and paleostress field with time in the baikal rift system during the cenozoic reduced paleostress tensors were determined by inversion from fault slip data measured in the central part of the rift and its southwestern termination between 1991 and 1995 results show that the stress field varies as well in time as in space two major paleostress stages are determined corresponding broadly to the classical stages of rift evolution late oligocene early pliocene and late pliocene quaternary the first paleostress stage is related to the rift initiation and the second to the major stage of rift development similarities between the recent paleostress field and the present day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active therefore we propose to use proto rift for the late oligocene early pliocene stage and active rift for the late pliocene quaternary stage of rift development during the proto rift stage the stress field was characterized by a compressional to strike slip regime a progressive change from transpression to transtension is suspected for the central part of the rift baikal and barguzin basins during this period in the western termination of the rift sayan massif tunka depression a strongly compressional stress field with oblique thrusting kinematics is well constrained in the late miocene early pliocene interval the active rift stage was initiated by a marked change in fault kinematics and stress regime in the late pliocene in the central part of the rift the stress regime changed into pure extension while in the southwestern extremity it changed into pure strike slip fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far field compressional stress on a mechanically heterogeneous crust with the southwards pointing wedge of the siberian craton acting as a passive indentor the cenozoic time space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere on intraplate stresses generated by the india eurasia convergence and the west pacific subduction

origin and accumulation mechanisms of petroleum in the carboniferous volcanic rocks of the kebai fault zone western junggar basin china abstract the kebai fault zone of the west junggar basin in northwestern china is a unique region to gain insights on the formation of large scale petroleum reservoirs in volcanic rocks of the western central asian orogenic belt carboniferous volcanic rocks are widespread in the kebai fault zone and consist of basalt basaltic andesite andesite tuff volcanic breccia sandy conglomerate and metamorphic rocks the volcanic oil reservoirs are characterized by multiple sources and multi stage charge and filling history characteristic of a complex petroleum system geochemical analysis of the reservoir oil hydrocarbon inclusions and source rocks associated with these volcanic rocks was conducted to better constrain the oil source the petroleum filling history and the dominant mechanisms controlling the petroleum accumulation reservoir oil geochemistry indicates that the oil contained in the carboniferous volcanic rocks of the kebai fault zone is a mixture the oil is primarily derived from the source rock of the permian fengcheng formation p 1 f and secondarily from the permian lower wuerhe formation p 2 w compared with the p 2 w source rock p 1 f exhibits lower values of c 19 tt c 23 tt c 19 20 tt tt ts ts tm and 20r sterane c 27 c 28 ratios but higher values of tt c 23 c 21 hhi gammacerane c 30 hopane hopane 20s c 34 c 33 c 29 and c 29 20s 20s 20r ratios three major stages of oil charge occurred in the carboniferous in the middle triassic late triassic to early jurassic and in the middle jurassic to late jurassic periods respectively most of the oil charged during the first stage was lost while moderately and highly mature oils were generated and accumulated during the second and third stages oil migration and accumulation in the large scale stratigraphic reservoir was primarily controlled by the top carboniferous unconformity with better porosity and high oil enrichment developed near the unconformity secondary dissolution pores and fractures are the two major reservoir storage space types in the reservoirs structural highs and reservoirs near the unconformity are two favorable oil accumulation places the recognition of the large scale carboniferous volcanic reservoirs in the kebai fault zone and understanding of the associated petroleum accumulation mechanisms provide new insights for exploring various types of volcanic reservoir plays in old volcanic provinces and will undoubtedly encourage future oil and gas exploration of deeper strata in the region and basins elsewhere with similar settings

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

reevaluating the roles of eddies in multiple barotropic wind driven gyres multiple gyre ocean models have a weaker mean subtropical circulation than single gyre calculations with the same viscosity and subtropical forcing traditionally this reduction in circulation is attributed to an intergyre eddy vorticity flux that cancels some of the wind input part of which does not require a lagrangian mass exchange theory of dissipative meandering herein the intergyre eddy vorticity flux is shown to be a controlling factor in barotropic models at high reynolds number only with exactly antisymmetric gyres and slip boundary conditions almost no intergyre flux occurs when no slip boundary conditions are used yet the subtropical gyre is still significantly weaker in multiple gyre calculations sinuous modes of instability present only in multiple gyres are shown here to vastly increase the eddy vorticity transport efficiency this increase in efficiency reduces the mean circulation necessary for equilibrium with slip boundary conditions the intergyre eddy transport is possibly much larger however with wind forcing relevant for the ocean two unequal gyres a mean flow flux of vorticity rather than an eddy flux between the regions of opposing wind forcing is increasingly important with increasing reynolds number a physical rationalization of the differing results is provided by diagnosis of the equilibrium vorticity budget and eddy transport efficiency calculations varying 1 boundary conditions 2 sources and sinks of vorticity 3 eddy transport efficiency and 4 the degree of symmetry of the gyres are discussed

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

earthquakes stress and strain along an obliquely divergent plate boundary reykjanes peninsula southwest iceland earthquakes stress and strain along an obliquely divergent plate boundary the reykjanes peninsula southwest iceland

a substorm onset signature at the auroral zone as observed with superdarn and equatorial magnetometers ionospheric convection enhancement with high westward velocities exceeding 1500 m s 1 was observed in association with the particle injection event by superdarn in the dusk sector of the auroral zone such a convection enhancement could be attributable to the enhanced sunward plasma flows in the dusk sector of the magnetosphere at the dip equator a ground pi2 onset was found to occur in association with these convection transients the onset of the pi2 lagged behind the convection enhancement by 4 11 min it is argued that the convection transient could be the earliest indicator of the substorm onset among energetic particle injection low latitude pi2 onset and perhaps the auroral breakup

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

ion microprobe u pb dating of quaternary zircon implication for magma cooling and residence time abstract an ion microprobe u pb dating method of quaternary zircon is developed using the sensitive high resolution ion microprobe shrimp and applied to igneous rocks from japan total pb u isochron age of 1 36 0 23 ma 95 confidence limit cl mswd 1 03 was obtained for zircons extracted from takidani granodiorite in central japan this may provide a constraint on magma cooling when coupled with fission track ages of the zircon as well as rb sr and k ar ages on co existing minerals the closure temperature determined by each method has lead to exponentially declining cooling history the history is consistent with a simple conductive cooling model of a small magma body 10 2 10 3 km 3 a pb u isochron age of 271 58 ka 95 cl mswd 0 85 has been determined for volcanic zircons from torihama dacite sw japan the age is significantly older than the 238 u 230 th ionium age of 145 33 ka and fission track age of 163 8 ka found in the literature suggesting either the magma residence time of 100 kyr or a rapid cooling rate of about 6 c kyr in contrast a tephrochronological estimate of 240 10 ka agrees well with the observed pb u age implying that the ionium and fission track ages are incorrect the pb u isochron age of 90 29 ka 95 cl mswd 1 04 is obtained for zircons in pyroclastic rocks from the unzen eruption in 1991 sw japan the age is again older than the eruptive age which suggests a magma residence time of 90 kyr in a shallow reservoir

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

statistical analysis of dayside equatorial ionospheric electric fields and electrojet currents produced by magnetospheric substorms during sawtooth events 1 substorms cause significant disturbances in the ionosphere however it has not been well understood how the electric field and electrojet in the dayside equatorial ionosphere respond to substorm onset previous studies found that the equatorial electric field after substorm onset could be eastward or westward because the onset of isolated substorms is often related to a northward turning of the interplanetary magnetic field imf the measured total electric field is determined by contributions from both imf northward turning and substorm onset and is not necessarily the signature of the onset in order to exclude the effect of imf northward turning we analyze the variations of ionospheric electric field and electrojet during storm time substorms when the imf remains stable thus the ionospheric variations can be identified to be caused solely by substorms the electric field data are measured by the jicamarca radar and the electrojet is derived from magnetometers at jicamarca and piura it is found that substorm onset induces an eastward electric field and electrojet in the dayside equatorial ionosphere when the imf remains continuously southward across the onset the equatorial electrojet starts to increase at the onset reaches a maximum value 30 min after the onset and then decreases to the pre onset value 60 min after the onset westward electric field and counter electrojet occur only if the substorm onset is associated with a northward turning of the imf it is concluded that the effect of substorm onset on the dayside equatorial ionosphere without involvement of imf reorientations is an enhanced eastward electric field

implications of palaeozoic ophiolites from western junggar nw china for the tectonics of central asia western junggar nw china contains fragments of several palaeozoic island arcs accretionary prisms and ophiolites which were accreted to each other and possibly to the southern margin of eurasia by the end of the palaeozoic the ophiolites of western junggar are important in reconstructions of these events four ophiolite belts are recognized from south to north these are tangbale mayila dalabute and barleik honggulueleng hebukesair they record the production and destruction of oceanic tracts in this region from the cambrian to the carboniferous the following characteristics are representative of them all 1 the ophiolitic successions are strongly deformed 2 sheeted dyke swarms are not preserved 3 basic igneous rocks show a range of chemical characteristics from depleted island arc tholeiite to ocean island basalt altered n type mid ocean basalts may be represented 4 large scale thrusts typically directed towards the southeast lie at the base of many individual ophiolitic bodies 5 the ophiolites rarely mark the position of possible sutures between arcs they are normally highly deformed and imbricated with other lithologies accretion of palaeozoic arcs and obduction of ophiolites was complete by the end of the carboniferous but the timing and sequence of events remain obscure no continental collisions occurred during this period of assembly crustal growth occurred by the amalgamation of non continental blocks

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

simulated pathways of the overflow waters in the north atlantic subpolar to subtropical export motivated by previous observational and modeling studies that demonstrate interior pathways for the export of labrador sea water lsw into the subtropical north atlantic we analyze whether the deeper denser overflow water ow pathways are similarly distributed in the basin for this study we rely on the simulation of ow pathways in the context of an ocean general circulation model from our analysis we suggest that ow in the north atlantic is not restricted to the dwbc in its equatorward transport thus a measure of ow transport variability within the dwbc downstream of the subpolar gyre cannot be safely equated to transport variability of the lower limb of the amoc since this study of ow export pathways relies solely on simulated pathways the applicability of our results to the north atlantic rests on the representativeness of the model s deep flow field

late archaean granites a typology based on the dharwar craton india abstract extensive field work in the eastern dharwar craton associated with petrographic and geochemical major and trace elements investigations allows four main types of late archaean granitoids to be distinguished 1 na rich granitoids of trondhjemitic tonalitic and granodioritic composition ttg that are characterised by strongly fractionated ree patterns and low hree contents and generally interpreted as slab melts generated by partial melting of metamorphosed hydrated basalt most likely in a subduction environment 2 sanukitoids which are k and mg rich monzonites and granodiorites with ttg like ree patterns associated with marked lile enrichment and considered to result from the reaction of slab melts generated in a subduction environment with and assimilation of mantle wedge peridotite 3 uncommon high hfse mg and k granites with strongly ree and lile enrichment that probably formed by partial melting of an enriched mantle source unlike in the genesis of sanukitoids in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals resulting in mantle enrichment subsequent melting of this enriched mantle probably in a post subduction setting gives rise to high hfse mg and k magmas as demonstrated for the closepet granite dharwar craton the hot mantle derived magma can induce melting of continental crust and then mix with the anatectic products 4 k rich mg poor anatectic biotite granites with ree patterns that are less fractionated and show negative eu anomalies these granites result from the remelting of old basement or recently accreted plutons both with ttg compositions such anatexis can occur either in a subduction or in a post subduction environment this typology based on the well exposed dharwar craton can be extended to late archaean granitoids from all over the world about such 500 analysis were compiled and a number of discriminant diagrams based on both major and trace elements are proposed the most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system followed by thermal reworking of the newly accreted continental material however it has been found that archaean subduction related granitoids are significantly different from their modern counterparts implying progressive changes in the modes of magma generation at convergent margins from the archaean to the present

substorm associated changes in the high latitude ionospheric convection 1 a study of polar cap dayside convection response to substorm intervals using canadian advanced digital ionosondes cadis situated well within the polar cap and superdarn radars during steady and prolonged southward imf bz conditions showed three distinct features 1 gradual prolonged decrease of dayside polar cap convection speed until the substorm onset 2 sudden decrease of convection following the onset of the substorm and 3 increase of convection during the recovery phase of the substorm we hypothesize that the observed features of the convection are due to the modulation of the region 1 current system associated with the substorm the ground magnetic response of magnetometers situated inside the polar cap is at least consistent with our postulate

geochemical signature of porphyries in the baogutu porphyry copper belt western junggar nw china article i nfo the baogutu porphyry copper belt lies in the darbut transitional island arc of the western junggar in the western section of the central asian orogenic belt in nw china our new petrographic results for the ore bearing porphyry stocks in the baogutu porphyry copper belt recognize them as diorite porphyry stocks rather than the granodiorite porphyry stocks as previously identified the copper mineralization is hosted in the diorite diorite porphyries and related breccias of the diorite porphyry stocks geochemical data indicate that the ore bearing porphyries have a predominantly intermediate composition with a transitional character from tholeiite to calc alkaline and are enriched in large ion lithophile elements lile and depleted in highfield strength elements hfse with a clear negative nb anomaly ree patterns show distinctenrichmentsin lree relative to hree the rocks also exhibit high initial end t 2 7to 6 3 ratios and low initial 87 sr 86 sr values 0 70359 0 70397 many samples are chemically similar to adakites

geochronological and geochemical study of the darbut ophiolitic complex in the west junggar nw china implications for petrogenesis and tectonic evolution abstract the darbut ophiolitic complex consists of ultramafic rocks cumulates pillow lavas and abyssal radiolarian cherts and occurs in the sartuohai suluqiaoke area which is situated in the eastern part of the west junggar nw china southern altaids in the present study we report new geochronological and geochemical data obtained for gabbro and basalt blocks embedded in the darbut ophiolitic complex zircon u pb analyses from one representative gabbro sample by la icp ms yielded a weighted mean age of 391 6 ma showing that it was emplaced in the middle devonian the samples bear n morb e morb and arc like geochemical signatures they are tholeiitic in composition with low concentrations of na 2 o k 2 o varying from 1 36 to 4 09 wt mg and ti depleted large ion lithophile elements liles enriched with insignificant negative eu they also show negative nb ta anomalies and high positive e nd t values with relative high la nb ba nb ba la ba th and zr nb ratios we interpret that these volcanic rocks were derived from a slightly metasomatized depleted mantle source in a back arc basin setting which corresponds to 10 30 partial melting of a depleted spinel lherzolite mantle the data presented here in combination with previously published data suggest a northwestward subduction model for the west junggar during the period of the early devonian to early carboniferous the west junggar was dominated by normal northwestward subduction and the ridge subduction commenced in the late carboniferous

potential vorticity sources and sinks in a quasi geostrophic ocean beyond western boundary currents abstract potential vorticity dynamics for a quasi geostrophic eddy resolving general circulation model egcm are studied in order to determine the effects of mesoscale variability on the potential vorticity distribution of a wind driven ocean the study employs both eulerian and lagrangian analyses in the effort to describe the potential vorticity gain loss cycle along the path of a particle while the mean wind stress curl is the dominant potential vorticity source for the interior of the upper layer a redistribution of eddy potential vorticity creates sources of potential vorticity for the multiple gyres in the lower layers this redistribution is a result of the local generation of eddies via baroclinic instabilities these eddies are advected by the western boundary current into the midlatitude jet where they are responsible for a cross gyre potential vorticity exchange this exchange is concentrated at the entrance to the eastward jets where northward and southward boundary currents converge from

tectonic evolution of the west junggar region xinjiang china the west junggar region of northwestern china consists of a hercynian age fold belt occupying a late paleozoic zone of convergence between three major eurasian plates siberian tarim and kazakhstan the northern part of the west junggar area includes part of the irtysh zaysan fold belt that extends to the northwest into the ussr where it marks a broad boundary between the siberian and kazakhstan plates the central part of the west junggar area includes the southeastern extension of the chingiz tarbgatay early paleozoic island arc sequence the southern part of the west junggar area is truncated by the deep fundamental junggar alakol fault that has brought the yili microplate into juxtaposiition with the west junggar region in the vicinity of ebin nur lake to the east the junggar area is covered by sediments of the junggar basin paleogeographic reconstructions and geologic evidence indicates that these plates began converging in the middle carboniferous and by late carboniferous alkali granites began invading the imbricated leading edges of the coalescing plates paleomagnetic data reveals that the plates continued to move along strike slip faults bringing the tarim plate and yili microplate into their present positions sometime in the early mesozoic six separate ophiolite occurrences in the west junggar region are strongly tectonized and dismembered the tangbale ophiolite is considered to be late cambrian 508 20 ma and represents the earliest known oceanic crust in western china petrologic and chemical data indicate that the tangbale ophiolite represents a possible back arc or forearc tectonic setting situated close to a developing island arc the darbut ophiolite has been tectonized to a melange and is overlain by late devonian flysch ages of the radiolaria in the cherts associated with the pillow lavas indicate that it formed in the middle devonian other occurrences of ophiolite melange in the northern part of the west junggar region may be as young as late devonian these ophiolites record ocean crust formation in the early to middle paleozoic within the junggarian ocean and their present tectonic setting indicates continental accretion and imbrication shortly after their formation associated blueschist blocks in the ophiolite melange further supports the concept of imbrication brought about by b type subduction and associated obduction postcollisional alkali granites intrude the oceanic sequences of the west junggar region and are considered to represent melted lower crustal material lead isotopic systematics on feldspars from six separate granite plutons indicate ratios that could only be considered oceanic as far as the magmas are concerned the implications of this finding is that the junggar basin basement rocks in the north and west are probably oceanic evidence presented in this study shows a complex history for the development of the west junggar region major movements of consolidating plates in the late paleozoic followed by rejuvenation of these same zones of closure during the himalayan tectonic event of the late tertiary demonstrates that convergence zones and fold belts may respond to vertical and horizontal stress even after many million years of inactivity

origin of high magnesian andesites in the setouchi volcanic belt southwest japan i petrographical and chemical characteristics high magnesian andesites of middle miocene age occur in southwest japan forming an obvious volcanic belt these andesites have low feo mgo ratios 0 546 0 931 and are rich in ni 101 312 ppm co 30 0 45 1 ppm and cr 208 756 ppm they are relatively aphyric phenocrysts 10 vol and the phenocrysts of magnesian olivine fo88 are in equilibrium with the host high magnesian andesite magmas on the basis of the fe mg exchange partitioning these features suggest that the high magnesian andesites are not differentiated or accumulative they appear to represent primary andesites generated in the upper mantle these southwest japanese high magnesian andesites are rich in incompatible elements and show light rare earth enrichment relative to boninites suggesting that the former is derived from a less depleted mantle source than the latter

tectonic evolution of palaeozoic terranes in west junggar xinjiang nw china nine separate cambrian to carboniferous terranes recognized in west junggar northwest china they were amalgamated as part of the central asian orogenic belt which records accretion of continental island arc and oceanic terranes to archaean proterozoic continental nuclei tangbale kekesayi ebinur and mayila terranes cambrian silurian evolved in intra oceanic settings and docked along a series of north dipping subduction zones on to the laba terrane to their south this southern continent was contiguous with lithosphere of the kulumudi ocean to the north devonian subduction on the northern edge of this ocean resulted in formation of a continental arc toli terrane and accretionary complex kulumudi terrane the karamay terrane formed as an accretionary complex during the carboniferous the ophiolitic sartuohai terrane was emplaced as melange between kulumudi and karamay terranes during the late carboniferous subduction migrated southward continuing beneath these terranes resulting in the intrusion of i type granites into the toli kulumudi sartuohai and karamay terranes these granites are closely associated with epithermal and porphyry style gold mineralization composite terranes either side of the kulumudi ocean collided in the late carboniferous marking the final consolidation of central asia collision was accompanied by anorogenic granite and diabase dyke intrusion followed by widespread latest carboniferous to permian extension and subsequently the formation of the junggar basin west junggar has been further disrupted by cenozoic strike slip faulting along junggar and dalabute faults

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

eocene volcanism above a depleted mantle slab window in southern alaska the caribou creek volcanic fi eld lies along the continent side edge of forearc basin rocks in south central alaska and consists of over 1000 m of shallow dipping basalt and andesite lavas with minor mafi c pyroclastic deposits dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas the basalts are tholeiitic and strongly depleted in the light rare earth elements la yb 0 18 1 5 with concentrations of high fi eld strength elements e g zr hf ti y similar to mid ocean ridge basalt and with variable enrichment in fl uid mobile elements e g cs ba and pb intermediate and felsic rocks show enrichment in the rare earth elements and fl uid mobile elements plus rb and k but retain low la yb ratios 0 48 3 6 a few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites e g sr y up to 52 ten 40 ar 39 ar ages for the caribou creek volcanic rocks range from 49 4 2 2 to 35 6 0 2 ma an adakite like tuff beneath the other volcanic rocks yields an age of 59 0 0 4 ma caribou creek basalts were derived from mid ocean ridge like depleted mantle that was emplaced beneath the southern margin of alaska through a slab window following spreading ridge subduction caribou creek volcanism was coeval with oblique subduction oroclinal bending and right lateral strike slip faulting in south central alaska all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas the basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of jurassic arc basement rocks to form the intermediate and felsic magmas enrichment of the basaltic parent magmas in fl uid mobile elements occurred by contamination from the jurassic arc rocks and or by contamination with metasomatic mantle remnant from preceding subduction high heat fl ow through the slab window induced partial melting of garnet bearing mafi c parts of the jurassic arc basement to form the adakite like rocks the caribou creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime e g crustal extension and that the infl uence of a slab window on continental margin magmatism can be long lived 20 m y

contribution of storm time substorms to the prompt electric field disturbances in the equatorial ionosphere this study tries to bring out the fact that storm time substorms can compete and at times significantly contribute to the geomagnetically disturbed time prompt penetration electric field effects on low and equatorial latitudes observations of unusual equatorial plasma drift data from jicamarca unattended long term investigations of the ionosphere and atmosphere during two space weather events show that substorms can induce both eastward and westward penetration electric fields under steady southward interplanetary magnetic field imf bz conditions during the first event on 2 january 2005 the enhancement of the daytime eastward electric field over jicamarca due to substorm is found to be comparable with the sq and interplanetary electric field iefy generated electric fields combined during the second event on 19 august 2006 the substorm is seen to weaken the daytime eastward field thereby inducing a westward field in spite of the absence of northward turning of imf bz overshielding the westward electric field perturbation in the absence of any overshielding events is observationally sparse and contrary to the earlier results further the substorm induced field is found to be strong enough to compete or almost nullify the effects of storm time iefy fields this study also shows quantitatively that at times substorm contribution to the disturbed time prompt electric fields can be significant and thus should be taken into consideration in evaluating penetration events over low latitudes

palaeostress reconstructions and geodynamics of the baikal region central asia part i palaeozoic and mesozoic pre rift evolution abstract this paper presents the first palaeostress results obtained for the basement of the baikal rift system in southern siberia russia large scale structural analysis and palaeostress reconstructions show that the palaeozoic mesozoic kinematic history precursor of the baikal cenozoic rifting is characterized by the succession of six regional palaeostress stages stress inversion of fault slip data and earthquake focal mechanisms is performed using an improved right dieder method followed by rotational optimization d delvaux tensor program the results are interpreted in the light of recent developments in the investigation of regional intraplate stress field and used as additional constraints for palaeogeodynamic reconstruction of central asia after the final palaeozoic closure of the palaeo asian ocean on the southern margin of the siberian platform the marginal suture with the sayan baikal caledonian belt was repeatedly and preferentially reactivated during the subsequent palaeozoic and mesozoic history this suture zone also controlled the opening of the baikal rift system in the cenozoic the progressive closure of the palaeo asian and mongol okhotsk oceans generated successive continental collisions which were recorded in the baikal area by brittle ductile and brittle deformations the first two palaeostress stages correspond to the successive collage of precambrian microcontinents and caledonian terranes along the southern margin of the siberian platform 1 late cambrian early ordovician n s compression and 2 late silurian early devonian nw se compression the next two stages are related to the remote effects of the complex evolution of the western palaeo asian ocean southwest of the siberian continent 3 late devonian early carboniferous n s compression recorded only in the altai region and 4 late carboniferous early permian e w compression recorded both in the altai and baikal regions the last stages are the consequences of the mongol okhotsk oceanic closure 5 late permian triassic nw se extension with development of cordilleran type metamorphic core complexes and volcanism along the active margin of the mongol okhotsk ocean 6 initial development of early middle jurassic 10 15 km wide molassic basins in trans baikal and large foredeeps along the southern margin of the siberian platform in probable extensional context but for which no palaeostress data are available and 7 final closure of the mongol okhotsk ocean in the cretaceous this last event results from the collision between the mongol china and the siberian plates and is evidenced by the inversion of the middle mesozoic basins and by late jurassic early cretaceous coal bearing sedimentation in trans baikal this long tectonic history yielded a highly heterogeneous basement in the baikal area precursor of the cenozoic rifting

electrodynamic disturbances in the brazilian equatorial and low latitude ionosphere on st patrick s day storm of 17 march 2015 the st patrick s day storm of 17th march 2015 has a long lasting main phase with the dst reaching a minimum of 223 nt during the main phase two strong prompt penetration electric field ppef phases took place first with the southward turning of imf bz around 1200 ut and the second with the onset of a substorm around 1725 ut leading to strong equatorial zonal electric field enhancements the consequent spatio temporal disturbances in the ionospheric total electron content tec and the resultant modifications in the equatorial ionization anomaly eia over the brazilian longitudinal sector are investigated in detail the simultaneous measurements from a large network of gps receivers ionosonde and magnetometers over the brazilian longitudinal sector are used for this study in the presence of enhanced zonal electric field the equatorial f2 layer peak hmf2 experienced a rapid uplift without any significant change in the base height h f while the f2 layer is redistributed into f2 and f3 layers the enhanced zonal electric filed due to ppef led to the strong super fountain effect under which the anomaly crest departed poleward to 40o s latitude in the presence of westward and equatorward wind surge over brazil with the co existing disturbance dynamo fields strong hemispheric asymmetry is seen in the storm time response of eia during both the ppef phases

kinematics and age constraints of deformation in a late carboniferous accretionary complex in western junggar nw china abstract the karamay area situated in the eastern part of western junggar southern altaids contains an ophiolitic melange with ultramafic rocks gabbro basalt chert and limestone which show typical block in matrix structures and coherent turbidites and tuffs these lithological associations are interpreted as incoherent and coherent series formed in an accretionary complex on the basis of detailed field mapping and analyses of the asymmetry of imbricate thrusts duplexes tilted structures shear band cleavages and the nw verging inclined to overturned folds we conclude that the overall movement in the accretionary complex was top to the nw the youngest tuff involved in the deformation contains detrital zircons that have a u pb age la mc icp ms of 308 7 ma 39ar 40ar resistance furnace step heating of amphibole separates from a diorite dike which cuts the folded and imbricated rocks in the accretionary prism yielded a plateau age of 307 2 ma consequently the age of the deformation in the prism is tightly constrained at 307 308 ma implying that the deformation occurred in an extremely short time span during se ward subduction combined contemporaneous occurrence of baogutu adakite high mg sr enriched and y poor dioritic dikes miaoergou charnockite and maliya mafic rocks we further suggest the accretionary complex was cut by near trench volcanic rocks and plutons possibly due to interaction with a spreading ridge

pathways of cross frontal exchange in the north atlantic current the north atlantic current nac forms part of the boundary between the subtropical and subpolar gyres in the north atlantic ocean the current has topographically controlled stationary meanders that appear to grow and decay a region east of the current in the newfoundland basin contains water of mixed subpolar subtropical properties suggesting that there is exchange across the nac this study considers data from isopycnal rafos floats launched in the nac region from 1993 to 1995 we use the rafos data to define the frontal zone as a pressure range where the jet is most likely to be found this definition requires a latitudinal dependence as the nac shoals to the north floats shallower and deeper than this range are defined to be on the subpolar and subtropical side respectively these definitions are used to estimate mixing that occurs between the current and its surroundings and to estimate the relative quantity of exchange of water parcels between the two gyres only small quantities of mass exchange from one gyre to the other are found but there is a distinct asymmetry leading to a mean flux from the subpolar to subtropical sides we also find that floats spend significant time in the frontal region and are frequently exchanged between fast and slow moving waters particularly at the meander extrema diffusion while in the jet leads to eddy cross frontal exchange which is important for the exchange of properties across the nac

a superposed epoch study of superdarn convection observations during substorms 1 case studies of flow patterns during substorms have revealed characteristic patterns in the period leading to expansion onset velocities in the premidnight region have been observed to become strongly sheared and the premidnight convection reversal has been observed to extend across the midnight meridian at expansion onset the shear decreases and flow at midnight becomes meridional out of the polar cap the study presented here superposes flow patterns from 10 substorm intervals to determine the average flow patterns the study confirms the findings of the case studies and extends them to additional local time sectors in addition changes to the flow pattern after expansion onset are examined

polar equatorial ionospheric currents driven by the region 2 field aligned currents at the onset of substorms 1 the dawn to dusk convection electric field increases during the growth phase of substorms driving dp2 currents composed of two cell current vortices in high latitude and leading to an increase in the eastward electrojet at the dayside dip equator eej during the expansion phase of substorms electric field and currents are often reversed in direction to the normal dp2 currents at subauroral to equatorial latitudes when the convection electric field reduces abruptly the reversed current at the dayside dip equator appears as a counterelectrojet cej and causes an equatorial enhancement of the negative bay in the afternoon sector conversely the convection electric field increases during substorms as deduced from sawtooth events causing an increase in the eej in this study by analyzing isolated substorms with magnetometer array data and superdarn super dual auroral radar network convection maps we deduce that the reversed electric field and currents develop at the subauroral to equatorial latitudes at the onset of substorms while the convection electric field increases at auroral latitudes these observations imply that both the region 1 and region 2 field aligned currents r1 and r2 facs develop on the dayside concurrently with the current wedge responsible for the midlatitude positive bay at midnight the substorm associated r2 facs are strong enough to cause the reversed current at the subauroral latitude and the cej at the equator we also deduce that the dayside equatorial cej begins simultaneously with or even earlier than the midlatitude positive bay at midnight while the midlatitude positive bay onset is delayed by several minutes as the station departs from the midnight meridian these observations suggest that the substorm begins with the intensification of the r2 facs responsible for the equatorial cej

african stress pattern from formal inversion of focal mechanism data abstract the kinematic models and the associated orientation of extensional stress of the east african rift system have been subjected to much debate since a long time in the past decades the proposed models relied on the interpretation of the overall rift geometry geological fault slip data and the few focal mechanisms available these models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space in the recent years there has been a significant increase in the number of focal mechanisms available for the entire rift system and it is now possible to estimate the present day stress field in relative detail based on seismotectonic data alone we compile 347 focal mechanism data from the global harvard cmt catalogue and various other sources and grouped 332 of them in 24 distinct regions boxes on the basis of their geographical proximity kinematic homogeneity and tectonic setting for each box and for the same data set reduced stress tensors have been obtained by formal stress inversion using both the tensor program delvaux sperner 2003 and the slick method michael 1984 1987 both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes which stem for the robustness of the approach the obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces including plate boundary forces and 2nd and 3rd order effects as gravitational potential of topography intra lithospheric processes and the influence of structural heterogeneities of the rift structures the evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns

genesis of post collisional granitoids and basement nature of the junggar terrane nw china nd sr isotope and trace element evidence the post collisional magmatism of the junggar terrane is characterized by intrusion of large amounts of granitoids and minor basic ultrabasic rocks the granitoids comprise two magmatic suites calc alkaline and alkaline which were emplaced contemporaneously at about 294 ma the calc alkaline rocks are typically sodium rich na2o k2o 1 1 3 5 and metaluminous a nk 1 0 a cnk 0 78 1 04 they show mildly fractionated ree patterns la yb n 15 and spidergrams with strong depletion of nb ti and to a lesser extent sr the alkaline granites have high contents of sio2 75 78 alkalis nb hree y sn f and high feo mgo ratios and huge sr and eu depletion in the spidergrams moreover they display characteristic tetrad ree patterns and non charac trace element behaviour the two rock suites have similar initial nd and sr isotopic compositions with end t in the range 5 2 to 7 1 and isr mostly in the range 0 7031 0 7041 this points to a predominance of juvenile components in their sources the calc alkaline rocks are most probably derived by dehydration melting of a basic lower crust leaving behind a granulite residue the process was probably triggered by underplating of mantle derived basic magmas in an extensional regime the alkaline granites are considered to have formed by differentiation of the calc alkaline granitoids our study argues for a juvenile continental crust for the basement of the junggar terrane which is likely dominated by early paleozoic oceanic crust and arc complex that were deeply buried during the late paleozoic subduction and accretion

high mg diorites derived from sanukitic hma magmas kyushu island southwest japan arc evidence from clinopyroxene and whole rock compositions abstract high mg diorites that have similar whole rock composition to high mg andesites hmas should not be simply interpreted as rocks solidified from the hma magmas because the high mg diorites may be mafic cumulates derived from a different magma from the hmas the hmas contain unique clinopyroxenes with higher mg and si than those of other sub alkaline series igneous rocks the mg and si are controlled by the source magma composition rather than its crystallized conditions such as pressure and temperature the chemical composition of clinopyroxenes would present important information for the investigation of the source of high mg diorites we considered the source of early cretaceous high mg diorites on kyushu island southwest japan arc based on their clinopyroxene and whole rock compositions the clinopyroxenes have similar chemical characteristics to those in hmas rather than those in other sub alkaline rocks moreover the whole rock compositions are equivalent to the sanukitic hma and do not show features of mafic cumulates this indicates that the high mg diorites solidified from sanukitic hma magmas it is generally believed that the sanukitic hma magmas involve the subduction of a young and or hot oceanic slab was situated in their genesis therefore the occurrence of the high mg diorites suggests that kyushu was situated in the tectonic setting of young and or hot slab subduction in the early cretaceous

global equatorial plasma bubble occurrence during the 2015 st patrick s day storm an analysis of the occurrence of equatorial plasma bubbles epbs around the world during the 2015 st patrick s day geomagnetic storm is presented a network of 12 global positioning system receivers spanning from south america to southeast asia was used in addition to colocated vhf receivers at three stations and four nearby ionosondes the suppression of postsunset epbs was observed across most longitudes over 2 days the epb observations were compared to calculations of the linear rayleigh taylor growth rate using coupled thermosphere ionosphere modeling which successfully modeled the transition of favorable epb growth from postsunset to postmidnight hours during the storm the mechanisms behind the growth of postmidnight epbs during this storm were investigated while the latter stages of postmidnight epb growth were found to be dominated by disturbance dynamo effects the initial stages of postmidnight epb growth close to local midnight were found to be controlled by the higher altitudes of the plasma i e the gravity term modeling and observations revealed that during the storm the ionospheric plasma was redistributed to higher altitudes in the low latitude region which made the plasma more susceptible to rayleigh taylor growth prior to the dominance of the disturbance dynamo in the eventual generation of postmidnight epbs

late paleozoic paleogeographic reconstruction of western central asia based upon paleomagnetic data and its geodynamic implications abstract carboniferous to permian volcanoclastic rocks have been collected from south junggar and west junggar primary magnetizations have been observed from the characteristic components of 10 sites of early permian p1 and late permian p2 red beds of south junggar area the 14 early carboniferous sites from west junggar mountains expose post folding secondary magnetizations and according to their spatial distribution 9 remagnetized sites are related to late carboniferous early permian granite emplacement whereas 5 sites are located at the vicinity of late permian mafic dykes two new paleomagnetic poles have been consequently calculated for the periods of p1 at 79 5 n 36 6 e and of p2 at 60 4 n 4 7 e with a95 of 6 8 and 5 4 respectively they yield two paleomagnetic poles at 65 3 n 329 7 e with a95 of 6 3 and 64 8 n 179 5 e with a95 of 6 9 respectively compilation of available data shows stationary and consistent poles for south junggar area during the carboniferous and permian whereas nw junggar underwent a significant anticlockwise rotation between the late carboniferous early permian and the late permian indicating that junggar was not a rigid block up to the end of the paleozoic west junggar and south junggar may have experienced contrasting tectonic evolutions comparisons of late paleozoic poles of central asia blocks show 1 counterclockwise rotation of west junggar with respect to siberia contrasting with the clockwise rotation of north kazakhstan with respect to siberia 2 no significant movements between west junggar north kazakhstan and siberia since late permian indicating that they were rigidly welded since that time and 3 anticlockwise rotations of tarim yili and south junggar with respect to the welded siberia kazakhstan west junggar block such rotations may have been accommodated by late permian to early triassic strike slip faults with an estimation of the displacements of 1570 280 km along the irtysh gornotsaev shear zone 410 380 km along the nikolaiev nalati tectonic line and 490 250 km along the chingiz alakol north tian shan fault since late permian time

mantle derived archaean monozodiorites and trachyandesites the 2 680 2 750 myr old1 5 intrusive granite greenstone terranes6 8 in the rainy lake region ontario and in and adjacent to the vermilion district minnesota of the superior province in north america include a wide variety of volcanic and intrusive rocks with initial pb sr and nd isotope ratios close to those for the mantle4 5 9 12 the isotopic constraints require the production of large volumes of silica oversaturated magmas from silica undersaturated mantle within 100 200 myr we present here geochemical data on monzodiorites and trachyandesites from the rainy lake area which are strongly enriched in large ion lithophile elements lile we conclude that they are derived by direct melting of the mantle at depths of 50km in either anhydrous or hydrous conditions their rare earth element ree abundances and initial 143nd 144nd ratios suggest that their mantle sources were enriched in lile shortly before melting the monzodiorites trachyandesites and their granodioritic derivatives may comprise up to 20 of the exposed igneous rocks in the rainy lake region and vermilion district if these rocks are as abundant in other archaean terranes a significant part of the early continental crust could have formed by direct melting of lile enriched mantle

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

signatures for magnetospheric substorms in the geomagnetic field of dayside equatorial region origin of the ionospheric component 1 an event based study is made of the conditions that lead to the ionospheric current component in the geomagnetic field signature for magnetospheric substorms in the dayside dip equatorial region the substorm activity studied here is characterized by two onsets separated by 40 min the first one occurred during steady southward interplanetary magnetic field imf and steadily increasing cross polar cap potential and enhanced energy input into the magnetosphere while the second one occurred in close association with a northward transition of imf and a rapid reduction of the polar cap potential and energy input into the magnetosphere earlier studies using multispacecraft data as well as ground based optical radar and magnetic recordings ascertained the first onset to be associated with a pseudo breakup and the second onset with a true breakup magnetometer recordings from a meridional network of stations in the indian and pacific sectors revealed the response of daytime equatorial h field to the pseudo breakup to be weak and not readily identifiable in contrast a distinct negative bay like disturbance with an unambiguous dip equator enhancement prevailed in the h field starting with the onset of the expansive phase of the fully developed substorm this behavior indicative of a significant contribution of ionospheric currents to the negative h field bay is seen both in the indian and pacific sectors covering the 1200 1600 lt region the present case study thus suggests that a sudden and prominent reduction in the cross polar cap potential at the substorm expansive phase is responsible for the prevalence of the ionospheric current component in the geomagnetic disturbance in the dayside equatorial region it is also found for the first time that the amplitude of the negative h field disturbance in the afternoon sector exhibits a marked hemisphere asymmetry at midlatitudes it is higher by a factor of about 3 in the summer hemisphere than in the winter hemisphere the equatorial enhancement of the negative h component disturbance is interpreted as the signature of the over shielding effect namely direct penetration of the perturbation component of the large scale electric field associated with the rapid reduction in convection at the onset of the substorm expansion phase the marked summer winter asymmetry of the h field disturbance at midlatitudes is suggested as the outcome of the competing magnetic effects of field aligned currents and polar origin dp2 currents

characterizing the 18 april 2002 storm time sawtooth events using ground magnetic data 1 beginning during the last half of the main phase and extending for over 12 hours the 18 april 2002 magnetic storm is dominated by sawtooth oscillations we examine the ground magnetic signatures of these events at both high latitudes and low latitudes the first immediate observation is that the magnetic signatures from one sawtooth to another can show considerable variation thus the current systems associated with these events show considerable variation in location pattern and strength from one oscillation to the next the characteristics of the magnetic disturbance for each sawtooth has little to distinguish it from the typical substorm expansion magnetic disturbance field except that some of the sawteeth disturbance profiles are broader in local time than observed during typical substorms the injection region determined from the energetic particle flux measurements at geostationary orbit is generally broad extending across the entire tail and past the terminators into the dayside the injection region is usually wider than the magnetic disturbance profile at the peak of the dipolarization there appears to be no relationship between the magnetic disturbance profile and the location and strength of the injection boundary finally sawtooth events appear to be driven by a steady solar wind driver and to develop during a period of already enhanced ring current

elemental and nd sr isotopic geochemistry of granitoids from the west junggar foldbelt nw china with implications for phanerozoic continental growth we report elemental and nd sr isotopic data for two coeval postcollisional magmatic suites 300 ma the miaogou and karamay suites from west junggar foldbelt nw china aiming to determine their source regions and implications for continental growth the miaogou rocks monzonitic to granitic in composition show low mg 0 30 and depletion of hfse and are highly depleted in isotopic compositions with isr ranging from 0 7035 to 0 7045 end 300 ma from 8 4 to 6 6 and young nd model ages 0 37 0 70 ga that coincide with the ages of the ophiolites in the area these features suggest that the miaogou rocks were derived essentially by partial melting of a juvenile basaltic lower crust formed in the early to middle palaeozoic the karamay suite contains a variety of rock types from gabbroic diorite to granite these rocks show enrichment of lree and lile and depletion of hfse and have chemical composition e g mg 0 63 0 41 rather different from the miaogou suite though isotopically they are indistinguishable this suggests that the parental magma of the karamay suite originated from melting of a young lithospheric mantle that had previously been metasomatized during the palaeozoic subduction the west junggar magmatic rocks represent production of juvenile continental crust in the phanerozoic triggered by upwelling of asthenophere in an extensional regime the basement beneath the area is dominated by trapped palaeozoic arc series and oceanic crust

the petrology and geochemistry of high magnesium andesites at the western tip of the setouchi volcanic belt sw japan k ar ages and petrographical and geochemical characteristics of high magnesium andesites and plagioclase phyric andesites from the ne kyushu region japan are presented k ar ages range from 10 7 0 3 to 14 4 0 4ma overlapping those reported for lavas of the setouchi volcanic belt in other regions 11 16ma this together with major and incompatible trace element and sr nd pb isotopic characteristics confirms that the setouchi volcanic belt which is characterized by the occurrence of high magnesium andesites extends for 600 km along the sw japan arc thus it may be suggested that the unusual tectonic setting required for high magnesium andesite magma generation was attained only for this limited region possibly in association with subduction of the young hence hot lithosphere of the shikoku basin two types of high magnesium andesite possessing identical bulk rock compositions are recognized one contains olivine phenocrysts with chromian spinel inclusions showing compositional characteristics consistent with their crystallization as liquidus phases whereas the other contains niand fe rich olivine and fe3 rich spinel one of the possible causes for such unusual mineral compositions could be effective elemental diffusion within and through olivine crystals associated with a long residence time in a crustal magma chamber and the slow rate of cooling of the host magma the compositions of liquidus chromite and olivine as well as whole rock major element compositions suggest that the ne kyushu highmagnesium andesite magmas were produced by higher degrees of partial melting than those in other regions of the setouchi volcanic belt

the transition to overshielding after sharp and gradual interplanetary magnetic field northward turning overshielding is referred to a shielding status during which the dawnward shielding electric field dominates over the duskward penetration electric field in the inner magnetosphere typically appearing when the interplanetary magnetic field imf suddenly turns northward after a prolonged southward orientation it is expected that the transition to overshielding after imf northward turning can be affected by the shape of northward turning sharp or gradual moreover the initial shielding status undershielding or goodshielding prior to the transition may also have influence on the transition here we analyze two groups of cases in which the transitions appear after sharp duration less than 5 min and gradual duration more than 30 min northward turning each group includes two cases in which the transition initiated from undershielding and goodshielding these cases show that 1 the beginning of the transition to overshielding coincides with sharp imf northward turning but appears in the midst of gradual imf northward turning 2 the transition from goodshielding to overshielding is always associated with convection electric field drop and or polar cap shrinkage regardless of the shape of imf northward turning and 3 the typical solar wind condition in which the imf suddenly turns northward after a prolonged southward orientation is neither a necessary condition nor a sufficient condition for overshielding furthermore we will discuss the effect of substorm processes on overshielding

eastward electric field enhancement and geomagnetic positive bay in the dayside low latitude ionosphere caused by magnetospheric substorms during sawtooth events 1 substorms cause significant disturbances in the magnetosphere and ionosphere although many substorm induced phenomena have been relatively well understood a serious controversy arises as to how the magnetospheric and ionospheric convection responds to the expansion onset in this paper we present the observations of the low latitude ionospheric electric field and geomagnetic field at substorm onsets during sawtooth events when the interplanetary magnetic field imf remains steady southward so the ionospheric disturbances are attributed solely to the substorm effects in the substorm events we have analyzed the dayside equatorial ionospheric electric field shows an eastward enhancement at the onset and the geomagnetic field northward component at middle and low latitudes shows an increase positive bay on both the dayside and nightside we suggest that the magnetospheric convection is enhanced at substorm onset and that the enhanced magnetospheric convection electric field penetrates to the low latitude ionosphere the penetration electric field driven by substorm onset causes the eastward ionospheric electric field enhancement and geomagnetic positive bay on the dayside and the substorm current wedge causes the geomagnetic positive bay on the nightside our findings are significantly different from the results of most previous studies and provide new insight into the substorm dynamic process

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region