hf isotopic composition and age constraints on the evolution of the archean central uchi subprovince ontario canada the archean uchi subprovince located in the northwestern superior province comprises greenstone belts and granitoid complexes that formed in the course of a multistage history between about 3000 and 2690 ma the earliest volcanic plutonic complexes were assembled into a microcontinent which then progressively grew by the addition of magmatic arcs and tectonic slivers a major compressional orogeny concluded this evolution at around 2700 ma lu hf isotopic compositions for well dated zircon populations from volcanic and plutonic rocks of the central part of the subprovince have hf values that range from 6 1 to 1 4 these results show that the bulk of the province was derived predominantly from depleted mantle sources but there is a superimposed trend showing involvement of the oldest greenstone and crustal sequences in the generation of the youngest magmatic complexes the temporal and isotopic relationships of the uchi subprovince contrast with the situation observed in the southern abitibi greenstone belt which developed within a time span of less than 80 m y in an intraoceanic setting and exhibits quite uniform depleted mantle like isotopic values revealing only very subordinate degrees of recycling of older crust

source parameters of earthquakes and intraplate deformation beneath the shillong plateau and the northern indoburman ranges we determined the fault plane solutions and focal depths of 17 earthquakes beneath the shillong plateau and the northern indoburman ranges by combining results from the inversion of long period p and sh waveforms and amplitudes from polarities of first motions and from the identification of pp and sp phases on short period seismograms fault plane solutions of 15 earthquakes show mixtures of thrust and strike slip faulting but the p axes for these events are nearly horizontal and consistently oriented north northeast south southwest all of these earthquakes occurred at depths greater than 29 km beneath the shillong plateau one event occurred at a depth of 52 km the relatively large depths for earthquakes in an intraplate setting suggest that the indian lithosphere in this area is especially cold earthquakes beneath the northern indoburman ranges define a gently east southeast dipping zone from 30 to 45 km beneath the bengal basin to 40 to 90 km beneath the ranges this zone seems to steepen and connect with the zone of intermediate depth seismicity that dips eastward beneath burma these earthquakes cannot have occurred along the interface between a subducting indian plate and the overriding indoburman lithosphere because the p axes not the nodal planes are parallel to the north south trending seismic zone although a couple of the earthquakes might have occurred within the indoburman lithosphere most of this seismicity seems to have occurred within the indian plate recently and currently being subducted eastward beneath the indoburman ranges the consistent north northeast trend of the p axes implies that the orientation of maximum compressional strain in the indian plate throughout its northeastern pan is nearly perpendicular to that responsible for roughly north south trending folds of the indoburman ranges thus either recently in geologic time since 1 ma the orientation of maximum compression changed dramatically or more likely the deformation in the indoburman ranges is decoupled from that in the underlying indian plate meanwhile the seemingly identical northward displacement of india and the indoburman ranges with respect to south china must be accommodated farther east along the sagaing and other faults

nd isotope systematics of coarse and fine grained sediments examples from the middle proterozoic belt purcell supergroup sm nd isotopic compositions are reported for 20 fine and 5 coarse grained samples of the belt purcell supergroup of western north america varepsilon nd and sm nd ratios for fine grained belt purcell sediments are remarkably uniform indicating crustal residence ages ranging only from 1 6 to 2 1 ga varepsilon nd and sm nd ratios for coarse grained sediments however are much more varied and contain evidence of sm nd fractionation during sedimentary recycling assuming that this ree fractionation occurred between the time source areas were eroded and when sediment was deposited a new method of calculating crustal residence age intervals is derived for the coarse grained samples which serves to place constraints on the average time since source materials were extracted from the mantle the nd data from both sample groups provide an average age estimate of 2 0 pm 0 3 ga for the middle proterozoic crust from which most of the belt purcell supergroup was derived the isotopic data toge

geochemistry of proterozoic volcanic and granitic rocks from the gold hill wheeler peak area northern new mexico abstract early proterozoic supracrustal and plutonic rocks from the gold hill wheeler peak area in northern new mexico define three populations amphibolite diorite tonalite hornblendite cumulus amphibolite and felsic volcanics and porphyries also present are mid proterozoic granites amphibolites are similar in ti zr cr ni and ree contents to young calc alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high al2o3 tonalites respectively felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile element contents geochemical model studies suggest that the amphibolites hornblendites diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite amphibolites represent parent tholeiites modified by olivine removal hornblendite is an early solid residue comprised chiefly of hornblende clinopyroxene and olivine diorite and cumulus amphibolite represent respectively residual solid clinopyroxene plagioclase hornblende and liquid after 50 crystallization tonalite represents a residual liquid after 80 crystallization felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite facies mineralogy in the lower crust granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet the calc alkaline igneous rocks in the gold hill wheeler peak area suggest the presence of an arc system in northern new mexico during the early proterozoic the fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited eroded and buried by cratonic sediments from the north

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

provenance of pliocene sediments and paleoenvironmental changes in the southern north sea region using samarium neodymium sm nd provenance ages and clay mineralogy abstract an 800 m thick sedimentary succession from the late pliocene north sea has been investigated on the provenance of the sediments and on changes of the source regions related to transport processes and climate changes the lower zanclean to piacenzian part of the succession from 1250 to 1050 m depth and the middle early gelasian part from 1100 to 700 m depth belong to one distributory system with at least two contributing source areas provenance ages calculated on basis of sm nd content indicate one old archean and one young

hypocentral trend surface analysis probing the geometry of benioff zones a hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the middle of a benioff zone in this paper we take a relatively simple approach to hypocentral trend surface analysis the hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface the vertical deviation of a hypocenter from the hypocentral trend surface is called its residual consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution any analysis of benioff zone geometry trades off presumed thickness of the zone with its presumed shape this trade off can be investigated by examining suites of candidate hypocentral trend surfaces together with their associated residual distributions generated by varying the number of degrees of freedom available to the trend surface hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in honshu japan cook inlet alaska and south peru hypocenters beneath honshu generate a bimodal distribution of residuals about their trend surface the vertical separation of the upper and lower sheets of this double benioff zone averages about 33 km no double benioff zone configuration is discovered below cook inlet or south peru regional trend surfaces are established for intermediate depth teleseismic data from south and middle america in south america 0 40 s teleseismic data suggest that along strike transitions between flat and moderately steeply dipping sections of the benioff zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear bound flaps or separate tongues of lithosphere hypocentral trend surface analysis of teleseismic data from south peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of hasegawa and sacks an eastward shallowing benioff zone beneath southwest mexico is closely associated with the geometry of the trans mexican volcanic arc the benioff zone beneath central america is more steeply dipping the flat slab steep slab transition near 96 w may relate to the difference in age of oceanic lithosphere on either side of the tehuantepec ridge while the strong flexure of the subducted nazca plate beneath ecuador is closely associated with the carnegie ridge the nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath south peru the flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding south american plate

petrogenesis of the late cretaceous demirkoy igneous complex in the nw turkey implications for magma genesis in the strandja zone abstract the srednogorie zone in bulgaria and its eastern continuation in nw turkey the strandja zone consist of a late cretaceous magmatic belt comprised of plutonic volcanic and associated volcano sedimentary rocks the demirkoy igneous complex is the biggest cretaceous pluton within the strandja zone and its age is between 71 and 84 ma the demirkoy pluton and additional smaller plutons are intruded into triassic and older metamorphic basement rocks a contact metamorphic aureole with hornblende hornfels facies conditions was developed around the demirkoy pluton the wnw trending demirkoy pluton has an elliptical shape and a concentrically zoned internal structure in this structure gabbro diorites and quartz diorites are only seen in the sw corner of the pluton while granodiorites form the main body of the pluton surrounded by a zone of granite the granodiorites contain dioritic mafic microgranular enclaves small quartz monzonite lenses and or bodies and mafic dykes are also observed in the demirkoy igneous complex geochemically the demirkoy igneous complex has calc alkaline metaluminous and medium k characteristics there is a significant correlation between the lree hree ratios and sio 2 values chondrite normalized ree patterns are relatively flat la n lu n 2 in gabbros this ratio ranges from 3 to 11 for the diorite granodiorites and from 14 to 35 for the granites dioritic enclaves have ree patterns similar to those of the main body and have la n lu n values between 3 and 6 relatively enriched lile sr k rb ba and th and relatively depleted hfse ta nb and ti values indicate the classic subduction related origin of the complex the initial isotopic signatures range from e nd i 0 45 to 2 57 and 87 sr 86 sr i 0 7042 0 7064 for gabbros and e nd i 0 43 to 2 67 and 87 sr 86 sr i 0 7048 0 7059 for granodiorites and granites 18 o values range from 6 18 to 7 80 vsmow for gabbros and from 6 86 to 8 89 vsmow for granodiorites and granites based on geochemical and isotopic characteristics this magmatic complex can be subdivided into 4 groups a gabbro b diorite to alkali granite main body c syenite monzonite and d mafic dykes geochemical data and modelling results favour a typical subduction related enriched lithospheric mantle and lower crustal source for the demirkoy igneous complex slab derived fluids and crustal contamination also played a key role in the petrogenesis of this magmatic suite

revised sm nd systematics of kambalda greenstones western australia sm nd isotope systematics in mafic ultramafic lavas from the norseman wiluna greenstone belt western australia have been reinvestigated a previous age based on mixing lavas and granites is not confirmed instead a sm nd age of 3 262 44 myr is indicated for the lavas 400 myr older than intruding granites and predating gneisses that have been proposed as basement rocks

widespread basement erosion during the late paleocene early eocene in the laramide rocky mountains inferred from 87sr 86sr ratios of freshwater bivalve fossils reconstruction of the timing and pattern of exhumation of basement involved uplifts during the laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region we use reconstructions of the 87 sr 86 sr ratios of late cretaceous early cenozoic river water from fossil shells in six basins of the rocky mountains contextualized by modern river water sr chemistry studies to trace the erosion of precambrian basement cores in the laramide ranges the 87 sr 86 sr ratios and sr concentration of modern river water in the rocky mountains are controlled by river bedrock lithology weathering of precambrian silicate rocks in the cores of laramide ranges produces high 87 sr 86 sr ratios in highland rivers weathering of paleozoic and mesozoic carbonates along the basin margins reduces the 87 sr 86 sr ratios and increases sr concentration of rivers as they flow basinward lowland rivers that have headwaters in precambrian basement mostly have 87 sr 86 sr ratios 0 711 whereas rivers confined to or with very long reaches in basins have 87 sr 86 sr ratios between 0 709 and 0 711 river water 18 o values do not change in response to changes in catchment elevation our results from fossil shells show that carbonate weathering dominated the river water sr chemistry in the alberta foreland williston and crazy mountains basins and that proterozoic low grade metamorphic carbonates in the belt purcell supergroup were not exposed in the canadian rocky mountains during late cretaceous early paleocene time silicate weathering influenced the sr chemistry of surface waters in the powder river basin during the late paleocene and the washakie basin during the early eocene suggesting precambrian silicate basement rock was extensively exhumed and eroded the observation that precambrian basement was eroded earlier around the powder river basin than around the washakie basin is consistent with a previous study that suggests laramide ranges in northeast wyoming reached high elevation earlier than the ranges adjacent to the sevier thrust belt widespread basement erosion in late paleocene early eocene time was controlled by tectonic exhumation and may have been intensified by the wet and warm global climate

uniformly processed strong motion database for himalaya and northeast region of india this paper presents the first uniformly processed comprehensive database on strong motion acceleration records for the extensive regions of western himalaya northeast india and the alluvial plains juxtaposing the himalaya this includes 146 three components of old analog records corrected for the instrument response and baseline distortions and 471 three components of recent digital records corrected for baseline errors the paper first provides a background of the evolution of strong motion data in india and the seismotectonics of the areas of recording then describes the details of the recording stations and the contributing earthquakes which is finally followed by the methodology used to obtain baseline corrected data in a uniform and consistent manner two different schemes in common use for baseline correction are based on the application of the ormsby filter without zero pads trifunac 1971 and that on the butterworth filter with zero pads at the start as well as at the end converse and brady 1992 to integrate the advantages of both the schemes ormsby filter with zero pads at the start only is used in the present study a large number of typical example results are presented to illustrate that the methodology adopted is able to provide realistic velocity and displacement records with much smaller number of zero pads the present strong motion database of corrected acceleration records will be useful for analyzing the ground motion characteristics of engineering importance developing prediction equations for various strong motion parameters and calibrating the seismological source model approach for ground motion simulation for seismically active and risk prone areas of india

island arc character and late precambrian age of volcanics at wadi shwas hijaz saudi arabia geochemical and sr and nd isotopic evidence abstract a summary description of two volcanic sequences named the surgah and the shwas formation situated in the wadi shwas area at long 42 0 lat 20 0 in the arabian shield is given major and trace element concentrations including ree abundances are presented as well as sr and some nd isotope results one sequence the surgah formation shows a tholeiitic differentiation trend from basaltic to rhyolitic rocks whereas the shwas formation has calc alkaline affinities both sequences have low concentrations of incompatible elements notably rb nb zr and ti the coexistence of a tholeiitic and a calc alkaline suite along with these low concentrations is strongly reminiscent of some present day island arc environments an rb sr errorchron age of 721 55ma was obtained on the surgah formation with high initial87sr 86sr ratios around 0 705 the rare earth abundances are overall very low and the patterns show a relative depletion of the light ree in the least evolved rocks indicating a relatively light ree depleted mantle source a time integrated light ree depleted source is also indicated by the fairly radiogenic143nd 144nd ratios end values around 8 the nd model age of source depletion tchurnd is around 2 2 ga

the mean life of continents is currently not constrained by nd and hf isotopes the paper on crustal recycling by depaolo the mean life of continents geophys res lett 10 705 708 1983 contains errors in the listing and plotting of published hf isotopic data which materially affect the inferred pattern of mantle hf isotopic evolution more generally the impression given in the paper that it is now possible to use nd and hf isotopic data to constrain the degree of crust to mantle recycling is misleading because exactly the same observed effects would be produced by mixing within the mantle itself existing nd and hf isotopic data allow either crustal growth with no recycling of crust to mantle or a model of constant mass continental crust with extensive recycling this implies that all intermediate growth plus recycling combinations are also permitted

lead isotopic evidence for the origin of paleo and mesoproterozoic rocks of the colorado province u s a abstract lead isotopic ratios of k feldspars and whole rocks from 1 7 and 1 4 ga plutonic rocks of the colorado province are relatively non radiogenic for 207 pb 204 pb plotting below the average crust model curve of stacey and kramers 1975 indicating that the terrane was derived primarily from juvenile mantle material slightly more radiogenic ratios in the northern part of the terrane near the archean wyoming province suggest minor inclusion of an older component the data from 1 7 ga plutons plot in a broad field suggesting two episodes of re equilibration with whole rock pb probably related to heating events in the mesoproterozoic 1 4 ga and cretaceous 70 ma possible differences in calculated whole rock th u coupled with slight pb isotopic variations along the north south transect suggest either a terrane boundary through central colorado near salida and gunnison or fundamental differences in source rocks metasedimentary vs metavolcanic u pb analyses of multigrain splits of detrital zircons from quartzites throughout the colorado province have failed to identify archean detritus the oldest 207 pb 206 pb ages found in two samples of quartzite from northern colorado are about 2 0 ga perhaps derived from rocks of the trans hudson orogen in contrast to 2 75 ga detrital zircon in a paleoproterozoic quartzite from the southern part of the wyoming province while we are not yet able to discern if these ages are true provenance ages or mixtures of archean and paleoproterozoic components the absence of easily recognizable archean zircons supports other isotopic data and a conclusion that most of the paleoproterozoic crust of the colorado province was ultimately derived from a juvenile at 1 8 ga mantle reservoir

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

geochemistry and isotopic characteristics of mafic phulad ophiolite and related rocks in the delhi supergroup rajasthan india implications for rifting in the proterozoic abstract regionally metamorphosed mafic rocks phulad ophiolite in the delhi supergroup n w india have tholeiitic compositions the data indicate that there are two independent rock suites phulad barr jetgarh pbj meta basalts with element abundances and ratios resembling midocean ridge basalts morb and ranakpur desuri r d meta basalts and meta gabbros with relatively high al2o3 and low tio2 for similar mgo and high lil element rb ba sr coupled with low hfs zr nb and ree sm nd abundances the compositions of these rocks are characteristic of ocean arc basalts dioritic rocks at ranakpur exhibit low k tholeiitic differentiation trends and have close chemical affinity with the spatially associated meta basalts ranakpur diorites define a sm nd whole rock isochron corresponding to an age of 1012 78 ma with an initial nd isotopic composition that is similar to bulk earth sm nd mineral isochrons for a meta gabbro diorite and quartz diorite at ranakpur correspond to ages of 838 36 835 43 and 791 43 ma respectively these younger ages reflect metamorphic re equilibration of isotope compositions during widespread granite and granodiorite emplacement in this region during the late proterozoic model sm nd ages for the meta basalts calculated relative to undepleted mantle vary between 1300 1700 ma the chemistry of both mafic rock suites indicates derivation from source regions which had been depleted during previous melting episodes so the model ages are a maximum age constraint ranakpur desuri mafic rocks have negative initial nd 1 0 to 4 5 and low sm nd ratios suggesting crustal contamination or reworking with a few exceptions the pbj meta basalts have positive initial nd 1 5 to 4 8 indicating that they were derived from heterogeneously enriched although longterm depleted mantle the chemical data support the hypothesis that the pbj meta basalts are fragments of proterozoic ocean crust

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

active tectonics and earthquake potential of the myanmar region this paper describes geomorphologic evidence for the principal neotectonic features of myanmar and its immediate surroundings we combine this evidence with published structural geodetic and seismic data to present an overview of the active tectonic architecture of the region and its seismic potential three tectonic systems accommodate oblique collision of the indian plate with southeast asia and extrusion of asian territory around the eastern syntaxis of the himalayan mountain range subduction and collision associated with the sunda megathrust beneath and within the indoburman range and naga hills accommodate most of the shortening across the transpressional plate boundary the sagaing fault system is the predominant locus of dextral motion associated with the northward translation of india left lateral faults of the northern shan plateau northern laos thailand and southern china facilitate extrusion of rocks around the eastern syntaxis of the himalaya all of these systems have produced major earthquakes within recorded history and continue to present major seismic hazards in the region

locked and loading megathrust linked to active subduction beneath the indo burman ranges it is unclear whether subduction is still active beneath the indo burman mountain range analyses of gps measurements from this region reveal a locked megathrust fault implying that subduction is active and could generate a large earthquake

combined escape tectonics and subduction rollback back arc extension a model for the evolution of tertiary rift basins in thailand malaysia and laos the tertiary rift basins of thailand and adjacent countries show considerable variability in the timing of rift initiation termination the timing and magnitude of thermal subsidence and the timing and intensity of inversion episodes the rift basins developed on continental blocks that were extruded southeastwards hence their development must be tied into himalayan extrusion tectonics current tectonic models propose that the tertiary basins opened up as pull apart basins associated with strike slip faults published geochronology of strike slip fault zone rocks mapping of fault patterns in the tertiary basins and mapped releasing restraining bend geometries all indicate that in thailand major sinistral strike slip motion ceased at about 30 ma prior to the formation of most rift basins the thailand the effects of later dextral slip were minor and probably a result of reactivation during episodes of inversion during nw se to ne sw himalayan compression dextral slip was not responsible for opening most of the rift basins in thailand an alternative mechanism to open the rift basins is subduction rollback of the indian plate to the west of thailand it is proposed that subuction rollback can help explain some of the characteristics of the rift basins such as non uniform lithospheric extension deep sag basins and the diachronous onset and termination of rifting

subduction in the indo burma region is it still active the indo burma region which includes longitude 92 96 e latitude 20 26 n is a subduction zone where the indian plate underthrusts the south eastern asian plate but the nature of subduction is complex i examine here the distribution of the p t and b axes of 37 earthquakes the time period 1977 1995 depths 0 153 km and magnitudes mb 4 8 6 8 with respect to the geometry of the wadati benioif zone the analysis shows that the t axes are clustered close to the down dip direction of the subducting slab implying a predominant down dip tensional stress regime within the slab which is typical of intermediate depth seismicity of active subduction zones these results suggest that the subduction in the indo burma region is possibly active

early proterozoic continental tholeiites from western bergslagen central sweden ii nd and sr isotopic variations and implications from smnd systematics for the svecofennian sub continental mantle abstract a subset of nine least altered least fractionated within plate continental metatholeiites from western bergslagen yields a sm nd isochron age of 1886 48 ma with an nd t of 1 1 this age is in agreement with previous u pb zircon dating of a coeval felsic intrusion variation of nd t in other subsets is shown to be caused by crustal contamination of the basic magma to explain the remarkable isotopic homogeneity of the least altered least fractionated metatholeiites and their combined lree enrichment and isotopic depletion three petrogenetic models are discussed the first model involves incipient melting of a depleted mantle in the garnet stability field the second model presumes complete homogenization of a depleted mantle derived melt contaminated by older continental crust and the third model proposes melting of an enriched mantle source in light of the depleted nature of the baltic asthenosphere absence of enriched archaean crust in the bergslagen area negative hfse zr hf ti anomalies in the metatholeiites and enriched isotopic characteristics of lamprophyres occurring 50 km to the west model three is preferred in a tentative scenario three time scales are inferred from sm nd isotope systematics to generate the metatholeiites 1 long term 1 ga lree depletion of the asthenosphere by extraction of continental crust 2 shorter term pre 1 88 ga bp lree enrichment of this depleted mantle by supra subduction zone modification of the mantle wedge during protocrust formation and 3 short term around 1 88 ga bp generation of basic magma by melting of this enriched mantle as a result of continental rifting of the early proterozoic protocrust

age tectonic setting and provenance of ostfold marstrand belt supracrustals westward crustal growth of the baltic shield at 1760 ma the ostfold marstrand paragneiss belt is the oldest part of the sw scandinavian domain of the baltic shield in the swedish part of the belt minor mafic volcanics now amphibolites in the metagreywacke dominated stora le marstrand slm supracrustals are chemically similar to primitive modern oceanic arcs or back arc regions a wide compositional range is attributed mainly to fractional crystallisation yielding fe rich andesitic amphibolites diverging lree patterns between different localities are consistent with variations in the degree of partial melting in the source a nine point sm nd errorchron of 1758 78 ma mswd 9 4 for the amphibolites is interpreted to date the volcanism and coeval sedimentation and hence directly dates crustal growth in this part of the baltic shield the exclusion of two pillow lava samples which may have been contaminated yields an isochron with an identical though more precise age an nd value of 4 3 indicates a depleted mantle source similar to that of canada sw u s a s greenland and parts of finland but rather more depleted than that reported hitherto from eastern sweden at the same time nd isotopic data suggest a two component provenance for the slm sediments involving acid derivatives from the mafic volcanics and a continental source similar to the exposed 2 0 1 75 ga old svecofennian crust of eastern sweden with minimal contribution from substantially older crust the mafic volcanism took place during subduction of oceanic crust outboard of the svecofennian part of the baltic shield after substantial addition of 1650 ma old granitoids accretion to the baltic shield possibly took place at 1580 ma

infilling history of a neoproterozoic intracratonic basin nd isotope provenance studies of the uinta mountain group western united states abstract neodymium isotope and selected trace element concentration data were obtained from 27 samples of shales and arkosic sandstones comprising the neoproterozoic uinta mountain group umg in north eastern utah and northwestern colorado to determine the provenance of siliciclastic detritus deposited in this region and to reconstruct aspects of the proterozoic geologic history of the southern margin of the archean wyoming province in the east lobe of the uinta mountains coarse grained components of the jesse ewing canyon formation the oldest recognized formation of the umg have measured nd values of 23 to 28 and were most probably derived from archean basement of the wyoming province to the north fine grained sediment in the southern portions of the jesse ewing canyon formation and shales and quartz arenites from overlying undifferentiated umg all contain at least a component of detritus derived from proterozoic crust in the west lobe the oldest members of the umg the red castle lake gilbert peak and moosehorn lake formations and arkosic arenite comprising the northern portions of the overlying mount watson formation were also derived exclusively from archean sources however as in the east lobe the more mature sandstones that dominate the southern sections of the mount watson formation have isotopic compositions that require a significant proterozoic crust derived detrital component our data support a model in which sedimentary rocks with an archean provenance represent the preserved remnants of alluvial fans marking the northern margin of the basin in which the umg was deposited a source for these sediments within the archean wyoming province is consistent with the high light rare earth element lree and th abundances of finegrained low nd sedimentary rocks given that late archean granitic rocks within the wyoming province have similar trace element characteristics sediments with proterozoic components in both the east and west lobes were apparently deposited from a fluvial system that transported sediment along the basin axis the proterozoic detrital component in these sediments originated either through direct erosion of the colorado province in the neoproterozoic or through erosion of mixed archean and proterozoic crust located to the east of the umg basin that originally formed during the paleoproterozoic accretion of the colorado province to the southern margin of the wyoming craton

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

neodymium strontium and lead isotopes in the maloin ranch pluton wyoming implications for the origin of evolved rocks at anorthosite margins abstract neodymium strontium and lead isotopic data are used in this study to investigate the origin of chemically evolved rocks in the maloin ranch pluton a composite body that borders and intrudes the laramie anorthosite the range of rock types present is similar to that found at the margins of proterozoic anorthosites throughout the world in the maloin ranch pluton these include ferrodiorite at the base of the intrusion overlain progressively by fine grained monzonite monzosyenite and porphyritic granite biotite gabbro and fine grained granitic dikes are present locally at various levels of this sequence the origin of the evolved rocks and their possible relation to associated anorthositic bodies has been much debated in the maloin ranch pluton each rock type has distinct isotopic characteristics which together with trace element data previously reported suggest different source characteristics for each member strontium and neodymium isotopic data for maloin ranch ferrodiorite and laramie anorthositic rocks show considerable overlap consistent with a comagmatic relation biotite gabbro is chemically and isotopically the most primitive rock type in the maloin ranch pluton the data suggest that biotite gabbro has a mantle source but has undergone extensive fractionation in the crust our results suggest that the remainder and bulk of the intrusion formed by partial melting of the lower crust due to the emplacement of the laramie anorthosite maloin ranch fine grained monzonite is chemically more evolved than the underlying ferrodiorite i e lower mg higher silica and alkalis yet has more primitive isotopic characteristics lower isr higher epsilonnd formation of fine grained monzonite by in situ fractionation of ferrodiorite is precluded by both the isotopic data and by mass balance the isotopic data do not rule out formation of fine grained monzonite by fractionation or contamination of ferrodiorite at depth but this model requires that the ferrodiorite is isotopically heterogeneous and that very large volumes of ferrodiorite were present alternatively trace element and isotopic characteristics of the fine grained monzonite are explained by partial melting of mantle derived lower crust added to the margin of the archean wyoming craton at about 1 8 ga neodymium strontium and lead isotope data for maloin ranch monzosyenite and porphyritic granite also suggest a lower crustal source these data indicate a 1 8 ga source component for the maloin ranch monzosyenites but do not require input of archean crust from the wyoming province to the north

asymmetric radiation of seismic waves from an atoll nuclear tests in french polynesia seismic records of nuclear tests detonated in the mururoa atoll in french polynesia show large unpredicted arrivals 2 2 and 4 5 seconds x1 and x2 after the p wave at the australian warramunga array these arrivals are not observed at the canadian yellowknife array x1 and x2 are also absent on warramunga array recordings of tests carried out at the fangataufa atoll situated 40 km sse of mururoa array analysis shows that x1 and x2 are produced within the source area the layered crustal structure of the atoll significant local inhomogeneities and focusing effects due to the elongated shape and the steep flanks of the mururoa atoll are most likely responsible for x1 and x2 the form of mururoa 28 10 km and its east west orientation is due to its location on the austral fracture zone afz the fangataufa atoll on the other hand is almost circular 10 km diameter and is unaffected by the dynamics along the afz our observations demonstrate that complicated structures in the source area can significantly alter the wave field at teleseismic distances and produce a large magnitude mb bias a better understanding of the exact cause of these unusual seismic observations will only become possible if the coordinates of the tests and information on the detailed 3 d structure of the atolls are released

nd and sr isotopic study of volcanic rocks from japan two older granitic rocks and some selected quaternary volcanic rocks from the japanese islands were analyzed in a reconnaissance study for the purpose of examining the relationships between nd and sr isotopic abundances and the megatectonic structure around the japanese islands model ages of 0 9 ae were determined by the nd and sr methods on a paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the japanese islands the nd and sr isotopic data show that the cretaceous granodiorite is the result of partial melting of older crust the nd of tholeiitic rocks from the izu arc gives e nd ranging from 8 3 to 9 3 and with the corresponding e sr from 14 5 to 18 5 these results are identical to those found for the mariana arc these values are distinctly lower than typical morb by around 1 2 eu this difference in e nd between arcs and morb is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust the hakone volcano is clearly confirmed as belonging to an oceanic source by nd isotopic results e sr e nd values of the volcanics from a section along the fossa magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material the crustal component clearly increases in going from south to north volcanics across the northeast japan arc also show a distinct correlation of e sr e nd related to the position relative to the active subduction zone but with the opposite trend these relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the japan sea in general there appear to be regular isotopic relationships between the izu mariana oceanic island arc and the continental island arc of japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

measurement of vertical profiles of snow specific surface area with a 1 cm resolution using infrared reflectance instrument description and validation the specific surface area ssa defined as the surface area of ice per unit mass is an important variable characterizing the complex microstructure of snow its application range covers the physical evolution of snow metamorphism photochemistry and optical and microwave remote sensing this paper presents a new device posssum profiler of snow specific surface area using swir reflectance measurement designed to allow the rapid acquisition of ssa profiles down to 20 m depth and with an effective vertical resolution of 10 20 mm posssum is based on the infrared ir reflectance technique a laser diode operating at 1310 nm illuminates the snow at nadir incidence angle along the face of a drilled hole the reflected radiance is measured at three zenith angles 20 40 and 60 each for two azimuth angles 0 and 180 a second laser operating at a shorter wavelength 635 nm which is almost insensitive to ssa allows the distance to the snow face to be estimated the reflected ir radiance and the distance are combined to estimate bidirectional reflectances these reflectances are converted into hemispherical reflectances and in turn into ssa using a theoretical formulation based on an asymptotic solution of the radiative transfer equation the evaluation and validation of posssum s ssa measurements took place in spring 2009 in the french alps the new method was compared with the methane adsorption technique and dufisss another well validated instrument based on the ir technique the overall measurement error is in the range 10 15

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

new models for paleoproterozoic orogenesis in the cheyenne belt region evidence from the geology and u pb geochronology of the big creek gneiss southeastern wyoming the disputed age of the deep crust of the colorado province is central to hypotheses for paleoproterozoic crustal growth in the region we studied the high grade big creek gneiss southeastern wyoming as a potential exposure of pre 1780 ma basement rocks new geologic mapping and u pb geochronological data indicate that the big creek gneiss exposes a deeper but coeval level of the green mountain arc relative to the predominantly supracrustal section to the west the big creek gneiss is composed of supracrustal rocks a ca 1780 ma green mountain arc correlative bimodal intrusive suite a ca 1763 ma extensional bimodal intrusive suite and widespread ca 1630 ma pegmatitic leucogranite the mafic member of the younger bimodal suite is documented here for the first time u pb zircon ages from migmatite leucosomes indicate penetrative deformation of the big creek gneiss at ca 1750 ma we find that the postarc intrusive suite is mantle involved implying a second period of crustal growth shortening postdates arc magmatism by 20 m y implying that termination of arc magmatism and accretion were separate events finally criteria previously used to constrain the polarity of subduction for the green mountain arc are not reliable we propose two competing models 1 southward dipping green mountain arc subduction present coordinates with slab breakoff related magmatism following arc accretion or 2 northward dipping subduction with extensional postarc magmatism in both models high temperature deformation coincides with accretion along the cheyenne belt and extensional magmatism is an important component of crustal growth we prefer the northward dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks

measuring specific surface area of snow by near infrared photography the specific surface area ssa is considered an essential microstructural parameter for the characterization of snow photography in the near infrared nir spectrum is sensitive to the ssa we calculated the snow reflectance from calibrated nir images of snow pit walls and measured the ssa of samples obtained at the same locations this new method is used to map the snow stratigraphy the correlation between reflectance and ssa was found to be 90 calibrated nir photography allows quantitative determination of ssa and its spatial variation in a snow profile in two dimensions within an uncertainty of 15 in an image covering 0 5 1 0 m 2 even layers of 1 mm thickness can be documented and measured spatial maps of ssa are an important tool in initializing and validating physical and chemical models of the snowpack

geochemical and sm nd isotopic study of amphibolites in the cathaysia block southeastern china evidence for an extremely depleted mantle in the paleoproterozoic abstract geochemical and sm nd isotopic results are reported for late paleoproterozoic mafic amphibolites from sw zhejiang and nw fujian parts of the cathaysia block of se china two suites of contemporaneous amphibolites are distinct in their geochemical characteristics group 1 samples from nw fujian have chemical compositions of transitional and alkali basalts show lree enriched patterns and plot mainly in the field of within plate basalt on a number of trace element discrimination diagrams group 2 rocks from sw zhejiang have tholeiitic compositions and are characterized by flat to lree depleted patterns and fall into the morb field all the amphibolite samples have high end t values of 5 6 to 8 5 t 1766 19 ma a positive correlation between end t and nb th suggests possible mixing of a mantle derived magma and a crustal component with the least contaminated samples having very high end t values 8 8 5 and nb th ratios of 13 the geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto oceanic basin e g the gulf of tadjoura the exceptionally high end t values of up to 8 5 for some of the amphibolites suggest the presence of a time integrated extremely depleted mantle source beneath cathaysia during the paleoproterozic

texture and strength changes of buried surface hoar layers with implications for dry snow slab avalanche release buried layers of surface hoar are the failure plane for many slab avalanches including fatal human triggered avalanches in various mountain regions these layers may persist as weak layers in the snow cover for weeks or months it is therefore essential for operational avalanche forecasters to monitor the evolution of persistent weak layers such as buried surface hoar traditional grain shape observations of isolated grains with a magnifier and crystal screen do not show bonding that is decisive for strength in this study we used in situ microphotography and observations of texture to complement strength measurements from shear frame tests buried layers of surface hoar consist of crystals most of which extend from the layer below to the layer above and may exhibit a columnar or truss like structure observations and measurements show that texture and crystal size change little over periods of up to several months during which the snowpack remains dry under these conditions layer thickness decreases while density and strength increase based on field measurements we argue that the increase in strength is primarily due to penetration of the surface hoar crystals into the adjacent layers especially at the bottom of the buried surface hoar layer where bonding is critical the weak bonding at the bottom implies that shear failure occurs at the lower interface rather than within the weak layer on slopes we find that surface hoar crystals that were initially surface normal are tilted downslope faster than predicted by published shear strain rates for settled snow indicating that shear strain is concentrated in these layers the characteristic texture of buried surface hoar columnar or truss like permits collapsing at the time of fracture the gravitational energy released by the displacement of the slab may contribute to the extensive fracture propagation associated with buried surface hoar layers

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

accretionary tectonics of burma and the three dimensional geometry of the burma subduction zone the geometry of the burma wadati benioff zone wbz has been determined by fitting a trend surface parameterized with eight effective degrees of freedom to 184 well located hypocenters the dip of this surface which passes through the middle of the wbz varies from about 50 in the north near the eastern himalayan syntaxis to about 30 in the bay of bengal area the eastern edge of the indo burman ranges closely follows the map projection of the 60 km depth contour of the wbz the curvature of the indo burman ranges is controlled by the geometry of the interface between the more steeply dipping part of the indian plate and the leading edge of the overriding burma platelet shallow earthquakes beneath the indo burman ranges are primarily confined to the underthrusting indian plate their focal mechanisms indicate strike slip faulting and north south shortening parallel to the eastern margin of the indian plate

the 2016 mw 6 7 imphal earthquake in the indo burman range a case of continuing intraplate deformation within the subducted slab the 2016 m w 6 7 imphal earthquake is one of the largest and instrumentally well recorded seismic events to have occurred on the segment of the indo burman plate boundary where indian and burman plates converge in a roughly northeast direction similar to 3 4 cm yr this segment of the plate boundary has not generated any great earthquakes in the documented history but is noted for its complex intraslab deformation arc parallel compression and slip partitioning with the continental strike slip sagaing fault previous studies have noted a mix of faulting styles and hypocentral distribution indicating the inherent complexities within this segment based on the teleseismic moment inversion of 88 p and sh waveforms from 70 global broadband seismic stations we use the source model of the 2016 earthquake to corroborate the continuing intraslab deformation the earthquake is sourced at a depth of 55 km where the dip of the slab changes to similar to 50 degrees with the p axis oriented in the north northeast south southwest direction our analysis of azimuths and plunges of p axes of 214 earthquakes m w 4 5 global centroid moment tensor is consistent with this trend previous studies have noted two distinct types of intraplate earthquakes along this segment shallower events 25 75 km due to arc parallel bending and the deeper ones 90 150 km in response to slab pull interplate seismicity is almost absent and the potential for great earthquakes seems much lower compared with the rest of the eastern india plate boundary

geochemistry of the dubois greenstone succession an early proterozoic bimodal volcanic association in west central colorado abstract the dubois greenstone succession an early proterozoic supracrustal succession in west central colorado is composed of fine grained felsic volcaniclastic sediments ash flow tuffs and tholeiitic flows sills and dikes the volcanic rocks comprise a bimodal suite of tholeiite and rhyolite the tholeiites are divided into two groups group i exhibiting nearly flat ree patterns 20 30 chondrites and group ii with light ree enriched patterns 40 70 chondrites these groups can be produced by two stage melting of an undepleted or enriched garnet lherzolite source in which garnet remains in the residue of only the first melting stage which produces group ii magmas the felsic magmas can be produced by partial melting of a high grade gneiss in the lower crust in which zircon is entrained in the magma the dubois greenstone succession appears to have formed by felsic and mafic subaqueous eruptions in which some ash flows upon entering water underwent phreatomagmatic explosions giving rise to detritus which formed the volcaniclastic sediments although tectonic setting cannot be assigned to the succession with a high degree of confidence an immature back arc basin developed on sialic crust in consistent with most data

geodynamic evolution of the outer arc forearc belt in the andaman islands the central part of the burma java subduction complex the andaman islands the central part of burma java subduction complex expose tectonostratigraphic units of an accretionary prism in an outer arc setting and turbidites of a forearc setting a number of n s trending dismembered ophiolite slices of cretaceous age occurring at different structural levels with eocene trench slope sediments were uplifted and emplaced by a series of e dipping thrusts subsequently n s normal and e w strike slip faults resulted in the development of a forearc basin with deposition of oligocene and mio pliocene sediments metapelites and metabasics of greenschist to amphibolite grade occur in a melange zone of ophiolites the eocene mithakhari group represents pelagic trench sediments and coarser clastics derived from ophiolites evidence of frequent facies changes predominance of mass flow deposits syn sedimentary basinal disturbance and wide palaeogeographic variation indicate deposition of eocene sediments in isolated basins of an immature trench slope setting deposition of the oligocene andaman flysch group in a forearc setting is indicated by the large scale persistence of beds lack of small scale lithological variation bimodal provenance less deformation a well defined submarine fan sequence and development predominantly on the eastern part of the outer arc the mio pliocene archipelago group includes alternations of siliciclastic turbidites and subaqueous pyroclastic flow deposits in the lower part and carbonate turbidites in the upper part suggesting its deposition in the shallower forearc compared to the siliciclastic oligocene sediments

the mean life of continents estimates of continent recycling rates from nd and hf isotopic data and implications for mantle structure an analysis of the initial neodymium and hafnium isotopic compositions of modern and ancient mantle derived rocks shows that the divergence of the mantle 143nd 144nd and 176hf 177hf ratios from the chondritic ratios over the past 2 billion years has been much less than would be expected based on the sm nd and lu hf ratios of the mantle source of mid ocean ridge basalt the most likely explanation is that rock material from the continents has been recycled semi continuously back into the mantle over this time period the required rate of return of continents to the mantle calculated from a simple transport model is about 0 35 0 15 of the continental mass per billion years 2 5 km year this result is consistent with the hypothesis that the mass of the continents has been approximately constant since the late archean and implies that recycled continental material must be an important component of the upper mantle

the potential for giant tsunamigenic earthquakes in the northern bay of bengal the sumatra andaman earthquake and indian ocean tsunami of 2004 came as a surprise to most of the earth science community while it is now widely recognized that the risk of another giant earthquake is high off central sumatra just east of the 2004 earthquake there seems to be relatively little concern about the subduction zone to the north in the northern bay of bengal along the coast of myanmar formerly burma phil cummins suggests however that giant earthquake activity is possible in this region and that a large and vulnerable population may be facing a significant earthquake and tsunami hazard there is a high potential for giant earthquake activity along the coast of myanmar indications that this may be the case include the tectonic environment historical earthquake activity and stress and crustal strain observations these findings suggest that giant earthquakes do occur off the coast of myanmar and that a very large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard the great sumatra andaman earthquake and indian ocean tsunami of 2004 came as a surprise to most of the earth science community although it is now widely recognized that the risk of another giant earthquake is high off central sumatra1 just east of the 2004 earthquake there seems to be relatively little concern about the subduction zone to the north in the northern bay of bengal along the coast of myanmar here i show that similar indicators suggest a high potential for giant earthquakes along the coast of myanmar these indicators include the tectonic environment which is similar to other subduction zones that experience giant megathrust earthquakes stress and crustal strain observations which indicate that the seismogenic zone is locked and historical earthquake activity which indicates that giant tsunamigenic earthquakes have occurred there in the past these are all consistent with active subduction in the myanmar subduction zone and i suggest that the seismogenic zone extends beneath the bengal fan i conclude therefore that giant earthquakes probably occur off the coast of myanmar and that a large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard

the opening of the andaman sea where is the short term displacement being taken up the short term spreading velocity in the andaman sea a marginal basin inboard of the western sunda arc is calculated using the strain rate tensor which in turn is obtained from summing seismic moment tensors of normal faulting earthquakes a velocity of displacement is also obtained for the right lateral sagaing fault which is the northern continuation of the andaman spreading system using earthquakes in a 22 year period 1964 1986 the maximum spreading velocity in the andaman sea is found to be oriented in a n 19 w direction with a speed of 0 05 cm yr the north south relative velocity along the sagaing fault is 0 5 cm yr for strike slip earthquakes between 1971 and 1992 historical earthquakes in the andaman sea do not significantly add to the short term spreading velocity because there are no earthquakes with ms 6 0 for the sagaing fault however when moment tensors from earthquakes since 1908 are used the relative velocity is 5 7 cm yr a figure close to the 3 72 cm yr long term 13 ma rate of opening of the andaman sea these results suggest that strain due to the opening of the andaman sea is seismic along the sagaing fault and aseismic along the andaman sea spreading system probably because the spreading centers and transform faults in the andaman sea are short and cannot accomodate large earthquakes

the growth of the continent through geological time studied by nd isotope analysis of shales abstract we determined the nd isotopic composition and the sm nd ratios in a series of australian shales ranging from 0 2 gy to 3 3 gy the first result of this study is the constancy of the sm nd ratio in these shales as in granitoids secondly the initial 143nd 144nd ratio gives a regular curve decreasing through geological time both results confirm that shales are representative samples of the continental crust when insoluble elements like ree are studied we calculated their nd model ages of crustal differentiation the model ages regularly decrease with the stratigraphic ages and after 2 gy the curve flattens and tends to an asymptotic value at around 1 8 gy the significance of the shales is that they represent a mixture of continental materials and we consider the model age of such a mixture directly linked with the mean age of the continental portion feeding the sedimentary basin from these results we deduce a quantitative model of the growth curve of the continental source of the shales taking into account the effects of erosion which selectively sample recent mountains relative to shield areas we propose that the results obtained here are representative of the whole gondwana continent having studied the case of the australian shales we try to extend our study first we applied our quantitative approach to the recent results obtained by o nions et al on north atlantic provinces the continental growth curves obtained by our inversion procedure are quite distinct from the australian shales showing the regional character of shales with these two cases studied we try to compare our results with the already developed model for continental growth the consideration of the surface conservative versus surface non conservative models clearly show the non uniqueness of the geological interpretation of the growth curve on the other hand we have calculated for each case the recycling rate versus geological time by comparing the growth curve with the hurley rand province age curve such recycling increases with time in agreement with the data obtained on nd and sr initial ratios on granitoids

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc