the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

alkaline series related to early middle miocene intra continental rifting in a collision zone an example from polatli central anatolia turkey abstract a large volcanic area 7600 km2 the galatean volcanic province gvp developed in northwest central anatolia during the miocene along the neo tethys ocean suture zone possibly by post collisional processes the gvp mainly comprises 20 14 my old acid to intermediate volcanites with a geochemical signature indicating a mantle source modified by earlier late cretaceous subduction related events 100 km south of the gvp near polatli ankara basaltic rocks that cover large areas are intercalated with the miocene deposits of the beypazari basin an intra continental subsidence zone at the southwest of the gvp field observations geochemistry and k ar age dating of the polatli volcanites show that they are early 19 9 ma to mid 14 1 ma miocene in age covering an area as large as 215 km2 variations in lava thickness and the thickness of the underlying silicified baked zones suggest that the basaltic lavas erupted from a southern source possibly from the eskisehir fault zone and flowed northwards most polatli samples have chemical compositions that indicate derivation from a mantle source with crustal contamination during ascent they do not display any characteristic to suggest a subductional component although the gvp and polatli lavas formed close in time and space they were derived from different mantle sources considering the positions of these two magmatic regions with regard to the tethyan suture zone we propose that the mantle beneath the gvp and near the suture zone memorised the earlier subduction while the mantle beneath polatli that is located about 100 km further from the suture zone remained apparently unchanged after a significant volume of magma was consumed in the gvp a later 10 my and last activity guvem activity has produced quantitatively much less basaltic rocks where this subductional signature seems to completely disappear considering that the western anatolian crust is proposed to undergo extension since the late oligocene early miocene times the early miocene intra plate polatli activity may have developed within this extensional tectonic regime combined with regional data polatli data also provide broad estimations on how long a subductional event continues to modify the mantle after the subduction ceased at least 20 my how long the subductional signature is preserved during significant magmatism between 6 and 10 my and how far the subductional effect disappears laterally on the mantle with respect to the collision zone

the seismic cycle in southern california precursor or response the seismicity rate m 3 0 in southern california shows two cycles with periods of high activity 90 events year from 1945 1952 and 1969 1992 and lower activity 60 70 events year from 1952 1969 and 1992 present abrupt drops in the seismicity rate occur after the 1952 kern county m7 5 and the 1992 landers m7 3 earthquakes the sudden increase in 1969 does not coincide with any major event but approximates the time needed to reaccumulate the seismic moment released in the 1952 earthquake this temporal correlation with the preceding earthquake suggests that the seismic cycle lower seismicity after a major earthquake and higher seismicity before the next major earthquake should be interpreted as a response to the first earthquake rather than a precursor to the second southern california is now at a rate of seismicity as low as it experienced in the 1950s and 1960s

petrological and geochemical characteristics of cenozoic high k calc alkaline volcanism in konya central anatolia turkey abstract late miocene to pliocene volcanic rocks outcrop west northwest and southwest of the konya area in central anatolia turkey volcanic products are lava domes nuee ardentes and ignimbrite deposits predominantly andesitic to dasitic in composition together with rare basalt basaltic andesite basaltic trachyandesite and trachyandesite 50 35 69 39 sio2 the serie exhibits high k calc alkaline affinities fractional crystallization of pyroxene plagioclase and fe ti oxides is the main process in the magmatic evolution of konya volcanic rocks volcanic units exhibit typical high k calc alkaline character their geochemical characteristics e g enrichments in lil elements such as k rb ba sr depletion in hfse such as ti nb and high ba nb and low nb y ratios are consistent with those of active continental margin regions high 87 sr 86 sr 0 704841 0 707340 and low 143 nd 144 nd 0 512390 0 512618 ratios suggest crustal involvement in their petrogenesis correlations between 86 sr 87 sr isotope with rb rb nb rb ba and rb sr also emphasize the effect of crustal contamination on the andesitic and dacitic magmas as a consequence konya volcanic rocks are products of assimilation and fractional crystallization afc processes of a magma which seems to be linked to the subduction of the african plate underneath the anatolian plate during miocene

geochronology geochemistry and petrogenesis of late triassic dolerites associated with the nibao gold deposit youjiang basin southwestern china implications for post collisional magmatism and its relationships with carlin like gold mineralization abstract the relationship between gold mineralization and magmatism within the youjiang basin of sw china remains poorly constrained as a result of the lack of outcropping igneous units in this region this study presents new geological geochronological and geochemical data for recently discovered dolerite dykes associated with the nibao gold deposit which is located within the nw youjiang basin of sw china zircon laser ablation inductively coupled plasma mass spectrometry la icp ms u pb dating of these dykes yielded an age of 213 6 2 5 ma indicating that these dykes were emplaced during the late triassic and providing new evidence for late triassic magmatism within the study area these dykes also represent an opportunity to investigate the petrogenesis of mafic rocks and the tectonic setting of the late triassic magmatism in the youjiang basin these nibao dolerite dykes contain relatively low concentrations of sio 2 46 03 47 66 wt have moderate total alkali contents k 2 o na 2 o 2 71 3 15 wt and high total fe as fe 2 o 3 tfe 2 o 3 contents 13 49 14 96 wt their trace element compositions are indicative of an oib like geochemical affinity barring depletion in the high field strength elements hfse such as nb ta and th the nibao dolerites have homogeneous lu hf and sr nd isotopic compositions with 87 sr 86 sr i and 143 nd 144 nd i value of 0 70562 0 70677 and 0 51233 0 51239 respectively these dykes have associated e nd t and e hf t values that exhibit a narrow range from 0 70 to 0 51 and from 0 04 to 3 24 respectively these geochemical data indicate that the dolerites associated with the nibao gold deposit were derived from an enriched region of the asthenopheric mantle 90 km depth and the magmas that formed these dykes were generated by 5 10 partial melting of 75 garnet peridotite and 25 spinel peridotite material the ascending magmas underwent both crustal contamination and the fractional crystallization of olivine clinopyroxene and fe oxides e g magnetite or ilmenite prior to emplacement the nibao dolerites are geochemically distinct from mafic rocks related to the emeishan large igneous province suggesting that they are not linked to this mantle plume event instead they formed in an extensional setting generated by the subduction of the song ma oceanic plate beneath south china block and the subsequent collision between the indochina block and south china block the breakoff of the song ma oceanic slab was the process that caused the partial melting of the mantle source region for the nibao dolerite dykes these relationships suggest that the post collision extensional environment associated with this breakoff contributed to the deeply sourced mantle derived late triassic magmatism in this area and perhaps as well the gold mineralization in the youjiang basin most importantly the new geochronological data presented in this study the timing of extension in this region and the results of previous research together suggest that the carlin like gold deposits in the youjiang basin formed at around 213 ma i e the late triassic

the adaptation of pearce element ratio diagrams to complex high silica systems pearce element ratios pers of pearce 1968 express geochemical data in a form where variations in absolute compositions of an igneous suite can be evaluated generally the denominator value in the ratio is taken as a major element abundance but it is argued here that zr provides a more suitable choice zr remains incompatible in magmatic systems up to 68 wt sio2 because zircon fractionation can be suppressed by high melt temperatures and increased volatile contents the use of zr thus permits per modelling to be extended to much higher levels of silica than previously investigated however such systems are more complex than those just involving simple basaltic magmas besides fractionation the processes of magma mixing combined assimilation and fractional crystallization and the initial degree of partial melting in the mantle source must also be considered to distinguish and evaluate these processes a set of example suites are investigated from a complex synextensional calc alkaline province in the western usa samples within most individual suites can be modelled by fractionation however a significant trend orthogonal to the main fractionation vector is also apparent and open system processes are inferred successful modelling is achieved on an inter suite basis using diagrams with axis functions of 4 ca na 0 5 fe mg zr versus si al zr potential open system evolution paths between mafic end members and crustal contaminants are also displayed and evaluated on these same diagrams the encouraging results suggest that such per diagrams may be employed as a versatile tool for investigating the systematics of related igneous suites over a wide area

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

tertiary calc alkaline magmatism associated with lithospheric extension in the pacific northwest the long standing assumption that calc alkaline magmatism requires contemporaneous subduction is critically reviewed two of many widely spread fields of tertiary calc alkaline magmatism in the pacific northwest one eocene and one miocene provide clear examples of the association between calc alkaline magmatism and lithospheric extension we suggest that the calc alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle enriched by previous subduction events than by contemporaneous eocene subduction this model proposes that extension will cause calc alkaline dominated magmatism in areas of continental crust with a history of previous subduction as along the western margin of north america currently popular models for the tectonic magmatic evolution of the basin and range province in which subduction driven calc alkaline magmatism is followed by extension related bimodal magmatism may need modification much more of this magmatism may result from extension in the pacific northwest we suggest that subduction beneath the idaho batholith ceased at the end of the cretaceous blocked by the accreted terranes to be reactivated in its present position beneath the cascades during the eocene the various isolated eocene volcanic fields spread across the pacific northwest were due to lithospheric extension not to the presence of a subducting plate immediately below

silurian extension in the upper connecticut valley united states and the origin of middle paleozoic basins in the quebec embayment pre silurian strata of the bronson hill arch bha in the upper connecticut valley nh vt are host to the latest ludlow comerford intrusive suite consisting east to west of a mafic dike swarm with sheeted dikes and an intrusive complex the rocks are mostly mafic but with compositions ranging from gabbro to leucocratic tonalite the suite is truncated on the west by the monroe fault a late acadian thrust that carries rocks of the bha westward over silurian devonian strata of the connecticut valley gaspetrough cvgt dikes intrude folded strata with a pre intrusion metamorphic fabric taconian but they experienced acadian deforma tion twenty fractions of zircon and baddeleyite from three sample sites of gabbro diorite spanning nearly 40 km yield a weighted 207 pb 206 pb age of 419 1m a greenschist facies dikes sampled over a strike distance of 35 km were tholeiitic basalts formed by partial melting of asthenospheric mantle with little or no influence from mantle or crustal lithosphere the dike chemistry is similar to mid ocean ridge within plate and back arc basin basalts parent magmas originated in the astheno sphere and were erupted through severely thinned lithosphere adjacent to the cvgt extensive middle paleozoic basins in the internides of the appalachian orogen are restricted to the quebec embayment of the laurentian rifted margin and include the cvgt and the central maine trough cmt separated from the bha by a silurian tectonic hinge the ne trending comerford intrusions parallel the cvgt cmt and the tectonic hinge and indicate nw se extension during post taconian convergence the irregular margins of composite laurentia and avalon permitted continued colli sion in newfoundland st lawrence promontory and coeval extension in the quebec embayment extension may be related to hinge retreat of the northwest directed brunswick subduction complex and rise of the asthenosphere following slab break off an alternative hypothesis is that the basins originated as pull apart basins between northwest trending left stepping sinistral strike slip faults along the southern flanks of the new york and st lawrence promontories

double branching model to forecast the next m 5 5 earthquakes in italy abstract the purpose of this work is to set up a new forecasting model named double branching for large earthquakes in italy the model is time dependent since it assumes that each earthquake can generate or is correlated to other earthquakes through different physical mechanisms in a recent paper marzocchi w lombardi a m 2008 a double branching model for earthquake occurrence j geophys res 113 b08317 doi 10 1029 2007jb005472 we have shown that the model applied to two worldwide catalogs in different time magnitude windows shows a good fit to the data and its earthquake forecasting performances are superior to what is obtained by the etas single branching model and by the poisson models remarkably the model can be tested in a forward perspective which is the most straightforward way to evaluate the reliability of any forecasting model here we apply this model to the italian historical seismicity of the last four centuries with magnitude mw 5 5 this application shows that the time dependent features are comparable to what observed in other applications for different time space magnitude window moreover the comparison of the forecasting capability of the double branching model and of a spatially variable stationary poisson process used in italy for seismic hazard assessment shows that the former has significantly better performances finally we provide some probability maps for different temporal windows

geochemistry and sr nd pb hf isotopes of early cretaceous basalts from the great xinggan range ne china implications for their origin and mantle source characteristics abstract late mesozoic volcanism is widespread throughout northeastern china mainly clustering in the great xinggan range where the major period of volcanic eruption was during the early cretaceous the spatial distribution of volcanic rocks shows a nne extending pattern the lithology of mesozoic basaltic rocks in the great xinggan range ranges from trachytic basalts basalts to basaltic andesites on the basis of lithological associations and spatial relationships we divide the basalts into four groups i e tahe genhe zalute and xiwu the early cretaceous volcanism in the great xinggan range is characterized by significant large ion lithophile elements lile and light rare earth elements lree enrichment and high field strength elements hfse depletion with a slightly enriched to depleted nd hf and weakly enriched sr isotopic compositions initial 87sr 86sr 0 7046 0 7079 end t 3 6 to 6 9 and ehf t 8 9 to 4 6 the pb isotopic compositions are also variable with 206pb 204pb 207pb 204pb and 208pb 204pb values of 17 791 18 530 15 477 15 597 and 37 761 38 497 respectively the sr nd pb hf isotopic data indicate that the magmatic sources are heterogeneous showing an enriched to depleted signature of continental lithospheric mantle the magmatic sources of the genhe and zalute groups are characterized by mixing among pm em ii and dm whereas the tahe and xiwu groups derive from enriched lithospheric mantle that had experienced metasomatism by fluids or melts from subducted paleo asian and mongol okhotsk ocean slabs and possible influence of precambrian blocks the late mesozoic volcanic rocks in the great xinggan range resemble those of the cenozoic calc alkaline magmatism in the basin and range province of the united states we therefore suggest that the widespread mesozoic volcanic rocks in the great xinggan range were generated in an extensional tectonic setting

geochemical character of silurian volcanism in sw ireland the dunquin group mid silurian of dingle southwest ireland is a sequence of basaltic to rhyolitic lavas and pyroclastic rocks interbedded with shallow to non marine sediments that records one of only four known outbreaks of silurian volcanicity south of the iapetus suture in the british isles new geochemical petrographic and field evidence indicate that the volcanic sequences exposed in the dunquin inlier and the islands of beginish and inishvickillane could have formed by fractionational crystallization in a layered repeatedly replenished basaltic magma chamber the site of which is probably coincident with an intense aeromagnetic anomaly to the southwest of inishvickillane the volcanic suite is subalkaline transitional between calc alkaline and tholeiitic and is non bimodal high lile hfse ratios and high lree concentrations suggest that the parent basaltic magmas were generated at a destructive continental margin although subduction may have ceased before eruption

petrology and geochemistry of tertiary volcanic rocks from the ikizce ordu area ne turkey implications for the evolution of the eastern pontide paleo magmatic arc abstract tertiary volcanism in the ikizce region at the western edge of the eastern pontides paleo magmatic arc is represented by basaltic and andesitic rocks associated with sediments deposited in a shallow basin environment the basaltic rocks contain plagioclase an58 80 olivine fo82 84 clinopyroxene wo44 48en35 42fs7 17 hornblende mg 0 68 0 76 phenocrysts and magnetite microcrysts whereas the andesitic rocks include plagioclase an25 61 clinopyroxene wo46 49en38 43fs11 13 hornblende mg 0 48 0 81 biotite mg 0 48 0 60 phenocrysts titanomagnetite apatite and zircon microcrysts geochemical data indicate magmatic evolution from tholeiitic alkaline transitional to calc alkaline characteristics with medium k contents the geochemical variation in the rocks can be explained by fractionation of common mineral phases such as clinopyroxene olivine hornblende plagioclase magnetite and apatite the trace elements distributions of the volcanic rocks show similarities to those of e type morb have a shape that is typical of rocks from subduction related tectonic setting with enrichment in lile and to a lesser extent in lree but depletion in hfse the rocks evolved from a parental magma derived from an enriched source formed by subduction induced metasomatism of basaltic rocks the latter formed through clinopyroxene olivine controlled fractionation in a high level magma chamber the andesitic rocks developed through hornblende plagioclase controlled fractionation in shallow level magma chamber s

paleogene volcanism in central afghanistan possible far field effect of the india eurasia collision abstract a volcanic sedimentary succession of paleogene age is exposed in isolated patches at the southern margin of the tajik block in the ghor province of central afghanistan the volcanic rocks range from basalts and andesites to dacites including adakites they are intercalated with sedimentary rocks deposited in shallow marine environments dated biostratigraphically as paleocene eocene this age corresponds to the age of the asy b d andesites located in the western ghor province estimated by the 40ar 39ar method as 54 ma the magmatism post dates the cimmerian collision between the tajik block including the band e bayan block and the farah rod block located to the south while the investigated volcanic rocks apparently bear geochemical signatures typical to an active continental margin environment it is presumed that the magmatism was related to rifting processes most likely initiated by far field tectonics caused by the terminal collision of the indian plate with eurasia najman et al 2017 this event led to the dextral movement of the farah rod block particularly along hari rod herat fault system resulting in the development of a transtensional regime in the proximal southern margin of the tajik block and giving rise to a rift basin where marine sediments were interbedded with pillow lavas intruded by sheeted dyke series

a pan african alkaline pluton intruding the saramuj conglomerate south west jordan the geological setting petrography and bulk mineral chemistry of a monzodiorite and a presumably consanguineous megaporphyry with large up to 25 cm labradorite megacrysts both intruding the upper proterozoic saramuj conglomerate in south west jordan south eastern shore of the dead sea were examined the crystallization temperatures of the monzodiorite and the megaporphyry as determined from pyroxene thermometry and supported by contact metamorphic mineralogy are about 700 and 900 c respectively the intrusion depth of the monzodiorite is about 3 4 km the monzodiorite was emplaced in the saramuj conglomerate at about 595 2 ma ago according to rb sr and u pb age determinations

forecasts of the 1989 loma prieta california earthquake abstract the magnitude m w 6 9 loma prieta earthquake struck the san francisco bay area of central california at 5 04 p m local time on 17 october 1989 killing 62 people and generating billions of dollars in property damage scientists were not surprised by the occurrence of a destructive earthquake in this region and had in fact been attempting to forecast the location of the next large earthquake in the san francisco bay area for decades this article summarizes more than 20 scientifically based predictions made before the 1989 loma prieta earthquake for a large earthquake that might occur in the loma prieta region the predictions geographically closest to the actual earthquake primarily specified slip on the san andreas fault northwest of san juan bautista a number of the predictions did encompass the magnitude of the actual earthquake and at least one approximately encompassed the along strike rupture length post loma prieta studies of the 1906 san francisco california earthquake in the loma prieta region of the san andreas fault zone show the loma prieta and 1906 events with different senses of slip and fault plane dip therefore some have argued that the 1989 earthquake was not foreseen even though 1 this earthquake appears to have released much of the horizontal strain accumulated since 1906 and 2 not all of the forecasts were based on 1906 behavior

crustal contamination of late neogene basalts in the dien bien phu basin nw vietnam some insights from petrological and geochronological studies early pliocene zanclean basalts in the dien bien phu pull apart basin in nw vietnam associated with the presently sinistral dien bien phu fault zone have been dated by the k ar method at 4 4 4 9 and 5 4 5 2 ma rapid migration of basaltic magma to the surface in the dien bien phu fault zone may be due to pliocene transtension of the crust in this region resulting from asthenospheric upwelling induced by lateral displacement of the mantle the basalts are moderately phyric 10 and consist of olivine hyalosiderite plagioclase bytownite labradorite and orthopyroxene bytownite labradorite phenocrysts and a fine grained crystalline matrix olivine hortonolite plagioclase labradorite clinopyroxene pigeonite and augite k feldspar the presence of fe rich olivine and orthopyroxene phenocrysts indicates that the basalts are sio2 saturated oversaturated olivine tholeiites which formed under water undersaturated conditions the dien bien phu basalts contain both mantle derived pyroxenites dunites gabbros and crustal sillimanite mullite mg fe spinel wallrock xenoliths indicative of crustal contamination during the ascent of the basaltic magma the basalts show selective enrichment in some mobile elements k rb sr and th a feature considered to be a result of metasomatism these rocks classified on the basis of their normative composition as quartz tholeiites could represent primary olivine tholeiites basalts in which the geochemical signatures were modified by the processes of contamination 2006 elsevier ltd all rights reserved

resolution of fault slip along the 470 km long rupture of the great 1906 san francisco earthquake and its implications data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture because of varying network aperture and station density slip resolution is very uneven although slip is determined within uncertainties of 1 0 m along 60 of the fault constraints are poor on the remaining mostly offshore portions of the rupture slip decreases from maxima of 8 6 and 7 5 m at shelter cove and tomales bay to 4 5 m near mount tamalpais and 2 7 m at loma prieta the geodetically derived slip distribution is in poor agreement with estimates based on analysis of s wave seismograms probably because these waves register only 20 30 of the total seismic moment obtained from longer period surface waves consideration of a range of fault geometries for 1906 slip near loma prieta indicates right lateral motions lie between 2 3 and 3 1 m these values are considerably greater than the 1 5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based this factor along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts casts doubt on some claims of success in predicting the 1989 m 6 9 loma prieta earthquake

eocene k rich adakitic rocks in the central iran implications for evaluating its cu au mo metallogenic potential abstract the kal e kafi intrusive complex is located in the central iran magmatic zone and includes a series of granitoid rocks ranging in composition from gabbro to granite these granitoids show some affinities with adakites e g high sr y and la yb ratios coupled with low yb and y enrichment in lree and lile with no eu anomalies similar to products from all of which are compatible with derivation from slab melting however relatively their low end t values 0 21 to 2 35 and higher 87 sr 86 sr i 0 7041 0 7044 are inconsistent with an origin by slab melting with these rocks also showing shoshonitic affinities with high k 2 o na 2 o ratios geochemical characteristics indicate that the kal e kafi intrusive complex was probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thicknesses of 40 km generated near the boundary between garnet bearing amphibolite or amphibole bearing eclogite melting moreover the kal e kafi adakitic magmas must have interacted with the surrounding metasomatized mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity o 2 of the magma the high o 2 could have induced oxidation of metallic sulfides in the mantle resulting in the mobilization of chalcophile elements which are required to produce porphyry cu au and mo mineralization

geochemical approach to magmatic evolution of mt erciyes stratovolcano central anatolia turkey erciyes stratovolcano culminating at 3917 m is located in the cappadocian region of central anatolia during its evolution this quaternary volcano produced pyroclastic deposits and lava flows the great majority of these products are calc alkaline in character and they constitute kocdag and erciyes sequences by repeated activities alkaline activity is mainly observed in the first stages of kocdag and approximately first middle stages of erciyes sequences generally kocdag and erciyes stages terminate by pyroclastic activities the composition of lavas ranges from basalt to rhyolite 48 4 70 5 wt sio calc alkaline rocks are represented mostly by andesites and dacites some compositional 2 differences between alkaline basaltic basaltic and andesitic rocks were found while the composition of dacites remain unchanged all these volcanics are generally enriched in lil and hfs elements relative to the orogenic values except rb ba nb depleted alkaline basalt 87 srr 86 sr and 143 ndr 144 nd isotopic composition of the volcanics range between 0 703344 0 703964 0 512920 0 512780 for alkaline basalts and change between 0 704322 0 705088 0 512731 0 512630 for alkaline basaltic rocks whereas calc alkaline rocks have relatively high sr and nd isotopic ratios 0 703434 0 705468 0 512942 0 512600 low rb ba nb content with high zrrnb low barnb laryb ratio and low sr isotopic composition suggest an depleted source component while high ba rb nb content with high laryb barnb low zrrnb and low 87 srr 86 sr ratios indicate an oib like mantle source for the generation of erciyes alkaline magma these elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes the depletion in ba rb nb content may be explained by the removal of these elements from the source by slab derived fluids which were released from pre collisional subduction modified the asthenospheric mantle the chemically different mantle sources interacted with crustal materials to produce calc alkaline magma the barnb increase of calc alkaline samples indicates the increasing input of crustal components to erciyes volcanics sr and nd isotopic compositions and elevated lil and hfs element content imply that calc alkaline magma may be derived from mixing of an oib like mantle melts with a subduction modified asthenospheric mantle and involvement of crustal materials in intraplate environments q 1998 elsevier science b v all rights reserved

mesoproterozoic lamprophyres in the labrieville massif quebec clues to the origin of alkalic anorthosites mesoproterozoic lamprophyre dikes occur at four localities within the 1010 ma labrieville anorthosite in the grenville province of southern quebec all dikes are thin 20 cm wide oriented n57 w and steeply dipping three are biotite lamprophyres but the fourth is dominated by amphibole laser total fusion 40ar 39ar analyses on amphibole or biotite yield minimum ages for the dikes of 945 5 8 ma 907 0 5 9 ma 892 0 6 2 ma and 889 2 9 8 ma a crystallization age of 1 ga for all dikes is consistent with these data and rb sr isotopic systematics all lamprophyres contain high levels of cr 284 393 ppm ni 101 180 ppm ba 1380 5580 ppm sr 2359 3109 ppm and rare earth elements ree lan 191 288 with average lan lun 26 and no eu anomalies at 1 ga all lamprophyres have a similar initial sr isotopic ratio of 0 7034 similar to bulk earth in contrast e nd values vary considerably from 4 to 10 2 indicating sources with variable long term light ree enrichment negative correlations between 143nd 144nd and la nb and k suggest involvement of at least two source components probably asthenosphere and metasomatized subcontinental lithosphere the lamprophyres are only slightly younger than 1010 ma labrieville massif which is also alkalic and enriched in sr and ba relative to most other anorthosites the lamprophyres provide unequivocal evidence for a sr and ba rich component in the lithospheric mantle that may have contributed to the parental magmas of labrieville and other andesine anorthosites in this part of quebec

petrogenesis of extension related alkaline volcanism in karaburhan sivrihisar eskisehir nw anatolia turkey abstract alkaline lavas were erupted as phonolites and trachytes around karaburhan sivrihisar eskisehir nw anatolia within the izmir ankara erzincan suture zone these volcanic rocks were emplaced as domes close and parallel to the ophiolite thrust line according to 40ar 39ar geochronological analyses of sanidine crystals from the phonolites the age of the alkaline volcanics is 25 ma late oligocene early miocene the flow textured phonolites are porphyritic and consist mainly of sanidine clinopyroxene and feldspathoid crystals the clinopyroxenes show compositional zoning with aegirine na0 82 0 96fe 30 68 0 83 rims and aegirine augite cores containing calcium magnesium and fe 2 some aegirine augites are replaced with sodium calcium and magnesium rich amphibole hastingsite feldspathoid hauyne crystals enriched with elemental na and ca have been almost completely altered to zeolite and carbonate minerals the fine grained trachytes with a trachytic texture consist of feldspar oligoclase and sanidine phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites although there are some differences in their element patterns the phonolites and trachytes exhibit enrichment in liles sr k rb ba th and lrees la ce pr nd and negative anomalies in nb and ta these geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component in addition the high 87sr 86sr 0 706358 0 708052 and low 143nd 144nd 0 512546 0 512646 isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction derived fluids petrogenetic modeling indicates that the alkaline lavas generated from the subduction modified lithospheric mantle have undergone assimilation fractional crystallization and crustal contamination acquiring high pb ba rb and sr contents and pb isotopic compositions during their ascent through the thickened crust in an extensional setting

snowdon basalts and the cessation of caledonian subduction by the longvillian basalts of the bedded pyroclastic formation snowdon volcanic group north wales of longvillian caradoc age are heterogeneous in trace element composition forming a mixing trend between ocean island basalt oib type and magmatic arc basalt the two end member compositions were partial melts of two distinct upper mantle sources the oib end member is similar to icelandic oib and is the only known example of basalt of this type in the ordovician silurian welsh basin since ocean island basalt magma type is thought never to erupt above well established subduction zones the eruption of the longvillian oib provides a minimum age for the end of subduction of iapetus oceanic lithosphere below the welsh basin a model for the evolution of the mantle below the welsh basin is proposed which accounts for the geochemistry of magmas erupted from the tremadoc to the early silurian

tertiary minette and melanephelinite dikes wasatch plateau utah records of mantle heterogeneities and changing tectonics a swarm of minette and melanephelinite dikes is exposed over 2500 km2 in and near the wasatch plateau central utah along the western margin of the colorado plateaus in the transition zone with the basin and range province to date 110 vertical dikes in 25 dike sets have been recognized strikes shift from about n80 w for 24 ma dikes to about n60 w for 18 ma to due north for 8 7 m y these orientations are consistent with a shift from east west oligocene compression associated with subduction to east west late miocene crustal extension minettes are the most common rock type mica rich minette and mica bearing melanephelinite occurs in 24 ma dikes whereas more ordinary minette is found in 8 7 ma dikes one melanephelinite dike is 18 ma these mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions they appear to have crystallized from primitive magmas major trace element and nd sr isotopic data indicate that melanephelinite which has similarities to ocean island basalt was derived from small degree melts of mantle with a chondritic sm nd ratio probably located in the asthenosphere but it is difficult to rule out a lithospheric source in contrast mica bearing rocks mica melanephelinite and both types of minette are more potassic and have trace element patterns with strong nb ta depletions and sr nd isotopic compositions caused by involvement with a component from heterogeneously enriched lithospheric mantle with long term enrichment of rb or light rare earth elements ree epsilon nd as low as 15 in minette light ree enrichment must have occurred anciently in the mid proterozoic when the lithosphere was formed and is not a result of cenozoic subduction processes after about 25 ma foundering of the subducting farallon plate may have triggered upwelling of warm asthenospheric mantle to the base of the lithosphere melanephelinite magma may have separated from the asthenosphere and while rising through the lithosphere provided heat for lithospheric magma generation varying degrees of interaction between melanephelinite and small potassic melt fractions derived from the lithospheric mantle can explain the gradational character of the melanephelinite to minette suite

rethinking earthquake prediction we re examine and summarize what is now possible in predicting earthquakes what might be accomplished and hence might be possible in the next few decades and what types of predictions appear to be inherently impossible based on our understanding of earthquakes as complex phenomena we take predictions to involve a variety of time scales from seconds to a few decades earthquake warnings and their possible societal uses differ for those time scales earthquake prediction should not be equated solely with short term prediction those with time scales of hours to weeks nor should it be assumed that only short term warnings either are or might be useful to society a variety of consumers or stakeholders are likely to take different mitigation measures in response to each type of prediction a series of recent articles in scientific literature and the media claim that earthquakes cannot be predicted and that exceedingly high accuracy is needed for predictions to be of societal value we dispute a number of their key assumptions and conclusions including their claim that earthquakes represent a self organized critical soc phenomenon implying a system maintained on the edge of chaotic behavior at all times we think this is correct but only in an uninteresting way that is on global or continental scales the stresses in the regions surrounding the rupture zones of individual large earthquakes are reduced below a soc state at the times of those events and remain so for long periods as stresses are slowly re established by tectonic loading a region approaches a soc state during the last part of the cycle of large earthquakes the presence of that state can be regarded as a long term precursor rather than as an impediment to prediction we examine other natural processes such as volcanic eruptions severe storms and climate change that like earthquakes are also examples of complex processes each with its own predictable possibly predictable and inherently unpredictable elements that a natural system is complex does not mean that predictions are not possible for some spatial temporal and size regimes long term and perhaps intermediate term predictions for large earthquakes appear to be possible for very active fault segments predicting large events more than one cycle into the future appears to be inherently difficult if not impossible since much of the nonlinearity in the earthquake process occurs at or near the time of large events progress in earthquake science and prediction over the next few decades will require increased monitoring in several active areas

palaeozoic within plate volcanic rocks in nova scotia canada reinterpreted isotopic constraints on magmatic source and palaeocontinental reconstructions palaeozoic volcanism in the avalon terrane of northern nova scotia occurred during three time intervals cambrian early ordovician late ordovician early silurian and middle late devonian in the meguma terrane of southern nova scotia palaeozoic volcanism is limited to the middle ordovician geochemical data show that most of these volcanic rocks are bimodal within plate suites initial e nd signa tures range from 5 4 to 1 9 in the rhyolites and 6 8 to 2 7 in the basalts a difference attributable to the absence or presence respectively of a significant crustal component the data and regional tectonic settings of the avalon and meguma terranes suggest that the volcanism was generated in three different within plate settings 1 cambrian early ordovician volcanism related to thermal decay of late proterozoic arc magma tism during transtensional deformation 2 middle ordovician early silurian volcanism during sinistral tele scoping between laurentia and gondwana where extensional bends in the appalachians produced rifting and 3 devonian volcanism resulting from lithospheric delamination during dextral transpression and tele scoping in each setting active faults served as conduits for the magmas nd isotopic data indicate that the source of the palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern nova scotia and did not substantially change with time this crustal source appears to have separated from the mantle during the proterozoic a conclusion consistent with the hypothesis that the palaeozoic rocks in nova scotia were deposited upon a late proterozoic oceanic cratonic volcanic arc terrane the nd data when combined with published faunal palaeomagnetic and u pb isotopic data suggest that the avalon terrane was peripheral to gondwana off northwestern south america during neoproterozoic and early palaeozoic times

geochemical characteristics of igneous host rocks of lubin zardeh au cu deposit nw iran abstract the lubin zardeh au cu pb zn deposit is located in the azerbaijan western alborz zone 45 km ne of zanjan iran it is situated within the eocene oligocene volcanic plutonic belt of the tarom subzone of the alborz mountains range quartz monzonite and tuffs are the main host rock of the lubin zardeh au cu pb zn deposit granitoid rocks are predominantly high k calc alkaline with shoshonitic affinities they are i type metaluminous magnesian and are characterized by lile lree and th enrichment relative to hfse depletion in nb ta ti and weak depletion in hree and y these granitoids show some affinities with adakites e g high sr y and la yb ratios coupled with low yb and y and enrichment in lree and lile with weak eu anomalies their relatively low to moderate initial 87sr 86sr values 0 7047 0 7051 positive nd t 36 ma values 0 39 2 1 and tdm ages of 0 69 to 1 06 ga with pb isotopic ratios of initial 206pb 204pb 18 49 18 68 initial 207pb 204pb 15 58 15 61 and initial 208pb 204pb 38 33 38 77 are consistent with a phlogopite bearing amphibole rich and enriched lithospheric mantle source metasomatized by earlier subducted sediments the geochemical and isotopic compositions of the granitoids are similar to the host rocks of porphyry and epithermal deposits in the urumieh dokhtar zone

late paleoproterozoic charnockite suite within post collisional setting from the north china craton petrology geochemistry zircon u pb geochronology and lu hf isotopes abstract charnockites pyroxene bearing granitoids of magmatic origin in diverse tectonic settings and ranging in age from mesoarchean to cretaceous constitute important components of the continental crust here we report charnockites displaying both magnesian and ferroan compositions associated with gabbros from an amcg anorthosite mangerite charnockite granite suite in the north china craton the orthopyroxene in the magnesian charnockite is characterized by moderate x mg of 0 63 0 65 wo 1 2 en 62 63 fs 35 36 and low al 2 o 3 content of 0 59 0 71 wt the magnesian charnockites show medium to high k contents and high mg 47 69 similar to that of gabbros whereas the mg of the ferroan charnockites is low 6 28 the ferroan charnockites are alkali calcic to alkalic and weakly peralkaline to metaluminous whereas the magnesian charnockites are calcic to calc alkalic and metaluminous although magnesian charnockites are in general considered to have formed in subduction setting the medium to high k contents high mg values with a wide range and the highly negative ehf values of the zircons in these rocks 8 4 to 13 6 suggest inheritance of the arc signature from the melting of ancient arc related crustal material the ferroan charnockites show tholeiitic affinity and define a common differentiation trend with the gabbroic anorthosites and likely represent fractionated end members with or without crustal interaction in a post collisional rift setting we present u pb age data from zircon grains on seven samples including two ferroan charnockites three magnesian charnockites one gabbroic enclave in magnesian charnockite and one gabbroic anorthosite which show emplacement ages of 1748 8 6 4 ma 1747 1 9 5 ma 1756 4 7 3 ma 1756 7 9 2 ma 1731 17 ma 1731 6 8 2 ma and 1746 5 7 3 ma respectively the negative ehf values 1 2 to 13 6 of zircon grains from these rocks and the older crustal model ages ranging from mesoarchean to paleoproterozoic suggest that the magma sources of these rocks involved the melting of ancient crustal components the age data suggest that the magmatic suite was emplaced within a relatively short time interval between1 73 and 1 76 ga during late paleoproterozoic placing the rocks suite in a post collisional scenario following the amalgamation between the eastern and western blocks of the north china craton along the trans north china orogen at ca 1 85 1 80 ga

late paleozoic post collisional magmatism in the eastern tianshan belt northwest china new insights from geochemistry geochronology and petrology of bimodal volcanic rocks abstract the late carboniferous to permian marks an important phase of terrane amalgamation through accretion collision in the central asian orogenic belt caob the tianshan belt forms part of this sequence where the post collisional event generated an e w trending bimodal volcanic zone that extends for ca 500 km from the southern bogda mt to the easternmost tianshan belt here we report the petrology geochemistry and geochronology of the bimodal volcanic rocks to characterize the post collisional tectonic evolution in late paleozoic the major element chemistry of the basalt and rhyolite samples indicates a bimodal high k calc alkaline affinity the basalts are characterized by high abundance in tio2 th u and pb and depletion in nb ta and p together with slight enrichment in lree contents and low hfse lree ratios the rhyolites show a close affinity to aluminous a type granites with enrichment in k na zr ce y and depletion in p nb ta and ti they also exhibit fractionated ree patterns with prominent negative eu anomalies the petrographic and geochemical data suggest that this bimodal rock series was generated in a post collisional setting the mafic rocks are likely derived from a metasomatized lithospheric mantle assimilation of different crustal components played a minor role in the genesis of these basaltic rocks the la icp ms analysis on zircons from four rhyolites and two basalts yielded similar ages ranging from 295 8 2 8 to 293 3 1 7 ma suggesting an early permian event zircons from the basalts show a wide ehf t range from 0 49 to 13 whereas those from the rhyolites show two groups of ehf t values from 5 to 3 and from 6 to 12 suggesting a mixing of crust mantle material the magmatic zircons from the bimodal volcanic rocks yielded two peaks of tdm2 between 600 820 ma and 1160 1360 ma probably representing two significant episodes of continental crust growth the bimodal volcanic rocks are correlated with the early stage of post collision our results suggest that vertical crustal growth through mantle derived magmas in post collisional setting also played a major role in the construction of the caob

geochemistry of mafic lavas in the early rio grande rift yarmony mountain colorado u s a abstract mafic lavas high k basalts and shoshonites erupted 21 24 ma ago early miocene near state bridge nw colorado and now exposed on yarmony mountain were related to the initial phase of extension in the northernmost section of the rio grande rift the volcanism was of small volume consisting of eleven flows on yarmony mountain although much more voluminous contemporaneous lavas occur 50 km to the west all eleven flows have been analysed comprehensively for major and trace elements and five of the flows for nd and sr isotopes some samples experienced post eruptive carbonation and leaching of alkali elements the effects of these processes on normative compositions are examined and it is suggested that major element abundances are sufficiently well preserved to be fairly sure that the magmas experienced fractional crystallization at pressures appropriate for the mid lower part of the crust most of the basalts are chemically closely related and have the following characteristics 1 they are depleted in nb and ta relative to lree a feature of volcanic arc magmas 2 they have lower abundances of rb and k and to a lesser extent ba and th for a given lree content than typical magmatic arc magmas and 3 they have nd and sr isotopic ratios similar to both certain low 143 nd 144 nd oceanic basalts notably from the kerguelen area indian ocean and subduction related magmas from mature volcanic arcs notably from the andes south america we argue that the magmas erupted at yarmony mountain were generated by partial melting of asthenospheric mantle which had been modified by subduction of oceanic lithosphere during the cenozoic the relatively low abundances of the alkali elements appears to be a consequence of depletion of these elements in the mantle source by a previous episode of melt extraction before that which generated the yarmony magmas an alternative is that the mantle source was depleted in alkali elements by a dehydration event possibly involving a flow of co2 one of the lavas in the yarmony sequence is elementally and isotopically distinctive having higher k zr ba k 2 o na 2 o la ta and k la and lower 87 sr 86 sr than the rest of the samples it is argued in detail that this is not the result of contamination by any reasonable crustal composition and that this magma contained a component 10 20 of ultrapotassic mafic magma derived by partial melting of subcontinental lithospheric mantle there is no petrographic evidence for mixing and the hybridization probably took place in magma chambers situated in the mid lower crust

intraplate mafic magmatism degasification and deposition of mercury the giant almaden mercury deposit spain revisited abstract the giant almaden mercury deposit spain is hosted by the lower silurian criadero quartzite in turn this ore bearing rock unit is cross cut by the so called frailesca unit a diatreme body of basaltic composition the geochemical characteristics of the silurian to devonian almaden district volcanic units indicate that these rocks originated from an enriched evolving mantle source that ultimately yielded basanites nephelinites to rhyolites through olivine basalts pyroxene basalts trachybasalts trachytes very scarce rhyolites and quartz diabases the silurian intraplate alkaline volcanism developed in submarine conditions which triggered widespread hydrothermal activity resulting in hg ore formation and pervasive alteration to carbonates the 18 o 13 c and 34 s isotopic signatures for carbonates and pyrite suggest different sources for carbon and sulfur including magmatic and organic for the former and magmatic and sea water for the latter the most important and efficient natural source of mercury on earth is by far the volcanic activity which liberates mercury via quiescent degassing and catastrophic plinian events when eruptions can overwhelm the atmospheric budget of hg thus we suggest that co 2 degasification and coeval distillation of mercury from the volcanic rocks fed the huge hydrothermal system that led to massive deposition of mercury at almaden build up of hg 0 gas in magmatic chambers during waning rifting in the late ordovician followed by renewed volcanism in the early silurian would have resulted in massive degasification of the accumulated mercury part of this mercury went into the criadero quartzite leading to formation of the huge almaden deposit and others e g el entredicho along the same stratigraphic level progressive depletion of the deep seated magmatic hg stock would have resulted in a drastic reduction in ore deposit size after the lower silurian when smaller deposits formed e g las cuevas

acoustic emission microstructure and damage model of dry and wet sandstone stressed to failure twenty three uniaxial compression tests were performed on dry and wet flechtingen sandstone from germany compressive strength of wet core is 60 of the strength of dry core before fracture the transverse p wave speed drops by 13 and the pulse amplitude by 22 for wet and 37 for dry cores accumulated strain energy doubles for dry core acoustic emissions ae are detected with 10 sensors for 19 cores ae activity starts at 84 of the fracture strength of wet cores 55 mpa and at 91 of the strength of dry cores 87 mpa the ratio of located to recorded ae is 0 37 for dry and 0 13 for fully wet cores ae hypocenter patterns document the development of two opposite fracture cones the negative slope of cumulative ae amplitude frequency distribution drops by 50 before failure in dry cores the slope of the wet core drops and recovers energy discrimination of ae detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture using the analogy to visible light microfracturing events are separated into high energy short pulses blue ae and low energy pulses with long duration times red ae blue ae are explained by intragranular grain breakage red ae by multiple stick slip on crack planes or grain boundaries deformed cores show highly fractured calcite cement and mostly intact quartz grains the stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral calcite

frequency of occurrence of moderate to great earthquakes in intracontinental regions implications for changes in stress earthquake prediction and hazards assessments we investigate the departure of shallow intracontinental earthquakes from the frequency magnitude relationship log n m a bm we consider seven large continental regions but exclude subduction zones in each active and stable areas are differentiated within active areas we separately categorize earthquakes on faults with high slip rates we use data sets from 1978 1994 and 1900 1994 which are complete for moment magnitudes mw 5 3 and 7 0 to calculate changes in b value with mw for all active intracontinental regions combined except asia b changes at 98 confidence from 0 90 to 2 1 at a corner magnitude mc of 6 9 to 7 0 the distribution for stable regions indicates a similar change values of mc are associated with downdip widths of rupture of about 23 to 27 km which are smaller than those for subduction zones in each case mc marks a transition from unbounded to bounded earthquakes in asia events from 1958 to 1994 also delineate a similar change near mc 7 0 earthquakes in asia of 7 0 mw 8 1 from 1900 to 1957 however define a b value of 1 0 and a moment release rate 5 6 times higher than that from 1958 to 1994 those anomalous values indicate that a large region of asia was close to failure i e near a self organized critical state during the earlier period we associate those changes with the giant himalayan earthquake in 1950 of mw 8 6 regional variations in seismic strain release rates correlate with the types of processes at adjacent plate boundaries extrapolating rates of activity of the past 95 years to mw 8 0 indicates that such events should be very rare in stable areas

age petrogenesis and tectonic implications of early devonian bimodal volcanic rocks in the south altyn nw china abstract in this paper we report zircon u pb dating hf isotopes geochemical and sr nd isotopic data with the aim the petrogenesis and regional tectonic evolution of early devonian bimodal volcanic rock in the south altyn nw china new la icpms zircon u pb isotopic data constrained them at ca 406 ma the mafic samples are characterized by high fe cr and ni contents low ti and mg contents slightly enriched lree patterns and low la yb n la nb and la ta ratios and positive e nd t values 3 3 to 3 4 indicating that they were likely derived from strong batch melting of the asthenosphere in the spinel facies field the felsic rocks show an a type affinity with high alkalis fe ga zr nb ce and y contents low mg sr content high rb sr and ga al ratios enrichment in lile e g rb k th u and lree and depletion in ba sr nb ta p and ti and fractionated ree patterns with very strong negative eu anomalies these features along with distinct e nd t values 0 5 to 2 3 and mostly positive e hf t 0 29 to 5 18 indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions and simultaneously underwent slight magma mixing of such melts with mantle magma according to the petrogenetic schemes and geological background of the early devonian bimodal volcanic rocks tholeiite and a type dacite rhyolite they should have formed in a post collisional extensional setting moreover on the basis of spatial and temporal distribution and formation mechanism the tectonic magmatic evolution of the early paleozoic south altyn tagh could be divided into three stages 505 472 ma continental collision the magmatite formed under high pressure conditions due to the deep subduction and initial tearing of continental slab 467 450 ma continental slab break off the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating and 432 385 ma post collisional extension the magmatite consists of a type granites and bimodal volcanic rocks which are the products of the interaction between mantle and crust and asthenosphere upwelling

thermal structure beneath the snake river plain implications for the yellowstone hotspot basaltic magmatism associated with the yellowstone hotspot has been widely attributed to upwelling of a mantle plume yet the temporal and spatial distribution of these magmas and their compositional characteristics are distinctive from oceanic hotspot magmatism fundamental questions concern the influence of continental cratonic lithosphere in producing the differences and the extent to which upper plate processes contribute to magma production to better understand scenarios of melt generation p t conditions are estimated for segregation of primitive snake river plain srp basalts from the mantle combined with analysis of trace element and seismic constraints we conclude from this that 1 melt production was concentrated at depths between roughly 70 100 km 2 mantle temperature was only slightly higher than ambient conditions with a maximum potential temperature of 1450 c and 3 the mantle source was relatively fertile mg 90 these results suggest that the seismically imaged plume below yellowstone is significantly cooler than upwellings beneath hawaii iceland and many other oceanic hotspots our findings in combination with other geochemical and geodynamic considerations are permissive of magma generation within the ancient lithospheric mantle keel associated with the wyoming craton plume contributions while not excluded involve physical and geochemical implications that suggest they are subordinate

m 6 earthquakes in southern california during the twentieth century no evidence for a seismicity or moment deficit abstract a broadly based report on seismic hazards in southern california wgcep 1995 concluded that the predicted seismicity exceeds that observed since 1850 a subsequent independent analysis argued that infrequent huge m 8 earthquakes are needed to explain the low rate of large earthquakes jackson 1996 frequency magnitude relationships and earthquake reporting suggest that the 1903 to 1997 catalog we present here with a b value of 1 0 and a rate of m 6 shocks of 0 42 to 0 49 yr 1 is nearly complete in contrast the 1850 to 1994 catalog used by wgcep is incomplete before the turn of the century and thus its reported seismicity rate of 0 32 m 6 shocks yr 1 is too low principally because the wgcep 1995 model results in b values of up to 4 0 for regions of lesser and blind faults the rate of m 6 shocks off the san andreas system predicted by the wgcep 1995 model is three times greater than that observed in this century because they obtained b 0 4 for m b 2 2 for m 7 3 on major faults their expected rate of m 7 san andreas shocks is twice as high as observed thus part of the seismicity and moment discrepancy identified by wgcep was caused by use of an incomplete catalog and part was caused by inappropriate b values we obtain a southern california moment release rate of 8 to 12 10 18 n m yr 1 which cannot be distinguished from the moment release estimated by fault slip or the moment accumulation inferred from plate motions or geodetically measured shear strain we thus find no evidence for a moment deficit significant aseismic moment release or for rare m 8 earthquakes off the san andreas fault system finally the number of m 6 earthquakes per decade does not depart significantly from a poisson process during this century and thus we find no evidence that the rate of seismicity is increasing now or at any other time since 1900

intermediate and long term earthquake prediction abstract progress in long and intermediate term earthquake prediction is reviewed emphasizing results from california earthquake prediction as a scientific discipline is still in its infancy probabilistic estimates that segments of several faults in california will be the sites of large shocks in the next 30 years are now generally accepted and widely used several examples are presented of changes in rates of moderate size earthquakes and seismic moment release on time scales of a few to 30 years that occurred prior to large shocks a distinction is made between large earthquakes that rupture the entire downdip width of the outer brittle part of the earth s crust and small shocks that do not large events occur quasi periodically in time along a fault segment and happen much more often than predicted from the rates of small shocks along that segment i am moderately optimistic about improving predictions of large events for time scales of a few to 30 years although little work of that type is currently underway in the united states precursory effects like the changes in stress they reflect should be examined from a tensorial rather than a scalar perspective a broad pattern of increased numbers of moderate size shocks in southern california since 1986 resembles the pattern in the 25 years before the great 1906 earthquake since it may be a long term precursor to a great event on the southern san andreas fault that area deserves detailed intensified study