using 137cs and 210pbex measurements to estimate soil redistribution rates on semi arid grassland in mongolia abstract in this study two small experimental catchments were selected in semi arid grassland in mongolia the kherlen bayan ulaan kbu catchment has been subjected to intensive grazing as a wintering shelter for domestic livestock animals and the number of livestock animals in the baganuur bgn catchment has been increasing over the last few decades the rates and patterns of soil erosion within the catchments were estimated based on cesium 137 137 cs and lead 210 210 pb ex inventories in bulk soil core samples the medium term 40 years soil erosion rates based on the 137 cs measurements indicated that annual soil loss from the kbu catchment was approximately three times greater than that from the bgn catchment the 137 cs derived rates and patterns of soil erosion indicated that soil in the kbu catchment was susceptible to surface erosion with facilitated transport of eroded sediment to the catchment outlet the 210 pb ex derived soil erosion rates were generally greater than those derived from 137 cs in both catchments if we assume that the soil erosion rate has been increasing with increasing domestic livestock numbers the most plausible explanation is that the 210 pb ex derived estimates represent recent increases in soil erosion furthermore the 210 pb ex 137 cs inventory ratio showed a tendency to increase with downslope distance on the eroded hillslope in the bgn catchment assuming that the increase in inventory ratio is a result of constant fallout input of 210 pb ex on the eroded sediment during its transportation along the hillslope the 210 pb ex inventory may not be suitable to estimate soil erosion rates at sites where downslope movement of sediment is slow further discussion regarding interpretation of 210 pb ex derived soil loss is required and the applicability of the 210 pb ex technique on semi arid hillslopes remains to be determined

characteristics of late cenozoic volcanism along the archibarca lineament from cerro llullaillaco to corrida de cori northwest argentina abstract the archibarca lineament is one of several nw se trending transverse lineaments that cut across the central andes of argentina and chile central andean late miocene quaternary subduction related volcanism is mainly restricted to a 50 km wide arc forming the approximately n s axis of the cordillera but extends along the transverse lineaments for up to 200 km to the se lineaments are interpreted to be deep seated long lived basement structures or anisotropies which can control the localization of magmatism and in some cases magmatic hydrothermal ore deposits e g the escondida porphyry cu deposit chile as a first step towards exploring the regional scale controls on magmatism and related mineralization exerted by such structures the styles of volcanism and near surface hydrothermal activity along a segment of the archibarca lineament in the puna of northwest argentina are described here volcanic structures have been mapped and sampled along a 50 km transect from cerro llullaillaco a large medium k dacitic quaternary stratovolcano to corrida de cori a range of pliocene pleistocene high k andesitic vents apart from a southeastward increase in k content and the predominance of dacitic lavas at cerro llullaillaco the geochemical affinity of late cenozoic volcanic rocks varies little in time or space this uniformity extends further se to cerro galan where published data closely match the results from the study area in detail trace element compositions reveal the localized in both time and space effects of crustal contamination recognized as th 10 ppm and depth of fractionation 1 yb 0 7 ppm 1 reflecting garnet residue explosive volcanic rocks such as ignimbrites show the strongest indications of crustal contamination whereas the cerro llullaillaco dacite lavas mostly record significant garnet fractionation other lavas from the llullaillaco area including one flow from cerro llullaillaco do not show garnet control suggesting that different batches of magma stalled and fractionated at different levels in a thick 60 km crust prior to eruption the youngest volcanism in the corrida de cori area is represented by olivine phyric basaltic andesite cinder cones and flows the ascent of these relatively primitive magmas appears to have been controlled by late quaternary normal faults which directly tapped deeply derived melts the corrida de cori volcanic range has experienced intense fumarolic alteration with deposition of abundant sulfate and native sulfur previously mined at mina julia deeper levels of hydrothermal alteration have been sampled by an ignimbrite erupted from cerro escorial which among other lithic clasts contains numerous fragments of vein quartz fluid inclusions in this quartz record evidence for a boiling high salinity fluid which may represent a link between a high temperature magmatic hydrothermal system at depth i e a porphyry type system and shallow level fumarolic activity an ignimbrite erupted from cerro escorial preserves textures such as internal wave forms between flow units and surface wave morphologies at its distal limits that indicate flow as a series of dense turbulent pulses which interdigitated and interfered with one another lithic lag breccias occur near the base of the flow proximal to the vent but no air fall deposits are preserved probably due to transport of ash far from the vent by strong high altitude winds

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

moho topography in the central andes and its geodynamic implications p to s converted waves at the continental moho together with waves multiply reflected between the earth s surface and the moho have been used to estimate the moho depth and average crustal vp vs variations in the central andes our analysis confirms and significantly complements the moho depth estimates previously obtained from wide angle seismic studies and receiver functions the resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins 30 km further to the east in the chaco plains beneath the andean plateau the moho is deeper in the north altiplano and shallower in the south puna where the plateau attains its maximum elevation a non linear relation exists between crustal thickness and elevation and bouguer gravity suggesting that the crust shallower than 50 55 km is predominately felsic in contrast to a predominately mafic crust below such a relation also implies a 100 km thick thermal lithosphere beneath the altiplano and with a lithospheric thinning of a few tens of kilometers beneath the puna absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau in the subandean ranges at 19 20 s the relation between altitude and crustal thickness indicates a thick lithosphere up to 130 150 km and lithospheric flexure beneath a relative topographic low at the salar de atacama a thick crust 67 km suggests that the lithosphere in this region is abnormally cold and dynamically subsided possibly due to coupling with the subducting plate this may be related to the strongest ms 8 0 known intra slab earthquake in the central andes that happened very close to this region in 1950 the average crustal vp vs ratio is about 1 77 for the altiplano puna and it reaches the highest values 1 80 1 85 beneath the volcanic arc indicating high ambient crustal temperatures and wide spread intra crustal melting 2002 elsevier science b v all rights reserved

geochemistry and isotope systematics of small to medium volume neogene quaternary ignimbrites in the southern central andes evidence for derivation from andesitic magma sources the geochemical and isotopic compositions of small to medium volume 10 km3 50 km3 neogene quaternary ignimbrite sheets of the southern central andes 25 00 26 45 s 67 30 69 30 w have been analysed in order to assess the petrogenesis of this type of calc alkaline magma the ignimbrites are derived from the active volcanic zone around salar de la isla sis in chile and from behind the active arc in the area of the salar de antofalla saf argentina the ignimbrites in both areas are characterized by 87sr 86sr t ratios 0 7066 0 7090 that are lower and end t values 2 4 to 5 1 that are higher than in the large volume caldera sourced ignimbrites of cerro galan 100 km further east and those of ignimbrites from other megacalderas of the central andes this difference reflects smaller amounts of crustal material in the sis and saf ignimbrites rather than a change in basement composition as indicated by pb provinciality the isotope signature of the sis and saf ignimbrites is similar to that of late to post miocene composite calc alkalic andesite dacite stratovolcanoes of the frontal arc in this region this finding combined with major and trace element modelling lends support to a petrogenetic model for the sis and saf ignimbrites involving fractional crystallization from a lil depleted andesitic parental magma which was beforehand modified by crustal contamination

relative roles of source composition fractional crystallization and crustal contamination in the petrogenesis of andean volcanic rocks there are well established differences in the chemical and isotopic characteristics of the calc alkaline basalt andesite dacite rhyolite association of the northern n v z central c v z and southern volcanic zones s v z of the south american andes volcanic rocks of the alkaline basalt trachyte association occur within and to the east of these active volcanic zones the chemical and isotopic characteristics of the n v z basaltic andesites and andesites and the s v z basalts basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere conversely the alkaline lavas are derived from basaltic parent magmas formed from mantle of within plate character recent basaltic andesites from the cerro galan volcanic centre to the se of the c v z are derived from mantle containing both subduction zone and within plate components and have experienced assimilation and fractional crystallization a f c during uprise through the continental crust the c v z basaltic andesites are derived from mantle containing subduction zone components probably accompanied by a f c within the continental crust some c v z lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing the petrogenesis of the c v z lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation fractional crystallization and magma mixing during uprise through the continental crust

style and history of andean deformation puna plateau northwestern argentina topographically the puna plateau of northwestern argentina is the southern continuation of the bolivian altiplano its thickening and consecutive uplift result from the andean orogeny to better constrain the structural style and its progressive development we have studied field data topographic and satellite imagery balanced cross sections seismic reflection data kinematic analysis of fault slip data anisotropy of magnetic susceptibility ams paleomagnetic data and apatite fission track aft data across the puna plateau precambrian and paleozoic basement ranges bounded by high angle reverse faults dips 60 alternate with cenozoic intermontane basins major thrusts trend nne ssw and do not show a preferred vergence intermontane basins have various degrees of symmetry depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets there is a close correlation between the surface expression of a basin and the amount of internal deformation a line balanced cross section of the puna at 25 s has yielded a cenozoic shortening of 10 15 in a direction subperpendicular to the orogen by kinematic analysis of cenozoic fault slip data we have obtained principal directions of strain rate across the puna shortening axes are subhorizontal and trend on average wnw ese n110 stretching axes are subvertical and intermediate axes are subhorizontal and trend on average nne ssw strain ellipsoids are dominantly of plane strain type and they represent dip slip thrusting from paleomagnetic and ams data shortening axes form a radial pattern around the eastern edge of the central andes the pattern is attributed to an inhomogeneous stress field reflecting the eastward convex shape of the central andean thrust front from the history of burial and uplift andean shortening reached the northeastern part of the puna in the late eocene and the adjacent eastern cordillera in the late eocene or early oligocene this shortening was presumably due to the incaic phase of the andean orogeny in the eastern part of the orogen the onset of shortening was probably guided by preexisting paleozoic and mesozoic structures so that andean deformation propagated unevenly eastward

advection and removal of 210pb and stable pb isotopes in the oceans a general circulation model study abstract natural pb isotope variability in the oceans encodes information about the sources of continental material to the oceans about ocean circulation and about pb removal in order to use this information we must understand the natural cycle of pb in the oceans which is overprinted by large anthropogenic input in this study we use 210 pb which has not been significantly anthropogenically perturbed to investigate oceanic pb geosecs 226 ra and model derived atmospheric fluxes of 210 pb are used to input 210 pb into an ocean general circulation model irreversible scavenging of this 210 pb onto settling biogenic particles and at the seafloor are tuned so that the model replicates the observed pattern of 210 pb in the oceans the best fit model run provides a map of the variability of residence time for pb the global average residence time of pb in this model is 48 yr but there is over an order of magnitude variation between areas of high and low productivity this is expected to enhance provinciality of pb isotope ratios in the oceans because stable pb isotopes are expected to behave in seawater in a similar fashion to 210 pb the pattern of removal of 210 pb derived by the model can be used to investigate the behavior of stable pb isotopes we use a simplified input of pb consisting of five point sources representing major rivers and a disseminated dust input although this simplified input scheme does not allow precise reconstruction of pb concentration and isotopes in the oceans it allows us to answer some first order questions about the behavior of pb as an ocean tracer with a total pb input of 6 3 10 7 mol yr chow t j and patterson c c the occurrence and significance of pb isotopes in pelagic sediments geochim cosmochim acta 26 263 308 1962 the model predicts natural seawater pb concentrations averaging 2 2 pmol kg even in the absence of anthropogenic input the model ocean exhibits a near surface maximum in pb concentration and the model suggests natural pb concentrations in the northern hemisphere an order of magnitude higher than in the southern hemisphere a point source of pb is suggested to dominate the seawater pb inventory close to the source but is reduced to typically less than 10 of the inventory by the time that pb is advected out of the originating ocean this length scale of advection for pb isotope signals confirms their potential as tracers of ocean circulation assuming an 8 dissolution of dust their input to the open ocean are seen to be a significant portion of pb inventory throughout the oceans and make up 50 of the pb inventory in the model s southern ocean finally a preliminary investigation of the response of pb isotope distributions to changes in boundary conditions between glacial and interglacial times illustrates that significant variation in the pb isotopes are expected in some regions even for reasonably small changes in climate conditions

late cenozoic tectonism collapse caldera and plateau formation in the central andes abstract the evolution of andean volcanism including the formation of late miocene to recent collapse calderas on the puna plateau is generally interpreted in terms of the kinematic framework of the nazca and south american plates we present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first order nw se striking zones of left lateral transtension on the local and regional scales consequently ages of collapse calderas indicate activity of these fault zones which initiated at about 10 ma on the puna plateau the onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen parallel stretching upon reaching maximum crustal thickness and critical surface elevation horizontal magma sheets that formed at mid crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub vertical faults this accounts well for the observed tectono magmatic phenomena at surface it follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the andean plateau rather than to changes in the geometry of the wadati benioff zone or plate boundary kinematics

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

crustal lineament control on magmatism and mineralization in northwestern argentina geological geophysical and remote sensing evidence abstract the relationship between long lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the puna region of northwestern argentina through the analysis of regional aeromagnetic surveys landsat images and geological information the good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study at a regional scale several contrasting magnetic domains are recognized which correlate with crustal geology two basement domains are separated by a nne trending boundary which is believed to correlate with a paleozoic suture zone between the pampia to the southeast and arequipa antofalla terranes to the northwest locally overlying these basement terranes is the cenozoic magmatic domain which is best developed in the n s trending volcanic arc at the western edge of the puna the cordillera occidental in addition four southeast trending volcanic zones extend for several hundred kilometers across the puna many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts and we have selected three such areas for more detailed study bajo de la alumbrera argentina s largest porphyry copper deposit cerro galan the largest ignimbrite caldera in argentina with associated hydrothermal alteration zones and el queva a historic polymetallic district located within a major volcanic range a comparison of lineament maps generated from aeromagnetic and landsat tm images reveals broad correlation between these different remote sensing techniques which respectively highlight subsurface magnetic and surface geological features in addition the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework and are seen to occur near the intersections of major lineament zones it is suggested that in three dimensions such intersection zones form trans lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain and may thereby serve as conduits for magma ascent to the shallow crust pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits it is important to note that in this model structural intersections serve as facilitators for magma ascent and volatile exsolution but do not in themselves cause this process other factors such as magma supply rate and tectonic stress are essential primary ingredients and local magmatic and volcanic processes affect the ultimate potential for ore formation nevertheless we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration other regional and local geological considerations must then be applied to identify priority targets within this framework

using 137cs and 210pbex to assess soil redistribution on slopes at different temporal scales abstract increasing risk of soil loss as a result of climate change has generated a need for reliable information on erosion rates at different temporal scales use of the fallout radionuclides 137 cs 210 pb ex and 7 be as tracers of sediment mobilization and redistribution makes it possible to obtain estimates of soil redistribution rates within both undisturbed and cultivated landscapes over a range of timescales mediterranean landscapes are characterized by a great diversity of physiography and land use and as a consequence erosion and deposition patterns are highly variable spatially to document such spatial variability a slope transect located in the subhumid pre pyrenean mountains ne spain was selected to use 137 cs and 210 pb ex to assess medium and longer term soil redistribution rates a total of 23 sectioned soil cores spaced 50 m apart were collected along the slope transect where 7 be had been previously used to document soil redistribution resulting from an individual storm event the inventories of both radionuclides varied markedly between 409 and 6080 bq m 2 for 137 cs and between 0 and 6734 bq m 2 for 210 pb ex estimates of soil redistribution derived from the 137 cs depth profiles using appropriate conversion models show that erosion rates along the transect vary between 2 6 and 31 9 mg ha 1 year 1 and that sedimentation rates vary between 0 2 and 24 5 mg ha 1 year 1 the highest soil losses occur in cultivated fields within the midslope zone of the transect while the highest deposition rates are found in tilled fields within the lower part of the transect erosion rates from 210 pb ex varied widely between 0 1 and 83 7 mg ha 1 year 1 on the lower slope whereas sedimentation rates ranged between 0 8 and 110 mg ha 1 year 1 also at the bottom slope the spatial distribution of the radionuclides along the transect reflects the effects of different land use and slope gradient on water erosion the results obtained confirm the potential for using 137 cs and 210 pb ex measurements for assessing soil redistribution on slopes in the mediterranean environment over different temporal scales

the relationship of soil organic carbon to 210pbex and 137cs during surface soil erosion in a hillslope forested environment the relationships of erosion induced displaced soil organic carbon soc to discharged 210pbex and 137cs in a hillslope forested area were investigated using runoff plots in three different forest types a significant correlation was found between the displaced soc and the radionuclides in each examined forest type compared with 137cs however the discharge of 210pbex is more closely correlated with the displaced soc and shows a far lower rmse value by approximately a factor of two this finding suggests that 210pbex and soc are more likely than 37cs and soc to move together in the course of soil erosion processes moreover because 137cs was introduced in the 1960s by thermonuclear bomb tests and is being depleted its availability in the environment could be limited in the foreseeable future 210pbex in contrast is continuously replenished from its ubiquitous natural source 222rn in a manner similar to the replenishment of soc from sources consisting of organic material e g litter in the light of this basic understanding our findings provide practical evidence of the potential of 210pbex application for assessing and documenting the water induced distribution of soc on forested hillslopes

implication of dating of early tertiary volcanic rocks from the north chilean precordillera abstract since the early jurassic subduction induced volcanic activity in the north chilean andes 21 23 5 s has migrated 200 300 km eastward from the present coast jurassic magmatic arc to the western cordillera miocene recent volcanic rocks due to the paucity of chronological data however the exact age and extent of various magmatic sub belts within this area are not well constrained in the precordillera a sequence of 1500 5000 m thick red bed sediments is partly intercalated with and mostly covered by predominantly intermediate calc alkaline tuffs and lavas presently the age of this sequence is considered to be upper jurassic and or cretaceous from this sequence eleven biotites and seven hornblendes were analyzed and yielded 40 ar 39 ar plateau ages of between 48 and 38 ma indicating upper eocene volcanic activity in the eastern part of the precordillera in contrast volcanic activity in the western part of the precordillera has been dated at 92 2 5 ma indicating an earlier onset of volcanism and therefore sedimentation of the red beds in the late eocene the intense volcanism ceased in this area contemporaneous with decreasing relative convergence rate of the nazca plate and the south american continent this final phase of eocene volcanism coincided with a period of important deformation the incaic phase biotites from two ash fall tuffs separated by a distinct angular unconformity yield identical ages of 38 50 0 90 ma and 38 45 0 60 ma these dates provide the best age estimate of the incaic phase in the north chilean precordillera

contemporaneous formation of adjacent porphyry and epithermal cu au deposits over 300 ka in northern luzon philippines there is commonly a close spatial relation between porphyry cu au and high sulfidation epithermal cu au deposits throughout the world although a genetic association has not been proven nowhere is this spatial association better seen than in northern luzon philippines where the lepanto epithermal cu au deposit overlies the far southeast fse porphyry cu au deposit both world class orebodies fresh rock and hydrothermal mineral separates yield k ar ages indicating that premineralization and postmineralization volcanism occurred at 2 2 1 8 ma and 1 2 0 9 ma respectively and that the hydrothermal system was active from 1 5 to 1 2 ma k ar ages of alunite from lepanto have the same range as those of hydrothermal biotite and illite from the fse deposit indicating that both epithermal and porphyry mineralization formed from an evolving magmatic hydrothermal system that was active for about 300 ka this temporal relation strengthens the argument for a genetic link between these two styles of ore deposit and has implications for exploration where one style of mineralization is found there is potential for the other nearby

u series evidence for crustal involvement and magma residence times in the petrogenesis of parinacota volcano chile in this study we present th u disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the nevados de payachata volcano which erupted through 70 km of continental crust in the central volcanic zone of the andes 18 s 69 w both lavas and mineral separates were analyzed by mass spectrometry for 238u 230th disequilibria the lavas are characterized either by 230th enrichment or depletion relative to its parent nuclide 238u mineral separates are used to derive u th isochron ages and these ages compare favorably with inferred stratigraphic ages or k ar ages although in one case the u th age is significantly older than the stratigraphic age despite relatively constant sr nd and pb isotope ratios the lavas display inverse trends in 230th 238u versus ce yb or ba hf diagrams these trends cannot be interpreted by simple two component mixing rather there must be three and perhaps four components involved in the genesis of the parinacota lavas a mantle wedge a slab fluid and a lower crustal component can be identified a sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence the existence of binary arrays can be explained by variable amounts of crustal material the process of crust mantle interaction must have been short enough to preserve u th disequilibrium 300 ka

intracrustal derivation of na rich andesitic and dacitic magmas an example from volcan ollague andean central volcanic zone andesitic and dacitic lavas erupted from quaternary stratovolcanoes in the central volcanic zone cvz of the andes have major and trace element compositions similar to proposed melts of subducted mid ocean ridge basalt and na rich trondjhemites tonalites and granodiorites ttg in early archean high grade gneiss terrains suites of lavas from individual volcanoes have average la yb of 25 45 sr y of 30 65 yb contents of 1 5 1 1 ppm and na 2 o contents 3 8 wt with lavas erupted on the volcanic front displaying the most affinity to the early archean suites because slab melting is not involved in the derivation of the cvz magmas these data indicate that rocks with early archean ttg compositions can also be produced through intracrustal differentiation processes isotopic major element and ree data from volcan ollague are consistent with a model incorporating contamination of mantle derived basaltic magmas in garnet bearing deep continental crust to produce basaltic andesite magma followed

the negra muerta volcanic complex southern central andes geochemical characteristics and magmatic evolution of an episodically active volcanic centre abstract this petrologic analysis of the negra muerta volcanic complex nmvc contributes to understanding the magmatic evolution of eruptive centres associated with prominent nw striking fault zones in the southern central andes specifically the geochemical characteristics and magmatic evolution of the two eruptive episodes of this complex are analysed the first one occurred as an explosive eruption at 9 ma and is represented by a strongly welded fiamme rich andesitic to dacitic ignimbrite deposit the second commenced with an eruption of a rhyolitic ignimbrite at 7 6 ma followed by effusive discharge of hybrid lavas at 7 3 ma and by emplacement of andesitic to rhyodacitic dykes and domes both explosive and effusive eruptions of the second episode occurred within a short time span but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber pressurising the magmatic system and triggering explosive eruption of rhyolitic magma chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition which could have been strong enough to inhibit the interaction between the two magmas after the initial explosive stages of the eruption at 7 6 ma the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber at 7 3 ma increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera assimilation during turbulent ascent ata is invoked to explain a conspicuous reversed isotopic signature 87sr 86sr and 143nd 144nd in the entire volcanic series therefore the 7 6 to 7 3 ma volcanic rocks of the nmvc resulted from synchronous and mutually interacting petrological processes such as recharge fractional crystallization hybridisation and assimilation during turbulent ascent ata geochemical characteristics of both volcanic episodes show diverse type and or depth in the sources and variable influence of upper crustal processes and indicate a recurrence in the magma forming conditions similarly other minor volcanic centres in the transversal volcanic belts of the central andes repeated their geochemical signatures throughout the miocene

the late miocene quaternary antofalla volcanic complex southern puna nw argentina protracted history diverse petrology and economic potential abstract the antofalla volcanic complex avc is located on the southern puna plateau of nw argentina 100 km east of the main axis of the cordillera occidental volcanic arc it lies on the nw se archibarca lineament one of several transverse structures that cut across the andes from chile and divert arc magmatism in se trending fingers across the puna 40ar 39ar dating of volcanic products from the avc reveals a protracted magmatic history from 10 9 to 1 6 ma initial volcanism was characterized by eruption of voluminous rhyolitic ignimbrites deposited directly onto exposed crystalline basement and supracrustal sedimentary rocks these rhyolites record significant geochemical evidence for crustal contamination or derivation by crustal melting which decreases upwards through the pyroclastic sequence they were followed by extrusion of potassic lavas of the shoshonite latite trachydacite suite from 10 1 to 9 5 ma subsequent magmatism 9 1 1 6 ma was of basaltic andesite andesite dacite composition with localized smaller volume dacitic and rhyolitic ignimbrites monogenetic basaltic andesite cinder cones and lava flows were erupted in the quaternary in response to a change to transtensional tectonics in the puna the volcanic sequence is interpreted to record the early development of a lower crustal mash zone at 11 ma where mantle derived arc magmas interacted extensively with felsic lower crustal rocks to produce evolved shoshonitic compositions and crustal melts that erupted to form rhyolitic ignimbrites common at this time throughout the puna after establishment of a steady state mash zone by 9 ma subsequent basaltic andesite andesite magmas show less evidence of felsic crustal contamination and may have developed largely by interaction with previous ultramafic mafic fractionates of early magmas in the lower crust more felsic dacitic magmas appear to have evolved largely by fractional crystallization from basaltic andesites and andesites with widespread evidence of mixing and mingling between magmas of different degrees of differentiation some contamination by upper crustal components is also evident in these dacites but is shown more clearly by small volume rhyolites some of which may again represent predominantly crustal melts epithermal deposits of au and pb zn ag occur within the avc apparently associated with the formation of rhyolite domes the mineralization style is typical of other andean volcanic centres and may be underlain by deeper seated porphyry styles of mineralization extensive blanketing by younger lavas and limited erosion restricts exposure of these altered and mineralized sequences however although the potential for further discoveries beneath cover or at depth in this system appears high

major element geochemistry of purple soils rocks in the red sichuan basin china implications of their diagenesis and pedogenesis major element compositions and chemical weathering features of the purple soils and their corresponding rocks in the red sichuan basin china were studied in this paper to infer the diagenetic features of the purple rocks and the pedogenetic features of the purple soils the results showed that all of the rock and soil samples except those from yibin have strongly similar major element compositions and the distribution patterns of their major element compositions are similar to those of the upper continental crust chemical index of alteration chemical index of weathering and na2o k2o indicate that the purple soils and rocks have similar chemical weathering intensities whereas the al2o3 cao na2o k2o a cn k relationship suggests that chemical weathering was strong during diagenesis of the purple rocks but weak during pedogenesis of the purple soils variations in major element compositions of the purplish rock samples except that from yibin do not primarily reflect differences in compositions of the sediment source sediment recycling and potassium metasomatism and therefore the effects of chemical weathering on the major element compositions were mainly dependent on climate condition and the residence time of material exposed to chemical weathering chemical weathering of the purple rocks reached only moderate degrees under the general influence of warm and humid climate during jurassic and cretaceous warmer and more humid climate conditions partly resulted in stronger chemical weathering of rocks in the southern region of the basin than in the central and western region whereas shorter residence time of material in upslope position partly resulted in weaker chemical weathering of rocks in the eastern region than in the central and western region the same climate and stable tectonic setting led to comparable chemical weathering intensities of rocks in the ziliujing and jiaguan formations gradually colder and dryer climate from early jurassic to middle jurassic then to late jurassic and shorter residence time in tectonically active setting of middle jurassic and late late jurassic resulted in the following order of chemical weathering intensity of the purple jurassic rocks ziliujing formation shaximiao formation suining formation penglaizhen formation

fallout 210pb as a soil and sediment tracer in catchment sediment budget investigations a review increasing anthropogenic pressures coupled with climate change impacts on natural resources have promoted a quest for innovative tracing techniques for understanding soil redistribution processes and assessing the environmental status of soil resources among the different existing tracers the fallout component of the radioisotope lead 210 also termed unsupported or excess lead 210 pb 210 ex when referring to its presence in soil or sediment arguably offers the broadest potential for environmental applications due to its origin and relatively long half life for more than five decades pb 210 ex has been widely used for dating sediments to investigate sedimentation processes and since the 1990s to provide information on the magnitude of soil and sediment redistribution the aim of this review is to provide a comprehensive evaluation and discussion of the various applications of pb 210 ex as a tracer in terrestrial and aquatic environments with particular emphasis on catchment sediment budget investigations this paper summarizes the state of the art related to the use of this tracer the main assumptions the requirements including the need for accurate analytical measurements and for parallel validation and the limitations which must be recognised when using this fallout radionuclide as a soil and sediment tracer lessons learned and current and future research needs in the environmental and radiochronological application of pb 210 ex are also presented and discussed c 2014 elsevier b v all rights reserved

partial melting at the earth s surface implications for assimilation rates and mechanisms in subvolcanic intrusions abstract the relatively recent routine availability of isotopic and elemental analytical results from large sample suites of igneous rocks has revolutionized the study of magmatic processes large uncertainties remain however with regard to the highly variable rates of magma differentiation and ascent through the crust although the repose interval prior to an eruption may be equivalent to magma residence time at a certain level in the crust this cannot be assumed a priori even where this happens to be the case uncertainties in determining such intervals for prehistoric eruptive sequences may be large with respect to process rates consequently independent insights from geologically well constrained situations must be sought this paper first presents documentation of an example wherein fragments of dacite incorporated by a mafic lava flow on the surface were melted to the point that they viscously deformed in response to stresses in this flow the transformation from the solid state to a largely molten state is inferred to have occurred within a few weeks these results are then discussed in the context of examples from the literature that support rapid melting and digestion of xenoliths in magma it is suggested that the rate of crustal assimilation may be sufficiently rapid that it can operate within the durations of extended eruptive episodes years decades which are generally much shorter than typical repose periods at arc volcanoes decades millennia

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

oligocene range uplift and development of plateau morphology in the southern central andes 1 the puna altiplano plateau in south america is a high elevation low internal relief landform that is characterized by internal drainage and hyperaridity thermochronologic and sedimentologic observations from the sierra de calalaste region in the southwestern puna plateau argentina place new constraints on early plateau evolution by resolving the timing of uplift of mountain ranges that bound present day basins and the filling pattern of these basins during late eocene miocene time paleocurrent indicators sedimentary provenance analyses and apatite fission track thermochronology indicate that the original paleodrainage setting was disrupted by exhumation and uplift of the sierra de calalaste range between 24 and 29 ma this event was responsible for basin reorganization and the disruption of the regional fluvial system that has ultimately led to the formation of internal drainage conditions which in the salar de antofalla were established not later than late miocene upper eocene oligocene sedimentary rocks flanking the range contain features that suggest an arid environment existed prior to and during its uplift provenance data indicate a common similar source located to the west for both the southern puna and the altiplano of bolivia during the late eoceneoligocene with sporadic local sources this suggests the existence of an extensive longitudinally oriented foreland basin along the central andes during this time citation carrapa b d adelmann g e hilley e mortimer e r sobel and m r strecker 2005 oligocene range uplift and development of plateau morphology in the southern central andes tectonics 24 tc4011 doi 10 1029 2004tc001762

radionuclide dating of recent sediment and the validation of pollen environment reconstruction in a small watershed reservoir in southeastern china abstract a 76 cm sediment core was collected from the huangdong reservoir southeastern china the radionuclides 137 cs and 210 pb and sediment grain size analysis were applied to establish the chronology and modern sedimentation rates the chronology established by the 137 cs time markers was consistent with the constant flux constant sedimentation cfcs model and the known time of dam construction 1960 ad recognized from the curve of sediment grain size at the depth of 54 cm the results show that the sedimentation rate of the huangdong reservoir is relatively high implying that soil erosion and water loss were strong the sedimentation rate before the construction of the reservoir was approximately 1 5 cm a which belongs to the fluvial flooding deposit the rate decreased to approximately 1 0 cm a during the reservoir deposition since 1960s pollen and micro charcoal analysis were performed to test the reliability of pollen based environment reconstruction including vegetation cover deforestation planting and fire disasters the results of pollen and micro charcoal indicated that the herbs primarily gramineae ferns such as dicranopteris and several trees mainly pinus varied following chorography or historical records we discovered that the most important indicators for local forest cover changes are pollen concentration and the ap nap value ratio of arboreal pollen to non arboreal pollen human impact on forests is manifested by a sharp decrease in pollen percentage of trees and absolute pollen concentration forest fire events were recorded by the peaks of charcoal concentration and the c p value ratio of charcoal concentration to pollen concentration for example the massive deforestation and intensive fires during the great leap forward movement and the cultural revolution are reflected by pollen and micro charcoal spectra this work testifies that radionuclide use as a dating method is applicable in southern china and the 137 cs method has demonstrated its reliability in the small reservoir setting in spite of low absolute values the environmental proxies such as grain size pollen and charcoal are sufficiently sensitive to reveal local ecosystem changes caused by natural disasters or human activity

average pleistocene climatic patterns in the southern central andes controls on mountain glaciation and paleoclimate implications abstract despite elevations of 5000 6800 m modern glaciers occur along the southern puna plateau and the northern sierras pampeanas in the southern central andes the modern snowline rises from 5100 m in sierra aconquija to 5800 m in the puna as a result of a westward decrease in precipitation from 450 to less than 100 mm yr during the pleistocene these arid highlands experienced multiple cirque and valley glaciation that likely postdate the last interglacial period although lack of age control prevents an absolute chronology glaciation in the puna and along the eastern puna edge produced a 300 m pleistocene snowline psl depression while in the sierras pampeanas the psl depression was at least 900 m the greater psl depression in the sierras pampeanas is best explained by a combination of cooling and increase of easterly moisture whereas the psl depression in the puna appears more sensitive to moisture increases than temperature previously glaciations in this region have been explained by increa

analogue models of pull apart basins sandbox analogue models of pull apart basins that developed in sedimentary strata above releasing steps in underlying basement faults are characterized by rhombic basins that are flat bottomed box grabens with a subhorizontal synkinematic basin infill steep to nearly vertical sigmoidal oblique slip and segmented oblique extensional faults are the dominant bounding structures of the pull apart basins cross basin short cut faults link the offset principal displacement zones that are characterized by flower structure development the structural architectures of the physical models compare directly in form and dimensions to natural examples of strike slip pull apart basins

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

partial assimilative recycling of the mafic plutonic roots of arc volcanoes an example from the chilean andes at long lived arc volcanic centers open system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust in the tatara san pedro complex 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine augite and plagioclase derived from solidified cumulates wide but poorly correlated ranges of incompatible and compatible major and trace elements and limited isotopic variability this mineralogical chemical signature reflects assimilation wherein grain boundary melting and disaggregation of xenoliths led to blending of incompatible element enriched melts derived mainly from hornblende phlogopite and plagioclase in combination with variable retention of xenocrysts hence variable but high compatible element concentrations

time scale of magma mixing between basalt and dacite estimated for the saga futagoyama volcanic rocks in northwest kyushu southwest japan abstract a composite lava extrusion of the saga futagoyama volcanic rocks consisting of a central unit of the dacite lava flanked by the andesite lava containing phenocrysts of quartz and plagioclase is exposed in northwest kyushu southwest japan field and microscopic observations indicate that the andesite and the dacite simultaneously emerged at the surface disequilibrium relationships between plagioclase and bulk rock compositions suggest that the andesite lava is a hybrid of more mafic and more silicic magmas the whole rock chemistry of the composite lava suggests that the contemporaneous eruption had two end members of basalt and dacite magmas the time scale of magma mixing between basalt and dacite is estimated using reaction coronas around quartz and dissolution textures of plagioclases in the andesite the typical reaction duration was estimated at 1 4 h to 12 days using reaction coronas around quartz and at 2 8 h to 1 day using dissolution textures of plagioclases these estimates indicate that the mixing essentially occurred during ascent in a conduit the concentration of the andesite compositions at sio2 59 wt requires other processes to homogenize mixed magma such as phenocryst stirring

high precision 40ar 39ar dating of two consecutive hydrothermal events in the chuquicamata porphyry copper system chile 40ar 39ar data have been used to resolve the relatively high temperature 350 c potassic alteration event at chuquicamata from the lower temperature 350 c quartz sericite alteration phase four k feldspars from the potassic zone yielded plateau large domain ages in the range 32 9 33 8 ma mean 33 4 0 3 ma the fact that apparent ages of co existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures especially if these minerals closed at substantially different temperatures multi domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid ca 60 80 my through closure temperatures of 330 440 c as these closure temperatures are substantially higher than the ones normally assumed for biotite i e about 300 c they and or the calculated cooling rates may be unreliable four sericites from the quartz sericite zone yielded variably discordant age spectra the least discordant of these has a plateau at 31 1 0 3 ma and this is interpreted as the time of this alteration event some k feldspars in the quartz sericite zone were completely overprinted at this later time and most of the other ones studied show partial overprinting overprinting appears to decrease as distance from the central quartz sericite zone increases in the more remote areas effects of the later event appear constrained to narrow fault controlled alteration zones the data are consistent with a conceptual model proposed by zentilli et al zentilli m leiva g rojas j graves m c 1994a the chuquicamata system revisited extended abstract society of economic geologists symposium copper deposits of the andes new developments concepcion chile october 17 18 1994 proceedings 2 1647 1651 wherein potassic alteration by late magmatic fluids occurred at depth prior to rapid exhumation of the system quartz sericite alteration perhaps caused by a new pulse of porphyry intrusion at depth occurred some 2 3 my later principally in the core of the deposit although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle relatively shallow fracture system

late cenozoic magmatism of the bolivian altiplano small basalt to dacite volcanic centers are distributed sparsely over the bolivian altiplano behind the andean volcanic front most are pliocene to recent in age and are characterized by textural and mineralogical disequilibrium with abundant xenoliths and xenocrysts true phenocrysts are rare in the more mafic samples compared with recent volcanic rocks from andean stratovolcanoes the bolivian centers overlap in major element trends incompatible element contents tend to be higher particularly in the eastern altiplano the ranges of isotopic compositions reflect ubiquitous crustal contamination pb isotope compositions are dominated by pb from isotopically heterogeneous basement resulting in a wide scatter of data lying between inferred crustal compositions and showing little overlap with possible mantle sources in the region rocks sampled from the bolivian altiplano were probably derived from asthenospheric mantle and subjected to extensive open system differentiation during ascent through the 70 km thick crust of the region major element trends are largely controlled by the fractionating phase assemblage olivine clinopyroxene and amphibole trace element and isotope systematics however defy realistic attempts at modeling due to the geographic scatter of samples the uniformity of compositions at a given center and the heterogeneity of the contaminant nevertheless there are first order systematic trace element variations that appear to relate to the geometry of the subduction zone in particular life hfse exemplified by ba nb and zr nb ratios decrease from the arc front eastward into the altiplano these variations are not easily reconciled with control by crustal contamination alone a model is proposed in which the asthenospheric source is fluxed by high ba nb slab derived fluid to induce melting beneath the arc high fluid flux increases the ba nb ratio of the asthenosphre and leads to high degrees of partial melting high zr nb to the east lower or no fluid flux leads to low ba nb and low degrees of partial melting low zr nb melting in the back arc region of the altiplano may be facilitated by lithospheric delamination that leads to decompression melting of counter flowing asthenosphere there is no unequivocal evidence that requires a significant role for the lithospheric mantle

volcanic rocks from the bolivian altiplano insights into crustal structure contamination and magma genesis in the central andes lavas from quaternary minor volcanic centers behind the central andean volcanic arc in bolivia form a compositional continuum with rocks from the arc front extending to more primitive compositions the lavas show petrographic and geochemical evidence for deep crustal contamination during differentiation sr nd and pb isotopic data from centers within 30 km of each other encompass the entire range found in the central volcanic zone of the andes the most mafic rocks are largely typical of arc magmas elsewhere and may represent parental magmas to the porphyritic andesites and dacites erupted from stratovolcanoes along the arc to the west andean arc magmas therefore undergo considerable contamination during ascent through the thick crust the only basalts lacking petrographic evidence for contamination are picrobasalts from chiar kkollu that form the most primitive end member of the compositional spectrum these rocks resemble ocean island basalts in some respects indicating that variations in crustal contamination across the strike of the arc may be superimposed on variations in the mantle source composition

volcanological and petrological evolution of volcan tata sabaya sw bolivia tata sabaya 19 08 s 68 32 w 5430 m a s l is located on the northern shores of salar de coipasa on the altiplano of bolivia it is a 1700 m high symmetrical composite cone one of two holocene centers in a chain of edifices trending e w from the main n s trending volcanic arc of the andean central volcanic zone cvz four main stages are envisaged for the evolution of the edifice stage 1 formation of a pyroclastic shield possibly around a central dome complex stage 2 formation of an andesitic composite cone on top of the early pyroclastic sequence stage 3 collapse of a 120 sector of the southern flank of the volcano which produced a prominent debris avalanche deposit covering 300 km2 of the salar de coipasa to the south collapse was induced by inflation of the edifice by an input of fresh magma into the sub volcanic magma chamber stage 4 flows and domes which have since healed the edifice tata sabaya shows a restricted range of whole rock compositions most units fall in the range 60 62 5 sio2 with a general evolution to the more silicic stage 4 lavas through time the typical phenocryst assemblage is plagioclase hornblende hypersthene augite oxide biotite is found in the stage 4 lavas and is dominant over pyroxene hornblende and biotite are thought to be a late stage sub liquidus products by far the most mafic compositions at tata sabaya are the hornblende and plagioclase inclusions in two of the stage 4 domes 52 sio2 macroscopic microscopic and chemical evidence for magma mixing mixed phenocryst and matrix populations resorption features mixed glass compositions contraindicative temperature indicators etc are found throughout tata sabaya the whole rock chemistry bears out that magma mixing was a dominant process and that it was responsible for buffering the composition of the magma in an open magmatic system despite being located 35km to the east of the main arc front the rocks of tata sabaya are compositionally similar to those from other centers in the northern 17 19 s segment of the cvz the high k2o calc alkaline suite is characterised by lree and extreme lile enrichments up to 1800 ppm ba and sr 87sr 86sr 0 7056 0 7068 and 143nd 144nd 0 51217 0 51227 are similar to other cvz lavas this is regarded as a baseline signature derived during deep crustal contamination this is corroborated by pb isotope data which indicates influence of the low 206pb 204pb precambrian basement that is thought to underlie the northern portion of the cvz modification of the isotope ratios during open system processes at shallow level is also indicated the most extreme incompatible element enrichments are found in the quenched mafic inclusions these characteristics have been reported from other holocene mafic lavas elsewhere in the northern cvz and may represent a distinct recent magmatic episode

generation of a crust mantle magma mixture magma sources and contamination at cerro panizos central andes cerro panizos a large caldera in the central andes mountains produced two large dacitic ignimbrites at 7 9 ma and 6 7 ma and many andesitic and dacitic lava flows and domes the older rhyodacitic cienago ignimbrite represents the most silicic magma erupted by the system the younger much larger volume dacitic cerro panizos ignimbrite is very crystal rich containing up to 50 biotite plagioclase and quartz crystals in the pumice it is weakly zoned with most of the zoning apparent between two main cooling units major and most trace elements show little variation through the cerro panizos ignimbrite but the small range of composition is consistent with typical fractionation trends sr nd and pb isotopic ratios are very crustal with initial 87sr 86sr values of 0 711 to 0 715 nd values of 7 5 to 10 2 and nearly invariant pb isotopic ratios 206pb 204pb 18 85 207pb 204pb 15 67 and 208pb 204pb 38 80 the limited zonation observed in the cerro panizos ignimbrite is explained by impeded crystal settling due to high crystal content the magma body was a crystal liquid mush before ascent to the pre eruption crustal levels crystals formed but did not separate easily from the magma limited fractionation of plagioclase and biotite may have occurred but the composition was largely controlled by lower crustal mash processes afc modeling shows that the cerro panizos magmas resulted from a mixture of roughly equal proportions of late miocene mantle derived basalts and melts from 1 0 ga grenville age lower crust this occurred in a mash zone in the lower crust and set the crustal isotopic ratios observed in the cerro panizos magmas the great thickening of the crust beneath the central andes mountains sent upper and middle crustal rock types to lower crustal and deeper depths and this explains the upper crustal isotopic signatures of the cerro panizos rocks minor upper crustal assimilation of early miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system the nearly invariant high pb isotopic values and high pb concentrations indicate that pb came almost entirely from the crustal source and was little altered by any subsequent upper crustal assimilation this pb signature is isotopically similar to that of the southern bolivian tin belt suggesting a widely distributed pb source the great difference between compositions of miocene and quaternary central andean volcanic rocks is explained by crustal thickening in early miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions the lower crust dried out considerably by quaternary time so less crustal component is present

application of the initial model feature of microtremor to investigate a part of the geothermal field of jiangsu region in china application of geothermal resources draws growing attention in recent years as geothermal resources are an emerging clean and renewable energy there are tremendous geothermal resources in jiangsu province which have been considered as significant contributions to regional economic development and climate variation controlled by tectonic structure the spatial distribution of geothermal resources is highly uneven because of the complexity in terms of thermal storage background and reservoir types successful application of geothermal resources is facing many practical difficulties such as determining the drilling location of geothermal well thus successful geothermal exploration is still of high risk firstly geothermal resources of jiangsu province can be categorized into four areas thus geothermal reservoirs of different areas are discussed in this paper according to the results of rock samples testing and analyzing a geothermal reservoir physical model of southern jiangsu geothermal resource area has been established which can provide an initial restriction for microtremor inversion two survey points with major inversion error under blind inversion are recalculated as well the results show that the initial inversion model can effectively improve the accuracy of inversion and thus benefit the sounding reservoirs by introducing the actual initial inversion model microtremor survey method can achieve more accurate results to provide critical information for the development of geothermal resources which reduce the risk of successful geothermal drilling

giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism cenozoic magmatic activity in northern chile led to the formation of two contrasting porphyry copper belts 1 a paleocene early eocene belt comprising small porphyry copper deposits e g lomas bayas of normal calc alkaline affinity and 2 a late eocene early oligocene belt hosting huge porphyry copper deposits e g chuquicamata of adakitic affinity although the first belt comprises both volcanic and plutonic rocks andesitic basaltic and rhyolitic lavas and tuffs and associated sub volcanic porphyries and felsic stocks the latter only includes intrusions mostly granodioritic types including porphyry copper deposits we suggest that the late eocene early oligocene belt formed when fast and oblique convergence between the south america and farallon plates led to flat subduction and direct melting of the subducting plate hence giving rise to plutonic rocks of adakitic affinity the absence of volcanism under prevailing compressional conditions prevented the escape of so2 from the adakitic sulfur rich highly oxidized magmas closed porphyry system which allowed formation of huge mineral deposits on the contrary coeval volcanic activity during formation of the paleocene early eocene calc alkaline porphyries allowed development of open systems hence to outgassing and therefore to small mineral deposits

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

the nevados de payachata volcanic region 18 s 69 w n chile ii evidence for widespread crustal involvement in andean magmatism volcanism extending over 11 ma is represented in the rocks of the nevados de payachata region culminating in the formation of two large composite stratocones within the last 500 000 years chemically distinct mafic magmas are erupted at a number of parasitic centers these cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones two distinct trends are defined by the intermediate and evolved rocks a high lile large ion lithophile element tio2 and ce yb lineage among the youngest rocks including the two major stratocones and a more typical calc alkaline trend among the older 1 ma rock types within individual volcanic centers differentiation involves fractionation of plagioclase pyroxene and hornblende with biotite and k feldspar in the more evolved rock types isotopic compositions sr pb nd o vary little with differentiation from basaltic andesite to rhyolite or with age contamination during differentiation from basalt to rhyolite may occur but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust the majority of data are similar to baseline compositions cenozoic parental magmas from other parts of the central andes and may reflect a relatively homogeneous magma source or source mixture throughout this central volcanic zone cvz which is distinct from the southern and northern andes and from island arc volcanic rocks

the relations between faulting and mafic magmatism in the altiplano puna plateau central andes mafic lava flows and scoria cone on the altiplano puna plateau correlate spatially and temporalily with late pliocene quternary strike slip faulting thrust faulting dominated deformation in the plateau and adjacent foreland during the miocene pliocene the relations between faulting and mafic magmatism may be explained by a change from vertical to horizontal least principal compression during the pliocene quaternary a model in which mantle derived mafic magma ascends through the thick less dense continental crust of the altiplano puna plateau suggests that nonlithostatic stress in the crust may have controlled the depth and geometry of intrusion horizontal nonlithostatic compression favored the formation of horizontal mafic intrusions in the lower crust which may have thermally weakened the crust localized crustal thickening and caused crustal anatexis horizontal nonlithostatic tension favored vertical intrusions in the crust and surface eruption

a preliminary assessment of the potential for using 210pbex measurement to estimate soil redistribution rates on cultivated slopes in the sichuan hilly basin of china in order to assess its potential for estimating soil redistribution rates the naturally occurring fallout radionuclide pb 210 ex has been used in parallel with cs c 137 derived from the atmospheric testing of nuclear weapon testing in the 1950s to 1970s to estimate rates of soil redistribution on a sloping field with traditional erosion control measures located near jiajia village jianyang county in the sichuan hilly basin of china the local pb 210 ex reference inventory of 12 860 bq m 2 is higher than those reported for many other areas of the world and may reflect the influence of cloudy weather in preventing pb 210 released to the atmosphere across the local region moving up into the upper troposphere where is would be more widely dispersed the mean pb 210 ex and cs 137 inventories measured in cores collected from the upper part of the field with an average slope of 10 degrees were 8028 bq m 2 and 993 bq m 2 respectively and the equivalent values for the lower part of the field where the slopes are steeper 20 degrees were 11 388 bq m 2 and 1299 bq m 2 the pattern of post fallout pb 210 ex and cs 137 redistribution on the sloping field reflects not only the effects of water erosion and redistribution by tillage but also the local traditional practice of tiaoshamiantu whereby sediment trapped in the ditches is returned to the fields by the fanner the estimates of annual rates of soil loss provided by the pb 210 ex measurement are closely comparable with those derived from the cs 137 measurements and are consistent with existing knowledge for the study area the results obtained from this study confirm the potential for using pb 210 ex measurement to estimate soil erosion rates over medium term timescale of 50 100 years by combining the estimates of erosion rates provided by the pb 210 ex and cs 137 measurements the weighted mean net soil loss was estimated to be 48 7 t ha 1 year 1 from the upper subfield and 16 9 t ha 1 year 1 from the lower subfield these rates are considerably lower than the erosion rates obtained from runoff plot measurements in the local area it is suggested that the traditional erosion control practices and the practice of tiaoshamiantu have a significant effect in reducing soil loss and conserving valuable cultivated soil on sloping fields in the sichuan hilly basin c 2006 elsevier b v all rights reserved

boron in central andean ignimbrites implications for crustal boron cycles in an active continental margin abstract calc alkaline pumices and lavas from the neogene pleistocene central andean altiplano puna volcanic complex apvc contain quartz phenocrysts with abundant primary melt inclusions melt inclusions represent the melt composition of dacitic to rhyodacitic magmas prior to eruption and they are widely protected from eruptive degassing low temperature vapor phase alteration and post depositional contamination with boron rich dusts and aerosols the average boron isotopic compositions of melt inclusion and matrix glasses are uniform in the units studied overall average 11 b 3 8 2 8 1 sd and overlap with the range of local basement rocks 11 b 11 to 5 b is enriched in glasses 55 14 ppm 1 sd whereas whole rock contents range between 10 and 40 ppm at similar b contents incompatible trace element ratios like b nb are generally higher in the ignimbrites and lavas b nb 2 4 and 4 0 than in crustally derived paleozoic to cretaceous granitoids from nw argentina b nb 1 0 2 7 this suggests a minor contribution of high b nb arc andesites and is consistent with petrogenetic models in which apvc dacites originated from hybridization of dominantly crustally derived melts with andesitic magmas boron rich hydrothermal brines and salt lake water from within the apvc have 11 b 4 1 to 5 7 these values are close to the apvc ignimbrite values and contrast with hydrothermal brines from the eastern puna and eastern cordillera which have lower 11 b 10 to 17 and occur in an area of little volcanic cover this regional pattern suggests that borate enrichments resulted from the leaching of local country rocks ignimbrites in the w altiplano puna and paleozoic basement in the eastern puna and eastern cordillera

a revised late eocene age for porphyry cu magmatism in the escondida area northern chile new u pb zircon and 40 ar 39 ar single crystal incremental heating biotite dates are reported for porphyritic rocks from three mineralized centers in the escondida region of northern chile all dates are reported with 2sigma errors at the chimborazo porphyry cu prospect a weighted mean 40 ar 39 ar plateau age of 38 09 or 0 30 ma has been obtained for igneous biotite from a syn or postmineralization feldspar biotite quartz porphyry intrusion at the zaldivar porphyry cu mine zircon and biotite have been analyzed from a similar synmineralization feldspar biotite quartz porphyry the llamo porphyry the zircon u pb age is 38 7 or 1 3 ma whereas the weighted mean 40 ar 39 ar plateau age of igneous biotite is slightly younger at 37 40 or 0 18 ma u pb dating of two other rhyolitic quartz feldspar porphyry bodies that host much of the ore at zaldivar shows that these rocks are paleozoic in age antigua porphyry 290 or 4 ma zaldivar porphyry broadly coeval with antigua but age poorly defined at the escondida deposit the reportedly synmineralization escondida porphyry is dated by u pb at 37 9 or 1 1 ma whereas the reportedly postmineralization rhyolitic porphyry returned a u pb age of 34 7 or 1 7 ma these results indicate a single pulse of synmineralization magmatism in the escondida district at approximately 38 ma followed by minor largely nonmineralizing intrusive events the timing of main stage porphyry mineralization at escondida is apparently bracketed by the ages of the escondida 37 9 or 1 1 ma and rhyolitic porphyries 34 7 or 1 7 ma previously published k ar dates for intrusive rocks and alteration minerals at escondida which range from 39 to 31 ma are interpreted to reflect variable degrees of disturbance of the k ar system by later intrusive events and or by supergene weathering processes an age of approximately 38 ma for ore forming porphyry magmatism in the escondida region is significantly older than previously accepted best estimates based on k ar and u pb dating 32 34 ma but still falls within the age range of other large porphyry systems within the north south trending west fissure zone of northern chile 31 42 ma a common feature of the porphyry intrusions at zaldivar and escondida and also at chuquicamata and el salvador is the presence of zircons with inherited upper paleozoic cores these cores are interpreted to reflect interaction between arc magmas and crustal rocks during melting assimilation storage and homogenization mash and assimilation and fractional crystallization afc processes at various crustal levels

upper cenozoic volcanism and tectonism in the andes of northern chile abstract recent geological and radiometric information of the upper cenozoic volcanic sequences of northern chile in conjunction with previous evidences point out the complex evolution of the central andes volcanism volcanic activity developed along the andean range during the last 23 m y appears to have been temporally and genetically related to tectonic movements that shaped the major morphostructural features of this region since the middle miocene 12 m y there has been a constant overlap both in time and space between lavas and ignimbrites however ignimbrite eruptions seem to predominate up to the upper miocene while lava eruptions are dominant from the pliocene to the quaternary small scale chronological and volumetric variations of the volcanic activity in the studied areas make difficult the correlations with volcanic episodes through long distances units older than pliocene have been folded and thrusted as a result of compressive movements that occurred ca 7 4 m y ago this deformation is mostly remarkable at the borders of major faulted blocks these blocks were structured along pre existing fractures caused by tectonic movements during the lower miocene pliocene and quaternary block movements developed mainly through n s fault systems these faults together with nw se and ne sw systems control the spatial distribution of most volcanic vents in this region locally the n s faults may correspond to reactivations of previous fault zones this allowed successive effusions of volcanic material at least during the last 17 m y

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

pure and simple shear plateau uplift altiplano puna argentina and bolivia abstract the concepts of pure and simple shear strain provide a meaningful way of discussing the alternative modes of lithospheric shortening which produced uplift of the central andean plateau since 10 ma discrete segments of the plateau exemplify pure or simple shear modes characteristics of the simple shear mode are a flanking thin skinned thrust belt and subdued relief and lack of significant late miocene to recent deformation within the plateau the pure shear mode is characterized by more rugged internal relief and active neotectonic deformation within the plateau and thick skinned foreland deformation involving seismic deformation of the crust to 30 km or more these modes of crustal thickening correlate with changes in lithosphere thickness modes of isostatic compensation broad wavelength topography and back arc magmatism the correlation of the boundary between pure and simple shear segments with major changes in pre existing anisotropies in the foreland of the orogen suggests that events which long pre date the andean orogeny indirectly controlled the along strike variation in the magnitude and style of shortening in the central andes and may have influenced the development of flat subduction

dubious case for slab melting in the northern volcanic zone of the andes it was first suggested by r w kay that adakites may represent melts of subducted slab since then the term adakite has become synonymous with slab melts based on their unusual geochemical signature this contribution using the northern volcanic zone of the andes as an example aims 1 to expose the weakness in simply associating a geochemical signature with a genetic mechanism and 2 to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting we conclude that although slab melting probably occurs in some arcs regional geochemical trends in the northern volcanic zone and their relationship to the subduction zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge derived basaltic magmas

giant evaporite belts of the neogene central andes large volumes of continental evaporites accumulated within the central andes during neogene uplift of the altiplano puna plateau and development of the andean volcanic arc halite and gypsum are dominant minerals along with local and economically important borates playa conditions have existed since ca 15 ma halite and borate deposition has occurred for the past 7 to 8 m y evaporites formed in salar environments e g playa lakes and are characterized by complex mineral assemblages occurrence zonation and geochemistry evaporite deposition was controlled by volcanism geothermal activity closed drainage and climate these andean deposits and their controls differ from evaporites in other continental and marine environments

the formation of high grade ignimbrites i experiments on high and low concentration transport systems containing sticky particles high grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature or even solidus temperature corresponding pyroclastic flow particles range from plastic to partially liquid and are able to aggregate or coalesce this contrasts with particles in pyroclastic flows producing unwelded ignimbrite which are capable of elastic grain interactions the low aspect ratio and great areal extent of high grade ignimbrites requires transport in a particulate state either by a high concentration mass flow facilitated by fluidizing gas reducing internal friction or by b expanded turbulent flow of low but downward increasing concentration this paper presents experiments designed to investigate the effects of plastic to liquid particles on these two contrasting transport mechanisms gas fluidization experiments using polyethyleneglycole peg powders heated above minimum sintering tms and melting tm temperatures cover a wide range of fluidization velocities umf ua 0 6 ut but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash where particle volume concentration outside the bubbles is high 10 1 when the powders reach a critical temperature tm t tms defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized beds as well as in experiments using mixtures of high and low tm 30 wt peg powders when t tms of the lower tm powder this indicates that extended particulate transport at t tms is not possible at such high particle concentrations in the turbulent flow experiments liquid sprays of molten peg or water vertically injected into a high re 104 horizontal air flow form a low concentration 10 5 to 10 4 turbulent suspension current proximal formation of partially coalesced aggregates which settle faster than individual particles causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid particle sedimentation from turbulent suspensions as particles become finer downstream and coalescence efficiency decreases in response to cooling more distally formed aggregates become too small and rare to modify sedimentation rate decay from that of suspension flows containing solid particles the key difference between the two transport systems is particle concentration c since particle collision rate rcoll c2 collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency 1 ecoal 0 is larger than 10 3 low concentration suspensions on the other hand require much higher values of ecoal for significant aggregation to occur dilute pyroclastic flows will have higher particle volume fractions 10 3 than the experimental currents but then viscous pyroclasts should have lower coalescence efficiencies than peg droplets experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high grade ignimbrite sheets

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

response of intracontinental deformation in the central andes to late cenozoic reorganization of south american plate motions new stratigraphic chronologic and fault kinematic data from the quebrada del toro in northwestern argentina provide a detailed view of late cenozoic kinematic reorganization in the southern central andes the quebrada del toro is a thrust bounded basin filled with 2 km of synorogenic clastic deposits ranging in age from early miocene to holocene as observed elsewhere in northwestern argentina faults in the quebrada del toro record an early phase of horizontal nw se contraction beginning between early and late miocene and ending after 0 98 ma and a later phase of horizontal ne sw contraction beginning between late miocene and 4 17 ma and still active these results require both kinematic regimes to have been active between 4 17 and 0 98 ma compatible with timing and kinematic data from the adjacent central andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes a hypothesis that intraplate deformation in the central andes is driven by the absolute motion of the south american plate with respect to the hotspot reference frame is tested by comparing quebrada del toro kinematics to plate reconstructions and space based geodetic results nazca south america relative motion and absolute nazca plate motion show consistent azimuths throughout the past 20 5 myr in contrast absolute south american plate motion azimuths mimic the changing contraction directions determined from intraplate andean faults although the timing of the south american plate motion change is poorly resolved by current data it is compatible with the quebrada del toro data absolute upper plate motion might control intraplate kinematics along many of the earth s convergent margins

using unsupported lead 210 measurements to investigate soil erosion and sediment delivery in a small zambian catchment abstract traditional techniques used to assemble information on rates of erosion and soil redistribution possess many important limitations as a result the use of environmental radionuclides and more particularly 137 cs measurements has attracted increasing attention in recent years as a means of obtaining spatially distributed information on rates of erosion and deposition the application of the 137 cs approach is however hampered in some areas of the world where 137 cs inventories are low and the low concentrations of 137 cs found in soils and sediments cause problems for laboratory analysis these problems will increase as time progresses due to the radioactive decay of the existing inventory most of which was deposited as fallout ca 40 years ago this contribution explores the potential for using another fallout radionuclide namely unsupported 210 pb as an alternative to 137 cs in the small 63 km 2 upper kaleya catchment in southern zambia where 137 cs inventories are already very low the approach employed with unsupported 210 pb is similar to that used for 137 cs although the essentially constant fallout of unsupported 210 pb through time means that the resulting estimates of erosion and soil redistribution rates reflect a longer period of time ca 100 years rather than ca 40 years the estimates of erosion and deposition rates derived from the unsupported 210 pb measurements are used to construct typical sediment budgets for the three main land use types in the upper kaleya catchment namely commercial cultivation communal cultivation and bush grazing the results obtained from the unsupported 210 pb are compared with equivalent results based on 137 cs measurements provided by a previous investigation undertaken in the study catchment the two sets of results are highly consistent the study reported confirms the viability of using unsupported 210 pb as an alternative to 137 cs in this environment and demonstrates that conjunctive use of both radionuclides can provide additional information on the erosional history of a study area