intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

a magnetotelluric study in the southern chilean andes long period magnetotelluric investigations were carried out along two transects traversing the southern chilean andes from the pacific ocean until the argentinian border near latitude 39 s to study the mid and deep crustal distribution of electrical conductivity dimensionality analysis showed that the mt data may be regarded as 2 d with the exception of areas in the longitudinal valley and the region east of the volcanic chain a subsequent multi site decomposition of magnetotelluric impedances yielded approx n s regional strike directions for most parts of the two profiles first 2 d inversion revealed a zone of moderately high conductivities approx 0 1 s m in the deep crust below the active volcanic arc extending eastwards to the argentinian border inversion results of both profiles are very similar except of a good mid crustal conductor in the longitudinal valley which only appears in the north and may image a fault zone running obliquely to the main south andean structures

seismotectonics of the easternmost transverse ranges california relevance for seismic potential of the southern san andreas fault earthquake locations depths and focal mechanisms from the southern california regional network 1977 1985 are used to identify the orientation and sense of slip of active subsurface faults in the easternmost transverse ranges etr the etr are separated from the salton trough province by the southernmost strands of the san andreas fault saf much of the seismicity in the etr is concentrated well northeast of the saf at relatively shallow depths under the little san bernardino mountains many of these earthquakes reflect slip on steeply dipping left lateral faults striking northeast to east at relatively high angles to the adjacent saf focal mechanisms in the etr show predominantly strike slip normal or oblique normal faulting and share common near horizontal t axes striking wnw p axes range from near vertical to near horizontal and strike mostly nne in contrast reverse and strike slip focal mechanisms that exhibit persistent north trending near horizontal p axes characterize the san gorgonio pass area immediately to the west these different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the saf this boundary which in the coachella valley may be the mission creek fault is also distinguished by abrupt changes in 1 rate and depth of seismic activity 2 topography 3 quaternary vertical deformation 4 strikes and dips of major branches of the saf and 5 seismic velocities in the crust and upper mantle the preponderance of secondary normal faulting in the etr versus secondary reverse faulting in the san gorgonio pass region suggests that fault normal stress is much less across the saf adjacent to the etr if a friction law where strength is proportional to normal stress applies to the saf then a smaller tectonic shear stress would be required tor slip in large earthquakes along the salton trough segment in this case the southernmost saf may have shorter repeat times smaller average displacements and lower moment release rates than earthquakes that rupture through the major restraining bend in san gorgonio pass

crease structures indicators of emplacement rates and surface stress regimes of lava flows crease structures are features commonly found on lava flow surfaces and consist of a fracture with curved walls that extend outward from a linear valley these crease structures are found on flows of nearly all compositions and crystallinities we have mapped the distributions of crease structures on many flows in the western united states and found that 1 axial length is not dependent upon composition and crystallinity 2 adjacent crease structures are generally aligned in an en echelon pattern 3 crease structures located adjacent to flow margins are generally perpendicular to these margins and 4 at mount st helens washington large lobe bisecting crease structures are found on lobes situated on slopes of less than 20 degrees a primary surface feature found on many crease structures is striations striations are sets of long stripes on the walls of the crease structure that extend approximately parallel to the axis of the central valley and they appear to be analogous to those found on the faces of cyclically fractured basalt columns observations of developing crease structures on 6 of the nearly 20 mount st helens dome lobes show that they form throughout the extrusion of flows situated on slopes of less than 20 degrees but only at the very beginning and or end of extrusion of flows on steeper slopes these observations imply that crease structures form when the lava flow is forced to spread laterally either as the flow advances over a flat area or as the down slope movement stagnates near the end of extrusion causing the rate of spreading to exceed the downslope rate of flow this lateral spreading of lava results in the concentration of tensile stress along a line oriented perpendicular to the direction of spreading the cooled crust of the extrusion is therefore torn apart about this line of tensile stress concentration forming a central valley that exposes hot ductile material from the flow interior to the atmosphere the presence of striations on many crease structure walls implies that the emplacement mechanism is similar to that suggested for columnar basalts where each striation is produced by incremental fracturing the stress responsible for crease structure formation is due to both cooling and lateral spreading of lava calculations show that stress due to spreading is three orders of magnitude higher than that due to cooling fracture advance times were calculated using a simple one dimensional conductive cooling model adjusted for the special geometry of the crease structure the model closely approximates measured crease structure emplacement times at mount st helens and gives reasonable estimates for crease structures found on older silicic lava flows we then used this model to calculate emplacement times of entire domes that consist of a single crease structure by dividing the dome volume by the formation time we are able to calculate extrusion rates for these older domes these values which include the first flow rate estimates reported for rhyolitic lavas 0 03 106 m 3 s similar to average flow rates of 0 7 40 m 3 s calculated for mount st helens dacite lobes from detailed topographic maps

lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots the fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material it has been proposed either that 1 secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or 2 that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly a parameterized method of modeling transient secondary convection is developed to examine this question calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity in addition vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like cape verde this conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the hawaiian swell

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

volcanism and erosion during the past 930 k y at the tatara san pedro complex chilean andes geologic mapping together with 73 new k ar and 40ar 39ar age determinations of 45 samples from 17 different volcanic units plus paleomagnetic orientations geochemical compositions and terrestrial photogrammetry are used to define the chronostratigraphy of the tatara san pedro complex an eruptive center at 36 s on the volcanic front of the andean southern volcanic zone the tatara san pedro complex preserves 55 km3 of lavas that erupted from at least three central vent regions remnant unconformity bound sequences of lavas are separated by lacunae that include significant periods of erosion quaternary volcanism commenced ca 930 ka with eruption of voluminous dacitic magma followed 100 k y later by the only major rhyolitic eruption from 780 ka onward more than 80 of the preserved volume is basaltic andesite 52 57 sio2 but petrographically and geochemically diverse dacitic magmas 63 69 sio2 erupted sporadically throughout this younger dominantly mafic phase of activity a few basaltic lavas 49 52 sio2 are present mainly in portions of the complex older than 230 ka the number of vents the petrologic and geochemical diversity and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the tatara san pedro complex over the past million years nearly two thirds of the preserved volume of the tatara san pedro complex comprises the two youngest volcanoes which were active between ca 188 83 ka and 90 19 ka repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre 200 ka volcanic record particularly on the south flank of the complex dating the inception of a glaciation on the basis of preserved material is difficult but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation on this basis the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca 830 790 610 400 330 230 110 and 17 ka these are broadly consistent with global ice volume peaks predicted by the oxygen isotope based astronomical time scale and with age brackets on north american glacial advances estimated growth rates for the two young volcanoes are 0 2 to 0 3 km3 k y these are three to five times greater than a growth rate estimated from all preserved lavas in the complex 0 06 km3 k y removal of up to 50 95 of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy and it raises the possibility that episodic erosion of mid latitude frontal arc complexes may be extensive and common

morpho structural evolution of the cordon caulle geothermal region southern volcanic zone chile insights from gravity and 40ar 39ar dating the cordon caulle geothermal region 40 5 s is a 13 km 6 km nw se elongate volcano tectonic depression hosting numerous monogenetic volcanic centers fumaroles and hot springs most of which occur spatially associated with the edges of the depression volcanism started at approximately 0 3 ma with dominantly mafic effusions and expressed ultimately in post glacial and historic dacite rhyolite eruptions 1921 1922 and 1960 three ne trending gravimetric profiles were carried out across the depression of cordon caulle yielding a residual negative anomaly with the greatest amplitude 20 mgal at the center of the depression a 2d gravity model given by a 490 m thick shallow body with a density contrast 650 kg m3 interpreted as a shallow package of dacite rhyolite lavas and tephra was found to match the residual anomaly a dense rock equivalent volume of felsic material on the order of 28 km3 25 of the total output of cordon caulle is derived from this thickness previous workers explain the origin of felsic rocks at cordon caulle in terms of the orientation of the borders of cordon caulle relative to the maximum stress direction 1 i e ne trending prevailing during the quaternary according to which nw trending domains were compressional favoring stagnation storage and differentiation of magmas the dominantly monogenetic style of volcanism the plateau like morphology and the negative bouguer anomaly on the contrary are interpreted as an indication of extensional tectonics intra arc extension is proposed to be related to coseismic or postseismic stress relaxation accompanying major thrust events such as the great 1960 chilean earthquake which is consistent at cordon caulle with the direction of propagation of dikes of the 1960 eruption i e coseismic extension and present day subsidence inferred from insar data i e postseismic extension prior to the quaternary 8 2 1 6 ma nearly east west compression prevailed at the intra arc zone a 5 44 0 04 ma old granodiorite intrusion bracketed by two sub parallel branches of the regional margin parallel liquine ofqui fault zone lofz in a prominent pop up structure 15 km to the east of cordon caulle accounts for reverse reactivation of the lofz and rapid fault related exhumation of basement rocks 1 mm year we argue that under compression reverse reactivation of the lofz was facilitated by the combined effect of cyclic increase in fluid pressure induced by magmatic hydrothermal input and removal of lithostatic load induced by glacial erosion 1 5 mm year in the vicinity of cordon caulle

relative and radiocarbon chronology of two former glaciers in the chilean lake district this paper investigates the relative importance of climatic and topographic factors on the fluctuations of two adjacent palaeoglaciers in the chilean lake district geomorphological mapping of the landforms around two lakes occupied by the palaeoglaciers has identified a series of intersecting moraine limits and ice marginal meltwater channels which allow the relative timing of glacier fluctuations to be established the broad pattern of advance and retreat is the same in the two basins with at least one synchronous major advance sometime after 19 500 yr bp but there is also evidence of discordant behaviour with one glacier advancing less often and lagging the other glacier during retreat seventeen radiocarbon dates suggest a similar chronology to other palaeoglaciers in the chilean lake district but the advance after 19 500 yr bp may have been 1500 14c yr later than major advances of palaeoglaciers situated immediately south the empirical evidence of differential behaviour can be simulated by a glaciological model which suggests that the differences in glacier response are due to contrasts in basin topography contrasts of this magnitude between the timing of some glacier advances has important implications for regional and interhemispheric correlation of chilean lake district glacier chronologies to other climate proxy records 1997 by john wiley sons ltd

recent volcanism in the puyehue cordon caulle region southern andes chile 40 5 s petrogenesis of evolved lavas

the southern andes between 36 and 40 s latitude seismicity and average seismic velocities abstract the project issa 2000 integrated seismological experiment in the southern andes consists of a temporary seismological network and a seismic refraction profile a network of 62 seismological stations was deployed across the southern andes at 38 s three hundred thirty three local seismic events were observed in a 3 month period p and s arrival times of a subset of high quality data were inverted simultaneously for 1 d velocity structure hypocentral coordinates and station delays seismic refraction data along a transect at 39 s provide further constraints on the crustal structure low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere the continental moho is not clearly observed in this region average velocities of the crust beneath the arc are higher than those beneath the forearc crustal thickness is about 40 km crustal seismicity concentrates in the forearc region along the bio bio and gastre fault zones the area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the benioff zone in the southern andes a maximum of seismicity at 60 km depth may be caused by dehydration embrittlement

pleistocene glaciation in the southern lake district of chile relative age criteria permit deposits of successive andean glacier advances in the southern lake district of chile to be divided into four mappable drift sheets the oldest two of which overlie tertiary bedrock along the eastern flank of the cordillera de la costa only the youngest drift llanquihue is datable by radiocarbon during the most extensive ice advance of the last glaciation the lago llanquihue glacier was about 95 km long and reached an estimated maximum thickness of between 1000 and 1300 m glacier equilibrium lines at that time lay about 1000 m below their present level and rose eastward with a gradient of about 5 m km successive ice advances in the lago llanquihue basin which resulted in construction of end moraines and associated outwash plains beyond the lake margin culminated sometime before about 20 000 yr ago and between 20 000 and 19 000 yr ago a later readvance inferred from the sedimentary record of lake level fluctuations in the basin had begun by about 15 000 yr ago and culminated shortly after 13 000 yr ago a comparable but less closely dated record of ice advances is found northwest of seno reloncavi and on isla chiloe deglaciation following the latest advance is likely to have been rapid for the major glacier lobes fronted on deep water bodies that would have promoted extensive calving

ar ar ages from transitionally magnetized lavas on la palma canary islands and the geomagnetic instability timescale 1 a detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of barranco de los tilos on the island of la palma canary islands reveals a complex temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events the matuyama brunhes m b reversal is recorded in five transitionally magnetized lava flows from the north tn section the isochrons obtained from three of the lower four m b lavas are defined by 14 incremental heating experiments that together with a previous age determination yielded a weighted mean of 798 4 6 2 ka all uncertainties 2s in addition a 780 3 10 3 ka isochron was determined for the overlying transitionally magnetized flow indicating that it was erupted during a distinctly younger portion of the transition near the base of the south ts section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole vgp positions in the southwest indian ocean between latitudes 56 s and 65 s suggesting instability of the geomagnetic field beyond that of typical secular variation 40 ar 39 ar isochrons from three of these flows defined by 11 separate incremental heating experiments gave a weighted mean of 822 2 8 7 ka this anomalous field behavior recorded 24 11 kyr prior to the m b reversal may coincide with an event featured in several marine sediment records directly above two normal polarity flows 40 ar 39 ar isochrons of 751 9 18 1 ka and 675 0 15 7 ka are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude vgps spanning 23 60 n these flows are then capped by a single flow possessing normal polarity based on 12 incremental heating experiments 40 ar 39 ar isochrons of five of these nine lavas along with the uppermost flow gave a weighted mean age of 580 2 7 8 ka for this period of transitional to normal field behavior from these same transitional lavas quidelleur et al 1999 reported three unspiked k ar ages with a weighted mean of 602 24 ka and proposed a new event called the la palma excursion however the 40 ar 39 ar age presented here is three times more precise than the k ar age and is indistinguishable at the 95 confidence level from the 40 ar 39 ar age of a lava from the snake river plain idaho that originally defined the big lost event transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the big lost event recorded at la palma was indeed global in extent rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process index terms 1560 geomagnetism and paleomagnetism time variations secular and long term 1520 geomagnetism and paleomagnetism magnetostratigraphy 1035 geochemistry geochronology 1513 geomagnetism and paleomagnetism geomagnetic excursions keywords argonargon dating lavas excursion paleomagnetism reversal

volcanic mixed avalanches a distinct eruption triggered mass flow process at snow clad volcanoes a generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at mount st helens nevado del ruiz and redoubt volcano between 1982 and 1989 these mixed avalanches traveled as far as 14 km at velocities up to 27 m s involved as much as 10 7 m 3 of rock and ice and left unmelted deposits of single flow units as thick as 5 m during flow downslope heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures apparently giving these mixed avalanches mobilities equal to or greater than those of dry debris avalanches of similar volume the avalanches studied for this report began as snow avalanches triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris erosion by the avalanches incorporated additional snow fragments of glacier ice and other rock debris mixed avalanche deposits were massive very poorly sorted and ungraded to inversely graded prior to melting although rock debris composed as much as 70 wt of the frozen samples the bulk volumetric content of rock debris was only 9 to 36 snow and ice composed between 36 and 72 of the frozen samples with void space making up the remainder of the volume deformation and consolidation after melting of the ice content reduced the deposits to loose porous layers that were only a fraction of their original thickness and that later gave little hint of their origin after melting and desiccation the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record

eruptive history geochronology and magmatic evolution of the puyehue cordon caulle volcanic complex chile forty three 40 ar 39 ar age determinations of lava flows domes ignimbrites and dikes plus 14 c dates from seven distal tephra layers combined with stratigraphy geochemistry and sr and th isotope data establish an eruptive chronology for the puyehue cordon caulle volcanic complex at 40 5 s in the andean southern volcanic zone svz the complex preserves 131 km 3 of lava and tephra that erupted from numerous vents widely separated in time and space approximately 80 of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka the modern puyehue stratovolcano was built on the southerly shield during the past 69 k y following a hiatus of 25 k y puyehue has erupted 15 km 3 of basaltic to rhyolitic magma that spans the entire compositional range found in the southern svz and evolved via at least six phases including 1 basaltic andesitic to dacitic lavas between 69 and 32 ka 2 a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of mgo rich basaltic andesite at 34 ka 3 dacitic to rhyolitic flows and domes from 19 to 12 ka 4 basaltic to basaltic andesitic flows between 15 and 12 ka 5 subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka followed by 6 a powerful series of phreatomagmatic and sub plinian eruptions at ca 1 1 ka that obliterated the preceding rhyolite domes and formed the present 2 5 km diameter 280 m deep summit crater along the cordon caulle fissure zone 5 km 3 of rhyodacitic to rhyolitic lavas domes and cones have formed during the past 16 5 k y including explosive and effusive eruptions in 1921 1922 and 1960 eruptive rates were nonuniform over time with background growth at 0 04 km 3 k y or less punctuated by spurts at up to 0 90 km 3 k y the time averaged rate 0 42 km 3 k y is nearly double that at the tatara san pedro complex 500 km to the north during the past 300 k y these findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable the last three stratocone building events on puyehue began during periods of deglaciation suggesting a relationship between unloading of ice and ease of magma ascent puyehue basalt exhibits subtle changes in 238 u 230 th 87 sr 86 sr and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge or the extent to which basalt was modified by assimilation of heterogeneous crustal melts the complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt despite the high flux of basalt during the past 300 k y no large silicic magma reservoir formed in the upper crust instead 238 u 230 th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust promoted perhaps by conduits that reflect strike slip faulting beneath the complex

uranium and 10 be enrichments by fluids in andean arc magmas volcanic rocks from subduction settings frequently contain 10be enrichments that have been attributed to sediment subduction1 6 and are often characterized by 238u 230th ratios greater than unity in contrast to mid ocean ridge and ocean island basalts7 8 here we report such an excess of 238u over 230th in historic lavas from the southern volcanic zone of chile 33 42 s the greatest uranium excess is found south of 39 s where 10be 9 be ratios are highest and the trench is filled with sediment these features suggest that both u and 10be enrichments result from preferential partitioning of u and be into a fluid phase during dehydration of the recently subducted sediments the presence of strong 238u 230th disequilibria in basaltic magmas of the southern volcanic zone suggests that the timescale for dehydration melting and eruption in this arc is probably less than 20 000 yr

temporal evolution of sediment supply in lago puyehue southern chile during the last 600 yr and its climatic significance short term climate changes in southern chile are investigated by a multi proxy analysis of a 53 cm long sedimentary sequence selected among eight short cores retrieved in lago puyehue chile 40 s this core contains a 600 yr long undisturbed record of paleo precipitation changes two measurement methods for sediment density organic matter and biogenic silica contents are compared and the most appropriate techniques are selected together with aluminium and titanium concentrations grain size and geochemical properties of the organic matter these proxies are used to demonstrate paleo precipitation changes around 40 s increase of terrigenous particle supply between a d 1490 and a d 1700 suggests a humid period contemporaneously y 13 c data show increasing lake productivity in response to the high nutrient supply the a d 1700 1900 interval is characterized by a decreasing terrigenous supply and increasing y 13 c values interpreted as a drying period the magnetic susceptibility signal reflecting the terrigenous biogenic ratio demonstrates that similar variations occur in all the undisturbed sedimentary environments of lago puyehue the a d 1490 1700 wet period is associated with the onset of the european little ice age lia andinterpreted asits local signature this worksupports the fact that the liawasaglobal event not only restricted to thenorthern hemisphere d 2005 university of washington all rights reserved

bromine as indicator of source of lacustrine sedimentary organic matter in paleolimnological studies bromine br to organic matter om concentration ratio is studied in lake sediment sequences to provide information on environmental changes modifying om production the sequences studied were extracted from shallow lakes morenito el trebol escondido and portezuelo and deep lakes futalaufquen moreno and traful north patagonia andean range lake morenito a former lake moreno bay until its closure in ad 1960 showed a decrease in br om ratios from 1 38 to 0 74 after lake closure associated with an increase of primary autochthonous productivity attributable to the development of submerged and emerging macrophytes sedimentary sequences from lakes escondido portezuelo and el trebol with large participation of macrophytes in primary productivity and from lakes moreno futalaufquen and traful with little development of littoral macrophytes showed br om ratios consistent with the lake morenito pattern consistently the morphometric parameters mean depth and shoreline development correlate with br om ratios therefore br om ratios can be associated with the composition of primary autochthonous productivity with values of about 0 7 associated to significant macrophyte contributions and higher values associated with more pelagic contributions accordingly br om variations along a sedimentary sequence can be associated with modifications on the composition of the primary autochthonous productivity of the water body providing information on environmental changes

co eruptive subsidence and post eruptive uplift associated with the 2011 2012 eruption of puyehue cordon caulle chile revealed by dinsar abstract the 2011 2012 eruption of the puyehue cordon caulle volcanic complex southern andes chile was associated with complex surface deformation affecting an area of roughly 50 by 50 km we report here differential sar interferometry dinsar results of pre co and post eruptive deformation from envisat asar cosmo skymed and alos 2 palsar scenes acquired between early 2011 and early 2017 no clear pre eruptive deformation is observed during five months before the eruption although some patterns could be interpreted as showing inflation occurring between april and may 2011 co eruptive interferograms show a complex deformation pattern consisting in a major deflation lobe 120 cm los lengthening centered 10 km nw of the eruption vent accompanied by smaller uplift and subsidence regions in the vicinity of the vent re inflation began immediately after the end of the eruption a first pulse lasted 3 years between 2012 and 2015 accumulating 70 cm uplift we detect here a second pulse beginning in june 2016 and still ongoing in february 2017 reaching 12 cm in half a year inverse modeling with spherical cavity and spheroidal sources locates re inflation sources at a depth ranging between 8 and 11 km under the surface it suggests re filling of the reservoir occurring after the draining of a shallow magma chamber during the 2011 2012 eruption

crustal sources involved in continental arc magmatism a case study of volcan mocho choshuenco southern chile volcan mocho choshuenco a pleistocene holocene basaltic andesite to dacite shield volcano located at lat 40 s in the southern volcanic zone svz of the andes is built on the thinnest svz continental crust 30 km which consists largely of paleozoic sedimentary and metamorphic rocks and mesozoic plutons sr pb nd and o isotopic ratios of mocho choshuenco lavas 87 sr 86 sr 0 70399 to 0 70422 206 pb 204 pb 18 57 to 18 61 207 pb 204 pb 15 60 to 15 63 208 pb 204 pb 38 45 to 38 57 143 nd 144 nd 0 512787 to 0 512881 18 o 5 8 to 6 8 standard mean ocean water are similar to those of other svz centers and exhibit no correlation with indices of differentiation however incompatible trace element systematics indicate that evolved lavas have been contaminated by crustal plutonic rocks by 5 to 15 rb ba k ba rb la and k la ratios increase with increasing sio 2 content as a result of assimilation of orogenic calc alkalic plutonic rocks enriched in rb and k relative to ba and la xenoliths of plutonic rocks recovered from mocho choshuenco lavas are slightly more radiogenic in 87 sr 86 sr less radiogenic in 143 nd 144 nd and comparable in pb isotopic ratios and they have variable 18 o contents 87 sr 86 sr 0 70482 to 0 70557 206 pb 204 pb 18 60 to 18 64 207 pb 204 pb 15 60 to 15 62 208 pb 204 pb 38 53 to 38 56 143 nd 144 nd 0 512686 18 o 3 3 to 5 5 smow contamination of mocho choshuenco parental magmas by crustal melts with small isotopic contrast imposed little variation on isotopic heterogeneities existing in the parental compositions these heterogeneities probably were produced during magma crust interaction at the crust mantle boundary or in the lower crust

rapid reinflation following the 2011 2012 rhyodacite eruption at cordon caulle volcano southern andes imaged by insar evidence for magma reservoir refill cordon caulle is a large fissural volcano that has erupted rhyodacitic magma of the same composition in its past three historical eruptions in 1921 1960 and 2011 2012 there was significant ground deformation observed before and during the 2011 2012 eruption here we use c and x band insar time series results to document post eruptive uplift up to 0 8 m between march 2012 and may 2015 with los rates up to 45 cm yr that have been largely aseismic along with subsidence in the 2011 2012 lava flow the 2012 uplift rate is one of the largest for silicic systems and was likely produced by the intrusion of 0 125 km3 of magma in the same tectonically controlled plumbing system that has been active during the historical eruptions nevertheless the uplift ended before the reservoir refilled with the erupted volume maybe due to a change in the pressure gradient produced by the 2011 2012 eruption

structural controls of volcanism in transversal chains resheared faults and neotectonics in the cordon caulle puyehue area 40 5 s southern andes abstract the nature of the relationship between tectonics and volcanism in a specific area is the result of the interaction between the crustal structures of the basement and ongoing regional stress field in the southern andes different kinematic environments represented by two nearly orthogonal active fissure systems show apparently incompatible arrays of volcanic chains moreover a local quaternary compressive regime is inferred from basement faults however these mutually orthogonal chains which are also oblique to the volcanic front have contrasting magma evolution and eruptive styles that can be related with their tectonic setting whereas northeast trending monogenetic cones carran los venados can be interpreted as resulting from magma upwelling along regional tension cracks in a quaternary dextral transpressive compressive domain northwest striking fissure system cordon caulle volcanic complex can be the result of extensional fault reactivation and long magma residence in the upper crust different kinematic environments cause contrasting magma evolution and eruptive styles a two way coupling mechanism is suggested for to explain local depart of the regional stress field geometric array of extensional features and final magmatic evolution in these volcanic domains of the southern andes

geochemical variations in andean basaltic and silicic lavas from the villarrica lanin volcanic chain 39 5 s an evaluation of source heterogeneity fractional crystallization and crustal assimilation at 39 5 s in the southern volcanic zone of the andes three pleistocene recent stratovolcanoes villarrica quetrupillan and lanin form a trend perpendicular to the strike of the andes 275 to 325 km from the peru chile trench basalts from villarrica and lanin are geochemically distinct the latter have higher incompatible element abundances and la sm but lower ba la and alkali metal la ratios these differences are consistent with our previously proposed models involving a a west to east decrease in an alkali metal rich high ba la slab derived component which causes an across strike decrease in degree of melting or b a west to east increase in the contamination of subduction related magma by enriched subcontinental lithospheric mantle silicic and mafic lavas from the stratovolcanoes have overlapping sr nd and o isotopic ratios silicic lavas also have geochemical differences that parallel those of their associated basalts e g rhyolite from villarrica has lower la sm and incompatible element contents than high sio2 andesite from lanin at each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing apparently crustal contamination was not an important process in deriving the evolved lavas basaltic flows from small scoria cones 20 35 km from villarrica volcano have high incompatible element contents and low ba la like lanin basalts but trend to higher k rb 356 855 and lower 87sr 86sr 0 70361 0 70400 than basalts from either stratovolcano however all basalts have similar nd pb and o isotope ratios the best explanation for the unique features of the cones is that the sources of svz magmas e g slab derived fluids or melts of the subcontinental lithospheric mantle have varying alkali metal and radiogenic sr contents these heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing this model is preferable to one involving crustal contamination because it can account for variable sr isotope ratios and uniform nd and pb isotope ratios among the basalts and the divergence of the cones from across strike geochemical trends defined by the stratovolcanoes

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

eruptive stratigraphy of the tatara san pedro complex 36 s southern volcanic zone chilean andes reconstruction method and implications for magma evolution at long lived arc volcanic centers

lava and ice interaction at stratovolcanoes use of characteristic features to determine past glacial extents and future volcanic hazards structures resulting from lava and ice interaction are common at glaciated stratovolcanoes during summit eruptions at stratovolcanoes meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers eroding the ice and enlarging preexisting glacial drainages as a result eruptions in this environment have produced few catastrophic floods lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker filling the available space and producing steep sided bodies with smooth bulbous contact surfaces quenching of lava against ice or by water forms small scale features such as tensional fractures and glass as the amount of meltwater in contact with the lava increases the type and abundance of smaller scale features become similar to those produced during subglacial eruptions into meltwater lakes identification of large and small scale lava ice contact features in the field can be used to reconstruct paleoglacial extent and combined with geochronology of lavas to determine past paleoclimate an understanding of lava ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes

magmatic evolution of the puyehue cordon caulle volcanic complex 40 s southern andean volcanic zone from shield to unusual rhyolitic fissure volcanism abstract magmas erupted from quaternary volcanoes of southern andes between 37 and 46 s latitude are mainly basaltic to andesitic however pccvc 40 s shows a singular magmatic evolution due to the abnormal evacuation of rhyolites especially in the last 100 ka in addition pccvc is the result of juxtaposing products from the nw trending alignment of cordillera nevada caldera cordon caulle fissure volcano and the puyehue stratocone using 40 ar 39 ar and 14 c geochronology it can be established that they evolved since ca 500 ka as coeval but separated vents with a first stage of shield volcanism followed by repeated collapses that formed an internal nw elongated graben from ca 100 ka volcanic activity occurred in both a fissure system cordon caulle and a central volcano puyehue holocene explosive eruptions mainly in the puyehue crater accompanied the dome growing along a nw trending fissure system last historical eruptions were in 1921 and 1960 when nw fissures of cordon caulle fed rhyodacitic lava flows in 1960 the fissure eruption was triggered by a remote mw 9 5 thrust earthquake cordillera nevada caldera presents a reduced compositional range 52 63 sio 2 and geochemical features of low pressure magma mixing and assimilation instead cordon caulle and puyehue volcanoes have a wide silica range 48 71 sio 2 and an outstanding affinity which can be modelled with initial high pressure fractional crystallization moderate magma mixing and subsequent low pressure fractional crystallization from a common parental source the exceptional magmatic evolution and eruptive style of pccvc in southern andes could be related with the physics of the plumbing system which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime

calbuco volcano and minor eruptive centers distributed along the liquine ofqui fault zone chile 41 42 s contrasting origin of andesitic and basaltic magma in the southern volcanic zone of the andes calbuco volcano is a late pleistocene holocene composite stratovolcano located at 41 20 s in the southern region of the southern volcanic zone of the andes ssvz 37 46 s in contrast to basalt and basaltic andesite which are the dominant lava types on the volcanic front from 37 to 42 s calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths they have sio2 contents in the 55 60 range and have comparatively low k2o rb ba th and lref abundances relative to other ssvz centers incompatible element abundance ratios are similar to those of most ssvz volcanics but 87sr 86sr and 143nd 144nd are respectively higher and lower than those of adjacent volcanic centers basalts from nearby osorno stratovolcano 25 km to the northeast are similar to other basaltic ssvz volcanoes however basalts from several minor eruptive centers mec located east of calbuco and osorno volcano along the liquine ofqui fault zone lofz are enriched in ba nb th and lree and have higher la yb and lower ba la k la and rb la 87sr 86sr and 143nd 144nd in mec basalts are respectively lower and higher than those of osorno and calbuco lavas we suggest that mec basalts were produced by lower extents of mantle melting than basalts from osorno and other ssvz stratovolcanoes probably as a result of lower water content in the source of mec basalts calbuco andesites formed from basaltic parents similar to osorno basalts by moderate pressure crystallization of a hornblende bearing assemblage accompanied by crustal assimilation hornblende stability in the calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks which are also an appropriate endmember for isotopic trends together with magma storage at mid crustal depths the unique characteristics of calbuco volcano i e the stability of hornblende at andesitic sio2 contents low 143nd 144nd and high 87sr 86sr and abundant crustal xenoliths provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the ssvz