no flat wadati benioff zone in the central and southern central andes abstract the wadati benioff zone wbz is an approximate plane defined by earthquakes hypocentres observed in convergent plate boundaries and that usually dips at angles greater than 30 in some areas of the andes where there are gaps in volcanic activity and where heat flow is abnormally low this plane in most studies has nearly horizontal dip at a depth of about 75 100 km and it has been associated to flat subduction of the oceanic lithosphere this situation has been taken as the present day analogue of the laramide orogeny of western north america for which a flat slab episode has been proposed in the past years in this work the observed low heat flow in areas of the andes is assumed to be due to low radiogenic heat generation in geologically old and allochthonous terranes constituting large regions of western south america on the basis of geotherms obtained for areas of ecuador peru chile and argentina and of rheological results describing the partition between brittle and ductile regimes the seismic activity observed both in the lower crust and at depths of about 75 100 km is thoroughly explained at these depths earthquakes occur within the subcontinental upper mantle and then there is no flat wbz associated to subduction of the oceanic lithosphere there is evidence from recent seismological observations that the real wbz lies not horizontally and deeper in the tectonosphere

lower cambrian paleosols from the cantabrian mountains northern spain a comparison with neogene quaternary estuarine analogues abstract the lower cambrian barrios beds cantabrian mountains consist of a siliciclastic succession deposited in deltaic environments punctuated by intertidal and fluvial systems in the barrios de luna area this succession displays a progradational depositional systems tract that contains linkages between upper offshore tidal sand bars fluvial channels and floodplain environments the uppermost fluvial channels are encased within floodplain or interfluvial deposits some of which develop muddy paleosols the detailed micromorphological scanning electron microscopy sem geochemical and mineralogical studies of their pedogenic profiles indicate the presence of three well differentiated horizons fragmentation and shrinking of the upper horizons originated a network of channel like to vuggy porosity occluded by microlaminated clays cyclically emplaced by tidal floods and seasonal oxidation the middle horizons have concretionary to vermicular textures associated with microbioturbation and bacterial colonization and the lower ones are mottled containing numerous preserved lithorelics the barrios sulfidic ustifluvent paleosols represent short time pedogenic events under semiarid subtropical conditions and support previous studies of cambrian microbial life on subaerially exposed microenvironments

seismic velocity structure of the slab and continental plate in the region of the 1960 valdivia chile slip maximum insights into fluid release and plate coupling the south central chilean subduction zone has witnessed some of the largest earthquakes in history making this region particularly important for understanding plate coupling here we present the results of a local earthquake tomography study from a temporary local seismic network in the villarrica region between 39 and 40 s where the largest coseismic displacement of the 1960 valdivia earthquake occurred a lowvelocity anomaly and high vp vs values occur under the coastal region indicating mantle serpentinisation and or underthrusting of forearc material further east a high velocity anomaly is observed interpreted as normal high velocity mantle under the active volcanic arc a low velocity anomaly together with high vp vs ratios 1 8 and higher likely images fluid ascent beneath the volcanoes close to the subducting valdivia fracture zone the coastal low velocity anomaly extends further inland where it interrupts and shifts the high velocity anomalies associated with normal fast mantle velocities this may indicate enhanced fluid presence along this part of the margin probably caused by a stronger hydration of the incoming plate along the valdivia fracture zone this is consistent with geochemical fluid proxies u th pb ce ba nb in young volcanic rocks displaying peak values along the volcanic front at llaima and villarrica volcanoes and with recent gps measurements which suggested a local reduction in plate coupling in this region the shift in the high velocity anomaly underlying the central part may be caused by a north to south decrease in plate age and hydration across the valdivia fracture zone and may explain why a central valley is absent in this segment of the margin the low la yb ratios in the volcanic rocks from villarrica and llaima suggest that the high slab derived fluid flux causes elevated degrees of melting beneath these volcanoes providing an explanation as to why these are amongst the most active volcanoes in south america

electromagnetic study of the active continental margin in northern chile magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern chile between 19 5 and 22 s to study the electrical conductivity structures of this active continental margin the instruments used covered a very broad period range from 10 4 s to approx 2 104 s and thus enabled a resolution of deep as well as shallow structures in this paper we focus on the interpretation of data from an east west profile crossing chile from the pacific coast to the western cordillera at 20 5 s a decomposition of the impedance tensors using the groom bailey decomposition scheme shows that a two dimensional interpretation is possible the resulting regional strike direction is n9 w two dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the andes the final model shows a generally high resistivity in the forearc and a very good conductor below the precordillera unlike earlier models from areas further south a good conductor is not observed below the magmatic arc itself this correlates with the so called pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas

crustal attenuation in the southern andean retroarc 38 39 30 s determined from tectonic and gravimetric studies the lonco luan asthenospheric anomaly abstract the western retroarc of the southern andes between 38 and 40 s is formed by a nnw elongated ridge not associated with stacked thrust sheets on the contrary during the last 4 3 ma this ridge was affected by extensional deformation regional uplift and related folding on a very broad scale receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust expected crustal thickness at these latitudes is around 38 km whereas in this part of the retroarc the thickness is less than 32 km the causes for such attenuation have been linked to a moderate steepening of the subducted nazca plate beneath the south american plate which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 ma gravimetric studies show that the upper plate did not react homogeneously to slab steepening but ancient sutures and lithospheric discontinuities deeply buried under mesozoic to cenozoic sequences in the retroarc were locally reactivated these processes resulted in an asthenospheric anomaly that correlates at the surface with the area of pliocene to quaternary doming widespread extension and three radial troughs two of the troughs have accommodated substantial amounts of extension but the third was probably aborted at an early stage moreover the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly and their age distribution may indicate minor migration of the asthenospheric anomaly between 4 and 2 ma through the western south american plate

a millennium of geomagnetism 1 the history of geomagnetism began around the year 1000 with the discovery in china of the magnetic compass methodical studies of the earth s field started in 1600 with william gilbert s de magnete gilbert 1600 and continued with the work of among others edmond halley charles augustin de coulomb carl friedrich gauss and edward sabine the discovery of electromagnetism by hans christian oersted and andre marie ampere led michael faraday to the notion of fluid dynamos and the observation of sunspot magnetism by george ellery hale led sir joseph larmor in 1919 to the idea that such dynamos could sustain themselves naturally in convecting conducting fluids from that came modern dynamo theory of both the solar and terrestrial magnetic fields paleomagnetic studies revealed that the earth s dipole had undergone reversals in the distant past and these became the critical evidence in establishing plate tectonics finally the recent availability of scientific spacecraft has demonstrated the intricacy of the earth s distant magnetic field as well as the existence of magnetic fields associated with other planets and with satellites in our solar system

a comparative study of results obtained in magnetotelluric deep soundings in villarrica active volcano zone chile with gravity investigations distribution of earthquake foci heat flow empirical relationships isotopic geochemistry 87sr 86sr and sb systematics abstract the first magnetotelluric deep soundings in chile were carried out during 1986 in the villarrica active volcano zone 39 25 s 71 57 w in the tm mode of polarization the curves show a distorted segment with dispersion a static distortion at long periods is observed in curves in the te mode of polarization the segment was shifted vertically to fit the geomagnetic global model values at daily periods this modified curve was used for 1d modelling to determine the electrical structure in the study area the upper level of the intermediate conducting layer of resistivity 20 60 m is found to be at 35 50 km depth a higher resistivity layer 600 m starting at 100 km depth may be resolved in the intermediate conducting layer a sharp decrease in the resistivity is shown by the model at 500 km large heterogeneities at the level of the conducting layer encountered in the 1d modelling and increased resistivity of the ultimate layer may account for distortion observed at long periods two dimensional test models show that the conducting layer in the area of villarrica volcano may be an anomalous heated layer surrounded by rocks of higher resistivity of about 2 10 3 m these features correspond to the interaction with a subsiding oceanic lithosphere resulting in a complex thermal structure and perturbed resistivity distribution in transition zones of the pacific type such as chile and to the existence of a megafault and a system of fractures in the sounding area these facts make it difficult to determine the conductance of the electrical asthenosphere the parameters of the model structure correlate well with geophysical and geochemical results obtained in the area by other workers gravity studies indicate a maximum crustal thickness of about 37 km which implies a non full compensation according to the airy hypothesis the morphology of the wadati benioff zone clearly shows a sharp decrease of earthquake foci at 50 km depth and a reinitiation of seismic activity from a depth of 100 km down to 160 km kaufman and keller and levi and lysak empirical relationships between heat flow and thickness of the lithosphere are somewhat consistent when the parameters of the model structure are assumed the strontium isotope ratios are indicative of minimal crustal contamination of mantle derived magmas thus allowing a relatively thin crust under the area furthermore the sb index of partial melting of mantle peridotite may suggest the contribution of uprising material from the deep asthenosphere through fractures extending to depth

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

lodestone natures only permanent magnet what it is and how it gets charged magnetite and titanomagnetite exhibit magnetic properties which are attributable to the micro structures developed during oxidation and exsolution all magnetite iron ores which are lodestones contain maghemite these lodestones have hc between 10 and 30 mt sirm between 8 and 18 am kg and ri between 0 10 and 0 26 magnetite titanomagnetite and metals have rem values ratio of nrm to sirm 0 05 samples called fulgarites obtained from the smithsonian institution have rem values ranging from 0 45 to 0 92 the rem value serves as a witness parameter to the magnetic fields associated with the lightning bolt if a high rem value say 0 1 can be verified as not to be due to contamination by man and does not contain md hematite then the rock has lrm lightning remanent magnetization the magnetic field associated with lightning can be revealed from an isothermal remanent acquisition ra curve

echinoderm stabilization associated with a paleokarst surface at the mississippian pennsylvanian boundary in tennessee u s a abstract pennington formation limestone deposits in tennessee immediately underlie a paleokarst surface that straddles the mississippian pennsylvanian unconformity vertic paleosols with significant gleying and coal seams overlie the paleokarst surface this study quantifies the fe loss associated with paleosol gleying and subsequent incorporation of fe into echinoderm grains that underwent stabilization echinoderm grains immediately below the paleokarst surface have lower mg and 13c values and higher fe concentrations compared to echinoderm grains meters below the paleokarst surface additionally echinoderm grains have variable 13c and invariant 18o values characteristic of alteration in a meteoric setting paleosols draping the paleokarst surface and coal seams that were deposited immediately above the mississippian pennsylvanian unconformity provided ample organic matter that could have driven heterotrophic microbial activity additionally the paleosols were an ample source of oxidized fe that could have been used as a terminal electron acceptor by an anaerobic microbial community therefore we suggest that fe reduction transferred fe from the paleosol to the limestone during meteoric stabilization of echinoderm grains fluid rock modeling constrains the magnitude of fe transfer that occurred during echinoderm stabilization stabilization can account for only a fraction 8 to 14 of the total quantity of fe lost from the gleyed paleosol thus supporting the premise of continued transfer of fe from the paleosol to limestone during burial diagenesis additionally this study constrains the timescale associated with echinoderm stabilization which was likely 12 ky finally stabilization timescale is consistent with echinoderm alteration associated with organic diagenetic conditions with elevated dissolved inorganic carbon in stabilizing pore fluids that were localized immediately below the paleokarst surface

distinguishing climate and time in the soil record mass balance trends in vertisols from the texas coastal prairie mass balance relationships of 10 major elements in a vertisol climosequence and chronosequence formed on alluvial terraces on the texas gulf coastal plain indicate that the soils develop characteristic element translocation patterns in response to climatic forcing and reach mass flux equilibrium within a relatively short period of time vertisols within the climosequence age 35 ka 800 1500 mm mean annual precipitation map approach a net mass flux of 16 3 of parent element contents corresponding to a weathering flux of 1 0 x 10 3 to 1 3 x 10 3 g cm 2 yr 1 when map exceeds 900 mm net mass flux assessments in a vertisol chronosequence 0 4 35 ka 1000 mm map show that this equilibrium is achieved within 5 6 k y below 900 mm map positive net mass fluxes indicate that vertisol profiles are gaining material at a rate of 0 2 x 10 3 g cm 2 yr 1 vertisols forming in drier map areas approach equilibrium with climate by different mechanisms and at different rates compared to their wetter counterparts and may not be as sensitive to millennial scale climate shifts

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

shear wave velocity structure of the chilean subduction zone 39 40 s based on rayleigh wave dispersion evidence of fluid release and melts in the mantle beneath the villarrica volcano a seismic network operated from december 2008 to november 2009 in south central chile covering the chile subduction zone from c 39 s to 40 s this segment of the subduction zone includes the highly active villarrica volcano and the maximum slip area of the 1960 mw 9 5 earthquake we applied surface wave dispersion analysis to data from a linear array of broadband stations and to records of four areal sub arrays fifty regional and teleseismic events were used to produce dispersion curves of rayleigh waves from the dispersion curves we determined depth functions of the shear wave velocity for 4 subregions of the subduction zone the coastal range the central valley the volcanic arc and the back arc region in argentina the resulting models reveal the structure of the crust and the depth of the moho discontinuity below the volcanic arc the shear wave velocities of the continental mantle are reduced by c 7 with respect to a background value of 4 3 km s 1 this low velocity zone coincides with a zone of reduced electrical resistivity that was previously determined from magnetotelluric measurements the combined occurrences of minima in the s wave velocity and resistivity can be interpreted as an indicator of partial melts

contrasting nature of deformation along an intra arc shear zone the liquine ofqui fault zone southern chilean andes abstract the liquine ofquifault zone lofz cuts the patagonian batholith and the modern volcanic arc of southern chile for ca 1000 km the rock fabric along three transects of the lofz combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike in the liquine transect 39 s a 1 km wide northeast striking subvertical mylonitic zone shows north plunging stretching lineations this mylonitic zone has been juxtaposed by high angle reverse faulting against a nearly undeformed miocene granitoid metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement deformation pre dates a 100 2 ma undeformed porphyritic dike hornblende 40ar 39ar mean age in the reloncavi transect 41 42 s deformation in cretaceous and miocene plutons is predominantly brittle kinematic analysis of two fault populations yields compressional and dextral strike slip stress regimes interpreted as late miocene in age in the hornopiren transect 42 43 s a 4 km wide mylonitic zone developed in plutons and wallrocks shows subhorizontal stretching lineations and dextral displacement a single fault population overprinting the mylonites supports a dextral strike slip stress regime available u pb k ar and 40ar 39ar dates on deformed cenozoic plutons and wallrock range from 9 to 13 ma on hornblende and from 6 to 3 ma on biotite microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies similar to the biotite ar closure temperature subparallel magmatic and solid state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons dextral oblique subduction of the nazca plate beneath western south america has driven long term intra arc deformation at the southern andes plate boundary zone ridge collision in turn has favored dextral displacement at the leading edge of the continent since the pliocene

refining the archaeomagnetic dating curve for the near east new intensity data from bronze age ceramics at tell mozan syria abstract uncertainty in radiocarbon dates for the near east caused by a bimodal distribution of ages due to the natural fluctuations of 14 c in the atmosphere has demonstrated the need for an alternative absolute dating technique to aid in the construction of site chronologies here we present a new archaeointensity reference curve model for the first three millennia bce for the levant syria israel jordan for use in archaeomagnetic dating and contribute twelve new intensity results to an increasingly dense geomagnetic field record for the period between 2400 and 1200 bce in the near east archaeomagnetic analysis was conducted on ceramic samples i e pottery sherds from seven sequential and well constrained occupational layers at the site of tell mozan bronze age urkesh in northeastern syria resulting in a 90 success rate by specimen n 42 for archaeointensity determination and an 86 correspondence between the model and the archaeologically derived dates within one standard deviation 1 age standard deviations as low as 24 years were obtained after integration with stratigraphic constraints we also outline the techniques and sampling procedures of archaeomagnetic dating in a manner suitable for the non paleomagnetist while detailing methodology for archaeomagnetic researchers

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

comparison of modern and ancient vertisols developed on limestone in terms of their geochemistry and parent material identification of paleosol parent material can be difficult a common simplistic assumption is that nonpedogenic strata directly beneath paleosols are representative of the parent material field relationships petrography and geochemical massbalance were used to test the simple hypothesis that the parent material of both an early carboniferous late mississippian vertic vertisol like paleosol and a modern analog soil vertisol was immediately subjacent marine carbonate rock an alternative hypothesis also tested was that the paleosol formed primarily from siliciclastic sediment deposited on top of the carbonate the modern ancient analog comparison is valid because both the paleosol and the modern soil have high clay contents extensive shrink swell features and rest directly on top of marine carbonate substrates field and thin section observations indicate minor assimilation of marine carbonate into both the paleosol and soil matrices mass balance of both the paleosol and soil calculated assuming that the subjacent carbonate was the parent material and that ti was immobile during weathering indicated 25 75 net volume loss during weathering and 25 99 net losses of many alkali alkaline earth and redox sensitive elements however calculations based on an alternative hypothesis of clayshale parent material for the paleosol indicated only 10 25 volume loss and lower translocation losses for weathering of this material the insoluble residue content of the dolostone would require that a fourfold volume reduction during weathering occurred if the dolostone were the only parent material if the paleosol formed from clayshale then significantly less volume loss characterised weathering these results collectively illustrate that the simplest interpretation of paleosol parent material may not always be correct and that paleosols can be derived from weathering of more than one parent material d 2002 elsevier science b v all rights reserved

physical compaction of vertic palaeosols implications for burial diagenesis and palaeo precipitation estimates burial compaction is one of several major obstacles to estimating palaeoprecipitation from depth to pedogenic carbonate in favourably preserved palaeosols palaeosols must be decompacted and the preburial depth to the pedogenic carbonate obtained vertic palaeosols may be particularly good candidates for palaeoprecipitation estimates because of their increased likelihood of preserving clastic dykes one of the best features for estimating burial compaction compaction estimates from clastic dykes and literature based depth of burial estimates suggest vertic palaeosols undergo significantly less burial compaction than may be commonly assumed late carboniferous vertic palaeosols buried to 2 5 3 0 km compacted to 93 of their original thickness in contrast clastic dykes in a nonpedogenic shale directly underlying one of the late carboniferous palaeosols records compaction to 70 of original thickness similarly obtained burial compaction and burial depth estimates for early carboniferous ordovician and proterozoic vertic palaeosols were used to test a burial compaction curve and equation specific to vertic palaeosols results suggest this vertic calibrated curve and equation can be used to estimate burial compaction for vertic palaeosols lacking clastic dykes but additional testing is needed naturally high bulk densities may have limited the compactibility of vertic palaeosols likewise high initial bulk density and an abundance of swelling clays may have severely limited the transmissivity of some vertic palaeosols as they passed from pedogenic to burial environments upon burial these vertic palaeosols may have behaved as closed systems which has implications for understanding their diagenetic modification additional efforts to understand burial compaction of vertic palaeosols also promises to improve our understanding of aquifer aquiclude and hydrocarbon reservoir seal relationships in sedimentary basins containing intercalated palaeosols

palaeosols and sequence stratigraphy of the lower permian abo member south central new mexico usa the relationship between palaeosols and sequence stratigraphy is tested in the lower permian abo member south central new mexico by comparing interfluve and fluvial terrace palaeosols with palaeosols that developed within lowstand fluvial deposits interfluve and fluvial terrace palaeosols consist of primary pedogenic features including vertical root traces vertic structures stage ii and iii pedogenic calcite and translocated clay argillans which are cross cut or replaced by low aluminium goethite gley colour mottling sparry calcite veins and ankerite the polygenetic character of the palaeosols is consistent with initial development for several thousand to tens of thousands of years on well drained interfluves or fluvial terraces followed by waterlogging due to invasion by a rising water table that locally may have been brackish in contrast lowstand fluvial sediment that filled incised valleys contains only rooted and vertic palaeosols whose immaturity resulted from high aggradation rates palaeosols similar to those in the abo member have been recognized in other ancient strata and when combined with highresolution correlation provide evidence for interpretation of sequencestratigraphic surfaces and systems tracts

the direction of the earth s magnetic field at london 1570 1975 the observations of magnetic declination and dip made in london over the past 400 years comprise one of the longest and most complete series of such data for anywhere in the world they give an indication of secular change on a time scale intermediate between that of recent observatory measurements and of archaeomagnetic measurements we have attempted to catalogue all direct measurements of declination and dip paying particular attention to completeness in the earlier years and tracing the data to their original source wherever possible sufficient details of the observations are given for an assessment of their reliability and accuracy the data are then corrected to a single site fitted with a continuous smooth curve and discussed

resistivity cross section through the southern central andes as inferred from magnetotelluric and geomagnetic deep soundings to investigate the upper lithospheric structure and subduction induced processes across an active continental margin magnetotelluric mt and geomagnetic deep sounding gds experiments were done in the southern central andes 21 25 s of northern chile southern bolivia and northwestern argentina using two dimensional modeling we have constructed an e w resistivity cross section at latitude 21 45 s that involves all the andean units from the pacific coast to the lowland plains the crust of the coastal cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted nazca plate the electrical resistivity of the normal andean crust seems to be low having values of only 200 ohm m a wide zone of extremely high electrical conductivity with an e w extent of more than 330 km was detected this high conductivity zone hcz is found at a depth of about 25 km under the western cordillera and has a total conductance of more than 23 000 s here it may be caused by partially melted crust an interpretation which would correlate with those from gravity and seismic data the hcz can be followed in the lower crust eastward below the altiplano where it has a total conductance of about 15 000 s in the western part of the eastern cordillera total conductance first increases and then drops abruptly at about longitude 65 10 w the latter hczs can be explained in terms of thrust tectonics where detachment zones may involve fluids as well as increased mineralizations the lowlands of the sub andean and the chaco are characterized by a cover of anisotropic low resistivity the resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m while the deeper mantle gets unexpectedly much more conductive

payenia volcanic province in the southern andes an appraisal of an exceptional quaternary tectonic setting abstract the southern volcanic zone of the andes has a quaternary basaltic province along the retroarc which has a unique tectonic setting the payenia volcanic province covers an area larger than 40 000 km2 between 33 30 and 38 south latitudes with an estimated volcanic volume of about 8387 km3 erupted through more than 800 volcanic centers in the last 2 ma the mainly basaltic province developed above the san rafael block is subdivided in three segments characterized by the cerro nevado llancanelo payun matru tromen and auca mahuida volcanic fields together with hundreds of minor monogenetic basaltic centers the analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2 0 and 1 7 ma which reached the 35 s to the north above this plateau monogenetic centers as nihuil vn 1 433 ma and cerro chato at 1 352 ma are developed followed by the large polygenetic center of cerro nevado 3980 m a s l at 1 320 ma this plateau was broken by a series of normal faults that produced volcanic cone alignments such as the nnw trending mancha jarilla lineament in the central part at about 1 0 ma extension shifted to the eastern margin of the san rafael block which concentrates tens of monogenetic centers between 0 9 and 0 7 ma extension then migrated towards the foothills in the west where many monogenetic cones were erupted through nw trending normal faults between 0 5 and 0 435 ma the collapse of the large diamante caldera at 0 445 ma coincides with that period subsequent volcanism was concentrated in 1 the payun matru volcanic field with the eruption of cerro payen between 0 272 and 0 261 ma the payun matru shield volcano with polygenetic eruptions at least since the last 0 233 ma and with the caldera formation bracketed between 0 168 0 004 ma and 0 082 0 001 ma followed by several eruptions until 7000 yrs and even historical ones and in 2 the tromen volcano where younger than 0 2 ma eruptions took place and historical eruptions were reported the understanding of these eruptions in time and space combined with geophysical data indicates the geometry of an important crustal attenuation beneath payenia associated with a hot sublithosphere the late miocene uplifted san rafael block collapsed in the early pleistocene as a consequence of the steepening of the subducted slab and the injection of hot asthenosphere produced the quaternary payenia volcanic province melts of the lower crust along the principal cordillera at these latitudes are responsible for the quaternary calderas ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the andes the payun matru volcanic field concentrates this asthenospheric flow in the present

on the possible occurrence of archaeomagnetic jerks in the geomagnetic field over the past three millennia abstract archaeomagnetism can provide a high resolution full vector description of the earth s magnetic field for the past several thousand years we analyse the bulk of archaeomagnetic data both direction and intensity obtained recently in western europe and the eastern mediterranean covering the past three millennia we demonstrate a remarkable coincidence between sharp cusps in geomagnetic field direction and intensity maxima two clear ones at ad 200 and 1400 two presently less well constrained at 800 bc and ad 800 these sharp changes may constitute a new feature of geomagnetic secular variation archaeomagnetic jerks with time characteristics intermediate between geomagnetic jerks and magnetic excursions

the study of earth s magnetism 1269 1950 a foundation by peregrinus and subsequent development of geomagnetism and paleomagnetism 1 this paper summarizes the histories of geomagnetism and paleomagnetism 1269 1950 the role of peregrinus is emphasized in the sixteenth century a debate on local versus global departures of the field from that of an axial dipole pitted gilbert against le nautonier regular measurements were undertaken in the seventeenth century at the turn of the nineteenth century de lamanon de rossel and von humboldt discovered the decrease of intensity as one approaches the equator around 1850 three figures of rock magnetism were fournet remanent and induced magnetizations delesse remagnetization in a direction opposite to the original and melloni direction of lava magnetization acquired at time of cooling around 1900 brunhes discovered magnetic reversals in the 1920s chevallier produced the first magnetostratigraphy and hypothesized that poles had undergone enormous displacements matuyama showed that the earth s field had reversed before the pleistocene our review ends in the 1940s when exponential development of geomagnetism and paleomagnetism starts

unbiased analysis of geomagnetic data sets and comparison of historical data with paleomagnetic and archeomagnetic records reconstructions of the past geomagnetic field provide fundamental constraints for understanding the dynamics of the earth s interior as well as serving as basis for magnetostratigraphic and archeomagnetic dating tools such reconstructions when extending over epochs that precede the advent of instrumental measurements rely exclusively on magnetic records from archeological artifacts and farther in the past from rocks and sediments the most critical component of such indirect records is field intensity because of possible biases introduced by material properties and by laboratory protocols which do not reproduce exactly the original field recording conditions large biases are usually avoided by the use of appropriate checking procedures however smaller ones can remain undetected in individual studies and might significantly affect field reconstructions we introduce a new general approach for analyzing geomagnetic databases in order to investigate the reliability of indirect records this approach is based on the comparison of historical records with archeomagnetic and volcanic data considering temporal and spatial mismatches with adequate weighting functions and error estimation a good overall agreement is found between indirect records and historical measurements while for several subsets systematic bias is detected e g inclination shallowing of lava records we also demonstrate that simple approaches to analyzing highly inhomogeneous and internally correlated paleomagnetic datasets can lead to incorrect conclusions about the efficiency of quality checks and corrections consistent criteria for selecting and weighting data are presented in this review and can be used to improve current geomagnetic field modeling techniques

stratigraphic implications of meteoric sphaerosiderite 18o values in paleosols of the cretaceous albian boulder creek formation ne british columbia foothills canada abstract the md 80 08 coal exploration core from the albian boulder creek formation of northeastern british columbia contains 90 m of alluvial strata that underwent extensive pedogenic modification gleyed colors pedogenic slickensides clay coatings mottling sphaerosiderite and sedimentary pyrite characterize the paleosols the isotopic compositions of 30 sphaerosiderite bearing horizons commonly show trends of invariant 18o and variable 13c values equal area plots of 18o vs 13c were used to construct meteoric sphaerosiderite lines msls the msls indicate that meteoric groundwater compositions ranged from approximately 11o oo to 16o oo smow the sphaerosiderite 18o values indicate formation in shallow groundwaters predominantly recharged by local precipitation the sphaerosiderite isotopic compositions are a proxy record of mid cretaceous precipitation and are useful for quantifying changes in the hydrologic cycle during humid greenhouse periods micromorphology of a 1 15 meter interval 69 75 68 6 m with a 1 31o oo increase in the average 18o values records a polygenetic history of paleosol development the pedocomplex has a four stage developmental history characterized by 1 an initial well drained phase with wet dry cycles and relatively low baselevel 2 erosion baselevel rise and coarser grained sedimentation 3 saturated reducing soil conditions high baselevel with some marine influence on pedogenesis and 4 meteoric water dominated hydromorphic soil conditions high baselevel and sphaerosiderite precipitation the 18o excursion recorded through this interval reflects minor mixing of marine and meteoric groundwaters the modal abundance of pyrite and increased mg ca mg and mg fe cation ratios in the sphaerosiderites further substantiate marine influence on pedogenesis the alluvial section of the boulder creek formation is aggradational and several changes in baselevel recorded in the paleosols are interpreted to have been driven by minor parasequence scale changes in relative sea level

the bolivian altiplano conductivity anomaly 1 a long period magnetotelluric study was carried out in the central andes between latitudes 19 5 s and 21 s along two almost parallel profiles of 220 and 380 km length respectively the investigation area extends from the pacific coast to the southern altiplano plateau in the back arc of the south american subduction zone the main geoelectrical structure resolved is a broad and probably deep reaching highly conductive zone in the middle and deeper crust beneath the high plateau although the data show deviations from two dimensionality a two dimensional approach is justified for large parts of the profiles sensitivity studies were carried out in order to constrain the depth extent another electrically conductive structure was resolved in the middle crust of the chilean forearc thought to be connected with the precordillera fault system the andean continental research program ancorp seismic reflection profile carried out along the same line at 21 s revealed highly reflective zones below the altiplano in good correlation with the upper boundary of the altiplano conductor this highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp vs ratio and although not well resolved with low qp seismic quality factors taking into account the enhanced heat flow and a derived temperature model the most probable explanation lies in the assumption of granitic partial melts the good conductor below the volcanic arc which was found in regions farther south at 22 s gradually vanishes toward the north this is consistent with the results of seismic tomography concerning qp values and a gap of recent volcanism

paleopedology and paleoclimatic implications of late ordovician vertic paleosols juniata formation southern appalachians abstract claystone paleosols with abundant vertic features occur in upper ordovician ashgillian red beds of the juniata formation in the southern appalachians the beans gap claystone paleosol formed during late ordovician pedogenesis of estuarine and upper tidal flat mud deposits pedogenic processes were dominated by shrinking and swelling of clay in a subtropical to warm temperate paleolatitude 30 35 south paleoclimate characterized by a seasonal moisture deficit dry 4 8 months year large scale pedogenic slickensides are abundant and locally define pseudo anticline structures the predominance of sepic paleosol micro fabrics compares favorably with holocene vertisol micromorphology post pedogenic marine transgression resulted in erosion of the upper parts of paleosols followed by marine bioturbation and introduction of marine fluids changes in whole rock chemistry associated with early marine diagenesis include elevated na2o cao mgo mno and p2o5 in the preserved top of each paleosol reduction of fe3 to fe2 resulted in a decrease in fe2o3 and formation of drab halos around burrows the original smectitic mineralogy was converted to a well ordered 2m1 illite through progressive burial diagenesis

tertiary tectonics of the patagonian andes retro arc area between 38 15 and 40 s latitude the structure and tectonics of the retro arc area of the andes between 38 degrees 30 and 40 degrees s remains poorly understood because of the scarcity of previous structural studies this paper analyzes the styles distribution and timing of the deformation in the eastern flank of the andes between these latitudes with focus in the tertiary tectonics of the area this andean section can be divided into an internal and an external domain based on its distinctive structural evolution the boundary between them coincides with the bio bio and alumine valleys which separate sectors of the fold and thrust belt with opposite vergence and different structural styles structures developed in the external domain during the andean cycle are associated with a west verging fold and thrust belt which was controlled geometrically by pre existing normal faults of the mesozoic rift phase we also differentiate three segments along the external domain of the inner retro arc zone at these latitudes each segment is associated with a tertiary depocenter related to the western border of a structural high copahue pino hachado block and cordilleras de catan lil and chachil which constitutes an outstanding feature within the study area the recognition and interpretation of the main structures and the spatial and temporal distribution of the tertiary sequences allowed us to establish a tectonic model in which the uplift of this nnw trending block during upper miocene times originated a series of intermontane depocenters well represented between 38 and 40 degrees s as a result of that syntectonic and synorogenic deposits accumulated in a compressive regime these depocenters can be integrated in a narrow deposition zone associated with the western slope of the copahue pino hachado block and its southward prolongation in the cordillera de catan lil and cordillera del chachil on the basis of tectonostratigraphic controls we define the last andean contractional phase of the inner retro arc area at these latitudes between 11 and 5 67 ma for the northern segment between 10 and 6 2 ma for the central segment and between 13 to 8 and 43 ma for the southern segment after this short lived compressional stage deformation migrated to the west associated with the onset of the liquine ofqui strike slip fault zone along the intra arc zone c 2010 elsevier b v all rights reserved

structural electrical anisotropy in the crust at the south central chilean continental margin as inferred from geomagnetic transfer functions abstract induction vectors as a visualization of geomagnetic deep sounding transfer functions display an unique pattern at the south chilean continental margin between latitudes 38 41 s and longitudes 71 74 w at long periods of approx 3000 s their real parts are uniformly deflected from the w e direction which would be expected due to the coast effect and or anomalies beneath the roughly n s striking andean mountain chain to the ne attempts to model this behavior with simple and geologically realistic 3 d models failed but a reasonable data fit was obtained by employing 2 d models with a structurally anisotropic lower crust this anisotropy hints at a deeply fractured fluid rich crust with a major strike direction of 40 50 sw ne oblique to the continental margin and in accordance with the regional stress field in the region of the volcanic arc a surprising result is that the anisotropy persists in the forearc and may even reach until the continental slope near the trench

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

three dimensional electrical resistivity image of the south central chilean subduction zone abstract based on isotropic 3 d inversion we re interpret long period magnetotelluric data collected across the geotectonic structures of the south central chilean continental margin at latitudes 38 41 s and summarize results of long period magnetotelluric mt investigations performed between 2000 and 2005 the new 3 d conductivity image of the south central chilean subduction zone basically confirms former 2 d inversion models along three profiles and complete the previous results the models show good electrical conductors in the tip of the continental crustal beneath the pacific ocean the frequently observed forearc conductor at mid crustal levels a highly conductive zone at similar levels slightly offset from the volcanic arc and a not well resolved conductor in the argentinian backarc the subducted nazca plate generally appears as a resistive but discontinuous feature unlike before we are now able to resolve upper crustal conductors interpreted as magma reservoirs beneath active lonquimay villarrica and llaima volcanoes which were obscured in 2 d inversion data fit is rather satisfactory but not perfect we attribute this to large scale crustal anisotropy particularly beneath the coastal cordillera which we cannot include into our solution for the time being

electrical conductivity beneath the bolivian orocline and its relation to subduction processes at the south american continental margin 1 a long period magnetotelluric data set was obtained during 2002 and 2004 in the central andes to study the deep electrical conductivity structure in the region of the bolivian orocline between latitudes 17 s and 19 s the profile extends from the coastal cordillera in northernmost chile crosses the volcanic arc and the altiplano high plateau in central bolivia and ends in the eastern cordillera two dimensional inversion revealed several well defined conductivity anomalies in upper crustal levels the conductive sedimentary basins of the central altiplano and the resistive arequipa block beneath the western altiplano are imaged earlier seismological and magnetotelluric investigations on the southern altiplano inferred a large highly conductive partially molten body in the mid to deep crust it was assumed that this structure would be underlying the entire plateau but this is not the case according to the new models instead the most prominent feature in the new investigation area is a high conductivity zone at upper mantle depths below the high plateau which may be interpreted as an image of partial melts and fluids triggered by water supply from the subducting nazca slab this conductor would be in accordance with the standard subduction scenario it is however laterally offset by almost 100 km from the volcanic arc in contrast the deep crust and upper mantle beneath the arc is moderately resistive both observations may hint at an emerging shift of the magmatic fluid system in the central andes

electrical conductivity of hydrous andesitic melts pertinent to subduction zones andesitic magmatism and rocks are widespread at convergent plate boundaries electrically conductive bodies beneath subduction zone arc volcanoes such as the uturuncu volcano bolivia may correspond to active reservoirs of h2o bearing andesitic magma laboratory measurements of electrical conductivity of hydrous andesitic melts are required to constrain the physicochemical conditions of these magma reservoirs in combination with magnetotelluric data this experimental study investigates electrical conductivity of andesitic melts with 0 01 5 9 wt of h2o at 1164 1573 k and 0 5 1 0 gpa in a piston cylinder apparatus using sweeping frequency impedance spectroscopy electrical conductivity of andesitic melt increases with increasing temperature and h2o concentration but decreases with pressure across the investigated range of h2o concentration electrical conductivity varies by 1 2 2 4 log units indicating stronger influence of h2o for andesitic melt than for rhyolitic and dacitic melts using the nernst einstein equation the principal charge carrier is inferred to be na in anhydrous melt but divalent cations in hydrous andesitic melts the experimental data are regressed into a general electrical conductivity model for andesitic melt accounting for the pressure temperature h2o dependences altogether modeling results show that the conductive layer at 20 km depths beneath the surface of the uturuncu volcano could be interpreted by the presence of less than 20 vol of h2o rich andesitic melt with 6 9 wt h2o

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

desiccation cracks in vertic palaeosols of the cretaceous hasandong formation korea genesis and palaeoenvironmental implications abstract the hasandong formation upper cretaceous korea contains diagnostic vertic features such as pedogenic slickensides pseudoanticlines and large desiccation cracks the hasandong vertic palaeosols developed on floodplain deposits and display a diverse range of desiccation cracks including calcite filled vertically stratified and thin varieties the calcite filled cracks are 5 10 cm wide on the surface and up to 60 cm deep and may have been formed by selective calcite precipitation in the cracks due to preferential vegetation the vertically stratified cracks are those with crack fillings having alternating sandstone and mudstone subparallel to crack walls they are interpreted to have been formed by repeated crack opening and filling at crack margins due to seasonal drying and wetting the thin cracks are less than 1 mm to a few mm wide and a few cm long and are recognized by light colour mottling around them thin crack development may have resulted from intermittent incomplete cracking during sedimentation the hasandong vertic palaeosols have stratigraphic and lateral variations in vertic features which are attributed to the changes in palaeoenvironments deep and large desiccation cracks with unbridged sediment infilling formed in flood basins the low topographic position of the distal floodbasin area resulted in weakly developed vertic palaeosols with calcite filled desiccation cracks and some rhizocretions and proximal location to a main channel may be responsible for rare development of deep and wide cracks and intercalation with calcrete intraclast conglomerates differences in these vertic features along with interbedded calcic palaeosols may provide useful criteria for the palaeoenvironmental changes in and correlation of the floodplain deposits

distinguishing climate in the soil record using chemical trends in a vertisol climosequence from the texas coast prairie and application to interpreting paleozoic paleosols in the appalachian basin u s a a suite of vertisols clay rich soils with high shrink swell potential were examined across a climosequence climatic transect in twelve soil pits from the coast prairie of texas in order to determine if mean annual precipitation map exerts a control on the chemistry of these soils and if the observed chemical trends are useful for interpreting paleoclimate records of paleovertisols in the geologic record the precipitation regime of the climosequence spans a range between 144 and 86 cm year with moisture regimes classified as udic udic ustic ustic and aridic ustic in a general northeast to southwest direction other soil forming factors such as soil age 35 40 ka parent material fluviodeltaic beaumont formation of late pleistocene age landscape low relief coastal plain and vegetation prairie or mixed woody shrubs are relatively constant across the climosequence climate sensitive chemical proxies of map identified include dithionite citrate extractable fe fedith acid oxalate extractable fe feoxal caco3 equivalent caco3equiv s and ammonium acetate extractable na k and mg naacet kacet and mgacet respectively which vary across the climosequence because of differences in effective depths of leaching and intensity of wetting and drying cycles these standard usda wet chemical climate proxies are related to bulk oxide or element chemistry of soils and paleosols measured using xrf which supports the use of geochemical climate proxies for interpreting the paleoclimate records of paleovertisols application of the chemical index of alteration minus potash cia k geochemical climofunction to late mississippian paleosols from the appalachian basin of the eastern u s demonstrates evidence for a shift from a lower to a higher map paleoclimate that is consistent with previous paleoclimate models and with observed morphological changes in the paleosols we advocate actualistic research using bulk chemistry of soils and paleosols as a means of obtaining soil information useful for interpreting paleosols in the geological record

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

paleoclimate and the global distribution of paleosols a theoretical model is developed for the relationship between paleoclimate and paleosols for the time interval following the widespread appearance of vascular land plants the model defines four paleoclimatic zones 1 a wet equatorial zone primarily characterized by oxisols and secondarily by argillisols and histosols 2 a subtropical dry zone containing calcisols and less commonly gypsisols and vertisols 3 a moist mid latitude zone with argillisols spodosols gleysols and histosols and 4 a polar zone featuring protosols and gleysols the model is general in nature and its application to the geologic past will be affected by variability through time in paleogeography and paleoclimatic parameters

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

petrophysical model of deep electrical conductors graphite lining as a source and its disconnection due to uplift petrophysical information including stress release data recently made available for low porosity 0 1 0 4 crystalline rocks has been used to develop a broken conductor model that may explain the origin of enhanced electrical conductivity of the intermediate to lower continental crusts of shields and its disappearance when the rocks are uplifted average pore size and pore density of their fluid flow paths are 0 1 0 4 m and 1 400 m respectively the model assumes that these paths are filled or lined with interconnected electrical conducting minerals such as graphite resulting in the rocks having resistivities as low as 40 to 800 m in situ graphite film thicknesses required to produce these resistivities are 5 20 nm 50 200 a the model also assumes that electrical conducting paths break up during uplift due to stress release resulting in a poorly conducting rock petrophysical data of granites and observations from deep drill holes suggest a porosity increase of the order of 3 from an uplift of 10 km or more however the newly formed pore space would fill with deposits or intruded material so that surface rock porosities would not be much larger than at depth electrical resistivities of rock samples containing disconnected grain boundary graphite presently exposed by the kapuskasing uplift of the canadian shield are in the range of 3000 12000 m for the direction of foliation these values are 10 100 times larger than the estimates for similar rocks containing interconnected graphite but are 10 100 times smaller than usually observed in the canadian shield for similar rocks absent of grain boundary graphite these observations are in good agreement with the results predicted by the broken conductor model for an uplift of 10 km or more if a gross linear relationship is assumed between crack porosity and uplift for the upper 10 km of the crust