reinterpretation of late quaternary sediment chronology of lake biwa japan from correlation with marine glacial interglacial cycles a review of published stratigraphic records of pollen sediment grain size diatoms and organic matter composition from lake biwa japan identifies four pre holocene episodes of milder climate increased surface runoff and enhanced aquatic productivity indicating intervals of warmer and wetter conditions which are interpreted as being interglacial correlation of these episodes to times of marine interglacial periods revises the age scale of the lake biwa sediment sequence which has been based on fission track dating the revised chronostratigraphic scale proposes an age of ca 430 000 yr b p for the base of the 250 m thick t bed instead of the former age of ca 700 000 yr b p

broad synchrony of a late glacial glacier advance and the highstand of palaeolake tauca in the bolivian altiplano the morphology and sedimentology of glacially influenced fan deltas on massifs at the margin of the southern altiplano bolivia suggest a broadly synchronous expansion of glaciers and palaeolake tauca during the late glacial interval this is shown by sedimentary successions of glacigenic glacifluvial and glacideltaic facies linking palaeoglaciers with palaeolake tauca on the flanks of cerro azanaques and cerro tunupa at altitudes of 3770 3720 m radiocarbon dates from peat overlain by glacial diamict and glacifluvial outwash indicate that glaciers in this area reached their last glacial maximum extent after ca 13 300 14 c yr bp glacifluvial fan deltas contiguous with the moraines confirm that the advance coincided with a highstand of palaeolake tauca radiocarbon dated to the interval ca 13 500 11 500 yr bp modeling of climatic controls on this glacier advance suggests the primary forcing was increased summer wet season moisture possibly amounting to 600 mm above the modern values of 200 400 mm greater cloud cover probably depressed local temperatures and reduced the evaporation rate the consequent rise in effective annual moisture p e comfortably accommodates a palaeolake 48 50 10 3 km 2 in area and up to 100 m deep in the southern altiplano because the palaeoglacier equilibrium line altitudes rose toward the south and west like the gradient of modern precipitation totals we conclude that the increased late glacial moisture was brought by weather systems similar to those of the present but that atmospheric conditions were cloudier and cooler 1997 john wiley sons ltd

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

magnetic fabrics and microstructures of the post collisional aegirine augite syenite triunfo pluton northeast brazil abstract anisotropy of magnetic susceptibility ams and microstructures of the neoproterozoic triunfo alkaline pluton situated in northeast brazil have been investigated in order to characterize fabrics formed during its emplacement bulk magnetic susceptibility in the pluton varies between 0 15 10 3 and 25 10 t 3 si and the mean magnetic anisotropy ratio is around 1 04 aegirine augite and secondary magnetite are the main carriers of the magnetic susceptibility with the former controlling the directions of the anisotropy magnetic foliation and lineation are consistently oriented throughout the pluton foliation typically dips gently to the northeast and lineation plunges gently to the east these structures were mostly acquired at the magmatic and sub magmatic stages the shape of the magnetic ellipsoid is dominantly oblate a feature that may reflect either the uniaxial oblate ams ellipsoid of the aegirine augite crystals and or a vertical compaction of the magma in its final stages of crystallization the foliation pattern suggests the pluton has a tabular shape within the basement rocks magma emplacement is inferred to have occurred by its lateral migration along a flat lying crustal structure after fracture propagation and ascension of the magma from a lower crust mantle source late fine grained vertical dykes with the same mineral composition as the triunfo syenite cross cut the wall rocks of the pluton and close the alkaline magmatism in the area the ne trending alignment of alkaline plutons and the nne trending syenitic dykes indicate that the emplacement of such bodies occurred along a crustal extension event with a minimum compressive stress horizontal trending roughly e w

shape anisotropy versus magnetic interactions of magnetite grains experiments and application to ams in granitic rocks the magnetic fabric of ferromagnetic granitic rocks results from both the shape preferred orientation of individual magnetite grains and their distribution anisotropy through magnetic interactions between neighbouring grains measurement of the low field magnetic anisotropy of single multi domain magnetite grains shows a linear correlation between their magnetic anisotropy degree and their aspect ratio interactions between two elongated grains were studied experimentally using two types of grain arrangement an aligned configuration and a side by side configuration for a distance between the grain centers equal to approximately twice the average grain size the magnetic susceptibility and its anisotropy are enhanced in both configurations and the direction of kmax the easiest magnetization axis is stable in the aligned configuration whereas it rotates toward an orthogonal direction in the side by side configuration depending on the distribution of the interacting magnetite grains magnetic interactions may therefore either increase the whole rock anisotropy magnitude or reduce it as in the given example of the granitic rocks from madagascar

a 350014c yr high resolution record of water level changes in lake titicaca bolivia peru abstract sediment cores collected from the southern basin of lake titicaca bolivia peru on a transect from 4 6 m above overflow level to 15 1 m below overflow level are used to identify a new century scale chronology of holocene lake level variations the results indicate that lithologic and geochemical analyses on a transect of cores can be used to identify and date century scale lake level changes detailed sedimentary analyses of subfacies and radiocarbon dating were conducted on four representative cores a chronology based on 60 accelerator mass spectrometer radiocarbon measurements constrains the timing of water level fluctuations two methods were used to estimate the14c reservoir age both indicate that it has remained nearly constant at 25014c yr during the late holocene core studies based on lithology and geochemistry establish the timing and magnitude of five periods of low lake level implying negative moisture balance for the northern andean altiplano over the last 3500 cal yr between 3500 and 3350 cal yr b p a transition from massive inorganic clay facies to laminated organic matter rich silts in each of the four cores signals a water level rise after a prolonged mid holocene dry phase evidence of other significant low lake levels occurs 2900 2800 2400 2200 2000 1700 and 900 500 cal yr b p several of the low lake levels coincided with cultural changes in the region including the collapse of the tiwanaku civilization

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

a multidisciplinary study on the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part i structural geology ams and paleomagnetism during the cretaceous the south china block scb experienced a widely distributed extensional event including numerous plutons emplacement and basin opening investigations on the tectonic regime coeval with pluton emplacement and emplacement mechanism of the pluton remain relatively rare in the scb in order to address these questions a multidisciplinary approach including field structural and petrographic observations anisotropy magnetic susceptibility ams and paleomagnetic analyses was carried out on the qingyang jiuhua granitic massif which intrudes into the lower yangtze fold belt in the northeastern part of scb the qingyang jiuhua massif is composed of the granodioritic qingyang and monzogranitic jiuhua plutons dated by zircon u pb method at ca 142 ma and ca 131 ma respectively our structural observations show that the intrusion of the qingyang jiuhua massif does not modify the fold strike a weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism in the contact aureole of the massif the foliation follows the pluton contour and the mineral lineation is rare when present it exhibits a down dip attitude field and microstructural observations indicate isotropic magmatic textures in most parts of the massif the ams analysis of 93 sites reveals weak values for the anisotropy degree pj 0 for most of the measured samples two principal foliation patterns are identified horizontal foliations in the center of the plutons and vertical foliations on the boundaries magnetic lineation strike is largely scattered and weakly inclined at the scale of the entire massif the paleomagnetic investigations indicate that a the younger jiuhua pluton did not produce a remagnetization in the older qingyang pluton b no relative movement can be observed between these two plutons c the entire massif did not experience any important relative movement with respect to south china considering the paleomagnetic uncertainties integrating the newly obtained results with previous observations our study favors a permissive emplacement mechanism for the two plutons i e vertical magma intrusion into an opening space controlled by the nw se brittle stretching of the upper crust which is in agreement with a weak extensional regional tectonic framework of the scb

impedance measurements of the complex dielectric permittivity of sea ice at 50 mhz pore microstructure and potential for salinity monitoring we report impedance measurements of the complex dielectric permittivity epsilon epsilon j epsilon of sea ice and laboratory grown nacl single crystals using 50 mhz stevens water monitoring systems hydra probes temperature cycling of the single crystal samples shows hydrohalite precipitation and hysteresis in epsilon and epsilon qualitatively consistent with the expected evolution of brine inclusion microstructure measurements parallel and perpendicular to intra crystalline brine layers show weak 10 anisotropy in epsilon and a 20 40 difference in epsilon due to enhanced d c conductivity along the layers measurements in landfast first year ice near barrow alaska usa indicate brine motion in warming ice as the brine volume fraction v b increases above 5 plots of v b derived from salinity profiles against epsilon and epsilon for these and previous measurements display too much variability between datasets for unguided inversion of v b contributing to this variability are intrinsic microstructural dependence uncertainties in v b and sub representative sample volumes a standard model of sea ice permittivity is inverted to derive the apparent brine inclusion aspect ratio and bulk d c conductivity at a spatial scale complementary to previous measurements we assess hydra probe performance in high salinity environments and conclude that they are not generally suited for autonomous sea ice salinity measurements partly due to the range of relevant brine pocket inclusion length scales

el nino southern oscillation events recorded in the stratigraphy of the tropical quelccaya ice cap peru snow accumulation measured during 1982 1983 on the quelccaya ice cap peru was 70 percent of the average from 1975 through 1983 inspection of 19 years 1964 through 1983 of accumulation measured near the summit of quelccaya reveals a substantial decrease 30 percent in association with the last five el nino southern oscillation enso occurrences in the equatorial pacific the enso phenomenon is now recognized as a global event arising from large scale interactions between the ocean and the atmosphere understanding this extreme event with the goal of prediction requires a record of past occurrences the quelccaya ice cap which contains 1500 years of annually accumulated ice layers may provide a long and detailed record of the most extreme enso events

tropical climate and glacier hydrology a case study in bolivia runoff from intertropical glaciers is highly variable indicating that they are greatly affected by climatic changes peculiar to tropical climates the 3 km2 basin presented in this case study lies in the cordillera real of bolivia and is 77 covered by glacier ice ranging in elevation from 6000 to 4830 m a s l a comparison of 2 years of study demonstrates the peculiar feature of intertropical glaciers that accumulation and melting periods coincide during the rainy season during the first hydrological year 1991 1992 runoff was 1793 mm for an average precipitation on the glacier of approximately 916 mm during the second year 1992 1993 runoff was 1080 mm for a precipitation of 1060 mm solar radiation and daily mean duration with positive temperature on the ablation zone are the parameters best explaining variation in monthly runoff gauge readings taken downstream enable discharge from the glacier to be established over a period of nearly 20 years the most prominent events coincide with significantly negative values of the southern oscillation index often linked to el nino southern oscillation enso phenomena in conclusion results show that these intertropical glaciers are receding rapidly as precipitation does not compensate for loss from melting

holocene hydrological reconstructions from stable isotopes and paleolimnology cordillera real bolivia multiproxy analyses of sediment cores from lago taypi chaka kkota ltck cordillera real bolivia provide a record of drier conditions following late pleistocene deglaciation culminating in pronounced aridity between 6 2 and 2 3 ka b p today ltck is a glacial fed lake that is relatively insensitive to changes in p e because it is largely bu ered from dry season draw down through the year round supply of glacial meltwater this was not the case during the middle to late holocene when glaciers were absent from the watershed lake water d18o values inferred from d18o analysis of sediment cellulose range from 12 9 to 5 3 and average 8 7 between 6 2 and 2 3 ka b p modern lake water d18o from ltck averages 14 8 which is compatible with the d18o 8 value of 14 3 for the surface sediment cellulose analyses of d18o from modern surface waters in 23 lakes that span the range from glacial fed to closed basin vary from 16 6 to 2 5 this approximates the magnitude of the down core shift in d18o 8 values in ltck during the middle to late holocene from 12 9 to 5 3 strong paleohydrologic change during the middle holocene is also evident in diatom assemblages that consist of shallow water non glacial periphytic taxa and bulk organic d13c and d15n that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions neoglaciation began after 2 3 ka b p as indicated by changes in the composition of the sediments lower d18o values and a return to diatom assemblages characteristic of the glacial sediments that formed during the late pleistocene collectively these data indicate that the past 2 3 ka b p have been the wettest interval during the holocene millennial scale shifts in the paleohydrologic record of ltck during the early to middle holocene conform to other regional paleoclimatic time series including lake titicaca and nevado sajama and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply in contrast an apparent 1200 year lag in the onset of wetter conditions at ltck 2 3 ka b p compared to lake titicaca 3 5 ka b p provides evidence for variable sub regional hydrologic response to climate change during the middle to late holocene 2000 elsevier science ltd all rights reserved

paleohydrology of lake victoria east africa inferred from 18o 16o ratios in sediment cellulose delta 18 o values of aquatic cellulose preserved in a 7 6 m sediment core from near the center of lake victoria east africa record a 13 000 yr hydrological history of the basin relative to the present value 3 4 highly elevated lake water 18 o values of 8 2 from 13 200 to 7250 14 c yr b p indicate closed basin conditions and precipitation evaporation ratios much smaller than at present an abrupt drop in the lake water 18 o to 0 1 marks the rapid transition to an open basin between 7250 and 7150 14 c yr b p from 7140 to 5400 14 c yr b p increasing lake water 18 o values from 0 1 to 5 1 5 are proposed to reflect outlet downcutting and a likely decrease in relative humidity these data have unexpectedly shifted our view of the timing of changes in the lake victoria water balance and imply a period of at least 10 000 years 17 000 to 7000 14 c yr b p when the lake victoria basin did not contribute waters to the main nile river

high resolution seismic reflection profiles from lake titicaca peru bolivia evidence for holocene aridity in the tropical andes high resolution seismic reflection profiles of the sediments of lake titicaca peru bolivia suggest that lake levels in the recent past were considerably lower than today incised channels on the major deltas extend to depths of 85 m below modern lake level erosional truncation of onlapping seismic reflectors is found at similar depths this interpretation of the seismic data is supported by analyses of sediment cores from the lake which indicate that there was a significant drop in lake level during the early to mid holocene

monthly balance and water discharge of an inter tropical glacier zongo glacier cordillera real bolivia 16 s measurements of mass balance were performed every month on zongo glacier bolivia simultaneously water discharge temperature and precipita tion data were obtained the first year of the survey 1991 92 was marked by an enso el nino southern oscillation event with high temperature and low precipitation whilst the following year 1992 93 was normal results point to the early and late wet season october december and march may as playing a critical role in the determination of the annual mass balance the wet season is the warmest period of the year and consequently the duration of the wet season is a highly relevant variable in determining mass balance both glaciological and hydrological methods for the determination of the mass balance provide similar results our study confirms that enso events have a major influence on the rapid glacier retreat currently affecting this part of the andes

carbon isotope composition of lake sediments in relation to lake productivity and radiocarbon dating abstract carbon 13 profiles and radiocarbon dates were obtained from two canadian kettle basins having similar geological and hydrological characteristics to develop criteria for evaluating the validity of radiocarbon dates on lake sediment from basins in calcareous terrain radiocarbon dates from a site in alberta show a variable hard water effect related to local hydrological changes during postglacial history of the lake basin whereas radiocarbon dates from the other site in ontario show no noticeable influence of old carbon during its history these differences are mainly related to lake water residence time which has influenced carbon isotopic exchange between atmospheric co2 and dissolved inorganic carbon in lake water 13c values for bulk organic sediment and terrestrial and aquatic macrofossils reveal that the main component of lake sediment at both sites is autocthonous in origin furthermore each site supported different submerged aquatic plant communities that used different sources of carbon for photosynthesis thereby imprinting the organic sediments with a characteristic 13c composition both sites reflect a clear relationship between 13c values and paleoproductivity this study shows the individualistic response of the developing lake system to the hydrology lake biota and local geology and demonstrates the problem of using 13c in lake sediments as a single criterion to recognize the validity of radiocarbon dates of lake sediment without supporting paleoecological information

magnetic fabrics in the bjerkreim sokndal layered intrusion rogaland southern norway mineral sources and geological significance abstract magnetic anisotropy can provide important information about mineral fabrics and thus magmatic processes particularly when it is known how multiple mineral species contribute to the anisotropy it may also affect the direction of induced or remanent magnetization with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies here we aim at describing the magnetic fabrics in the bjerkreim sokndal layered intrusion and identifying their carriers anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the bjerkreim sokndal layered intrusion the intrusion is characterized by magmatic layering and has a synform structure with strong foliation on the limbs detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial but the minimum susceptibility and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering the minimum susceptibility and anhysteretic remanence are associated with pyroxene 100 axes and the maximum susceptibility and anhysteretic remanence are sub parallel to the pyroxene 001 axes in layers mcu ivc and mcu ive for which electron backscatter data are available however the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy we propose that the magnetic anisotropy of magnetite free specimens is carried by hemo ilmenite exsolutions within pyroxene in addition to pyroxene itself when present multi domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence due to shape preferred orientation and distribution anisotropy the orientation of the magnetic fabric appears independent of carrier due to their common deformation history but the degree of anisotropy is stronger for magnetite bearing specimens the results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection

lake level chronology on the southern bolivian altiplano 18 23 s during late glacial time and the early holocene stratigraphic analyses of outcrops shorelines and diatoms from the southern bolivian altiplano uyuni coipasa basin reveal two major lacustrine phases during the late glacial period and the early holocene based on a chronology established by radiocarbon and u th control a comparison of14c and230th 234u ages shows that during times of high lake level radiocarbon ages are valid however during low water periods 14c ages must be corrected for a reservoir effect the lacustrine tauca phase started a little before 16 00014c yr b p and the lake level reached its maximum between 13 000 and 12 00014c yr b p a dry event ticana occurred after ca 12 000 and before 950014c yr b p a moderate lacustrine oscillation coipasa event occurred between ca 9500 and 850014c yr b p using a reservoir corrected conventional14c chronology comparisons between the lake level chronology in the uyuni coipasa basin and data from other southern tropical areas of south america suggest that the lacustrine evolution may reflect large scale climatic changes

holocene palaeohydrology of lake titicaca estimated from an ostracod based transfer function lacustrine deposits of holocene age 8000 14c yr bp present were studied in lake titicaca bolivia ostracod assemblages were identified in six cores taken in the three basins which today form lake titicaca factorial correspondence and multiple linear regression analysis of the ostracods have permitted the reconstruction of 1 lake level 2 water salinity and 3 water mg ca ratio variations palaeolake levels indicate a complex evolution of the three basins with some major phases after a flooding at about 8000 yr 14c bp the lake level fluctuated around a mean position until 3900 14c yr bp when a second rise took place it seems however that the present day lake level was not reached until after 680 yr 14c bp some major dry events are also recorded at about 5300 14c yr bp and 2300 14c yr bp the lake titicaca data are consistent with other records from south america and also seem to reflect some world wide events

magnetic fabric vs magnetite and biotite shape fabrics of the magnetite bearing granite pluton of gameleiras northeast brazil abstract a well defined orientation pattern has been evidenced by the magnetic fabric lineation and foliation of the granite pluton of gameleiras northeast brazil this magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves and by the planar arrangement of tabular k feldspar phenocrysts low field anisotropy of magnetic susceptibility properties as well as magnetic mineralogy studies and optical microscopy observations indicate that multidomian iron rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy to examine in more detail the relationships between the magnetic and magmatic fabrics measurements of two dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections angular departure has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen we found that the fabric of biotite correlates strongly with the magnetic fabric 7 8 the fabric of magnetite although not as well defined mainly because of its weak shape anisotropy still correlates with the magnetic fabric 12 18 origin of low field magnetic fabric in the pluton of gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains magnetic interactions between the grains of magnetite where they form clusters may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy concerning the orientation of the magnetic fabric as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals particularly biotite and titanite the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite however as no correlation exists between shapes or clustering of magnetites and strain undergone by the deforming magma the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity

astronomical forcing of contrasting rainfall changes in tropical south america between 12 400 and 8800 cal yr b p abstract today precipitation over tropical south america is largely controlled by the seasonal movements of the inter tropical convergence zone itcz during the summer the itcz is shifted southward due to the warming of the continent paleoclimate data from southeastern amazonia and the central andes indicate that these two areas evolved similarly during the last 30 000 yr however between 12 400 and 8800 cal yr b p eastern amazonia received substantial moisture whereas the bolivian altiplano was arid this suggests that the itcz during summer was then farther north than it is today

isotope hydrology of groundwaters in the pampa del tamarugal chile the water resources in northern chile are extremely scarce and development is limited by this this paper discusses the isotope hydrology of groundwaters in the pampa del tamarugal an area of extreme aridity rainfall 1 mm yr whose groundwater resources are mined for the town of iquique as well as for industrial and agricultural purposes the aim of the project was to obtain information on modern recharge and to delineate if possible recharge environments to obtain the necessary background information a precipitation survey in the high andes as well as spring and surface water studies were carried out the results show that a well defined meteoric water line exists where 2h 7 8 18o 10 3 and altitude effects depend on air mass movements and cannot be defined without a broader regional and detailed sampling programme however it is still possible to assign maximum altitudes of recharge to springs in the andes and at the eastern border of the pampa del tamarugal comparison of these data with groundwater compositions show that these groundwaters originate from infiltrating surface water rather than directly infiltrated precipitation a dependence of individual groundwater systems on specific quebradas river valleys is recognized however low 14c activities indicate that most of the waters pumped today are fossil and at least several hundreds if not thousands of years old some minor subsurface recharge does occur at the foot of the andes especially at pica where high altitude waters discharge but even there groundwater appears to be a diminishing resource

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

magnetic fabric in the biella and traversella stocks periadriatic line implications for the mode of emplacement abstract the biella and traversella stocks forming part of the intrusions along the periadriatic line intruded into the sesia lanzo zone italian western alps 30 ma ago samples were collected from 12 localities in the traversella massif diorites and 22 in the biella massif granites syenites monzonites in most localities the fabric is primarily planar with a distinct foliation of intermediate to subvertical dip the magnetic lineation is less developed its plunge is very variable marking a girdle pattern the mean foliations and lineations per locality are scattered this is probably due to the fact that the outcropping parts of the two stocks are summital i e nearer the roof and hence the magma movements may have been less regular in the structurally deepest part of the biella massif the magnetic foliation dips regularly to the northwest its direction could reflect the pattern of the deep fractures along which the magma rose the classic models of owens to describe the process of magnetic mineral orientation do not apply to the features of these two stocks a new model has been constructed to take account of the differences in viscosity contrast between the magnetic mineral crystals and the matrix referred to as the ductile model it offers a suitable representation of the typical situation of crystal mushes and is the most appropriate for the biella and traversella stocks

albedo changes milankovitch forcing and late quaternary climate changes in the central andes late quaternary humidity changes resulted in substantial modifications of the land surface characteristics in the altiplano of the atacama desert central andes reconstructions of surface albedo top of atmosphere toa albedo and short wave net radiation in the andes of northern chile for 20 14 10 7 and 0 ka suggest that surface and toa albedo increased substantially during periods of relatively humid environmental conditions i e with large palaeolakes glaciers and dense vegetation the decrease of summer shortwave net radiation and seasonality during the late glacial early holocene humid phase 14 to 10 ka due to earth s surface and atmospheric characteristics added to the effect of orbitally driven negative deviations of southern hemisphere austral summer insolation and minimum seasonality at 20 s therefore in situ radiative forcing is in contrast to the northern hemisphere tropics not a suitable explanation for enhanced convective precipitation and ultimately humid climatic conditions our results suggest that late quaternary humidity changes on the altiplano reflect a collective response to 1 environmental changes in the source area of the moisture e g re expansion of the rain forest and increased release of latent heat over amazonia and the chaco warm sea surface temperatures in the e pacific and 2 large scale circulation patterns and wave structures in the upper troposphere strength and position of the bolivian high divergent flow stimulating convection over the altiplano or that they even reflect a response to 3 interhemispherical teleconnections

magnetic anisotropy in clinopyroxene and orthopyroxene single crystals pyroxenes constitute an important component in mafic igneous and metamorphic rocks they often possess a prismatic habit and their long axis the crystallographic c axis helps define a lineation in a textured rock anisotropy of magnetic susceptibility ams serves as a fabric indicator in igneous and metamorphic rocks if a rock s ams is carried by pyroxenes it can be related to their crystallographic preferred orientation and degree of alignment this requires knowing the intrinsic ams of pyroxene single crystals this study provides a comprehensive low field and high field ams investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure chemical composition oxidation state of fe and the possible presence of ferromagnetic inclusions the paramagnetic anisotropy extracted from high field data shows clear relationships to crystallographic directions and fe concentration both in clinopyroxene and orthopyroxene in the diopside augite series the intermediate susceptibility is parallel to b and the maximum is at 45 to the c axis in aegirine the intermediate axis remains parallel to b while the maximum susceptibility is parallel to c the ams of spodumene depends on fe concentration in enstatite the maximum susceptibility aligns with c and the minimum with b and in the case of hypersthene the maximum susceptibility is normal to the exsolution lamellae magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities which dominates the low field anisotropy these results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene bearing rocks

late quaternary spatio temporal limnological variations in the altiplano of bolivia and peru abstract between the western and eastern andean cordilleras in peru and bolivia there are three main lacustrine basins lake titicaca lake poopo and the group of coipasa uyuni for the past few millennia highly variable environmental conditions have been recorded in their sediments today a latitudinal meteorological gradient influences the lakes status leading to specific deposits and ostracod communities lake titicaca in the north is oligohaline whereas lake poopo further south is polyhaline in the south the coipasa uyuni depression is characterized by a 12 000 km2 surficial salt crust during the late pleistocene ca 40 000 to 25 000 yr b p the water depth and salinity in paleolake poopo fluctuated widely and paleolake titicaca was slightly larger than at present sedimentation was mostly biocarbonate in the shallower areas and it was detrital organic in the deepest zones during the holocene a dry period transformed lake poopo into a salar with evaporite precipitation lake titicaca registered a large decline in water level 8100 3600 yr b p initially inducing gypsum precipitation followed by short influxes of water with an ostracod faunal composition similar to that of the modern brines of lake poopo lake titicacas present condition only appeared between 2200 and 1500 yr b p

paleohydrology of a canadian shield lake inferred from 18o in sediment cellulose oxygen and carbon isotope analyses on cellulose in the postglacial sediment of weslemkoon lake southern ontario show that the cellulose came mainly from aquatic plants or algae rather than from terrestrial sources if a wholly aquatic source is assumed the oxygen isotope content permits inferences of lake water 18o values over the past 10 000 years by accounting for the isotopic fractionation that occurs during cellulose synthesis chronological control is provided by pollen analysis and six 14c dates our reconstruction shows lake water 18o fluctuated from about 5 lower than present in the early postglacial to 5 or more above present values during the mid postglacial these broad secular shifts reflect a combination of fluctuating mean annual 18o of local precipitation evaporative isotopic enrichment of surface waters and snowmelt bypass effects the first two factors reflect the changing paleotemperature and paleohydrology respectively whereas the third factor is a more speculative interp

large scale silicate liquid immiscibility a possible example from northeastern brazil abstract ultrapotassic peralkalic silica saturated plutons 580 ma are widespread in the cachoeirinha salgueiro foldbelt northeastern brazil they consist of alkali feldspar syenites with pyroxenite as co magmatic inclusions and syn plutonic or late stage dikes pyroxenite and syenite have the same mineral phases aegirine augite microcline sphene apatite blue amphibole magnetite but only in different proportions rare inclusions of a mixed rock about 60 syenite 40 pyroxenite in an emulsion like texture are also present pyroxenes in the three units are all only slightly zoned silica saturated and extremely low in al2o3 0 2 1 4 amphiboles are mostly k rich richterite characterized by high sio2 low al2o3 and tio2 contents and low mg the three rock types have similar ree chondrite normalized patterns with negative slopes and lack of eu anomaly with the total ree in the pyroxenite greater than that of the syenite trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs the rocks have similar high 18o values avg w r 8 smow corrected from pyroxene high initial 87 sr 86 sr ratios about 0 710 and low 143 nd 144 nd avg 0 51104 field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite the mixed rock representing the original magma composition

deformation history inferred from magnetic fabric in the southwestern okcheon metamorphic belt korea abstract magnetic fabric and rock magnetic studies have been carried out for the non fossiliferous low to medium grade metasedimentary rocks in the southwestern part of the okcheon metamorphic belt omb two major metamorphic events in the study area were previously recognized regional metamorphism m 1 in the late carboniferous to early permian and contact metamorphism m 2 due to the intrusion of granite in the middle jurassic the metamorphic grade of the study area increases from the biotite zone in southeast through the garnet zone to the staurolite zone towards the northwest magnetic fabrics of the study area are generally well defined and can be characterized according to the metamorphic zones magnetic foliation is the dominant magnetic fabric in the biotite zone while magnetic lineation prevails in both garnet and staurolite zones we interpret the metamorphism related deformation history of the study area based on magnetic fabrics magnetic mineralogy and previously reported metamorphic evolution as follows penetrative nw dipping cleavage represented by magnetic foliation was formed in the study area by prevailing nw se shortening event during the m 1 regional metamorphism in the late carboniferous early permian this shortening event is interpreted to be associated with the collisional event between the north and south china blocks cleavages dipping steeply to the southeast in the staurolite zone are attributed to the pressure exerted from the intrusion of jurassic granite in the northwestern area

paleohydrology of the laguna lejia north chilean altiplano and climatic implications for late glacial times paleoenvironmental and sedimentological data from laguna lejia 23 30 s 67 42 w 4325 m in the high altiplano of the chilean atacama desert indicate that climatic conditions during late glacial times were significantly wetter than today a water and energy budget model was used to simulate climatic scenarios that would have resulted in the observed changes in the water levels water surfaces water volumes and salinity during the time interval different climatic scenarios for the laguna lejia catchment include precipitation increases of over 100 up to 400 500 mm yr today 200 mm yr to account for the late glacial lake levels with shorelines 15 25 m higher than at present and a lake surface of 9 11 km2 compared to the present 2 km2 during the lake maximum about 13 500 11 300 yr b p finely laminated sediments consisting of mg calcite diatoms and sometimes gypsum were deposited the paleoenvironmental evidence indicates that north easterly wind direction prevailed and the summer rainfall resulted from a seasonal poleward shift of the tropical circulation the lake history of laguna lejia is representative for the altiplano area between 21 and 24 s

origin and intensive parameters in the crystallization of ultrapotassic syenites the serra do man pluton northeastern brazil abstract the late neoproterozoic ultrapotassic serra do man pluton intruded supracrustal schists and igneous rocks of the alto pajeu tectonostratigraphic terrane in the borborema province of northeastern brazil this quartz alkali feldspar syenite is composed of microcline albite normally zoned aegirine augite winchite formed after pyroxene titanite and apatite with the major element geochemistry in consonance with early fractionation of pyroxene titanite apatite assemblage chondrite normalized ree patterns are lree enriched la lu 20 with discrete negative eu anomaly and primitive mantle normalized elemental concentration diagrams display enhanced negative nb p and ti anomalies that are also observed in patterns of similar rocks in ne brazil whole rock d18o values are rather high avg 8 7 smow and in the 7 8 to 8 6 smow range when clinopyroxene corrected a five point whole rock rb sr isochron yields an age of 580 28 ma with high sro 0 7108 which together with end 0 58 ga value of 17 and tchur of 2 3 ga allow hypothesizing a paleoproterozoic enriched mantle source for the syenites in the ultrapotassic province of the alto pajeu terrane the aegirine augite titanite and quartz assemblage attests to a high oxygen fugacity prevailing during magmatic crystallization the almost complete absence of primary hydrated phases assures that the magma was rather anhydrous and its viscosity at the liquidus temperature was relatively high density is slightly higher than expected for syenitic magmas 2 8 g cm3 being explained by a relatively high iron content of the magma geothermobarometry suggests shallow emplacement and temperatures of 700 750 c and 850 900 c respectively for the solidus and liquidus of the magma

holocene paleohydrology of the tropical andes from lake records two century scale time series in northern bolivia constrain the ages of abrupt changes in the physical geochemical and biological characteristics of sediments obtained from lakes that formed during deglaciation from the late pleistocene glacial maximum the watersheds of laguna viscachani 16 12 s 68 07 w 3780 m and lago taypi chaka kkota 16 13 s 68 21 w 4300 m located on the eastern and western slopes of the cordillera real respectively contain small cirque glaciers a high resolution chronology of the lake sediments is provided by 23 ams14c dates of discrete macrofossils late pleistocene glaciers retreated rapidly exposing the lake basins between 10 700 and 970014c yr b p the sedimentary facies suggest that after 890014c yr b p glaciers were absent from the watersheds and remained so during the middle holocene an increase in the precipitation evaporation balance is indicated above unconformities dated to 230014c yr b p in both lago taypi chaka kkota and laguna viscachani an abrupt increase in sediment accumulation rates after 140014c yr b p signals the onset of neoglaciation a possible link exists between the observed millennial scale shifts in the regional precipitation evaporation balance and seasonal shifts in tropical insolation

rock and magnetic fabric of the renca batholith sierra de san luis argentina constraints on emplacement abstract the renca batholith belongs to a group of post collisional plutons that intruded the early paleozoic basement of the sierra de san luis during late devonian to early carboniferous combined microstructural petrofabric and anisotropy of magnetic susceptibility ams studies indicate that interactions between magmatism and emplacement related fabrics played the main role in the strain patterns with a very subordinate far field control by regional deformation this concentrically foliated zoned elliptical granitoid is made up by porphyric granodiorites and equigranular monzogranites that are associated with syenites and monzonites two major magmatic associations are present i a mg k monzonitic association represented by syenitic monzonitic and monzodioritic enclaves and synplutonic dykes and a porphyroid monzodioritic to melagranodioritic facies and ii a high k calc alkaline association that constitutes the major part of the concentrically zoned batholith with an external porphyroid granodiorite to monzogranite unit 1 and a central equigranular monzogranite unit 2 bulk susceptibility k shows a bimodal distribution that corresponds respectively to unit 1 ferromagnetic and unit 2 paramagnetic similar distribution is observed in the intensity of natural remanent magnetization nrm and ams degree p the steep to vertical inward dipping foliation that is recognized in the porphyroid granitoids is parallel to the x y flattening plane of the enclaves the synplutonic dykes attitude and the contact between the two major units of the batholith the ams defines a predominantly foliated fabric with a distribution of magnetic foliation planes generally subvertical and parallel to the boundaries of the ring like unit 1 these internal features are concordant with the axial planes of the meso and microscale folding of the regional nne s2 s3 in the ductile aureole that developed in the medium grade metamorphic ordovician country rocks foliation results from a combination of both magmatic and high temperature solid state flows the latter as the result of the emplacement of the central monzogranite which caused a radial compression k 0 as measured using strain indicators suggests flattening unit 2 shows a planar magnetic fabric shallowly dipping outwards and dominant subhorizontal lineations the magnetic fabrics of this body agree with other fabric parameters and are consistent with a magmatic fabric there is a strong correlation between rock and magnetic fabrics these fabrics reflect internal magma dynamics whereas regional tectonic exerted a far field control on the expansion of the pluton

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

brasiliano crustal extension and emplacement fabrics of the mangerite charnockite pluton of umarizal north east brazil abstract shear zone kinematics and anisotropy of magnetic susceptibility ams data have been used in a brasiliano pan african fayalite clinopyroxene orthopyroxene syenite of umarizal ne brazil in order to reveal its emplacement mechanisms the pluton is located at the intersection of two major shear zones a ne trending transcurrent portalegre shear zone psz and a nw to e trending frutuoso gomes shear zone fgsz the fgsz dips gently ne in its nw trending branch and n in the e trending branch asymmetric porphyroclasts and s c shear planes observed in mylonitic gneisses along the fgsz point to a top to the east extensional movement hysteresis and low field susceptibility measurements on mangerites and charnockites indicate that magnetic susceptibility is principally carried by multidomain magnetite in the core of the pluton the magnetic anisotropy is weaker p f ams ellipsoids the magnetic anisotropy increases south westward p 1 10 approaching the fgsz and the ams ellipsoid becomes dominantly disc shaped f l magnetic lineations plunge gently to nw in the western part of the pluton and to ene on the east the absence of steep lineations except around a large xenolith in the center of the pluton exclude the presence of magmatic feeding zones at the actual level of exposure foliations are roughly parallel to the pluton contact but most of them dip moderately towards north east in the sw border the magnetic foliation is parallel to the mylonitic foliation displayed in the fgsz although emplaced within migmatized host rocks the kinematics of the shear zones and the magmatic fabric pattern rule out a buoyancy driven diapiric emplacement of the pluton syn magmatic shear deformation associated with e w crustal stretching accounts for the fabrics of emplacement recorded in the umarizal pluton

extrusion tectonics inferred from fabric study of the guanzizhen ophiolitic melange belt in the west qinling orogen central china abstract the recently identified guanzizhen ophiolitic melange belt gomb in the west qinling orogen is regarded by many geochemical and geochronological works as the westward extension of the shangdan belt in the east qinling that separated the north and south china blocks the gomb trends generally nw se and comprises ophiolites and forearc volcanic sedimentary complex the liziyuan group ductile shear zones with various widths are a striking structural feature of the gomb and studying them could contribute to a better understanding of the tectonic processes of the collisional orogeny in this area here we characterize the deformation style of these shear zones by combining the low field anisotropy of magnetic susceptibility ams data microstructural and field observations the new dataset demonstrates that the deformation of these ductile shear zones is dominated by dextral transpressional shearing under low amphibolite grade condition combining the dextral transpression of the gomb and the coeval sinistral transpression of the shangdan belt in the east qinling we propose a late triassic bilateral extrusion tectonics of the qinling orogen in response to the collision between the ordos and sichuan blocks

a new estimate of the holocene lowstand level of lake titicaca central andes and implications for tropical palaeohydrology new evidence from piston cores and high resolution seismic reflection data shows that water levels in lake titicaca were as much as 100 m below the present level during the early to mid holocene between 6 and 3 8 14c kyr bp climatological and modelling studies indicate that lake titicaca rainfall depends on convective activity in upwind amazonia the lake level data therefore suggest a drier amazon basin during this time this view is bolstered by an excellent match between the titicaca lake level curve and decreased methane concentrations in greenland ice previously ascribed to drying of low latitude wetlands blunier et al 1995 the postglacial history of lake titicaca fits a global pattern of lake level change in the tropics characterized by opposite phasing between the southern and northern hemispheres this pattern is most likely the result of orbital controls over the intensity of summer insolation

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

submagmatic microfractures in granites microstructures in hercynian granite plutons of the pyrenees indicate fracturing of plagioclase crystals in the presence of melt this is represented mainly by quartz and or feldspar that fills fractures electron microprobe analyses demonstrate that in compositional agreement with the rims of the zoned plagioclase the feldspar infillings are more sodic with decreasing melt fraction fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic i e when the melt was below the critical fraction for magmatic flow material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures because during cooling the volume fraction of melt remains low for a large temperature range a strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas

isotopic evidence for late quaternary climatic change in tropical south america the tropical hydrologic cycle affects atmospheric trace gases and global climate change and thus records of hydrologic change encompassing a variety of time scales from the low latitudes are important in paleoclimatology isotopic analysis of calcite from lake junin peru provides a record of hydrologic variability that spans the last glacial interglacial transition in the southern tropics the record reveals a 6 enrichment in 18 o calcite during the late glacial followed by a gradual depletion during the holocene which can be interpreted as a decrease followed by a long term increase in effective moisture close agreement between 18 o calcite and rainy season insolation indicates that long term changes in tropical hydrology were linked to orbital variations furthermore hydrologic change was out of phase in the northern and southern tropics over this time period

magnetic and magmatic structures of the emas granodioritic pluton cachoeirinha belt ne brazil relationships with pan african strike slip fault systems abstract the emas granodiorite was intruded during the brasiliano pan african orogeny into the metapelites of the cachoeirinha belt ne brazil this pluton was chosen for a petrofabric study of magmatic emplacement structures using magnetic anisotropy because of its conspicuous position in the junction area between the large e w trending patos shear zone and the ne sw trending cachoeira da mina strike slip fault the magnetic behavior of this calc alkaline granite is dominated by paramagnetic minerals which is optimal for the application of the anisotropy of magnetic susceptibility technique used for this study almost all microstructures correspond to the magmatic state so the measured magnetic lineations and foliations may be interpreted respectively as directions and planes of magmatic flow these magnetic magmatic lineations and foliations are dominantly n s and ne sw oriented and are gently to moderately dipping the anisotropy of magnetic susceptibility ratios range from 1 to 5 4 most of these values corresponding to magmatic alignment the lowest ratios are located chiefly in the centre of the pluton and the highest on the northern and western margins from these data we propose that the emplacement of the emas pluton was controlled by a ne sw trending left lateral strike slip zone parallel to the cachoeira da mina fault giving rise to openings in the metapelitic country rocks infilled by two succesive magmas the principal result of this study is the unexpected independence between the emplacement of the emas pluton and the functioning of the patos dextral shear zone

late quaternary glaciation of the cordillera real bolivia glacial geological studies in tropical mountain areas of the southern hemisphere can be used to address two issues of late pleistocene climate change the global synchroneity of deglaciation and the magnitude of temperature reduction in the tropics radiocarbon dates from the cordillera real and from other areas in peru and bolivia suggest that late pleistocene glaciation culminated between 14 000 and 12 000 yr bp followed by rapid deglaciation because deglaciation was apparently synchronous with that in northern hemisphere regions insolation change at high latitudes may not have been the only factor that produced global deglaciation at this time late pleistocene glaciation in the cordillera real culminated when precipitation was 200 mm yr 1 higher and temperatures were 3 5 1 6 c lower than today this produced an equilibrium line altitude depression of about 300 100 m on the western side of the cordillera prior to this conditions were drier and probably at least as cold however the lack of moraines in the cordillera real dated to the last glacial maximum ca 18000 yr bp precludes using the equilibrium line altitude method to quantitatively evaluate the discrepancy between warm sea surface temperatures and cold terrestrial conditions reconstructed with other proxies for this time period

on the effect of phase transformations on saline ice compliance to obtain a better understanding of the compliance of saline ice we developed a simple conceptual model of a process that presumably takes place in saline ice using elastic models we assessed the role that phase transformations might play when brine is sealed into small cells during crystal growth cooling of a sealed brine cell leading to the precipitation of ice provides a mechanism for the accumulation of large localized stresses based on our analysis this mechanism has the potential to be a significant source for the nucleation of dislocations and can conceivably make a significant contribution to the greater compliance softness of saline ice relative to non saline ice the results of this model are consistent with the observation that laboratory grown saline crystals sometimes display extensive differences in mechanical behavior that appear to be due to variations in the growth and storage conditions experienced by the crystals

recent glacial history and paleoclimate of the peruvian bolivian andes abstract radiocarbon dates of glacial deposits in the peruvian bolivian andes are used to place the observed phases of glaciation in a common chronological framework the climatic conditions that may determine modern snowline are examined in order to understand the types of climatic forcing to which glaciers may have been sensitive in the past also considered are independent proxies of climatic change including pollen sequences lake level history and ice core data late pleistocene glaciation in central peru between 24 ka and 12 ka bp appears to have occurred during colder and perhaps drier conditions in bolivia a glacial maximum is dated to phases of late holocene glaciation in peru that correspond in part to the little ice age in the northern hemisphere appear to have been caused by a combination of colder and wetter conditions an overall warming trend may be the cause for the retreat of many glaciers in the region since the mid 19th century

production sedimentation and isotopic composition of organic matter in lake ontario organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the rochester basin of lake ontario to evaluate their usefulness in reconstructing historic changes in lake productivity the greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events indicating that calcite precipitation is an effective mechanism for sedimenting organic matter carbon isotopes of organic matter were low prior to the onset of stratification increased to maximum values in late summer and then decreased following fall overturn this pattern is controlled mainly by the timing of stratification and primary productivity which preferentially removes 12co2 from the epilimnion the physiological effect of decreased carbon isotopic fractionation with decreasing supplies of co2 aq may have also contributed to increased 13corgc nitrogen isotopes showed a seasonal pattern opposite to that of carbon whereby 15n values were low during the summer stratified period and high for the remainder of the year seasonal variability in 15norgn probably reflects changes in the source of sedimented organic particles which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period a comparison of organic carbon accumulation rates and 13corgc between sediment cores collected in 1987 and 1993 1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters but will not change the 13corgc values all cores analyzed for 13corgc display the reproducible pattern of a progressive increase in the 19oos peaking in the early to mid 1970s and then decreasing to the present this pattern matches the historical trends of phosphorus loading to the basin suggesting that 13c of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column the 15n of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0 3ppt per decade and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment the increase in 15norgn reflects a combination of factors including early forest clearance by europeans increased sewering by municipalities after 1940 and increased nitrate utilization as productivity increased in the lower great lakes increased rates of denitrification in the central basin of upstream lake erie from the 1930s to the early 1970s may have also contributed to the rise in 15norgn values

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

observations of the internal structure of sea ice by x ray computed tomography x ray computed tomography ct was used to observe nondestructively the internal structure of sea ice grown in the laboratory the ct images were compared with photographs taken by the conventional method both brine layers and brine drainage channels which are characteristic sea ice features were clearly recognizable on the ct images the ct scanner is an extremely useful tool for examinations of sea ice structure and can possibly be applied to three dimensional measurements of ice core density

formation processes of brine drainage channels in sea ice this paper is an experimental study of the crystallographic circumstances governing the formation of brine drainage channels in sea ice the study shows that brine channels always form in the grain boundaries toward which the ice platelets grow from both sides and that they do not form at the boundaries from which the platelets slant away these observations imply that brine expulsion and gravity drainage are the major contributors to sea ice desalination

an early extensional event of the south china block during the late mesozoic recorded by the emplacement of the late jurassic syntectonic hengshan composite granitic massif hunan se china continental scaled extension is the major late mesozoic jurassic and cretaceous tectonic event in east asia characterized by faulting magmatic intrusions and half grabens in an area with a length of 5000 km and a width of 1000 km numerous studies have been conducted on this topic in the south china block scb however the space and time ranges of the compressional or extensional regimes of the scb during the jurassic are still unclear partly due to the lack of structural data the emplacement fabrics of granitic plutons can help determine the regional tectonic background in this study a multidisciplinary approach including anisotropy of magnetic susceptibility ams macro and micro structural analyses quartz c axis preferred orientation gravity modeling and monazite epma dating was conducted on the hengshan composite granitic massif in scb that consists of the triassic nanyue biotite granitic pluton and the late jurassic baishifeng two mica granitic pluton the magnetic fabrics are characterized by a consistent nw se oriented lineation and weakly inclined foliation a dominant high temperature deformation with a top to the nw shear sense is identified for both plutons the deformation increasing from the center of the baishifeng pluton to its western border is associated to the development of the west hengshan boundary fault whbf the gravity modeling shows a saw tooth shaped ne sw oriented structure of the baishifeng pluton which may be considered as ne sw oriented tension gashes formed due to the nw se extension all results show that the triassic nanyue pluton was deformed under post solidus conditions by the whbf coeval with the emplacement of the late jurassic baishifeng pluton all these observations comply with the nw se extensional tectonics coeval with the emplacement of the baishifeng pluton which argues that the nw se crustal stretching started since the late jurassic at least in this part of the scb

assessment of petrophysical surface data of the harsit granitoid in trabzon in northeastern turkey the harsit granitoid in northeastern turkey comprises four separate granite units that are apparently unfoliated the harsit granitoid was investigated here by using microstructural petrofabric and anisotropy of magnetic susceptibility ams data the structural data of the granitoid were found to be highly compatible with the zonation recognized from ams measurements the orientation of magnetic fabrics within the granite units indicates that tectonic deformation might have occurred coevally with the magmatic emplacement of the intrusion when we evaluated the manners on the scale of the pluton that the disruption took place in the form of uplifting probably related to a rapid migration of the volcanic front and the documented change from deep sea sedimentation predominant until late cretaceous to shallower environments during the paleocene the possible tectonic control of fault systems on the ascent and emplacement of the granitic magma in the study area however cannot be completely ruled out because the harsit granitoid is situated at or very near the naf systems in northern anatolia in any case the intrusion is clearly discordant to the regional deformational features formed during the collisional events between the eurasia and anatolia plates in northern turkey

climate variation and the rise and fall of an andean civilization paleolimnological and archaeological records that span 3500 years from lake titicaca and the surrounding bolivian peruvian altiplano demonstrate that the emergence of agriculture ca 1500 b c and the collapse of the tiwanaku civilization ca a d 1100 coincided with periods of abrupt profound climate change the timing and magnitude of climate changes are inferred from stratigraphic evidence of lake level variation recorded in14c dated lake sediment cores paleo lake levels provide estimates of drainage basin water balance archaeological evidence establishes spatial and temporal patterns of agricultural field use and abandonment prior to 1500 b c aridity in the altiplano precluded intensive agriculture during a wetter period from 1500 b c to a d 1100 the tiwanaku civilization and its immediate predecessors developed specialized agricultural methods that stimulated population growth and sustained large human settlements a prolonged drier period ca a d 1100 1400 caused declining agricultural production field abandonment and cultural collapse