late collision rhyolitic volcanism in the north eastern part of the armenian highland abstract plio pleistocene acidic volcanism of the northeastern part of the armenian highland is related to a continental collision zone as a result of convergence between eurasia and arabia lithosphere plates the development of volcanism is divided into three stages of 10 17 4 5 7 5 and 0 1 2 8 ma a crustal origin model is proposed to explain the geochemical and petrogenetic aspects of the acidic volcanism formation spatially separated ba rich and rb rich geochemical types of rhyolites were revealed indicating more evolved and primitive rhyolitic magma types respectively the variation interval of the primary 87sr 86sr ratio was found to be from 0 70438 to 0 70636 age and lateral variation of geochemical and isotopic parameters were determined by differences of initial substrates and degrees of their partial melting the mechanisms of rhyolitic magma evolution were the remelting of heterogeneous sialitic materials and low pressure fractional crystallization in isolated magmatic chambers these facts confirm the model of crustal anatexis origin of the eutectic melts the eutectic rhyolitic magma was generated by an open system melting in the presence of deep horizon mobile k rb fluids initiated by mantle resources

the influence of petrophysical properties on the salt weathering of porous building rocks the influence of pore structure water transport properties and rock strength on salt weathering is evaluated by means of a thorough rock characterisation and a statistical analysis the pore structure was described in terms of its porosity pore size distribution quantified by mean pore radius and specific surface area density and water transport was characterised by means of water permeability saturated flow and capillary imbibition unsaturated flow whilst the rock strength test was carried out using uniaxial compressive strength compressional and shear wave velocities dynamic elastic constants and waveform energy and attenuation were obtained from the digital analysis of the transmitted signal a principal component analysis and a stepwise multiple regression model was carried out in order to examine the direct relationships between salt weathering and petrophysical properties from the principal component analysis two main components were obtained and assigned a petrophysical meaning the first component is mostly linked to mechanical properties porosity and density whereas the second component is associated with the water transport and pore structure salt weathering quantified by the percentage of weight loss after salt crystallisation was included in both principal components showing its dependence on their petrophysical properties the stepwise multiple regression analysis found that rock strength has a predominant statistical weight in the prediction of salt weathering with a minor contribution of water transport and pore structure parameters

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

40ar 39ar and k ar chronology of pleistocene glaciations in patagonia during the pleistocene east of lago buenos aires argentina at 46 5 s at least 19 terminal moraines were deposited as piedmont glaciers from the patagonian ice cap advanced onto the semi arid high plains adjacent to the southern andes exceptional preservation of these deposits offers a rare opportunity to document ice cap fluctuations during the last 1 2 m y 4 0 ar 3 9 ar incremental heating and unspiked k ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence the 4 0 ar 3 9 ar isochron ages of three lavas that overlie till 90 km east of the cordillera at lago buenos aires and another 120 km from the andes along rio gallegos at 51 8 s that underlies till strongly suggest that the ice cap reached its greatest eastward extent ca 1100 ka at least six moraines were deposited within the 256 k y period bracketed by basaltic eruptions at 1016 10 ka and 760 14 ka similarly six younger more proximal moraines were deposited during an 651 k y period bracketed by an underlying 760 14 ka basalt and the 109 3 ka cerro volcan basalt flow that buried all six moraines coupled with in situ cosmogenic surface exposure ages of moraine boulders the 109 ka age of cerro volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6 further westward toward lago buenos aires six additional moraines younger than the cerro volcan basalt flow occur surface exposure dating of boulders on these moraines combined with the 1 4 c age of overlying varved lacustrine sediment indicates deposition during the last glacial maximum lgm 23 16 ka although antarctic dust records signal an important patagonian glaciation at 60 40 ka moraines corresponding to marine oxygen isotope stage 4 are not preserved at lago buenos aires apparently these were overrun and obliterated by the younger ice advance at 23 ka notwithstanding the overall pattern of glaciation in patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m y we hypothesize that tectonically driven uplift of the patagonian andes which began in the pliocene yet continued into the quaternary in part due to subduction of the chile rise spreading center during the past 2 m y maximized the ice accumulation area and ice extent by 1 1 ma subsequent deep glacial erosion has reduced the accumulation area resulting in less extensive glaciers over time

records of the cobb mountain subchron from the bermuda rise odp leg 172 abstract duplicate records of the cobb mountain subchron were obtained from two holes holes 1063b at 33 41 204 n 57 36 w and 1063c at 33 41 181 n 57 36 w drilled in a sediment drift at ocean drilling program odp site 1063 on the northeast bermuda rise we sub sampled the cores from both holes using u channels up to 1 5 m long samples with 2 2 cm cross section focusing on continuous sampling through the late matuyama interval containing the cobb mountain subchron results of standard progressive alternating field demagnetization experiments reveal that these sediments generally have a stable characteristic component after removal of a low coercivity drilling overprint additional discrete samples 7 cm 3 oriented cubes used in progressive thermal demagnetization studies display similar behavior by taking duplicate sets of u channels we were able to identify and remove several measurement artifacts such as edge effects that affect measurements made near the ends of the u channel samples the upper normal to reverse polarity transition was not obtained because of a coring gap or an interval of coring deformation after removing measurement artifacts the records from the two holes were stacked to obtain a composite record virtual geomagnetic poles vgps computed from the composite record follow a path through the central pacific during the lower polarity transition very similar to vgp paths from cobb mountain subchron records from other sites around the world furthermore the vgps cluster near southern africa early in the reversal process and then group in a nearly antipodal patch in the north pacific late in the reversal taken together these records indicate that the transitional field remained nearly dipolar through the polarity reversal or at least during the beginning and end of the reversal when the field was dominated by an axial dipole tilted about 20 40 to the spin axis if a tilted dipole is an inherent part of the initiation and ending of a reversal it may help explain why many sediment recorders which are thought to smooth the field record antipodal vgp paths through many reversals

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited new 40 ar 39 ar age determinations on sanidine from the rhyolite of alder creek california indicate a 1 186 0 006 ma age for the cobb mountain normal polarity subchron the new age is statistically older 0 05 than the previously reported k ar age 1 12 0 02 ma and agrees with the age suggested by the astronomical polarity time scale incomplete extraction of radiogenic 40 ar 40 ar from the sanidine is the most likely reason for the disparity between the 40 ar 39 ar and k ar ages because the cobb mountain subchron is a worldwide short duration event and because no widely used interlaboratory 40 ar 39 ar standard younger than 27 ma exists we propose that sanidine from the rhyolite of alder creek be considered for use as a new quaternary 40 ar 39 ar mineral standard

arabia eurasia collision and the forcing of mid cenozoic global cooling the end of the eocene greenhouse world was the most dramatic phase in the long term cooling trend of the cenozoic era here we show that the arabia eurasia collision and the closure of the tethys ocean gateway began in the late eocene at 35 ma up to 25 million years earlier than in many reconstructions we suggest that global cooling was forced by processes associated with the initial collision that reduced atmospheric co2 these are 1 waning volcanism across southwest asia 2 increased organic carbon storage in paratethyan basins e g black sea and south caspian 3 increased silicate weathering in the collision zone and 4 a shift towards modern patterns of ocean currents associated with increased vigour in circulation and organic productivity

capillary water absorption characteristics of some cappadocian ignimbrites and the role of capillarity on their deterioration cappadocia stands out as one of the most important regions in turkey for its natural historical and cultural heritages the region is under the influence of wetting and drying and freezing and thawing weathering processes which are largely controlled by the water content water migrates through rock pores via different processes such as capillary water absorption which is one of the most common ways observed in the cappadocia region in this study a research program consisting mainly of laboratory studies was carried out to investigate the capillary water absorption characteristics of ignimbrites which are the host rocks of many natural and historical heritage structures in cappadocia accordingly xrd analyses mercury porosimeter and capillary water absorption tests were performed the pore size distribution which is a function of welding degree controls the capillary water absorption process of ignimbrites particularly ignimbrites that have a uniform pore size distribution of around 5 10 m reveal higher capillary water absorption potential ignimbrites have a considerable and variable potential in terms of capillarity and the capillarity plays a major role on the deterioration and decomposition of many of the historical and natural heritage structures in the cappadocia region

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

s wave velocity structure of the canadian upper mantle abstract we investigate mantle structure beneath the canadian landmass using 500 surface wave waveforms recorded at stations of the canadian national seismic network and global seismic network the data are processed using the method of partitioned waveform inversion and afford a comprehensive sampling of s velocity structure beneath the north american craton and adjacent orogenic belts our study thereby extends results from previous regional s wave studies north of the coterminous us and provides a characterization of mantle structure beneath canada which is complementary to the body wave model of grand j geophys res 99 1994 11591 the resulting 3d velocity model is well resolved to a depth of 400 km over most of the region and displays two prominent large scale anomalies a high velocity structure beneath the canadian shield and platform associated with the cratonic keel and pronounced low velocities beneath the cordillera the contrast between these features is strong up to 10 and at more southerly latitudes is sharp with the transition occurring over distances of 600 km or less laterally coherent high velocities persist to depths of ca 250 km beneath the north american craton which we interpret to represent the base of the continental keel smaller scale variations are observed within the cratonic mantle but in general show little correlation with surface geological provinces moderately low velocities beneath the st lawrence valley region and labrador may be related to intracontinental volcanism and rifting respectively

plio pleistocene time averaged field in southern patagonia recorded in lava flows 1 paleomagnetic directions were obtained from stepwise alternating field or thermal demagnetization of 53 lava flows from southern patagonia latitudes 49 5 52 1 s that include the pali aike volcanic field and the meseta viscachas plateau lavas in addition to previous miocene late quaternary ages of these flows 40ar 39ar dates spanning from 0 1 to 15 4 ma were obtained for 17 of the sites all except one of the magnetic polarities coincide with the expected polarities of the magnetic polarity timescale cande and kent 1995 for the obtained 40ar 39ar ages the mean direction from 33 sites eliminating sites 4 ma that pass a selection criteria of 95 5 is dec 358 7 inc 68 2 95 3 5 a value that coincides within the statistical uncertainty with the direction of the geocentric axial dipole for that area inc 68 1 likewise the mean virtual geomagnetic pole vgp coincides within the statistical uncertainty with the geographic north pole the secular variation described by the vgp angular standard deviation for these sites is 17 1 a value expected for that latitude according to model g of paleosecular variation mcfadden et al 1988 the characteristics of the data presented are optimum for time averaged field taf studies because of the good age control and good quality of the paleomagnetic data 1 primary components of magnetization were obtained using principal component analysis kirschvink 1980 from at least five points and maximum angular deviation 5 2 site means were calculated with fisher statistics using at least three samples and c 38 of the 53 flows had 95 5 no results five sites or high 95 values 5 were obtained primarily from sites affected by lightning

paleomagnetic secular variation of the honolulu volcanic series 33 700 ka o ahu hawaii abstract thirty years after the precursory study j geophys res 13 1972 2129 19 dated sites with ages between 33 and 700 ka from the honolulu volcanic series hvs southeastern part of the island of o ahu in hawaii have been revisited a minimum of 16 samples per site have been studied using alternating field and thermal demagnetization both techniques yielded similar results coeval flows could be identified by relying on their k ar ages similar rock magnetic parameters and identical directions reliable paleomagnetic directions have been obtained for all sites two site mean directions which deviate significantly from the geocentric axial dipole gad correspond to periods of low dipole intensity and may thus reflect a larger proportion of non dipole components the average value of the site mean declinations is 358 6 while the mean inclination of 34 5 is 3 5 lower than the value of the geocentric dipole at the site the north geographic pole is largely contained within the uncertainties surrounding the position of the mean virtual geomagnetic poles vgp latitude the deviation of 2 3 in latitude is further reduced by 1 if we consider the existence of a standing small quadrupole component that would represent 5 of the dipole the angular dispersion of the virtual geomagnetic poles 12 is only 1 7 smaller than expected at the site latitude and thus not a firm indication of low secular variation thus there is no significant difference with the global time averaged field for the same period other recent results from the same island for an older time period display similar characteristics but with reverse and normal polarities that are not perfectly antipodal it is suggested that this could result from changes in the mean dipole field intensity during these time intervals this points out to the importance of scrutinizing the entire field vector in any analysis of paleosecular changes these results do not bring any evidence for a persistent field anomaly under hawaii

paleosecular variation from easter island revisited modern demagnetization of a 1970s data set abstract sixty two brunhes age lava flows from easter island in the eastern pacific ocean collected and processed for paleomagnetic and secular variation studies in the 1970s have been reprocessed to meet current standards of demagnetization alternating field demagnetization in 7 10 steps was performed on all samples previously undemagnetized and characteristic directions determined from principal component analysis the final data set gives a mean direction of i 46 1 and d 357 4 with 95 3 9 eliminating sites with 95 confidence circles greater than 10 0 gave an angular dispersion of virtual geomagnetic poles vgps of 14 1 with respect to the rotation axis this value is slightly larger than that determined prior to detailed demagnetization and agrees with values predicted for this latitude by model g there is no compelling evidence from this study supporting the existence of a secular variation low in the southeastern pacific region

the santa rosa event 40ar 39ar and paleomagnetic results from the valles rhyolite near jaramillo creek jemez mountains new mexico the jaramillo event was originally defined by doell and dalrymple in 1966 on the basis of k ar ages from sanidine in the normally transitionally and reversely magnetized rhyolite domes named cerro del abrigo cerro santa rosa i and cerro santa rosa ii respectively that erupted following collapse of the archetypal valles caldera new mexico we have collected new paleomagnetic data from the three domes and new 40 ar 39 ar laser fusion and furnace incremental heating experiments on sanidine crystals from the cerro santa rosa i rhyolite step wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of doell and dalrymple science 152 1966 1060 1061 are valid cerro del abrigo is normally magnetized whereas the santa rosa i dome is transitional with an inclination of 363 toward the east and the santa rosa ii dome is of reversed polarity twenty five laser fusion experiments on sanidine crystals from the the cerro santa rosa i dome together with the saddle shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption our preferred age of 936 8 ka 2c for the santa rosa i rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages this age is significantly older than was inferred on the basis of earlier laser fusion 40 ar 39 ar results that suggested a trapped component characterized by a 40 ar 36 ar ratio lower than the atmospheric value of 295 5 our new age distinguishes the santa rosa i dome by 65 kyr from the termination of the jaramillo normal subchron 1001 10 ka and by 37 kyr from basaltic lavas at haleakala and tahiti which record the kamikatsura event 899 6 ka moreover this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period the cerro santa rosa i rhyolite dome once intimately linked with the end of the jaramillo event and the acceptance of plate tectonic theory now defines a highly resolved feature the santa rosa event in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments fl 2002 elsevier science b v all rights reserved

clay swelling mechanism in tuff stones an example of the hilbersdorf tuff from chemnitz germany hydric expansion is considered an important factor for the weathering behavior and deterioration of tuffs which are utilized as building stones and typically associated with the presence of swellable clay minerals two types of swelling mechanisms are discussed when swellable clay minerals are present stepwise intracrystalline swelling and continuous osmotic swelling a mechanism that can cause expansion in the absence of swellable clay minerals which is characterized by interaction of surface forces is the disjoining pressure the identification of the primary mode of swelling is important for understanding and finally preventing the swelling damage in tuff stones the hydric expansion of the hilbersdorf tuff can exceed the values of typical volcanoclastic materials by multiples but partly shows the absence of swellable clay minerals in this study extensive mineralogical and petrophysical investigations on five varieties of the hilbersdorf tuff were performed swelling experiments demonstrated that intracrystalline swelling plays a major role when swellable clay minerals are present furthermore the importance of the structure and location of the clay minerals in the rock fabric is highlighted the investigation suggests that the disjoining pressure plays an important role for the hydric expansion of two varieties which are free of swellable clay minerals but show high microporosity and pores smaller than 2 nm

non antipodal directions in magnetostratigraphy an overprint bias s u m m a r y paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal as one would expect for an axial geocentric dipole model specifically in magnetostratigraphic studies of tertiary sediments which include large data sets of paleomagnetic directions of both polarities it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions we investigate the conventional explanation for such asymmetry which involves the presence of a persistent partially unremoved present day field magnetization in the studied rocks both plate motion history and inclination shallowing which we evaluate in detail can play an important role in the paleomagnetic record our analysis shows that alternatively it is plausible that the inclination anomaly is due to a contribution of an axial octupole in accordance with observations of the recent 0 5 ma geomagnetic field recorded in lava flows

high and low resolution images of the earth s mantle implications of different approaches to tomographic modeling recently published images of the earth s mantle are characterized by a nominal resolution much higher than that used in previous studies where substantially different techniques were employed the agreement between such high resolution and low resolution images often seems very poor in an attempt to determine the reason for this discrepancy we analyze how the choice of inversion algorithm exact or iterative regularization norm or roughness minimization and parameterization spherical harmonics up to a variable degree blocks affects a tomographic model we also investigate the effects of the varying density of the data coverage on the final solution in our experiments we employ two seismic data sets rayleigh wave phase velocity at 75 s period and p wave travel times we construct a new model of p velocity in the mantle bdp98 based on the international seismological center bulletins 1964 1992 we use our findings in an evaluation of recent mantle models including our own focusing on similarities and discrepancies between models of different nominal resolution in all the models the long wavelength component is the most stable however consistent high resolution details probably corresponding to features of the real earth are also seen in general we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem rather than from actual errors or approximations

40ar 39ar ages and paleomagnetism of sao miguel lavas azores we present new 40 ar 39 ar ages and paleomagnetic data for sao miguel island azores paleomagnetic samples were obtained for 34 flows and one dike successful mean paleomagnetic directions were obtained for 28 of these 35 sites 40 ar 39 ar age determinations on 12 flows from the nordeste complex were attempted successfully ages obtained are between 0 78 ma and 0 88 ma in contrast to published k ar ages of 1 ma to 4 ma our radiometric ages are consistent with the reverse polarity paleomagnetic field directions and indicate that the entire exposed part of the nordeste complex is of a late matuyama age the duration of volcanism across sao miguel is significantly less than previously believed which has important implications for regional melt generation processes and temporal sampling of the geomagnetic field observed stable isotope and trace element trends across the island can be explained at least in part by communication between different magma source regions at depth the 40 ar 39 ar ages indicate that our normal polarity paleomagnetic data sample at least 0 1 myr 0 0 1 ma and up to 0 78 myr 0 0 78 ma of paleosecular variation and our reverse polarity data sample approximately 0 1 myr 0 78 0 88 ma of paleosecular variation our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long term geomagnetic field behavior negative inclination anomalies are observed for both the normal and reverse polarity time averaged field within the data uncertainties normal and reverse polarity field directions are antipodal but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction 1998 elsevier science b v all rights reserved

contrasts in lithospheric structure within the australian craton insights from surface wave tomography abstract contrasts in the seismic structure of the lithosphere within and between elements of the australian craton are imaged using surface wave tomography new data from the wacraton and tigger experiments are integrated with re processed data from previous temporary deployments of broad band seismometers and permanent seismic stations the much improved path coverage in critical regions allows an interpretation of structures in the west of australia and a detailed comparison between different cratonic regions improvements to the waveform inversion procedure and a new multi scale tomographic method increase the reliability of the tomographic images in the shallowest part of the model 75 km a region of lowered velocity is imaged beneath central australia and confirmed by the delayed arrival times of body waves for short paths within the cratonic lithosphere there is clearly structure at scale lengths of a few hundred kilometres resolution tests indicate that path coverage within the continent is sufficient to reveal features of this size in the upper part of our model in western australia differences are seen beneath and within the archaean cratons at depths greater than 150 km faster velocities are imaged beneath the yilgarn craton than beneath the pilbara craton in the complex north australian craton a fast wavespeed anomaly continuing to at least 250 km is observed below parts of the craton suggesting the possibility of archaean lithosphere underlying areas of dominantly proterozoic surface geology

revised and updated paleomagnetic results from costa rica 1 paleomagnetic results from globally distributed lava flows have been collected and analyzed under the time averaged field initiative tafi a multi institutional collaboration started in 1996 and designed to improve the geographic and temporal coverage of the 0 5 ma paleomagnetic database for studying both the time averaged field and its very long term secular variations paleomagnetic samples were collected from 35 volcanic units either lava flows or ignimbrites in costa rica in december 1998 and february 2000 from the cordilleras central and guanacaste the underlying canas liberia and bagaces formations and from volcano arenal age estimates range from approximately 40 ka to slightly over 6 ma although initial results from these sites were used in a global synthesis of tafi data by johnson et al 2008 a full description of methodology was not presented this paper documents the definitive collection of results comprising 28 paleomagnetic directions 24 normal 4 reversed with enhanced precision and new geological interpretations adding two paleointensity estimates and 19 correlated 40ar 39ar radiometric ages the average field direction is consistent with that of a geocentric axial dipole and dispersion of virtual geomagnetic poles 17 3 4 6 is in general agreement with predictions from several statistical paleosecular variation models paleointensity estimates from two sites give an average field strength of 26 3 t and a virtual axial dipole moment of 65 zam2 the definitive results provide a useful augmentation of the global database for the longer term goal of developing new statistical descriptions of paleomagnetic field behavior

a morphotectonic analysis of central patagonian cordillera negative inversion of the andean belt over a buried spreading center 1 we use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the central cordillera of patagonia between 46 s and 48 s in a region overlying subducted segments of the chile spreading ridge this region shows a number of peculiar features not expected in a normal subduction related belt located away from a buried spreading center first it is characterized by overall short wavelength topography and highly contrasted reliefs typical of deeply incised young mountain belts but surprisingly also by an average very low elevation of 900 m second the cordillera lacks central high reliefs instead there is a central depressed domain with average low slope gradients including the deepest point of the study area 300 m below sea level in the bottom of lake general carrera buenos aires this depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips to the east the central patagonian cordillera is now separated from the flat plains of the pampean domain representing its former piedmont and several elevated basaltic plateaus mesetas by 160 n 180 n trending morphotectonic corridors and the western border scarps of the mesetas these scarps correspond to neotectonic features controlled by recent vertical faults mostly active after 3 ma remnants of a former peneplain the avellanos surface are widespread within the entire cordilleran region from geological evidence this peneplain formed between 15 and 4 ma and was connected originally with the mesetas domain and now is dissected by these corridors it is shown that a negative inversion related to extensional and strike slip faulting occurred in the frontal region of the cordillera at 3 ma synchronously with the emplacement of magmatic products timing of the extensional deformation in central patagonia is concomitant with the migration at depth of different segments of the south chile ridge which entered the trench at 6 and 3 ma respectively and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions finally migration of segments of the chile spreading ridge since 6 ma triggered regional uplift of the cordillera followed by limited extension in relation with increased thermal weakening of the crust

age and magnetism of lavas in jokuldalur area eastern iceland gilsa event revisited abstract we present results of paleomagnetic measurements and k ar age determinations of 38 lava flows collected in five separate sections in the jokuldalur area of eastern iceland including one section previously studied by watkins et al watkins n d kristjansson l mcdougall i 1975 a detailed paleomagnetic survey of the type location for the gilsa geomagnetic polarity event earth planet sci lett 27 436 444 these sites are close to the type locality of the normal gilsa event in the matuyama chron first identified by mcdougall and wensink mcdougall i wensink h 1966 paleomagnetism and geochronology of the pliocene pleistocene lavas in iceland earth planet sci lett 1 232 236 using new experimental results as well as field observations we could correlate the sequence of lava flows in the jokuldalur area spanning the time interval between 1 8 and 0 5 ma the magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for brunhes matuyama ages given by cande and kent cande s c kent d v 1995 revised calibration of the geomagnetic polarity timescale for the late cretaceous and cenozoic j geophys res 100 6093 6095 based on marine magnetic anomalies except that the gilsa event needs to be added existence of olduvai age lavas under gilsa was inferred but not certain our results are fairly in good agreement with the former studies by wensink wensink h 1964a secular variation of earth magnetism in plio pleistocene basalts of eastern iceland geol mijnbouw 43 403 413 wensink h 1964b paleomagnetic stratigraphy of younger basalts and intercalated plio pleistocene tillites in iceland geol rund 54 364 384 and watkins et al watkins n d kristjansson l mcdougall i 1975 a detailed paleomagnetic survey of the type location for the gilsa geomagnetic polarity event earth planet sci lett 27 436 444 but the assignment to the polarity zones is different because of the newly obtained k ar ages based on the present study we propose that the gilsa event is a short normal subchron in the matuyama chron distinct from and above the more well established oldvai subchron

pore property as an indicator of macro deterioration in slightly weathered tuffs abstract tuff rock is highly heterogeneous due to volcanic activity hydrothermal alteration and weathering it is difficult to explain the deterioration mechanisms of the weathered rocks merely based on mineralogical and chemical parameters it has been reported that subtle weathering can modify pore structure and subsequently affect the rock mechanical behavior of tuffs in this work the pore characteristics in slightly weathered tuffs and fresh tuffs are systematically analyzed based on nuclear magnetic resonance nmr and mercury intrusion porosimetry mip tests additionally meso level uniaxial compression tests were conducted on the tuff samples by utilizing the mti lm miniature tensile instrument light microscope system to detect the crack propagation and deformation process the results demonstrated that additional mesopore 10 50 nm increases with pore throat expansion during weathering and the change of pore structure influences the tuff failure mode specifically for the slightly weathered tuff crack initiates in the altered minerals or matrix and results in shear failure however as for the fresh tuff crack usually initiates in the intact and fresh minerals and matrix and eventually leads to tensile failure therefore we propose that tuff pore property can serve as potential indicator of the micro mechanism of substantial macro deterioration due to weathering

paleomagnetism and 40 ar 39 ar geochronology of the plio pleistocene boring volcanic field implications for the geomagnetic polarity time scale and paleosecular variation abstract paleomagnetic directions and 40ar 39ar ages have been determined for samples of lava flows from the same outcrops where possible for 84 eruptive units ranging in age from 3200 ka to 60 ka within the boring volcanic field bvf of the pacific northwest usa this study expands upon our previous results for the bvf and compares the combined results with the current geomagnetic polarity time scale gpts lava flows with transitional directions were found within the bvf at the matuyama brunhes and jaramillo matuyama polarity boundaries and replicate ages corresponding to these and other boundaries have been newly ascertained although the bvf data generally agree with gpts chronozone boundaries they indicate that onset of the gauss matuyama transition and olduvai subchron occurred significantly earlier than given in the current time scale calibration additional comparisons show that the bvf results are consistent with recent statistical models of geomagnetic paleosecular variation

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

re os isotopic evidence for archean lithospheric mantle beneath the kimberley block western australia we report sm nd and the first re os isotopic data as well as platinum group element concentrations for two of the highest grade diamond deposits in the kimberley block of western australia whole rock sm nd isotopic data for the 1200 ma argyle olivine lamproite and an argyle peridotite xenolith yield unradiogenic initial isotopic compositions nd 3 2 to 6 0 and depleted mantle model ages of 1750 to 2000 ma these data indicate that shallow mantle light rare earth element enrichment of the argyle lamproite source probably occurred during the proterozoic hooper orogeny sm nd isotopic data for the 800 ma seppelt kimberlite yield a radiogenic initial isotopic composition nd 1 8 and a t dm model age of 1200 ma isotopic features consistent with worldwide group i kimberlite occurrences re os isotopic data yield unradiogenic initial isotopic compositions for the argyle lamproite and peridotite xenoliths os 2 to 6 and seppelt kimberlite and kimberlitic chromites os 7 to 8 whereas data for a picroilmenite megacryst from the 800 ma maude creek kimberlite yield a radiogenic initial os isotopic composition os 27 our modeling suggests that the os isotopic composition of the argyle lamproite and peridotites may be explained as mixtures of a ca 3000 ma refractory mantle component as represented by the kimberlitic chromites and a ca 1500 ma enriched mantle component as represented by the picroilmenite megacryst the xenoliths and chromites yield an imprecise re os isochron age of 3400 ma giving a strong indication that the kimberley block is underlain by archean continental lithospheric mantle and that this mantle was largely unaffected during early proterozoic orogenesis

moraines and marls giant landslides of the lago pueyrredon valley in patagonia argentina abstract giant catastrophic landslides 108 m3 dot the formerly glaciated mountain forelands of the eastern patagonian andes from geomorphic mapping sedimentological logs and radiocarbon dating we infer the emplacement kinematics and approximate timing of giant landslides in moraines and other glacial deposits in the lago pueyrredon valley lpv argentina for the first time we report in detail examples of giant low gradient landslides with hummocky lobes derived from unconsolidated glacial deposits and weak bedrock we find that at least 4 5 km3 of debris and weak bedrock were mobilized by slope failures in an area of 500 km2 since the last glacial maximum lgm 25 18 ka nearly 90 of this landslide volume originated along the shores of or as subaqueous failures in a postglacial moraine dammed meltwater lake the larger landslides 1 km3 detached from moraines fringing the lake whereas other landslides displaced glacial and lake deposits either on the paleolake bed or in a river gorge that was cut upon drainage of the glacial lake sequences of till glaciofluvial and glaciolacustrine deposits overlying weak early miocene marlstone are mostly conducive to major landslides in the lpv cross cutting relationships indicate that largest landslides in the area originated during rapid glacial lake level drops distribution and internal structure of hummocks within landslide lobes suggest that these landslides were emplaced as catastrophic debris avalanches it suggests that giant catastrophic landslides in the glacier forelands of patagonia can result from layered weak rocks changes in meltwater lake levels and possibly as a consequence of strong earthquakes linked to rapid post glacial rebound following the retreat of the patagonian ice sheet pis

comparison of remote sensing observations of planetary wind streaks abstract wind streaks are common aeolian features observed on terrestrial planets they were first identified on mars thus the associated terminology is based on mars related observations usually describing surface patterns with distinct albedos however terms related to reflected tones are only relevant to past knowledge for mars and not necessarily suitable for description of radar visible streaks located on venus titan or earth furthermore the sensor type used to observe wind streaks can influence the subsequent data analysis the purpose of this study was to examine the effects of sensor type on wind streak identification and interpretation six case studies featuring wind streaks on earth were performed using imaging by both radar and optical sensors the results indicate that wind streak identification is constrained to a specific combination of sensor settings appropriate for the local surface properties only half the optically visible wind streaks considered in this study were also radar visible but all the wind streaks observable in the radar images also appeared in the optical images furthermore bright and dark reflectance and backscatter are relative terms and should be used with caution these results suggest that the venusian wind streak database is most likely far from complete and that many more streaks exist

miocene to late quaternary patagonian basalts 46 47 s geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction miocene to quaternary large basaltic plateaus occur in the back arc domain of the andean chain in patagonia they are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the south chile ridge scr we have investigated three volcanic centres from the lago general carrera buenos aires area 46 47 s located above the inferred position of the slab window corresponding to a segment subducted 6 ma ago 1 the quaternary rio murta transitional basalts display major trace elements and sr and nd isotopic features similar to those of oceanic basalts from the scr and from the chile triple junction near taitao peninsula e g 87sr 86sr o 0 70396 0 70346 and end 5 5 3 0 we consider them as derived from the melting of a chile ridge asthenospheric mantle source containing a weak subduction component 2 the plio quaternary 1 and tio2 2 wt these intermediate basalts differ from their strictly alkalic equivalents by having lower high field strength element hfse and higher end up to 5 4 these features are consistent with their derivation from an enriched mantle source contaminated by ca 10 rutile bearing restite of altered oceanic crust the petrogenesis of the studied mio pliocene basalts from mlba and mcc is consistent with contributions of the subslab asthenosphere the south american subcontinental lithospheric mantle and the subducted pacific oceanic crust to their sources however their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment scr 1 which entered the trench at 6 ma indeed magmatic activity was already important between 12 and 8 ma in mlba and mcc as well as in southernmost plateaus i e 6 ma before the subduction of the scr 1 segment we propose a geodynamic model in which oib and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear in the slab which formed when the southernmost segments of the scr collided with the chile trench around 15 ma during their ascent they interacted with the patagonian supraslab mantle and locally with slivers of subducted pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

intracanyon basalt lavas of the debed river northern armenia part of a pliocene pleistocene continental flood basalt province in the south caucasus abstract late pliocene to early pleistocene 3 25 2 05 ma 200 400 m thick basalt lavas outcrop in the south caucasus region including the kars erzurum plateau northeastern turkey the javakheti plateau georgia armenia and the lori plateau northern armenia these fissure fed rapidly erupted fluid lavas filled pre existing river valleys over many tens of kilometres the basalts exposed in the debed river canyon northern armenia are 200 m thick and of three morphological types 1 basal pillow basalts and hyaloclastites overlain by 2 columnar jointed pahoehoe sheet flows in turn overlain by 3 slabby pahoehoe and rubbly pahoehoe flows the lower and middle lavas show evidence for damming of river drainage like many lavas of the columbia river flood basalt province scotland ireland and iceland there is also evidence for syn volcanic faulting of the early lavas related basalts also outcrop in the gegham uplands and the hrazdan river basin in armenia this 3 25 2 05 ma south caucasus basalt province covering parts of turkey georgia and armenia has an estimated areal extent of 15 000 km2 and volume of 2250 km3 because its main geological features are remarkably like those of many continental flood basalt cfb provinces we consider it a true albeit small cfb province it is the smallest and youngest cfb in the world an analogue closely similar in major features is the late miocene altos de jalisco cfb province in the western trans mexican volcanic belt both provinces formed during lithospheric pull apart and transtensional faulting their broader significance is in showing flood basalt size distribution to be a continuum without natural breaks with implications for geodynamic models

subduction of the south chile active spreading ridge a 17 ma to 3 ma magmatic record in central patagonia western edge of meseta del lago buenos aires argentina abstract the chile triple junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent the mlba plateau meseta del lago buenos aires is one of the neogene alkali basaltic plateaus located in the back arc region of the andean cordillera at the latitude of the current chile triple junction the genesis of mlba can be related with successive opening of slabs windows beneath patagonia within the subducting nazca plate itself and between the nazca and antarctic plates detailed 40ar 39ar dating and geochemical analysis of bimodal magmatism from the western flank of the mlba show major changes in the back arc magmatism which occurred between 14 5 ma and 12 5 ma with the transition from calc alkaline lavas cerro plomo to alkaline lavas mlba in relation with slab window opening in a second step at 4 3 ma alkaline felsic intrusions were emplaced in the western flank of the mlba coevally with the mlba basalts with which they are genetically related these late oib like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting nazca plate during the opening of the south chile spreading ridge related slab window these basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones the close association of bimodal magmatism with extensional tectonic features in the western mlba is a strong support to the model of patagonian collapse event proposed to have taken place between 5 and 3 ma as a consequence of the presence of the asthenospheric window scr 1 segment of south chile ridge below the mlba area

salt weathering of sandstone at the angkor monuments cambodia identification of the origins of salts using sulfur and strontium isotopes abstract the angkor monuments in northwestern cambodia which are primarily made of sandstone are suffering from deterioration due to salt weathering in order to elucidate the sources of the salts and salt weathering process this paper analyzed bulk chemical compositions and s sulfur and sr strontium isotopic ratios for the salts and surrounding environmental materials at places where bats inhabit the structures various kinds of sulfate and phosphate salts are formed on the surface of the sandstone the results of the analysis demonstrate that the s and p phosphorus components in the salts are primarily derived from bat guano moreover ca calcium which is the major element in the salts is suggested to be derived from both bat guano and the sandstone the isotopic data strongly suggests that bat guano is related to salt weathering removal of these animal excrements is essential for future conservation of these monuments on the other hand exfoliation of sandstone commonly occurs due to crystallization of calcite caco 3 on platform surfaces of pyramid type monuments a large amount of ca in calcite is suggested to be derived from calcite cement of the building sandstone

deterioration of volcanic tuff rocks from armenia constraints on salt crystallization and hydric expansion volcanic tuffs are widely used in the armenian architecture and represent building stones of the country s most precious cultural heritage sites for the very first time extensive investigations regarding their chemical and mineralogical composition as well as the influence of petrophysical properties on their weathering behavior were realized different groups of tuff rocks could be identified which differ greatly in their chroma texture their chemical and mineralogical composition as well as their weathering behavior at 30 porosity and 25 micropores the tuff rocks show a sharp limit of changing water transport and retention behavior swellable clay minerals the amount of micropores and the hygroscopic sorption value show a direct relation with the hydric expansion and proved to be reliable parameters for its estimation zeolite minerals proved to drastically increase the sorption values of the tuff rocks and are discussed as a cause for potential disjoining pressure furthermore the application of the salt bursting test by european standard on these glass rich volcanic tuff rocks is questioned due to considerable different mineralogy and fabric in comparison to classic ash rich tuffs

the behaviour of consolidated volcanic tuffs weathering mechanisms under simulated laboratory conditions five volcanic tuffs ranging from dacitic tuffs of hungary to rhyolite phonolite and basaltic tuffs of germany were consolidated under laboratory conditions prior to consolidation an anti hygro a hydrous consolidant which reduces the swelling ability of clay minerals was applied the three consolidants a silicic acid ester sae an elastic silicic acid ester esae and an acrylate resin pmma were applied on test specimens under vacuum petrographic characterisation polarizing microscopy xrd sem provided data for fabric analyses and the mineral composition of the tuffs changes in fabric effective porosity density tensile strength ultrasonic wave velocity were evaluated after the treatment weathering simulation tests such as hygric dilatation and thermal dilatation aimed to prove the effectiveness of consolidation and the durability of consolidated tuff samples more than 500 samples were analysed the tests showed that sae caused the highest increase in indirect tensile strength the water absorption and the pore size distribution of the tuffs were modified by consolidation the pmma reduced the water absorption the most whereas sae modified it the least all the tested consolidants increased the thermal dilatation of the tuffs the changes in hygric dilatation were not uniform for most tuffs sae increased and pmma decreased the hygric dilatation although the clay rich habichtswald tuff showed the opposite trend the changes in hygric and thermal behaviour of consolidated tuff require special care when specific consolidants are chosen these products modify the physical properties of consolidated tuffs and change the behaviour of weathering

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

insolation weathering and hygric dilatation two competitive factors in stone degradation in the natural environment temperature and moisture are very important parameters in stone degradation therefore on 28 samples from different lithologies magmatic metamorphic and sedimentary rocks thermal and hygric expansion measurements were performed in order to constrain their behaviour related to environmental changes it was shown that marbles even those that are dolomitic are sensitive to temperature impacts for rocks with clay minerals the effect of temperature may be responsible for shrinking finally the amount of residual strain i e the formation of microcracks due to temperature action controls the rate of stone degradation for most sedimentary rocks no residual strain after heating was observed in contrast to most magmatic and metamorphic low porosity rocks sedimentary rocks contain swelling minerals like smectite and even phyllosilicates which are very sensitive to hygric attacks the cycles of wetting and drying i e the natural environmental effects may significantly control the deterioration velocity caused by hygric expansion

moisture expansion as a deterioration factor for sandstone used in buildings the expansion processes that develop in building stones upon changes of moisture content may be an important contributing factor for their deteriorations until recently few data could be found in the literature concerning this parameter and weathering processes moreover the processes that may be responsible for the moisture related expansion of natural building stones are not yet completely understood to further elucidate this process extensive mineralogical petrophysical and fabric investigations were performed on eight german sandstones in order to obtain more information regarding the weathering process and its dependence on the rock fabric the analysed sandstones show a wide range of pore size distributions and porosities a positive correlation with the fabric and the pore space can be found for all studied petrophysical parameters the intensity of the expansion and related swelling pressure cannot be attributed only to the swelling of clay minerals the investigations suggest that the micropores and the resulting disjoining pressure during wet dry cycles also play an important role the results obtained suggest that the mechanism is related to the presence of liquid water within the porous material

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

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

new isotopic ages and the timing of orogenic events in the cordillera darwin southernmost chilean andes abstract the cordillera darwin a structural culmination in the andes of tierra del fuego exposes an orogenic core zone that has undergone polyphase deformation and metamorphism some of the classic problems of orogenic zones have remained unanswered in the cordillera darwin the age of deformed plutonic rocks the distinction of structurally reactivated basement and metamorphosed cover rocks and the timing of orogenic events this study addresses and partially answers these questions a well constrained rb sr isochron age of157 8m y and an initial 87 sr 86 sr ratio of 0 7087 obtained from a pre tectonic granitic suite suggest a genetic relation between this suite and upper jurassic silicic volcanic rocks in the cover sequence tobifera formation and also suggest involvement of continental crust in formation of these magmas a poorly constrained rb sr isochron age of240 40m y obtained from supposed basement schists is consistent with field relations in the area which suggest a late paleozoic early mesozoic metamorphism for these pre late jurassic rocks however because of scatter in the data and the uncertainties involved in dating metasedimentary rocks the significance of the isotopic age is dubious compilation of previously published ages in the area 9 with new mineral ages reported here indicate that early andean orogenic events occurred between 100 and 84 m y ago and that subduction related magmatism has contributed probably discontinuously to the crustal evolution of the region throughout the mesozoic

the geomagnetic field over the past 5 million years summary the modern geomagnetic field is usually expressed as a spherical harmonic expansion although the palaeomagnetic record is very incomplete in both space and time sufficient data are available from a span of ages to generate time averaged spherical harmonic field models with many degrees of freedom here three data sets are considered directional measurements from lavas inclination measurements from ocean sediments and intensity measurements from lavas individual data are analysed as well as siteaverages using the same methods that have been developed for the modern field to give models for the past 5 myr the normal polarity field model has an axial dipole intensity similar to that of the modern day field whilst the equatorial dipole component is very much smaller the field is not axisymmetric but shows flux concentrations at the core s surface under canada and siberia similar to those observed in the field over historical timescales tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples the reverse polarity field model does not show such obvious features but this may be due to the sparsity of the data the patterns observed in the normal polarity field with persistent features in the northern hemisphere and a smooth southern hemisphere could be explained if the present pattern of secular variation is typical of the past several million years this would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation with relatively little change over quieter regions however we have been unable to find any evidence for a geographical pattern of secular variation in the data

paleomagnetism of the southwestern u s a recorded by 0 5 ma igneous rocks 1 the issue of permanent nondipole contributions to the time averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field in this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern u s a in the age range 0 5 ma and investigate both the time averaged field and its variability about the mean value several decades of work in the southwestern united states have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites as part of a new investigation of the san francisco volcanics we collected paleomagnetic samples from 47 lava flows many of which have been previously dated the new data combined with published data are highly scattered contributions to the scatter were considered and we find that removal of data sets from tectonically active areas and judicious selection according to fisher s 1953 precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal monte carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector average directions for the southwestern u s a show small deviations from a geocentric axial dipole field but these cannot be considered statistically significant virtual geomagnetic pole vgp dispersions are consistent with those from globally distributed observations analyzed by mcelhinny and mcfadden 1997 however a systematic investigation of the effect of imposing a cutoff on vgps with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction such a procedure can result in severe underestimates of the variance in the geomagnetic field a more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution

neogene kinematic history of nazca antarctic phoenix slab windows beneath patagonia and the antarctic peninsula article i nfo the patagonian slab window is a subsurface tectonic feature resulting from subduction of the nazca antarctic spreading ridge system chile rise beneath southern south america the geometry of the slab window had not been rigorously defined in part because of the complex nature of the history of ridge subduction in the southeast pacific region which includes four interrelated spreading ridge systems since 20 ma first the nazca phoenix ridge beneath south america then simultaneous subduction of the nazca antarctic and the northern phoenix antarctic spreading ridge systems beneath south america and the southern phoenix antarctic spreading ridge system beneath antarctica spreading ridge paleo geographies and rotation poles for all relevant plate pairs nazca phoenix antarctic south america are available from 20 ma onward and form the mathematical basis of our kinematic reconstruction of the geometry of the patagonia and antarctic slab windows through neogene time at approximately 18 ma the nazca phoenix antarctic oceanic ridge ridge ridge triple junction enters the south american trench we recognize this condition as an unstable quadruple junction heat flow at this junction and for some distance beneath the forearc would be considerably higher than is generally recognized in cases of ridge subduction from 16 ma onward the geometry of the patagonia slab window developed from the subduction of the trailing arms of the former oceanic triple junction the majority of the slab window s areal extent and geometry is controlled by the highly oblique near parallel subduction angle of the nazca antarctic ridge system and by the high contrast in relative convergence rates between these two plates relative to south america the very slow convergence rate of the antarctic slab is manifested by the shallow levels achieved by the slab edge b45 km thus no point on the antarctic slab is sufficiently deep to generate normal mantle derived arc type magmas upwelling beneath the region may have contributed to uplift and eastward transfer of extension in the scotia sea

salt bursting tests on volcanic tuff rocks from mexico during the pre colonial period all the architectural constructions of the maya and the later aztec culture were made from different types of rocks available in the nearby surroundings of the settlements later the architecture characterizing the spanish colonization utilized volcanic tuff rocks for ashlars and masonry facades volcanic tuffs were used in large quantities and are still the main natural building rock used in most of the country tuffs are mostly soft and porous rocks that can be easily cut and reworked they are however very susceptible to the effects of weathering especially the deterioration caused by salt crystallization seventeen volcanic tuff samples from mexico used as natural building stones were studied these tuffs show highly variable compositions and different porosities and textural properties as well as very different behaviors in regard to salt crystallization the petrographic petrophysical mineralogical and geochemical properties of the 17 samples were analyzed before and after a large number of salt bursting tests in order to try and determine the main factors leading to the weathering caused by salt crystallization some factors appear to have a real importance in regard to the resistance against salt crystallization two of the most important are the water vapor diffusion value and the pore radii distribution however microporosity also becomes a relevant factor for rapid salt destruction if it reaches more than 50 of the pore volume all these effects related to the salt crystallization are dependent on the presence of water in any form so the climatic conditions must also play a principal role

anisotropic paleosecular variation models implications for geomagnetic field observables abstract we present a family of statistical models for paleosecular variation psv of the geomagnetic field that are compatible with paleodirectional and paleointensity variations in lava flows sampling the last 5 ma and explore what paleomagnetic observables might be used to discriminate among the various family members we distinguish statistical models with axial anisotropy which provide a suitable description for an earth with homogeneous boundary conditions at the core mantle interface from those with more general anisotropy corresponding to geographically heterogeneous boundary conditions the models revise and extend earlier ones which are themselves descendants of cp88 devised by constable and parker constable c g parker r l 1988 statistics of the geomagnetic secular variation for the past 5 m y j geophys res 93 11569 11581 in cp88 secular variation is described by statistical variability of each gauss coefficient in a spherical harmonic description of the geomagnetic field with each coefficient treated as a normally distributed random variable the gauss coefficients of the non dipole part of the field exhibit isotropic variability and the variances are derived from the present field spatial power spectrum the dipole terms have a special status in cp88 with a non zero mean for the axial dipole and lower variance than predicted from the spatial power spectrum all non dipole terms have zero mean except the axial quadrupole cp88 is untenable for two reasons it fails to predict the observed geographic dependence of directional variability in the magnetic field and it grossly underpredicts the variance in paleointensity data the new models incorporate large variance in the axial dipole and in the non axial quadrupole gauss coefficients g21 and h21 the resulting variance in paleomagnetic observables depends only on latitude zonal models unless the variance in h21 is different from that in g21 non zonal models non zonal longitudinal variations in psv such as the flux lobes seen in the historical magnetic field are simulated using the non zonal models both the zonal and non zonal models fit summary statistics of the present dataset we investigate the influence of persistent non zonal influences in psv on various paleomagnetic observables it is shown that virtual geomagnetic pole vgp dispersion is rather insensitive to longitudinal variations in structure of psv and that inclination dispersion has the potential to be more informative given the right site distribution there is also the possibility of using paleointensity and geographic variations in the frequency of occurrence of excursional directions to identify appropriate psv models

a method for intercalibration of u th pb and 40ar 39ar ages in the phanerozoic the increasing analytical precision of 40ar 39ar geochronological data and its use in studies that combine u pb and 40ar 39ar data has necessitated derivation of a means to intercalibrate ages between the two isotopic systems the primary determinants inhibiting accurate cross calibration are uncertainty in decay constant values and uncertainty in 40ar 40k usually presented as apparent age for 40ar 39ar standards notably those derived from the fish canyon tuff by utilizing a monazite and biotite bearing oligocene ash flow tuff mac 83 as the primary standard material precise u th pb and 40ar 39ar ages can be derived because both isotopic systems should reflect closure during the instantaneous eruption event the 24 21 0 10 ma 207pb 235u age should match the age resulting from 40ar 39ar biotite analysis this forced equivalence will partly offset the effects of decay constant uncertainty furthermore by co irradiating fish canyon tuff sanidine with mac 83 biotite the apparent age for fish canyon tuff sanidine is back calculated to 27 98 0 15 ma relative to the mac 83 207pb 235u age hence intercalibration between the two isotopic systems in a relative sense can be accomplished allowing for a direct and valid comparison of ages it is also shown that forcing equivalence of the two isotopic systems minimizes the effect of decay constant uncertainty for samples of phanerozoic age

geomagnetic paleosecular variation recorded in plio pleistocene volcanic rocks from possession island crozet archipelago southern indian ocean possession island in the crozet archipelago consists of volcanic units erupted mainly between 5 and 0 5 ma a paleomagnetic sampling was carried out along several sections distributed near the northern eastern and southeastern coasts a total of 45 independent flows were sampled 320 samples for each flow a precisely defined characteristic remanence direction was usually isolated after a careful progressive cleaning in alternating fields however particularly complex remanence behavior is often observed the magnetostratigraphy of the lava pile is quite simple with reversed rocks in the lower part and normal units in the upper part of two sections a third section is of normal polarity throughout its whole thickness including three excursional directions we did not find any intermediate directions between the normal and reverse magnetozones thus we have no evidence for the recording of the matuyama brunhes transition expected from a previous study watkins 1972 the amplitude of paleosecular variation estimated from between flow dispersion from the field of an axial dipole is 11 8 with 95 confidence limits between 9 3 deg and 14 0 deg this value is consistent with the general anisotropic statistical model for paleosecular variation of constable and johnson 1999

late miocene earliest pliocene glaciation in southern argentina implications for global ice sheet history abstract in south america at latitude 47 s till was deposited sometime between 7 m y and 4 6 m y ago at a time when the local climate was colder than today s during this same interval glaciers extended to sea level in southeast alaska and widespread cooling of the ocean surface in middle latitudes worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred from the last three events major late miocene expansion of the antarctic ice sheet has been inferred on the assumption that the history of north atlantic ice rafting precludes the existence of northern hemisphere ice sheets until 3 m y ago this is disputed first because precipitation in antarctica would probably have decreased at temperatures below today s second because the antarctic ice sheet cannot expand appreciably until buildup of northern hemisphere ice sheets has lowered sea level third because virtually no late miocene sediments are present at the labrador sea dsdp sites that are critical to the reconstruction of north atlantic ice rafting history and fourth because the scale of late miocene glaciation in alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east if global ice sheet volume at the end of the miocene was greater than it is today by an amount that would have lowered sea level by several tens of meters the excess ice did not accumulate in antarctica but in the northern hemisphere chiefly in north america away from the atlantic coast

weathering behaviour and construction suitability of dimension stones from the drei gleichen area thuringia germany the construction suitability of a dimension stone depends on its weathering properties along with the petrology and the petrophysical properties the aim of this study was to evaluate the suitability of the dimension stones from the drei gleichen area for construction and replacement purposes in total six sandstones ingersleben wachsenburg hindfelden seeberg rohnberg gleichenberg upper triassic as well as two carbonates wachsenburg sinter quaternary wandersleben dolomite middle triassic were analysed the results from our laboratory and on site studies of the dimension stones show that rocks from the same stratigraphic layer like the sandstones from the upper triassic can show major differences in their petrophysical and weathering properties these differences are attributed to their different diagenesis resulting e g in varying pore space water balance and strength properties the pore size distribution can be divided into four different groups based on their occurring maxima and micropore content the determined water balance properties as well as moisture expansion and salt attack depend on these groups next to this the mineralogical composition significantly influences the weathering resistance sandstones with a high content of altered lithoclasts show a high amount of moisture expansion low strength and in consequence a low weathering resistance against salt attack based on the results of the present study an evaluation of construction suitability could be accomplished from the analysed sandstones only the seebergen sandstone is suitable for construction purposes due to its good availability good strength properties high compressive and tensile strength low softening degree as well as a low porosity furthermore the wachsenburg sandstone also shows good petrophysical and petrological properties but exploitable deposits are too sparse to be of commercial interest from the carbonates the wachsenburg sinter shows very suitable rock parameters but only sparse outcrops occur which are not appropriate for mining

paleosecular variation of brunhes age lava flows from british columbia canada 1 brunhes age lava flows have been sampled for paleosecular variation studies from several volcanic fields of southern british columbia silverthrone garibaldi lake mt meager clearwater wells gray park and kelowna area a total of 52 lava flows were sampled and 7 to 10 samples were drilled at each site previous radiometric studies indicate that the ages of these lava flows range from 2 3 to 760 ka stepwise thermal demagnetization 14 to 21 steps was carried out for all the samples in each site and af demagnetization was performed on one sample per site forty five sites were selected based on rigorous criteria 95 5 to calculate a mean direction d 356 9 i 70 2 95 2 8 that is statistically indistinguishable from the direction of the dipole field at the area i 68 3 virtual geomagnetic poles vgps do not show the far sided effect and the angular standard deviation is 17 5 a value in agreement with the paleomagnetic field dispersion for that latitude these high quality results are expected to improve the time averaged field taf and secular variation models

paleomagnetism and 40ar 39ar ages from la palma in the canary islands 1 the structure of the time averaged geomagnetic field has been known forcenturies to be approximately dipolar significant departures of the time averagedfield from that of an axial geocentric dipole however have been reportedfor decades the data on which time averaged field models are based must beof the highest quality in order to document subtle long term features wepresent here new paleomagnetic data and 40ar 39ar ages for the island of la palma the canary islands paleomagnetic samples were obtained from 28 lava flows of these 21 met ourminimum acceptance criteria for use in time averaged field models the 40ar 39ar age determinationswere successfully carried out on samples from eight of the flows our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales the reversed data in the updated databaseare antipodal to the normal data within the uncertainties and the time averageddirection is indistinguishable from that expected from a geocentric axialdipole

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

time averaged field at the equator results from ecuador seventy sites were collected from pliocene to recent lavas in ecuador between 0 19 and 1 1 s latitude lightning affected many of the sites sampled and the final data set consisted of 51 sites with 95 of 10 or less 21 flows yielded normal directions of magnetization declination 354 6 inclination 5 95 7 2 and 30 yielded reverse declination 183 5 inclination 5 6 95 5 these sites pass a reversal test at a high level the combined data give a direction of declination 359 9 inclination 5 4 and 95 4 2 which is just significantly different from the gad field but is in agreement with a gad field plus 5 quadrupole field mean vgps are not significantly different from the geographic poles the asd for the combined sites is 13 3 which is in agreement with model g paleointensity measurements yield values of vadm which range from 1 39 1022 to 22 6 1022 am2 the mean value for the reverse field is 3 2 1022 am2 while for the normal field it is 10 2 1022 am2 the 40ar 39ar dates on selected sites show that they are 2 6 ma and younger the age dates and magnetic polarity indicate that the chacana structure is brunhes matuyama in age

volcanic tuffs from hesse germany and their weathering behaviour in the hessian region of germany numerous deposits of tuffs are known from which natural building stones were extracted many buildings show examples of the use of this material in the past in contrast to their attractive appearance and ease of extraction from quarries tuffs are liable to destruction by weathering most common damages are disintegration crumbling cracking scaling and flaking in this paper the causes of damage of selected varieties have been analysed using petrographical and petrophysical methods the measured values are disadvantageous in comparison to other natural building stones large amounts of swellable clay minerals mostly forming the cementing material cause scaling due to repeated humidification high porosity including negative pore structures and high adsorption effects results in structural damages while freezing consequently most varieties of the described tuffs possess a limited usage as an outdoor building material

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

age dependent seismic thickness and mechanical strength of the australian lithosphere 1 we present constraints on the regional variations of the seismic and mechanical thickness of the australian lithosphere we infer the seismic thickness from a waveform tomographic model of s wave speed and as a proxy for the elastic thickness we use the wavelength at which the coherence of surface topography and bouguer gravity drops below half of its long wavelength maximum our results show that on scales 1000 km the relationship between the age of the crust and the thickness of the lithosphere is more complicated than longer wavelength or global averages suggest recent geochemical and geodynamical evidence for small scale secular variations of the composition and stability of continental cratons further illustrates the complexity of the age dependence of seismo mechanical lithospheric properties on regional scales

paleomagnetism and 40ar 39ar chronology of lavas from meseta del lago buenos aires patagonia 1 as part of a larger project investigating paleosecular variation in south america 36 paleomagnetic and geochronology sites were sampled on meseta del lago buenos aires 47 s 289 e 200 km east of the active plate boundary in southern patagonia basaltic lavas sampled range from late miocene to late pleistocene yet most of the individual lavas are younger than 3 3 ma thirty eight isochron ages determined via 40ar 39ar incremental heating experiments frame a detailed stratigraphy the isochron ages range from 66 9 4 1 ka for scoria cones and a youthful lava flow in the rio pinturas valley to 10 12 1 35 ma for thick plateau forming lavas exposed along the se edge of the meseta the last 3 3 myr is characterized by seven episodes of volcanism at ca 3 2 3 0 ma 2 4 ma 1 7 ma 1 35 ma 1 0 ma 750 ka 430 330 ka and 110 ka the bulk of lavas forming the surface of the meseta erupted in the last 1 7 my all sites have stable magnetization and after step demagnetization using either thermal or alternating field techniques yield characteristic directions held by magnetite and or titanomagnetite ten sites have distinct transitional directions defined by pole latitudes 55 and associated ages indicate possible connections to known reversals within the matuyama chron including the onset of the jaramillo subchron 1 016 0 01 ma the cobb mountain subchron 1 25 0 03 ma the ontong java 1 event 1 37 0 03 ma and the termination of the olduvai subchron 1 72 0 02 ma remaining sites are divided into normal 14 sites and reversed polarity 12 sites at 95 confidence mean normal and reversed directions overlap the mean direction for 26 sites is i 63 0 d 3 4 95 5 4 which is indistinguishable from the expected geocentric axial dipole paleosecular variation measured by the dispersion of virtual geomagnetic poles about the rotation axis is higher than expected at 20 0 model g using data for the past 5 myr predicts a dispersion of 17 this discrepancy may be due to true dispersion of the field in southern south america or it may be an artifact of inadequate sampling

magnetic properties of anorthosites a forgotten source for planetary magnetic anomalies 1 anorthosites igneous rocks very rich in plagioclase rarely considered to be strongly magnetic are common on earth and the moon and inferred to be on other planets magnetic properties of anorthosites could be important in investigating associated mineral deposits and in studying magnetic anomalies especially on mars here we investigate three late proterozoic anorthosites in rogaland norway for magnetic and petrographic properties two of the anorthosites have large natural remanent magnetization nrm with intensities comparable to tertiary basalts susceptibility nrm and hysteresis properties provide information about the magnetic minerals present and their response to inducing fields microscopic observations show ubiquitous hemo ilmenite in the anorthosites whereas magnetite is common in the haland helleren but rare in the ana sira and egersund ogna bodies this study illustrates that anorthosites can be important sources of magnetic anomalies and can retain a remanent field over geologic time it also supports the recently described property of lamellar magnetization

fine tuning the radiometrically derived geomagnetic polarity time scale gpts for 0 10 ma new 40ar 39ar step heating ages for geomagnetic field reversals at 3 2 and 4 5 ma along with previous data at 1 2 10 and 16 ma are used to derive a geomagnetic polarity time scale for 0 10 ma these ages for chron boundaries are fitted to the relative widths of the polarity intervals on the ocean floor using a cubic spline technique the interpolated ages of chrons for 0 6 ma are in good agreement with the astronomical time scale and differ considerably from earlier attempts based in part on k ar dates significant differences between the interpolated ages and the astronomical time scale are noted for the onset of the olduvai and reunion sub chrons there is better agreement if 40ar 39ar data obtained from direct dating of these chrons is utilized this time scale is estimated to be accurate to 1 for 0 6 ma for 6 10 ma the lack of a tie point limits the accuracy to about 2

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

postglacial patagonian mass movement from rotational slides and spreads to earthflows abstract many of the volcanic plateau margins of the eastern formerly glaciated foreland of the patagonian andes are undermined by giant landslides 108 m3 one cluster of such landslides extends along the margin of the meseta del lago buenos aires mlba plateau that is formed mainly by neogene quaternary basalts the dry climate is at odds with numerous 2 km long earthflows nested within older and larger compound landslides we present a hydrological analysis a detailed geomorphic map interpretations of exposed landslide interiors and radiocarbon dating of the el mirador landslide which is one of the largest and morphologically most representative landslide we find that the presence of lakes on top of the plateau representing low infiltration rates correlates negatively with the abundance of earthflows on landslide debris along the plateau margins field outcrops show that the pattern of landslides and earthflows is likely controlled by groundwater seepage at the contact between the basalts and underlying soft miocene molasse numerous peat bogs store water and sediment and are more abundant in earthflow affected areas than in their contributing catchment areas radiocarbon dates indicate that these earthflows displaced metre thick layers of peat in the late holocene

paleomagnetism and 40ar 39ar ages from volcanics extruded during the matuyama and brunhes chrons near mcmurdo sound antarctica maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis these lobes persist in field models for the last few millenia the anomalous lobes are associated with observation sites at extreme southerly latitudes to determine whether these features persist for millions of years paleomagnetic vector data from the continent of antarctica are essential we present here new paleomagnetic vector data and ar 40 ar 39 ages from lava flows spanning the brunhes and matuyama chrons from the vicinity of mcmurdo sound antarctica oriented paleomagnetic samples were collected from 50 lava flows by e mankinen and a cox in the 1965 1966 austral summer season preliminary data based largely on the natural remanent magnetization nrm directions were published by mankinen and cox 1988 we have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection of these only one site fails to meet our acceptance criteria for directional data seven sites are reversely magnetized the mean normal and reverse directions are antipodal the combined mean direction has d over bar 12 i over bar 86 alpha 4 kappa 37 and is indistinguishable from that expected from a gad field we obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 zam 2 and a standard deviation of 28 zam 2 ar 40 ar 39 age determinations were successfully carried out on samples from 18 of the flows our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

weathering of volcanic tuff rocks caused by moisture expansion moisture expansion in natural building stones is considered one of the most important factors affecting their weathering and deterioration the processes that may be responsible for the expansion under determinate relative humidity hygric dilatation and water saturated conditions hydric dilatation are generally attributed to the presence of swellable clay minerals in contrast to this assumption our investigations show that moisture expansion also takes place in volcanic tuff building stones almost free from clay minerals to provide a deeper understanding of the processes swelling and deterioration were performed on 14 volcanic tuffs used as important building stones of different ages compositions and weathering stages from mexico germany and hungary the investigations undertaken include extensive chemical petrophysical and fabric analyses the samples show a wide range of effective porosity microporosity capillary water absorption moisture expansion and cec values high moisture expansion does not seem to depend on clay mineral content alone we also observed that there is no significant effect on dilatation if clay minerals are present but only form a thin coat on the outer shell of bigger pores moreover we identified a correlation between microporosity average pore radius and moisture expansion the investigations highlight the fact that moisture expansion cannot only be attributed to swellable clay minerals and suggest that the presence and accumulation of micropores and their average radius and distribution play an important role for non clay associated swelling intensity which can most probably be attributed to the disjoining pressure

salt and ice crystallisation in porous sandstones salt and ice crystallisation in the pore spaces causes major physical damage to natural building stones the damaging effect of these processes can be traced back to physically induced stress inside of the rock while crystallizing the increasing scientific research done during the past century has shown that there are numerous parameters that have an influence on the weathering resulting from these processes however the working mechanisms of the stress development within the rock and its material dependency are still subject to discussion this article gives an overview of salt and ice weathering additionally laboratory results of various sandstones examined are presented salt crystallisation tests and freeze thaw tests were done to obtain information about how crystallisation weathering depends on material characteristics such as pore space water transportation and mechanical features simultaneous measuring of the length alternating during the salt and ice crystallisation has revealed detailed information on the development of crystal in the pore spaces as well as the development of stress these findings can help to understand the damaging mechanisms

long term cosmogenic 3 he production rates from 40 ar 39 ar and k ar dated patagonian lava flows at 47 s exceptionally well preserved basaltic lava flows on the cool arid eastern side of the southern andes near lago buenos aires lba at lat 47 s provide ideal sites for calibration of in situ produced cosmogenic 3he we report new 3he measurements and independent age determinations using unspiked k ar and 40ar 39ar incremental heating measurements from two basalt flows both age techniques give concordant ages with analytical uncertainties of 5 demonstrating their utility for dating surfaces older than 35 ka the limit of 14c dating the weighted mean age of the cerro volcan flow is 109 3 ka and of the rio pinturas flow is 68 3 ka scaled to sea level and high latitude d lal earth planet sci lett 104 1991 424 439 and standard atmospheric pressure j o stone j geophys res 105 2000 23 753 23 759 3he production rates on the cerro volcan flow are 130 4 atoms g 1 a 1 in clinopyroxene and 139 4 atoms g 1 a 1 in olivine 2 consistent with a small compositional dependence of the production rate the 3he production rate in olivine from the younger rio pinturas flow is 129 6 atoms g 1 a 1 which is indistinguishable from that obtained on cerro volcan the 3he production rate is at least 11 higher than previous determinations from low mid latitudes in the northern hemisphere corrections for magnetic field strength variability lower the scaled lba production rates by only 1 and do not account for the discrepancy we infer that the enhanced production rates are caused by lower atmospheric pressure in patagonia during glacial periods the southern westerlies which are associated with the steep circumpolar atmospheric pressure gradient are presently centered on lat 50 s but based on independent marine and terrestrial evidence are thought to have migrated north at least 5 during glacial periods the inferred paleobarometry provides independent evidence for northward shifts in the patterns of southern hemisphere atmospheric circulation

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

patterns and controls on the distribution of diamondiferous intrusions in australia abstract the distribution of kimberlite lamproite and related alkaline volcanism in australia can be broadly related to the structure of the australian continent and lithosphere diamondiferous kimberlites and lamproites with the apparent exception of the weakly diamondiferous orrorro kimberlites in the adelaide fold belt lie within the large precambrian shield where seismic tomographic models and heat flow data indicate the presence of relatively cold high seismic wave speed lithosphere tectosphere typically some 200 km thick or more beneath the archaean cratons and up to 300 km in parts of central australia many of the diamondiferous intrusions appear to lie at the margins rather than in the centre of the lithosphere domains the highest concentration of diamondiferous intrusions kimberlites and lamproites is on and around the kimberley craton where seismic data indicate crustal thicknesses of 35 40 km and a lithosphere up to 275 km thick that is distinct from proterozoic northern australia many but clearly not all of the intrusions show evidence of regional and local structural controls some are spatially associated with known crustal structures especially regional faults others are aligned either singly or in clusters along or near discontinuities and or gradients evident in regional scale potential field data especially the total horizontal gradients of gravity data continued upward tens to hundreds of kilometres many of these features are not evident in the original datasets as their signatures are masked by shorter wavelength near surface anomalies in some cases the kimberlites and associated rocks lie within crustal blocks and domains defined by discontinuities in the potential field data rather than at domain boundaries our overview suggests that analysis of potential field data especially horizontal gradients in upwardly continued potential field data at all scales can assist definition of crustal and potentially lithospheric structures that may influence the distribution of diamond pipes however more definitive mapping of australia s diamond prospective regions requires the integration of data on crustal structures especially trans lithospheric faults and geodynamic settings with high resolution tomographic models and other geophysical petrologic and isotopic information on the nature of the lithosphere beneath the australian continent

persistently anomalous pacific geomagnetic fields a new average geomagnetic field model for the past 3kyr als3k helps bridge a large temporal sampling gap between historical models and more traditional paleo magnetic studies spanning the last 5 myr a quasi static feature seen historically in the central pacific has the oppo site sign in als3k its structure is similar to but of larger amplitude than that in the time averaged geomagnetic field for the last 5 myr anomalous geomagnetic fields exist be neath the pacific over timescales rangingfrom 10 2 10 6 years it is unlikely that bias over such long time scales arises from electromagnetic screening but conceivable that the lorentz force is influenced by long wavelength thermal variations and or localized regions of increased electrical conductiv ity associated with compositional anomalies and possibly partial melt this is consistent with recent seismic obser vations of the lower mantle

40ar 39ar chronology and paleomagnetism of quaternary basaltic lavas from the persani mountains east carpathians abstract quaternary volcanism in the persani mountains forms an na alkali basaltic province inside the bend area of the carpathians in the southeastern part of europe previous k ar ages and paleomagnetic data reveal several transitional virtual geomagnetic poles which were tentatively associated with the cobb mountain subchron and a brunhes chron excursion we report a new paleomagnetic and rock magnetic study coupled with 40ar 39ar geochronology to better constrain the age of geomagnetic reversals or excursions that might be recorded and the timing of volcanism of the paleomagnetic directions obtained from sampled lava flows 4 are reversed polarity 19 are normal polarity and 16 have transitional polarity 40ar 39ar plateau ages determined from incremental heating experiments on groundmass indicate that two of the reversely magnetized lavas erupted at 1142 41 and 800 25 ka four of the normally magnetized lavas erupted at 1060 10 1062 24 684 21 and 683 28 ka and two transitionally magnetized lavas formed at 1221 11 and 799 21 ka both the new 40ar 39ar ages and the paleomagnetic data suggest at least five episodes of volcanic activity with the most active periods during the jaramillo and brunhes chrons this results shows that the last phases of alkalic and calc alkaline magmatism in the south east carpathians were contemporaneous the age of the older transitionally magnetized lava flow is within error of recent unspiked k ar and astrochronologic ages for the reversal that defines the onset of the cobb mountain normal polarity subchron the age of the younger transitional lava is similar to that of an excursion that preceded the matuyama brunhes polarity reversal and which has come to be known as the matuyama brunhes precursor omitting the excursion data the dispersion of the virtual geomagnetic poles around 19 is larger than the expected value around 45 n from the global compilation but closer to the value obtained only from the time averaged geomagnetic field initiative studies

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors