ordering and exsolution processes in or rich alkali feldspar megacrysts from the eldzhurtinskiy granite caucasus the extremely young 2 5 ma i type eldzhurtinskiy granite complex central caucasus is uniform with respect to modal composition major and trace element chemistries of bulk rocks and mineral phases in contrast it reveals two types of alkali feldspar megacrysts differing in tetrahedral al content 2t1 and exsolution microtextures

a new method for the estimation of cooling and denudation rates using paramagnetic centers in quartz a case study on the eldzhurtinskiy granite caucasus we present a new method for the assessment of the most recent cooling and denudation rates using paramagnetic centers in quartz measured by electron spin resonance esr spectroscopy these centers have a relatively low thermal stability for cooling rates of 40 o and 1000oc myr effective closure temperatures vary between 55 o and 82oc ti center and 49 o and 64 oc a1 center respectively samples were collected from two cores that were drilled into the eldzhurtinskiy granite which has an emplacement age of 2 ma as measured by u pb analyses of zircons one 1500 rn core was taken from a drill hole into the dome of the granite a second core of 4000 rn from a drill hole at the base of the baksan valley our results yield cooling rates of between 160 and 250oc myr for the upper core and between 570 o and 600oc myr for the lower core the corresponding denudation rates are 2 5 upper core and 5 5mm a lower core the shape of the temperature profile of the lower core indicates recent erosion when fitting the temperature data with a two dimensional heat transfer model we obtain a net denudation rate of 10 mm a and cooling rates in the range of 500oc myr thus confirming the cooling rates estimated by esr however the esr denudation rates underestimate the erosion rate of the baksan valley because the geothermal gradient is not equilibrated between the surface and the depth of the annihilation temperatures 950 and 1800 rn for the a1 and ti centers respectively we conclude that esr measurements of paramagnetic centers in quartz will allow the recon struction of landscape dynamics for the past 10 1000 kyr and that in conjunction with u pb fission track and ar ar analyses it will be possible to develop dynamic models for quaternary tectonic movements

recent tectonics of east iranian azerbaijan from stress state reconstructions abstract iranian east azerbaijan e azerbaijan in northwestern iran is characterized by relatively complex active tectonics the area is shaped like a lozenge and is bordered by strike slip faults this region includes three fold and thrust belts the arasbaran ghoshe dagh and bozkosh mountain ranges and includes a set of n s trending compressive thrusts and folds and a set of e w trending compressive structures the stress inversion analysis and separation of stress tensors performed in this study indicate that the region is characterized by n s to ne sw n018 thrusting compression and nw se n287 and n155 strike slip compression the first stress system was responsible for the development of the three fold and thrust belts while the second system formed the n s striking structures such as those in the talesh salavat moro and kiamaki ranges the stress reconstructions indicate that e azerbaijan is colliding with the south caspian basin and the lesser caucasus in the ne and n due to the compressive stress s2 and with the central iranian block and western alborz in the southeast and east respectively due to the strike slip stress mode s3 and s1 respectively

past present and future of thermochronology in one form or another geochronologists have been practicing thermochronology 1 the use of radioisotopic dating to constrain thermal histories of rocks and minerals for over 40 years building from lessons learned over these four decades thermochronology continues to evolve due to technical developments increasingly sophisticated theoretical models and an expanding range of applications in geologic and planetary science most recently interest in earth surface processes and interactions between tectonics erosion and climate has drawn attention to techniques that can address the timing and rates of processes operating at temperatures below about 300 c the purpose of this rimg volume is to assess the current state of thermochronology as of circa 2005 which is coincidentally the 100th anniversary of the first radioisotopic date rutherford 1905 1906 excellent review papers and books on specific topics within this field have been published but no single volume has yet provided a comprehensive review of current practices basic theory and illustrative examples the motivation for this volume stems from these considerations knowing that in a fast developing field a book like this can quickly become dated we tried to include sufficient review of fundamentals and the literature to offer students and new users a useful introduction to thermochronology that may have some staying power in this chapter we first review the salient points of thermochronology s history before assessing our current capabilities and challenges and then taking the risk of suggesting where the field is headed we do not provide a comprehensive history of the method that does full justice to the work of the large and growing cohort of thermochronologists in this short space we instead opted to give our perspectives on where the intellectual and technological roots of the discipline lie which run deeper and go back farther than is sometimes appreciated

dawn and dusk sector comparisons of small scale irregularities convection and particle precipitation in the high latitude ionosphere received 7 june 2001 revised 31 october 2001 accepted 4 december 2001 published 19 september 2002 1 small scale ionospheric irregularities and auroral precipitation are common features of the auroral ionosphere but their spatial association has not been examined on global scales in this paper we compare electron and ion precipitation from individual passes of the defense meteorological satellite program dmsp spacecraft with concurrent observations of ionospheric irregularities and plasma convection from the northern hemisphere component of the super dual auroral radar network superdarn because of the nature of the dmsp orbits and the spatial resolution of the superdarn measurements these comparisons have been limited to the dusk and postdawn sectors and spatial dimensions greater than 100 km we have found that the superdarn radars generally observe ionospheric irregularities over a greater latitudinal extent than the dmsp satellites observe particle precipitation specifically ionospheric irregularities are observed both equatorward and poleward of the convection reversal boundary crb in the dawn and dusk sectors whereas particle precipitation is only observed equatorward of the crb under conditions where the radars can detect the true equatorward boundary of the irregularities they are observed to extend equatorward of the particle precipitation both irregularities and particle precipitation expand equatorward with increasing geomagnetic activity and there is evidence that precipitation regions with higher energy flux are associated with regions of stronger sunward convection these results suggest that superdarn can provide a coarse determination of the auroral oval position that will complement measurements with optics and particle detectors more importantly they demonstrate the spatial relationships between precipitation electric fields and ionospheric irregularities that result from the electrical coupling between the magnetosphere and ionosphere and the dependence of ionospheric plasma instabilities on the ionospheric electric field and precipitation induced electron density gradients index terms 2760 magnetospheric physics plasma convection 2407 ionosphere auroral ionosphere 2704 2463 ionosphere plasma convection 2455 ionosphere particle precipitation 2439 ionosphere ionospheric irregularities

comparison of superdarn radar boundaries with dmsp particle precipitation boundaries 1 auroral boundary algorithms for superdarn ionospheric convection observations are introduced both the equatorward boundary of scatter ebs and the convection reversal boundary crb are found and compared with boundaries based on dmsp particle precipitation observations these comparisons provide an intercalibration between the two schemes and insight into the relationship between disparate phenomena the ebs is reliably poleward of the equatorward boundary of precipitation ebp at most local times but centers well on the equatorwardmost precipitation between 1600 and 2200 mlt when within 8 latitude of the radar station indicating a correspondence with the slerps slow long lived e region plasma structures of jayachandran near dawn the ebs coincides better with the structured unstructured boundary sub than the ebp as one might expect if the ionospheric irregularities responsible for this scatter were produced by the gradient drift instability the crb correlates well with the open closed boundary ocb as seen by dmsp there is however an equatorward offset of the crb relative to the ocb that varies from zero near noon to 1 near dawn and dusk and is largest near midnight this offset is consistent with a small viscous like interaction between the magnetosheath and the low latitude boundary layer resulting in a degree of antisunward flow on closed field lines the offset decreases under the influence of southward interplanetary magnetic field as one might expect if increased dayside merging interfered with this effect

rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

high resolution 40ar 39ar dating of the oldest oceanic basement basalts in the western pacific basin 1 we report new 40ar 39ar ages for the oldest pacific oceanic floor at ocean drilling program site 801c in the pigafetta basin and site 1149d close to the izu bonin subduction zone in the nadezhda basin these ages were determined by applying high resolution incremental heating experiments including 15 30 heating steps to better resolve the primary argon signal from interfering alteration signatures in these low potassium ocean crust basalts combined with previous results from pringle 1992 for site 801b and 801c we arrive at a multistage history for the formation of the pigafetta ocean crust the oldest part of the pacific plate was formed at the spreading ridges at 167 4 1 4 3 4 ma n 2 2 internal absolute error offering an important calibration point on the geological reversal timescale grts since it represents the old end of the mesozoic magnetic anomalies this mid ocean ridge basalt sequence however is overlain by more tholeiites and alkali basalts that were formed 7 3 1 5 myr later around 160 1 0 6 ma n 7 2 internal error the older age group is confirmed independently by radiolarian ages ranging from late bajocian to middle bathonian 167 173 ma bartolini and larson 2001 and by profound differences in the structural characteristics of this basement section pockalny and larson 2003 thin layers comprising hydrothermal deposits separate these sequences which in addition to the difference in isotopic age show distinct major and trace element compositions this indicates that key volcanic and hydrothermal activity took place 400 600 km away from the spreading ridges on the basis of a jurassic 66 km myr half spreading rate in the pacific it remains unclear if these processes were active continuously after the initial formation of the pacific oceanic crust but all our observations seem to point to an episodic history site 1149d gives another important calibration point on the grts of 127 0 1 5 3 6 ma n 1 2 internal absolute error for anomaly m12 that is slightly younger when compared to current timescale compilations 134 2 2 1 ma gradstein et al 1995 this might suggest that the dated basalt from site 1149d does not represent the age of the ocean crust formed at its ridge axis it may also be part of the early cretaceous intraplate events that have produced dolerite sills in the pacific crust at sites 800 and 802 around 114 126 ma

structural and geochronological constraints on the tectono thermal evolution of the danba domal terrane eastern margin of the tibetan plateau the songpan ganze terrane of the tibetan plateau is underlain by neoproterozoic crystalline basement rocks of the yangtze block these basement rocks are exposed as a series of extensional tectonic domes that form a nearly north south trending extensional belt more than 1000 km long in the eastern margin of the tibetan plateau in the danba area detachment faults separate the basement core complexes e g the gezong and gongcai complexes from the paleozoic strata which have been thinned or removed completely the cover sequences have undergone upper greenschist to lower amphibolite facies metamorphism to form the danba schist and are overlain by the triassic xikang group a thick flysch sequence both the basement rocks and the paleozoic rocks have undergone multiple stages of deformation and thus provide an excellent opportunity to study the tectono thermal evolution of the eastern margin of the tibetan plateau two stages of deformation corresponding to three generations of foliation s1 view the mathml source and view the mathml source have been recognized on the basis of structural and microscopic observations we selected amphibole and biotite separates associated with distinct generations of foliation for 40ar 39ar dating using laser microprobe incremental heating technique to place numerical constraints on the major tectono thermal events within the danba area the geochronogical results reveal an earliest metamorphic event at 258 6 0 5 ma s1 biotite and 263 6 0 8 ma s1 amphibole coinciding temporally with the mantle plume that produced the voluminous emeishan flood basalts the second event was a progressive extensional deformation first occurred at 159 166 ma view the mathml source amphibole responsible for the earlier tectonic doming of the crystalline basement and then the final tectono thermal overprint recorded by view the mathml source foliation and view the mathml source metamorphism locally in the core complexes at 47 58 ma for the gezong complex and 64 81 ma for the gongcai complex this major post orogenic extensional event is believed to be a consequence of collision between the north china and south china blocks the apparent discrepancy of the 40ar 39ar ages observed between localities suggests a slow cooling process associated with progressive uplift

recrystallization or cooling ages in situ uv laser 40ar 39ar geochronology of muscovite in mylonitic rocks the intra grain 40 ar 39 ar age distributions of muscovite from greenschist facies mylonite zones are shown to be sensors of metamorphic history textures compositions and in situ uv laser ablation 40 ar 39 ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo or recrystallization ages deformed muscovite porphyroclasts and recrystallized shear band muscovite from the pogallo shear zone ivrea zone southern alps reveal variable intra grain 40 ar 39 ar ages with internal age variations of more than 65 ma the wide range of 40 ar 39 ar ages within compositionally homogeneous grains is consistent with diffusion dominated argon loss controlled by observable intra grain microstructures in contrast intra grain 40 ar 39 ar ages of muscovite fish from proterozoic mylonite of the porsgrunn kristiansand shear zone southern norway display only minor age dispersion and age variations are correlated with changes in muscovite composition the sympathetic age composition correlations in different muscovite microstructures directly relate to sequential neo and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the porsgrunn kristiansand shear zone

lithostratigraphy and paleomagnetism of pre fraser glacial deposits in south central british columbia lithostratigraphic records spanning considerable parts of the pleistocene were studied at three sites in south central british columbia a sedimentary succession near pavilion includes three distinctly different till units while the surface till can be associated with the last glaciation the fraser glaciation 18o stage 2 the ages of the two older till units are presently unknown however optical dating of outwash silt resting on the oldest till indicates that this outwash unit and all the overlying units are younger than 160 ka in big bar creek valley about 50 km north of pavilion an aggradational sequence of indurated glaciofluvial sand gravel and till is exposed a silt unit and a sand lens within an overlying till bed near the top of the section have reversed magnetization indicating deposition prior to 780 ka probably during the matuyama chron the big bar creek sequence also includes glacio fluvial sediments near the base that are normally magnetized suggesting that they were deposit

paragenetic specularite and adularia elba italy concordant u th he and k ar ages abstract in order to test the usefulness of the helium method for detecting young geological events in hematite u th and 4 he concentrations were analyzed in five specularite samples from rio marina type hematite deposits northeastern elba italy four specularite samples define a u 0 235th 4 he isochron age of 5 39 0 46 ma 2 the isochron intercepts the 4 he axis at 49 88 pl g 2 adularia which is paragenetic with specularite in one sample used for the isochron gives a meank ar age of 5 32 0 11 ma in addition two specularite samples one from rio marina and one from terra nera southeastern elba give helium model ages of 6 4 0 4 and 7 3 0 4 ma respectively depending on the model of genesis the high ages reflect incomplete 4 he degassing and or 4 he excess the study demonstrates that geologically meaningful helium ages as young as pliocene can be obtained from specularite in spite of expected interferences from initial helium if the 4 he axis intercept is a general measure of initial 4 he in hematite its effect on the calculated ages in mesozoic hematites in previous studies is negligible the age concordance implies that similar concordance should exist in samples with a known thermal history much older than pliocene moreover the helium method could also work on specularite with a thermal history younger than pliocene

geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin 1 we report natural remanent magnetization nrm directions and geomagnetic paleointensity proxies for part of the matuyama chron 0 9 2 2 ma interval from two sites located on sediment drifts in the iceland basin at ocean drilling program sites 983 and 984 mean sedimentation rates in the late matuyama chron are 15 9 and 11 5 cm kyr 1 respectively for the older part of the record 1 2 ma oxygen isotope data are too sparse to provide the sole basis for age model construction the resemblance of the volume susceptibility record and a reference 18o record led us to match the two records to derive the age models this match based on site 983 984 susceptibility is consistent with available site 983 984 benthic 18o data paleointensity proxies were derived from the slope of the nrm versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30 60 mt peak field range paleointensity lows correspond to polarity reversals at the limits of the jaramillo olduvai cobb mountain and reunion subchrons and to seven excursions in nrm component directions magnetic excursions defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator are observed at 932 1048 1115 1190 1215 cobb mountain subchron 1255 1472 1480 1567 1575 gilsa subchron and 1977 ka the results indicate that geomagnetic directional excursions associated with paleointensity minima are a characteristic of the matuyama chron and probably of polarity chrons in general

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

40ar loss in experimentally deformed muscovite and biotite with implications for 40ar 39ar geochronology of naturally deformed rocks abstract the effects of deformation on radiogenic argon 40ar retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite cylindrical cores 15 mm in length and 6 25 mm in diameter were drilled from granite collected from the south armorican massif in northwestern france loaded into gold capsules and weld sealed in the presence of excess water the samples were deformed at a pressure of 10 kb and a temperature of 600 c over a period 29 of hours within a solid medium assembly in a griggs type triaxial hydraulic deformation apparatus overall shortening in the experiments was approximately 10 transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking cracking and grain segmentation of the micas infrared ir laser co2 heating of individual 1 5 2 5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well defined 40ar 39ar plateau ages of 311 2 ma 2 identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40ar loss of 0 35 in muscovite and 2 3 40ar loss in biotite intragrain in situ ultraviolet uv laser ablation 40ar 39ar ages 4 10 1 of deformed muscovites range from 309 13 to 264 7 ma consistent with 0 16 40ar loss relative to the undeformed muscovite the in situ uv laser ablation 40ar 39ar ages of deformed biotite vary from 301 to 217 ma consistent with up to 32 40ar loss no spatial correlation is observed between in situ 40ar 39ar age and position within individual grains using available argon diffusion data for muscovite the observed 40ar loss in the experimentally treated muscovite can be utilized to predict average 40ar diffusion dimensions maximum 40ar 39ar ages obtained by uv laser ablation overlap those of the undeformed muscovite indicating argon loss of

cooling history of nested plutons from the variscan tichka plutonic complex morocco four imbricated mafic to felsic plutons of variscan age from morocco have been investigated for their cooling history and geochemical interactions with surrounding continental rocks oxygen isotope compositions of whole rocks and minerals have been used to model the cooling rates of these kilometer sized intrusions by combining both the knowledge of oxygen self diffusion data of rock forming minerals and the determination by ir spectroscopy of the water content of quartz the cooling times are estimated ranging from 10 to 5 10 years in agreement with the shallow emplacement 4 6 km depth of these intrusions into the continental crust such fast cooling rates could explain why after assimilation of the various country rocks heterogeneities of both neodymium and strontium isotope ratios were still preserved a progressive o increase from the mafic to felsic terms of the plutonic suite which does not excess 1 to 1 5 could be explained by the assimilation of metamorphosed pelitic and volcanic rocks that constitute the basement of the tichka plutonic complex

k ar dating of magmatic sericite crystallites for determination of cooling paths of metamorphic overprints a combined optical and x ray characterisation of fine grained micas in granodioritic to tonalitic dikes and intrusions of the eastardennes eifel nappe linksrheinisches schiefergebirge germany belgium was completed with k ar dating of separated size fractions from these rocks in each sample the k ar ages decrease with decreasing size fraction but the same size fractions of the different samples yield different ages application of the closure temperature concept to the k ar ages of the various size fractions indicates that the crystallite size expressed as cis fwhmser001ad and coherent scattering domain csd thickness acts as the effective radius of ar diffusion a linear model may describe the correlation between k ar ages and crystallite thickness the ages younger than the model prediction are caused by the occurrence of microcline as an accessory k bearing mineral phase the obtained k ar ages were interpreted as representative of the cooling path of the magmatic components in the eastardennes eifel nappe below 330 c the k ar age of the largest size fraction of the tonalitic dike v11 313 6 7 2 ma is the closest value for the age of the variscan cleavage development the oldest age of the hydrothermally altered tonalitic dike 300 2 8 4 ma might represent an episode of hydrothermal fluid migration after cleavage development it represents at least a minimum age the microcline age 242 8 6 0 ma of the hill intrusion may represent the closure temperature of this mineral at about 140 c

petrography and uplift history of the quaternary takidani granodiorite could it have hosted a supercritical hdr geothermal reservoir abstract the quaternary takidani granodiorite japan alps is analogous to the type of deep seated 3 5 km deep intrusive hosted fracture network system that might support supercritical hot dry wet rock hdr hwr energy extraction the i type takidani granodiorite comprises porphyritic granodiorite porphyritic granite biotite hornblende granodiorite hornblende biotite granodiorite biotite hornblende granite and biotite granite facies the intrusion has a reverse chemical zonation characterized by 70 wt sio2 at its inferred margin and 600 c hypersaline 35 wt nacleq fluids of magmatic origin with inferred minimum pressures of formation being 600 750 bar which corresponds to fluid entrapment at 2 4 3 0 km depth al in hornblende geobarometry indicates that hornblende crystallization occurred at about 1 45 ma 7 7 9 4 km depth in the marginal eastern takidani granodiorite but later at 1 25 ma and shallower 6 5 7 0 km near the core of the intrusion the average rate of uplift across the takidani granodiorite from the time of hornblende crystallization has been 5 1 5 9 mm yr although uplift was about 7 5 mm yr prior to 1 2 ma which is faster than average uplift rates in the japan alps 3 mm yr during the last 2 million years a temperature depth time window when the takidani granodiorite had potential to host an hdr system would have been when the internal temperature of the intrusive was cooling from 500 c to 400 c taking into account the initial 7 5 mm yr rate of uplift and effects of erosion an optimal temperature time depth window is proposed for 500 c at 1 54 1 57 ma and 5 2 0 9 km drilling depth and 400 c at 1 36 1 38 ma and 3 3 0 8 km drilling depth which is within the capabilities of modern drilling technologies and similar to measured temperature depth profiles in other active hydrothermal systems e g at kakkonda japan

the low latitude boundary layer and the boundary plasma sheet at low altitude prenoon precipitation regions and convection reversal boundaries the dayside zone of soft precipitation can be divided into four distinct types of plasma regimes each corresponding to the respective magnetospheric source region the cusp the mantle the low latitude boundary layer llbl and the dayside extension of the bps based on a detailed spectral study including comparisons with nonsimultaneous isee 1 satellite llbl data we identify regions of llbl type plasma in the dmsp data set and compare these plasma boundaries with convection reversal boundaries crbs as determined by either sondrestrom or the drift meter instrument on board the dmsp f9 spacecraft the nine cases considered are all in the prenoon local time sector we find that in eight of the nine cases the crb occurs within the llbl as expected generally near to but not coincident with the equatorward edge of the llbl type plasma in our sample set chosen for cases with latitudinally wide easily identifiable llbl signatures the average latitudinal width was 1 85 magnetic latitude the crb defined as the onset of steady antisunward convection occurred about 30 of this width beyond the equatorward onset of llbl type particles the most equatorward portion of the region with llbl type plasma usually had near zero or erratic convection and may correspond to the stagnation region reported from isee observations the potential drop observed across the low altitude llbl is roughly estimated to be typically 5 kev a summary is given on how the various high altitude sources can be identified when plasma regions are observed at low altitude in the dayside auroral oval

an extensive late cenozoic terrestrial record of multiple glaciations preserved in the tintina trench of west central yukon stratigraphy paleomagnetism paleosols and pollenthis is a companion paper to barendregt et al also in this issue geological survey of canada contribution 20100035 the tintina trench in west central yukon is a late miocene graben formed along the antecedent early tertiary tintina fault since its formation the trench has served as a sediment trap for alluvial and glacial deposits an extensive record of preglacial glacial and interglacial sediments spanning the late pliocene to late pleistocene has been preserved and is exposed today in modern landslide scars this sedimentary record comprises multiple sequences of tills outwash mudflows loess and paleosols the glacial sediments are the product of both local montane and regional cordilleran ice advances that channeled into the trench while loess and well developed paleosols brunisols and luvisols reflect nonglacial and interglacial conditions respectively the tintina trench exposures provide the most complete record of glaciations for the region paleomagnetism paleosols and palynology provide age constraints for the geological events a formal stratigraphic nomenclature is proposed for this region

mid cretaceous oblique rifting of west antarctica emplacement and rapid cooling of the fosdick mountains migmatite cored gneiss dome abstract in marie byrd land west antarctica the fosdick mountains migmatite cored gneiss dome was exhumed from mid to lower middle crustal depths during the incipient stage of the west antarctic rift system in the mid cretaceous prior to and during exhumation major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle derived diorite in dikes a succession of melt and magma related structures formed at temperatures in excess of 665 50 c based on ti in zircon thermometry these record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique south fosdick detachment zone solid state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels to determine the duration of exhumation and cooling we sampled granite and gneisses at high spatial resolution for u pb zircon geochronology and 40 ar 39 ar hornblende and biotite thermochronology u pb zircon crystallization ages for the youngest granites are 102 ma three hornblende ages are 103 to 100 ma and 12 biotite ages are 101 to 99 ma all overlap within uncertainty the coincidence of zircon crystallization ages with 40 ar 39 ar cooling ages indicates cooling rates 100 c m y that when considered together with overprinting structures indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting orientations of structures and age constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the marie byrd land margin the rotation may be a result of localized extension arising from unroofing and arching of the fosdick dome extensional opening within a pull apart zone or changes in plate boundary configuration the rapid tectonic and temperature evolution of the fosdick mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension transtension and accords with numerous studies of migmatite cored gneiss domes in transcurrent settings

constraints on the timing of late eburnean metamorphism gold mineralisation and regional exhumation at damang mine ghana the damang gold deposit in southwest ghana is unique among known deposits in ghana comprising gold mineralisation in two distinct styles early gold hosted in a stratigraphically controlled auriferous quartz pebble conglomerate horizon is overprinted by later gold mineralisation contained in a sub horizontal fault fracture quartz vein array a multi system geochronological study is used to constrain the timing of igneous activity regional metamorphism gold mineralisation and the thermal history at damang u pb analysis of zircons from birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178 0 9 3 ma and 2164 6 8 0 ma respectively which is consistent with previous studies the age of formation of staurolite grade amphibolite facies peak metamorphic mineral assemblages at 2005 26 ma is provided by u th total pb epma analysis of metamorphic monazite grains in the tarkwa phyllite measured 40ar 39ar biotite ages range between 1980 9 ma and 1898 11 ma argon diffusion modelling with the programme diffarg suggests that this age range could be achieved by a period of rapid cooling at a rate of approximately 17 c ma followed by a prolonged period of much slower cooling at a rate of 0 15 c ma the period of rapid cooling is interpreted to represent localised exhumation of the damang host rocks during the latest stage of the eburnean orogeny at the time of hydrothermal gold mineralisation given these age constraints hydrothermal gold mineralisation is inferred to have occurred between approximately 2030 ma and 1980 ma these ages constrain metamorphism fluid flow and gold mineralisation at damang and are the youngest currently recognised in the birimian of sw ghana

cooling pattern and mineralization history of the saint sylvestre and western marche leucogranite pluton french massif central i 40ar 39ar isotopic constraints abstract the saint sylvestre western marche leucogranite complex lc is a composite pluton emplaced 324 m y ago in the variscan continental crust of the northwestern limousin french massif central petrographic structural and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for ar retention in muscovite part i and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40 50 m y part ii a suite of thirty seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40 ar 39 ar technique muscovite is characterized by well defined plateau ages ranging between 314 and 301 ma while biotite gives less well behaved age spectra with apparent ages varying between 320 and 300 ma the muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton field petrographic gravimetric and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional post cooling fault network cutting through the laccolith when restored to their approximate pre faulting geometry the age variations are shown to correlate positively with sample elevation with a top to base difference as large as 10 14 m y over a mean intrusion thickness of 3 km this pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened variscan crust during the westphalian and is confirmed by two dimensional thermal modelling part ii the data show that 40 ar 39 ar mica ages can postdate the time of crystallization of the host pluton by up to 23 ma depending on the level sampled at the surface they warn against interpreting k ar mica ages from similar low relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure cooling mechanism and post cooling faulting of the pluton in turn provided that the pre faulting geometry of the intrusion is properly assessed the vertical cooling pattern can reveal fine scale variations in the thermal structure of old eroded intrusions that may be extremely useful for recovering quantitative information on the mechanisms and rates of cooling in ancient orogens

40ar 39ar thermochronology of the kampa dome southern tibet implications for tectonic evolution of the north himalayan gneiss domes structural and thermochronological studies of the kampa dome provide constraints on timing and mechanisms of gneiss dome formation in southern tibet the core of kampa dome contains the kampa granite a cambrian orthogneiss that was deformed under high temperature sub solidus conditions during himalayan orogenesis the kampa granite is intruded by syn tectonic leucogranite dikes and sills of probable oligocene to miocene age overlying paleozoic to mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite staurolite grade at the base of the sequence to unmetamorphosed at the top the kampa shear zone traverses the kampa granite metasediment contact and contains evidence for high temperature to lowtemperature ductile deformation and brittle faulting the shear zone is interpreted to represent an exhumed portion of the south tibetan detachment system biotite and muscovite 40 ar 39 ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14 22 0 18 to 15 54 0 39 ma cooling ages and concordant spectra with 14 64 0 15 to 14 68 0 07 ma cooling ages petrographic investigations suggest disturbed samples are associated with excess argon intracrystalline deformation mineral and fluid inclusions and or chloritization that led to variations in argon systematics we conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at 14 6 ma the kampa granite yields the youngest biotite 40 ar 39 ar ages of 13 7 ma immediately below the granite metasediment contact we suggest that this age variation reflects either varying mica closure temperatures re heating of the kampa granite biotites above closure temperatures between 14 6 ma and 13 7 ma or juxtaposition of rocks with different thermal histories our data do not corroborate the inverse mica cooling gradient observed in adjacent north himalayan gneiss domes instead we infer that mica cooling occurred in response to exhumation and conduction related to top to north normal faulting in the overlying sequence top to south thrusting at depth and coeval surface denudation

the cooling history of the late pliocene eldzhurtinskiy granite caucasus russia and the thermochronological potential of grain size age relationships abstract isotopic age investigations have been carried out on samples from the late pliocene eldzhurtinskiy granite great caucasus russia in order to elucidate the cooling history of this kilometere sized plutonic body the samples were taken from a horizontal profile from the edge to the centre of the granite and from a vertical profile from the top to a depth of 4570 m the 40ar 39ar results on biotites ranging from 2 5 to 1 2 ma indicate a vertical age zoning of the granite caused by regional uplift an uplift rate of 4 mm a is calculated from the age differences together with the results of fission track dating on apatite and zircon and the present day rock temperatures the biotite ages can be used to evaluate cooling rates below 400 c the cooling rates of about 180 c ma are relatively high and spatially and temporarily fairly uniform throughout the granite a geothermal gradient of 45 c km is derived from uplift and cooling rates this is identical to the present day borehole gradient the 40ar 39ar ages of distinct biotite grain fractions from one and the same sample specimen are discordant generally they show clear age grain size correlation as expected from ar diffusion theory for cooling systems thus the biotite results support the additional detailed thermochronological information the cooling rates in the temperature range of biotite closure based on the grain size age correlations of the biotites are close to 180 c ma they provide evidence that within a certain temperature interval the time temperature path of a rock may be determined exclusively by isotopic dating of different grain size fractions of only one type of mineral

the caucasus an actual example of the initial stages of continental collision abstract a field study of the neotectonics of the georgian caucasus is combined with geophysical geodetic and geological information in order to obtain a mechanical model of actual deformation the analysis of satellite photos together with topographical and geological maps shows that the great caucasus is bisected by a nne trending sinistral strike slip fault zone the western great caucasus is thrusted to the south over the rioni basin while on the other hand its northern flank is a monocline that descends gently into the russian platform active volcanism is present from elbruz to kazbeg the eastern great caucasus is limited by thrusts to the north and to the south and intermediate structures are disposed in a fan like shape no recent volcanism is observed the eastern limit of the chain is a nnw trending dextral strike slip fault zone seismicity from a local network illustrates the activity of the thrusts and gives evidence for the strike slip faults microtectonic data confirm the recent activity and the sinistral character of the borjomi kazbeg strike slip fault the strike slip faults bordering the eastern caucasus may be seen as the limits of a plastic wedge in front of the arabian plate at an early stage of continental collision

partial melting and recrystallization of granite and their application to deep disposal of radioactive waste part 2 recrystallization abstract a previously published scheme for the high temperature disposal of nuclear waste in deep boreholes requires that granite host rock partially melted by the heat from the waste can be completely recrystallized on the time scale of the cooling of the waste experimental studies of dynamic recrystallization of a typical s type crustal granite were therefore undertaken under the pressure 0 15 gpa and other conditions likely to arise during implementation of this scheme linear cooling experiments at 1 c h 0 1 c h and 1 c day were carried out over the temperature range 800 560 c these demonstrate that the partially melted granite can be fully recrystallized when cooled at 0 1 c h or slower crystal growth rates for the main phases formed during cooling are linearly proportional to total h 2 o content and decrease from alkali feldspar through biotite to quartz recrystallizing the partially melted granite at 0 1 c h compared with 1 c h increases grain size by a factor of 2 5 on the basis of partial melting experiments reported previously by us lithos 2003 this issue for the same granite and the results presented here it is concluded that an s type granite similar to the one studied would be a suitable host rock for the proposed high temperature very deep disposal scheme for radioactive waste

spatial relationship between large aspect angle vhf radio aurora and 557 7 nm emissions evidence for refraction a recently proposed explanation for large aspect angle vhf backscatter observations based on refraction through auroral ionization structures in the e region moorcroft 1989 has been confirmed by an experimental comparison of observations of auroral e region coherent backscatter at 48 5 mhz made with the bistatic auroral radar system or bars with simultaneous 557 7 nm emissions detected with a ccd based all sky imager the apparently large aspect angle radio auroral echoes are found to occur predominantly on the side of the visual aurora away from the radar for both the premidnight and the postmidnight sectors for the red lake bars radar this means that the strongest echoes occur to the north of the visual form in sharp contrast with previous comparisons particularly for the premidnight sector which found that radar echoes occur predominantly southward of the visual aurora these results are consistent with the proposed explanation based on refraction in the auroral e region

cenozoic recent tectonics and uplift in the greater caucasus a perspective from azerbaijan the greater caucasus is europe s highest mountain belt and results from the inversion of the greater caucasus back arc type basin due to the collision of arabia and eurasia the orogenic processes that led to the present mountain chain started in the early cenozoic accelerated during the plio pleistocene and are still active as shown from present gps studies and earthquake distribution the greater caucasus is a doubly verging fold and thrust belt with a pro and a retro wedge actively propagating into the foreland sedimentary basin of the kura to the south and the terek to the north respectively based on tectonic geomorphology active and abandoned thrust fronts the mountain range can be subdivided into several zones with different uplift amounts and rates with very heterogeneous strain partitioning the central part of the mountain range defined by the main caucasus thrust to the south and backthrusts to the north forms a triangular shape zone showing the highest uplift and fastest rates and is due to thrusting over a steep tectonic ramp system at depth the meridional orogenic in front of the greater caucasus in azerbaijan lies at the foothills of the lesser caucasus to the south of the kura foreland basin

the metamorphic complex of beloretzk sw urals russia â a terrane with a polyphase meso to neoproterozoic thermo dynamic evolution abstract an integrated geological study of the tectono metamorphic evolution of the metamorphic complex of beloretzk mcb which is part of the eastern bashkirian mega anticlinorium bma sw urals russia shows that the main lithological units are neoproterozoic riphean and vendian age siliciclastic to carbonate successions granitic syenitic and mafic intrusions together with subaerial equivalents comprise the neo and mesoproterozoic magmatic rocks the metamorphic grade ranges from diagenetic and very low grade in the western bma to high grade in the mcb the n s trending zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern bma structural data pb pb single zircon ages 40 ar 39 ar cooling ages and the provenance signature of riphean and vendian siliciclastic rocks in the western bma give evidence of mesoproterozoic grenvillian rifting deformation and eclogite facies metamorphism in the mcb and a neoproterozoic cadomian orogenic event in the sw urals three pre ordovician deformation phases can be identified in the mcb the first sse vergent isoclinal folding phase d 1 is younger than the intrusion of mafic dykes pb pb single zircon 1350 ma and older than the eclogite facies metamorphism high p low t eclogite facies metamorphism is bracketed by d 1 and the intrusion of the achmerovo granite pb pb single zircon 970 ma an extensional sinistral top down to nw directed shearing d 2 is correlated with the first exhumation of the mcb e vergent folding and thrusting d 3 occurred at retrograde greenschist facies metamorphic conditions the tremolite 40 ar 39 ar cooling age 718 5 ma of amphibolitic eclogite and muscovite 40 ar 39 ar cooling ages about 550 ma of mica schists indicate that a maximum temperature of 500 50 c was not reached during the neoproterozoic orogeny the style and timing of the neoproterozoic orogeny show similarities to the cadomian aged timan range nw of the polar urals geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the zuratkul fault suggest that the mcb is exotic with respect to the se margin of the east european platform thus the mcb is named the beloretzk terrane recognition of the beloretzk terrane and the neoproterozoic orogeny at the eastern margin of baltica has important implications for neoproterozoic plate reconstruction and suggests that the eastern margin of baltica might have lain close to the avalonian cadomian belt

paleomagnetic evidence for multiple late cenozoic glaciations in the tintina trench west central yukon canadathis article is a companion paper to duk rodkin et al also in this issue the tintina trench in west central yukon has preserved an extensive record of late cenozoic preglacial glacial and interglacial deposits these deposits comprise multiple sequences of tills outwash loesses and paleosols the sediments that were laid down directly by ice tills are of both local montane and regional cordilleran provenance the tintina trench area was impacted repeatedly by montane ice from the southern ogilvie mountains to the northwest 2500 m above sea level asl and also repeatedly along its southern extent by cordilleran ice from the selwyn mountains to the east 2759 m asl the latter forming the continental divide in this region we report here the magnetostratigraphy of three sections rock creek 64 13 n 139 07 w west fifteenmile river 64 29 n 139 55 w and east fifteenmile river 64 23 n 139 48 w the majority of the units identified at these sections record late pliocene to mid pleistocene glaciations although relatively thin surficial sequences of late mid

anticorrelated rb sr and k ar age discordances leuchtenberg granite ne bavaria germany the leuchtenberg granite oberpfalz ne bavaria displays a continuous differentiation trend ranging from mildy peraluminous coarse grained porphyritic biotite granites bg to strongly peraluminous medium to fine grained garnet bearing muscovite granites gmg the rb sr and k ar age determinations of whole rock and mineral samples from the granite and associated intermediate rocks redwitzites have revealed two divergent age gradients rb sr wholerock dates decrease and initial 87sr 86sr ratios increase for successively more evolved subsets of the granite all bg samples 87rb 86sr 2 16 yield a date of 326 2 ma with a low initial 87sr 86sr ratio of 0 70778 0 00013 1 while all gmg samples 87rb 86sr 70 to 1000 yield a younger date of 317 2 ma with an enhanced initial 87sr 86sr ratio of 0 7146 0 0039 the k ar measurements on biotites and muscovites give closely concordant dates for the gmg 326 323 ma and the southern lobe of the bg 324 320 ma the northern lobe of the bg including the redwitzites shows a well defined trend of decreasing k ar dates from 320 ma to 300 ma towards the northwest critical consideration of both isotope systems leads to the conclusion that the rb sr system of the gmg was disturbed by a later hydrothermal event the ca 326 ma whole rock rb sr date for the bg is not in conflict with any of the k ar mineral dates and is taken as approaching the crystallization age of the leuchtenberg granite the k ar age progression within the northern lobe of the bg indicates that this part either cooled down over a protracted period of some 20 ma or experienced reheating at ca 300 ma the study highlights the potential of combined rb sr and k ar dating in deciphering detailed chronology on the scale of a single igneous intrusion

comparison between grain size and multi mineral 40ar 39ar thermochronology abstract in the case of volume diffusion the closure temperature of a mineral is function of among other factors the characteristic diffusion dimension which can be approximated by the grain size of the mineral analysed for grains smaller than or similar in size to the diffusion domains the theoretical possibility of single mineral grain size thermochronology had been demonstrated empirically in earlier studies mostly using biotite in order to examine the potential of this method it was tested alongside the widely used multi mineral 40 ar 39 ar thermochronology the sample comes from the granitic mclean pluton in the south section of the grenville orogeny seven grain size separates of biotite ranging between 90 and 1000 m eight size fractions of amphibole between 63 and 1000 m and three size fractions of k feldspar 250 600 m were extracted and dated by the laser step heating 40 ar 39 ar method the total gas ages obtained behave as theoretically predicted with increasing ages for increasing grain sizes including for k feldspar but with the exception of the smallest and the largest grains for biotite and amphibole the calculated cooling rates are ca 0 7 c ma for k feldspar ca 2 5 c ma for biotite and ca 11 c ma for amphibole corresponding very well to a monotonic cooling of the mclean pluton a quick initial thermal re equilibration with the cooler host rocks is followed by a much slower cooling on a thermal path parallel to that of the frontenac terrain situated immediately to the southeast the validity of the single mineral grain size thermochronology is demonstrated by comparison with the thermal evolution of the adjacent units and with the cooling history derived from a multi mineral thermochronology suggesting that it can be routinely used the application of this method can be hampered by insufficiently low analytical uncertainties

alpine deformation and 40ar 39ar geochronology of synkinematic white mica in the siviez mischabel nappe western pennine alps switzerland we explore the timing of deformation and exhumation of the siviez mischabel nappe western swiss alps which has been considered a classic example of a midcrustal crystalline nappe since the studies of argand 1916 this study presents 40ar 39ar ages obtained on both synkinematic white mica from permo triassic cover sediments and more complex white mica populations from basement gneisses of the siviez mischabel and middle pennine nappes primary foliation developed in cover units by nucleation growth and rigid rotation of mica grains during noncoaxial alpine deformation although some samples show a crenulation of this primary foliation mica growth appears to have occurred only during the development of primary foliation the main phase of greenschist facies deformation related to imbrication of the siviez mischabel nappe and other middle pennine nappes good agreement exists between independent estimates of the timing of deformation and reported 40ar 39ar white mica ages from cover units of the central and southern siviez mischabel nappe in cover units from the central and southern siviez mischabel regions of the study area 40ar 39ar ages appear to date synkinematic white mica growth results suggest that the siviez mischabel nappe was emplaced and developed foliation during a 5 m y period from 41 to 36 ma in cover units from the eastern siviez mischabel however 40ar 39ar white mica ages appear to date postkinematic thermal events these thermal events may be related to oligocene magmatic activity in the lower pennine nappes or to miocene development of the simplon fault zone variations in the relation between alpine age and grain size for cover samples from the central eastern and southern siviez mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism when considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western swiss alps these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high pressure units in the suture zone of the western alps are intimately related and synchronous on the scale of a few million years

holocene glacier activity in the british columbia coast mountains canada abstract the coast mountains flank the pacific ocean in western british columbia canada subdivided into the southern pacific ranges central kitimat ranges and northern boundary ranges the majority of large glaciers and icefields are located in the boundary and pacific ranges prior descriptions of the holocene glacial history of this region indicate the holocene was characterized by repeated episodes of ice expansion and retreat recent site specific investigations augment our understanding of the regional character and duration of these events in this paper previously reported and new radiocarbon evidence is integrated to provide an updated regional assessment the earliest evidence of glacier expansion in the coast mountains comes from the boundary ranges at 8 9 and 7 8 ka and in the pacific ranges at 8 5 8 2 ka with the latter advance corresponding to an interval of rapid global climate deterioration although generally warm and dry climates from 7 3 to 5 3 ka likely limited the size of glaciers in the region there is radiocarbon evidence for advances over the interval from 7 3 to 6 0 and at 5 4 5 3 ka in the pacific ranges following these advances glaciers in the pacific ranges expanded down valley at 4 8 4 6 4 4 4 0 3 5 2 6 1 4 1 2 and 0 8 0 4 ka while glaciers in boundary ranges were advancing at 4 1 4 0 3 7 3 4 3 1 2 8 2 3 1 7 1 1 and 0 8 0 4 ka after 0 4 ka it appears that most glaciers in the coast mountains continued to expand to attain their maximum holocene extents by the early 18th to late 19th centuries this enhanced record of holocene glacier activity highlights the temporal synchrony in the coast mountains individual expansion events in the mid to late holocene broadly correspond to intervals of regional glacier activity reported in the canadian rocky mountains in alaska and on high elevation volcanic peaks in washington state

seismic structure of the crust and uppermost mantle beneath caucasus based on regional earthquake tomography abstract we present a new seismic model of the crust beneath the caucasus based on tomographic inversion of p and s arrival times from earthquakes occurred in the region recorded by regional seismic networks in the caucasian republics the resulting p and s velocity models clearly delineate major tectonic units of the study area a high velocity anomaly in transcaucasian separating the great and lesser caucasus possibly represents a rigid crustal block corresponding to the remnant oceanic lithosphere of tethys another high velocity pattern coincides with the southern edge of the scythian plate strongly deformed areas of great and lesser caucasus are mostly associated with low velocity patterns representing thickened felsic part of the crust and strong fracturing of rocks most cenozoic volcanic centers of caucasus match to the low velocity seismic anomalies in the crust for example the kazbegi volcano group is located above an elongated low velocity anomaly squeezed between high velocity segments of transcaucasian and scythian plate we propose that mantle part of the arabian and eurasian plates has been delaminated due to the continental collision in the caucasus region as a result overheated asthenosphere appeared nearly the bottom of the crust and facilitated melting of the crustal material that caused the origin of recent volcanism in great and lesser caucasus

the exhumation of the western greater caucasus a thermochronometric study this study provides 39 new thermochronometric analyses from the western part of the greater caucasus a region in which existing data are extremely limited and of questionable quality the new results are consistent with field studies that identify triassic to middle jurassic cimmerian and oligo miocene alpine orogenic erosional events an inverse relationship between the fission track and depositional ages of oligo miocene sedimentary samples also implies some degree of eocene erosion of the greater caucasus and intermediate sediment storage cooling ages and field relationships within the core of the range west of mt elbrus require 5 km of permo triassic exhumation and restrict the overall amount of cenozoic exhumation to 2 5 km current exhumation rates are typically low and do not support a plio pleistocene increase in climate driven denudation high 1 km ma 1 rates of exhumation are restricted to the southern flank of the range in northwest georgia despite a general lack of significant seismicity within the study region this exhumation peak is close to the largest instrumentally recorded earthquake in the caucasus ms 7 0 this may suggest that exhumation is associated with the decoupling of the sedimentary succession from its crystalline basement in the southern part of the range and the inversion of the largely jurassic fill of the greater caucasus basin rates of exhumation are compatible with this being driven by active shortening further sampling and analysis are required to provide a higher resolution low temperature thermochronology of alpine exhumation to isolate the drivers for palaeogene dziruli massif cooling and uplift and to constrain better the extent of the current localized phase of rapid exhumation

large scale imaging of high latitude convection with super dual auroral radar network hf radar observations the hf radars of the super dual auroral radar network superdarn provide measurements of the e b drift of ionospheric plasma over extended regions of the high latitude ionosphere with the recent augmentation of the northern hemisphere component to six radars a sizable fraction of the entire convection zone approximately one third can be imaged nearly instantaneously 2 min to date the two dimensional convection velocity has been mapped by combining line of sight velocity measurements obtained from pairs of radars within common volume areas we describe a new method of deriving large scale convection maps based on all the available velocity data the measurements are used to determine a solution for the distribution of electrostatic potential expressed as a series expansion in spherical harmonics the addition of data from a statistical model constrains the solution in regions of no data coverage for low order expansions the results provide a gross characterization of the global convection we discuss the processing of the radar velocity data the factors that condition the fitting and the reliability of the results we present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field imf in the case of a sudden polarity change from northward to southward imf the convection is seen to reconfigure globally on very short 6 min timescales

the relation between grain size and 40ar 39ar date for alpine white mica from the siviez mischabel nappe switzerland abstract variations in alpine white mica 40 ar 39 ar dates from the cover units of the siviez mischabel nappe relate to regional variations in the thermal history of the nappe we focus on three regions within the nappe the central siviez mischabel csm the southern siviez mischabel ssm and the eastern siviez mischabel esm our approach weaves together observations of quartz and mica textures in thin section the variation of 40 ar 39 ar date with grain size considerations of the effective diffusion dimension edd of argon in white mica and a comparison of dates with diffusion model results in the csm pressure solution of quartz and dislocation glide in mica accommodated alpine deformation dates record mica growth during nappe emplacement from 40 to 36 ma and do not vary with grain size in the ssm and esm both mica and quartz show textures associated with dynamic recrystallization and dates decrease with grain size in the ssm dates also agree with the timing of nappe emplacement but in the esm dates significantly post date the timing of nappe emplacement a comparison of dates with diffusion model results supports inferences from rock fabrics that the ssm experienced higher peak temperatures than the csm even though dates from both units approximate the timing of mica growth dates obtained from the esm however do not compare well with simple models and the thermal evolution of this region of the nappe in the neighborhood of the simplon fault zone is not well understood

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

evidence for an early pleistocene glaciation in the okanagan valley southern british columbia depositional evidence of early pleistocene glaciations in british columbia are documented at only a few sites near kelowna in southern british columbia a construction project exposed glacial sediments beneath lambly creek basalt providing a minimum age for this glaciation the basalt is composed of a number of flows yielding ages that range from 0 76 0 11 to 1 5 0 1 ma the sediments consist of a diamicton interpreted to be till up to 3 m thick mantled by a weakly developed paleosol the diamicton is underlain by fluvial sands up to 5 m thick in places revealing injection features and minor faulting a unit of stratified gravel underlain by grey clay is inferred to underlie the exposed sediments based on nearby outcrops and excavations sediments and overlying basalts are normally magnetized and are assigned to the jaramillo normal subchron 1 069 0 987 ma the till is here referred to as the westbank first nation till it is early pleistocene in age and represents the earliest evidence of g

40ar 39ar and 18o 16o studies of the chegem ash flow caldera and the eldjurta granite cooling of two late pliocene igneous bodies in the greater caucasus mountains russia volcanic and intrusive rocks of the chegem caldera and the nearby eldjurta eldzhurtinskiy granite record a late pliocene episode of silicic magmatism in the north central caucasus mountains surface exposures created by the recent rapid uplift and erosion of the caucasus mountains span a 2 km vertical section of chegem caldera fill and 1150 m of the eldjurta granite cored mineral exploration drillholes in the eldjurta granite extend the sampling to a depth of 4 km the unique sampling range available in these two young igneous bodies affords an excellent opportunity to study their denudation and cooling histories which we examine by means of 40 ar 39 ar and 18 o 16 o measurements on an extensive sample suite total fusion biotite and sanidine ages from eight chegem tuff samples both intracaldera and outflow are analytically indistinguishable with a weighted mean of 2 82 0 02 ma a cross cutting granodiorite porphyry intrusion has a sanidine total fusion age of 2 84 0 03 ma and whole rock incremental heating of a post caldera andesite flow which caps the caldera fill yields an age of 2 82 0 02 ma thus caldera formation and post caldera resurgence and volcanism all occurred within a very short time 50 000 yr biotite total fusion ages of ten eldjurta granite samples including seven from 500 m intervals in the 4 km deep drillhole show a systematic linear decrease in age with depth from 1 90 ma near the roof contact of the granite to 1 56 ma at a depth of 3700 m assuming these ages were set at the same temperature this age depth gradient implies an isotherm migration rate of 13 mm yr between 1 90 and 1 56 ma this migration rate is due to a combination of rapid denudation and downward relaxation of isotherms with cooling rates between 200 and 500 c ma during this period oxygen isotopic compositions of quartz k feldspar plagioclase and biotite from the drillhole samples below the 800 m depth are fairly uniform and record primary igneous 18 o values with little evidence for subsolidus hydrothermal activity however in surface outcrop samples and in the shallowest drillhole sample mineral 18 o values have been lowered by up to 3 by interaction with an external meteoric hydrothermal fluid the primary mineral 18 o values of the eldjurta granite are distinctly higher than the corresponding phenocryst 18 o values in the chegem volcanic rocks indicating that the two bodies evolved as separxate magma batches

partial melting and recrystallization of granite and their application to deep disposal of radioactive waste part 1 rationale and partial melting abstract a high temperature deep borehole disposal scheme for nuclear waste is outlined this requires that granite host rock can be partially melted by the heat from the waste then completely recrystallized both on the time scale of the cooling of the waste experimental studies of non equilibrium partial melting of a typical s type crustal granite were carried out under the conditions likely to arise during implementation of this scheme p 0 15 gpa t 700 850 c the melting relations were determined under h2o saturated and h2o undersaturated conditions melting begins just under 700 c and the amount of melt increases with temperature and especially h2o content up to saturation such that with no added h2o only 5 of melt is produced even at 850 c but the addition of 1 h2o will generate 40 of melt 50 c lower at 800 c the partial melts formed at high degrees of melting e g h2o saturated t 800 c approximate to equilibrium melting but the smaller amounts of melt produced at lower temperatures under h2o undersaturated conditions clearly do not on the basis of these experiments it is concluded from the point of view of partial melting that an s type granite similar to the one studied would be a suitable host rock for the proposed high temperature very deep disposal scheme for radioactive waste

the tectonic significance of k ar illite fine fraction ages from the san luis formation eastern sierras pampeanas argentina the sierra de san luis forms the southern tip of the eastern sierras pampeanas in central argentina two narrow belts of low grade phyllites and quartz arenites i e the san luis formation have accommodated part of the strain related differential exhumation of the medium to high grade metamorphic domains that constitute to the basement complex of the sierra eleven phyllite samples were subjected to the k ar fine fraction dating technique results are interpreted in relation to the kubler index of the illites which indicate epimetamorphic conditions for the majority of the samples obtained ages between 330 and 290 ma cover a period of compressional tectonics in the late mississippian visean serpukhovian boundary followed by the subsidence during the formation of the paganzo basin in the provinces of la rioja and san luis these tectonic movements are coincident with the toco orogeny in northern chile and southern bolivia this suggests that the older k ar ages document the compressional stage and that younger ages record the cooling of the basement during the subsequent extensional uplift of the basement

empirical high latitude electric field models electric field measurements from the dynamics explorer 2 satellite have been analyzed to extend the empirical models previously developed from dawn dusk ogo 6 measurements j p heppner 1977 the analysis embraces large quantities of data from polar crossings entering and exiting the high latitudes in all magnetic local time zones paralleling the previous analysis the modeling is based on the distinctly different polar cap and dayside convective patterns that occur as a function of the sign of the y component of the interplanetary magnetic field the objective which is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns is met by deriving one pattern model bc for the northern hemisphere with a y interplanetary magnetic field imf and southern hemisphere with a y imf and two patterns models a and de for the northern hemisphere with a y imf and southern hemisphere with a y imf the most significant large scale revisions of the ogo 6 models are 1 on the dayside where the latitudinal overlap of morning and evening convection cells reverses with the sign of the imf y component 2 on the nightside where a westward flow region poleward from the harang discontinuity appears under model bc conditions and 3 magnetic local time shifts in the positions of the convection cell foci the modeling above was followed by a detailed examination of cases where the imf z component was clearly positive northward neglecting the seasonally dependent cases where irregularities obscure pattern recognition the observations range from reasonable agreement with the new bc and de models to cases where different characteristics appeared primarily at dayside high latitudes the analysis of these differences for strong bz conditions using passes through different mlt zones led to finding that the deviations from both the bc and de models could be modeled by extending the evening cell foci toward and beyond noon with a deformation that roughly resembles a rotational twist of the extended evening cell the morning cell accommodation to the model bc deformation produces a narrow tongue of eastward convection near 75 invariant latitude on the dayside the model de deformation compresses the morning cell toward the night hours such that the sunward convection in the morning cell is diverted to polar latitudes in the 0400 to 0700 mlt sector in both cases the deformations of the two cell patterns lead to sunward convection in dayside polar regions while maintaining the integrity of the nightside convection pattern thus the nightside dilemmas that plague three and four cell models designed to explain sunward convection in polar regions under bz conditions do not appear