the dynamics of the suture between the kohistan island arc and the indian plate in the himalaya of pakistan abstract the pressure temperature and temperature time paths derived for rocks in the kohistan arc and adjacent nanga parbat haramosh massif record the dynamics of the collision between the island arc and the indian plate studies of p t t paths show that the kohistan arc was thrust over the nanga parbat haramosh massif at least 25 ma ago but not more than 30 35 ma ago rocks in the kohistan arc followed decreasing pressure paths with the early metamorphism beginning at high pressures 9 5 kbar and later metamorphism occurring at 8 0 kbar in contrast rocks in the nanga parbat haramosh massif indian plate experienced increasing pressure and temperature paths prior to thrusting the massif was at low pressures 4 0 kbar and low temperatures 450 c later the pressure and temperature increased to 8 kbar and 580 c the authors interpret the convergence to approximately the same pressure and temperature of the p t paths in the two terranes as being the result of thrusting and thermal equilibration between the thrust sheets 40ar 39ar cooling ages of hornblendes and other geochronological data suggest that the time of peak metamorphism and hence the completion of thickening was approximately 30 35 ma ago temperature time paths show that after thrusting during the period 25 10 ma the kohistan arc and nanga parbat haramosh massif were uplifted at similar rates 0 5 km ma however in the past 10 ma the nanga parbat haramosh massif has been uplifted more rapidly than the adjacent kohistan arc rapid uplift has been accommodated by late faults along the edge of the massif

direct dating of mid crustal shear zones with synkinematic allanite new in situ u th pb geochronological approaches applied to the mont blanc massif dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes this objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ u th pb geochronology of allanite a greenschist facies shear zone within the mont blanc massif is documented allanite is synkinematic and belongs to the mylonitic assemblage la icp ms u th pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common pb component the use of measured pb isotope compositions of associated minerals feldspars and chlorite is critical for accurate common pb correction and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage a mean 208pb 232th age of 29 44 0 95 ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions this age reflects the timing of the mont blanc underthrusting below the penninic front and highlights the potential of directly dating deformation with allanite

geological significance of 40ar 39ar mica dates across a mid crustal continental plate margin connemara grampian orogeny irish caledonides and implications for the evolution of lithospheric collisions1 the connemara region is a world class example of a regional scale high temperature metamorphic terrain its rock record documents formation of a bi vergent orogenic wedge and associated calkalkali

contact metamorphism and depth of emplacement of the manaslu granite central nepal implications for himalayan orogenesis abstract the manaslu massif central nepal provides a well exposed example of a deeply eroded pluton its contact aureole can be followed from the base to the top along medium to low grade mesozonal to anchizonal tethyan metasediments contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18 21 km for the base and 9 13 km for the roof calculated temperatures within the aureole at 550 40 c are compatible with the intrusion temperature of a leucogranitic magma microstructural evidence shows that the temperature remained high 500 c at the base of the massif during and after granite emplacement whereas towards the top of the granite deformation proceeded rapidly at lower temperature heating of the abundant calcareous rocks in the contact aureole released a local co2 rich fluid whereas a h2o and boron rich fluid seems to have pervaded the whole zone this fluid is probably exsolved from the granitic melt during its crystallization the depth of emplacement of the massif has an important implication for the reconstruction of the himalayan geodynamic evolution implying a burial of the tethyan metasediments by a major refolding of the sedimentary cover or more probably by extensive development of the north himalayan nappes towards the south for more than 70 km before granite emplacement i e before the miocene the young age of the granite and its depth of emplacement suggest a rapid tectonic denudation in the order of 1 mm a 1 probably by normal faulting north of the massif during its cooling

relative sea level changes from ne ireland during the last glacial termination deglacial sea level records from ne ireland between 21 and 11 cal ka bp record marine transgressions and sensitive lithospheric responses to ice loading the sawtooth sea level curve contains four intervals characterized by 1 strong net uplift subaerial channelling and a global meltwater pulse 21 19 cal ka bp 2 ice loading isostatic depression and high relative sea level 19 17 5 cal ka bp 3 ice loading renewed isostatic depression and high relative sea level for 1000 years 17 14 5 cal ka bp 4 catastrophic ice wastage rapid uplift and lowstand 14 5 13 cal ka bp geophysical models do not capture these fluctuations

deglacial chronology from county donegal ireland implications for deglaciation of the british irish ice sheet in the north of ireland ice advanced northwards onto the continental shelf from inland centres of dispersion at the last glacial maximum we constrain the timing of three subsequent events by accelerator mass spectrometry 14 c dating of marine microfaunas from muds at corvish county donegal early and rapid deglaciation of the continental shelf off northwestern ireland occurred before 17 1 k 14 c years bp when marine muds formed in trawbreaga bay ice subsequently readvanced into the bay some time between 15 0 and 14 0 k 14 c years bp and compressed contemporary sea stacks and beach cobbles into a ridge at ballycrampsey early deglacial muds were deformed during this ice readvance and redeposited with stratigraphically inverted 14 c ages between 16 0 and 15 2 k 14 c years bp the deformed mud is overlain by undeformed laminae deposited during the final deglaciation of the area after 14 1 k 14 c years bp the early deglaciation in the north at c 17 k 14 c years bp is correlative with deglaciation recorded in the irish sea basin western ireland and northeastern scotland indicating a response of the entire british irish ice sheet to early warming of the north atlantic ocean that resulted in a loss of up to two thirds of its mass subsequent ice sheet readvance between 15 0 and 14 1 k 14 c years bp in the north is part of a widespread response of the british irish ice sheet to heinrich event 1 during the killard point stadial high relative sea levels that accompanied deglaciation at 17 k 14 c years bp and again at 14 1 k 14 c years bp indicate substantial loading and attendant isostatic depression by the british irish ice sheet

isotopic age constraints and metamorphic history of the talladega belt new evidence for timing of arc magmatism and terrane emplacement along the southern laurentian margin abstract u pb and 40ar 39ar dates from the talladega belt southernmost appalachians provide insight into the timing and nature of pre alleghanian tectonism in this region low grade metasedimentary rocks of the talladega belt represent the outermost preserved portions of the southern laurentian margin thus recording the earliest orogenic events that affected the margin in addition to later overprinting events these rocks are structurally overlain by the hillabee greenstone metavolcanic sequence from which metadacite yields an age of 470 4 ma ion microprobe u pb zircon age hillabee geochemistry indicates formation in an arc or back arc setting we suggest correlation of the hillabee greenstone with 460 470 ma arc related rocks in the dahlonega gold belt which extends along strike with the hillabee through georgia and into north carolina and interpret the hillabee as the southernmost volcanic expression of an early middle ordovician arc that formed outboard of laurentia rocks of the talladega be

vertical dimensions and age of the wicklow mountains ice dome eastern ireland and implications for the extent of the last irish ice sheet abstract patterns of erratic distribution show that the wicklow mountains formerly supported an independent ice cap or ice dome geomorphological mapping of the upper limits of evidence for glaciation ice scoured and moulded bedrock perched boulders and the distribution of features indicative of prolonged periglacial conditions tors frost shattered rock blockfields indicates that along the main axis of high ground erosive warm based ice buried all but the highest 725 m summits and over ran adjacent lower peaks and cols the presence of gibbsite in soil samples from above the inferred upper limit of glacial erosion and its absence in all samples from below this limit is consistent with the geomorphological evidence and implies removal of gibbsitic soils below 725 m by glacial erosion during the last glacial stage cosmogenic 10 be exposure ages for rock outcrops above the inferred upper limit of glacial erosion yield pre last glacial maximum lgm ages of 46 9 3 0 ka to 95 9 6 1 ka whereas rock outcrops on summits over ridden by warm based ice give post lgm ages of 18 2 1 2 to 19 1 1 2 ka all geomorphological and dating evidence thus indicates an lgm age for the ice dome the thickness of the ice dome and limited lateral dispersal of erratics indicate that at the lgm the wicklow ice was encircled by and confluent with thick powerful ice streams moving se from the irish midlands and southwards down the irish sea basin this conclusion is irreconcilable with the traditional view that the last irish ice sheet terminated at the south ireland end moraine but consistent with recent proposals that the last ice sheet was much more extensive than previously believed and over ran the south coast of ireland

peak metamorphic temperatures from cation diffusion zoning in garnet a model that relates the characteristic diffusion length and average cooling rate to peak temperature was developed for chemical diffusion in spherical geometries on the basis of geospeedometry principles and diffusion theory the model is quantitatively evaluated for cation diffusion profiles in garnet important model parameters were calibrated empirically using diffusion zoning of ca in garnet from the pikwitonei granulite domain a terrane for which the thermal history has been well characterized the results are used i to empirically test diffusion parameters for mg and fe ii and ii to develop a tool that uses the diffusion zoning of these cations in garnet to constrain peak temperature conditions for garnet bearing rocks the thermometric approach was externally tested by applying it to garnet crystals from various metamorphic terranes worldwide and comparing the results to published peak temperature estimates the results overlap within uncertainties in all cases but result that are based on fe ii and mg chemical diffusion profiles are up to three times more precise than those acquired by conventional methods the remarkable consistency of the results implies that the model is robust and provides a reliable means of estimating peak temperatures for different types of high grade metamorphic rock the tool could be of particular advantage in rocks where critical assemblages for conventional thermometry do not occur or have been replaced during retrogression

crustal shortening and eocene extension in the southeastern canadian cordillera some thermal and rheological considerations metamorphic core complexes in the southeastern canadian cordillera were formed during eocene crustal extension shortly within a few millions of years after late jurassic paleocene crustal shortening thermal rheological modeling constrained by geological and geochronological studies of the valhalla core complex and other core complexes in this region is used to investigate two major problems concerning the formation of these core complexes 1 the dynamic links between crustal shortening and extension and 2 the cooling history and unroofing rates during extension thermal rheological effects associated with crustal shortening are integrated through the history of crustal compression since crustal shortening in this region was a long and slow process and cannot be treated as an instantaneous event our results suggest that crustal shortening may have played an important role in eocene extension in the southeastern canadian cordillera by 1 producing a thickened and therefore unstable crust and 2 thermally weakening the lithosphere however heat generated by crustal shortening is not enough to account for the thermal state of the valhalla complex and additional heat sources at depth may be necessary we then investigate thermal evolution during extension in both a simple shear model and a progressive pure stretching model we show that the geotherm in an extensional region is time and space dependent and is affected by many variables including the preextensional thermal history and the mode of extension thus caution needs to be exercised when inferring unroofing rates from thermochronologic data the cooling history of the valhalla core complex may be explained by unroofing at rates of 1 2 mm yr

exhumation of high grade terranes a review high grade granulite facies terranes are brought to the surface by a combination of uplift and erosion exhumation the reported mechanisms and durations of exhumation are variable and depend pa

microstructure metamorphism thermochronology and p t t deformation history of the port aux basques gneisses south west newfoundland canada the port aux basques gneisses comprise three lithostratigraphic units separated by major fault zones the grand bay complex the port aux basques complex and the harbor le cou group a similar regionally developed polyphase history of penetrative deformation characterizes each of these units thickening during d1 produced rare recumbent folds f1 and an axial planar schistosity s1 overprinted by d2 recumbent folds f2 and transposed during development of a locally penetrative differentiated crenulation cleavage s2 in western sectors of the area d2 was associated with nw directed reverse shearing the ne trending structural grain reflects d3 transpression partitioned into dextral transcurrent movement along major shear zones and development of upright to steeply inclined periclinal folds f3 and a variably penetrative schistosity s3 amphibolite facies metamorphism increases in grade from west to east across the area microstructures including porphyroblast matrix foliation relations and internal textural unconformities in garnet indicate episodic porphyroblast nucleation and growth which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism garnet kyanite and staurolite porphyroblasts are wrapped by the s2 foliation but each may contain trails of inclusions that define s1 commonly these trails preserve early stages of s2 crenulation cleavage development progressive and sequential reaction out of kyanite staurolite and muscovite in favour of sillimanite garnet biotite and k feldspar and the development of an increasing volume of anatectic migmatite in south eastern sectors of the area record syn to late d2 peak metamorphic conditions microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in p t space for units of the port aux basques gneisses peak metamorphic conditions are estimated to have been 620 650 c at 8kbar in the west and 700 750 c at 8 kbar in the east titanite from an upper amphibolite facies calc silicate gneiss yields u pb ages of c 420 ma interpreted to date cooling shortly after the thermal peak in these gneisses variable d3 strain was associated with some recrystallization of hornblende and micas 40ar 39ar hornblende plateau isotope correlation ages range from c 419 to c 393 ma from east to west across the area and are interpreted to record cooling through c 500 c coeval with or soon after d3 deformation the range in ages may record the effects of heterogeneous d3 deformation and differential uplift from south east to north west associated with displacement on major shear zones 40ar 39ar muscovite plateau ages cluster at c 390 ma and date cooling through c 375 c during regional exhumation cooling rates are moderate to fast and may indicate a component of tectonic exhumation the port aux basques gneisses are a product of silurian collisional tectonics the higher grade of metamorphism in comparison with adjacent areas of the canadian appalachians is interpreted to reflect greater thickening due to juxtaposition of the st lawrence promontory laurentian margin with the cabot promontory avalonian margin during closure of the iapetus ocean

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

radiocarbon constraints on readvances of the british irish ice sheet in the northern irish sea basin during the last deglaciation moraines deposited by the dundalk bay ice lobe record two readvances of the irish ice sheet into the northern irish sea basin during the last deglaciation these readvances overrode and incorporated fossiliferous marine muds from the floor of dundalk bay ams 14c dates from monospecific microfaunas obtained from these muds indicate that the earlier clogher head readvance occurred sometime between 15 0 and 14 2 14c ka bp thus identifying a previously unrecognized ice margin fluctuation in the irish sea basin that is correlative with a readvance in northwest ireland the younger readvance occurred after 14 2 14c ka bp and is equivalent to the killard point readvance identified elsewhere in the irish sea basin these readvances occurred during the oldest dryas cold interval and bracket heinrich event 1 raised marine muds that were deposited between ice readvances require that a substantial ice sheet remained on ireland throughout much of the last deglaciation with attendant isostatic depression of at least 110 m

rapid rise of sea level 19 000 years ago and its global implications evidence from the irish sea basin supports the existence of an abrupt rise in sea level meltwater pulse at 19 000 years before the present b p climate records indicate a large reduction in the strength of north atlantic deep water formation and attendant cooling of the north atlantic at this time indicating a source of the meltwater pulse from one or more northern hemisphere ice sheets warming of the tropical atlantic and pacific oceans and the southern hemisphere also began at 19 000 years b p these responses identify mechanisms responsible for the propagation of deglacial climate signals to the southern hemisphere and tropics while maintaining a cold climate in the northern hemisphere

pressure temperature time paths from two dimensional thermal models prograde retrograde and inverted metamorphism two dimensional thermal models of time transitive crustal thickening and subsequent unroofing in large scale overthrust terrains generate pressure temperature time ptt paths that generally resemble those produced by earlier one dimensional instantaneous models but that differ in detail these differences in ptt path morphology are most pronounced proximal to major fault zones where postthrusting geotherms are characterized by large temperature inversions in one dimensional models but generally lack inversions in two dimensional models tests using the two dimensional fault model developed here indicate that 1 burial rate proportional to dp dt not thrust fault geometry dip angle controls the topology of synthrusting pt paths but plays only a minor role in determining the maximum temperature that rocks attain during the later unroofing stage in their thermal histories 2 normal ranges of thrusting and erosion rates and fault parameters result in pt paths with the usual sense clockwise on conventional pt diagrams counterclockwise in our diagrams 3 the amount of heating and duration of heating following the end of thrusting are a function of the rate of unroofing dp dt during this period 4 fast unroofing rates lead to the attainment of lower maximum temperatures after greater amounts of unroofing 5 the initial thermal state of the lithosphere prior to thrusting has a profound effect on pt path morphologies and on the peak metamorphic conditions attained by samples 6 for excess heat distributed across the entire lithosphere e g due to increased background thermal gradients a plot of peak temperatures experienced by metamorphic rocks versus structural depth tmax plot closely represents the initial geotherm 7 excess heat in the crust only e g increased radioactive heating in a layer yields a different result with the tmax plot corresponding to the initial geotherm only near the top of the hanging wall tmax plots for both 6 and 7 show no temperature inversion which exceeds the nominal uncertainties 50 k for geothermometric data 8 shear heating can lead to significant temperature inversions at the fault zone if the frictional coefficient is 0 6 or greater and 9 simultaneous thrusting and erosion produce pt loops significantly narrower than those resulting from sequential thrusting and erosion suggesting that any formulation which fails to account for some degree of simultaneity between thrusting and erosion represents a far endmember model forward models like those presented here provide important guidelines for understanding the sensitivity of metamorphic ptt paths to various thermal mechanical and geometric factors related to tectonism but they are generally inappropriate for reconstructing metamorphic thermal histories from actual petrologic and geochronologic data analytical inversion techniques that use a postthrusting thermal regime consistent with two dimensional forward models and that integrate values of dp dt dt dt and radioactive heating rates extracted from suites of metamorphic rocks provide the best hope for furthering our understanding of the thermal evolution of metamorphic terrains

cenozoic thermal evolution and tectonics of the coast mountains of british columbia 1 fission track dating apparent uplift rates and patterns of uplift the high relief of the coast mountains of british columbia has been produced in part by rapid late neogene uplift the relationship of this uplift to the presently diverse and complex plate tectonic regime off the west coast of british columbia however is not clear cut to study this problem the areal patterns rates and total amounts of cenozoic uplift from 50 to 55 n have been determined by using fission track dating of zircon and apatite separated from rocks collected at high and low altitudes along several traverses through the mountains accepting closure temperatures of 105 c 10 c and 175 c 20 c for apatite and zircon respectively and a likely closure temperature difference between zircon and apatite of 70 15 c paleogeothermal gradients have been measured at kemano c km 35 ma ago and near ocean falls king island c km 20 ma ago subcrustal heat flow is suggested to have significantly increased in the late neogene near king island uplift rates in the axial part of the northern 52 55 n coast mountains were 0 1 0 2 km ma from 25 to 15 ma ago this uplift was temporally and tectonically related to part of the subsidence in the adjacent queen charlotte basin uplift rates increased somewhat in the late miocene to about 0 4 km ma probably due to passage of the anahim hot spot beneath the area late miocene recent erosion in the north 52 55 n has reduced summit altitudes and been more extensive than farther south despite this relicts of miocene river valleys and topography are still locally preserved partly beneath 7 10 ma basalts the area of the present southern 50 to 52 n coast mountains was near sea level and experienced very low uplift rates 0 5 km ma of the southern coast mountains the profile of this uplift was broadly plateaulike and this uplift led to the inversion of the miocene coastal topography middle cenozoic uplift from 52 to 55 n was probably the result of denudation and diminishing uplift after the terminal paleocene eocene orogenic episode which grossly thickened the crust this middle cenozoic erosion resulted in the restoration of a normal crustal thickness late neogene uplift in the coast mountains was the result of thermal expansion in the mantle caused by a mantle hot spot near 53 n and changes in the geometry of the subducted slab farther south

metamorphic p t paths from pelitic schists and greenstones from the south west tauern window eastern alps petrological data from intercalated pelitic schists and greenstones are used to construct a pressure temperature path followed by the upper schieferhulle ush series during progressive metamorphism and uplift in the south west tauern window italy pseudomorphs of al epidote fe epidote albite oligoclase chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite albite chlorite subfacies of the blueschist facies at p 7 8 kbar geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475 25 c 5 6 kbar calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of 7 5 kbar garnet zoning patterns are complex and reversals in zoning can be correlated between samples thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity fluid inclusion data from coexisting immiscible h2o co2 nacl fluids constrain the uplift path to have passed through temperatures of 380 30 c at 1 3 0 2 kbar there is no evidence for metamorphism of ush at pressures greater than 7 5 kbar in this area of the tauern window this is in contrast to pressures of 10 kbar recorded in the lower schieferhulle only 2 3 km across strike a history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the p t data obtained from these adjacent tectonic units

radiocarbon constraints on the history of the western irish ice sheet prior to the last glacial maximum accelerator mass spectrometer c 14 dating of in situ monospecific marine microfaunas and reworked shells records high relative sea level on the western coast of ireland between ca 40 and 19 call calibrated ka requiring substantial isostatic loading by the irish ice sheet iis during that interval glacial and marine deposits also record a rapid fluctuation of the us margin onto the continental shelf ca 28 cal ka evidence that the coast was not subsequently overridden by the iis indicates that at least for this sector of the ice sheet the ice margin during the last glacial maximum lgm was less extensive than the 28 cal ka event our data from western ireland support c 14 and cl 36 chronologies from the irish sea basin in identifying an us that was as extensive and thick as the lgm iis for much of the 20 k y interval prior to the lgm

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

the geounri shear zone in the paleozoic taebaeksan basin of korea tectonic implications abstract the songrim and daebo orogenies represent two major phanerozoic tectonic events that are well preserved across much of the present day korean peninsula the songrim orogeny corresponds to the late permian triassic collision of the north and south china cratons whereas the jurassic daebo tectonic event represents a thin skinned contractional deformation in a continental arc setting it is well established that the songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the korean peninsula e g the pyeongnam basin and the imjingang belt with only a minor impact on the geology of southern korea e g the okcheon and taebaeksan basins it was the daebo tectonic event however which generated most of the deformational structures observed within the okcheon and taebaeksan basins the deokpori thrust in the taebaeksan basin is a significant fault structure that formed during the daebo tectonic event no other regional structures related to the songrim orogeny have been found in the taebaeksan basin in the vicinity of the deokpori thrust we have identified in this study a previously undocumented reverse slip shear zone which is named the geounri shear zone microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at 400 c in contrast with the nearby deokpori thrust which formed in a brittle deformation regime a geochronological analysis of muscovite isolated from phyllonite samples of the geounri shear zone yielded two 40 ar 39 ar age spectra with a combined weighted mean age of 209 5 2 ma thermal modeling of the ar data combined with structural interpretations suggests that this muscovite 40 ar 39 ar age represents a robust lower age limit for the timing of the geounri shearing event therefore linking formation of the shear zone with deformational events caused by the songrim orogeny these results imply that structural features caused by songrim deformation were developed at higher temperatures than structures caused by daebo deformation as found within the taebaeksan basin

the tectonothermal evolution of the kirwanveggen h u sverdrupfjella areas dronning maud land antarctica abstract three episodes of deformation spanning a period of 900 ma are recognised the first deformation at 1000 ma involved progressive yet distinct stages within a protracted composite event involving recumbent folding low angle thrust faulting and locally highly oblique transpressive strike slip shearing these structures suggest tectonic transport from the south and southeast during d 1 d 2 at 500 ma is variable in the different areas and involved thrust faults and folding with locally developed axial planar foliations the orientations of the fault planes and axial planar structures suggest transport from the west and northwest in western sverdrupfjella and from the southeast in kirwanveggen d 3 involved normal faulting and jointing adjacent and parallel to the jutulstraumen glacier in the west the joints affect the alkaline complexes some of which are jurassic in age and consequently d 3 is related to the breakup of gondwana four phases of metamorphism related to the deformation are recognised the dominant mineral assemblages m 1 3 are typical of medium to high grade metamorphism discordant mafic intrusions provide evidence of a long history of metamorphism mafic nodules from the eastern part of the area contain high pressure assemblages representing m 1 assemblages and textures indicate that m 2 was related to isothermal decompression related to the uplift caused by thrust faulting during d 1 the period between m 2 and m 3 appears to have involved annealing at mid crustal levels m 3 mineral development is dominated by biotite which is oriented axial planar to d 2 folds m 4 assemblages are typically low grade and involve hydrothermal alteration resulting in chloritisation and sausseritisation related to d 3

p t paths derived from garnet growth zoning in an extensional setting an example from the tormes gneiss dome iberian massif spain the tormes gneiss dome nw iberian massif variscan belt of spain comprises a metamorphic core complex lower unit bounded by a major extensional detachment despite metamorphic temperatures in the upper amphibolite facies 700 740 c metapelites from the highest levels of the lower unit contain garnet with preserved growth zoning these rocks were used for reconstruction of quantitative p t paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning the results are consistent with a two stage tectonothermal evolution under high grade conditions 1 an early compressional phase of deformation that led to upper amphibolite facies barrovian type metamorphism and to p increase and t rise to approximately 9 kbar and 700 725 c 2 a subsequent major extensional phase of deformation that led to quasi isothermal decompression from 8 9 to 3 kbar at t conditions between 700 and 740 c several lines of structural textural and petrological evidence suggest that up to 15 20 km of overburden was removed from the lower unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions a final stage of quasi isobaric cooling to greenschist facies conditions is locally recorded in late low grade detachments

applications of diffusion data to high temperature earth systems diffusion in general is such a widespread phenomenon in nature that a basic knowledge can serve very effectively to address well known applications not only to thermochronology and petrology but also to environmental geology e g paleoclimate sedimentology and applied geosciences e g assessing contamination of gas stations indeed the list of potential applications of diffusion phenomena is so long that an exhaustive treatment would quickly exceed the scope of this review the reader is therefore referred to reviews covering various aspects of diffusion in the earth sciences including diffusion in melts watson 1994 chakraborty 1995 diffusion processes during diagenesis berner 1980 krom and berner 1980 boudreau 2000 and diffusion phenomena in general e g lasaga 1998 ganguly 2002 watson and baxter 2007 zhang 2008 all of these treatments are based on the same general concepts of diffusion which do not need restating here in great detail applications of diffusion profiles in igneous systems have been thoroughly reviewed a recent reviews in mineralogy and geochemistry volume rimg 69 putirka and tepley 2008 and there is also an excellent volume on low temperature thermochronology rimg 58 reiners and ehlers 2005 describing the impact and application of diffusion theory for age and thermal history determination with these precedents in mind we focus the present review on the application of diffusion in high temperature regimes i e systems at t 500 c both igneous and metamorphic systems are addressed but because of the recent precedents noted above we emphasize recent developments and challenges in metamorphic studies in a broad sense diffusion describes the statistical non directional movement of molecules or heat through a medium of interest as independent theories of diffusion were developed in the early 19th century and compared with the principles of statistical variance it was

thermobarometric profile through the caledonian nappe stack of western ofoten north norway p t estimates from pelite thermobarometry along a transect through the caledonian nappe stack in ofoten north norway 68 30 n plot as a trend in p t space ranging from 443 c 6 9 kb to 688 c 10 6 kb the p t range for each nappe overlaps the others except for those from the lowest unit narvik group which has significantly higher values within the p t ranges for each nappe the position on the trend of a given sample is independent of its tectonic position results support our earlier conclusion based on structural and petrographic grounds that the regional metamorphic peak was reached during or after stacking of the nappes in the siluro devonian scandian orogeny

ice sheet variability around the north atlantic ocean during the last deglaciation millennial scale variability in the flux of ice rafted detritus to north atlantic sediments during the last glacial period has been interpreted to reflect a climate forced increase in the discharge of icebergs from ice sheet margins surrounding the northern north atlantic ocean 1 but the relationship between ice sheet variability and climate change is not clear as both the sources of ice rafted detritus and the ice marginal processes are varied and complex 2 4 terrestrial records are helpful in unravelling this complexity because they can demonstrate the scale of ice sheet oscillations and whether the ice sheet or sector was advancing or retreating with respect to climate change here we constrain the age and anatomy of a prominent readvance of the british ice sheet in the northern irish sea region at similar to 14 14 c kyr bp similar to 16 4 calendar kyr bp the analysis indicates that the british ice sheet participated in an iceberg discharge episode known as heinrich event 1 comparison with other terrestrial and marine ice sheet records suggests that the dynamic collapse of the laurentide ice sheet beginning at 14 6 15 0 c 14 kyr bp1 4 similar to 17 2 17 6 calendar kyr bp 5 initiated varied responses from other ice sheet margins around the northern north atlantic region these observations support the argument that the release of icebergs and meltwater during heinrich event 1 disrupted the north atlantic thermohaline circulation 6 8 leading to a delay or reversal of deglaciation of the northern hemisphere and at least as far south as 40 degrees s for two to three thousand years 5 9 10 suggesting a climate forcing and response similar to that of the ensuing younger dryas cold snap 11 12

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

post acadian sediment recycling in the devonian old red sandstone of southern ireland abstract the upper devonian munster basin of southern ireland has traditionally been viewed as a post orogenic molasse deposit that was sourced from the caledonides of central ireland and subsequently deformed by the end carboniferous variscan orogenic event the basin fill is composed of super mature quartz arenite sandstone that clearly represents a second cycle of deposition the source of this detritus is now recognized as lower devonian dingle basin red bed sequences to the north this genetic link is based on the degree of similarity in the detrital mica chemistry in both of these units micas plot in identical fields and define the same trends in addition the two sequences show increased textural and chemical maturity up sequence and define indistinguishable 40 ar 39 ar age ranges for the detrital mica grains partial resetting of the ar ages can be attributed to elevated heat flow in the region caused by munster basin extension and subsequent variscan deformation the combined evidence from southwest ireland therefore points to a caledonian or possibly taconian primary source area that initially shed detritus into the lower devonian dingle basin which was subsequently recycled into the upper devonian munster basin following mid devonian acadian basin inversion

the neoproterozoic brasiliano orogeny in northeast brazil 40ar39ar and petrostructural data from ceara abstract the borborema province in ne brazil comprises paleoproterozoic basement and its proterozoic cover that experienced a complex tectonometamorphic history during the brasiliano pan african orogeny in ceara four major domains separated by continental scale steeply dipping dextral shear zones have been identified 40 ar 39 ar step heating of mineral separates and laser probe fusion of single grains have been used in conjunction with petro structural observations to delineate the cooling tectonic history of these domains to the north of the senador pompeu shear zone spsz amphibole and biotite pairs from low pressure granulite belts exposed along the granja shear zone and spsz record plateau ages in the range 575 560 ma corresponding to cooling rates close to 20 c m y 40 ar 39 ar laser probe muscovite dates from the chaval augen gneiss point to a similar age for shearing responsible for the exhumation and fast cooling of the granulite belts a muscovite biotite pair from the central ceara high pressure nappes lying between the shear zones documents a lower cooling rate in the interval 534 525 ma related to the decompression and uplift of previously deelly buried rocks to the south of the spsz only moderate crustal thickening occurred within the various schist belts a distinct cooling pattern is indicated for these belts and the western patos shear zone attesting to a different crustal development 40 ar 39 ar dates of amphibole phlogopite muscovite and biotite range from 550 to 522 ma recording a postmetamorphic cooling rate of 6 9 c m y slower cooling rates of 3 4 c m y between amphibole and biotite closure ages in the range 540 500 ma are reported for the eastern part of the palos shear zone system the diachronous cooling recorded along this shear zone is interpreted to reflect the variations in thickness of the high temperature mylonitic belts earlier more rapid cooling occurred where the belts are narrow in the west late slower cooling occurred in wider zones to the east these thermochronological data reveal an age and cooling rate distribution at the scale of the borborema province that provides new constraints on the development of the brasiliano orogeny between 580 and 500 ma

dome formation and extension in the tethyan himalaya leo pargil northwest india metamorphic dome complexes occur within the internal structures of the northern himalaya and southern tibet their origin deformation and fault displacement patterns are poorly constrained we report new fi eld mapping structural data and cooling ages from the western fl ank of the leo pargil dome in the northwestern himalaya in an attempt to characterize its post middle miocene structural development the western fl ank of the dome is characterized by shallow west dipping pervasive foliation and wnw ese mineral lineation shear sense indicators demonstrate that it is affected by east west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting sustained top to northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation garnet and kyanite indicate that the leo pargil dome was exhumed from the mid crust 40 ar 39 ar mica and apatite fi ssion track aft ages constrain cooling and exhumation pathways from 350 to 60 c and suggest that the dome cooled in three stages since the middle miocene 40 ar 39 ar white mica ages of 16 14 ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation aft ages between 10 and 8 ma suggest that ductile fault displacement had ceased by then and aft track length data from high elevation samples indicate that the rate of cooling had decreased signifi cantly we interpret this to indicate decreased fault displacement along the leo pargil shear zone and possibly a transition to the kaurik chango normal fault system between 10 and 6 ma aft ages from lower elevations indicate accelerated cooling since the pliocene that cannot be related to pure fault displacement and therefore may refl ect more pronounced regionally distributed and erosion driven exhumation

the 40ar 39ar ages from the west sudetes ne bohemian massif constraints on the variscan polyphase tectonothermal development

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

a revised model for the last deglaciation of eastern scotland accelerator mass spectrometry 14 c ages on monospecific marine microfauna from raised mud record initial deglaciation of the eastern coast of scotland before 21 0 cal ka bp two subsequent ice margin readvances occurred prior to the loch lomond advance and are identified from ice contact deposits overlying marine mud the lunan bay readvance dates to 20 2 cal ka bp and possibly 18 2 cal ka bp the younger perth readvance occurred between c 17 5 cal ka bp and 14 5 10 be ka within dating uncertainties these readvances are similar in age to ice margin fluctuations documented from the irish sea basin and northwestern ireland and record three near synchronous fluctuations of much of the british irish ice sheet margin during the last deglaciation suggesting a common response to regional climate forcing

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

40ar 39ar thermochronological constraints on the retrogression and exhumation of ultra high pressure uhp metamorphic rocks from xitieshan terrane north qaidam china abstract paleozoic ultra high pressure uhp eclogite bearing mafic boudins are exposed in gneiss in the xitieshan terrane north qaidam petrological and thermobarometric studies show that garnet amphibolite and amphibolite rocks record upper amphibolite facies and lower amphibolite facies retrograde metamorphic conditions of 8 1 9 1 kbar 630 670 c and 5 0 7 2 kbar 530 630 c respectively to better constrain the timing of retrogression and exhumation 40ar 39ar stepwise heating analyses have been performed on minerals that were extracted from these uhp mafic rocks and their country rocks amphibole yields disturbed release spectra marked by anomalously old apparent ages in low temperature steps suggesting the presence of extraneous 40ar isochron ages of 425 ma and 419 ma are obtained for amphibole from the garnet amphibolite and amphibolite respectively which are interpreted as the best estimate for the age of amphibolite facies retrogression during exhumation muscovite from the gneiss yields consistent plateau and isochron ages of 409 ma k feldspar from the same gneiss yields rising staircase shaped age spectra which are characterized by younger apparent ages in the first steps 263 281 ma and higher plateau ages in the last segment 294 321 ma indicating partial diffusive loss of argon from the k feldspar samples biotite from schist yields flat age spectra with a plateau age of 397 ma that is interpreted as a cooling age our new 40ar 39ar data together with published la icpms ages of zircon from a coesite bearing amphibolite indicate that the xitieshan uhp metamorphic rocks may have experienced two stages of exhumation during the first stage from 95 31 km depth the uhp metamorphic rocks were exhumed with a relatively rapid velocity of 4 0 km myr resulting from buoyancy forces subsequently the rate of exhumation decreased gradually from 1 1 to 0 1 km myr between 31 7 km depth this later gentle exhumation was probably governed by extension and or erosion

relative sea level observations in western scotland since the last glacial maximum for testing models of glacial isostatic land movements and ice sheet reconstructions observations of relative sea level change and local deglaciation in western scotland provide critical constraints for modelling glacio isostatic rebound in northern britain over the last 18 000 years the longest records come from skye arisaig and knapdale with a shorter holocene record from kintail biostratigraphic diatom pollen dinoflagellate foraminifera and thecamoebian lithological and radiocarbon analyses provide age and elevation parameters for each sea level index point all four sites reveal relative sea level change that is highly non monotonic in time as the local vertical component of glacio isostatic rebound and eustasy or global meltwater influx dominate at different periods copyright 2006 john wiley sons ltd

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

isotopic constraints on the cooling history of the nanga parbat haramosh massif and kohistan arc western himalaya nineteen 40ar 39ar and 22 rb sr mineral ages are presented for metamorphic rocks and granites collected from the northern part of the nanga parbat haramosh massif nphm and adjacent parts of the kohistan arc western himalaya the majority of these ages are interpreted as exhumation related cooling ages and they therefore place important constraints on the exhumation history of the region the 40ar 39ar and rb sr mica cooling ages for areas of the nphm to the north of the indus river lie in the range 2 8 ma and are similar to previously published ages from the indus gorge itself cooling histories for these areas show little systematic variation from east to west suggesting that the basement may have been exhumed as a relatively rigid block after the main stages of folding mica ages increase dramatically from the nphm westward into the kohistan arc where ages lie in the range 13 30 ma the contrast in both cooling ages and the age differential between mineral pairs between the two terranes indicates rapid cooling of the nphm in the last 10 m y compared with the kohistan arc the relationship between rates of cooling and exhumation is poorly constrained in regions of rapid uplift where the characteristics of the evolving geotherm are not well understood however the results are consistent with an anomalously rapid exhumation history of the nphm during the last 10 m y compared with the adjacent kohistan arc the interpretation of some of the ages documented in this study is problematical two biotite samples from contrasting lithologies within the nphm have yielded higher 87sr 86sr ratios than the corresponding whole rocks but the cause of this isotopic disequilibrium remains unclear other biotite samples from the region have consistently yielded younger rb sr ages than 40ar 39ar ages which may be a reflection of either preferential leaching of 87sr during fluid infiltration on a regional scale or the presence of excess argon alternatively the relative ages indicate slightly lower closure temperature for strontium diffusion in biotite than for argon diffusion in biotite where samples have undergone rapid cooling

implications for organic maturation studies of evidence for a geologically rapid increase and stabilization of vitrinite reflectance at peak temperature cerro prieto geothermal system mexico discussion a short term rapid heating and cooling of the rock in well m 94 below 1300 m was caused by a pulse of hot water passing through the edge of the cerro prieto mexico geothermal system below 1300 m the peak paleotemperatures were about 225 250 degrees c but equilibrium well log temperatures indicate a decrease to 150 210 degrees c at present this hot water pulse sharply increased vitrinite reflectance to levels comparable to those measured in the central part of the system even though studies of apatite fission track annealing indicate that the duration of heating was only 10 0 10 1 yr in m 94 in contrast to 10 3 10 4 yr in the central part of the system these data indicate that sedimentary organic matter chemically reacts quickly to temperature increases of about 125 degrees c above ambient even when the higher temperature existed for only 10 0 10 1 yr the quick change of the vitrinite reflectance geothermometer indicates that thermal maturation reactions can stabilize after a geologically short period of heating to a level consistent with peak temperature under moderate to high temperature diagenesis in open fluid rich geothermal systems cerro prieto is one of the most intensively studied and well known geothermal systems in the world thus data from cerro prieto are a benchmark to compare with the predictions of published thermal maturation models such as those formulated by j karweil n v lopatin and a k burnham and j j sweeney these thermal maturation models inaccurately predict duration of heating at cerro prieto the kinetic equations used in these models explicitly allow thermal maturation to continue indefinitely at peak temperature which does not seem to be the case at cerro prieto

glaciation and sea level change for ireland and the irish sea since late devensian midlandian time the sea level change around the coast of ireland and the irish sea for the past 20 000 years is primarily the combined result of the glacio isostatic adjustment of the crust to the removal of ice over the british isles and the total eustatic change from the global ice sheets however the isostatic effects due to the removal of ice from northern europe and north america and the addition of meltwater into the oceans also make a significant contribution predictions of sea level change based on glacio hydro isostatic models are compared with observations to constrain the ice volume over ireland at the time of the last glacial maximum and the maximum ice height appears to have been of the order of 600 m the models predict well the spatial variability in sea level change observed across the region for holocene and lateglacial time with levels above present being predicted only for northeastern ireland and north of about morecambe bay the models do not support suggestions that lateglacial levels along the east coast of ireland or the coast of wales were 50 150 m above their present levels consistent models that would produce such large lateglacial highstands are incompatible with all other sea level and glacial evidence for the british isles palaeobathymetry and palaeoshoreline reconstructions for the irish sea indicates that a tenuous landbridge between britain and ireland developed only across the celtic sea between about 18000 and 14000 years bp

the paleozoic metamorphic history of the central orogenic belt of china from 40ar 39ar geochronology of eclogite garnet fluid inclusions abstract the pressure temperature time evolution of the uhp eclogites of dabie sulu in the eastern sector of the central orogenic belt of china shows a complex pattern of predominantly triassic and to a lesser extent early paleozoic ages 40 ar 39 ar stepwise crushing analysis of eclogitic garnets yielded consistently paleozoic ages because our initial results from the bixiling eclogite were of limited areal extent their significance required further testing data for garnet bearing gneiss and eclogites from new locations in northern and southern dabieshan shuanghe ganghe and hongmiao confirm the paleozoic garnet results obtained from bixiling eclogite we also tested the technique on a garnet sample of known paleozoic age from altyn tagh in the western central orogenic belt these new results indicate that light colored garnets from dabieshan and altyn tagh yield concordant 40 ar 39 ar ages ranging from 445 465 ma by in vacuo crushing and confirm that a paleozoic u hp metamorphic event took place in the central orogenic belt from altyn tagh in western china to dabieshan in eastern china in contrast dark colored garnets from mafic metavolcanics do not yield 40 ar 39 ar plateaux or isochrons ages

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

constraints on unroofing rates in the high himalaya eastern nepal thermobarometric data for samples across the main central thrust zone in eastern nepal show an inversion in temperature but not in pressure these data have been interpreted to represent a portion of the paleogeotherm at the time of main central thrust deformation a 40ar 39ar age on hornblende closure temperature tc 500 50 c constrains the timing of this deformation to be 21 0 2 ma the 40ar 39ar ages of other minerals muscovite tc 350 c age t 12 0 0 2 ma k feldspar tc 220 c t 8 0 0 2 ma from the same location further constrain the cooling history of this region together the geochronologic and thermobarometric data yield an average unroofing rate of 1 2 0 6 mm yr for the high himalaya of eastern nepal simple thermal models show that these geochronologic and thermobarometric data are consistent with a wide range of different initial geotherms applied boundary conditions and magnitude of radiogenic heat production the variation through time of the unroofing rates can only be poorly constrained however the unroofing histories were found to be largely insensitive to the details of the assumed initial geotherm fairly sensitive to the magnitude of radiogenic heat production and extremely sensitive to the nature of the boundary conditions applied below the fault zone this study underscores the difficulty in constraining uplift histories on the basis of cooling rates even when thermobarometric data are available to supplement geochronologic constraints on the cooling history of the region

late pleistocene tidewater glaciers and glaciomarine sequences from north county mayo republic of ireland late pleistocene deposits in north county mayo were deposited in three main glacigenic environments 1 drumlins and basal tills were formed when ice moved from the irish lowlands and local mountain catchments into donegal bay 2 gilbert type deltas accumulated up to 80m i o d on the lowlands and subaqueous moraines formed across minor valleys when marine based ice grounded inland 3 a thick drape of fossiliferous glaciomarine mud along the coastal fringe was deposited from meltwater plumes and by ice rafting immediately outside of these ice limits the muds contain an elphidium clavatum dominated low diversity microfauna which is characteristic of cold water conditions adjacent to glacier termini valves of macoma calcarea from the mud have been 14c dated at 16940 120 and 17300 100 bp the high level delta complex was deposited from tidewater glaciers in a peripheral depression adjacent to the drumlin ice limits of north county mayo although the field evidence cannot be used to determine former sea level history with any accuracy it poses general problems for sea level history and isostatic effects of the last major ice sheet in the west of ireland raised glaciomarine sequences commonly occur in close association with drumlin ice limits elsewhere in ireland and represent marine transgressions prior to glacial unloading it is suggested that the magnitudes and patterns of crustal depression are greater and geometrically more complex at the margins of ice sheets in ireland than hitherto realised

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

cenozoic thermal evolution of the central sierra nevada california from uth he thermochronometry apatite u th he cooling ages are reported for igneous apatite samples from the central sierra nevada and compared to published apatite fission track ages and track length data from the same mineral separates helium ages are youngest at low elevations and increase systematically toward higher elevations ranging from 43 to 84 ma at yosemite valley 32 to 74 ma at kings river canyon and 23 to 75 ma at mt whitney helium ages from high elevation samples are generally concordant with corresponding fission track ages while lower elevation helium ages are substantially younger cooling histories inferred from present laboratory derived fission track annealing and helium diffusion models do not match well suggesting that either helium diffusion rates or fission track annealing rates are miscalibrated at temperatures below about 60 c for geologic exposure periods unlike the fission track results the helium data do not indicate a very low geothermal gradient in the sierra nevada during early to middle tertiary time

tectonic evolution of proterozoic rocks in the cimarron mountains northern new mexico usa abstract portions of three proterozoic tectonostratigraphic sequences are exposed in the cimarron mountains of new mexico the cimarron river tectonic unit has affinities to a convergent margin plutonic volcanic complex igneous hornblende from a quartz diorite stock records an emplacement pressure of 2 2 6 kbar rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4 5 kbar and 330 50 c the tolby meadow tectonic unit consists of quartzite and schist mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 20 c a low angle ductile shear zone separates this succession from gneisses of the structurally underlying eagle nest tectonic unit gneissic granite yields hornblende pressures of 6 8 kbar pelitic gneiss records regional metamorphic conditions of 6 7 kbar and 705 15 c overprinted by retrogression at 4 kbar and 530 10 c comparison of metamorphic and retrograde conditions indicates a p t path dominated by decompression and cooling the low angle ductile shear zone represents an extensional structure which was active during metamorphism this extension juxtaposed the tolby meadow and eagle nest units at 4 kbar and 520 c both units were later overprinted by folding and low grade metamorphism and then were emplaced against the cimarron river tectonic unit by right slip movement along the steeply dipping fowler pass shear zone an argon isotope correlation age obtained from igneous hornblende dates plutonism in the cimarron river unit at 1678 ma muscovite associated with the greenschist facies metamorphic overprint yields a 40 ar 39 ar plateau age of 1350 ma by contrast rocks within the tolby meadow and eagle nest units yield significantly younger argon cooling ages hornblende isotope correlation ages of 1394 1398 ma are interpreted to date cooling during middle proterozoic extension muscovite plateau ages of 1267 1257 ma appear to date cooling from the low grade metamorphic overprint the latest ductile movement along the fowler pass shear zone post dated these cooling ages argon released from muscovites of the eagle nest tolby meadow composite unit at low experimental temperatures yields apparent ages of c 1100 ma similar ages are not obtained north east of the fowler pass shear zone suggesting movement more recently than 1100 ma

evidence for heinrich event 1 in the british isles in the north irish sea basin isb sedimentary successions constrained by ams 14c dates obtained from marine microfaunas record three major palaeoenvironmental shifts during the last deglacial cycle i marine muds cooley point interstadial dated to between 16 7 and 14 7 14c kyr bp record a major deglaciation of the isb following the late glacial maximum lgm ii terminal outwash and ice contact landforms killard point stadial were deposited during an extensive ice readvance which occurred after 14 7 14c kyr bp and reached a maximum extent at ca 14 14c kyr bp at this time the lowlands surrounding the north isb were drumlinised coeval flowlines reconstructed from these bedforms end at prominent moraines killard point bride st bees and indicate contemporaneity of drumlinisation from separate ice dispersal centres substrate erosion by fast ice flow and subglacial sediment transfer to ice sheet margins in north central ireland bed reorganisation associated with this fast ice flow phase involved overprinting and drumlinisation of earlier transverse ridges rogen type moraines by headward erosion along ice streams that exited through tidewater ice margins this is the first direct terrestrial evidence that the british ice sheet bis participated in heinrich event 1 h1 iii regional mud drapes directly overlying drumlins record high relative sea level rsl with stagnation zone retreat after 13 7 14c kyr bp rough island interstadial elsewhere in lowland areas of northern britain ice marginal sediments and morainic belts record millennial scale oscillations of the bis which post date the lgm advance on to the continental shelf and pre date the loch lomond stadial younger dryas advance in the highlands of western scotland ca 11 10 14c kyr bp in western northwestern and northern ireland killard point stadial h1 ice limits are reconstructed from ice flow lines that are coeval with those in the north isb and end at prominent moraines on the scottish continental shelf possible h1 age ice limits are reconstructed from dated marine muds and associated ice marginal moraines it is argued that the last major offshore ice expansion from the scottish mountains post dated ca 15 14c kyr bp and is therefore part of the h1 event in eastern england the stratigraphic significance of the dimlington silts is re evaluated because evidence shows that there was only one major ice oscillation post dating ca 18 14c kyr bp in these lowlands in a wider context the sequence of deglacial events in the isb widespread deglaciation of southern part of the bis major readvance during h1 ice sheet collapse is similar to records of ice sheet variability from the southern margins of the laurentide ice sheet lis well dated ice marginal records however show that during the killard point readvance the bis was at its maximum position when retreat of the lis was well underway this phasing relationship supports the idea that the bis readvance was a response to north atlantic cooling induced by collapse of the lis 1998 john wiley sons ltd

thermobarometric constraints on the thermal history of the main central thrust zone and tibetan slab eastern nepal himalaya the main central thrust mct south of mt everest in eastern nepal is a 3 to 5km thick shear zone separating chlorite bearing schist in the lower plate from sillimanite bearing migmatitic gneiss in the overlying tibetan slab the metamorphic grade increases through the mct zone toward structurally higher levels previous workers have suggested that either post or synmetamorphic thrust movement has caused this inversion of metamorphic isograds in an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault four well calibrated thermobarometers were applied to pelitic samples collected along two cross strike transects through the mct zone and tibetan slab results show an increase in apparent temperature up section in the mct zone from 778 k to 990 k and a decrease in temperature to 850 k in the lower tibetan slab which is consistent with synmetamorphic thrust movement a trend in calculated pressures across this section is less well defined but on average decreases up section with a gradient of 28mpa km resembling a lithostatic gradient pressure temperature paths for zoned garnets from samples within the mct zone modelled using the gibbs method show a significant decrease in temperature and a slight decrease in pressure from core to rim which might be expected for upper plate rocks during synmetamorphic thrust movement samples from the uppermost tibetan slab yield higher temperatures and pressures than those from the lower tibetan slab which may be evidence for later resetting of thermobarometers by intrusion of the large amounts of leucogranite at that structural level

widespread inclination shallowing in permian and triassic paleomagnetic data from laurentia support from new paleomagnetic data from middle permian shallow intrusions in southern illinois usa and virtual geomagnetic pole distributions abstract recent paleomagnetic work has highlighted a common and shallow inclination bias in continental redbeds the permian and triassic paleomagnetic records from laurentia are almost entirely derived from such sedimentary rocks so a pervasive inclination error will expectedly bias the apparent polar wander path of laurentia in a significant way the long standing discrepancy between the apparent polar wander paths of laurentia and gondwana in permian and triassic time may be a consequence of such a widespread data pathology here we present new middle permian paleomagnetic data from igneous rocks and a contact metamorphosed limestone from cratonic laurentia the exclusively reversed middle permian magnetization is hosted by low ti titanomagnetite and pyrrhotite and yields a paleomagnetic pole at 56 3 s 302 9 e a95 3 8 n 6 this pole which is unaffected by inclination shallowing suggests that a shallow inclination bias may indeed be present in the laurentian records to further consider this hypothesis we conduct a virtual geomagnetic pole distribution analysis comparing theoretical expectations of a statistical field model tk03 gad against published data sets this exercise provides independent evidence that the laurentian paleomagnetic data is widely biased likely because of sedimentary inclination shallowing we estimate the magnitude of this error from our model results and present and discuss several alternative corrections

the timing of strike slip shear along the ranong and khlong marui faults thailand 1 the timing of shear along many important strike slip faults in southeast asia such as the ailao shan red river mae ping and three pagodas faults is poorly understood we present 40ar 39ar u pb shrimp and microstructural data from the ranong and khlong marui faults of thailand to show that they experienced a major period of ductile dextral shear during the middle eocene 48 40 ma centered on 44 ma which followed two phases of dextral shear along the ranong fault before the late cretaceous 81 ma and between the late paleocene and early eocene 59 49 ma many of the sheared rocks were part of a pre kinematic crystalline basement complex which partially melted and was intruded by late cretaceous 81 71 ma and early eocene 48 ma tin bearing granites middle eocene dextral shear at temperatures of 300 500 c formed extensive mylonite belts through these rocks and was synchronous with granitoid vein emplacement dextral shear along the ranong and khlong marui faults occurred at the same time as sinistral shear along the mae ping and three pagodas faults of northern thailand a result of india burma coupling in advance of india asia collision in the late eocene 37 ma the ranong and khlong marui faults were reactivated as curved sinistral branches of the mae ping and three pagodas faults which were accommodating lateral extrusion during india asia collision and himalayan orogenesis

on the occurrence and boron isotopic composition of tourmaline in ultra high pressure metamorphic rocks abstract the extensive p t stability and the high chemical variability of tourmaline tur together with its common occurrence in metasediments proves its high potential for petrological and isotope geochemical studies on fluid rock interaction in subduction and collision related rocks this paper reviews the occurrence major element chemistry and boron isotopic composition of tur in high and ultrahigh pressure metamorphic uhpm rocks in addition it presents a new discovery of coesite bearing tur schorl from the erzgebirge germany as well as tur dravite related to the retrograde history of coesite and diamond bearing rocks from the erzgebirge and the kokchetav massif kazakhstan the scarce data on worldwide occurrences of u hpm tur reveal a high occupation of the crystallographic x site dominated by na and the possible presence of excess b with little further distinctiveness in its major element composition when compared with tur from medium grade rocks high k 2 o contents in tur are probably not related to uhp growth or equilibration the b isotopic composition of u hpm tur ranges in 11 b from 16 to 1 with many samples in or below the range of continental crust in contrast tur formed during retrograde fluid influx typically shows high 11 b values up to 28 suggesting heavy b fluids infiltrating the exhuming u hpm units coesite inclusions in tur characterized by raman spectroscopy are regarded as the best indicator for its uhp stability supplementary material analytical methods tourmaline compositions and boron isotope values are available at http www geolsoc org uk sup18354

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

40ar 39ar age constraints on deformation and metamorphism in the main central thrust zone and tibetan slab eastern nepal himalaya in eastern nepal structural and petrologic observations suggest that movement on the main central thrust mct the development of an inverted metamorphic sequence and leucogranite plutonism were genetically related samples from across the mct zone and its hanging wall the tibetan slab were collected from four locations in the everest region for 40ar 39ar studies 1 the mct zone 2 the lower tibetan slab 3 the upper tibetan slab away from intrusive rocks and 4 the upper tibetan slab adjacent to and including leucogranitic material within the mct zone muscovite and hornblende yield cooling ages of 12 0 0 2 ma and 20 9 0 2 ma respectively k feldspar gives a minimum age of 8 0 0 2 ma closure temperature tc 220 15 c in the lower tibetan slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7 7 0 4 ma and 7 5 0 6 ma and a k feldspar minimum age of 6 4 0 8 ma tc 225 20 c an adjacent gneiss yields a 9 1 0 2 ma biotite isochron and a k feldspar minimum age of 3 6 0 2 ma tc 210 50 c in the upper tibetan slab samples collected 200 m from visible intrusives yield a biotite isochron age of 20 2 0 2 ma and a complex hornblende age spectrum suggestive of excess argon at the fourth location an augen gneiss a pegmatite and a tourmaline bearing leucogranite all in mutual contact yield indistinguishable mineral ages the average biotite and muscovite isochron ages for these samples are 17 0 1 4 and 16 6 0 2 ma respectively the minimum age for k feldspar from the leucogranite is 15 5 1 8 ma as thermobarometric data from the mct zone in this area suggest synmetamorphic deformation at temperatures of 500 550 c the mct hornblende tc 500 c dates this event at 21 ma diffusion experiments on hornblendes from the mct zone provide data which support a maximum duration of peak metamorphic temperatures of 2 ma biotite ages as old as 20 ma from the upper tibetan slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the mct zone very young ages in a ductile shear zone in the lower tibetan slab suggest that there has been deformation within the tibetan slab that postdates major movement on the mct

new age constraints on the metamorphic evolution of the high pressure low temperature belt in the western tianshan mountains nw china abstract the western tianshan high pressure low temperature orogenic belt in nw china contains eclogite facies metavolcanic rocks and omphacite bearing blueschists previous sm nd omphacite garnet glaucophane whole rock and 40ar 39ar crossite dating of eclogite facies rocks has suggested an age of ca 345 ma as the best approximation for the timing of peak metamorphic conditions the samples described here are blueschist facies rocks that formed during or after the transition from the eclogite facies to the epidote blueschist facies and subsequently experienced an incipient greenschist facies overprint by use of white mica geochronology k ar 40ar 39ar rb sr an attempt is made to date postpeak metamorphic stages of the complex pt path rb sr and 40ar 39ar ages range between 313 and 302 ma and 323 and 312 ma respectively but mostly cluster at ca 310 311 ma indicating that the studied samples recrystallized at this time however the 40ar 39ar age spectra show complex release patterns that a

environmental impact of the laacher see eruption at a large distance from the volcano integrated palaeoecological studies from vorpommern ne germany abstract in order to investigate the environmental impacts of the laacher see eruption 12 900 calendar years b p and the deposition of the laacher see tephra at a large distance from the volcano fine resolution micromorphological diatomological and palynological investigations were carried out on closely spaced areas in the reinberg basin vorpommern ne germany the lower part of the lst originates from atmospheric input whereas the upper part was washed in from the surrounding basin slopes diatom populations expanded as a consequence of the input of silica while simultaneously dissolution of dead diatoms decreased the tephra provided a favourable habitat for many epilithic and epipelic diatom taxa the diatoms show a slight acidification and an increase in specific conductivity of the water whereas no eutrophication is indicated these effects disappeared after lake sediments covered the tephra the pollen diagrams from the reinberg basin as well as many other pollen diagrams from ne germany indicate that the vegetation reacted to rising temperatures after the cooler gerzensee fluctuation to a short lived water level rise connected with higher precipitation after the volcanic eruption and to minor openings in the forest vegetation due to increased fire probably connected with severe thunderstorms

biotite and muscovite 40ar 39ar geochronological constraints on the post svecofennian tectonothermal evolution forsmark site central sweden in order to characterize the post svecofennian tectonothermal evolution of the fennoscandian shield 40ar 39ar biotite and some 40ar 39ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1000 m long borehole samples at forsmark central sweden the thirteen surface samples were collected across three branches of a major wnw to nw trending system of deformation zones whereas the boreholes were drilled within a tectonic lens in between two of these zones the 40ar 39ar biotite ages indicate that the present erosion surface in central sweden cooled below 300 c at 1 73 1 66 ga and that the rocks could respond to stress by brittle deformation between 1 8 and 1 7 ga the variation in surface ages is suggested to be due to faulting along the large wnw to nw trending deformation zones following the establishment of a sub cambrian peneplain the minor variation of ages within a single crustal block may be due to disturbance along ene to nne trending fracture zones possible cooling paths derived from 40ar 39ar hornblende muscovite and biotite ages were calculated for the time interval from 1 80 to 1 66 ga when the area cooled from 500 c to 300 c between 1 70 and 1 64 ga uplift rates of c 22 m m y were calculated from borehole 40ar 39ar biotite data tectonothermal histories inferred from the combined cooling and uplift rates are related to simple slow cooling after the svecofennian orogeny to crustal movement in response to far field effects of c 1 7 ga orogenic activity further to the west or to a combination of these possibilities less

metamorphic evolution and thermobaric structure of the subduction related bacariza high pressure granulite formation cabo ortegal complex nw spain abstract the high pressure bacariza granulite formation comprises various lithostratigraphic units of granulite orthogneisses ultramafic mg rich mafic intermediate and common mafic granulites as well as of more exotic intercalations mineral assemblages in equilibrium in ultramafic to intermediate rocks contain garnet clinopyroxene and plagioclase with different amounts of zoisite clinozoisite kyanite quartz scapolite rutile and ilmenite depending on the granulite lithotype whereas granulite orthogneisses contain garnet phengite biotite k feldspar antiperthitic plagioclase quartz and rutile as primary phases thermobarometry of these rocks supports the existence of a high pressure metamorphism for which near peak p t conditions have been estimated at ca 790 c and 1 6 gpa the preserved fabrics and structures enable us to relate the metamorphism to coeval polyphasic deformational processes dynamic retrogression began under still high pressure granulite facies conditions 1 4 gpa and ca 740 c and is postdated by symplectitization 1 3 gpa and ca 715 c subsequent retrogression under medium pressure amphibolite facies conditions at similar temperature was either widespread and static or localized and dynamic as a result of intense deformation partitioning during uplift loading heating and subsequent decompression cooling are related to a single cycle in a subduction conduit setting this study suggests that high pressure granulite metamorphism might not be as uncommon in the high pressure metamorphic series as previously thought moreover it might constitute a diagnostic feature of convergent lithospheric settings whether or not associated with eclogite facies metamorphism in adjacent units

radiocarbon dating of irish sea sediments abstract radiocarbon dating has been carried out on three cores from areas of muddy sediments in the n irish sea to estimate rates of sediment accumulation 14c age profiles of the two eastern basin cores revealed a near constant age from the sediment surface to the base of the core 12 500 1000 years bp the 14c age profile of the western basin core revealed a zone of apparent mixing to a depth of 55 cm underlain by a zone of constant sedimentation rate 0 018 cm y 1 to 160 cm these data are discussed in relation both to previously reported sedimentological studies of the area and to the authorised discharges of low level radioactive waste from the sellafield nuclear fuel reprocessing plant

pressure temperature time paths the past decade has been a time of unprecedented improvement in our ability to reconstruct the thermal evolution of orogenic belts like many instances of rapid advancement in science this achievement can be traced to the convergence of disparate research directions the new field of thermochronology the study of the time temperature evolution of rocks derived from the realization by geochronologists that many mineral iso topic systems could be used to determine the time of cooling of a terrain through a series of predictable temperatures e g armstrong 1966 wagner et ai 1977 harrison mcdougali 1980 berger york 1981 petrologists have made great progress in their ability to discern the pressure tem perature history of metamorphic rocks through the improvement of con ventional thermobarometric techniques see bohlen lindsley 1987 and essene 1989 for reviews and the development of important new approaches to relating changes in mineral chemistry to changes in environ mental parameters spear selvcrstonc 1983 st onge 1987 from a theoretical standpoint several geophysical papers in the midto late 1970s and early 1980s explored heat transfer processes in orogenic belts and the role of these in the development and preservation of metamorphic mineral assemblages oxburgh england 1974 bickle et a1 1975 bird et a1 1975 england richardson 1977 toksoz bird 1977 england thompson 1984 among others the integration of geochronologic and petrologic techniques aliows us

synorogenic extension quantitative constraints on the age and displacement of the zanskar shear zone northwest himalaya structural thermobarometric and geochronological data place limits on the age and tectonic displacement along the zanskar shear zone a major north dipping synorogenic extensional structure separating the high grade metamorphic sequence of the high himalayan crystalline sequence from the overlying low grade sedimentary rocks of the tethyan himalaya a complete barrovian metamorphic succession from kyanite to biotite zone mineral assemblages occurs within the 1 km thick zanskar shear zone thermobarometric data indicate a difference in equilibration depths of 12 3 km between the lower kyanite zone and the garnet zone which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the zanskar shear zone for the present day dip of the structure 20 a simple geometrical model shows that a net slip of 35 9 km is required to regroup these samples to the same structural level because the kyanite to garnet zone rocks represent only part of the zanskar shear zone and because its original dip may have been less than the present day dip these estimates for the finite displacement represent minimum values field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the zanskar shear zone occurred as a continuous process starting at the barrovian metamorphic peak and lasting throughout the subsequent extension induced exhumation geochronological dating of various leucogranitic plutons and dikes in the zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19 8 ma and that extension most likely initiated shortly before 22 2 ma

diachronous deformation along the base of the himalayan metamorphic core west central nepal geologic mapping combined with microstructural and geochronologic analyses from the lower himalayan metamorphic core in west central nepal record along strike similarity in flow style despite variability in the timing of metamorphism and deformation the main central thrust zone at the base of the himalayan metamorphic core varies in thickness tectonostratigraphy and metamorphic gradient along the 250 km of strike length studied in situ u th pb geochronology of monazite sampled from an along strike transect at the top of the high strain zone records eocene oligocene prograde metamorphism followed by miocene retrograde metamorphism the timing of prograde and retrograde metamorphism and the muscovite 40 ar 39 ar dates gradually decrease along strike from northwest to southeast this age trend is punctuated by an abrupt 3 8 m y decrease in the age of prograde and retrograde metamorphism and muscovite 40 ar 39 ar dates near the marsyandi river in central nepal quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high strain zone document similar flow style at 440 c in central nepal muscovite 40 ar 39 ar ages interpreted to approximate the age of deformation at this structural level decrease from ca 7 to 4 ma along strike from northwest to southeast diachronous deformation and metamorphism along strike in west central nepal demonstrate the necessity of incorporating more than a single transect into tectonic models along strike tectonometamorphic variability in west central nepal spatially corresponds to faults in the indian basement bounding the subsurface faizabad ridge highlighting the possible influence of inherited basement faults on the geometry of the basal himalayan detachment the main himalayan thrust as well as the tectonic evolution of the structurally overlying himalayan metamorphic core this study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along strike diachroneity of deformation in the himalaya

comment postglacial relative sea level observations from ireland and their role in glacial rebound modelling a j brooks s l bradley r j edwards g a milne b horton and i shennan 2008 journal of quaternary science23 175 192

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

a model of subaqueous sedimentation at the margin of the late midlandian irish ice sheet connemara ireland and its implications for regionally high isostatic sea levels abstract the geomorphology and sedimentology of a series of late midlandian glacigenic sites in connemara western ireland are described and a set of commonly occurring sedimentary assemblages identified these form components of a simple model of ice contact subaqueous sedimentation involving progressive down current transition from subglacial esker tunnel fill through proximal tunnel mouth fans proximal and distal subaqueous fans into basin floor sediments in two of the sites aggradation to the water line passes into fan delta topsets and foresets all sites abut into inferred water bodies to their west and imply minimum water surface heights of between 35 and 65 m irish od or in the case of the two deltaic sites shoreline heights of between 75 and 80 m these high water levels very close to and facing directly into the immediately adjacent open atlantic ocean are difficult to explain as ice dammed glaciolacustrine water bodies without invoking improbable ice sheet marginal geometries they may however be explained as a set of successive fjord head grounding line depositional systems that trace temporary still stands in the phased retreat of the irish ice sheet from its offshore maximum this possibility not yet confirmed by the discovery of in situ marine fauna implies a degree of late midlandian isostatic depression and subsequent response unexplained by currently applied crustal response models but supported by previously constructed glacio sedimentary models

apparent isobaric cooling paths from granulites two counterexamples from british columbia and new hampshire textural evidence for the reaction histories of pelitic granulites from british columbia and new hampshire shows that pressure temperature paths obtained from thermobarometry on these samples are an artifact of the calculation technique samples from both localities show breakdown of garnet to form cordierite a reaction that occurs during decompression additional reaction textures and fluid inclusion data also imply decompression at high temperatures for both localities however core rim thermobarometry yields nearly isobaric cooling paths similar cooling paths have been used by other authors to argue for a magmatic underplating model for regional mafic granulites we show that mineral homogenization in pelitic granulites can lead to erroneous isobaric cooling paths and hence to incorrect conclusions concerning tectonic similarities between different types of granulites

helium and argon thermochronometry of the gold butte block south virgin mountains nevada one of the largest exposures of precambrian crystalline rock in the basin and range province of the southwestern usa is the gold butte block of the south virgin mountains about 15 km west of the colorado plateau it has been interpreted as a largely continuous crustal cross section about 15 20 km thick that was exhumed by a deeply penetrating normal fault during miocene extension to test this interpretation as well as the use of the newly developed titanite u th he thermochronometer we examined the low temperature thermal history of the gold butte block with the apatite and titanite u th he and muscovite 40 ar 39 ar thermochronometers apatite he ages average 15 2 1 0 2 ma throughout the block indicating that the entire section was warmer than 70 c prior to miocene exhumation titanite he ages increase from 18 6 1 5 ma near the paleobottom west end of the block to 195 15 ma near the paleotop east end a rapid change from mid tertiary to increasingly older titanite he ages to the east is observed at about 9 3 km paleodepth and is interpreted as a fossil he partial retention zone for titanite assuming a pre exhumation geotherm of 20 c km consistent with earlier apatite fission track work this depth would have corresponded to 196 c prior to exhumation indicating that laboratory derived he diffusion characteristics for titanite that yield a closure temperature of about 200 c are applicable and correct muscovite 40 ar 39 ar ages are 1 0 1 4 ga near the paleotop of the block and 90 ma near the paleobottom together with 207 pb 206 pb ages on apatite and titanite and an earlier apatite fission track transect across the gold butte block our data indicate that the continental crust at the western edge of the colorado plateau resided at moderate geothermal gradients and slowly declined in temperature from 1 4 ga to about 100 200 ma a 90 ma cooling event clearly affected the mid crust deepest portions of gold butte which may reflect accelerated cooling or a brief heating and cooling cycle at this time after which gradients returned to about 20 c km prior to rapid exhumation in the miocene this work thus supports previous structural and thermochronologic studies that suggest that the gold butte block is the thickest largely continuous cross section of crust exposed in the southwestern usa

petrologic and age constraints on the origin of a low pressure high temperature metamorphic complex southern alaska the interrelationships between metamorphism deformation magma intrusion and 40ar 39ar geochronology were determined for a low pressure high temperature metamorphic complex which formed from an accretionary prism in the chugach mountains southern alaska compressional deformation which first produced south verging folds and associated thrusts was followed by magma intrusion and development of north verging folds synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall as documented by the distribution of mineral assemblages and by pressure temperature modeling of garnet growth the initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust regionally developed amphibolite facies metamorphism followed intrusion of felsic sills the peak metamorphic conditions derived from geothermobarometry mineral assemblages and fluid inclusions ranged from 400 to 600 c at a depth of 10 km the increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite bearing migmatites subsequent cooling was initially rapid 55 c ma to 350 c based on 40ar 39ar dates of 53 ma for hornblende and 50 ma for biotite and may have slowed to 11 c ma until 200 c based on an 40ar 39ar date of 35 ma for plagioclase intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds this suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism

zoned poikiloblastic garnets p t paths and syn metamorphic uplift through 30 km of structural depth wopmay orogen canada