metamorphic evolution partial melting and miocene exhumation of lower crust in the edough metamorphic core complex west mediterranean orogen eastern algeria abstract the edough massif is the easternmost crystalline massif of the maghrebides which represent the southeastern part of the west mediterranean orogen it emerges from below the greenschist facies tellian units that represent the thrusted mesozoic to eocene passive paleomargin of northern africa deposited on the thinned continental crust the top of the crystalline pile of the edough massif is characterised by metasedimentary rocks affected by medium to high pressure 12 14 kbar and medium temperature metamorphic conditions of ca 600 c the northern part of the massif as well as the overlying allochthonous amphibole peridotite unit in the north is characterised by high pressure granulite facies t 800 c and p 12 kbar nearly isothermal decompression is documented in metapelites by late to post kinematic crystallisation of sillimanite and andalusite overprinting kyanite the core of the dome comprises essentially diatexite and granite gneisses a late low pressure evolution and overprinting 2 3 kbar took place in paleozoic rocks in proximity to granite gneisses the predominant igneous mineralogy of granite gneisses suggests that they represent synmetamorphic and syn kinematic sheeted intrusions generated through a high degree melting of metasediments geochronology of the metamorphic rocks u pb on monazite sm nd on garnet wr pairs 40 ar 39 ar on amphiboles and micas yields ages bracketed between 24 and 16 ma suggesting that regional metamorphism of the edough massif took place during the late tertiary synchronous with the greenschist facies metamorphism recorded in the overlying mesozoic tellian units the edough massif represents a metamorphic core complex exhumed through tectonic denudation assisted by syn to late metamorphic low angle n dipping normal ductile shear zones an exhumation rate of 6 8 km ma and associated cooling rate of 200 300 c ma is implied we conclude that the edough massif represents part of the former thinned continental crust of africa involved in a late eocene northward subduction beneath the suboceanic domain that separated paleo europe and africa in mesozoic time the upward syn metamorphic tectonic emplacement at 40 45 km depth of slices of spinel peridotites detached from the lower mantle may be responsible for the local granulite facies occurrences their emplacement is a consequence of the long lived transtensional sinistral then dextral kinematics having taken place between the african and european plates

from orogenic hinterlands to mediterranean style back arc basins a comparative analysis hinterland plateaux and mediterranean style back arc basins both form behind active subduction zones or collisional megathrusts and share many characteristics 1 early crustal thickening to about twice normal continental thickness 2 thin lithospheric mantle 3 mixed magmatism including asthenospheric lithospheric and crustal melts 4 late stage horizontal extension accompanied by vertical shortening horizontal extension and vertical shortening are driven by high gravitational potential energy gpe contrasts between the hinterland and surrounding lithosphere which may reach 7 10 12 n m 1 equivalent to about 2 5 times the ridge push force if extension is rapid relative to the rate of lithospheric cooling gpe may remain positive even as extension continues declining to the mid ocean ridge value as crustal thickness approaches zero this suggests that hinterland plateaux could ultimately evolve into oceanic back arc basins the rate direction and amount of extension and the rate of vertical shortening depend on the plate boundary conditions and the gpe of the surrounding lithosphere vertical shortening in tibet is limited by work required to deform the surrounding asian lithosphere whereas mediterranean back arc basins can extend at the expense of regions of thin continental or oceanic crust

automated mapping of k feldspar by electron backscatter diffraction and application to 40ar 39ar dating abstract the ability to quantify feldspar microstructure using the electron backscatter diffraction ebsd method has direct application in the study of rock deformation and strain kinematics however automated ebsd analysis of low symmetry phases such as feldspar has previously proven difficult here we successfully apply the ebsd method to a number of granitic feldspars and develop automated phase and orientation mapping to discriminate k feldspar and plagioclase and quantify orientation variations within individual k feldspar grains these results represent the first automated quantitative mapping of orientation microstructure in k feldspar we use the method to evaluate the relationship between microstructure and 40 ar 39 ar age a controversial problem in thermochronology in a granitic k feldspar from central australia the range of observed orientation domains matches the small to intermediate and largest domain sizes predicted from multiple diffusion domain modeling in situ ultra violet laser microprobe analyses show that the youngest ages from the 40 ar 39 ar age spectra are recorded by grain mosaic k feldspars with diameter around 10 50 m these k feldspars are the smallest coherent microstructural features observed on scales of 1 m large 250 1000 m diameter microstructurally simple grains record the oldest ages observed in the age spectrum these results suggest a first order relationship between k feldspar microstructure and 40 ar 39 ar age and demonstrate a microstructural control on multidomain diffusion

middle oligocene extension in the mediterranean calabro peloritan belt southern italy insights from the aspromonte nappes pile the calabro peloritan belt constitutes the eastward termination of the southern segment of the alpine mediterranean belt this orogenic system was built up during the convergence between the eurasian and the african plates roughly directed north south since the upper cretaceous it was subsequently fragmented during the opening of the western mediterranean basins since oligocene times the curved shape of the calabro peloritan belt was acquired during the opening of the tyrrhenian basin since tortonian the origin kinematics and significance of the calabro peloritan tectonic pile are still debated our data in the aspromonte massif of southern calabria reveal an alpine history marked by two main superimposed kinematic regimes i a first phase corresponds to the piling up of basement nappes with a top to the se vergence i e in a direction orthogonal to the belt trend and towards the adriatic foreland this external vergence is similar to what is observed in both northeastern sicily and northern calabria in sicily the age of nappe piling is alpine as evidenced by pinched slices of mesozoic sediments in the aspromonte massif thrusting age is less constrained our data suggest remnants of late hercynian structuration before the alpine stacking ii a second phase corresponds to the thinning of the continental crust dated at around 30 ma by both geochronological and stratigraphical data this extension is mainly localized on two low angle detachment contacts with top to the ne displacement the lower one corresponds to the reworking of the former main nappe contact the upper one is a large detachment fault cutting across the pile from upper sedimentary levels down to metamorphic basement extension of similar alpine age and similar internal vergence has been already recognized in other parts of the calabro peloritan arc in the basement nappes of northeastern sicily and in the ophiolitic units of northern calabria coming back to the original geometry and position of the calabro peloritan belt before its bending and the opening of the liguro provencal and tyrrhenian basins we evidence a homogeneous oligocene ne sw extension all along the calabro peloritan segment of the alpine mediterranean belt this tectonometamorphic history is best explained within the framework of the continuous tertiary westward dipping subduction of the tethyan oceanic domain below the european active margin and the progressive eastward retreat of the apennine trench since oligocene times

reply to the discussion on orbital calibration of the late campanian carbon isotope event in the north sea journal of the geological society london 173 504 517 smith bailey 2017 a have criticized the methods applied for time series analysis in our study of a late campanian chalk core in the north sea perdiou et al 2016 these critics rely essentially on two points 1 the resolution spacing that can be applied to our gamma ray data 2 the use of the autoregressive order 1 ar1 noise model which allows calculation of significance thresholds

alpine extensional detachment tectonics in the grande kabylie metamorphic core complex of the maghrebides northern algeria abstract the maghrebides are part of the peri mediterranean alpine orogen they expose in their inner zone inliers of high grade crystalline rocks surrounded by oligo miocene and younger miocene cover detailed mapping coupled with structural and petrological investigations in the grande kabylie massif and the reinterpretation of the available geochronological data allow us to refute the traditional concept of rigid behaviour of this massif during alpine events we show that the dome geometry the kinematic and metamorphic evolutions and the age pattern are typical of metamorphic core complexes exhumed by extension a major low angle detachment fault defined by mylonites and by younger cataclasites has been traced in the massif the upper unit encompasses pre permian phyllites with variscan 40 ar 39 ar cooling ages capped by unconformable mesozoic to tertiary cover of the calcareous range both mainly affected by extensive tertiary brittle deformation and normal faulting the lower unit exposes in two half domes a continuous tectonic pile 6 8 km thick of amphibolite facies rocks and orthogneisses affected by syndashmetamorphic ductile deformation devoid of retrogression the regular increase of paleotemperature downward and the 40 ar 39 ar plateau ages around 80 ma suggest that the high temperature foliation and associated wnw directed shear under a high geothermal gradient relate to extensional tectonics developed during mesozoic lithospheric thinning of the variscan south european margin to the north the sidi alli bou nab massif exposes another crustal section affected throughout by wnw directed extensional shear during h p h t syndashmetamorphic thinning and with overall 40 ar 39 ar plateau ages of 25 ma the eocene oblique collisional event responsible for crustal thickening was totally overprinted by this new extensional regime synchronous with the beginning of the opening of the western mediterranean oceanic basin this was also coeval with south directed thrusting of foreland nappes to the south post miocene tectonic events cause significant overprinting

applying the multiple inverse method to the analysis of earthquake focal mechanism data new insights into the active stress field of italy and surrounding regions abstract in order to obtain new insights into the active tectonic setting of the italian territory and surrounding regions the multiple inverse method mim has been applied to the analysis of fault plane solutions from more than 700 earthquakes with m w 4 the active stress field in the top 40 km of the lithosphere has been defined for four 10 km thick layers each including 810 square cells of 1 5 side the obtained stress field maps point out that most of the upper crustal seismicity of the western and central alps is controlled by a strike slip regime which is dominant also in part of the dinarides albanides and hellenides and in a large sector encompassing eastern sicily and the malta area to the eastern tunisia offshore on the other hand the well known extensional belt occurring in the interior of the apennines appears to extend well beyond the backbone of italy potentially reaching the outer foothills of the northern marche region while the adjacent adria block extending to the eastern po plain and the outer dinarides sticks out as a major area characterised by dominant thrust faulting in the upper crust a similar regime characterises also a large sector of the western tyrrhenian sea from ne tunisia through western sicily and the west coast of sardinia to the provence coast besides lateral variations our analysis also points out a significant vertical heterogeneity of the stress field the deeper levels 20 to 40 km investigated in this study being characterised by dominant horizontal maximum compression even in areas of upper crustal extension the application of the mim to a large seismological dataset providing basic information for the compilation of active stress maps contributes to a better understanding of active tectonic processes and may be used for improving seismotectonic zoning and reservoir management

low pressure metamorphism of palaeozoic pelites in the aspromonte southern calabria constraints for the thermal evolution in the calabrian crustal cross section during the hercynian orogeny during hercynian low pressure high temperature metamorphism of palaeozoic metasediments of the southern aspromonte calabria a sequence of metamorphic zones at chlorite biotite garnet staurolite andalusite and sillimanite muscovite grade was developed these metasediments represent the upper part of an exposed tilted cross section through the hercynian continental crust p t information on their metamorphism supplements that already known for the granulite facies lower crust of the section and allows reconstruction of the thermal conditions in the calabrian crust during the late hercynian orogenic event three foliations formed during deformation of the metasediments the peak metamorphic assemblages grew mainly syntectonically s2 during regional metamorphism but mineral growth outlasted the deformation this is in accordance with the textural relationships found in the lower part of the same crustal section exposed in the northern serre pressure conditions recorded for the base of the upper crustal metasediments are c 2 5 kbar and estimated temperatures range from 350 c in the chlorite zone increasing to 500 c in the lower garnet zone and reaching 620 c in the sillimanite muscovite zone geothermal gradients for the peak of metamorphism indicate a much higher value for the upper crust c 60 c km 1 than for the granulite facies lower crust 30 35 c km 1 the small temperature difference between the base of the upper crust 620 c at c 2 5 kbar and the top of the lower crust 690 c at 5 5 kbar can be explained by intrusions of granitoids into the middle crust which in this crustal section took place synchronously with the regional metamorphism at c 310 295 ma it is concluded that the thermal structure of the calabrian crust during the hercynian orogeny as it is reflected by peak metamorphic assemblages was mainly controlled by advective heat input through magmatic intrusions into all levels of the crust

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

muscovite recrystallization and saddle shaped 40 ar 39 ar age spectra example from the blond granite massif central france muscovites from the blond granite west french massif central were dated by the 40ar 39ar single grain method the 40ar 39ar ages obtained vary from 305 5 0 3 ma to 311 3 0 5 ma and most of the age spectra are slightly saddle shaped the analyzed muscovites show phengitic recrystallization under optical microscope observations sem images and electron microprobe chemical analyses it is proposed that the saddle shaped age spectra result from a partial recrystallization which produced three different isotopic reservoirs in the analyzed white mica single grains domains of early muscovite domains of neocrystallized muscovite formed by phengitic and al fe substitutions and low activation energy sites

the western mediterranean extensional basins and the alpine orogen the western mediterranean late oligocene miocene basins alboran valencia and provencal basins are a coherent system of interrelated troughs in all basins normal faults and thermal subsidence migrated toward the east progressively moving to the miocene to pleistocene algerian and tyrrhenian basins all those troughs appear elements of the back arc opening related to the eastward roll back of the w directed apennines maghrebides subduction zone similarly to western pacific back arc settings these late oligocene early miocene basins nucleated both within the betic cordillera e g alboran sea and in its foreland valencia and provencal troughs the n40 70 direction of grabens is oblique to the coexisting n60 80 trending orogen and shows its structural independence from the orogenic roots thus as the extension cross cuts the orogen and developed also well outside the thrust belt front the westernmost basins of the mediterranean had to develop independently from the alps betics orogen therefore the alboran extension considered a classic example of a basin generated by the collapse of an orogen cannot be ascribed to the detachment or annihilation of the lithospheric root in contrast with the eastward migrating extensional basins the betic balearic thrust front was migrating westward producing interference or inversion structures

exhumation of the schistes lustres complex in situ laser probe 40ar 39ar constraints and implications for the western alps the schistes lustres sl suture zone occupies a key position in the alpine chain between the high pressure hp brianconnais domain and the ultrahigh pressure uhp dora maira massif and reached subduction depths ranging from c 40 65 km cottian alps in order to constrain the timing of hp metamorphism and subsequent exhumation several phengite generations were differentiated on the basis of habit texture paragenesis and chemistry as belonging to the first or second exhumation episode respectively d2 or d3 or to earlier stages of the tectono metamorphic evolution ten carefully selected samples showing d2 d3 d2 d3 or earlier mostly peak temperature phengite population s were subjected to laser probe 40ar 39ar analysis the data support the results of the petrostructural study with two distinct age groups crystallization ages for d2 and d3 phengite at 51 45 and 38 35 ma respectively the data also reveal a coherent age cluster at 62 55 ma for peak temperature phengite associated with chloritoid which were preserved in low strain domains the age of the d3 event in the sl complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs exhumation rates of the order of 1 2 mm yr 1 are obtained for the sl complex which are compatible with velocities documented for accretionary wedge settings similarly cooling velocities are only moderate c 5 c myr 1 which is at variance with recent estimates in the nearby uhp massifs

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

alpine tectonics in the calabrian peloritan belt southern italy new 40ar 39ar data in the aspromonte massif area this study provides new ar 40 ar 39 geochronological constraints on the age of the alpine tectonics in the aspromonte massif southern part of the calabrian peloritan belt this massif exposes the upper units of the calabride complex which originated from the european continental margin the calabride complex was incorporated in the alpine orogenic wedge and then integrated into the apennines and maghrebides fold and thrust belts throughout the calabride complex there is evidence for a two stage tectonic history which remains however rather poorly dated alpine nappe stacking is followed by extensional reworking along the former thrust contacts or along new detachment surfaces our new ages suggest that exhumation of the uppermost units which accompanied nappe stacking probably started at 45 ma and that the deepest units were almost completely exhumed at 33 ma this kinematics probably corresponds to syn orogenic extension while the end of exhumation is clearly related to the extensional tectonics dated at 28 6 ma along detachment structures our geochronological data reveal a very short lag time between accretional and extensional processes in this part of the mediterranean alpine orogenic belt the direction of extension when the units are restored to their initial position i e before the opening of the western mediterranean basins and the bending of the arc is nne ssw such a direction does not fit with the eastward slab retreat model generally put forward to explain extension in the western mediterranean in contrast we provide evidence for roughly n s middle oligocene extension in the accretionary prism not previously described in this part of the mediterranean domain

alpine orogenic p t t deformation history of the catena costiera area and surrounding regions calabrian arc southern italy the nappe edifice of north calabria revised with insights on the tyrrhenian apennine system formation 1 the nappe structured belt of calabria constitutes the eastward termination of the southern branch of the alpine mediterranean belt that delimits the northern edge of the africa plate contrasting hypotheses for the origin and tectonic significance of the north calabrian nappe edifice have been proposed and kinematic data from north calabria have been used to support different interpretations of the alps apennines linkage and the polarity of the tethyan subduction in the apennine region we reconstruct the architecture of the north calabria nappe edifice through a multidisciplinary approach which integrates structural investigations with metamorphic thermobarometry and 40ar 39ar geochronology results from this study indicate that north calabria consists of a tertiary nappe stack resulting from superimposed top to the west extensional shearing late oligocene to middle miocene in age onto a previously structured top to the east compressional belt eocene to oligocene in age this study also documents that the top to the west extensional tectonics was achieved by means of regionally sized extensional detachment fault systems stretching apart and translating as allochthonous fragments the previously accreted units thinning operated by top to the west extensional detachment tectonics also resulted in the direct juxtaposition of non alpine or slightly alpine metamorphosed units upper plate complex onto the previously exhumed deep seated portions of the orogenic wedge metamorphosed under blueschist facies metamorphic conditions lower plate complex these findings support a new tectonic scenario for the orogenic history of north calabria which may be adequately framed within the tertiary apennine tyrrhenian system evolution

radar altimetry of large martian craters abstract mars topography in the 23 to 22 latitude band has been measured by the goldstone 13 cm radar during the 1971 1982 oppositions a total of 108 scans were produced each scan spans several tens of degrees in longitude the longest scan spans 133 8000 the vertical resolution generally is better than 200 m with some exceptions the areal resolution is 0 16 8 km in longitude and 1 30 80 km in latitude on regional scales elevation differences of as much as 3 km over less than 10 longitude have been observed aeolis memnonia most local relief is provided by structural elements of impact craters one hundred fifty two complex degraded craters some as small as 25 km in diameter have been identified in this data set repeated scans of the same craters yield nearly identical elevation profiles measured crater depths rarely exceed 2 5 km depths of the most degraded craters in the sample are a weak function of crater diameter depths of the largest craters d 125km show no functional relationship to diameters measured rim heights rarely exceed several hundred meters structures with no discernible rim e g ladon have retained a basin shape low exterior relief rather than overall low relief seems to be the distinguishing characteristic of large degraded craters included in the goldstone sample craters or basins that serve as sinks for major channel systems e g gusev ladon appear to be as deep as comparable structures not associated with such systems suggesting that processes other than fluvial sedimentation contributed to crater shallowing

40ar 39ar and rbsr analyses from ductile shear zones from the atacama fault zone northern chile the age of deformation abstract the influence of deformation on the k ar and the rb sr isotope system is investigated it is assumed that due to the diffusion processes involved deformation has a similar effect on isotopic equilibrium as has temperature in order to examine the influence of deformation on the k ar and the rb sr isotope systems two shear zones from the atacama fault zone afz situated in the north chilean coastal cordillera have been investigated the afz which was active as a sinistral strike slip fault during the mesozoic has two sets of shear zones one formed under amphibolite sz1 one under greenschist facies conditions sz2 rb sr and 40ar 39ar age determinations were conducted on samples from cross sections of each set in sz1 the hornblendes and bioties from a weakly deformed sample reveal cooling ages of 153 152 and 150 1 ma respectively biotite from the center of the shear zone of sz1 gave an isochron of 143 9 0 3 ma mswd 0 04 which is interpreted as the age of deformation which produced resetting of the mineral system in sz2 hornblendes yielded 40ar 39ar plateau cooling ages of 138 ma biotites from undeformed samples gave rb sr and 40ar 39ar total degassing ages of 130 1 ma whereas biotite from the mylonitic rocks yielded 126 125 ma which dates the time of deformation sr isotope homogenization occurred in the mylonitic rocks and is most likely a result of deformation the formation of sz1 can be correlated to the araucanian nevadan phase the deformation in sz2 is related to the onset of uplift and cooling of the coastal cordilleran magmatic arc

the transition between orogenic and anorogenic magmatism in the western mediterranean area the middle miocene volcanic rocks of isola del toro sw sardinia italy the isola del toro is a small island 1k m 2 about 5 nautical miles sw of sardinia entirely made up of mildly alkaline sodic evolved rocks benmoreites and trachytes related to each other by fractionation processes 40 ar 39 ar dating of an anorthoclase mineral separate from a trachyte gave an age of 11 83 0 10 ma this age estimate falls within an interval previously considered a magmatic in sardinia the results of this study shift the beginning of the middle miocene quaternary anorogenic volcanism of sardinia back to the late serravalian the transition between orogenic and anorogenic activity in sardinia is markedly different from that recorded in the rest of the circum mediterranean area where this transition is much more gradual both in time and in terms of magma composition on the other hand the transition between orogenic and anorogenic magmatism in sardinia is rather sharp and lacks hybrid compositions the isola del toro volcanic rocks represent the first anorogenic products produced during the opening of the western mediterranean sea

timing duration and role of magmatism in wide rift systems insights from the jiaodong peninsula china east asia in east asia widespread extensional sedimentary basins together with the close association of metamorphic core complexes and magmatism are prominent features of the large scale wide rift system developed during late mesozoic times this region thus appears as a proper place to study continental extension as well as questions about the link between continental extension and magmatism this paper primarily provides pioneer exhumation time constraints on the linglong mcc jiaodong peninsula including cooling and deformation ages mcc cooling at mid crustal levels occurred at ca 143 ma besides last ductile deformation occurred at ca 134 ma while final exhumation stages under brittle 2 conditions occurred as late as ca 128 ma continuous crustal stretching were then recorded by the emplacement of a synkinematic pluton in the upper crust at ca 128 ma that cooled fast below an intracrustal shear zone crossing the ductile brittle transition at ca 123 ma the ca 120 118 ma age cluster is ascribed to the fast cooling of undeformed plutons marking the end of extension that lasted in the area over a minimum period of ca 30 my combining other mcc exhumation constraints and the onset of subsidence in the sedimentary basins total duration of late mesozoic extension in east asia could be estimated at ca 60 my related with a rather long process for extension from 160 to at least 100 ma east asian continental extension is heterogeneously distributed in space and time as revealed by fundamental differences between two end member classes of migmatite cored mcc already described in other wide rift systems extension seems to have first favoured partial melting which subsequently in turn maintained continental extension

neocrystallization or cooling 40ar 39ar ages of white micas from low grade mylonites abstract 40 ar 39 ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or alternatively ages of deformation induced neocrystallization to address these interpretations three types of information are needed 1 the effective diffusion length scale controlling volume diffusion of argon in white micas 2 an estimate of the diffusion parameters for volume diffusion of argon and 3 the thermal history completely recrystallized quartzites exhibit smooth grain size distributions for white micas with good exponential or power law fits suggesting that the grains grew under similar conditions the microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history measured grain size distributions provide a maximum limit for the diffusion length scale thermal histories are estimated by 40 ar 39 ar diffusion experiments on k feldspars using the multi diffusion domain method of lovera lovera o m richter f m harrison t m 1989 the 40 ar 39 ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes j geophys res 94 17917 17935 these results are consistent with the expected temperature time path for the thrusting environment and predict maximum temperatures of 250 350 c during the time span recorded by the white micas experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages forward modeling of the passage of the white micas through the closure window for argon diffusion using the thermal histories derived from the k feldspars indicates that the white micas record neocrystallization rather than cooling and supports the idea that the white micas record the timing of when ductile deformation ceased

a chlorine disinfectant for excess argon released from k felsspar during step heating abstract the release of excess 40 ar 40 ar e from virtually all alkali feldspars at low temperatures 40 ar 39 ar step heating experiments obscures potentially valuable age and thermal history information we report a method that takes advantage of the differential release of 40 ar e between contiguous isothermal heating steps due to the decrepitation of fluid inclusions and which permits derivation of a correction for the contaminating argon differences in age and cl k between successive temperature pairs commonly yield a single well correlated array on a 40 ar k versus cl k plot that identifies the cl correlated composition of 40 ar e 40 ar e cl affecting that sample eighteen of the twenty k feldspar samples we have measured yield a single 40 ar e cl component that varies from 1 1 10 3 to 4 2 10 6 making possible recovery of otherwise unobtainable thermochronological information the upper bound on the 40 ar e cl distribution of 10 3 may reflect ar saturation of pore fluids or the maximum duration between deuteric alteration and cooling below ar closure in k feldspar

tectonothermal history of the hercynian continental crust of the serre southern calabria italy monitored by rb sr biotite resetting in the serre mountains of calabria italy an exposed section of the continental crust as left by the hercynian orogeny consists of intermediate lower to upper crustal units huge masses of granitoids separate the lower from the upper crustal units many mica ages have been obtained from metamorphic and plutonic rocks which have been interpreted as reflecting continuous cooling or discrete mesozoic events a reappraisal of previously determined isotopic data integrated with new rb sr biotite ages is presented and assessed at regional scale to better constrain the post hercynian geological evolution of the continental crust of the serre the ages cover a wide span of time and form clusters which fit a model involving magmatic hydrothermal and tectonic events preceding and accompanying the opening and closure of the tethyan ocean

seismic anisotropy reveals the long route of the slab through the western central mediterranean mantle abstract in the south eastern corner of the tyrrhenian basin in the central mediterranean sea a tight alignment of earthquakes along a well defined benioff zone marks one of the narrowest active trenches worldwide where one of the last fragment of the former tethys ocean is consumed seismic tomography furnishes snapshot images of the present day position of this slab and seismic anisotropy allows to reconstruct the past kinematics of the subduction process using seismic anisotropy fast directions as a proxy for the present and past mantle flow we look backward for the seismic traces of the slab motion through the western central mediterranean mantle from the starting locus of subduction toward its present day position the result of combining independent data sets provides a coherent pattern of anisotropy that illustrates an example of slab rollback from its initiation point to its present day position

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

the stability of ground ice in the equatorial region of mars consideration of the partial pressure of h2o in the martian atmosphere and the range of mean annual temperatures at the martian surface suggests that the occurrence of ground ice in equilibrium with the atmosphere is restricted to latitudes poleward of 40 however there is a growing body of morphologic evidence that indicates that substantial quantities of ground ice may have been present in the equatorial regolith throughout martian geologic time the accepted explanation for this apparent contradiction has been that the h2o found near the equator is a relic emplaced very early in martian geologic history 3 5 b y ago and under substantially different climatic conditions it is generally believed that this fossil ground ice layer has been preserved to the present day by the diffusion limiting properties of a relatively shallow layer 10 m of fine grained regolith to evaluate this hypothesis the lifetime of a 200 m layer of ground ice buried below 100 m of ice free regolith has been examined for the latitudes between 30 on mars twelve model pore size distributions representative of silt and clay type soils found on earth were selected to simulate the pore structure of the martian regolith the parallel pore model of gaseous diffusion was then used to calculate the flux of escaping h2o molecules from the buried ground ice layer the potential effects of such factors as depth of burial the martian geothermal gradient regolith porosity adsorption surface diffusion and climatic change were also considered based on our analysis the most important factors affecting the stability of equatorial ground ice appear to be 1 soil structure 2 the magnitude of the geothermal gradient and 3 the climatic desorption of co2 from the regolith we conclude that while the conditions necessary for the long term survival of ground ice may exist in isolated regions within the equatorial regolith on the global scale it appears unlikely that a fossil ground ice layer has been preserved throughout martian geologic history if massive ground ice does presently exist in the equatorial regolith then its presence may imply the existence of some continuous or periodic process of replenishment

pre metamorphic 40ar 39ar and u pb ages in hp metagranitoids from the hercynian belt france in most orogenic belts the age of hp metamorphism and subsequent exhumation events still remain open to debate since geochronology can yield results which appear to conflict with the closure temperature concept dodson m h 1973 contrib mineral petrol 40 259 274 and because the behaviour of daughter radiogenic isotopes under hp to uhp conditions is poorly constrained to obtain new data on isotope migration under high pressure conditions two undeformed hp metagranites with partially preserved magmatic assemblages from the french variscan belt were investigated using the 40ar 39ar laser probe and u pb ion probe methods in both cases 40ar 39ar biotite and u pb zircon ages are consistent and could be related to the emplacement of pre orogenic granites despite petrological evidence of a strong metamorphic overprint during the variscan orogeny temperatures experienced by the granites during subduction and exhumation events were more than 400 c above that normally estimated for argon retention in biotite but failed to cause significant resetting of the mica 40ar 39ar chronometer only a weak intragrain redistribution of argon is evidenced with the laser probe up to the contact with metamorphic garnet fringing biotite by contrast a complete resetting of biotite ages occurs in gneisses enclosing the metagranites these results suggest that in these dry undeformed hp metagranites the thermally activated diffusion was relatively ineffective and that recrystallisation is the main process which controlled isotopic exchanges of ar and pb it is likely that the absence of pervasive deformation and fluid circulation has also exercised some control on the preservation of pre metamorphic isotopic signature in the studied rocks the possible influence of several other parameters is also discussed this example reveals that recovering thermochronological information in high pressure metamorphic rocks is not straightforward and that great caution must be paid in the use of ages for the reconstruction of p t t paths

the topography of impact craters in softened terrain on mars abstract terrain softening is a distinct style of landform degradation observed in high resolution viking orbiter images of mars to remove the subjectivity involved in identifying terrain softening we apply photoclinometry to 20 viking orbiter images to derive topographic profiles across 86 martian impact craters the resulting measurements provide strong evidence that craters on terrain that has been designated softened or unsoftened by visual inspection are topographically distinct we quantitatively compare craters using four measurements the crater depth the degree to which crater walls are concave or convex upward the degree of rounding of crater rims and the crater rim height our results indicate that compared to unsoftened craters both simple and complex softened craters are characterized by a rounding of the crater rims and by crater wall slopes that are more convex softened complex craters are modestly shallower than unsoftened complex craters with a fractionally greater drop in the rim height than in the bowl depth softened simple craters are much shallower than unsoftened simple craters with a fractionally greater drop in the bowl depth than in the rim height the observed crater morphologies are consistent with the interpretation that terrain softening results from the relaxation of a deformable regolith layer of restricted thickness a comparison with simple theoretical modeling results suggests that this layer is approximately 1 km in thickness

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

reply to the comment on astronomical constraints on the duration of the early jurassic pliensbachian stage and global climatic fluctuations earth planet sci lett 455 2016 149 165

extensional tectonics on sardinia italy insights into the arc back arc transitional regime although the tectonic features and stress regime typical for accretionary complexes and back arc domains have been widely documented so far few are known on the transitional zone separating these two systems here we report on structural analysis and anisotropy of magnetic susceptibility ams results from eocene pliocene sediments exposed in western sardinia from late oligocene to middle miocene the studied area was located between the alpine apennine wedge to the east which was undergoing shortening and accretion and the liguro provencal basin undergoing extension and spreading we find that prior to the formation of the liguro provencal basin the middle eocene lower oligocene sediments cropping out at the southwesternmost edge of sardinia were subjected to ne sw shortening in present day coordinates in agreement with recently reported geological information conversely the upper oligocene pliocene sedimentary sequences record a different evolutionary stage of extensional processes upper oligocene middle upper burdigalian sediments clearly show a n s oriented magnetic lineation that can be related to extensional direction along the prevalent e w oriented normal faults on the other hand no magnetic lineation has been detected in upper burdigalian serravallian sediments which mark the end of the first rifting process in sardinia which likely coincides with the rift to drift transition at the core of the liguro provencal basin finally a ne sw extension is observed in two tortonian pliocene sites at the northwestern margin of the nnw sse oriented campidano graben our study confirms that ams may represent a valuable strain trajectory proxy and significantly help to unravel the characters of temporally superimposed tectonic events

geochronology of the medium to high grade metamorphic units of the peloritani mts sicily the peloritani mountains are a fragment of an orogen variably attributed to the alpine or hercynian orogeny on the basis of 39ar 40ar u pb and rb sr dating the main metamorphism of the two medium high grade metamorphic units the mela and aspromonte units and most of the thrusting responsible for stacking the orogenic edifice are seen to be hercynian the main thrusting of the aspromonte unit over the lower grade units took place at 301 2 ma brittle deformation during tertiary reactivation of hercynian thrust planes did not generate any rejuvenation of white micas in the studied sector our dataset shows a great complexity and we propose to unravel it by considering different levels of information to first order the mela and aspromonte units differ in their metamorphic paths and their geochronological evolution the mela unit shows generally younger ages carboniferous than the aspromonte unit and unlike the latter was extensively retrogressed in greenschist facies the aspromonte unit is itself geochronologically heterogeneous proterozoic ages are preserved both in titanite and in amphibole relics of one tectonic subunit devonian to carboniferous amphibole ages are found in different other subunits tertiary overprint is minor and spatially limited we propose to consider the chronologically heterogeneous subunits as accreted pre hercynian terranes amalgamated late during the hercynian orogeny micas in both units give scattered mesozoic 39ar 40ar and rb sr ages with evidence for heterochemical mica generations we interpret them as a result of widespread hydrothermal circulation event s tertiary overprint is generally absent with the exception of a small area near messina where biotite and muscovite underwent a complex recrystallisation history in the interval between 48 and 61 ma

40ar39ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the klokken syenite south greenland laser probe andin vacuo crushing have been used to apply the40ar39ar technique to microtexturally well characterised pristine and turbid alkali feldspar from the 1166 ma klokken syenite individual crystals are several millimetres in size but sem and tem studies reveal complex microstructures which define a range of sub grain sizes over three orders of magnitude from 100 m in pristine areas with fine scale cryptoperthite down to 200 nm in turbid areas with coarsened perthites and numerous micropores laser probe40ar39ar measurements of turbid feldspar gave low apparent ages indicating significant40ar loss 40 average which can be accounted for by sustained heating of the small sub grains 1 m for 1166 ma at low temperature 150 c pristine feldspar gives high apparent ages indicating the presence of excess40ar this component was readily lost from the feldspar by laboratory heating at low temperature 600 c and was also released by crushing which indicates that it was released from fluid inclusions during crushing release of excess40ar is correlated with cl release and the40ar cl ratio of the fluid is similar to that of mantle fluids turbid feldspar also released a fluid during crushing but the40ar cl ratio was lower probably as a result of evolution of the original fluid during boiling anomalously high apparent ages are often a feature of low temperature ar release in stepped heating age spectra of feldspars the presence of excess40ar in fluid inclusions may substantially affect the lower temperature release in feldspar stepped heating spectra the character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

direct evidence of middle oligocene extension in the calabria peloritani terrane from co seismic faulting the pseudotachylyte bearing shear zones of palmi southern calabria italy the variscan crystalline basement of the calabria peloritani terrane cpt in southern italy was partly reworked by ductile and brittle shear zones throughout the alpine tectonic evolution from thickening to exhumation although evidence of extensional tectonics in the cpt has already been found and roughly constrained to the oligocene onward no attempt has ever been made to directly date brittle fault movements structural meso and micro scale kinematic and petrographic analyses and 40ar 39ar laser experiments reveal that the pseudotachylyte bearing shear zones of the palmi area in southern calabria formed in response to extensional shearing 33 5 ma ago and overprinted compressional tectonic structures results provide the first direct evidence of middle oligocene co seismic faulting in the area and confirm the role of extensional tectonics in promoting the oligocene exhumation of the calabria basement

history of subduction and back arc extension in the central mediterranean summary geological and geophysical constraints to reconstruct the evolution of the central mediterranean subduction zone are presented geological observations such as upper plate stratigraphy hp lt metamorphic assemblages foredeep trench stratigraphy arc volcanism and the back arc extension process are used to define the infant stage of the subduction zone and its latest back arc phase based on this data set the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross sections from the northern apennines and from calabria to the gulf of lyon can be derived further the reconstruction is used to unravel the main evolutionary trends of the subduction process results of this analysis indicate that 1 subduction in the central mediterranean is as old as 80 myr 2 the slab descended slowly into the mantle during the first 20 30 myr subduction speeds were probably less than 1 cm year x1 3 subduction accelerated afterwards producing arc volcanism and back arc extension and 4 the slab reached the 660 km transition zone after 60 70 myr this time dependent scenario where a slow initiation is followed by a roughly exponential increase in the subduction speed can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull finally the third stage is controlled by the interaction between the slab and the 660 km transition zone in the southern region this results in an important re shaping of the slab and intermittent pulses of back arc extension in the northern region the decrease in the trench retreat can be explained by the entrance of light continental material at the trench

evidence for excess argon during high pressure metamorphism in the dora maira massif western alps italy using an ultra violet laser ablation microprobe 40 ar 39 ar technique ultra high pressure eclogite amphibolite grade metamorphism of the dora maira massif in the western alps is a well established and intensively studied event however the age of peak metamorphism and early cooling remains controversial the 40ar 39 ar step heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 ma which have been interpreted as the time of early cooling after the high pressure event recent u pb and sm nd results challenge this assertion indicating a much younger age for the event around 45 ma and hence a radically different timing for the tectonic evolution of the western alps in a new approach to the problem samples from the undeformed hercynian metagranite brossasco were studied using an ultra violet laser ablation microprobe technique for 40ar 39 ar dating the new technique allowed selective in situ analysis at a spatial resolution of 50 urn of quartz phengite biotite and k feldspar the results demonstrate the frequent occurrence of excess argon with high 40ar 39 ar ratios 1000 10000 and a strong relationship between apparent ages and metamorphic textures the highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases the best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event then mixed with an atmospheric component during rapid cooling and retrogression producing a wide range of argon concentrations and 40ar 36ar ratios step heating analysis of minerals with this mixture would produce linear arrays on a40ar 36ar versus 40ar 36 ar correlation diagram leading to geologically meaningless plateau ages older than the true closure age in the present case some ages in the range 60 110 ma could be explained by the presence of excess argon incorporated around 40 50 ma ago similar results found in other high pressure terrains in the alps may reconcile the argon geochronometer with other systems such as rb sr u pb or sm nd this study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40ar 36 ar ages in many high pressure terrains

excess argon in phengite from eclogite evidence from dating of eclogite minerals by smnd rbsr and 40ar39ar methods the existence of excess ar in phengites from coesite bearing ultrahigh pressure eclogites of the dabie mountains and su lu terrain china has been confirmed by dating of minerals using sm nd rb sr and 40ar39ar methods 40ar39ar ages of phengites for two samples studied are much older than the sm nd and rb sr mineral isochron ages sample ql 1 has rb sr age of 219 5 0 5 ma sm nd age of 226 3 4 5 ma and phengite 40ar39ar age of 877 5 15 9 ma the corresponding ages for sample zb 4 are 223 9 0 9 ma rb sr 228 4 6 0 ma sm nd and 943 3 12 4 ma 40ar39ar phengite an age of 225 ma based on the sm nd and rb sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the north china and yangtse blocks and formation of these eclogites we call for greater caution in 40ar39ar dating of phengites from other occurrences of ultrahigh pressure environments

crater relaxation on ganymede implications for ice rheology controversy has existed over whether or not viscous relaxation is an important process on the icy satellites previous models involved large extrapolations of newtonian flow laws for ice whereas ice is known to exhibit non newtonian behavior recently the flow law parameters for ice at the appropriate temperatures and stresses have been measured numerical modelling of the viscous relaxation of basins on ganymede using these parameters has given implausibly short relaxation times however this model treated ice as a purely viscous substance so that no elastic lithosphere could develop near the surface here we treat ice as a maxwell visco elastic material and numerically model the relaxation of basins on ganymede we find that realistic young s moduli lead to little relaxation occurring in basins even 4 0 ga after their formation further we demonstrate that within broad limits the near surface temperature gradient has little effect on this result finally through examination of the distribution of maxwell times in the vicinity of the crater we show that most viscous relaxation occurs early in the basin s history when stresses are high and thus maxwell time is short as stresses are relieved the maxwell time becomes long and relaxation essentially ceases

geomorphic evidence for the distribution of ground ice on mars high resolution viking orbiter images show evidence for quasi viscous relaxation of topography the relaxation is believed to be due to creep deformation of ice in near surface materials the global distribution of the inferred ground ice shows a pronounced latitudinal dependence the equatorial regions of mars appear to be ice poor while the heavily cratered terrain poleward of 30 latitude appears to be ice rich the style of creep poleward of 30 varies with latitude possibly due to variations in ice rheology with temperature the distribution suggests that ice at low latitudes which is not in equilibrium with the present atmosphere has been lost via sublimation and diffusion through the regolith thereby causing a net poleward transport of ice over martian history

fission track analysis of the crystalline basement rocks of the calabrian arc southern italy evidence of oligo miocene late orogenic extension and erosion abstract fission track analysis is used to provide low temperature and time constraints on the late orogenic cooling and exhumational history of the crystalline basement rocks of the calabrian arc southern italy 65 samples yielded 57 apatite fission track ages 54 zircon fission track ages and 25 apatite confined fission track length distributions interpretation of this data reveals a phase of increased cooling rates related to exhumation between approximately 35 ma mid oligocene and 15 ma middle miocene evidence from the sedimentary record indicates significant oligo miocene erosion related to this period of exhumation new chronological constraints are also applied to localised late orogenic extensional tectonism that has recently been identified within the basement rocks of the calabrian arc this information is used to produce a new model of the oligo miocene tectonic evolution of the calabrian arc it proposes that the crystalline basement rocks were part of a critical orogenic wedge between the mid oligocene and the middle miocene

high cooling and denudation rates at kongur shan eastern pamir xinjiang china revealed by 40ar 39ar alkali feldspar thermochronology orthogneiss samples taken from the kongur antiform show ages varying from 2 ma to 1 ma for 40ar 39ar ages of biotites and muscovites and fission tracks on apatites leading to cooling rates of 150 c m y modeling of k feldspars highlights the effect of a range of diffusion domains with contrasting diffusion characteristics yielding closure temperatures from 400 to 150 c the feldspar data document the cooling history since 5 ma and indicate a sudden change in cooling rates of the antiform at 2 ma at that time cooling increases by a factor of 5 from an average of 20 c m y to a minimum of 150 c m y consideration of the regional thermal history ongoing uplift and erosional history of the antiform during the quaternary suggests that denudation rates have been of the order of 5 7 km m y since 2 m y ago and could be associated with significant upward surface movement triggered by major normal faulting the antiform is interpreted to have formed during thrusting at the pamir front as a result of the development of thrust ramps and normal faulting at the crustal scale ramp stacking is an important process of mountain building and normal faulting in this context must be regarded as a very efficient way of building high relief

sources of error in planetary photoclinometry photoclinometry has become a popular technique for the extraction of topography from digital images of planetary surfaces the technique however is subject to a number of error sources that can degrade results significantly we present here a quantitative analysis of error sources in one dimensional planetary photoclinometry the technique is affected by at least seven error sources which can be broken down into three categories those arising from 1 the spacecraft image 2 the planetary body and 3 the scan line orientation spacecraft image error sources include noise background offset and quantization errors deriving from the planetary body include those induced by variations in photometric properties such as albedo by incorrectly compensated atmospheric effects and by an incorrectly determined photometric function finally errors will result for scan lines which do not lie perpendicular to topographic strike we calculate slope errors for each of these sources using the examples of voyager imaging of ganymede and viking orbiter imaging of mars slope errors are investigated for a variety of viewing and lighting geometries slope angles and slope orientations the results can be broken down into nonsystematic and systematic errors nonsystematic errors are introduced by image noise and quantization and affect the slope calculation for each picture element independently other error sources are systematic these errors are more insidious since they may retain the general appearance of the topography while approximately scaling all relief by a multiplicative constant we present derivations that allow the calculation of photoclinometric slope errors for any photometric function and also briefly discuss the implications of our results for two dimensional photoclinometric techniques

crenulation cleavage formation evolving diffusion deformation and equilibration mechanisms with increasing metamorphic grade abstract an integrated microstructural and chemical study of crenulation cleavages developed in chlorite to garnet zone schists from the wenchuan maowen shear zone central china has resulted in the recognition of the deformation and equilibration mechanisms operating during their formation at a range of temperatures deformation during crenulation cleavage formation at chlorite to biotite zone temperatures 520 c is essentially a solution deposition process in which transport occurred by a combination of localized advection and grain boundary diffusion recrystallization and hence equilibration of muscovite in discrete cleavages is recognized by distinct compositional differences between muscovite from different domains crenulation cleavages developed in garnet zone schist are characterized by homogeneous muscovite compositions across domains and microstructures related to intracrystalline deformation indicating an increasing importance of volume diffusion and diffusive equilibration of mica at temperatures greater than 520 c

comment on astronomical constraints on the duration of the early jurassic pliensbachian stage and global climatic fluctuations earth planet sci lett 455 2016 149 165

global distribution and migration of subsurface ice on mars a thermal diffusive model of h2o kinetics and equilibrium was developed to investigate the long term evolution and depth distribution of subsurface ice on mars the model quantitatively takes into account 1 obliquity variations 2 eccentricity variations 3 long term changes in the solar luminosity 4 variations in the argument of subsolar meridian in planetocentric equatorial coordinates 5 albedo changes at higher latitudes due to seasonal phase changes of co2 and the varying extent of co2 ice cover 6 planetary internal heat flow 7 temperature variations in the regolith as a function of depth time and latitude due to the above factors 8 atmospheric pressure variations over a 104 year time scale 9 the effects of factors 1 through 5 on seasonal polar cap temperatures and 10 knudsen and molecular diffusion of h2o through the regolith the migration of h2o into or out of the regolith is determined by two boundary conditions the h2o vapor pressure at the subsurface ice boundary and the annual average h2o concentration at the base of the atmosphere these are controlled respectively by the annual average regolith temperature at the given depth and seasonal temperatures at the polar cap starting from an arbitrary initial uniform depth distribution of subsurface ice h2o fluxes into or out of the regolith are calculated for 100 selected obliquity cycles each representing a different epoch in mars history the h2o fluxes are translated into ice thicknesses and extrapolated over time to give the subsurface ice depth as a function of latitude and time the results show that obliquity variations influence annual average regolith temperatures in varying degrees depending on latitude with the greatest effect at the poles and almost no effect at 40 lat insolation changes at the pole due to obliquity argument of subsolar meridian and eccentricity variations can produce enormous atmospheric h2o concentration variations of 6 orders of magnitude over an obliquity cycle superimposed on these cyclic variations is a slow monotonic change due to the increasing solar luminosity albedo changes at the polar cap due to seasonal phase changes of co2 and the varying thickness of the co2 ice cover are critically important in determining annual average atmospheric h2o concentrations despite the strongly oscillating character of the boundary conditions only small amounts of h2o are exchanged between the regolith and the atmosphere per obliquity cycle 10 g cm2 the net result of h2o migration is that the regolith below 30 40 lat is depleted of subsurface ice while the regolith above 30 40 lat contains permanent ice due to the depth of penetration of the annual thermal wave this result is supported by recent morphological studies the rate of migration of h2o is strongly dependent on average pore capillary radius for which we have assumed values of 1 and 10 m we estimate that the h2o ice removed from the regolith would produce a permanent ice cap with a volume between 2 106 and 6 106 km3 this generally agrees with estimates deduced from deflationary features at lower latitudes depositional features at higher latitudes and the mass of the polar caps

the liguride complex of southern italy a cretaceous to paleogene accretionary wedge abstract the ophiolite bearing allochthonous flysch liguride complex of the southern apennines italy has traditionally been divided into two tectonic units the metamorphic frido unit and the unmetamorphosed cilento unit the two units have hitherto been considered to derive from two distinct paleogeographic domains however the identification of cilento unit sediments within frido unit successions implies that the sediments of both units were coeval deposits within the same sedimentary basin cilento unit sediments also occur as the sedimentary cover of kilometre scale oceanic slices within the liguride complex indicating deposition on oceanic basement and not continental basement as was previously considered thrust transport directions and sediment provenance indicate that during late jurassic time this ocean basin lay to the east of the calabrian terrain and to the west of apulia calabria and apulia representing the european and african margins of neotethys respectively northwestwards subduction of oceanic crust beneath the calabrian terrain from late cretaceous time onwards produced an accretionary wedge which was later emplaced onto the apulian margin during the burdigalian collision of calabria and apulia the liguride complex represents the obducted remains of this accretionary wedge this interpretation contains three important implications for pre tertiary plate tectonic reconstructions of the western mediterranean region these are 1 1 the calabrian continental terrain formed part of the iberian plate on the north margin of neotethys this may have implications for the former location and continuation of the north pyrenean fault 2 2 the existence of a continuation of the eo alpine belt through calabria and the kabylies is placed in doubt thus requiring only one constant polarity nw dipping subduction direction to explain the structures now seen in these regions 3 3 the existence of a transform fault between the europe vergent structures of corsica and the africa vergent structures of calabria this transform must have been active from at least late cretaceous time in a position now occupied by the tyrrhenian sea

geochronological constraints on the timing of granitoid magmatism metamorphism and post metamorphic cooling in the hercynian crustal cross section of calabria exposed cross sections of the continental crust are a unique geological situation for crustal evolution studies providing the possibility of deciphering the time relationships between magmatic and metamor phic events at all levels of the crust in the cross section of southern and northern calabria u pb rb sr and k ar mineral ages of granulite facies metapelitic migmatites peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late hercynian peak metamorphism and granitoid magmatism as well as on the post metamorphic cooling monazite from upper crustal amphibolite facies paragneisses from southern calabria yields similar u pb ages 295 293 4 ma to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust 300 10 ma monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303 302 0 6 ma zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290 2m a which may be interpreted as the minimum age for metamorphism or intrusion u pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern calabria sila also point to a near synchronism of peak metamorphism and intrusion at 304 300 0 4 ma at the end of the granulite facies metamorphism the lower crustal rocks were uplifted into mid crustal levels 10 15 km followed by nearly isobaric slow cooling c 3 cm a 1 as indicated by muscovite and biotite k ar and rb sr data between 210 4 and 123 1 ma the thermal history is therefore similar to that of the lower crust of southern calabria in combination with previous petrological studies addressing metamorphic textures and p t conditions of rocks from all crustal levels the new geochronological results are used to suggest that the thermal evolution and heat distribution in the calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late hercynian orogeny

when did sardinia stop rotating new palaeomagnetic results previous palaeomagnetic results from tertiary volcanic rocks of sardinia suggest that the island underwent a quick counterclockwise rotation between 20 5 and 19 ma in order to test this hypothesis a new palaeomagnetic study was carried out on sediments and volcanic rocks with well controlled biostratigraphic or radiometric ages younger than the suggested age for the end of the rotation unrotated palaeomagnetic directions obtained from the upper ignimbrites radiometric age 18 5 19 7 myr and the aquitanian tuffs collected in the anglona region n sardinia would suggest that the rotation ended earlier than 20 5 myr the directions found in the volcanics are perhaps the result of secular variation rather than differential rotation of the anglona region with respect to surrounding regions two sites belonging to the upper ignimbritic unit si2 in the monte traessu monte rispisu area logudoro region exhibited a large amount of rotation d 320 4 i 60 which may be age related radiometric age 19 9 myr four dikes radiometric age 18 3 16 7 myr in the arcuentu volcanic complex arburese region yield results d 348 4 i 33 8 that indicate a further amount of rotation after their emplacement stable end point directions were successfully isolated from three upper burdigalian langhian sedimentary sites biostratigraphic age 18 15ma from the marmilla and logudoro regions consistent paleomagnetic directions obtained from these sites d 352 i 49 indicate that a small amount of rotation was still to be completed in the langhian time this result is consistent with the history of sardinian volcanism which did not end until 13 ma

evolution of the sardinia channel western mediterranean new constraints from a diving survey on cornacya seamount off se sardinia abstract sarcya 1 dive explored a previously unknown 12 my old submerged volcano labelled cornacya a well developed fracturation is characterised by the following directions n 170 to n s n 20 to n 40 n 90 to n 120 n 50 to n 70 which corresponds to the fracturation pattern of the sardinian margin the sampled lavas exhibit features of shoshonitic suites of intermediate composition and include amphibole and mica bearing lamprophyric xenoliths which are geochemically similar to ti poor lamproites mica compositions reflect chemical exchanges between the lamprophyre and its shoshonitic host rock suggesting their simultaneous emplacement nd compositions of the cornacya k rich suite indicate that continental crust was largely involved in the genesis of these rocks the spatial association of the lamprophyre with the shoshonitic rocks is geochemically similar to k rich and tio2 poor igneous suites emplaced in post collisional settings among shoshonitic rocks sample sar 1 01 has been dated at 12 6 0 3 my using the 40ar 39ar method with a laser microprobe on single grains the age of the cornacya shoshonitic suite is similar to that of the sisco lamprophyre from corsica which similarly is located on the western margin of the tyrrhenian sea thus the cornacya shoshonitic rocks and their lamprophyric xenolith and the sisco lamprophyre could represent post collisional suites emplaced during the lithospheric extension of the corsica sardinia block just after its rotation and before the tyrrhenian sea opening drilling on the sardinia margin odp leg 107 shows that the upper levels of the present day margin hole 654 suffered tectonic subsidence before the lower part hole 652 the structure of this lower part is interpreted as the result of an eastward migration of the extension during late miocene and early pliocene times data of cornacya volcano are in good agreement with this model and provide good chronological constraints for the beginning of the phenomenon

a new proposal for the late cenozoic geodynamic evolution of the western mediterranean a simple mechanism of arcuate fold belt and back arc basin formation is presented based on the opening of mega continental tension gashes along pre existing deep parallel and steep faults that separate lithospheric units with different properties if plate convergence is parallel to these faults the fault bounded units open at right angles to the convergence vector adopting an arcuate shape with thrusting in front of the bowed out units and extensional basin opening between the separated units this model is applied to the cenozoic geodynamic evolution of the western mediterranean after the iberian collision 35 30 ma several ellipsoidal basins valencia alboran north algerian and liguro provencal developed by 10 ma along the eastern margin of the iberian plate the formation of these basins is attributed to an increase in ne sw horizontal tectonic palaeostress during early middle miocene times resulting from the post subduction collision of the tethyan oceanic lithosphere

mid ordovician u pb ages of porphyroids in the peloritan mountains ne sicily palaeogeographical implications for the evolution of the alboran microplate petrographic geochemical and geochronological work on a metavolcanic suite interbedded in a metapelitic sequence of the palaeozoic basement in the peloritan mountains sicily is used to contribute to a better understanding of the pre variscan geodynamic evolution of the former alboran microplate major and trace element data show that the volcanic suite consists of two main groups an intermediate unit of andesites and dacites with volcanic arc affinity and a felsic porphyroid unit of rhyolites with peraluminous composition isotope dilution thermal ionization mass spectrometry u pb analyses for single and multigrain zircon fractions from four samples of the felsic porphyroids yield results that are tightly clustered on or near concordia giving caradoc ages in the range of 456 452 ma the consistent morphological characteristics and oscillatory zoning of the zircons indicate single stage magmatic crystallization xenocrystic zircon grains with ages of 1150 and 2015 ma indicate that these rocks were developed upon a cratonic basement with grenvillian and transamazonian components hence supporting a peri gondwanan origin from the combined evidence we conclude that this portion of the alboran microplate formed as a mid ordovician volcanic arc in a sector of the peri gondwanan terranes that was affected by a tectonothermal caradoc event

tectonic history of the syria planum province of mars we attribute most of the development of extensive fractures in the tharsis region to discrete tectonic provinces within the region rather than to tharsis as a single entity one of these provinces is in syria planum faults and collapse structures in the syria planum tectonic province on mars are grouped into 13 sets based on relative age areal distribution and morphology according to superposition and fault crosscutting relations and crater counts we designate six distinct episodes of tectonic activity in the following chronologic order stage i is an early to late noachian deformation forming mostly east west structures fault set ia some large volcanoes also formed faults were produced possibly by flexural uplift also arcuate north trending grabens set ib indicate that faulting is transitional to the next stage stage ii is late noachian to early hesperian radial faulting centered in syria planum possibly due to isostatic uplift in late noachian set iia to early hesperian set iib time stage iii is early to late hesperian faulting tangential to syria planum that was related to local centers of uplift sets iii1 iii3 on the periphery of syria planum stage iv is a late hesperian graben formation that was circumferential to syria planum set iva caused either by collapse associated with eruption of magma or by flexure of the lithosphere due to volcanic loading in association with volcanism minor faulting occurred producing an oval pattern of faults in southwest syria planum set ivbl and a radial pattern south of the planum set ivb2 that apparently rejuvenated buried stage ii faults stage v is a late hesperian to early amazonian development of grabens and troughs of noctis labyrinthus and western valles marineris set v that was probably instigated by local uplift exposure of groundwater or ground ice zones may have produced further collapse and trough enlargement stage vi is early amazonian northwest trending faulting in noctis fossae set vil perhaps due to tharsis monies centered tectonism and north northwest normal faulting along the eastern side of the claritas rise set vi2 that was due to tectonic subsidence the duration of tectonic activity in the syria planum province was perhaps 2 3 b y photoclinometric topographic profiles across 132 grabens and fault scarps show that syria planum grabens have widths average of 2 5 km and most range from 1 to 6 km similar to lunar grabens but the martian grabens have slightly higher side walls average about 132 m and gentler wall slopes average of 9 and range of 2 25 than lunar grabens 93 m high and 18 slopes scarp degradation on mars has progressed through quakes impact shaking and dry slope processes the lower slopes may be due to mars higher gravity estimates of the amount of extension for individual grabens range from 20 to 350 m most estimates of the thickness of the faulted layer range from 0 5 to 4 5 km average is 1 5 km this thickness range corresponds closely to the 0 8 to 3 6 km range in depth for pits troughs and canyons in noctis labyrinthus and along the walls of valles marineris we propose that the predominant 1 to 1 5 km values obtained for both the thickness of the faulted layer and the depths of the pits troughs and theater heads of the canyons reflect the initial depth to the water table in this region as governed by the depth to the base of ground ice maximum depths for these features may indicate lowered groundwater table depths and the base of ejecta material

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

the cyclostratigraphy intercomparison project cip consistency merits and pitfalls abstract cyclostratigraphy is an important tool for understanding astronomical climate forcing and reading geological time in sedimentary sequences provided that an imprint of insolation variations caused by earth s orbital eccentricity obliquity and or precession is preserved milankovitch forcing numerous stratigraphic and paleoclimate studies have applied cyclostratigraphy but the robustness of the methodology and its dependence on the investigator have not been systematically evaluated we developed the cyclostratigraphy intercomparison project cip to assess the robustness of cyclostratigraphic methods using an experimental design of three artificial cyclostratigraphic case studies with known input parameters each case study is designed to address specific challenges that are relevant to cyclostratigraphy case 1 represents an offshore research vessel environment as only a drill core photo and the approximate position of a late miocene stage boundary are available for analysis in case 2 the pleistocene proxy record displays clear nonlinear cyclical patterns and the interpretation is complicated by the presence of a hiatus case 3 represents a late devonian proxy record with a low signal to noise ratio with no specific theoretical astronomical solution available for this age each case was analyzed by a test group of 17 20 participants with varying experience levels methodological preferences and dedicated analysis time during the cip 2018 meeting in brussels belgium the ensuing analyses and discussion demonstrated that most participants did not arrive at a perfect solution which may be partly explained by the limited amount of time spent on the exercises 4 5 hours per case however in all three cases the median solution of all submitted analyses accurately approached the correct result and several participants obtained the exact correct answers interestingly systematically better performances were obtained for cases that represented the data type and stratigraphic age that were closest to the individual participants experience this experiment demonstrates that cyclostratigraphy is a powerful tool for deciphering time in sedimentary successions and importantly that it is a trainable skill finally we emphasize the importance of an integrated stratigraphic approach and provide flexible guidelines on what good practices in cyclostratigraphy should include our case studies provide valuable insight into current common practices in cyclostratigraphy their potential merits and pitfalls our work does not provide a quantitative measure of reliability and uncertainty of cyclostratigraphy but rather constitutes a starting point for further discussions on how to move the maturing field of cyclostratigraphy forward

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

sense of overthrust shear in the alpine nappes of calabria southern italy abstract southern italy consists tectonically of ophiolite and basement nappes thrust over the apenninic sedimentary nappes whilst all more recent authors agree that the sediments of the apenninic nappes were deposited on apulian basement i e on african continental crust and that the ophiolites were associated with the oceanic basement of the mesozoic tethys the provenance of the basement nappes is still debated new data based on microstructural criteria have shown that the main shear sense of the ophiolite nappes and of the overlying basement nappes in northern calabria is from west to east in today s co ordinate system the basement nappes might not therefore be of austroalpine african provenance but could be of european origin

sidewall crystallization in the klokken intrusion zoned ternary feldspars and coexisting minerals the syenitic layered series in the klokken intrusion is surrounded by a zone 500 m thick of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro 200 m thick at the outer rim the unlaminated syenite is intrusive into the gabbro and develops a thin 2 m transition zone of syenodiorite at the contact a traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill mafic silicates olivine clinopyroxene biotite show inward evolution in fe fe mg across the syenodiorite unlaminated syenite zones feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase in some cases together with two alkali feldspars one a mesoperthite or cryptomesoperthite the other a cryptoperthite to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals plagioclase is absent from the layered series

map view restoration of aegean west anatolian accretion and extension since the eocene 1 the aegean region greece western turkey is one of the best studied continental extensional provinces here we provide the first detailed kinematic restoration of the aegean region since 35 ma the region consists of stacked upper crustal slices nappes that reflect a complex paleogeography these were decoupled from the subducting african adriatic lithospheric slab especially since 25 ma extensional detachments cut the nappe stack and exhumed its metamorphosed portions in metamorphic core complexes we reconstruct up to 400 km of trench perpendicular ne sw extension in two stages from 25 to 15 ma the aegean forearc rotated clockwise relative to the moesian platform around euler poles in northern greece accommodated by extensional detachments in the north and an inferred transfer fault se of the menderes massif the majority of extension occurred after 15 ma up to 290 km by opposite rotations of the western and eastern parts of the region simultaneously the aegean region underwent up to 650 km of post 25 ma trench parallel extension leading to dramatic crustal thinning on crete we restore a detachment configuration with the mid cycladic lineament representing a detachment that accommodated trench parallel extension in the central aegean region finally we demonstrate that the sakarya zone and cretaceous ophiolites of turkey cannot be traced far into the aegean region and are likely bounded by a pre 35 ma n s fault zone this fault became reactivated since 25 ma as an extensional detachment located west of lesbos island the paleogeographic units south of the izmir ankara sava suture however can be correlated from greece to turkey

a seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite facies rocks of calabria southern italy pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses metamonzodiorites as well as felsic and mafic granulites at the base of the section of the hercynian lower crust exposed in calabria southern italy the crustal section is tectonically superposed on lower grade units ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses whereas sparse fault related pseudotachylytes and thin mylonite ultramylonite bands occur in granulite facies rocks the presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high temperature conditions preceded the formation of pseudotachylytes we have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust different protoliths were selected to test how lithology controls pseudotachylyte composition and textures in migmatites and felsic granulites euhedral or cauliflower shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition this indicates that pseudotachylytes originated at deep crustal conditions 0 75 gpa in mafic protoliths quenched needle to feather shaped high alumina orthopyroxene occurs in contact with newly crystallized plagioclase the pyroxene crystallizes in garnet free and garnet bearing veins the simultaneous growth of orthopyroxene and plagioclase as well as almandine suggests lower crustal origin with pressures in excess of 0 85 gpa the existence of melts of different composition in the same vein indicates the stepwise non equilibrium conditions of frictional melting melt formed and intruded into pre existing anisotropies in mafic granulites brittle faulting is localized in a previously formed thin high temperature mylonite bands migmatitic gneisses are deformed into ultramylonite domains characterized by s c fabric small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high strain rate leading to seismic events and melting

structural signature of tectonic processes in the calabrian arc southern italy evidence from the oceanic derived diamante terranova unit the diamante terranova unit is an oceanic derived element within the thrust and nappe structure of the northern sector of the calabrian arc its metamorphic and structural signature reveals a long history of se dipping subduction and collision related deformation of an oceanic lithospheric section underlying the narrow diamante terranova basin the deformation history of the diamante terranova unit may be related to three main tectonic phases the first one is characterized by a structural association which includes features that developed during a progressive deformation related to the evolution of a deep seated shear zone kinematic indicators characterizing subduction related structures clearly show a top to the northwest sense of overthrust shear the second tectonic phase that can be recognized in the area is characterized by structures that may be related to the onset of shear deformation induced by a postcollisional northeastward motion of the overriding plate the third tectonic phase is recorded by brittle features related to the latest tectonic events affecting the calabrian arc during the opening of the tyrrhenian sea shear sense indicators and available radiometric and stratigraphic information together with plate kinematic data allowed us to infer the time evolution of this sector of the calabrian arc and to assess that the diamante terranova unit cannot be considered as an eoalpine element since its deformation related to subduction collision processes occurred prior to the end of the cretaceous

alpine structural and metamorphic signature of the sila piccola massif nappe stack calabria italy insights for the tectonic evolution of the calabrian arc combined structural and petrographical investigations coupled with 40ar 39ar geochronology were carried out in the sila piccola massif of the calabrian arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence on the basis of the contrasting alpine pressure temperature p t and structural signatures the nappe stack can be divided in two major tectonic complexes bounded by a flat lying ductile to brittle extensional shear zone the upper complex consists of a nappe like structure where a major top to the east compressional shear is recorded the lower tectonic complex consists of an ophiolite bearing sequence showing typical high p low t parageneses mg carpholite and na amphibole the 40ar 39ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the oligocene eocene boundary around 35 ma ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks reworking the previous nappe contacts with shear localization along the upper lower tectonic complex discontinuity the 40ar 39ar dating indicates that this postnappe stacking tectonic evolution took place from 30 ma onward it is proposed that exhumation of the deep seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation synorogenic extension the age of this detachment is bracketed between 30 ma and the post orogenic neogene basin sedimentation middle upper miocene the revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction which involves an early high p low t top to the east crustal thickening episode during the construction of the apennine orogenic wedge eocene oligocene followed and overprinted by a top to the west extensional shear probably active from the late oligocene

neogene and quaternary rollback evolution of the tyrrhenian sea the apennines and the sicilian maghrebides reconstruction of the evolution of the tyrrhenian sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 ma it involved two episodes of back arc extension which were induced by the rollback of a west dipping subducting slab the first period of extension 10 6 ma was prominent in the northern tyrrhenian sea and in the western part of the southern tyrrhenian sea the second period of extension mainly affected the southern tyrrhenian sea began in the latest messinian 6 5 ma and has been accompanied by subduction rollback at rates of 60 100 km myr 1 slab reconstruction combined with paleomagnetic and paleogeographic constraints indicates that in the central apennines the latest messinian 6 5 ma arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike slip faults these processes resulted in the formation of a narrow subducting slab beneath the ionian sea that has undergone faster subduction rollback and induced extreme rates of back arc extension

ground ice on mars inventory distribution and resulting landforms abstract most 90 of the estimated original volume of outgassed water on mars cannot be satisfactorily accounted for by exospheric escape or storage in the atmosphere as frost or in the permanent north polar ice cap the balance may be stored as ground ice in the martian cryosphere a zone of permanently frozen ground that is protected from the atmosphere by a debris cover ground ice can exist throughout the entire cryosphere but it need not fill it if the ground ice does fill the cryosphere then excess water can exist in a confined aquifer the theoretical distribution of ground ice can be tested by identification of forms on the martian surface that may be related to the presence of subsurface ice the observed features that are most likely to reflect ground ice are thermokarst like pits and debris flows landforms with ambivalent origins include polygonally patterned ground lobate ejecta blankets craters with central pits and curvilinear features the most persuasive morphologic evidence for ground ice is thermokarst pits and debris flows the thermokarst pits are primarily located in the volcanic regions of tharsis and elysium the association of ice related features with these volcanic areas suggests that these forms are not directly latitude dependent activation by orbital variations could produce periodic multiple episodes of melting that are dependent upon latitude the presence of ice related features in both hemispheres and the equatorial region of mars indicates that ground ice may be or have been present over the entire planet as predicted by the cryosphere model

viscous relaxation of impact craters on icy planetary surfaces determination of viscosity variation with depth spacecraft images show that the icy galilean satellites have surfaces with very low topographic relief impact craters on ganymede and callisto are anomalously shallow and are characterized by sharp well defined rims and domed floors these morphological characteristics can be explained by viscous relaxation of topography on an icy crust in which the viscosity is uniform or decreases with depth under these conditions large craters relax more rapidly than small craters therefore explaining a possible underabundance of large craters viscous relaxation on an icy crust that is thin compared to the crater diameter or on a thick icy crust in which viscosity increases with depth could not produce this crater morphology and would result in the more rapid relaxation of small craters rather than large craters the results of this study suggest that more detailed analysis of relaxing impact crater morphology may resolve the rate of viscosity decrease with depth and so provide evidence on the interior thermal evolution of icy planetary bodies

late proterozoic grenville deformation in central new mexico determined from single crystal muscovite 40ar 39ar age spectra abstract 40 ar 39 ar age spectrum analyses on deformed and undeformed single crystal muscovites from the manzano mountains in central new mexico provide insights on the argon systematics of deformed muscovite the post 1 4 ga cooling history of the region and the timing of low temperature deformation microstructures in narrow east side up shear zones in and around the 1427 ma priest pluton indicate post pluton greenschist facies 350 c deformation single crystal 40 ar 39 ar analysis of three samples containing undeformed muscovite yield plateau spectra with ages of 1390 3 1395 2 and 1401 3 ma suggests that by this time the pluton had cooled below 350 400 c single crystal muscovites from within the shear zones have markedly different age spectra and reveal well defined age gradients from 1 1 to 1 4 ga the age gradients are interpreted as resulting from deformation induced argon loss or reduction of the effective radius for argon diffusion thereby decreasing the closure temperature of the originally pristine magmatic muscovites these data and previously published results suggest a long period of crustal stability ca 1 4 to 1 1 ga in which average cooling rates were probably about 1 c ma the 1 1 ga ages in the deformed muscovite age spectra are interpreted as indicating that grenvillian tectonism to the southeast affected central new mexico it can not be unambiguously determined if the consequences of the grenville orogeny in new mexico was uplift and erosion cooling a discrete thermal pulse reheating or if the shear zones record grenville age contractual deformation

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

tomographic constraints on the geodynamic evolution of the italian region in this paper we present p wave tomographic images of the mantle beneath italy obtained by inverting 6000 teleseismic p and pkp wave arrival times accurately repicked recorded in the time period 1988 1994 by the stations of the national seismic network of the istituto nazionale di geofisica we pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set the data were inverted with the well established aki christofferson husebye tomographic technique different reference models and residuals computation have been tried to verify the stability of the results the high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the alps and the apennines looking for their lateral and vertical continuity down to 800 km depth the main finding of this study is a continuous high velocity body located between 250 and 670 km depth beneath the entire apenninic system dipping toward the tyrrhenian area which continues upward segmented in two main anomalies in the northern apenninic and the calabrian arcs we interpret this high velocity feature as the subducted oceanic lithosphere between the eurasian and african plates dipping down to the upper lower mantle boundary beneath the tyrrhenian sea the retrieved images of the lithosphere subducting beneath apennines are reliable in terms of thickness about 80 90 km and p wave velocity contrast 2 4 higher than the normal mantle furthermore our tomographic images which focus on the deep geometry and continuity of the velocity structures provide new keys to understanding the geodynamic evolution of the italian region the segmentation of the high velocity slab upward suggests a complex evolution of the arc trench system and the initially continuous subduction of the ionian adriatic plate progressively developed in subordinate arcs probably due to lateral heterogeneity of the subducting lithosphere

new39ar 40ar ages of hercynian and alpine thermotectonic events in grande kabylie algeria abstract the grande kabylie massif is situated within the internal zone of the alpine chain of northern algeria new 39 ar 40 ar ages which have been obtained from several key areas within the massif are compared with previous u pb and rb sr data the distinct isotopic signatures obtained lead to the recognition of several units with different tectonometamorphic histories that were intimately imbricated during the alpine orogeny in the north three structurally bound zones occur that form a core complex which is tectonically overlain by a nappe of low grade and non metamorphosed rocks from north to south the core complex is composed of 1 1 the naciria massif composed of high temperature paragneisses intruded by peraluminous granites measurements on biotites and feldspars from these series indicate that high temperature post kinematic recrystallization of these minerals occurred ca 80 ma ago before a low temperature event that produced some argon loss at ca 27 ma ago 2 2 structurally underlying the naciria zone is the sidi ali bou nab massif which consists of biotite bearing micaschists intruded by a heterogeneously mylonitized hercynian granite biotites and muscovites yield convergent 39 ar 40 ar and rb sr ages at 25 ma for this mylonitization synchronous with southeasterly overthrusting 3 3 further south orthogneisses anatectic paragneisses marbles and minor kinzigites form a second high temperature gneissic complex as for the naciria massif 39 ar 40 ar data on biotites and muscovites indicate that isotopic closure occurred 80 ma ago above the core complex is a thick series of epizonal phyllites that are unconformably overlain by unmetamorphosed permo triassic red beds of the dorsale calcaire low grade metamorphism in the phyllitic series is dated at 295 315 ma and thus reflects the influence of hercynian metamorphism from these results we consider the phyllitic series and its dorsale calcaire cover to be allochthonous units thrust upon a high temperature metamorphic basement during the early miocene

miocene thrusting in the eastern sila massif implication for the evolution of the calabria peloritani orogenic wedge southern italy abstract alpine orogens in the central mediterranean region have revealed the concomitance of crustal extension in back arc domain and crustal shortening in frontal domain quantitative data of deformation in the frontal orogenic wedges are necessary to understand how the shortening extension pair evolves in terms of structures orogenic transport timing and exhumation rate this paper deals with kinematics and ages of the frontal thrust systems of the calabria peloritani arc italy exposed in the eastern sila massif we first present structural fieldwork onshore and offshore well log data and new apatite fission track aft thermochronology then we describe the structural architecture of the studied area as an ene verging stacking of thrust sheets involving basement units and syn orogenic sediments the aft study documents that thrust sheets entered the partial annealing zone from 18 ma to 13 ma this early middle miocene thrusting phase was coeval with exhumation of high pressure low temperature metamorphic rocks in the hinterland of the orogen coastal chain area mainly driven by top to the w extensional tectonics opposite kinematic shear senses contractional top to the e and extensional top to the w and different exhumation rates slow in the frontal more rapid in the hinterland are framed in a tectonic scenario of a critically tapered orogenic wedge during the eastward retreating of the apennine slab

on the post 25 ma geodynamic evolution of the western mediterranean during the neogene and quaternary western mediterranean geodynamics were dominated by the eastward migration of the apenninic arc and associated back arc basins the migration was controlled by retreat of the apenninic slab and was associated with boudinage of the lithosphere in the back arc area palaeo reconstruction of the kinematics of the arc suggests about 775 km of migration from the late oligocene to present along a transect from the gulf of lions to calabria a maximum of 135 km of n s converge occurred between africa and europe during the same time span the western mediterranean was thus mainly shaped by the migration of the slab related to west directed subduction it is hypothesized that minor n s convergence deformed the arc but was not the cause of its formation 1998 elsevier science b v all rights reserved