effect of the greenhouse gases co2 h2o so2 on martian paleoclimate there is general agreement that certain surface features on mars are indicative of the presence of liquid water at various times in the geologic past in particular the valley networks are difficult to explain by a mechanism other than the flow of liquid water it has been suggested in several studies that a thick co2 atmosphere on mars early in its history could have provided a greenhouse warming that would have allowed the flow of water either on the surface or just below the surface however we have examined this effect with a detailed radiation model and have found that if reduced solar luminosity early in the history of the solar system is taken into account even three bars of co2 will not provide sufficient greenhouse warming the addition of water vapor and sulfur dioxide both plausible gases that may have been emitted by martian volcanoes to the atmosphere also fail to warm the surface above 273k for reduced solar luminosity conditions the increase in temperature may be large enough however for the formation of these features by brines

metamorphic evolution of the sesia lanzo zone western alps time constraints from multi system geochronology the sesia lanzo zone is a polymetamorphic unit containing hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the alpine orogeny different parts of the unit record different stages on the p t deformation evolution allowing multi system isotopic studies to unravel the precise timing of the metamorphic history new rb sr white mica and u pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60 70 ma this substantially alters the common view of early to mid cretaceous eclogite facies metamorphism in this unit the new results are more consistent with the established geotectonic framework for the alpine orogeny since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension it is also more concordant with recent studies of other units that demonstrate post cretaceous high pressure metamorphism step heated 40ar 39ar analysis of phengites yields good plateaux giving ages older than the corresponding rb sr age such anomalously high ages indicate the presence of radiogenic argon rich fluids in the grain boundary network under the fluid pressure conditions acting during this high pressure metamorphic event the u pb sphene ages are variable in polymetamorphic rocks and show inheritance of older pb or sphene crystals into the high pressure event two monometamorphic assemblages yield concordant ages at 66 1 ma reflecting crystallisation of the eclogite facies assemblage the gneiss minuti complex gmc lies structurally below the eclogitic micaschists and its pervasive greenschist facies fabric yields tightly clustered rb sr white mica ages at 38 39 ma this greenschist event did not affect the majority of the emc the 40ar 39ar ages of micas formed at this time were very disturbed whereas micas surviving from an earlier higher pressure assemblage had their 40ar 39ar system reset the greenschist event did not strongly affect u pb systematics in hercynian age sphenes suggesting that the gmc did not uniformly suffer an eclogite facies metamorphism during the alpine cycle but was juxtaposed against the emc later in the orogeny this model still requires that the locus of deformation and metamorphism and possibly fluid flux moved outboard with time leaving the sesia lanzo basement as a shear bounded unreactive block within the orogenic wedge

tectonic evolution of the cenozoic olympic subduction complex washington state as deduced from fission track ages for detrital zircons

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

mars change in axial tilt due to climate the average tilt of mars equator with respect to its orbital plane may have increased significantly over the age of the solar system obliquity oscillations might have induced changes in the climate which altered the mass distribution and hence the solar torque on the planet viscous deformation attributable to loading by the large polar caps expected at low obliquity may have induced secular changes in the axial tilt earth like effective viscosities can account for virtually the entire present obliquity of 24 4 degrees thus the present average tilt of mars may not be primordial

exhumation of the central alps evidence from 40ar 39ar laserprobe dating of detrital white micas from the swiss molasse basin 40ar 39ar single grain laserprobe dating of detrital white micas from early oligocene to middle miocene 31 14 ma sedimentary rocks of the central swiss molasse basin reveals three distinct clusters of cooling ages for the hinterland two palaeozoic age clusters reflect cooling after the variscan orogeny with only limited reheating during the alpine orogeny the third tertiary age cluster reflecting late alpine cooling is restricted to sediments younger than 20 myr old micas with cooling ages 30 myr are interpreted to originate from the footwall of the simplon detachment fault thus representing formerly exposed upper levels of the present day lepontine metamorphic dome erosion of these levels is reflected by an increase of low grade metamorphic lithic grains in the sandstones this interpretation puts constraints on the timing of exhumation as well as on the evolution of the drainage pattern of the central alps

has climate changed the earth s tilt have the ice ages secularly altered the obliquity axial tilt of the earth over geologic time the waxing and waning of ice caps in response to obliquity oscillations plus mantle adjustment to the weight of the caps alter the earth s dynamical flattening this affects the lunar and solar torques on the earth so as to secularly change the earth s axial tilt this mechanism is dubbed climate friction since analogous to tidal friction it arises from lags in the earth s response to cyclic external forcing but the existence of two processes ice cap waxing and waning and mantle viscous flow can lead to either an increase or decrease in the obliquity evidence indicates that the growth and decay of the ice caps greatly lag the orbital forcing this causes the axial tilt to increase with time but the ice cap effects are partly canceled by viscous compensation in the mantle low mantle viscosities about 1021 pa s lead to rapid compensation and have only a slight effect on obliquity high viscosities about 1022 pa s slow the compensation enough so that there could be significant secular change in the obliquity over geologic time perhaps explaining all of the present tilt of 23 5 however in addition to the current uncertainty as to the effective viscosity of the mantle knowledge of past ice ages is incomplete so that the amount of obliquity increase presently remains unknown

deformation in a partially molten mantle constraints from plagioclase lherzolites from lanzo western alps abstract we studied the deformation in a partially molten mantle through detailed petrostructural analysis of plagioclase lherzolites from the northwestern part of the central lanzo massif these peridotites display a pervasive anastomosed network of millimetre to centimetre scale plagioclase rich bands which grades locally into a planar layering marked by an intercalation of plagioclase rich and olivine rich layers up to tens of centimetres wide both the anastomosed and the planar layering are subparallel to the deformation fabric characterized by a foliation defined by the shape preferred orientation of olivine and by crystal preferred orientation cpo of olivine orthopyroxene and clinopyroxene this parallelism the coexistence of microstructural evidence for melt rock reactions and for deformation by dislocation creep and the predominance of axial 010 olivine cpo patterns characterized by 010 axes clustered normal to the layering indicate that deformation and the reactive melt percolation that formed the layering were coeval strong heterogeneity in mineral composition in the planar layering domains with fe enrichment in olivine and spinel and highly variable ti content in spinel in cm scale plagioclase rich bands implies that the latter interacted with higher melt volumes high ti contents in pyroxenes and spinel in the anastomosed layering domains point to changes in melt composition hinting to less effective melt transport we propose therefore that the planar layering records melt segregation in layers parallel to the shear plane whereas the anastomosed layering results from melt alignment along grain boundaries subparallel to the shear plane without segregation finite strain in the different layers cannot be quantified but comparison of the present observations with data from experiments and from other mantle outcrops displaying shear controlled melt organization suggests that the transition from the anastomosed to the planar layering might record an increase in finite strain that is strain localization associated with variations in the instantaneous melt fraction

two stage erosion and deposition in a continental margin setting an 40ar 39ar laserprobe study of offshore detrital white micas in the norwegian sea detrital white micas from a restricted region of the norwegian sea have been analysed by the 40ar 39ar laserprobe examining the ages of individual white mica grains from offshore triassic jurassic and cretaceous sandstones reveals a narrow range of detrital white mica ages 369 424 ma moreover there is an inverse relationship between sediment age and detrital white mica age with the triassic sands containing a larger proportion of younger white micas and the cretaceous sands containing the oldest white micas in addition residence times of the detritus which is the age of the sediment subtracted from the age of the detrital white mica range from c 150 to c 300 ma for the triassic and cretaceous sands respectively the inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion deposition model involving at least two stages taking into account previous provenance studies in the lofoten region of the norwegian sea these new data suggest that detritus shed initially from the exhuming caledonian nappes of nw norway in late silurian early devonian times was deposited in intramontane basins that were themselves subsequently unroofed and re eroded from permo triassic times this caused the recycling of the caledonian detritus from the devono carboniferous basins and led to the inverted detrital age distribution ovserved in the triassic cretaceous sediments examined in this study while sedimentary recycling is not a surprising feature for mesozoic basins in the north and norwegian seas this study documents a previously untapped method 40ar 39ar laserprobe provenance studies by which to identify its effect and extent

a gravity model of the crust beneath the tertiary piemonte basin northwestern italy abstract a gravity model of a crustal section crossing the tertiary piemonte basin from north to south is presented it was constructed by combining geological data and the results of the application of several geophysical methods with particular reference to the seismic findings a model is proposed for the morphology of the deep crust crust mantle transition identified by the wide angle reflection seismic data from the european geotraverse egt project and its geological consequences are discussed the existence of substantial displacements at the crust mantle boundary is stressed they do not appear to be directly related to superficial structures but underscore the wedging of the ligurian mantle on the insubrian crust compared with the observed bouguer gravity field gravity modelling of the section has supplied a calculated anomaly with a persistent short wavelength negative residual anomaly curve in the area of the asti basin it is suggested that this anomaly is due to a wedge of sedimentary rocks within the crust the wedge consists of both apennine and padane units and is located along a major structural boundary where metamorphic crust and tertiary sediments belonging to the alpine domain overthrust apennine and padane units during the tertiary

the obliquity of mars and climate friction the mean obliquity of mars may have increased over geologic time due to climatic changes associated with obliquity oscillations this mechanism is dubbed climate friction given here is the mathematical theory of climate friction also a 10 m y numerical integration of the equations for a hypothetically large amount of climate friction is performed for the cases of the obliquity oscillations being 1 a single sinusoid and 2 a sum of three sinusoids theory and numerics agree in both cases within about 12 on the size of the secular increase in obliquity with most of the discrepancy coming from the theory giving obliquity amplitudes 10 too large further one possible mechanism of climate friction is investigated here postglacial rebound on mars the idea is that giant polar caps form when the obliquity is low these caps slowly squeeze out an equatorial bulge when the obliquity is high the caps disappear but the bulge takes some time to collapse due to mantle viscosity thus the equatorial bulge oscillates but is out of phase with the obliquity oscillations like tidal friction this causes a secular increase in the average obliquity using the preferred climate model of francois et al 1990 the total secular increase in the obliquity over the age of the solar system can amount to about 10 if mars mantle has an earthlike effective viscosity of 3 1021 pa s 3 1022 p the secular change is negligible for effective viscosities which differ by more than a factor of 10 from this value last how much climate friction tipped the earth is presently unknown

40ar 39ar laser probe dating of detrital micas from the montgomery creek formation northern california clues to provenance tectonics and weathering processes generic provenance data from the montgomery creek formation northern california even in light of paleocurrent data and regional stratigraphic patterns do not uniquely establish its sediment source dating individual detrital mica grains mica with the sup 40 ar sup 39 ar laser probe shows that the source area for the upper montgomery creek included the idaho batholith unaltered muscovite grains yield data that provide limits on timing and rate of uplift of the batholith combined with paleocurrent data the inferred provenance constrains the timing of tectonic rotation in the southeastern klamath orogen dating variably altered biotite grains elucidates the age and environment of alteration and establishes a distinct source for some of the lower montgomery creek

mars secular obliquity change due to the seasonal polar caps there is a weak positive feedback mechanism between the astronomy and meteorology of mars the mechanism is this the seasonal waxing and waning polar caps cause small changes in mars dynamical flattening because the changes in flattening are out of phase with the sun there is a net annual solar torque on the planet which increases the angle between the equatorial and orbital planes on the basis of viking observations of the present climate and simple atmospheric models of past climates these seasonal shifts of mass between the atmosphere and polar caps are capable of secularly increasing mars obliquity by about 1 or 2 since the origin of the solar system thus the climate driven largely by the axial tilt reacts back on the planet and slightly enhances the seasons on mars as time progresses more sophisticated models will probably not change this result much therefore this mechanism probably produced only minor changes in mars climate it causes negligible changes in the axial tilt and climate of the earth

seasonal carbon dioxide exchange between the regolith and atmosphere of mars experimental and theoretical studies measurements of the rate of penetration of a co2 pressure wave through basalt clay soils have been conducted under conditions of pco2 6 mbar pco2 2 mbar soil density 1 3 and t 40 to 70 c which simulate the penetration of the cap induced seasonal co2 pressure wave through the topmost regolith on mars results suggest that existing theoretical models for diffusion of a gas through a porous highly adsorbing medium may be reliably used to assess the importance of martian seasonal regolith atmosphere co2 exchange results also suggest that although mars topsoils with a substantial component of fine grained weathering products could harbor within a few meters of the surface a substantial fraction of the atmospheric co2 inventory seasonal cap atmosphere co2 exchange is still much more important than seasonal regolith atmosphere exchange this is so because the high adsorptive capacity of such soils greatly slows co2 diffusion through them owing to the typically high ratio of mean residence time for co2 in the static adsorbed phase to that in the pore gas at t 40 c at such temperatures typical of the martian regolith the co2 diffusivity of such soils may be 1 10 2 cm2 s 1 only somewhat higher than the thermal diffusivity and not high enough to allow seasonal isothermal regolith buffering of the cap induced co2 pressure wave to be effective on the other hand even the adsorption slowed diffusivities are still high enough to allow thermally driven regolith atmosphere exchange to be effective on a time scale of the obliquity cycle 105 yr we also examined the maximum effect of thermally driven exchange between the topmost seasonally thermally affected regolith and the atmosphere while of somewhat greater potential importance than the isothermal exchange the thermally driven exchange would be recognizable only if the pressure wave from co2 thus exchanged at high latitudes did not propagate though the atmosphere at a faster rate than that at which the exchange itself occurred an unreasonable assumption our results are insensitive to soil geometry and temperature and suggest that both isothermal and thermally driven regolith atmosphere seasonal co2 exchange effects are likely to be greatly dominated by cap atmosphere exchange in principle it is conceivable that a more profound understanding of the cap atmosphere regolith interaction could eventually allow us to use the externally imposed pressure waves as a natural experiment to set limits on global or regional soil properties however the regolith effects while quite possibly dominant for h2o are seemingly too weak and too complex for co2 to allow this to be accomplished

laser 40ar 39ar dating of single detrital muscovite grains from early foreland basin sedimentary deposits in india implications for early himalayan evolution in india the dagshai and overlying kasauli formations represent the oldest exposed continental foredeep sediments eroded from the himalayan orogen 40ar 39ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of 28 ma for the dagshai formation at one locality and 25 ma at a second locality whereas deposition of the kasauli formation occurred after 28 ma at two localities and after 22 ma at a third locality this timing suggests that in india the start of substantial exhumation and erosion from the rising himalayan orogen was delayed until 28 ma

mars the regolith atmosphere cap system and climate change abstract a new model for predicting the behavior of the martian regolith atmosphere cap co 2 regime and describing its role in climate change is derived the model describes the time temperature histories of 90 regolith chunks on a latitude depth grid accounting for co 2 exchange by means of laboratory derived expressions relating temperature co 2 pressure p co 2 and adsorbed co 2 mass co 2 polar cap formation and sublimation is accounted for subject to the constraint of constant total co 2 in the atmosphere regolith cap system the influence of differences in regolith adsorption laws for basalt and clay and the influence of variations in regolith depth with latitude regolith thermal diffusivity and total exchangeable co 2 inventory on predicted variations in atmospheric pressure and cap mass are examined the following results are indicated 1 the atmosphere acts as a low capacity conduit through which flows between 10 and 100 times the current atmospheric mass of co 2 between two reservoirs a regolith ocean of adsorbed co 2 and a polar cap cryosphere 2 exchange between these reservoirs is driven by variations of obliquity with the polar cap the dominant co 2 sink at low and the regolith dominating at high atmospheric pressure ranges between 0 1 low and 15 mbar high pressures of up to 25 mbar could result from pre tharsis variations pressure up to 40 mbarare achieved only in models which also call for a huge quasi permanent present co 2 cap which is not apparent 3 at high the atmospheric pressure is buffered by the regolith and satisfies adsorption equilibrium for the 90 chunks as well as conservation of co 2 mass at low a potetially large co 2 cap appears and the atmospheric pressure is buffered by the cap not the regolith 4 the periodic reservoir flushing can account for the observation of simultaneous nonenrichment of 18 o and enrichment of 13 n 5 mars climate history may be divided into three parts a post tharsis formation history as described above b pre tharsis formation with qualitatively similar processes but larger oscillations between reservoirs and larger atmospheric pressure and cap mass variation and c the very early period of channel formation 6 the nature of the response of the three part regolith atmosphere polar cap martian climate system to surface insolation variations may be analogous to that of earth s ocean atmosphere cryosphere system

kinematics of the alpine arc and the motion history of adria the alps have been interpreted traditionally in terms of north or northwest directed convergence between africa and europe1 with adria the continental underpinnings of the adriatic sea and northern italy acting as a promontory of the african continent2 this has been supported by recent analyses of africa europe relative plate motions3 5 which indicate dominantly north south directed convergence during the tertiary phase of intracontinental shortening we show here however that kinematic indicators from alpine tectonites are difficult to reconcile with this motion history figs 1 and 2 and are explained best by roughly west northwest directed adria europe motion since about 70 million years ago combined with an arc normal pattern of motion driven by local body forces this implies that adria has moved independently of both africa and europe since at least the late cretaceous

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less

episodic exhumation in the western alps oligocene to miocene clastic sediments of the tertiary piedmont basin northwest italy were derived from erosion of western alps source rocks detrital white micas from dif ferent stratigraphic units and from sands of three present day rivers draining the internal western alps have been analyzed by 40 ar 39 ar geochronology our data suggest a wide spread fast cooling and exhumation event prior to ca 38 ma followed by a 30 m y period of slower cooling and exhumation combined with erosion of crustal rocks with uniform 40 ar 39 ar signatures these processes have resulted in a pattern of regularly increasing lag time up section