present day stress field changes along the baikal rift and tectonic implications intraplate extension in a frame of a global compressional stress field seems linked to local lithospheric perturbations lithospheric thinning or thickening able to modify the resulting state of stress zoback 1992 the baikal rift zone brz siberia is located north of the india asia collision zone and exhibits no direct communication with any oceanic domain it can thus be fully considered as an area of continental extension dominated by the global compressional intraplate stress field resulting from plate driving forces in order to address the problem of its dynamics and kinematics and their links with the india asia collision a comprehensive stress tensor analysis is presented based on 319 focal mechanisms of earthquakes located along the whole baikal rift the stress field is varying at different scales of observation when looking at central asia several thousands kilometers the maximum horizontal stress shmax directions remain rather constant with a fan shape geometry when the tectonic regime goes from compressional himalayas to extensional baikal when observing the baikal rift about 1000 km long clear variations of the stress regime are observed from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends finally at the scale of 100 km systematic shmax reorientations occur close to major rift faults we thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics we then use computed stress tensors to predict slip vectors on major rift faults deformation patterns show two distinct parts of the rift the south baikal rift sbr is characterized by a constant trending around n100 e slip vector meanwhile the north baikal rift nbr exhibits a complex block rotation behavior involving at least three crustal blocks we propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the india asia collision and the geometry of the hardly deformable siberian platform this particular setting can explain most of the surficial deformation patterns which suggest a large scale cracking of the lithosphere in the baikal region other possible sources of stress could also be considered like deep mantellic upwelling or trench suction linked to the pacific subduction

rheological transitions in progressive melting of rock and their geological constraints from the fuhu metatexite diatexite profile in guangdong province se china abstract integration of the published experimental data of partially melted amphibolite orthogneiss granite and aplite reveals the existence of three rapid strength drops of the rock assemblage at melt fraction around 7 21 and 41 respectively the first and the last drops are equivalent to the well known melt connectivity transition mct and solid to liquid transition slt the drop at melt fraction around 21 is newly identified and termed as the framework melting transition fmt the fmt is defined as the transition from eutectic quartz feldspars melting to refractory biotite melting the three rheological transitions are well constrained by the geological data from the fuhu profile in guangdong province se china this profile is a 15 m high cliff rocks exposed on the profile are zoned as mottled stripped narrow banded and wide banded migmatites and diatexitic granite from the top of the profile downwards the leucosome proportions of the zones measured on the outcrops combined with the experimental data of the partially melted rocks have significant implications for understanding the role of rheological transitions during crustal melting the mct which corresponds to the boundary between the mottled and the narrow banded migmatites suggests the segregation of melt within the partially melted system along with the compaction of matrix thus the mct is deemed to be the transition of the partially melted system from matrix supported melt stored in pore space to framework supported melt stored in foliation bedding space of protolith the fmt corresponding to the boundary between the narrow and the wide banded migmatites denotes the inception of melting of the solid framework mechanically the fmt can be regarded as the transition of the framework from compaction to fusion and geochemically the transition from eutectic melting of quartz feldspars to that of refractory biotite the increasing melt due to framework melting after the fmt is basically limited within the solid framework until the melt fraction exceeding the slt the slt is geologically constrained by the in situ metatexite diatexite transition that is also termed as magma interface mi the slt mi represents the upper limit of a crustal magma layer because of the downward increasing temperature and melt fraction there should be no consecutive layer of felsic rock existed in the space between the slt mi and the mafic lower crust except the isolated rock blocks within the magma layer

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

rift flank segmentation basin initiation and propagation a neotectonic example from lake baikal new surficial data field landsat tm and topography define morpho tectonic domains and rift flank segmentation in the ol9khon region of the central baikal rift deformation drainage and depositional patterns indicate a change in the locus of active extension that may relate to a recent

evidence for small scale mantle convection in the upper mantle beneath the baikal rift zone 1 inversion of teleseismic p wave travel time residuals collected along a 1280 km long profile traversing the baikal rift zone brz reveals the existence of an upwarped lithosphere asthenosphere interface which causes a travel time delay of about 1 s at the rift axis central high an area with early arrivals relative to the stable siberian platform of up to 0 5 s is observed on each side of the rift about 200 km from the rift axis flank lows while the location of the central high is approximately fixed in the vicinity of the rift axis those of the flank lows vary as much as 200 km with the azimuth of the arriving rays we use three techniques to invert the travel time residuals for velocity anomalies beneath the profile two of the techniques assume an isotropic velocity structure and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry we use independent geophysical observations such as gravity active source seismic exploration and crustal thickness measurements to compare the applicability of the models other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2 5 lateral velocity reduction and a velocity increase due to transverse isotropy with a vertical axis of symmetry we consider the anisotropy to be the result of the vertical component of a lithosphere asthenosphere small scale mantle convection system that is associated with the rifting

three dimensional laboratory modelling of rifting application to the baikal rift russia abstract continental rifting is treated as a mechanical instability developing under horizontal tectonic tension the instability results in strain localization and the formation of a neck which is interpreted as a rift zone at the scale of the whole lithospheric plate this process occurs in plane stress conditions and can therefore be modelled to a first approximation by a one layer plate whose properties represent integral over thickness properties of the real lithosphere we have designed a scaled experimental single layer lithosphere model having elasto plastic rheology and lying upon a liquid substratum to study its behaviour under axial horizontal tension in a homogeneous plate the instability develops along a linear zone oriented at an angle of 60 to the tension axis this orientation is preserved even when the divergent displacement of the plate boundaries is not plain parallel but rotational in the latter case the strain localization zone is rapidly propagating when the plate length to width ratio is less than 2 5 the necking develops along two branches conjugated at an angle of about 120 which is frequently observed in actual rift systems if the model contains a local weak zone hot spot or fault zone the rift junction is located at this zone in the lithospheric models comprising strong cratonic and weak segments strain localization depends on the configuration of the boundary between different lithospheres the necking starts to form within the weak segment in the vicinity of the cratonic promontories and propagates in opposite directions again at an angle of ca 60 to the tension axis in the models containing both a strong lithosphere and local weak zones the rift configuration depends on their shape and relative positions with necking always going through the weak zones in a set of models we have reproduced the geometry of the boundary between the siberian craton and the thermally much younger 100 ma sayan baikal lithosphere in the baikal rift area in these models we were able to obtain the well known three branch configuration of the baikal rift system only by introducing a weak zone in the area of lake baikal such a zone simulates the paleozoic suture existing in this area as in nature two wide outer branches eastern and western are oblique to the regional tension axis whereas the central one is narrow and orthogonal to the tension direction in nature and in the model rifting starts in the central branch corresponding to lake baikal the modelling also predicts the formation of a fourth oblique ns trending branch to the south of baikal although poorly expressed in the field this branch has some seismotectonic and magmatic manifestations the orientations of all four branches with respect to each other and with respect to the regional tension direction are remarkably similar in nature and in the model

mantle structure and rifting processes in the baikal mongolia region geophysical data and evidence from xenoliths in volcanic rocks abstract the origin of the baikal rift zone brz has been debated between the advocates of passive and active rifting since the 1970s a re assessment of the relevant geological and geophysical data from russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many active rifting models results of a large number of early and recent studies favour the role of far field forces in the opening and development of the brz this study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern siberia and central mongolia these xenoliths are direct samples of the upper mantle in the vicinity of the brz of particular importance are suites of garnet bearing xenoliths that have been used to construct p t composition lithospheric cross sections in the region for the depth range of 35 80 km xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient siberian platform sampled by kimberlites and beneath younger mobile belts south of the platform the uppermost mantle in southern siberia and central mongolia is much hotter at similar levels than the mantle in the siberian craton and also has significantly higher contents of basaltic major elements ca al na and iron higher fe si and fe mg the combination of the moderately high geothermal gradient and the fertile compositions in the off cratonic mantle appears to be a determining factor controlling differences in sub moho seismic velocities relative to the siberian craton chemical and isotopic compositions of the off cratonic xenoliths indicate small scale and regional mantle heterogeneities attributed to various partial melting and enrichment events consistent with long term evolution in the lithospheric mantle age estimates of mantle events based on os sr nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long term crust mantle coupling the ratios of 143 144nd in many lree depleted xenoliths are higher than those in morb or oib source regions and are not consistent with a recent origin from asthenospheric mantle mantle xenoliths nearest to the rift basins 30 50 km south of southern lake baikal show no unequivocal evidence for strong heating unusual stress and deformation solid state flow magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the moho comparisons between xenoliths from older and younger volcanic rocks east of lake baikal together with observations on phase transformations and mineral zoning in individual xenoliths have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small scale ascent of deep mantle material overall the petrographic p t chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies which found no evidence for a large scale asthenospheric upwarp beneath the rift and lend support to passive rifting mechanism for the brz

pleistocene deglaciation and the earth s rotation implications for mantle viscosity summary recent results from the analysis of postglacial rebound data suggest that the viscosity of the earth s mantle increases through the transition region models which fit both relative sea level and free air gravity data have viscosities which increase from a value near 1022 poise in the upper mantle beneath the lithosphere to a value of about 1023 poise in the lower mantle in this paper we analyse the effect of deglaciation upon the earth s rotation and thereby show that the observed secular trend polar wander evident in the ils ipms pole path and measurements of the non tidal acceleration of the length of day are both consistent with the viscosity profile deduced from postglacial rebound the two analyses are therefore mutually reinforcing

late pliocene sedimentation in lake baikal implications for climatic and tectonic change in se siberia abstract within the framework of the baikal drilling project bdp a 192 m long sediment core bdp 96 1 was recovered from the academician ridge a submerged topographic high between the north and central basins of lake baikal sedimentological clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth corresponding to ca 2 4 3 4 ma the aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of northern hemisphere glaciation in late pliocene time a major climate change occurred in the lake baikal area at about 2 65 ma enhanced physical weathering in the catchment mirrored in the illite to smectite ratio and temporarily reduced bioproduction in the lake reflected by the diatom abundance evidence a change towards a colder and more arid climate probably associated with an intensification of the siberian high in addition the coincident onset of distinct fluctuations in these parameters and in the zr al ratio suggests the beginning of the late cenozoic high amplitude climate cycles at about 2 65 ma fluctuations in the zr al ratio are traced back to changes in the aeolian input with high values in warmer more humid phases due to a weaker siberian high assuming that the sand content in the sediment reflects tectonic pulses the lake baikal area was tectonically active during the entire investigated period but in particular around 2 65 ma tectonic movements have likely led to a gradual catchment change since about 3 15 ma from the western towards the eastern lake surroundings as indicated in the geochemistry and clay mineralogy of the sediments the strong coincidence between tectonic and climatic changes in the baikal area hints at the himalayan uplift being one of the triggers for the northern hemisphere glaciation

the skaergaard layered series part iii non dynamic layering layering in the skaergaard intrusion has been divided into two introduction general types one produced by magmatic flow and another by recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe skaergaard intrusion mcbirney nicolas 1997 tallization and in the case of the layered series by compactionhave distinguished two broad types of layering one related processes modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which in the layered and upper another by processes that operate in situ such as varied border series are diffuse and normally lack strong foliation and nucleation and growth of crystals recrystallization or by lineation in the marginal border series the layers are thinner and compaction related mechanisms we refer to layering sharper possibly because the rate of accumulation was slower formed by these latter processes as non dynamic to oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers but it was less important than solution and processes although most layering combines elements of reprecipitation during slow cooling and ostwald ripening evidence more than one process the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor and setting layering related to compaction becomes sharper with increasing the distinguishing features of dynamic magmatic layheight in the layered series until it suddenly disappears above the ering have been described in part ii of this series mcbirtrough horizon near the base of upper zone b mechanical sorting ney nicolas 1997 they are best seen near the steep during compaction may have produced crude layering but if it did margins of the layered series in what wager brown the evidence has long since been destroyed by the superimposed effects 1968 referred to as the cross bedded zone where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re equilibrated with them numerical the rocks have a marked foliation and lineation simulations illustrate how small differences of surface energy caused layering not associated with magmatic flow takes a by variations of grain size textural dependence of solubility and variety of forms but we can distinguish two general pressure solution can cause segregation of minerals into layers during end members each of which has a distinct origin and solution and reprecipitation characteristics the first results from varied rates of

a rock magnetic record from lake baikal siberia evidence for late quaternary climate change rock magnetic measurements of sediment cores from the academician ridge region of lake baikal siberia show variations related to late quaternary climate change based upon the well dated last glacial interglacial transition variations in magnetic concentration and mineralogy are related to glacial interglacial cycles using a conceptual model interglacial intervals are characterized by low magnetic concentrations and a composition that is dominated by low coercivity minerals glacial intervals are characterized by high magnetic concentrations and increased amounts of high coercivity minerals the variation in magnetic concentration is consistent with dilution by diatom opal during the more productive interglacial periods we also infer an increased contribution of eolian sediment during the colder windier and more arid glacial conditions when extensive loess deposits were formed throughout europe and asia eolian transport is inferred to deliver increased amounts of high coercivity minerals as staining on eolian grains during the glacial intervals variations in magnetic concentration and mineralogy of lake baikal sediment correlate to the specmap marine oxygen isotope record the high degree of correlation between baikal magnetic concentration mineralogy and the specmap oxygen isotope record indicates that lake baikal sediment preserves a history of climate change in central asia for the last 250 ka this correlation provides a method of estimating the age of sediment beyond the range of the radiocarbon method future work must include providing better age control and additional climate proxy data thereby strengthening the correlation of continental and marine climate records

mantle transition zone discontinuities beneath the baikal rift and adjacent areas like most other major continental rifts the baikal rift zone brz in siberia ispresumably underlain by a hot and partially molten mantle which has a reduced seismicvelocity relative to surrounding areas recent seismic tomography studies however gaveconflicting results about the depth extent and even the existence of the low velocityanomaly beneath the brz suggesting that additional constraints are needed here wepresent results from stacking of about 1700 radial p to s receiver functions from asingle long running seismic station tly located at the sw tip of lake baikal a clearuplift of the 410 km discontinuity d410 with a magnitude of about 47 km relative tothe south margin of the siberian platform is observed beneath the rift currently availableseismic results suggest that the uplift is unlikely to be caused by addition of water tomantle transition zone mtz silicates but is the result of a 550 c reduction in temperaturein the vicinity of the d410 in addition the 660 km discontinuity d660 shows adownward trend toward the rift from the south suggesting that the entire mtz might havea low temperature beneath the rift the thickening of the mtz suggests a high velocityanomaly of about 2 in the mtz and rules out the possibility that the rifting is caused bya mantle plume originated in or beneath the mantle transition zone

magma ascent by porous flow porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle recent work has identified the importance of matrix deformation and compaction in this process we present finite difference calculations based on a generalized form of darcy s law that includes matrix deformation in two dimensional cartesian and cylindrical coordinates we emphasize the existence of solitary wave solutions called magmons these waves are regions of locally high porosity that ascend through regions of low uniform porosity they differ from diapirs where the liquid and matrix ascend together the one dimensional waves that were previously reported are found to be unstable in two dimensions breaking down to form two dimensional waves of circular cross section both the development of the instability and the form of the two dimensional waves are supported by theoretical analysis numerical experiments in which two dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves the conservation can be very good in other cases larger waves appear to grow at the expense of smaller waves although the quantification of the relevant permeability and rheological parameters remains uncertain geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year in our preliminary assessment of the relevance of these results to igneous processes we find that magmons are unlikely to be important in regions of broad upwelling such as beneath spreading centers here the transport of liquid adjusts to match the supply by melting and compaction processes are not important if liquid is supplied from below to a stable partially molten region of the asthenosphere it is expected to ascend in magmons this may occur beneath oceanic volcanic centers or in subcontinental mantle it is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity if higher level processes do not obscure this influence the waveform of a magmon ascends faster than the liquid within the magmon so new liquid is taken in from above while original liquid is lost from below consequently magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface

fractals in studies of faulting and seismicity in the baikal rift zone abstract fractal analyses have been carried out for spatial patterns of faulting including over 3000 faults and seismicity of more than 60 000 earthquakes in the baikal rift zone brz it is concluded that faulting and seismicity are in balance in the whole brz and out of balance in its flanks by analysing fractal dimensions of seismicity in time it has been revealed that the out of balance state of the processes is predetermined by the fact that development of the spatial patterns of epicentral fields has not been completed yet in the brz flanks

spatial changes of seismic attenuation and multiscale geological heterogeneity in the baikal rift and surroundings from analysis of coda waves abstract the baikal rift system is undergoing an active tectonic deformation expressed by a high level of seismic activity this deformation leads to physical and mechanical changes of crustal properties which can be investigated by the seismic quality factor and its frequency dependence using a single backscattering model a seismic quality factor qc a frequency parameter n and an attenuation coefficient have been estimated by analyzing coda waves of 274 local earthquakes of the baikal rift system for nineteen lapse time windows w from 10 to 100 s every 5 s and for six central frequencies 0 3 0 75 1 5 3 6 and 12 hz the average qc value increases with the frequency and lapse time window from 46 52 at 0 75 hz to 502 109 at 12 hz for w 10 s and from 114 49 at 0 3 hz to 1865 679 at 12 hz for w 100 s the values of qc f and were estimated for the whole baikal rift system and for separate tectonic blocks the stable siberian platform main rift basins spurs and uplifts along the rift system the q0 value qc factor at the frequency f 1 hz varies within 72 109 and the frequency parameter n ranges from 0 87 to 1 22 whereas q0 is 134 and n is 0 48 for the stable siberian platform vertical variations of attenuation reveal that sharp changes of and n are confined to the velocity discontinuities the comparison of lateral variations of seismic wave attenuation and geological and geophysical characteristics of the baikal rift system shows that attenuation is correlated with both seismic activity and heat flow and in a lesser degree with the surface fault density and the age of the crust seismic wave attenuation found across the main shear zones of the south western baikal rift main sayan strike slip fault zone and tunka obruchev and primorsky normal faults is increased by more than 25 60 compared to the neighboring areas

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

solitary waves in magma dynamics we investigate the stability of the one dimensional solitary waves solutions of the equations proposed by mckenzie to model the ascent of melts in the earth interior we show that for small porosity and two dimensional horizontal disturbances with long wavelength these solitary waves are unstable we also exhibit two and three dimensional solitary wave solutions of the mckenzie equations

melt segregation and matrix compaction the mush continuity equation compaction segregation time implications summary two phase aggregates a special case of which is partially molten systems are commonly modelled as an interpenetrating flow of two viscous liquids and are therefore described in terms of the fluid mechanics the governing equation set of fluid mechanics describing a two phase aggregate is incomplete as it consists of nine equations momentum and mass conservation equations for each phase and an energy conservation equation in ten unknowns six velocity components two pressures temperature and melt fraction in this paper the general equation closing the governing equation set is derived it explicitly expresses the condition of the coupling between the liquid percolation and porosity volume change due to which the medium continuity is maintained therefore the equation may be referred to as the mush continuity equation the character of the closed set solutions is demonstrated using 1 d isothermal models by examples of melt segregation inside a bounded low melting degree zone and sediment compaction at the bottom of a reservoir the compaction segregation style and time are controlled by the compaction segregation parameter c l c 2 and compaction segregation length depending on the mushy layer thickness l matrix permeability k characteristic porosity 0 and matrix and fluid viscosities at low c which approximately corresponds to low viscosity liquids kimberlites carbonatites the compaction segregation time scales as l 1 which effectively constrains the molten region thickness at the moment of the segregation and appears to explain the small volume and clustering of low viscosity magma eruptions characteristic of kimberlites at high c the porosity evolves through a formation of a series of isolated waves which could provide an explanation for the rhythmicity observed in large layered plutons

speckle imaging of titan at 2 microns surface albedo haze optical depth and tropospheric clouds 1996 1998 we present results from 14 nights of observations of titan in 1996 1998 using near infrared centered at 2 1 microns speckle imaging at the 10 meter w m keck telescope the observations have a spatial resolution of 0 06 arcseconds we detect bright clouds on three days in october 1998 with a brightness about 0 5 of the brightness of titan using a 16 stream radiative transfer model disort to model the central equatorial longitude of each image we construct a suite of surface albedo models parameterized by the optical depth of titan s hydrocarbon haze layer from this we conclude that titan s equatorial surface albedo has plausible values in the range of 0 0 20 titan s minimum haze optical depth cannot be constrained from this modeling but an upper limit of 0 3 at this wavelength range is found more accurate determination of titan s surface albedo and haze optical depth especially at higher latitudes will require a model that fully considers the 3 dimensional nature of titan s atmosphere

faults and tectonic stresses of the baikal rift zone abstract the cenozoic baikal rift system consists of a central extensional segment which links a southwestern and a northeastern sinistral shear segment statistical studies on fault length amount and sense of slip depth of crustal penetration and spacing has led to the establishment of empirical relationships which appear to apply also for non extensional regimes results are graphically displayed in figs 4 7 tectonic stress regimes were determined by means of fault and joint analyses and on the basis of seismological data results of both approaches are compatible and explain the evolution of the baikal rift zone in terms of tensional reactivation of pre existing crustal discontinuities

accommodating large scale intracontinental extension and compression in a single stress field a key example from the baikal rift system the baikal rift system in southern siberia is one of the main intracontinental extensional features on earth the rift system represents the northwestern boundary of the amuria plate and in that respect can be considered as an evolving plate boundary the baikal rift system has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution however the age of the initiation of deformation and the mechanism driving this deformation are still largely debated while major extension has occurred since the late miocene pliocene the onset of extension seems older than the india asia collision implying that several driving mechanisms may have acted together or in relay through time in this work we review the available data and models for deformation in an area encompassing the baikal rift system the sayan ranges to the west and the transbaikal to the east using a synthesis of this data and our own field and mapping observations we show that the baikal rift system along with transpressional deformation in the sayan ranges and transtension in the transbaikal area can be explained through major left lateral strike slip systems the deformation is strongly controlled by inherited crustal and lithospheric structures and is distributed over a wide area within the western amuria plate that consequently cannot be considered as a rigid block such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust

development of asymmetric basins along continental transform faults abstract many basins associated with wrench or transform faults are asymmetrical both longitudinally and laterally the sense of basin asymmetry changes along the strike of transform faults geophysical data from the dead sea rift and other transforms indicate that wrench induced asymmetric basins are bounded on only one side by a transform fault this implies that during the evolution of such basins maximum compression was parallel to the transform the direction of extension can change during the geologic evolution of a basin a possible explanation of this observation takes the strength of the fault into into consideration in the case of a weak fault in a strong crust the horizontal principal stresses would probably rotate to orientations parallel and perpendicular to the fault thus minimizing the shear stress on the main fault basins formed by regional extension such as in the east african rift system sometimes show similar features recent studies have shown that these basins often undergo oblique extension this suggests that the asymmetry in these basins may result from simultaneous strike slip motion and fault normal extension similar to the way asymmetry is produced in large transform basins periods of oblique extension can alternate with periods of orthogonal extension the asymmetry formed during periods of oblique extension is often preserved in the stratigraphic record

evolution of major and trace element composition during melt migration through crystalline mush implications for chemical differentiation in the crust we present the first quantitative model of heat mass and both major and trace element transport in a mush undergoing compaction that accounts for component transport and chemical reaction during melt migration and which is applicable to crustal systems the model describes the phase behavior of binary systems both eutectic and solid solution with melt and solid compositions deter mined from phase diagrams using the local temperature and bulk composition trace element concentration is also determined the results demonstrate that component transport and chemical reaction generate compositional variation in both major and trace elements that is not captured by existing geochemical models in particular we find that even for the simplest case of a homogenous insulated column that is instantaneously melted then allowed to compact component transport and reaction leads to spatial variations in major element composition that in this case produces melt that is more enriched in incompatible elements than predicted by batch melting in deep crustal hot zones dchz created by the repeated intrusion of hot mantle derived magmas buoyant melt migrating upwards accumulates in high porosity layers but has a composition corresponding to only a small fraction of batch melting because it has locally equilibrated with mush at low temperature moreover melt migration and chemical reaction in a layered protolith may lead to the rapid formation of high porosity melt layers at the interface between different rock compositions in both of these cases the melt in the high porosity layer s is less enriched in incompatible trace elements than predicted if it is assumed that melt with the same major element composition was produced by batch melting this distinctive decoupling of major and trace element fractionation may be characteristic of magmas that originate in dchz application of the model to a number of crustal systems including the ivrea verbano zone the rum layered intrusion and the holyoke flood basalt suggests that composi tional heterogeneity can be explained by buoyancy driven melt migration and compo nent transport through a reactive crystalline mush

chemical differentiation cold storage and remobilization of magma in the earth s crust the formation storage and chemical differentiation of magma in the earth s crust is of fundamental importance in igneous geology and volcanology recent data are challenging the high melt fraction magma chamber paradigm that has underpinned models of crustal magmatism for over a century suggesting instead that magma is normally stored in low melt fraction mush reservoirs 1 9 a mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space1 10 however many common features of crustal magmatism have not yet been explained by either the chamber or mush reservoir concepts1 11 here we show that reactive melt flow is a critical but hitherto neglected process in crustal mush reservoirs caused by buoyant melt percolating upwards through and reacting with the crystals10 reactive melt flow in mush reservoirs produces the low crystallinity chemically differentiated silicic magmas that ascend to form shallower intrusions or erupt to the surface11 13 these magmas can host much older crystals stored at low and even sub solidus temperatures consistent with crystal chemistry data6 9 changes in local bulk composition caused by reactive melt flow rather than large increases in temperature produce the rapid increase in melt fraction that remobilizes these cool or cold stored crystals reactive flow can also produce bimodality in magma compositions sourced from mid to lower crustal reservoirs14 15 trace element profiles generated by reactive flow are similar to those observed in a well studied reservoir now exposed at the surface16 we propose that magma storage and differentiation primarily occurs by reactive melt flow in long lived mush reservoirs rather than by the commonly invoked process of fractional crystallization in magma chambers14 magma storage and differentiation in the earth s crust mainly occurs by reactive melt flow in long lived mush reservoirs rather than by fractional crystallization in magma chambers as previously thought