an assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz bearing mylonite zones abstract recrystallized grainsize has been measured in quartz and in some cases feldspar in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings 1 1 supracrustal low temperature shallow dipping thrusts 2 2 deeper level major 100 km thrusts in high grade metamorphic rocks and 3 3 steeply dipping fault zones in granite within a low grade palaeozoic fold belt the concept of a steady state dynamically recrystallized grainsize largely unaffected by post creep processes e g static grain growth receives strong support especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones however the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize the available theoretical and empirical stress grainsize relationships do not account for these effects the concept of a steady state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation predicted deviatoric stresses have been computed using twiss theoretical model for comparison with other crustal stress estimates and most samples fall in the range from 20 to 150 mpa in a few cases where discrete high strain mylonite zones occur within contemporaneously regionally deformed rocks the recrystallized grainsize indicates that stresses were 1 5 to 2 times higher within the zones there is a clear need for improved theoretical and or empirical stress grainsize relationships for quartz and other minerals before extensive further application of this technique

modelling plastic deformation of peridotite with the self consistent theory theories for deformation of polycrystals have been substantially refined enabling us to model deformation of metals and minerals with considerable sophistication so far most modelling has been confined to single phase aggregates such as quartzite and limestone we present the first results for a polyphase aggregate peridotite consisting of 70 olivine and 30 enstatite the problem is approached with a viscoplastic self consistent theory satisfying stress equilibrium and strain compatibility for the average polycrystal and taking account of anisotropic neighbor interactions it is assumed that olivine deforms by 010 100 001 100 and 010 001 slip and enstatite deforms by 100 001 slip simulated textures for olivine and enstatite in peridotite resemble simulated textures in the pure phases indicating that for this system and for these volume fractions there is little influence of the different phases upon each other in our model the harder mineral enstatite deforms at a slower rate than olivine interaction between neighboring grains appears to be minimal which may be due to model assumptions predicted pole figures with olivine 010 axes and enstatite 100 axes aligning with the direction of shortening are in good agreement with preferred orientations in naturally and experimentally deformed peridotites

grain boundary sliding and development of grain boundary openings in experimentally deformed octachloropropane abstract in situ observations of grain boundary sliding and associated accommodation mechanisms in experimentally pure and simple sheared octachloropropane at 75 85 of its absolute melting temperature and at strain rates of 10 5 10 6 s 1 are discussed discontinuities in the strain rotation and or translation components of deformation across the grain boundary induce grain boundary sliding the influence of crystallographic orientation of grains on grain boundary sliding is so strong that grains unfavorably oriented for basal slip contribute to deformation mainly by grain boundary sliding grain boundary diffusion and intragranular plastic deformation are observed to accommodate grain boundary sliding grain boundary diffusion also causes grain boundary migration types ii and iii that is different from conventional grain boundary migration type i in being non conservative and in the details of boundary movement with respect to material points within grains grain boundary openings filled with fluid octachloropropane vapor are strongly associated with grain boundary sliding openings develop preferentially along grain boundaries at low angles to the shortening direction once openings grow they are closed by thrusting of sliding grains and by diffusion in faster strain rate experiments and entirely by diffusion in slower strain rate experiments an approximately steady openings ratio of 0 5 3 of the sample volume persists without the development of any large scale fracture all grain boundary openings disappear during static readjustment of the microstructure after deformation

creep mechanisms in the lithospheric mantle inferred from deformation of iron free forsterite aggregates at 900 1200 c abstract to further constrain the plasticity of rocks in the uppermost lithospheric mantle deformation experiments were carried out on forsterite aggregates using a gas medium apparatus paterson press at 300 mpa 900 1200 c and nearly constant strain rates of 10 5 s 1 the starting material was a synthetic iron free forsterite aggregate with an average grain size of 2 8 m and 2 3 of iron free enstatite eight deformation experiments were performed as well as an additional static annealing to characterize grain growth the maximum stresses obtained range from 480 to 1870 mpa below 1000 c where stress significantly exceeds confining pressure and based on microstructural observations grain boundary mediated creep is observed with evidences of sliding and cavitation gaping at grain boundaries at 1050 1200 c where pseudo steady state could be achieved the microstructures are very different and show evidences of dislocation activity resulting from the activation of several dislocation slip systems with increasing temperature when compared to rheology laws previously obtained from similar experiments the temperature dependence of iron free olivine creep is similar to the one of its iron bearing counterpart at high temperature 1200 c at temperatures 1000 c however the strength of iron free olivine is higher than for iron bearing olivine the deformation induced textures obtained show that grain boundary sliding gbs lead to cavitation which was activated in response to large differential stresses i e beyond the goetze criterion given these high stress conditions our results cannot be directly applied to deformation of the earth s mantle at large scale nevertheless they highlight the key role played by grain boundaries in producing strain at lithospheric temperatures when crystal plastic mechanisms remain inefficient

denitrification in the eastern arabian sea evaluation of the role of continental margins using ra isotopes abstract the 228 ra 226 ra activity ratios henceforth denoted as 228 226 have been measured both at the surface and at 300 m depth as a function of distance from the western continental margins of india to the open arabian sea between 15 n and 21 n a two dimensional eddy diffusion model is used to fit the measured 228 226 profiles to deduce horizontal eddy diffusivities in both zonal towards open ocean and meridional towards the equator directions these range between 1 3 106 and 3 1 106 cm2 s 1 assuming that the organic rich and deeply anoxic continental margin sediments at 300 m depth can be a source of organic c to the core of the denitrification layer and that the deficit flux in the carbon budget can be covered by this supply the required gradient dc dz in doc content of waters from coast to open arabian sea is computed using the kx value the required gradient is computed to be 4 mol dm 3 100 km 1 which is within the error associated with doc measurements

the water column distributions of thorium isotopes in the western north pacific abstract four vertical profiles of 230 th and 228 th were determined using large volume water samples in the western north pacific an almost linear increase of 230 th with depth was observed for all of the profiles for which the unidirectional first order scavenging model was difficult to explain we developed a model which included a dissolved particulate transformation as well as parameters of the scavenging model application of the model to the vertical distributions of total and the geosecs particulate th isotopes 230 th and 234 th yielded the residence time of dissolved th with respect to adsorption to particles and the turnover time of particulate th to be 235 days and 57 days respectively the th isotopes appeared to be carried down the water column by fine particles with a mean settling velocity of 1 m day which continually release th into sea water as well as pick up th from the water along their journey to the bottom for 228 th a large excess over 232 th was observed throughout the water column with pronounced high concentrations in surface and bottom waters suggesting that the 228 th was derived from 228 ra diffused out of sediments the vertical distributions of 228 th seemed to be significantly influenced by lateral mixing along isopycnals

detection of mantle earthquakes beneath the east african rift summary recordings from a temporary seismic network in the rwenzori region of the east african rift show seven earthquakes at a depth range between 53 and 60 km evidence from receiver functions indicates a crustal thickness of about 32 km in this region suggesting that the earthquakes are located well within the mantle beneath the rift the occurrence of faulting at this depth is of relevance for models of continental rift initiation and related deep magmatic processes

direct shear of olivine single crystals abstract knowledge of the strengths of the individual dislocation slip systems in olivine is fundamental to understanding the flow behavior and the development of lattice preferred orientation in olivine rich rocks the most direct measurements of the strengths of individual slip systems are from triaxial compression experiments on olivine single crystals however such experiments only allow for determination of flow laws for two of the four dominant slip systems in olivine in order to measure the strengths of the 001 100 and 100 001 slip systems independently we performed deformation experiments on single crystals of san carlos olivine in a direct shear geometry experiments were carried out at temperatures of 1000 to 1300 c a confining pressure of 300 mpa shear stresses of 60 to 334 mpa and resultant shear strain rates of 7 4 10 6 to 2 1 10 3 s 1 at high temperature 1200 c and low stress 200 mpa conditions the strain rate of crystals oriented for direct shear on either the 001 100 or the 100 001 slip system follows a power law relationship with stress whereas at lower temperatures and higher stresses strain rate depends exponentially on stress the flow laws derived from the mechanical data in this study are consistent with a transition from the operation of a climb controlled dislocation mechanism during power law creep to the operation of a glide controlled dislocation mechanism during exponential creep in the climb controlled regime crystals oriented for shear on the 001 100 slip system are weaker than crystals orientated for shear on the 100 001 slip system in contrast in the glide controlled regime the opposite is observed extrapolation of flow laws determined for crystals sheared in orientations favorable for slip on these two slip systems to upper mantle conditions reveals that the 001 100 slip system is weaker at temperatures and stresses that are typical of the asthenospheric mantle whereas the 100 001 slip system is weaker at conditions typical of the lithospheric mantle these observations demonstrate that the relative strength of the dislocation slip systems in olivine and thus the development of lattice preferred orientation and anisotropic viscosity in olivine rich rocks are strongly dependent upon temperature and stress

radium redux in the dead sea profiles and transient ra ba models abstract two radium 226 profiles were measured on 31 dead sea water samples collected in february 1978 before the 1979 overturn these two profiles located in the central northern basin are practically identical and show a distinct two layer feature the upper water mass uwm has a uniform ra activity at 114 5 dpm kg and the lower water mass lwm has a fairly constant value at 97 8 dpm kg being separated by a 25 m transition layer 150 175 m depth radon measurements in the lwm indicate that there is no radon in excess over radium except for the very bottom samples at 300 m depth these ra activity levels are at least two orders of magnitude greater than that of the pacific deep water that the uwm has significantly higher ra activity than the lwm is a unique feature suggesting major inputs into the uwm with very limited mixing between the two water masses this feature was used to estimate duration of the meromixis since the last overturn based on ra decay in a closed system stiller and chung limnol oceanogr 29 1984 574 586 a first order removal rate or particulate scavenging process was invoked later to interpret the observed barium and radium data chan and chung earth planet sci lett 85 1987 41 53 these two studies assumed that no mixing occurred between the two water masses and the lwm was thus in a closed system in this study we apply a transient box model in a closed system with varying scavenging rates of ra and ba to evaluate the isolation time since the previous overturn based on the ra and ba data measured in 1978 and 1979 as reported in this paper and in chan and chung earth planet sci lett 85 1987 41 53 we also evaluate the fluid residence time of the lwm with respect to mixing and exchange with the uwm m which characterizes the mixing rate between the two water masses in an open system with particulate scavenging assuming that the 1978 data represent a steady state condition and that the 1979 data represent the previous overturn condition the transient ra ba model in a closed system shows that the isolation time of the lwm is a linear function of the scavenging rate ratio of ra to ba with a maximum of 317 years as the ratio reaches zero and 207 years as the ratio becomes 1 and the scavenging residence time for ba is about 20 times the isolation time at any scavenging rate ratio the scavenging residence time for ra increases exponentially as the scavenging rate ratio decreases in the open system steady state model m is also a linear function of the scavenging rate ratio with a maximum of 339 years as the ratio vanishes the scavenging residence time for ba is also about 20 times greater than m similarly the scavenging residence time for ra increases exponentially as the ratio decreases since the same set of data are used in these calculations the isolation time in a closed system is quite comparable to but slightly less than m in an open system at any given scavenging rate ratio both the isolation time and m are quite sensitive to the changes of ra and ba values in the lwm and the overturned water if the mean ra value of the 1979 overturned water is increased by 2 and that of the 1978 lwm is decreased by 2 while the ba values remain unchanged this 4 deviation in ra values between the overturned water and the lwm leads to an increase of 28 30 in the isolation time and m respectively in comparison with the earlier calculations stiller and chung limnol oceanogr 29 1984 574 586 chan and chung earth planet sci lett 85 1987 41 53 the isolation time calculated with no ra scavenging is about 55 years longer probably because the concentrations of ba and ra are used for the present calculations with no constraint on the inventories and the lake volume changes

helium isotopes in dead sea transform waters abstract the concentrations and isotopic compositions of helium and relative abundances of co2 were studied in brines and freshwater springs groundwater wells and gas bubbles in and around the dead sea transform dst fault system the latter forms part of the plate boundary between the african and arabian plates along which intensive tectonic and volcanic activity occurred and where a range of freshwater to hypersaline water bodies percolate the atmosphere normalized 3he 4he ratios r ra show a distinct geographic gradient whereby fluids in the north part of the dst near lake kinneret are characterized by r ra 2 these values decrease gradually southward until the arava valley where they are approximately 0 15 reflecting a corresponding decrease in the mantle derived he fraction from about 30 in the north to a few percent in the south freshwater springs outside the dst in the galilee and judea mountains are characterized by r ra ratios close to unity reflecting the dominance of atmosphere derived helium although they still contain a few percent of a mantle component the spatial pattern of change in r ra ratios is independent of regional tectonic activity rifting which is similar around the lake kinneret basin and the dead sea and a correlation does not exist between such activity and he concentrations and co2 3he ratios in contrast helium concentrations and to a lesser extent co2 he ratios correlate with temperatures of the sampled fluids reflecting a dilution process of a deep hot mantle component as it percolates upwards toward the surface the preferential injection of a mantle he toward the surface in the north dst is facilitated by deep faults and a shallower moho boundary which are also associated with enhanced volcanism in the north dst and is superimposed over a regionally scaled mantle background value regionally the observations reflect a transition from a domination of a dilute mantle end member in the north which itself is the product of a mixture between mantle and crustal helium to a dominant crustal radiogenic end member in the south the r ra ratio in basaltic olivines and diopside from the golan is on average 6 6 0 7 lower than typical mantle values 8 but corresponding to comparable sub continental mantle compositions

viscoplasticity of polycrystalline olivine experimentally deformed at high pressure and 900 c abstract we have performed tri axial compression experiments on olivine aggregates at 900 c and at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus deformation at two different constant strain rates 1 1 10 5 s 1 and 3 4 10 4 s 1 yields continuous hardening reaching maximal differential stresses of 930 and 1076 mpa respectively before sample failure at ca 10 strain the deformed samples were characterized by electron backscatter diffraction ebsd and transmission electron microscopy tem crystallographic preferred orientations cpo are weak but reveal an alignment of the 010 axes around the compression direction and some concentration of 001 and 100 axes normal to it tem observations confirm plastic deformation by dislocation glide with both 001 and 100 burgers vectors electron tomography performed for the first time on deformed fined grained olivine shows that 001 dislocations glide extensively on 110 and 130 planes in complement to the 010 and 100 glide planes often reported at high temperatures 100 dislocations glide mostly on 010 but 011 and 041 planes are also activated the experimental cpo and relative activity of slip systems are well reproduced by viscoplastic self consistent simulations in which all 100 and 001 slip systems have similar strengths they are also consistent with olivine cpo from natural samples deformed at low temperature comparison of the present mechanical results to those obtained for olivine single crystals at similar conditions shows that at low temperature below 1100 c and high stress 300 mpa polycrystalline olivine has a more complex mechanical behavior than single crystals however both datasets advocate for a much weaker dry mantle lithosphere than previously thought

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

low strength of earth s uppermost mantle inferred from tri axial deformation experiments on dry olivine crystals abstract we have performed tri axial compression experiments on single crystals of san carlos olivine with various orientations at temperatures relevant for the uppermost mantle between 800 and 1090 c the experiments were carried out at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus at various constant strain rates from 7 10 6 s 1 to 1 10 4 s 1 they yield differential stresses ranging from 88 to 754 mpa unpolarized infrared spectroscopy analyses indicate that hydrogen concentration in the olivine lattice is very low e 1 0 2 10 6 exp 450 60 10 3 rt 1 15 4 1 2 2 where t is the absolute temperature r is the gas constant and is the differential stress in gpa subsequently small fractions of melt or incorporation of hydrogen in olivine may not be required to reconcile natural observations indicating a limited strength of earth s uppermost mantle lithosphere

flow behavior and microstructures of hydrous olivine aggregates at upper mantle pressures and temperatures deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation dia apparatus combined with synchrotron in situ x ray observations at pressures of 1 5 9 8 gpa temperatures of 1223 1800 k and strain rates ranging from 0 8 10 5 to 7 5 10 5 s 1 the pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 6 cm3 mol 1 and a stress exponent of 3 2 0 6 at temperatures of 1323 1423 k a comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323 1423 k is much weaker than that for the dislocation creep of water saturated olivine and is similar to that for diffusional creep and dislocation accommodated grain boundary sliding while dislocation microstructures showing the 001 slip or the 001 100 slip system were developed at temperatures of 1633 1800 k a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 7 cm3 mol 1 assuming a stress exponent of 3 5 water fugacity exponent of 1 2 and activation energy of 520 kj mol 1 i e power law dislocation creep of hydrous olivine because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures water weakening of olivine could be effective in the deeper and colder part of earth s upper mantle

seismological evidence for shallow crystalline basement in southern uplands scotland new evidence is presented and old data re appraised to indicate that the southern uplands of scotland contain crystalline rocks of continental affinity at shallow depth 1 5 km in at least two zones roughly parallel to the caledonoid strike this is in contrast with earlier interpretations especially of the lispb seismic profile1 2 which suggested that the lack of such rocks implied a major discontinuity across the caledonides near the southern uplands fault the new interpretations suggest that basement indistinguishable seismologically from that below the midland valley continues 15 20 km southwards below the southern uplands and that another such crustal block underlies the eskdalemuir eka seismological array further to the south the existence of such blocks radically affects plate tectonic reconstructions of the caledonides especially the margins of the lapetus ocean3 6 only drilling is likely to show whether the contact between basement and the overlying lower palaeozoic greywackes and shales is structural igneous or least likely simple overstep

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

deformation of olivine orthopyroxene aggregates at high pressure and temperature implications for the seismic properties of the asthenosphere abstract the effect of pressure temperature and composition on the development of crystal preferred orientations cpo and seismic properties of olivine orthopyroxene aggregates were investigated using samples containing olivine and 12 5 25 and 50 vol of orthopyroxene the samples were deformed in simple shear at a constant strain rate of 10 4 s 1 with total shear strains between 0 5 and 1 3 at pressures of 3 5 and 8 gpa and temperatures of 1300 1400 and 1500 c respectively olivine s cpo vary as a function of the orthopyroxene content all samples have their olivine 010 axes normal to the foliation samples with 12 5 and 25 orthopyroxene have their 001 axes parallel to the lineation b type whereas the samples with 50 orthopyroxene have their 100 axes oriented parallel to the lineation a type at 3 gpa we propose that olivine cpo may result from a variation between two types of diffusion accommodated grain boundary sliding difgbs mechanisms at higher pressure the relative contribution of difgbs and dislocation related mechanisms depends on the volume of secondary phases for low orthopyroxene contents dislocation related mechanisms prevail and induce the development of b type cpo whereas for higher amount of orthopyroxene difgbs controls the deformation and leads to a type cpo orthopyroxene s cpo strength increases with increasing pressure and temperature and is characterized by the concentration of 100 and 010 axes normal to the foliation and 001 close to the lineation the seismic properties show that deformation in pyroxene poor and rich peridotites are consistent with the seismic anisotropy observed in intraplate regions where the mantle flow is horizontal conversely only pyroxene rich peridotites deformed through difgbs could explain the vs h vs v

groundwater ages and reaction rates during seawater circulation in the dead sea aquifer the dead sea system provides a unique opportunity to study flow velocities and reaction rates during seawater circulation in the aquifer we present here a novel application of calculating groundwater age and velocity along the flow path of the hypersaline water from the dead sea into the aquifer using the buildup rate of 228 ra in this water the calculated circulation velocities are 1 10 m y which is in agreement with estimates based on the na cl ratios in this water 1 5 4 m y the latter is unique to the dead sea aquifer system where the na cl ratio has been decreasing during the past 50 years due to the precipitation of halite in the lake the velocity estimates facilitated the calculation of the rates of water rock interactions in the dead sea aquifer

mantle fault zones beneath the himalayan collision flexure of the continental lithosphere abstract the himalayas and the tibetan plateau are the result of the continental collision between india and eurasia the indian plate underthrusts the himalayan mountains and the southern tibetan plateau recorded seismicity at the himalayan collision zone suggests that earthquakes occur mainly at upper crustal depths and near the crust mantle boundary the question of whether the near moho earthquakes are in the crust or in the upper mantle has been controversial and has raised another question about the role of the mantle in the support of mountain loads and its ability to deform by brittle processes earthquake locations from several experiments place seismic events in the upper mantle using a finite element model we establish a link between the recorded upper mantle seismicity beneath the himalayan collision zone and flexural bending of the indian lithosphere earthquake locations focal mechanisms and seismic imaging results from the himnt experiment combined with previous constraints on the geometry and deformation of the himalayan collision are used to set up the finite element models of lithospheric loading our purpose is to infer the mechanical state of the lithosphere beneath the himalayas and to evaluate the role of the lithospheric mantle in the support of the loads the pattern of mantle seismicity can be explained by modeling the response of the indian plate to loads corresponding to the weight of the sediments of the ganga basin the himalayan mountains and the southernmost tibetan plateau combined with the effects of a horizontal force per unit length acting upon the lithospheric plate we calculated the steady state stress field in the indian lithosphere where the lithospheric mantle is assumed to be viscoelastic and non newtonian and the asthenosphere is modeled as viscoelastic and newtonian two model suites were tested one with an elastic crust model suite 1 and one with a viscoelastic crust model suite 2 both model suites provide a good fit to the observed patterns of seismicity but model suite 2 is the one that best reproduces the observations high differential stresses concentrate in the upper mantle and predicted principal stress orientations match those inferred from focal mechanisms in the area our models show that beneath the ganga basin and the southernmost himalaya earthquakes at near moho depths do not need to show extension nor is the lower crust required to be weak in order to infer that the uppermost mantle yields by brittle failure even when flexural stresses can generate the background stresses responsible for the generation of upper mantle earthquakes mohr coulomb theory suggests that additional factors such as the presence of lateral heterogeneities or the action of pore fluids are playing a fundamental role in bringing the upper mantle materials to brittle failure

intragranular plasticity vs grain boundary sliding gbs in forsterite microstructural evidence at high pressures 3 5 5 0 gpa

deep resistivity imaging across the northern and central belts of the southern uplands magnetotelluric data from 16 soundings have provided a northwest southeast traverse across the northern and central belts of the southern uplands the 34 km profile extends from the ordovician portpatrick formation northwest of the thornhill basin across the silurian gala group onto the permian lochmaben basin in the southeast the survey traverses and has particular relevance to the ordovician silurian boundary orlock bridge fault the associated moniaive shear zone and the moffat valley lineament the data obtained have been successfully modelled inverted to provide a crustal scale resistivity cross section the two upper palaeozoic basins are resolved as relatively conducting features basin depths are about 200 m thornhill and 1100 m lochmaben elsewhere through the upper crustal interval resistivities over 500 ohm m are associated with the lower palaeozoic greywackes the silurian formations appear more resistive and slightly thicker than their ordovician counterparts depth to basement base lower palaeozoic is not a well resolved characteristic but resistivity gradients observed below 5 km suggest a depth of this order concealed beneath the thornhill basin lie two narrow vertical conductive zones the more substantial feature lies towards the northwest margin of the basin and is probably associated with the orlock bridge fault the second zone lies towards the southeast margin of the basin the conductivity minima in both cases are located in the depth range 2 to 3 km they are separate and distinct from the overlying basin sediments if the two anomalies some 5 to 6 km apart can be considered an expression of the moniaive shear zone then the major conductive constituents are concentrated at the margins two dipping resistive features in the upper kilometre are imaged in the vicinity of the moffat valley lineament

upper greenschist facies intragrain deformation of albite in mylonitic meta pegmatite and the influence of crystallographic anisotropy on microstructure formation abstract we studied the deformation behaviour of albite from permian meta pegmatite in cretaceous upper greenschist facies shear zones from the austroalpine matsch unit in the eastern alps italy sodium feldspars from these rocks provide excellent natural examples for studying mechanisms of intragranular deformation under mid crustal conditions in grains with different angular relations between their 010 planes and the kinematic frame the studied rocks were deformed at c 500 c in localized shear zones with well characterized top w shear kinematics supposedly during the cretaceous upper greenschist facies tectonometamorphic event microstructural and chemical data suggest that crystallographic anisotropies in albite exert a strong control on microstructure formation and that albite primarily deformed by a combination of brittle fracturing dissolution precipitation and incipient crystal plasticity as a function of the orientation of the crystallographic anisotropy relative to the supposed shortening direction dissolution along discontinuities forming stylolites perpendicular to the shortening direction is associated with the precipitation of fine grained albite with some compositional variability ab96 98 and ab89 91 in cracks new albite precipitates form aggregates with straight segments of high angle grain boundaries nearly 120 dihedral angles and only a poor or no orientation relation to the hosting clast intragranular kinking is related to continuous lattice rotation of up to 15 by a misorientation axis close to albite 100 and the formation of subgrain boundaries with maximum misorientations of 7 synthetic microshear zones supposedly nucleated on pre existing cracks and are associated with formation of subgrain boundaries in shortening quadrants and cracks together with precipitates of potassium feldspar in extensional quadrants adjacent to the microshear zone new microstructural and textural data from mylonitic permian meta pegmatites document various closely linked crystal plastic and brittle deformation mechanisms and highlight the role of crystallographic anisotropies and their orientation with respect to the kinematic frame in microstructure formation

a re assessment of focal depth distributions in southern iran the tien shan and northern india do earthquakes really occur in the continental mantle summary we investigate the depth distribution of earthquakes within the continental lithosphere of southern iran the tien shan and northern india by using synthetic seismograms to analyse p and sh body waveforms in the zagros mountains of southern iran earthquakes are apparently restricted to the upper crust depths of 20 km whereas in the tien shan and northern india they occur throughout the thickness of the continental crust to depths of 40 45 km we find no convincing evidence for earthquakes in the continental mantle of these regions in spite of previous suggestions to the contrary and question whether seismicity in the continental mantle is important in any part of the world in some regions such as iran the aegean tibet and california seismicity is virtually restricted to the upper continental crust whereas in others including parts of east africa the tien shan and northern india the lower crust is also seismically active although usually less so than the upper crust such variations cannot reliably be demonstrated from published catalogue or bulletin locations even from ones in which depth resolution is generally improved in contrast to the oceanic mantle lithosphere in which earthquakes certainly occur the continental mantle lithosphere is we suggest virtually aseismic and may not be significantly stronger than the lower continental crust these variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness suggesting that the strength of the continental lithosphere resides in the crust and require some modification to prevalent views of lithosphere rheology

unusually deep earthquakes in east africa constraints on the thermo mechanical structure of a continental rift system shudofsky 1985 has established that earthquakes associated with the east african rift system have well constrained focal depths as great as 25 30 km using published heat flow measurements as a guide to the local geotherm we find through simple stress envelope calculations that the deepest earthquakes probably occur in the lower crust in a region where the lithosphere is strong these results are at odds with the commonly held idea that seismicity in zones of continental extension is limited to the shallow upper crust because of elevated temperatures accompanying lithospheric thinning any model of the rifting process in east africa must account for the fact that these regions exhibit considerable strength down to lower crustal levels

heterogeneous deformation and texture development in halite polycrystals comparison of different modeling approaches and experimental data modeling the plastic deformation and texture evolution in halite is challenging due to its high plastic anisotropy at the single crystal level and to the influence this exerts on the heterogeneity of deformation over halite polycrystals three different assumptions for averaging the single crystal responses over the polycrystal were used a taylor hypothesis a self consistent viscoplastic model and a finite element methodology the three modeling approaches employ the same single crystal relations but construct the polycrystal response differently the results are compared with experimental data for extension at two temperatures 20 and 100 degreesc these comparisons provide new insights of how the interplay of compatibility and local equilibrium affects the overall plastic behavior and the texture development in highly anisotropic polycrystalline materials neither formulation is able to completely simulate the texture development of halite polycrystals while at the same time giving sound predictions of microstructural evolution results obtained using the finite element methodology are promising although they point to the need for greater resolution of the individual crystals to capture the full impact of deformation heterogeneities c 2003 elsevier b v all rights reserved

basement control on the location of strike slip shear in the southern uplands of scotland in the sw part of the southern uplands terrane the moniaive shear zone has been superimposed above a geophysically defined basement discontinuity deformation was a protracted event commencing in the early wenlock and ending with granite intrusion at about 392 ma at surface the shear zone separates contrasting suites of granitic plutons these have age diverences and isotopic characteristics indicative of a lower crustal and possibly mantle break coincident with the crustal discontinuity deduced from geophysical modelling it is proposed that sinistral reactivation of the basement structure caused the development of the shear zone within the overlying allochthonous thrust belt this would require the basement structure to have been established beneath the developing southern uplands terrane by the wenlock

the role of pre existing mechanical anisotropy on shear zone development within oceanic mantle lithosphere an example from the oman ophiolite structural and fabric analysis of the well exposed hilti mantle section oman ophiolite suggests that shear zone development which may have resulted from oceanic plate fragmentation was influenced by pre existing mantle fabric present at the paleoridge detailed structural mapping in the mantle section revealed a gently undulating structure with an east west flow direction a nw se strike slip shear zone cuts across this horizontal structure the crystal preferred orientation cpo of olivine within the foliation is dominated by 010 axial patterns rather than more commonly observed 010 100 patterns suggesting that the horizontal flow close to the moho involved non coaxial flow olivine cpo within the shear zone formed at low temperature is characterized by 001 100 patterns and a sinistral sense of shear the olivine cpo becomes weaker with progressive mylonitization and accompanying grain size reduction and ultimately develops into an ultra mylonite with a random cpo pattern the olivine 010 axis is consistently sub vertical even where the horizontal foliation has been rotated to a sub vertical orientation within the shear zone these observations suggest that the primary mechanical anisotropy mantle fabric has been readily transformed into a secondary structure shear zone with minimum modification this occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling we propose that primary olivine cpo fabrics may play a significant role in the subsequent structural development of the mantle thus the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites abstract we use an anisotropic viscoplastic self consistent vpsc model to simulate the development of omphacite lattice preferred orientations lpos in response to deformation by dislocation glide in these simulations we consider slip systems identified either in naturally deformed omphacite or in experimentally deformed diopside simulated lpos reproduce very well the characteristic omphacite lpo pattern in naturally deformed eclogites a strong concentration of 001 axes sub parallel to the lineation and of 010 poles sub perpendicular to the foliation these models reconcile the interpretation of omphacite lpos in eclogites and tem observations of naturally deformed omphacite since they show that omphacite lpos in naturally deformed eclogites may develop by dislocation glide on 1 2 110 110 001 110 and 001 100 we also investigate the effect of the strain regime on the omphacite lpo development the simulations show that changes in deformation regime lead to second order variations in omphacite lpo patterns similar to those observed in eclogites such as an asymmetry of the lpo relative to the structural frame a stronger concentration of the 001 axes relative to the 010 pole concentration or a dispersion of 001 axes in the foliation plane this suggests that omphacite lpo patterns may carry information on the deformation regime simple shear transtension active during the high pressure events

deep crustal earthquakes and active faults along the rukwa trough eastern africa summary since june 1992 a seismic network of five digital stations has been operating in southwestern tanzania with the aim of investigating the seismic activity and its partitioning with regard to depth in the area between lake rukwa and lake malawi during the period 1992 june 15 to 1994 december 31 a reliable location was obtained for 199 local earthquakes 13 were recorded by all five seismic stations 71 by four stations and 115 by three seismic stations with both p and s wave arrival time measurements many earthquakes arc located in the south eastern part of the rukwa basin along its eastern border in a 20 km narrow zone centred on the lupa fault but the more active area is the songwe valley characterized by a narrowing of the basin in the proximity of the important uplift of the mbeya range seismic activity is also present inside intrabasinal faulted blocks the earthquake depth distribution presents an upper seismicity cut offal a depth of 10 to 12 km this upper stability zone is followed by the peak of seismic activity in the depth range 12 to 14 km where 20 per cent of the earthquakes occurred at greater depths the level of activity continuously decreases up to a depth of 34 km a crustal thickness of 42 4 5 km is estimated using arrivals identified as the pmp waves reflected at the base of the crust indicating that the deeper earthquakes are really occurring in the lower crust and not in the upper mantle this analysis confirms at a local scale what is suggested by geophysical and geological observations at a global scale in the western branch of the east african rift system border faults penetrate to lower crustal levels in a lithosphere that is cold and mechanically strong

dislocation creep of fine grained olivine 1 deformation experiments conducted in a gas medium apparatus at temperatures from 1200 to 1350 c with a fine grained solution gelation derived fe bearing olivine show a stress dependence of the strain rate at stresses above 150 mpa which is much stronger than previously reported for polycrystalline samples the data can be fit by a power law with n with n 7 8 or equally well by a peierls creep law with exponential stress dependence due to the observed strong stress dependence the samples deform at significantly higher strain rates at a given stress than single crystals or coarse grained polycrystals with n 3 5 tem observations indicate the presence of dislocations with at least two different burgers vectors with free dislocations predominantly of screw character subgrain walls are present but are only weakly developed and have small misorientation angles both the rheology and dislocation structures are consistent with creep rate limited by dislocation glide or cross slip for aggregates with grain sizes smaller than or approaching the recrystallized grain size deformation mechanism maps extrapolated to lithospheric temperatures using the melt free diffusion creep rheology of faul and jackson 2007 the dislocation creep rheology of hirth and kohlstedt 2003 and the results described here indicate that deformation conditions of ultramylonitic shear zones fall near the triple point of peierls dislocation and diffusion creep

strain induced seismic anisotropy of wadsleyite polycrystals and flow patterns in the mantle transition zone 1 we use forward models based on recent high pressure experimental data on mantle minerals to predict the seismic anisotropy produced by plastic strain of orthorhombic wadsleyite the dominant mineral in the upper transition zone these models predict a weak seismic anisotropy for a polycrystal of pyrolitic composition 60 wadsleyite 40 garnet at transition zone conditions 2 for p and 1 for s waves for a shear strain of 1 both p and s wave anisotropy patterns show an orthorhombic symmetry p waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane s wave anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction horizontal shearing results therefore in higher velocities for horizontally propagating p waves ph and horizontally polarized s waves sh as well as in weak azimuthal variation of sv and sh velocities on the other hand vertical flow leads to higher velocities for vertically propagating p waves pv and vertically polarized s waves sv and to a weak azimuthal variation of sv velocity but to a roughly constant sh velocity analysis of global observations of seismic anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone in agreement with predictions of geodynamical models that explicitly introduce phase transitions

a sequential back arc and foreland basin thrust duplex model for the southern uplands of scotland thrust imbrication of ordovician and silurian submarine fan sequences overlapping pelagic deposits in the southern uplands has been interpreted in terms of an accretionary prism formed above a nw directed subduction zone structural features invoked to support accretion are not definitive and could be explained in terms of a thin skinned thrusting model new palaeocurrent and compositional evidence from llandeilo to mid llandovery age turbidites in the northern part of the southern uplands proves interdigitation of sediments with strongly contrasting petrography turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz rich formations this implies a back arc situation the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south oblique collision of the opposing continental margins of the iapetus ocean during the llandovery caused the cessation of subduction underthrusting of the southern margin initiated a se propagating thrust stack which deformed the back arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc a southward migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late llandovery hawick group and wenlock sequences mid to end silurian sinistral strike slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep seated structures allowed the intrusion of lamprophyre dykes and granites

the extent of seawater circulation in the aquifer and its role in elemental mass balances a lesson from the dead sea abstract this paper shows for the first time a field based estimation of the volume of dispersive density driven long term seawater circulation in coastal aquifers which is crucial to the understanding of water rock interaction and to the assessment of its potential impact on elemental mass balances in the sea the dead sea is an ideal place for studying this type of circulation due to the absence of tides and the accessibility of the shallow fresh saline transition zone the unique antithetical behavior of 226 ra and 228 ra during seawater circulation in the dead sea aquifer where 228 ra is added and 226 ra is removed provides a robust new method for quantifying aquifer circulation here we estimate water circulation through the dead sea aquifer to be 400 million m 3 yr 2 5 million m 3 yr per 1 km of shoreline which is 20 of the fresh water inflow prior to the 1960s this large volume can affect trace element concentrations in the dead sea e g it is a sink for 226 ra ba and u and a source for 228 ra and fe these results suggest that dispersive density driven seawater circulation in aquifers may play an important role in mass balances in other lacustrine and oceanic settings

rift nucleation rift propagation and the creation of basement micro plates within active rifts abstract in this contribution we study the dynamics of rift nucleation and the interaction of propagating rift segments we use the east african rift system ears as a case study with special emphasis on the albertine rift system situated within the western branch of the ears with the 5000 m high rwenzori horst a basement block that was captured by two rift segments fieldwork in the rwenzori mountains shows that the range is not only affected by range parallel faults but that major fault sets cut across the rwenzoris in order to understand these complex fault patterns and the dynamics of the process that leads to the capturing of basement blocks within the extending rift system we developed a two dimensional visco elasto plastic spring model the model shows that propagating rift faults commonly capture micro plates with shapes similar to the rwenzori mountains these micro plates rotate during successive rift opening in a clockwise or anti clockwise sense depending on the direction of the rift segment overstep a left lateral overstep as is the case for the albertine rift system produces a clockwise rotation of the associated micro plate indicating that the rwenzori mountains rotate clockwise during the rotation of the micro plate the local stress field at the rift segment tips is perturbed so that the rifts turn 70 to 90 and propagate towards each other to capture the plates completely this perturbed stress field produces an extensional fault set that is oriented almost parallel to the far field extension direction and explains the range cutting faults that we find in the rwenzori mountains the large scale geometry of the rift system seems to be strongly controlled by rigid cratons in the lithosphere leading to a curvature of the western branch of the ears and a sinistral wrench component in the lake albert rift north of the rwenzori mountains capturing of micro plates of various sizes within the rift system is a common phenomenon and leads to strong perturbations of the local stress field during micro plate rotation the resulting structures are normal and strike slip faults that cut through the micro plates and local reverse faults

mantle fault zone beneath kilauea volcano hawaii relocations and focal mechanism analyses of deep earthquakes 13 kilometers at kilauea volcano demonstrate that seismicity is focused on an active fault zone at 30 kilometer depth with seaward slip on a low angle plane and other smaller distinct fault zones the earthquakes we have analyzed predominantly reflect tectonic faultingin the brittle lithosphere rather than magma movement associated with volcanic activity the tectonic earthquakes may be induced on preexisting faults by stresses of magmatic origin although background stresses from volcano loading and lithospheric flexure may also contribute

intermediate depth earthquakes linked to localized heating in dunite and harzburgite earthquakes can occur at great depths in the earth within subducting tectonic plates deformation experiments suggest these deep earthquakes can be triggered by localized heating of the slabs under high pressures

a high resolution magnetotelluric survey of the iapetus suture zone in southwest scotland summary magnetic variation mv surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the southern uplands of scotland from the north sea to galloway a new magnetotelluric mt survey across the conductor in galloway was designed to improve the resolution of structure in the depth range 1 15 km and at the same time assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition processing analysis and modelling a 40 km long profile was measured doubled for part of its length at a separation of 1 km with a typical site spacing of 2 km or less it was supplemented by shorter parallel and orthogonal lines giving a total of 40 sites galvanic distortion models involving regional 2 d structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites confirming the picture from induction arrow maps that many sites were influenced by 3 d structures on scales of a few kilometres and upwards other indicators of the electrical strike regional and local mv measurements the spatial structure of the groom bailey regional impedance phase and of the rotated off diagonal phase gave values more in agreement with the geological strike of n52 e which was adopted as the most appropriate coordinate system into which to resolve the data the mt impedances were accordingly rotated into directions n38 w tm and n52 e te and inverted using the 2 d code of rodi mackie the investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters and to different data subsets the te mode data were insensitive to structural detail and the best fit achieved was unacceptable suggesting the influence of structure outside the plane of the section the tm mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies forward modelling showed that the model generated from the tm mode also fitted the mv data adequately the inversion resolved the conductive zone into distinct blocks with edges matching faults mapped in the surface geology the leadhills fardingmullach and orlock bridge faults and the moffat valley lineament in the resistivity image the faults are vertical structures disrupting other features to a depth of 15 km which agrees with the view that although they originated as thrusts they were reactivated by strike slip motion during oblique closure of the iapetus ocean within some of the blocks are highly conductive regions no more than 4 km wide the most likely explanation is graphite or other metallic mineralization localized in shear zones

228ra in the dead sea abstract the 228 ra concentrations of the dead sea waters range from 0 13 to 1 48 dpm kg 1 two to three orders of magnitude higher than those of ocean waters and lake waters however the 228 ra 226 ra activity ratios 0 12 1 29 10 2 are in the range reported for the hydrosphere the surface waters of the dead sea are enriched in 228 ra by a factor of about three over the near bottom waters there is a factor of about two spatial variability in the mid depth ra concentrations at the two profile stations the near bottom 228 ra gradients yield vertical eddy diffusivity coefficient k of 2 0 and 0 4 cm 2 s 1 at profile locations 1 and 2 respectively these values are comparable to those measured in oceans and lakes

the development of discrete shear zones in a granite stress strain and changes in deformation mechanisms abstract the development of millimetre scale continuous shear zones in the sidi ali bou nab algeria granite and their progressive transformation into discrete shear zones are presented increases in strain stress and strain rate magnitudes as well as changes of deformation mechanisms are suggested by an evolution of the microstructure across these narrow shear zones temperatures are assumed to be constant from boundaries to core the most striking aspects of this evolution are 1 1 a decrease in the angle between the foliation and the shear zone 2 2 a decrease in the recrystallized grain size of quartz together with the development of a crystallographic orientation characterized by a strong concentration of c axes around the y axis of the finite strain ellipsoid 3 3 the development of very fine grained feldspathic aggregates or layers probably through dynamic recrystallization of original large crystals and 4 4 the replacement of original biotite by muscovite and a new generation of biotite in the continuous shear zones the deformation is controlled by the behaviour of quartz this mineral undergoes extensive dynamic recrystallization which probably favours a higher strain rate and a change in the active slip system from a dominant basal one in the outermost part of the structures to a dominant prismatic one in its core paleopiezometry and strain computations show a shear strain up to 10 and an inferred steady state differential stress to 100 130 mpa respectively the high stress level is suspect because the estimates are derived from recrystallized grain size which may be sensitive to strain and variations of chemical environment discrete shear zones develop in relation with dynamic recrystallization of feldspars in the core of continuous shear zones dynamic recrystallization of feldspars produces very fine grained aggregates grain size of 6 7 m which coalesce and form a continuous layer at the central part of the shear zones this fabric may result from 1 1 a change in the driving flow mechanism from dislocation creep of quartz to dislocation creep or and grain boundary sliding of feldspar neoblasts and 2 2 a large strain softening that would favour strain concentration in the fine grained matrix and the development of sharp shear zone boundaries

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

lateral variations of conductivity structure across southern scotland and northern england abstract high frequency 100 0 01 hz magnetotelluric mt soundings have been carried out along a 140 km profile across southern scotland and northern england where previously longer period soundings had been undertaken examples of the results of the processing of data from 15 sites along the profile and of the application of a number of 1d inversion procedures are presented the latter include the joint inversion of d c resistivity and mt data at rookhope where borehole stratigraphy is also available although the data were severely contaminated by near field sources at some sites and the station spacing was too large in parts of the traverse for good resolution of model parameters by incorporation of results from the earlier studies it is possible to obtain a preliminary 2d model of the electrical resistivity structure of the upper crust in this region significant differences have been found in the three main geological zones crossed by the survey the alston block the northumberland basin and the southern uplands of scotland in the discussion special attention has been paid to four sites in the neighbourhood of the southern uplands fault for which the data have been subjected to tensor decomposition this has revealed two dominant regional azimuths one of which dominates at the shorter periods and corresponds to the strike of the southern uplands fault and the other at longer periods which corresponds to the strike of the well known southern uplands magnetovariational mv anomaly moreover in this region the electrical resistivity models indicate the presence of a highly conducting zone beginning at a depth of about 4 km at station cap which is located on the axis of the mv anomaly and extending to a depth of at least 25 km the depth to this conducting zone increases both to the north and the south of the mv anomaly axis resulting in a structure which has many features in common with those along parallel traverses in se scotland and in ireland in discussing the implications of the irish model another researcher proposed either metamorphosed sediments and or serpentinised island are crust as being possible sources of the high conductivity and other geophysical and geological data the probability that the arguments in that study are equally applicable to southern scotland is currently under consideration

experimental constraints on the strength of the lithospheric mantle 1 to provide a better understanding of rheological properties of mantle rocks under lithospheric conditions we carried out a series of experiments on the creep behavior of polycrystalline olivine at high pressures 4 9 gpa relatively low temperatures 673 t 1273 k and anhydrous conditions using a deformation dia differential stress and sample displacement were monitored in situ using synchrotron x ray diffraction and radiography respectively experimental results were fit to the low temperature plasticity flow law on the basis of this analysis the low temperature plasticity of olivine deformed under anhydrous conditions is well constrained by our data with a peierls stress of p 5 9 0 2 gpa a zero stress activation energy of ek 0 320 50 kj mol 1 and ap 1 4 10 7 s 1 mpa 2 compared with published results for high temperature creep of olivine a transition from low temperature plasticity to high temperature creep occurs at 1300 k for a strain rate of 10 5 s 1 for a geological strain rate of 10 14 s 1 extrapolation of our low temperature flow law to 873 k the cutoff temperature for earthquakes in the mantle yields a strength of 600 mpa the low temperature high stress flow law for olivine in this study provides a solid basis for modeling tectonic processes occurring within earth s lithosphere

rift induced delamination of mantle lithosphere and crustal uplift a new mechanism for explaining rwenzori mountains extreme elevation with heights of 4 5 km the topography of rwenzori mountains a large horst of old crustal rocks located inside a young passive rift system poses the question why are the rwenzori mountains so high the cenozoic western rift branch of the east african rift system is situated within the late proterozoic mobile belts between the archean tanzania craton and congo craton the special geological setting of the massif at a rift node encircled by the ends of the northern western rift segments of lake albert and lake edward suggests that the mechanism responsible for the high elevation of the rwenzoris is related to the rifting process our hypothesis is based on the propagation of the rift tips surrounding the stiff old lithosphere at rwenzori region thereby triggering the delamination of the cold and dense mantle lithosphere ml root by reducing viscosity and strength of the undermost lower crust as a result this unloading induces fast isostatic pop up of the less dense crustal rwenzori block we term this rid rift induced delamination of mantle lithosphere the physical consistency of the rid hypothesis is tested numerically viscous flow of 2d models is approximated by a finite difference method with markers in an eulerian formulation the equations of conservation of mass momentum and energy are solved for a multi component system based on laboratory data of appropriate rock samples a temperature pressure and stress dependent rheology is assumed assuming a simple starting model with a locally heated ml the ml block between the weakened zones becomes unstable and sinks into the asthenosphere while the overlying continental crust rises up thus rid seems to be a viable mechanism to explain geodynamically the extreme uplift important conditions are a thermal anomaly within the ml a ductile lower crust with visco plastic rheology allowing significant strength reduction and lateral density variations the special situation of a two sided rifting or offset rift segments to decouple the ml laterally from the surrounding continental lithosphere seems to be most decisive further support for the rid mechanism may come from additional crustal thickness and an extensive stress field some parameters such as the excess temperature and yield stress are very sensitive small changes determine whether delamination takes place or not

experimental deformation of olivine single crystals at lithospheric temperatures 1 rheological properties of mantle minerals and rocks at temperatures t appropriate to much of earth s lithosphere have remained poorly constrained even though past experimental studies on olivine single crystals and polycrystalline aggregates have quantified the high temperature creep mechanisms t 1200 c consequently we have performed deformation experiments on crystals of san carlos olivine at lower temperatures from 900 to 1200 c in triaxial compression along the 101 c direction the experiments were carried out at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus at differential stresses of 100 to 500 mpa several samples were deformed at constant displacement rate and others at constant load in order to provide insight into possible effects of work hardening under the deformation conditions investigated little evidence of work hardening was observed the data follow a power law dependence on stress as in previous high temperature deformation studies the samples were however considerably weaker than predicted by the experimentally determined high temperature constitutive equation for olivine crystals of this orientation from the study of bai et al 1991 the mechanical behavior correlates instead with the weaker of the two mechanisms flow laws that contribute to the high temperature constitutive equation thus our experiments demonstrate that published high temperature constitutive equations overestimate the strength of lithospheric mantle and that the transition to low temperature creep occurs at lower temperatures than previously inferred

lispb revisited a new look under the caledonides of northern britain summary the lithospheric seismic profile in britain lispb shot in 1974 included a 700 km long north south profile of the crust and upper mantle of northern britain the wide angle seismic data from lispb was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile this reinterpretation shows that the original crustal model published by bamford et al 1978 satisfies the observed first arrival traveltimes but that further detail of crustal velocity structure may be obtained by model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface the new lithospheric model is compatible with the observed gravity field when converted to appropriate densities and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium profiles of the variation of velocity and density with depth from the new model are compared with measurements of lewisian rocks made at confining pressures up to 8 kbar allowing the seismic model to be interpreted geologically a synthetic normal incidence profile of the new model is superimposed on the nearby nec deep seismic reflection profile and the features imaged by the two techniques are compared

inversion of electromagnetic induction data for the iapetus suture zone in the uk abstract the iapetus suture zone isz corresponding to the join between the european crust of northern england and the north american derived crust of scotland has been examined by numerous seismic and induction experiments over a 20 year period the university of edinburgh electromagnetic em induction group is in the process of building a database comprising magnetotelluric and magnetic variation stations from this region in this paper we present 2d inversions of selected induction sites along roughly north south traverses across the isz in northern england and southern scotland the first inversion involves both the transverse electric te and transverse magnetic tm modes for 10 audio magnetotelluric approximately 100 0 1 hz sites care has been taken to remove the effects of static shifts this inversion resolves a conductive feature beginning under the southern northumberland trough at depths of 10 km dipping to the south below the alston block in the second inversion 12 longer period magnetic variation sites were added to the original data set improving the resolution of this feature which correlates well with a dipping bright reflector detected by the north east coast nec seismic reflection experiment in addition the southern uplands anomaly is imaged no north dipping mid to deep crustal conductor is observed which would correlate to north dipping palaeozoic subduction under laurentia