upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

differential stress strain rate and temperatures of mylonitization in the ruby mountains nevada implications for the rate and duration of uplift kno ledge of the m agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low angle detachment faults we have analyzed the differential flow stress during the mylonitization of quartzose rocks in the ruby mountains nevada using grain size piezometers and kinetic laws for grain g owth quartzites from mylonitic shear zones in lamoille canyon and secret creek gorge have grain sizes of 91 151 m and 42 64 m respectively the peak temperature during mylonitization was 630 i 50 c and analysis of grain growth kinetics indicates that mylonitization continued during cooling to temperatures s450 c quartz grain size piezometers suggest that the mylonitization occurred under differential stresses crl cr3 of 38 64 mpa or maximum shear stresses of 19 32 mpa extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10 10 and 10 13 1 arguments presented m the paper suggest that the likely range of strain rates is 10 11 to 10 12 1 these strain rates are compatible with displacement rates of the order of 23 mm yr l along a 1 5 km thick simple shear zone such a shear zone dipping 15 would produce an uplift rate of 5 8 km m y 1 and a horizontal extensioo rate of 22 km m y l this uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1 5 km thick simple shear zone dipping 15 in 2 6 m y

subgrain rotation and dynamic recrystallization of olivine upper mantle diapirism and extension of the basin and range province abstract on the basis of an abrupt change of olivine grain size at certain depths in the upper mantle beneath the basin and range province in the western u s a mercier 1980b proposed that subgrain rotation sgr recrystallization had taken place above the grain size discontinuity and grain boundary migration gbm recrystallization at depths greater than the discontinuity the rotation of subgrains and the characteristics of dynamically recrystallized neoblasts of olivine were analyzed in a series of dunite samples deformed experimentally to compressive strains of about 15 60 whereas the rotation angle between adjacent subgrains increases with strain the mean rotation angle did not exceed 2 even in the most heavily deformed samples rotation angles between kink bands which formed at all experimental conditions also increase with strain at the highest strains rotations of up to 110 are not uncommon at low strains the dominant rotation axis between kinks is 001 and less frequently ovw at high strains uvw rotation axes are common neoblasts did form mainly on old grain boundaries and less frequently within old grains rotation axes between intragranular neoblasts and their hosts are of the type uvw and the rotation angles are always large features which seem to be inconsistent with sgr recrystallization and suggest gbm recrystallization neoblasts may have formed in highly strained regions of old grains where slip occurs with uvw rotation axes the olivine textures and fabrics of lherzolite nodules originating in the uppermost mantle above the grain size discontinuity are similar to those of nodules from the dreiser weiher germany mercier 1980b a study of dreiser weiher samples shows however that they have features difficult to reconcile with the sgr mechanism it is proposed here that gbm recrystallization occurred throughout a rising upper mantle diapir beneath the basin and range extension zone as the upward flow crossed the regional lithosphere asthenosphere boundary at about 65 km depth it diverged causing flow velocities to decrease abruptly and the strain rate dropped approximately by an order of magnitude as a consequence the differential stress decreased by about a factor of two and the olivine grain size increased by a factor of two

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

ductile faulting dynamic recrystallization and grain size sensitive flow of olivine abstract twiss 1976 has suggested that the ductile faulting events observed by post 1973 during high temperature creep of dunite are due to a transition from creep by dislocation movement to a diffusion accommodated grain boundary sliding mechanism following a reduction in grain size by dynamic recrystallization similarly goetze 1978 has explained both ductile faulting and water weakening of dunite by transition to a nonlinear coble creep mechanism however the fundamental assumption made by twiss 1976 that the stress exponent n reduces to unity during ductile faulting events is questionable if the stress exponent remains high n 3 then a diffusion accomodated grain boundary sliding mechanism is excluded nonlinear coble creep would remain a viable alternative however this model fails to adequately explain the water weakening phenomenon and the available data do not constrain us to this model assuming that the water weakening phenomenon can be explained by other models e g blacic 1972 it will be shown by analogy with the behavior of metals that a third model also consistent with the available data also qualitatively explains the observations associated with ductile faulting without appeal to a transition in creep mechanisms the model is similar to one for metals undergoing deformation by dislocation movement and recovery by dynamic recrystallization which commonly exhibit behavior virtually identical to that observed in dunite during ductile faulting events without transition to grain size sensitive creep mechanisms

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

high temperature deformation of enstatite aggregates synthesized polycrystalline enstatite samples were deformed in a paterson gas medium apparatus at 1200 1300 c oxygen fugacity buffered at ni nio and confining pressures of 300 mpa protoenstatite field or 450 mpa orthoenstatite field at both confining pressures the mechanical data display a progressive increase of the stress exponent from n 1 to n 3 with increasing differential stress suggesting a transition from diffusional to dislocation creep nonlinear least squares fits to the high stress data yielded dislocation creep flow laws with a stress exponent of 3 and activation energies of 600 and 720 kj mol for orthoenstatite and protoenstatite respectively deformed samples were analyzed using optical microscopy and scanning and transmission electron microscopy microstructures show undulatory extinction and kink bands evidence of dislocation processes crystallographic preferred orientations measured by electron backscatter diffraction are axisymmetric and indicate preferential slip on 100 001 most deformed grains comprise an interlayering of orthoenstatite and clinoenstatite lamellae while many lamellae may have formed during quenching from run conditions those in samples deformed in the orthoenstatite field are often bordered by partial 001 dislocations suggesting transformation due to glide of partial 001 dislocations in 100 planes comparison of our orthoenstatite creep law with those for dislocation creep of olivine indicates that orthoenstatite deforms about a factor of 2 slower than olivine at our experimental conditions however as orthoenstatite has a higher activation energy and smaller stress exponent than olivine this strength difference is likely smaller at the higher temperatures and lower stresses expected in much of the upper mantle

deformation of single crystal clinopyroxenes 2 dislocation controlled flow processes in hedenbergite laboratory deformation experiments were carried out on two single crystal clinopyroxenes chrome diopside and hedenbergite the tests were made in a griggs solid medium triaxial not creep tester a confining pressure of 1000 mpa was applied in all the experiments the crystals were deformed at strain rates from 10 4 s 1 to 10 8 s 1 and at temperatures from 400 c to 1200 c two orientations of the crystals with respect to the maximum principal compressive stress were tested the first orientation subjects the 100 001 and 001 100 mechanical twinning systems in clinopyroxene to a high resolved shear stress appropriate for mechanical twinning to occur the second orientation subjects these systems to an equal resolved shear stress but in a way that mechanical twinning is not possible the effects of temperature and strain rate on the flow stress were observed for the two clinopyroxene compositions mechanical twinning on the system 100 001 was observed to be the primary deformation mechanism in crystals oriented favorably for twinning above 850 c deformation occurs by dislocation glide and is strongly temperature and strain rate dependent in the orientation for which mechanical twinning is not possible crystals deformed by two different dislocation glide mechanisms depending on temperature from 400 c to 900 c deformation in hedenbergite occurs by dislocation glide controlled by lattice resistance this thermally activated mechanism has an activation energy of 285 kj mol 68 kcal mol and an activation area that expresses the effect of the applied stress on the total activation energy this mechanism only operates at stresses above 520 mpa at temperatures above 900 c and applied stresses less than 520 mpa flow in hedenbergite occurs by climb controlled dislocation glide this leads to a thermally activated power law flow equation the activation energy is 523kj mol 125 kcal mol strain rate depends on the stress raised to the power 3 6 the flow law derived for hedenbergite at high temperatures and low stresses may be extrapolated to a strain rate of 10 14 s 1 thought to be appropriate for flow in the mantle this flow law predicts an equivalent viscosity of hedenbergite of 1021 poise at 800 c and 1019 poise at 1200 c

experimental deformation of a synthetic dunite at high temperature and pressure i mechanical behavior optical microstructure and deformation mechanism abstract we have performed a series of 27 deformation experiments on a very dry synthetic dunite using the griggs solid medium apparatus strain rates ranged from 1 10 4 to 1 10 7 sec 1 temperatures varied from 1100 to 1300 c and the confining pressure was maintained at 10 or more usually 15 kbar macor im a commercially available anhydrous machinable glass ceramic with a low melting point was used in many of the experiments we find that strength measurements performed using macor as a confining medium are similar to results obtained using sodium chloride as the confining solid tests performed in undried sample assemblies using air dried dunite specimens resulted in creep strengths greater than those found in any earlier studies except those of post 1973 1977 a single test on a sample for which both sample and assembly were dried at a temperature sufficiently high to drive off any adsorbed water resulted in a creep strength comparable to that determined by post 1973 1977 for very dry mt burnett dunite despite our experimental difficulties we are led to believe that our synthetic dunite exhibits mechanical behavior consistent with that determined in other experimental studies in which natural dunites and peridotites were used furthermore we conclude that dry dunite is very probably as strong as the oft disputed results of post earlier indicated finally we show that the optical textures of our highly recrystallized experimental specimens are essentially identical to naturally produced porphyroclastic textures and that the deformation mechanism in both the experimental and natural specimens is probably dislocation creep with recovery by dynamic recrystallization

upper mantle anisotropy a preliminary model seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion the physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide system of each mineral the dominant glide systems are assumed to be 100 0l0 for olivine and 001 100 for the pyroxenes these yield a predominantly orthorhombic fabric with the olivine 100 and the pyroxene 001 axes aligned in the upper mantle flow direction and the olivine 010 and the pyroxene 100 axes aligned normal to the upper mantle flow plane these glide systems have a threshold temperature of enhanced mobility of 1100 1200 k which yields a solid state thermally defined lithosphere asthenosphere boundary in an olivine pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere mantle anisotropy due to mineral alignment is then actively maintained below this boundary asthenosphere and mesosphere and is a fossil state above this boundary lithosphere we use the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models pyrolite and piclogite for the upper mantle by an extrapolation of single crystal anisotropic mineral elastic properties to a depth of 400 km the real earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy isotropy from random mineral alignment the seismic anisotropy of the upper 220 km is best represented by the pyrolite model which reduces to quasi hexagonal symmetry with the unique axis in the direction of mantle flow the lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy the piclogite model has orthorhombic symmetry distinctly different from that of the pyrolite the piclogite anisotropy model can appear quasi isotropic when measured by transverse isotropy parameterization if the mantle flow is mainly horizontal with a horizontal shear plane that is the shear flow gradient duh dr dominates

experimental deformation of a synthetic dunite at high temperature and pressure ii transmission electron microscopy abstract we have performed detailed transmission electron microscope on most of the deformed synthetic dunite specimens prepared in the study by zeuch and green 1984 we have identified three basic types of sub boundaries simple tilt walls in 100 and 001 composed by b 100 and b 001 edge dislocations respectively and twist boundaries in 010 composed of b 100 and b 001 screws we have also observed more complex asymmetric lilt boundaries in 100 and 001 like the 010 twist boundaries these asymmetric tilt walls are common only at the highest temperatures and lowest strain rates subgrain development is extensive at the higher temperatures and lower strain rates and subgrains are composed of the above mentioned three types of sub boundaries edge components in 100 and 001 ire knitted to screw components in 010 as described by kirby and wegner 1978 for naturally deformed olivine in many areas of the samples which we studied subgrain development is not observed but parallel arrays of tilt boundaries of one type or the other are present at higher temperatures and lower strain rates 100 organization durham et al 1977 is common this structure consists of parallel arrays of 100 tilt boundaries with b 100 screws connecting the sub boundaries at lower temperatures we have observed an analogous arrangement of 001 sub boundaries and b 001 screws which we refer to as 001 organization under all experimental conditions dislocations with b 100 and b 001 are present in approximately equal numbers however the two types of dislocations also have distinctly different geometries under all test conditions we suggest that the transition from slip parallel to 001 to slip parallel to 100 with increasing temperature which has been reported in earlier studies may also depend upon water content the substructures which we observe are virtually identical to those seen in many naturally deformed peridolites and we conclude that the mechanisms involved in both natural and laboratory deformation of olivine polycrystals are similar on the other hand the substructures reported here are very different from those observed in experimentally deformed olivine single crystals it seems likely that these substructural differences reflect fundamental differences in the behavior oh single crystals and polycrystals which are in turn reflected in different measured creep strengths

heat flow in the nipigon arm of the keweenawan rift northwestern ontario canada 1 in the archean superior province the nipigon embayment in the area of lake nipigon north of lake superior is covered by midproterozoic sediments intruded by keweenawan diabase sills it has been interpreted as a failed arm of the ca 1100 ma keweenawan rift six new heat flow values in this area show that the region of low heat flow associated with the keweenawan rift in lake superior extends northwards along the western margin of the nipigon embayment the average heat flow in the nipigon area 39 5 mwm 2 is only slightly lower than

structure and petrology of peridotites clues to their geodynamic environment the first part of this paper is a review on the main mineralogical petrological and structural criteria which can be used to unravel the history of the various types of mantle peridotites met at the earth s surface these criteria are applied in the second part to show that most peridotites share a common history which is that of an asthenospheric uprise followed by specific fates corresponding to their lithospheric history the unifying concept is that of adiabatic ascent rate related to rifting and spreading rates rates lower than 0 5 cm yr in continental graben correspond to departure from adiabatic conditions at depths greater than 30 km this inhibits further melt extraction with as consequences a limited melt extraction and comparatively fertile spinel lherzolites as residue higher rates probably not exceeding 1 cm yr correspond to oceanic rifts with the main melt extraction completed around 15 km generating a thin oceanic crust and plagioclase lherzolites as residue finally rates greater than 1 2 cm yr correspond to oceanic ridges with melt being extracted at moho depth thus generating a 6 km thick crust and leaving depleted harzburgites as residue thus examination of the peridotite type and associated crustal formations give some clue to trace back the environment of origin this conclusion must however be tempered by the fact that the spreading rate envisioned here as the principal controlling parameter can change with space and time in a given environment mantle diapirism rifting seafloor spreading crust generation

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

new heat flow density and radiogenic heat production data in the canadian shield and the quebec appalachians new heat flow density hfd measurements were performed at 10 sites in quebec for five of the sites located in the superior province the heat flow density varies between 24 and 35 mw m2 26 and 37 mw m2 after adjustment for pleistocene climatic variations in the grenville province the values obtained range between 25 and 28 mw m2 29 and 31 mw m2 after adjustment for two nearby sites in the gaspe region appalachians the heat flow density is 47 mw m2 48 mw m2 after adjustment radiogenic heat production was also measured at the sites located in the meta volcano sedimentary sequences of the superior province the heat production is low 0 1 0 6 w m3 and it does not always correlate with the surface heat flow in the grenville province the hfd is close to slightly higher than the reduced heat flow of the superior the higher hfd in the appalachians is partly explained by the higher crustal heat production and partly by higher deep heat flow

accurate differential stress measurement using the molten salt cell and solid salt assemblies in the griggs apparatus with applications to strength piezometers and rheology abstract calibrations have been developed for the molten salt cell and solid salt assemblies used in tri axial griggs type piston cylinder deformation apparatus griggs apparatus that enable accurate differential stress determinations in both assemblies at high pressures and high resolution differential stress measurements in the molten salt cell elastic response of the apparatus was characterized by loading alumina and wc rods and flow strengths were measured for high purity nickel and molybdenum and a ti zr mo alloy in replicate experiments in a griggs apparatus and a heard tri axial gas apparatus gas apparatus at identical pressures temperatures and strain rates the elastic response of the griggs apparatus is nonlinear at low total loads 1 760 mpa on a 5 mm diameter sample but becomes linear at loads greater than 15 kn plastic deformation experiments on water insensitive metals indicate that differential stresses measured in the molten salt cell msc and solid salt ssa assemblies in the griggs apparatus are greater than those measured in the gas apparatus at all conditions but vary systematically and reproducibly a simple linear calibration gas apparatus 0 73 griggsmsc 10 mpa where differential stress 1 3 can be applied to the differential stresses obtained using the msc in the griggs apparatus to obtain sample flow strength a similar linear calibration gas apparatus 0 73 griggsssa 48 mpa 30 mpa can be applied to the differential stresses obtained using the griggs apparatus with a solid salt assembly to obtain accurate sample flow strength these calibrations have been tested by applying them to mechanical data for carbonates deformed in the griggs apparatus and gas apparatus the calibrations are also applied to data from previous studies performed in the griggs apparatus on clinopyroxenites and quartzites and compared to results from similar experiments using the gas apparatus when the experimental starting materials and conditions are the same the recalculated flow strengths measured in the griggs apparatus experiments agree with those measured in the gas apparatus however discrepancies in differential stress are still observed in cases where starting materials differ or water available to silicate samples varies presumably through hydrogen diffusion through sample jackets and hydrogen fugacities of the surrounding confining media

flow behavior of fine grained synthetic dunite in the presence of 0 5 wt h2o the deformation behavior of synthetic dunite mg forsterite plus 2 5 vol enstatite with a grain size of 1 m has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850 to 1000 c strain rates of 10 7 to 10 5 s 1 and a confining pressure of 600 mpa nominally dry material behaved purely elastically in contrast samples with 0 5 wt added water flowed at stresses between 9 and 81 mpa reaching strains up to 12 fitting a dorn type power law to the wet data yielded stress exponents n of 1 7 0 4 and apparent activation energies in the range 302 22 kj mol 1 the wet deformed samples showed polygonal grains no crystallographic preferred orientation no subgrains low dislocation densities and evidence for grain boundary cavitation grain growth was minor it is concluded that the wet samples probably deformed by a grain size sensitive grain boundary sliding dominated mechanism though the nature of the accommodation process remains unclear and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

shear localisation in upper mantle peridotites upper mantle peridotite bodies at the earth s surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle deformation which occurred at high temperatures t 950 50 c is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres below 950 50 c strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites the microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes the most important strain softening processes are inferred to be structural and reaction induced softening structural softening processes include dynamic recrystallisation and strain induced transitions from dislocation creep to some form of grain size sensitive gss creep reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by gss creep and the formation of weak sheet silicates such as phlogopite chlorite talc and antigorite from experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910 160 c this temperature range is inferred to be a significant rheological transition in the upper mantle below 910 160 c deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct perhaps weak rheology at higher temperatures strain is accommodated in much wider deformation zones

on the inter relationship between grain size sensitive creep and dynamic recrystallization of olivine abstract the recent theoretical results of twiss 1976 and goetze 1978 suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain boundary sliding or nonlinear coble creep mechanisms this would result following the production of a fine grain size by dynamic recrystallization for the high temperature experimental work performed by karato et al 1982 dry single crystals of olivine were almost totally recrystallized during creep and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates however the activation energy for creep determined by karato et al 1982 was far higher than that predicted by the models of twiss 1976 or goetze 1978 although the conditions required for operation of at least the model of twiss 1976 apparently were satisfied the data for the highly recrystallized specimens from the higher stress lower temperature experiments of zeuch and green 1979 and zeuch 1980 are in good agreement with the results of karato et al 1982 these latter experiments were conducted under conditions where either the model of twiss 1976 or goetze 1978 should have been applicable i tentatively conclude that although fine grain sizes were produced in the experiments of karato et al 1982 zeuch and green 1979 and zeuch 1980 by dynamic recrystallization there is no evidence for a transition to grain boundary diffusional creep mechanisms at high or low stresses despite the predictions of twiss 1976 or goetze 1978 instead deformation is dominated by dislocation movement with recovery by dynamic recrystallization

climate science solution proposed for ice age mystery the ice sheets retreated 10 000 years ago during a peak in solar radiation but this peak was no larger than previous ones a modelling study suggests why the ice sheets were unusually vulnerable to melting at that time see letter p 190

deformation of single crystal clinopyroxenes 1 mechanical twinning in diopside and hedenbergite laboratory deformation experiments were carried out on two single crystal clinopyroxenes chrome diopside and hedenbergite tests were made in a griggs solid medium triaxial hot creep tester a confining pressure of 1000 mpa was applied in all experiments crystals were deformed at strain rates from 10 4 to 10 8 s 1 and at temperatures from 400 c to 1200 c two orientations of the crystals with respect to the maximum principal compressive stress were tested the first orientation subjects the 100 001 and 001 100 mechanical twinning systems in clinopyroxene to a high resolved shear stress in the sense appropriate for mechanical twinning to occur the second orientation subjects these systems to an equal resolved shear stress but in the opposite sense so that mechanical twinning is not possible effects of temperature and strain rate on the flow stress are observed for the two clinopyroxene compositions mechanical twinning on the system 100 001 is observed to be the primary deformation mechanism in crystals oriented favorably for twinning the resolved shear stress required for mechanical twinning on this system is 140 mpa for hedenbergite and 100 mpa for chrome diopside the smaller twinning stresses for diopside are considered to be due to abundant inclusions in the crystals the mechanical twinning stress is nearly independent of temperature from 400 c to 850 c and strain rate from 10 4 to 10 8 s 1 above 850 1000 c for diopside and 1000 c for hedenbergite deformation occurs by dislocation glide and is strongly temperature and strain rate dependent crystals tested in the orientation for which mechanical twinning is not possible deform by two different dislocation glide mechanisms depending on temperature details of these experiments are reported in part 2 results for mechanical twinning predict that this is not an important deformation mechanism in the earth unless resolved shear stress on the twinning system exceeds 140 mpa lack of dependence of twinning stress on strain rate implies that large rapid strains will occur in this mineral if the twinning stress is reached independence of twinning stress in clinopyroxenes from temperature strain rate or composition allows use of this mineral as a geopiezometer at plate margins where stresses are high presence of mechanically twinned clinopyroxene grains allows determination of the orientation and magnitude of stresses responsible for the deformation

long term deformation and fluid enhanced mass transport in a variscan peridotite shear zone in the ivrea zone northern italy a microtextural petrological and geochemical study of a reactivated shear zone the microtextural petrological and geochemical study of a ductile shear zone in the phlogopite peridotite of finero ivrea zone northern italy reveals the long term deformation of this zone the zone is divided into a protomylonitic and an ultramylonitic part both parts reflect different periods of deformation although the orientation of the mineral lineations does not change in the coarse grained part period 1 the deformation started under granulite facies conditions about 775 c olivine ortho and clinopyroxene and phlogopite recrystallized dynamically in the ultramylonitic part relics of the granulite facies event and evidence for a continuous or two stage deformation history under amphibolite facies minimum 640 c to upper greenschist facies conditions maximum 520 c are preserved period 2 amphibolite facies conditions are indicated by olivine recrystallization the monoclinization of orthopyroxene porphyroclasts and the recrystallization and chemical changes of clinopyroxene the greenschist facies final stage of period 2 is characterized by decreased x co 2 and the syntectonic formation of antigorite tremolite and phlogopite at the expense of recrystallized and porphyroclastic olivine and pyroxene between both deformation periods a short break in deformation continuity is probable continuous deformation or reactivation in shear zones of the ivrea zone has not been described so far during the granulite facies shearing small amounts of channelized fluid flow led to a slight mass transfer the shear zone shows a moderate enrichment of na ba cu cs h2o and co2 and a small loss of p the mass balance of the ultramylonite indicates a significant increase in mass transport a mass gain can be inferred for h na k ba al ti p s cs sr rb c zn zr s sc light rare earth elements nb cl and au the zone is depleted in ca cu co f and ni si mg cr mn y nb and v are constant within analytical error deformation and fluid infiltration led to a change in volume which increases during the granulite facies event by 5 7 and during the lower temperature phase by 3 3 the calculated fluid to rock ratios by standard equations results in unrealistically high values for the interpretation of highly deformed rocks with drastic grain size reduction it is therefore necessary to consider the enhanced diffusion which is mainly controlled by the increased grain boundary surface

seismic properties of the upper mantle beneath lanzarote canary islands model predictions based on texture measurements by ebsd abstract we present a petrophysical analysis of upper mantle xenoliths collected in the quaternary alkali basalt fields series iii and iv from the island of lanzarote the samples consist of eight harzburgite and four dunite nodules 5 to 15 cm in size and exhibit a typical protogranular to porphyroclastic texture an anomalous foliation resulting from strong recovery processes is observed in half of the specimens the lattice preferred orientations lpo of olivine orthopyroxene and clinopyroxene were measured using electron backscatter diffraction ebsd in most samples olivine exhibits lpos intermediate between the typical single crystal texture and the 100 fiber texture occasionally the 010 fiber texture was also observed simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime probably dominated by a simple shear component under low strain rate and moderate to high temperature orthopyroxene and clinopyroxene display a weaker but significant texture the lpo data were used to calculate the seismic properties of the xenoliths at pt conditions obtained from geothermobarometry and were compared to field geophysical data reported from the literature the velocity of p waves 7 9 km s obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation our results are consistent with a roughly w e oriented fastest p wave propagation direction in the uppermost mantle beneath the canary islands and with the lithosphere structure proposed by previous authors involving a crust mantle boundary at around 18 km in depth overlaid by intermediate material between 11 and 18 km

a reevaluation of the flow data for olivine abstract the solid medium deformation experiments on polycrystalline olivine at pressures of 500 1500 mpa show steadily increasing creep strength with decreasing water content thus water content can significantly influence the strength of olivine room pressure experiments on olivine single crystals have been conducted under conditions of low p h 2 o and therefore should be compared with solid medium experiments on dried samples deformed under anhydrous conditions the single crystals flow more rapidly by 2 3 decades in strain rate for a given stress three hypotheses regarding the origin of this strength discrepancy are 1 1 solid medium sample strengths are higher due to the finite strength of the confining medium 2 2 temperature gradients in the solid medium samples render them stronger 3 3 the application of high confining pressure has significantly increased strength these hypotheses have been analyzed and none of them appears to alter strain rates by more than a decade another hypothesis that dislocations are pinned by iron oxide precipitates in the dry solid medium experiments also does not appear likely unless some unknown effect in the solid medium apparatus results in anomalously high strength for olivine the discrepancy must be attributed to fundamentally different behavior of the single crystal and the aggregate

the role of fluids in the metamorphism of komatiites agnew nickel deposit western australia the agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy olivine tremolite chlorite cummingtonite enstatite assemblages occur at the margin of the ultramafic lens giving way to olivine anthophyllite olivine talc and olivine antigorite assemblages successively inwards these rocks are interpreted as having crystallized from komatiitic lavas and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates the relative modal abundances of metamorphic olivine tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith peak metamorphic conditions are estimated at 550 c based on garnet biotite thermometry at a maximum pressure of 3 kb this temperature falls within the narrow range over which metamorphic olivine may co exist with enstatite anthophyllite talc or antigorite depending upon the fugacity of water in the metamorphic fluid the observed mineralogical zonation is therefore attributed to infiltration by co2 rich fluids generated by decarbonation of talc carbonate rocks formed during pre metamorphic marginal alteration of the ultramafic lens metamorphic fluids were essentially binary mixtures of water and co2 with minor h2s having a maximum partial pressure less than 1 percent of total pressure enstatite bearing assemblages formed in the presence of co2 rich fluids at fluid rock volume ratios close to one while anthophyllite talc and antigorite bearing assemblages formed in the presence of progressively more water rich fluids at progressively lower fluid rock ratios

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

kinking in mg2geo4 olivine an ebsd study we have used electron backscatter diffraction ebsd to characterize the internal deformation and orientation of olivine grains within an experimentally deformed mg2geo4 olivine aggregate we observe that a significant fraction of the grains contain kink bands our observations of kink band geometry combined with the work of earlier authors indicate that 001 slip is the dominant slip system operating in mg2geo4 olivine at 1000 1473 k at 0 6 1 3 gpa since mg2geo4 is a high pressure analog for natural compositions of olivine this is consistent with the observations made by others that the dominant slip system for olivine in the mantle switches from 100 to 001 as pressure increases we also observed that kink bands formed in all measured olivine grains with 001 axes oriented within 35 of the compression direction we thus conclude that in a randomly oriented polycrystal undergoing uniaxial deformation at low to moderate temperatures 18 of all grains will deform by kinking and that kink band formation is an important accommodation mechanism for olivine at these conditions

creep mechanism in mg2geo4 effects of a phase transition we deformed polycrystalline mg2geo4 olivine and spinel in a griggs type solid medium apparatus flow of mg2geo4 olivine can be represented by 6 5 l07 3 5 exp 105 kcal mol rt where kbar is the differential stress and s 1 is the natural strain rate for mg2geo4 spinel the flow law is 2 6 104 2exp 73 kcal mol rt the low stress exponent and activation enthalpy coupled with fine grain size 3 m suggest that mg2geo4 spinel deformed by a superplastic mechanism flow parameters for the olivine phase suggest a dislocation creep mechanism comparison of theoretical superplastic flow laws for mg2geo4 olivine with the spinel phase data suggests that strain rates in mg2geo4 spinel are only about a factor of 3 lower than for mg2geo4 olivine of the same grain size a similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion transformation from mg2geo4 olivine to spinel reduced grain size to approximately 3 m thus we might expect a similar reduction in grain size in the earth s transition zone which could result in superplastic deformation of the transformed phase and cause a weak decoupling zone at the transition boundary in the mantle superplasticity brought about by transformation induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

microstructures of olivine and stresses in the upper mantle beneath eastern china abstract we have examined the microstructure of spinel and garnet peridotite xenoliths from eleven basalt localities in eastern china the dislocation substructures of olivine in all of the specimens are similar to those reported previously from other areas of the world and are strongly reminiscent of the substructures produced in laboratory deformation of polycrystals at high temperature and pressure extensive measurements of 100 and 001 subgrain dimensions demonstrate that in almost all cases the frequency of subgrain spacing follows a lognormal distribution with geometric mean spacings of 30 60 m in one locality mingxi two xenoliths show significantly narrower spacings of 15 20 m indicating a much higher stress while others from the same locality have the normal wider spacing and one specimen has a mixed distribution indicating that the strain produced at higher stress was not sufficient to re equilibrate the microstructure even higher stresses but probably trivial strains are suggested by 110 slip bands sporadically superposed on the normal microstructure which presumably were produced during extraction of the xenoliths by the magma the general correspondence between these results and those of previous studies conducted elsewhere in the world suggests that the general level of lithospheric stress of c 10 50 mpa has been essentially constant since the middle tertiary or that the dynamic process of magma generation and or extraction always leads to similar microstructures

constraints on the mechanism of deformation of olivine imposed by defect chemistry abstract more is known about diffusion electrical conductivity and creep in forsterite and olivine than any other mineral this paper examines these data to see what constraints are placed on the mechanisms of deformation in these minerals experimental observations show a creep rate for forsterite independent of the activity of oxygen a o 2 in olivine the creep rate is proportional to a 1 6 o 2 and in addition is ten times faster for the enstatite excess situation than for the magnesiowustite excess situation these data are consistent with positively charged kinks being the rate controlling species in olivine the data for forsterite are also compatible with a kink controlled mechanism although some other jog and diffusion controlled mechanisms remain as possibilities the activation energy for creep is not related to that for diffusion of silicon or oxygen for kink controlled mechanisms the activation energy is approximately given by the sum of the fermi level and the activation energy for charged kink migration minus the ionization energy of the kink the fermi level can be changed by minor changes in impurities and by changes in the imposed thermodynamic environment so unless these parameters are controlled during deformation variations in observed activation energies for creep are to be expected from one set of experiments to another