fabric evolution in salem attur shear zone south india and its implications on the kinematics abstract domains of opposing shear sense relative to the overall kinematic framework often add to the ambiguities in the interpretation of movement picture along major shear zones rheologic properties strain partitioning as well as rotation of fabric are the principal mechanisms that contribute to the evolution of fabric during shearing the relative dominance of these mechanisms varies with progressive strain leading to the development of domainal fabric departing from the regional stress strain vectors the salem attur shear zone exhibits imprints of multiphase shearing and domains of meso to micro fabrics indicating opposing shear sense resulting in conflicting interpretations of the regional kinematics since the sense of movement is a major constraint in matching the shear zones in gondwana fragments this paper focuses on understanding the movement picture of the shear zone and the mechanism of evolution of domainal fabric

microstructures and c axis fabrics indicative of quartz dissolution in sheared quartzites and phyllonites abstract quartzites sheared in low metamorphic grade shear zones that extend along the contact with migmatitic gneissic complexes in the quadrilatere ferifero region southeastern brazil show quartz microstructures indicating deformation by dissolution dissolution occurred adjacent to the foliation planes preferentially in quartz porphyroclasts oriented with their 0001 planes parallel to the foliation dissolution worked as an efficient mechanism to remove porphyroclast edges smoothing foliations and enabling strain accommodation both dissolution and crystal plasticity were competitive crystallographically controlled processes operating in different domains of the evolving microstructure dissolution occurred along the foliation planes while isovolume crystal plasticity and accompanying recrystallization operated essentially in the quartzose domains between folia phyllonites formed in the most strained domains of the shear zones fine grained quartz grains in phyllonites show uncommon c axis fabrics c axis oriented at low angles to the stretching lineation which were interpreted to form either by direct reprecipitation or by residual concentration of grains unfavourably orientated to undergo dissolution domainal c axis fabrics reflect a deformation partitioning path with increasing participation of solution transfer strengthening c axis maxima at low angles with the stretching lineation at the expense of crystal plastic processes during progressive phyllonitization

basal ice facies and supraglacial melt out till of the laurentide ice sheet tuktoyaktuk coastlands western arctic canada glacially deformed massive ice and icy sediments mi is in the eskimo lakes fingerlands and summer island area of the tuktoyaktuk coastlands western arctic canada show in the same stratigraphic sequences features characteristic of both basal glacier ice and intrasedimental ice basal ice features comprise 1 ice facies and facies groupings similar to those from the basal ice layers of contemporary glaciers and ice sheets in alaska greenland and iceland 2 ice crystal fabrics similar to those from basal ice in antarctica and ice cored moraines on axel heiberg island canada and 3 a thaw or erosional unconformity along the top of the mi is buried by glacigenic or aeolian sediments intrasedimental ice consists of pore ice and segregated ice formed within pleistocene sands deposited before glacial overriding the co existence of basal and intrasedimental ice within the mi is records their occurrence within the basal ice layer of the laurentide ice sheet stagnation of the ice sheet and melt out of till from the ice surface allowed burial and preservation of the basal ice layer on a regional scale the widespread occurrence of supraglacial melt out till with clast fabrics similar to those in the underlying ice suggests that such till can be well preserved during partial thaw of a continental ice sheet in lowlands underlain by continuous permafrost

syntectonic development of a strain independent steady state grain size during mylonitization abstract michibayashi k 1993 syntectonic development of a strain independent steady state grain size during mylonitization tectonophysics 222 151 164 for a distance of greater than 50 km along the median tectonic line japan syntectonic recrystallization of quartz has resulted in the development of a strain independent steady state grain size within the core of the kashio shear zone that is also independent of rock type the mean grain size of quartz in recrystallized aggregates progressively decreases towards the shear zone core becoming stable at around approximately 37 m even though the strain increases within more strongly mylonitized rocks some variation of mean grain sizes may result from secondary effects due to hydrothermal activity associated with the median tectonic line despite the development of a steady state mean grain size individual grains were not stable as revealed by log normal grain size distributions consequently dynamic recrystallization and grain growth were competing processes that achieved a balance and hence resulted in the development of the steady state grain size the actual grain size that resulted probably depended on both the water content and strain rate as well as the flow stress approaching the centre of the shear zone mylonitic fabrics e g s c fabrics mica fish were established well before a steady state grain size was reached a steady state grain shape shape orientation and crystallographic fabrics may not be developed until even more mylonitization had occurred variable grain sizes result from heterogeneous deformation as a consequence of deformation partitioning even under a stable stress prior to the development of a steady state grain size consequently it should be determined whether a steady state grain size has been achieved within a shear zone before the grain size is used as a paleopiezometer

discharge of debris from ice at the margin of the greenland ice sheet sediment production at a terrestrial section of the ice sheet margin in west greenland is dominated by debris released through the basal ice layer the debris flux through the basal ice at the margin is estimated to be 12 45 m 3 m 1 a 1 this is three orders of magnitude higher than that previously reported for east antarctica an order of magnitude higher than sites reported from in norway iceland and switzerland but an order of magnitude lower than values previously reported from tidewater glaciers in alaska and other high rate environments such as surging glaciers at our site only negligible amounts of debris are released through englacial supraglacial or subglacial sediment transfer glaciofluvial sediment production is highly localized and long sections of the ice sheet margin receive no sediment from glaciofluvial sources these findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin these data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the greenland ice sheet

two stage collision related extrusion of the western dabie hp uhp metamorphic terranes central china evidence from quartz c axis fabrics and structures abstract the western dabie orogen also known as the hong an block forms the western part of the dabie sulu hp uhp belt central china rocks of this orogen have been subjected to pervasive ductile deformation and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones quartz textures in these mylonites contain important clues for understanding the movement sense of late collisional extrusion and exhumation of high pressure ultrahigh pressure hp uhp rocks from the lower crustal level to the upper crustal level during middle triassic and early jurassic the orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen the asymmetry of c axis patterns consistently indicates top to the southeast thrusting across the orogen in early structural stages later stages of deformation show different senses of movement in northern and southern parts of the orogen with top to the northwest sinistral shearing recorded in rocks north of the xinxian hp uhp eclogite facies belt and top to the southeast dextral shearing south of the same unit based on our study on quartz c axis fabrics and marco to micro scale structures simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the xinxian hp uhp unit is explained by upward extrusion in early stages of deformation the extrusion process has been attributed to syn and late collisional processes accounting for some earlier deformation in the western dabie orogen such as metamorphic sequences around the core of the xinxian hp uhp eclogite facies unit much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field an orogen parallel eastward extrusion of the xinxian hp uhp eclogite facies unit however occurred diachronously in later stages of deformation therefore a tectonic model combining an early upward extrusion with a later eastward extrusion is presented two different stages and types of extrusion for exhumation of hp uhp rocks are suitable to all of east central china geochronological data shows that the first upward extrusion occurred during middle triassic the second eastward extrusion occurred during late triassic to early jurassic these two extrusions are correlative with two stages of rapid exhumation of the dabie hp uhp rocks respectively these two stage late collisional middle triassic to early jurassic extrusion events bridge the gap between syn collisional early to middle triassic vertical extrusion and post collisional cretaceous eastward directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of hp uhp rocks

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

microstructural evolution of the serido belt ne brazil the effect of two tectonic events on development of c axis preferred orientation in quartz the polyphase evolution of the serido belt ne brazil includes d crust formation at 2 3 2 1 ga d 2 thrust tectonics at 1 9 ga and crustal reworking by d 3 strike slip shear zones at 600 ma microstructural investigations within mylonites associated with d 2 and d 3 events were used to constrain the tectono thermal evolution of the belt d 2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions 600 650 degreesc through grain boundary migration subgrain rotation and operation of quartz q prism slip continued shearing and exhumation of the terrain forced the re equilibration of high t fabrics and the switching of slip systems from c prism to positive and negative a rhombs during d 3 enhancement of ductility by dissipation of heat that came from syn d 3 granites developed wide belts of amphibolite facies mylonites continued shearing uplift and cooling of the region induced d 3 shear zones to act in ductile brittle regimes marked by fracturing and development of thinner belts of greenschist facies mylonites during this event switching from a prism to a basal slip indicates a thermal path from 600 to 350 degreesc therefore microstructures and quartz c axis fabrics in polydeformed rocks from the serido belt preserve the record of two major events which includes contrasting deformation mechanisms and thermal paths c 2003 elsevier ltd all rights reserved

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

shear sense a new approach that resolves conflicts between criteria in metamorphic rocks investigation of microstructural relationships in major movement zones in metamorphic rocks where the sense of displacement is known from regional geological relationships indicates numerous problems with current concepts of shear sense criteria and their application the direction of apparent shearing commonly conflicts from one criterion to another e g from the symmetry of quartz c axis orientation diagrams to the asymmetry of extensional crenulation cleavages this implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous a new approach to interpreting shear sense involving the use of strain fields resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear sense histories extending back before the last shearing event for any foliated metamorphic rock it also provides a powerful tool for determining the structural metamorphic path that a rock has followed within an orogen for determination of the shear sense on the last foliation developed in a rock this approach uses geometries developed around competent heterogeneities such as quartz pebbles pegmatite pods veins porphyroclasts porphyroblasts and breccia clasts a shear sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows

strain induced phase changes within cold basal ice from taylor glacier antarctica indicated by textural and gas analyses this paper reports detailed textural and gas measurements conducted in cold basal ice 178c from the margin of taylor glacier an outlet glacier of the east antarctic ice sheet the analyzed samples were retrieved from a basal ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout the basal sequence exhibits two contrasting ice facies defined as the englacial and stratified facies on the one hand analysis of ice crystal textures from the basal ice sequence provides evidence for localized ductile deformation especially within the stratified facies where significant dynamic recrystallization was detected on the other hand high resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies these gas composition changes are typical of melting refreezing processes but are not associated with any significant loss of gas volume given the specific subglacial thermal conditions at the margin of taylor glacier we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre melt phase it is argued that such processes may play an important role in the post genetic geochemical evolution of cold debris laden ice and may be enhanced through intense strain conditions

the deforming bed debris rich basal ice continuum and its implications for the formation of glacial landforms flutes and sediments melt out till abstract there are two potentially important shear zones at the base of the glacier the deforming layer and or the debris rich basal ice layer these two layers have a significant effect on glacier motion and glacial erosional depositional and sedimentary processes however it is shown that most of the processes operating within these two layers are very similar this includes compression at the ice margin longitudinal extension and simple shear upglacier similar styles of shear zone and associated fabric development and similar incorporation transport and depositional processes in the past many criterion have been developed to identify distinct deposition from the deforming layer and the debris rich basal ice layer however if the processes within these layers are similar then it makes the distinct identification of these two separate layers in the geological record very difficult however it is also shown that the main differences between the two layers include rates of sediment movement processes and preservation potential the later is very important as it appears that the two layers are interconnected and that material is melted out from the debris rich basal ice to form a significant component of the deforming layer in this way the final depositional imprint is from the deforming layer and there is little likelihood of the debris rich basal ice layer being preserved undeformed this is demonstrated from modern glacial environments pleistocene flute studies and studies of pleistocene melt out tills from the u k u s a and germany

on the development of zones of reverse shearing in mylonitic rocks abstract most shear zones contain a minor proportion of domains recognizable from thin section to outcrop scale which show an opposing shear sense relative to the overall kinematic framework in this paper we discuss how these universal features result from the evolving deformation strain partitioning that generally accompanies progressive general shear principally in low metamorphic grade shear zones because the rheologic properties and consequently the contributions of the different deformation mechanisms change continuously with progressive strain an individual domain with a predominant deformation mechanism much faster or slower than the average may depart from the general kinematics enhancing the viscosity contrast between adjacent layers and causing locally reverse shear in the domain boundaries care is needed therefore in interpreting opposing shear senses in the same shear zone as indicating superimposed deformation events

cretaceous deformation chegitun river area chukotka peninsula russia implications for the tectonic evolution of the bering strait region 1 the koolen metamorphic dome of chukotka peninsula has been interpreted as a mid cretaceous extensional core complex on the nw flank of the dome three lithotectonic units are exposed 1 high grade unit composed of sillimanite grade rocks 2 tanatap unit composed of polydeformed greenschist grade phyllites and marbles 3 chegitun unit composed of unmetamorphosed early paleozoic limestones four early cretaceous to tertiary deformational events are documented d1 is attributed to the collisional closure of the south anyui suture the 40ar 39ar ages from the tanatap unit constrain this event at 117 124 ma relict kyanite bearing assemblages in the high grade unit probably formed during this event under moderately high pressure conditions d2 is related to extensional deformation in the koolen dome in the high grade unit sillimanite grade metamorphism was followed by rapid cooling and exhumation at 109 104 ma in the tanatap unit early fabrics underwent isoclinal folding and a new cleavage s2t was developed the kinematics of the high grade unit indicates ductile deformation by top to the south shear as is the case in the dome core this suggests that the koolen dome is a unidirectional core complex formed by south directed extension we believe collapse of overthickened crust caused extension in the koolen dome the crust was thickened during early cretaceous collision and during subsequent oroclinal bending of the structures in the bering strait region the youngest structures in the chegitun valley are nw striking normal faults kinematically linked to ne striking right lateral strike slip faults they are probably the continuation of structures found offshore in the hope basin

effect of water on the rheology of experimentally deformed quartzite empirical flow laws have been determined for simpson quartzite samples deformed to mechanical steady state in the quartz stability field using griggs blacic solid medium deformation apparatus experiments were conducted on samples both as received dry and with water added via the dehydration of a talc confining medium wet best fits of the power law type yield a stress exponent of 2 72 0 19 an activation enthalpy of 134 32 kj mol 1 and a preexponential constant of 1 16 1 15 0 58 10 7 mpa 2 72 s 1 for the dry quartzite law and are 2 61 0 15 145 17 kj mol 1 and 5 05 5 00 10 6 mpa 2 61 s 1 for the wet quartzite the enhanced hydrolytic weakening of the wet experiments appears to affect the flow laws mostly in the preexponential constants possibly as a defect concentration term that is higher in the wet than in the dry law over the range of experimental conditions 750 900 c 10 7 to 10 4 s 1 and 1 0 1 25 gpa confining pressure dry specimens are 1 5 2 5 times stronger than wet samples deformed at the same conditions the microstructures produced are analogous to those observed in many natural quartz tectonites recrystallization is well developed in the wet but not the dry specimens at flow stresses below 500 mpa microstructures indicative of recovery are well developed a comparison of our creep activation enthalpies with those determined from diffusion experiments are consistent with deformation of wet simpson quartzite that is limited by oxygen self diffusion inconsistencies between the present study and previous studies are probably the result of differences in sample material sample treatment and sample assemblies chemical environment

two facies interpretation of the basal layer of the greenland ice sheet contributes to a unified model of basal ice formation two exposures of basal ice in alaska and greenland which have previously provided the basis for contrasting models of basal ice development are in fact directly comparable reinterpretation of previous field descriptions combined with new structural and sedimentological data from west greenland facilitates a simple unification of previous models of basal ice development and a common terminology for the field description of basal ice sequences the stratigraphy of the basal ice sequence at any site indicates the subglacial conditions and processes operating up glacier of that site presented here are interpretations of the major sequence types

the tertiary geodynamical evolution of the aegean arc a paleomagnetic reconstruction abstract the paleomagnetic results obtained in the last 5 6 years from tertiary formations in the aegean domain indicate that the lower miocene arc was almost rectilinear with an e w trend and that its curvature has been acquired tectonically in two major phases during the middle miocene a first phase of deformation is characterized by rotations occurring at the two terminations of the arc clockwise in the west epirus anticlockwise in the east southeastern anatolia a second phase of rotation occurring in the last 5 ma about a pole situated in the southern adriatic sea has affected only the northwestern part the data also indicate that the antalya basin has not been involved in the geodynamical evolution of the arc so that the bey daglari represent its easternmost unit a more complex rotational pattern is observed in the internal zones the large rotations measured in evia 48 and skyros 26 probably occurred in the last 5 ma and result from rotation of fault bounded blocks in a shear zone connecting the north aegean trough to the external arc in the izmir region in the karaburun peninsula of western anatolia and in the island of lesbos significant counterclockwise clockwise or null rotations of coherent blocks have taken place during the neotectonic extensional regime furthermore the values of the inclination obtained from both sedimentary and volcanic oligo miocene formations are systematically shallower than expected on the basis of a geocentered dipole field this suggests that the entire studied area has undergone a northward drift of over 1000 km since the middle miocene

tectonic stresses on the mainland of greece in situ measurements by overcoring abstract direct in situ measurements of the horizontal stresses in continental greece indicate a predominantly n s extensive stress considerable scatter of the azimuth of the axis of extension is found in thrace in contrast to a more homogeneous pattern in macedonia and peloponnesus these results are obtained with an overcoring technique in boreholes of a few meters they can be compared with neotectonic and seismic data to evaluate the present deformation process in this basin

the aegean region deep structures and seismological properties summary 3 d p wave velocity anomaly patterns the geometry of the hellenic wadatibenioff zone and of the seismically active fracture zones in the overlying continental wedge and the depth distribution of seismogenic properties seismic activity energy and b value for the aegean region are examined in this study the western and eastern parts of the area studied differ significantly in terms of the depth distribution of the considered seismological parameters and the velocity structures the results indicate different physical conditions in the western and eastern parts of the region in question data for the surface heat flow and the evolution of the volcanism in the aegean region since the early eocene are employed to interpret the results of the present study possible geodynamic processes at the plate boundaries between africa and eurasia are discussed

basal ice microbiology at the margin of the greenland ice sheet basal ice at the margin of the greenland ice sheet was studied with respect to its physical characteristics and microbiological community the basal ice contained high concentrations of dissolved ferrous fe and must therefore be anoxic oxygen consumption experiments indicate that 50 of the oxidation was due to biological activity while the rest could be attributed to chemical processes most likely weathering reactions with ferrous fe at least six different fe containing mineral sources were detected in basal ice together with potential bioavailable fe nanoparticles an active denitrifier population was identified due to formation of 30 n dinitrogen gas after amendment of anoxic sediment slurries with 15 n no3 sulfate reduction could not be detected the solid ice facies contained an abundant 10 8 cells cm 3 and complex microbial community that harbored representatives of at least eight major phyla within the domain bacteria the clone library was dominated by members of the subdivision of proteobacteria of which the largest proportion was affiliated to the genus rhodoferax that comprises facultative aerobic iron reducers the second most abundant phylum was bacteroidetes the solid ice facies had many physical similarities with the overlying debris rich banded ice facies indicating that they formed by similar subglacial processes and harbor similar microbial communities this study extends our knowledge of life in subglacial environments such as beneath ice sheets genbank accession numbers hm439882 hm439950 hq144215 hq144221

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

strain measurements with the borehole slotter abstract in the past the doorstopper method has been used successfully for determining in situ tectonic stresses here an example is given about the application of the borehole slotter in the domain of tectonic stress determinations which is an innovative strain relief technique without overcoring data are presented that test the reliability of the strain measurements by the borehole slotter under well known stress conditions that indicate the reproducibility of borehole slotter strain data compared with those from doorstopper measurements and that demonstrate the utility of the borehole slotter even for the determination of weak tectonic strains in northern switzerland the borehole slotter was used in a horizontal borehole under lithostatic loading conditions only in a rock slope area with very heterogeneous stress conditions and in a vertical borehole where tectonic stress should be dominant the results show that the borehole slotter is a reliable technique which allows precise determination of in situ strains in addition borehole slotter measurements are less time consuming and therefore less costly than most other stress measuring techniques an important aspect in a future of reduced research budgets summary the borehole slotter is a strain relief technique using a recoverable strain sensor that need not be glued to the rock surface and need not be overcored it is quicker to use than most other stress or strain measuring techniques especially in comparison to standard overcoring techniques fig 14 the borehole slotter supplied convincing results with respect to principal stress orientations in good agreement with i principal stress orientations related to lithostatic stress conditions dittingen 2 site ii palaeostress orientations during the main stage of jura folding only 13 to 7 ma laufen 1 site iii the results of neighboring door stopper measurements dittingen 1 site the borehole slotter tests do not supply stress magnitudes directly thus the elastic properties of the rock mass have to be determined independently stress ratios from borehole dotter tests in dittingen 2 are realistic in conclusion the borehole slotter is a fully developed probe to measure in situ strains in boreholes it allows the reliable determination of a large number of data within a short time even in weakly stressed rocks this makes the borehole slotter a powerful tool for the determination of contemporary regional tectonic stress fields

geology and thermochronology of tertiary cordilleran style metamorphic core complexes in the saghand region of central iran an 100 km long north south belt of metamorphic core complexes is localized along the boundary between the yazd and tabas tectonic blocks of the central iranian micro continent between the towns of saghand and posht e badam amphibolite facies mylonitic gneisses are structurally overlain by east tilted supracrustal rocks including thick 1 km steeply dipping nonmarine siliciclastic and volcanic strata near the detachment the neybaz chatak fault the gneisses are generally overprinted by chlorite brecciation crosscutting relationships along with u pb zircon and 40 ar 39 ar age data indicate that migmatization mylonitic deformation volcanism and sedimentation all occurred in the middle eocene between ca 49 and 41 ma the westernmost portion of the tabas block immediately east of the complexes is an east tilted crustal section of neoproterozoic cambrian crystalline rocks and metasedi mentary strata 10 km thick the 40 ar 39 ar biotite ages of 150 160 ma from structurally deep parts of the section contrast with ages of 218 295 ma from shallower parts and suggest late jurassic tilting of the crustal section these results define three events 1 a late jurassic period of upper crustal cooling of the western tabas block that corresponds to regional jurassic cretaceous tectonism and erosion recorded by a strong angular unconformity below mid cretaceous strata throughout central iran 2 profound approximately east west middle eocene crustal extension plutonism and volcanism ca 44 40 ma and 3 2 3 km of early miocene ca 20 ma erosional exhumation of both core complex and tabas block assemblages at uppermost crustal levels resulting from significant north south shortening the discovery of these and other complexes within the mid tertiary magmatic arcs of iran demonstrates that cordilleran style core complexes are an important tectonic element in all major segments of the alpine himalayan orogenic system

a quantitative framework for interpretation of basal ice facies formed by ice accretion over subglacial sediment 1 we have constructed a numerical model of basal ice formation for glacier ice in contact with subglacial sediment the model predicts four different ice facies whose formation is controlled by availability of subglacial water to satisfy the basal freeze on rate clean or clotted facies may result from congelation or frazil ice growth occurring when supercooled meltwater separates ice base from substrate or if a groundwater source can supply water to the ice base at a velocity equal to the freezing rate laminated facies develops when the supply of subglacial water is sufficiently constrained for cryostatic suction to raise the subglacial effective stress above a threshold for intrusion of ice into sediment by regelation debris laminas 1 mm are entrained by short periodic regelation events of a few hours separated by longer periods days to months of congelation further meltwater limitation produces a massive dirty ice facies due to stacking of debris laminas the model predicts growth of solid dirty ice facies when the bed is meltwater depleted with fast freezing 5 mm yr 1 causing enhanced erosion 30 mm yr 1 we find that basal ice facies and sediment entrainment are controlled mainly by the ratio of freezing rate to water supply rate the predicted ice facies compare favorably with borehole camera imagery of the basal ice layer in kamb ice stream west antarctica facies variability in this layer suggests complex hydrologic history for the west antarctic ice sheet with significant changes occurring over a period of several thousand years

quartz plastic segregation and ribbon development in high grade striped gneisses quartz microstructures and c axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high grade alem paraiba shear zone in southeastern brazil are documented cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons these ribbons are separated by feldspar rich domains devoid of quartz the stage at which individual stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode which is reflected by an abrupt increase in the smoothness of the ribbon boundaries this change is interpreted to represent a strain softening kink in the stress strain time path progressive ribboning is accompanied by strengthening of the c axis fabric z maximum indicative of continued plastic flow by basal glide operation of basal glide at these high temperature conditions 680 700 c is interpreted to be a consequence of relatively dry deformation conditions a model is then proposed for development of straight quartz ribbons in high grade striped gneisses where scattered quartz grains are continuously stretched and segregated by crystal plastic processes the small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons

role of myrmekite and associated deformation fabrics in controlling development of granitic mylonites in the pofadder shear zone of southern namibia myrmekites developed within metagranites of the pofadder shear zone psz in southern namibia were investigated to understand how alkali feldspar deforms under fluid poor mid crustal conditions myrmekites showed two different morphologies 1 wormy myrmekite with a fine grained vermicular structure that occurred closest to the alkali feldspar host and 2 recrystallised myrmekite composed of regularly sized quartz grains that formed 120 boundaries with the plagioclase and occurred along the exterior of alkali feldspar and in contact with the mylonitic matrix these textures were interpreted to represent two generations of myrmekite growth recrystallised myrmekite masses were dismembered and incorporated into the fine grained matrix this suggests that myrmekite formation during psz deformation contributed to the grain size reduction of alkali feldspar and aided in the generation of the fine grained polymineralic matrix which accommodated most of the strain syn kinematic reactions such as myrmekite and muscovite production were internally balanced and t mh2o pseudosections indicated that progressive deformation could have occurred under closed conditions the biotite muscovite plagioclase alkali feldspar quartz equilibrium assemblage was stable with decreasing fluid contents until fluid saturated or slightly fluid undersaturated conditions were reached around 2 mol h2o the variability of the quartz rod spacing and the wide range of results from two feldspar thermometry and phengite barometry on associated muscovite growth indicates that peak metamorphic conditions were around 500 c 0 5 gpa and that myrmekite growth continued as the shear zone was exhumed the myrmekite forming reaction is a deformation induced interface coupled dissolution precipitation reaction with relict magmatic plagioclase supplying the needed cationic inputs these results indicate that deformation within granitic units of the psz was aided by the formation and deformation of myrmekite and could have taken place under closed relatively fluid poor conditions

kinematic vorticity and strain rate patterns associated with ductile extrusion in the chelmos shear zone external hellenides greece abstract kinematic vorticity strain rate finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal scale shear zone the chelmos shear zone csz of the external hellenides the csz has a curved shape which is marked by a frontal flat and a steep zone the spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone the pure shear component of deformation decreases from back to front and from top to bottom the ductile thinning normal to the shear zone s boundaries is higher in the steep zone and decreases westwards quartz grain size and strain rate in the csz is distributed into two approximately homogeneous domains the higher strain rate is recorded in the steep zone and the base of the frontal flat zone with lower rates in the upper parts of the frontal flat zone based on mesoscopic structural data and the spatial distribution of the shear strain rate the existence of a west dipping back flow zone in the boarder zone between these two domains is inferred the overall pattern of deformation in the csz implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone ductile deformation of rocks probably started close to simple shear on a planar low angle shear zone progressively became penetrative and more general shear occurred as the csz steepened up this progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal flat zone solid state extrusion in the csz was also associated with a heterogeneous and progressive increase in strain rate

extensional compressional tectonics associated with the aegean arc comparison with the andean cordillera of south peru north bolivia extensional compressional tectonics are analysed along the convergent boundaries of the aegean arc and of the andean cordillera of south peru and north bolivia this paper deals only with deformations of miocene pliocene and pleistocene ages the prevailing regime in the aegean basin is extensional yet two compressional tectonic phases of u miocene l pliocene and of l pleistocene age have been recorded they are also known in many places all around the mediterranean thus forces that induce compressional tectonics in the aegean seem to have their origin outside the region they seem to be linked with changes in the conditions of convergence change in rate between the arabo african and eurasian plates beneath the andean cordillera of south peru north bolivia the subduction zone plunges at an angle of 25 30 as it does under the aegean arc there as in the aegean sea tectonics are predominantly extensional and several short lived compressional phases break up this extensional regime the latter may also be induced by changes of the conditions of convergence here between the nazca and south american plates beneath central peru north of the nazca ridge the subduction zone plunges at a small angle about 10 there neotectonic deformations are different in the recent they are essentially compressional or expressed by strike slip faulting geological data from the aegean and andes seem to demonstrate that the compressional deformation in the crust above subduction zones is strongly controlled by the stress conditions along convergent boundaries

tectonic stresses on the boundary of the aegean domain in situ measurements by overcoring abstract direct in situ measurements of the horizontal stresses on the border of the aegean domain obtained by overcoring in boreholes a few meters deep revealed the various characteristics of plate boundaries continent continent collision subduction inter arc basin etc

preservation of basal ice sediment texture in ice sheet moraines ice sheet moraines near kangerlussuaq in west greenland inherit distinctive particle size distributions from basal ice although debris structures from the basal ice are commonly destroyed by deposition and resedimentation processes the abundance of clay and silt in the dispersed faciesa basal ice at the ice sheet margin is clearly re ected in the sedimentology of the ice sheet moraine geographical variations in the texture or grain size of moraine sediments may thus re ect geographical variations in basal ice this o ers a new approach to reconstructing the basal ice characteristics and hence the thermal and dynamic properties of former ice sheets 2000 elsevier science ltd all rights reserved

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

natural annealing of dynamically recrystallised quartzite fabrics example from the cevennes se french massif central abstract quartzite samples from the sw cevennes french massif central have been investigated in order to reveal the effects of annealing on a previously dynamically recrystallised quartz fabric the studied quartzite is interlayered with turbiditic micaschist series and after regional deformation the whole sequence was intruded by the st guiral granodiorite at 1 3 kbar and up to 650 700 c recorded crystallographic preferred orientation cpo patterns of the dynamically recrystallised quartzite are classified as type i with a monoclinic symmetry and type ii with an orthorhombic symmetry type i is related to simple shear dominated deformation and type ii to apparent constriction these two fabric types are consistent with structural observation that a constrictive and coaxial deformation regime controlled the formation of non cylindrical and refolded folds in the metamorphic aureole which produced essentially static annealing cpo patterns intensify slightly and grain size increases towards the granodiorite until the temperature at which inversion from low to high quartz is triggered this inversion caused a volume increase and subsequent intergranular stresses activated slip systems fitting the temperature and water content this study shows that annealing of dynamically recrystallised quartz crystals can activate previously inactive slip systems reactivate slip systems and amplify older cpo fabrics this conclusion identifies limits for the use of annealed quartz fabrics in reconstructing structural histories when earlier dynamic recrystallisation has occured

kinematics of rock flow and the interpretation of geological structures with particular reference to shear zones abstract current interpretations of structures are generally based on homogeneous and steady deformation models despite the fact that both the heterogeneity of rocks materially rheologically and geometrically and the time dependence of imposed geological conditions give rise to significant heterogeneous and non steady flow in concert with field observations we emphasize that the expectation of heterogeneity and non steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation it is necessary to recognize the first order distinction between imposed boundary conditions typically used to define the tectonic regime e g transcurrent transpression and the response recorded by rocks within the zone structures and fabrics using s c fabric as an example it is demonstrated how flow with a non zero spinning component resulting from the rheological contrasts and or geologically realistic time dependent boundary displacement can drastically change the ideal geometric and kinematic relations between the fabric and the host zone in agreement with both theoretical analysis and field observation it is shown that natural flow regimes range from pure shear to pure rotation including super simple shear in consideration of the heterogeneity and non steadiness of natural deformation kinematic analysis is justifiable only within a homogeneous domain and steady period flow kinematics and mechanisms are interrelated in that firstly mechanisms provide internal constraints on kinematics ensuring that only certain flows are possible and secondly flow kinematics will favour development of certain mechanisms

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

identification of basal layer debris in ice marginal moraines russell glacier west greenland different processes of ice formation and deformation at the glacier bed create distinctive basal ice facies the geographical distribution of these facies at the glacier margin can indicate the distribution of subglacial environments previous research has shown that debris in dispersed facies basal ice at the margin of the greenland ice sheet has a distinctive particle size distribution that can also be recognised in moraines deposited where the dispersed facies is present this permits reconstruction of the distribution of basal ice characteristics and subglacial conditions from moraine sediments here we study stratified and debris band facies of the basal ice layer at the same site we find that contrary to previous suggestions these two facies can also be distinguished sedimentologically co variant plots of the c40 aggregate shape and ra aggregate roundness indices show that pebble and cobble sized clasts 2 to 7 in the debris bands are statistically more angular than those in the stratified facies sedimentological characteristics of the parent facies are retained in ice marginal moraines combined with previous work these new observations suggest that the geographical distribution of basal ice facies at a glacier margin can be reconstructed from sedimentological characteristics preserved in moraines

till deposition by glacier submarginal incremental thickening macro and micro scale sedimentological analyses of recently deposited tills and complex push squeeze moraines on the forelands of icelandic glaciers and in a stacked till sequence at the former younger dryas margin of the loch lomond glacier lobe in scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris rich basal ice by cumulative freeze on processes glacier submarginal processes are dictated by seasonal cycles of refreezing and melt out of tills advected from up ice by a combination of lodgement deformation and ice keel and clast ploughing although individual till layers may display typical a and b horizon deformation characteristics the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record at the macro scale icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings very strong fabrics typify unequivocally lodged clasts despite previous interpretations of these tills as subglacial deforming layers micro morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes and water escape and sediment flowage features are widespread a model of submarginal incremental thickening is presented as an explanation of these data involving till slab emplacement over several seasonal cycles each cycle involves 1 late summer subglacial lodgement bedrock and sediment plucking subglacial deformation and ice keel ploughing 2 early winter freeze on of subglacial sediment to the thin outer snout 3 late winter readvance and failure along a decollement plane within the till resulting in the carriage of till onto the proximal side of the previous year s push moraine 4 early summer melt out of the till slab initiating porewater migration water escape and sediment flow and extrusion repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements

morphologically ductile criteria for the sense of movement on slickensides from an extensional detachment fault in southern spain abstract new criteria for the sense of movement on slickenside surfaces displaying evidence for mesoscopic ductility are described in the neogene jaloche extensional detachment fault in southern spain this detachment develops spectacular slickensides and fault gouges and four of the criteria for the sense of movement observed along the highly polished slip surfaces are new to the literature they are 1 synthetic secondary fractures with hangingwall drag effects in the form of roll over microsynclines 2 metre scale oval shaped asymmetric culmination and depression features 3 microthrusts verging towards the direction of motion of the opposite block and dragging overthrusting previous planar elements on the fault surface and 4 flakes of fault surface material trailed and plastered in the direction of the missing block the geometry and microstructure of these features reveal that they deformed in a morphologically ductile manner by the coupled processes of cataclastic flow and shear localization probably during aseismic sliding during transient seismic pulses deformation occurred along shiny polished slickenside fault planes occasionally bearing jigsaw implosion breccias fluids under high pressure present initially may have promoted distributed cracking whereas the process of dilatancy hardening may have served to facilitate cataclastic flow in the deformation history

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

influence of temperature on the distribution of stress and displacement in a volcano a numerical approach a previously published geological model and numerical finite element simulation of a basaltic vollano is extended by introducing temperature dependent elastic parameters the geological model is composed of a shield with a central volcanic cone resting on oceanic crust the magma system consists of a 3 km deep upper reservoir from which rises a central column the model of the temperature field inside the volcano is based on geological observations and comprises 1 a hot zone 200 300 m thick around the magma system where temperature drops rapidly by conduction and 2 a cooler zone where temperature gently decreases by convection of circulating fluids the temperature dependent parameters the young modulus e the poisson coefficient v and the coefficient of linear thermal expansion of which the first two show a non linear variation with temperature introduce heterogeneity in the medium three zones with different mechanical behaviour are distinguished in the volcanic edifice shearing and strong deformation processes are prominent next to the magma system in the less rigid hot confined zone radial cracks are likely to be initiated in the central part of the volcano where tiffness increases and finally concentric vertical or outward dipping fissures are associated with horizontal extension towards the surface with cold and stiff matenal and a low state of thermal stress the results are used to propose a revised model of the internal structure of basaltic oceanic intraplate volcanoes

tectonic stresses and volcanism in situ stress measurements and neotectonic investigations in the etna area italy abstract mt etna has been building up for about 500 000 years on the eastern margin of sicily at the external border of the apenninic maghrebian mountain chain and at the crossing of several important regional fault systems the tholeiitic and alkaline magmatism of mt etna represents the northernmost manifestation of present day volcanism which was continuous in space and time from the upper miocene to the lower pleistocene in central eastern sicily in situ stress measurements and neotectonic investigations have been carried out on mt etna on its periphery and on the hyblean mountains to obtain a better understanding of the stress field which is connected to the present day activity of the volcano and a clearer picture of the regional geodynamical evolution the investigations allow a comparison of the present stress field with that acting during the quaternary the neotectonic analysis has given evidence of strong compressional stresses exerted on the pleistocene sediments overlying the substratum of the volcano and its immediate surroundings in addition to evidencing the already known extensional deformation the compressional forces were oriented approximately n s at the beginning of the volcanic activity the in situ stress measurements show now that this n s compression is today active only in the northernmost part of the etna area whereas in the remaining area extensional stresses oriented approximately wnw ese prevail it is concluded that the compressional stresses near the surface today limited to the northernmost part of the etna area had a much more widespread distribution at the beginning of the volcanic activity this conclusion is important for understanding the deep seated magmatic process focal mechanisms of earthquakes indicate a complex tectonic situation with normal faulting down to a depth of 5 km and reverse faulting at deeper levels

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

new insights into phanerozoic tectonics of south china early paleozoic sinistral and triassic dextral transpression in the east wuyishan and chencai domains ne cathaysia the northeast cathaysia area is characterized by an archetypical transpressional system with widespread strike slip shear zones whose geometries kinematics and ages are critical for deciphering the phanerozoic tectonic evolution of south china we present new structural geochronological and thermochronological data from the shear zones in the east wuyishan and chencai domains which record two phases of deformation the first phase corresponds to sinistral oblique shearing along arrays of nne oriented steep dipping zones under amphibolite facies conditions the sinistral oblique shearing commenced at 451 ma concurrently with regional nw se directed thrust shearing and folding the coexistence of sinistral and thrust structures indicates nw se transpressive shortening deformation dating by 40ar 39ar shows that such deformation terminated before 400 ma and was followed by cooling through 450 350 c at 400 370 ma our results merged with published data aid in tracing an early paleozoic orogen that extends through the jiangnan domain into the northeast cathaysia with the southeast yangtze acting as a foreland belt the synorogenic shortening was interpreted as resulting from underthrusting of the cathaysia beneath the east yangtze the second phase involved dextral oblique shearing associated with nne ssw transpression under greenschist to amphibolite facies conditions at 245 228 ma which was followed by postkinematic magmatism and cooling at 221 200 ma in the cathaysia similar middle triassic dextral shear zones were widespread and operated with approximately east striking thrusts as mutually complementary structures their kinematic coupling can be explained by a contractional termination model geodynamically we attributed middle triassic dextral transpression to the collisions of south china with north china and indochina

partitioning of non coaxiality in deforming layered rock masses abstract partitioning of non coaxiality and other flow parameters in deforming layered rock masses with nearly incompressible newtonian rheology is examined in two dimensions by the finite element method the results show that layers with viscosity contrast in suitable orientation deform by different and even by a reverse sense of non coaxiality this remarkable partitioning of non coaxiality is due to a large rotation rate of instantaneous stretching axes in a heterogeneous body as a result of geometry changing through deformation rather than through rotation of material lines

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

a model for the development of a domainal quartz c axis fabric in a coarse grained gneiss abstract quartz c axis fabrics in high grade coarse grained layered gneisses reveal a distinctive pattern with three perpendicular maxima which can be interpreted in terms of syntectonic recrystallization and growth mapping of the distribution of c axes within a single thin section demonstrates that the fabric is domainal fabric domains are on the scale of millimetres and are associated with individual quartz rich patches no single domain exhibits the overall fabric of the complete thin section within each domain weighting of the c axis fabric by the area of each quartz grain demonstrates that the largest grains have their c axes preferentially at the maxima a model is presented to account for the development of this fabric grains which can keep their stored strain energy to a minimum by the use of a single glide system oriented approximately parallel to the plane and direction of maximum resolved shear stress grow at the expense of other grains the largest most successful grains deform by glide on only one of either the basal a or the prism a systems depending on the crystallographic orientation of the grain each domain develops its own glide direction by co operation of the glide systems of its constituent grains the successful grains in a domain are those which can accommodate that direction the bulk strain of the rock is achieved by the addition of the strains distinct to each domain

geometry classification and kinematics of s c and s c fabrics in the mushandike area zimbabwe planar fabrics in small dextral shear zones in the mushandike area of the zimbabwe archaean craton consist of a sigmoidal primary foliation s displaced by discrete shears c or c these planar fabrics can be divided into porphyroclastic megacrystic and banded types depending mainly on the values of the angles between s and c or c surfaces the structures that define s surfaces and the spacing of c or c surfaces the values of the angles between the foliations are proposed as the basis for a general classification of planar fabrics in shear zones the values of the angles depend on microstructures and strain microstructural evidence shows that slip did not occur on s surfaces but only on c or c surfaces a model for the development of the fabrics proposes that s surfaces lie parallel to the xy plane of the local finite strain ellipsoid in domains of simple shear between c or c surfaces c surfaces may form in the orientation of a coulomb failure surface the field evidence suggests that an increase in bulk strain is at least partly accommodated by an increase in local strain in domains between c or c surfaces as the s surfaces rotate with the local finite strain ellipsoid

the tectonic evolution of the quadrilatero ferrifero minas gerais brazil abstract the structural framework of the quadrilatero ferrifero is the result of two main deformational events the first is an extensional event of palaeoproterozoic age between 2 100 and 1700 ma the transamazonian event which resulted in the formation of archaean granite gneiss domes and the nucleation of regional synclines in the overlying rio das velhas and minas supergroups strata such regional synclines are continuous with each other and are bordered by structural granite gneiss highs at the contact between these units ductile brittle to ductile extensional shear zones are developed representing the dislocation surfaces of archaean blocks this tectonism is probably related to the evolution of a metamorphic core complex formed in the hinterland to the west of a transamazonian collision zone which structured the paramirim craton the second event is compressive and associated with the closure of the pan african brasiliano proto ocean 650 500 ma situated to the east of quadrilatero ferrffero qf during this event a west verging fold and thrust belt ftb developed causing inversion amplification translation and rotation of the basinal synclines the ftb affected mainly the eastern portion of qf and obliterated many of the tectonic features of the extensional event

a new geodynamic model for the seismicity and in situ stresses of the mediterranean region abstract the application of plate tectonic concepts to mediterranean seismicity involves contradictions and anomalies which explains why there is such a great number of conflicting theories each of them based on different geophysical data a new model designed to take into account and explain all the data known to date is presented this is a magmatic vertical chimney where the magma is moved upwards while rotating thus exerting vertical pressure on the periphery of the lithospheric lid moreover it has some relevance to geological structures if this model is correct earthquake prediction can be viewed from an entirely different angle

melt out till and ribbed moraine formation a case study from south sweden abstract hummocky moraine with dispersed agglomerations of ribbed moraine here named asnen type ribbed moraine forms a 20 40 km wide zone over smaland south sweden terminated to the north by a sharp boundary to streamlined terrain the hummocky ribbed moraine zone can be geomorphologically linked to the subaqueous goteborg moraine formed at an oscillation stand still phase during the deglaciation of the swedish west coast based on detailed sedimentological and structural investigations of ribbed moraine ridges it is concluded that diamict sequences and associated sorted sediment were deposited due to passive melt out from stagnant debris rich ice with synsedimentary deposition of sorted sediments preferentially at a melting ice bed interface to accommodate for the geomorphological expression such an interpretation further implies that debris rich ice formed in an intermediate adfreezing zone between ice at the pressure melting point and a frontal frozen zone at deglaciation basal debris rich ice was stacked into transversally arranged zones controlled moraine forming ribbed moraine embryos the active phase of ridge formation the stacked sequences of debris rich ice eventually melted out beneath a stable and melt retarding supraglacial ablation complex to form the final moraine ridges the passive phase of ridge formation in areas with no stable supraglacial ablation complex the resulting landform after final de icing was a hummocky moraine landscape internal composition of moraine hummocks suggests that most of them are composed of sediment gravity flow sediments intercalated with stream deposited sediments all resting on a platform of subglacial melt out till de icing of the zone now occupied by hummocky and ribbed moraine took a considerable time deglacial 14 c age differences from lake basins on either side of the geomorphic boundary to the streamlined terrain indicate a separation between active and stagnant ice along that boundary and that it took another 200 300 yr for the stagnant ice zone to melt during the final formation of present day landforms

local variations in quartz c axis orientations in non coaxial regimes and their significance for the mechanics of s c fabrics abstract the quartz c axis orientations of quartz bearing s c mylonites vary on the mm scale two specimens from different tectonic settings the variscan schwarzwald west germany and the eastern alps austria are investigated in the first specimen prism glide along a predominates and in the second specimen basal glide along a predominates in both specimens the pattern of c axis variations is similar it is suggested to be characteristic of s c mylonites on the local scale there are two patterns of preferred orientation 1 a crossed girdle type i which is inclined in a sense opposite to the inferred overall sense of shear this occurs in moderately strained domains 2 a single girdle which is slightly inclined in the same sense as the inferred overall sense of shear this occurs in domains that suffered higher strain both types are considered to reflect stable orientations of the easy glide direction crystallographic a direction at different stages of the deformational history two kinematic interpretations are discussed quartz c axis fabric variations reflect a spatial and possibly also temporal partitioning in flow path between domains that underwent either 1 spinning coaxial flow and non spinning non coaxial flow or 2 spinning non coaxial flow and non spinning non coaxial flow associated with variations in parameters of the deformation such as magnitude of strain and strain rate and perhaps also rate of recrystallization and recovery in either interpretation in moderately strained domains oblique c axis girdles are generated that cannot be used as shear sense indicators

modern tectonic stress field in the mediterranean region evidence for variation in stress directions at different scales summary a compilation of more than one thousand stress indicators which include in situ stress measurements focal mechanisms microtectonic and other geological data allowed us to reconstruct the modern stress field in the mediterranean region and the surrounding area average stress directions at different scales have been reconstructed by means of a linear interpolation method this method takes into account the distribution scale and quality of stress data the results of the interpolation at plate scale allow us to recognize slightly deformed regions such as the northwestern european platform where average maximum horizontal stress direction is oriented roughly nnw sse subparallel to absolute and relative plate velocity directions other regions such as the caucasus alps and pyrenees where recent tectonic deformation and seismicity are present display important variations of stress directions the reconstruction of the average stress directions at different scales within the french alps pointed out that the average stress field pattern may vary from one scale to another nevertheless variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale

basal ice sequences in antarctic ice stream exposure of past hydrologic conditions and a principal mode of sediment transfer 1 the brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in kamb ice stream the upper sequence 7 3 m comprises clear ice and layers with dispersed and stratified debris the lower sequence 8 2 m consists of accretion ice with alternating layers of stratified and solid debris the two sequences have volumetric debris contents of about 5 and 20 respectively we infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the siple coast plain where basal freezing currently dominates the basal thermal regime the basal ice layer contains the equivalent of 2 1 0 4 m of frozen sediment the high volume of sediment is a result of debris rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on off cycles fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the ross sea our results show that sediment wedges near grounding lines may form by melt out of basal ice debris this is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override coulomb plastic substrates

coeval brittle and ductile structures associated with extreme deformation partitioning in a multilayer sequence an investigation on the effects of a strong rheological contrast in the deformation of layered anisotropic rocks is presented the study focuses on the geometric and kinematic analysis of complex structures developed within and adjacent to a thin marble metapsammite multilayer unit from the cap de creus tectonometamorphic belt ne spain zones of high ductile strain localise in the marble layers which exhibit complex folds whereas metapsammites show mostly brittle boudinage structures these structures strongly contrast with coeval retrogressive discrete shear zones developed in the surrounding migmatitic schists the extreme strain partitioning is due to the rheological contrast between different lithological layers in addition the specific orientation of this multilayer unit induces a reversal of local kinematics with regard to bulk kinematics consequently caution should be exercised when interpreting regional tectonics in highly partitioned domains associated with rheological heterogeneities

the basal ice layer of glaciers and ice sheets abstract in many glaciers and ice sheets there is a basal ice layer bil in which the ice is conditioned primarily by processes operating at the bed the bil is chemically and physically distinctive and is characterised by a component of basally derived sediment the bil is a rheological control on ice sheet dynamics an indicator of subglacial conditions and processes an agent of subglacial geologic processes the source of a substantial proportion of glacial sediments a limit to the downward extension of the climate record from deep ice cores if debris characteristics of the bil are preserved in glacial sediments former glacier conditions can be inferred analysis of stable isotopes gas and cation composition ice crystallography debris composition and structural glaciology has elucidated many mechanisms for bil formation common elements can be identified from a range of locations key issues are entrainment of ice and debris from the bed formation of new ice metamorphism of existing ice in the bil and thickening of the bil by deformation a distinction can be made between basal ice formed by accretion of material from the bed stratified facies and basal ice formed by metamorphic processes within the ice close to the bed dispersed facies

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

v pull apart microstructures a new shear sense indicator abstract granitoids deformed in a low grade shear zone show conspicuous development of asymmetric pull apart microstructures in feldspars characterized by v shaped gaps between the separated fragments v pull aparts they occur beside active shear planes and result from rigid body rotation of one fragment relative to the other the shear sense in the adjacent flow surface can be easily determined through the asymmetry normally shown by v pull aparts where the fracture wall at a lower angle to the shear plane indicates which fragment has rotated synthetically to the adjacent shearing another diagnostic feature is the foliation deflection that occurs adjacent to the rotated fragment the v shaped gaps are infilled by recrystallized quartz mica aggregates showing quartz c axis patterns indicative of strong participation of crystal plastic processes in these domains evidence of crack seal by precipitation c axes parallel to the direction of separation have also been found the rigid body rotation indicated by these microstructures worked as a mechanism of removal of protruding porphyroclast edges enabling the smoothing of the shear planes rigid body rotation should operate in circumstances where other mechanisms such as dissolution or crystal plasticity did not succeed in eliminating undulations along the shear planes

deformation mechanisms in the mylonite ultramylonite transition abstract the deformation mechanisms and controls that operate in the mylonite ultramylonite transition are interpreted from microstructural observation the investigated mylonites and ultramylonites were derived from a granitic protolith which was deformed under greenschist facies conditions and in the presence of fluid in a regional scale shear zone from northwest argentina several deformation mechanisms were recognized to operate simultaneously in different domains of the microstructure at each particular stage of the microstructural evolution this continuously mobile deformation partitioning present throughout the microstructural evolution ceases abruptly in the ultramylonite stage where a stable state microstructure is achieved domainal quartz c axis fabrics indicate that quartz deforms by crystal plastic processes at the initial and intermediate stages of deformation but solution transfer processes become predominant in the ultramylonite stage plagioclase is progressively transformed into muscovite through retrograde softening reactions k feldspar is progressively transformed into fine grade aggregates via cataclastic flow and incipient recrystallization mica deforms by kinking and basal slip with progressive development of fine grained morphologically oriented aggregates plagioclase disappearance as well as the development of intrafolial microfolds characterize the transition between the mylonitic and ultramylonitic domains disruption of these microfolds is interpreted to represent the ultimate control on the localization of the ultramylonite bands

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

late paleozoic strike slip shear zones in eastern central asia nw china new structural and geochronological data 1 new structural studies and 40ar 39ar dating in northwest china provide information about late paleozoic strike slip motions subsequent to accretional events which built eastern central asia during the paleozoic two principal areas were affected by these large transcurrent motions first in the tianshan range main east west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism associated biotites give ages ranging from 290 ma to 245 ma the earlier n110 shearing occurred in western tianshan while the last one dextral in whole tianshan occurred 250 245 myr ago second in the chinese altay region several nw se shear zones structured the area the main motion is sinistral and occurred along the erqishi zone at 280 290 ma it is followed by a complex succession of dextral and sinistral shearing episodes leading to the northwestward structuring dated at 245 ma of a metamorphic zone that was folded during a compressive event

late cainozoic geodynamic evolution of thessaly and surroundings central northern greece abstract in the framework of the late alpide deformation of greece and of the recent and active extensional tectonism of the aegean region the geotectonic evolution of the thessaly region central northern greece has been examined using a quantitative and qualitative structural analysis stratigraphie sedimentological morphotectonic and seismological data the geometry of the faults their architecture and the knowledge of the stress pattern are used to explain some aspects of the tectonics and crustal dynamics of thessaly and the surrounding area the oldest compressional phases taken into account show a mean ene wsw trending direction of shortening and have been defined as late alpide early aquitanian and langhian a later late miocene pliocene ne sw oriented extensional phase has been related to the hellenic post orogenic collapse which developed behind the collisional front between the aegean eurasia and african plates this phenomenon diachronically migrated from the east central macedonia thermaikos basin towards the west epirus albania where it is still active as a consequence of this second phase the area forms a basin and range like structure the third and last phase middle pleistocene present is characterized by a n s direction of extension and affects the entire aegean region it generated new e w trending basins superimposed on the inherited ones this gave as a final result the complex block pattern we can see today the recent and active right lateral strike slip movements along the north aegean trough seem to stop in the sporades basin and do not affect the uppermost crust of mainland greece a further wnw ese directed extension observed occasionally in central and northern greece could be explained by local events or as block related deformation

basal ice facies a review and unifying approach abstract over the past 30 years numerous basal ice facies have been identified named and classified however the resulting facies descriptions and names are inconsistent and no single scheme encompasses all of the different ice types that exist at different ice masses in this paper we review and critique existing basal ice facies names descriptions and classification schemes and propose a new non genetic approach that has the capacity to name and describe all basal ice types we define six fundamental basal cryofacies and a further 12 composite cryofacies which can all be defined on the basis of a cursory evaluation of debris disposition and concentration more detailed cryofacies description is based on characterizing three sets of ice properties that can also be estimated visually in the field a the thickness of the basal ice facies and its constituent sub layers b the concentration and texture of debris included within any or all of those layers and c the concentration and size of bubbles included within any or all of those layers we also propose a shorthand method for the presentation of this descriptive information here codes for the layer thicknesses are presented as standard text and codes for the included debris characteristics and included bubble characteristics are presented as superscripts and subscripts respectively sub layers are characterized similarly but within a nested sequence of brackets we evaluate the effectiveness of these two schemes by using them to rename and reclassify several existing basal ice facies results indicate that the schemes are robust and that they provide a coherent non genetic framework for the effective naming and description of basal cryofacies

quartz c axis fabrics in deformed conglomerates some support for a skeletal approach to fabric analysis abstract preferred orientation patterns of c axes in deformed quartzite rocks are conveniently characterized by 1 their skeletal outline and 2 by the way this skeleton is populated by c axes it has been recognized that the fabric skeleton principally reflects the kinematics of the flow during at least the later part of the deformation history whilst the distribution of c axes over the fabric skeleton is less reliable in this respect because the c axes orientation distribution may be dominated by the effect of an uneven and non random distribution of c axes prior to deformation little evidence has been presented however to confirm this interpretation quartz c axis fabrics in quartzite pebble conglomerates from southern spain show variable and incomplete patterns in individual pebbles which however all have a similar fabric skeleton and link to form more complete patterns when c axes from several pebbles are plotted together in view of the common deformation history of the pebbles within a given sample the variable and incomplete fabrics in single pebbles can only be explained by the effect of initial non randomness of c axes prior to deformation it follows that the c axis orientation distribution in this case bears little relationship with the inferred kinematics of the flow whilst the fabric skeleton shows a consistent relationship with the kinematic framework

mylonitization in the lower plate of the buckskin rawhide detachment fault west central arizona implications for the geometric evolution of metamorphic core complexes abstract in metamorphic core complexes it is commonly unclear whether lower plate mylonites formed as the down dip continuation of a detachment fault or whether they represent a subhorizontal shear zone that was captured by a more steeply dipping detachment fault detailed microstructural fabric and strain data from mylonites in the buckskin rawhide metamorphic core complex west central arizona constrain the structural development of the lower plate shear zone widespread exposures of 22 21 ma granitoids of the swansea plutonic suite enable us to separate miocene strain coeval with core complex extension from older deformation mylonites across the lower plate consistently record top to the ne directed shear miocene quartz and feldspar deformation recrystallization mechanisms indicate 450 500 c mylonitization temperatures that were relatively uniform across a distance of 35 km in the extension direction quartz dynamically recrystallized grain sizes do not systematically vary in the extension direction strain recorded in the swansea plutonic suite is also relatively uniform in the extension direction which is incompatible with models in which lower plate mylonites form as the ductile root of a major detachment fault altogether these data suggest the mylonitic shear zone initiated with a 4 dip and was unroofed by a more steeply dipping detachment fault system lower plate mylonites in the buckskin rawhide metamorphic core complex thus represent a captured subhorizontal shear zone rather than the down dip continuation of a detachment fault

fabric domains in quartz mylonites localized three dimensional analysis of microstructure and texture fabric domains of a quartz vein deformed under greenschist facies conditions arc analysed with domainal x ray texture goniometry optical microscopy computer integrated polarization microscopy cip and shape analysis in two mutually perpendicular sections the analysed domains consist of large non recrystallized deformed clast fragments prolate non recrystallized ribbon grains and prolate recrystallized domains in terms of texture monomaximal and polymaximal domains have been distinguished pronounced partitioning of deformation is observed between these domains this strain partitioning asks for caution when making kinematic inferences for fault zones based on a study of domainal fabrics domains persist as fabric elements up to high strains this emphasizes the often neglected role of a non random initial fabric neighbouring domains with alternating c axis orientations and grain boundary preferred orientations indicating opposing senses of shear occur in both sections the bulk texture is controlled by clast and ribbon domains rather than being strictly deformation induced we infer that prolate elongation of clasts and ribbons is achieved mainly by combined prism and basal slip recrystallized domains are characterized by different textures they are deformed by intracrystalline slip combined with grain boundary migration and sliding inclined preferred grain boundary orientations are continuously maintained by grain boundary migration copyright 0 1996 elsevier science ltd

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

seismotectonics of the aegean region abstract fault plane solutions neotectonic field observations and in situ stress measurements have been used to determine the stress field associated with the active deformation of the lithosphere in the aegean and surrounding regions a stress gradient and a zonal pattern of different tectonism away from the trench axis has been postulated pure thrust faulting occurs in the fore arc side while the back arc side is mainly dominated by almost n s extensional tectonics a narrow belt of strike slip faults with predominantly thrust component separates these tectonic zones this belt seems to represent a transition from the thrust type faults to normal faults fault plane solutions of the whole region in question indicate that both the large and small magnitude shocks are caused by the same regional stress field a body of 33 focal mechanisms indicates that the intermediate depth shocks along the sinking mediterranean slab are associated with thrust type fracturing orientation of kinematic axes favours the suggestion that shear zones within the descending slab cause these shocks the active crustal shortening along the hellenic consuming boundary and the sinking of the mediterranean lithosphere beneath the south aegean area explain satisfactorily the seismotectonic features of the fore arc and the southern back arc sides on the contrary geophysical evidence concerning the cenozoic geodynamic evolution of the aegean is needed to interpret features of the north back arc area

structural analysis of recent and active faults and regional state of stress in the epicentral area of the 1978 thessaloniki earthquakes northern greece during spring and summer 1978 macedonia greece suffered damaging seismic activity during the two main shocks which occurred on may 23 mb 5 7 and june 20 mb 6 1 seismic open cracks and faults were formed in the epicentral area 25 km nne from thessaloniki they have been mapped and their kinematics analyzed these studies have shown that they result from the reactivation of existing faults of pleistocene recent and historical age and that the crust is stretching along a n s to nne ssw direction in that part of the north aegean principal stress directions have been computed from the slip vectors measured on recent neotectonic faults and seismic cracks and faults this computation utilizing a simple mechanical model homogeneous deformation through the addition of small and independent displacements of rigid blocks in a highly fractured body a striking feature of this analysis is that the tensional directions 3 deduced from seismic faulting and from recent neotectonic faulting are nearly the same they are compatible with the regional tension axis deduced from focal mechanisms of north aegean and with the focal mechanisms of the 1978 thessaloniki main shocks they are also compatible with in situ stress measurements in this region studies of superficial seismic faulting strongly support previous assumptions proposed from aftershock distribution the 1978 thessaloniki earthquakes resulted from reactivation of several different faults situated within the same ne dipping fault zone of upper eocene lower oligocene age and older separating the serbo macedonian massif from the peonias east vardar zone

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations

quantitative microstructural analysis of mylonites formed during alpine tectonics in the western mediterranean realm abstract quantitative microstructural investigations involving new gis based tools are applied to three different types of mylonitic rocks these techniques enable mineral shape distribution maps to be produced thereby allowing the nature and relative percentage of the weakening vs hardening layers as well as their kinematics to be obtained within a statistically meaningful dataset selected rock types are derived from shearing associated with the meso alpine strike slip tectonics of the western mediterranean realm which since the paleocene has involved skarn migmatitic paragneiss and tonalites these rocks belong to a deep seated portion of the original southern european variscan chain known as the calabrian pelortani orogen microstructural maps reveal porphyroclastic domains represented alternatively by feldspar amphibole pyroxene or scapolite that can be distinguished from weakening phase areas characterised by quartz biotite plus quartz or alternatively by calcite when the weakening layer is controlled predominantly by skarns image elaboration of porphyroclastic domains was employed to estimate the dominant shear type with rigid grain analysis indicating a pure shear component of 66 68 for the mylonitic tonalites 62 66 for the mylonitic paragneisses and 58 62 for the mylonitic skarn grain boundary mapping of the quartz rich domains also permits an estimate of shear strain rate which ranges from 1 14 10 12 1 s for mylonitic paragneiss to 5 91 10 12 1 s for mylonitic tonalite and is in accord with high strain zones in natural settings finally our results opened up new perspectives in the interpretation of the kinematics and rheology of this exhumed sector of the southern european variscan chain reflecting the deep seated meso alpine strike slip tectonics of the western mediterranean realm