defect structures in naturally deformed clinoamphiboles a tem study abstract naturally deformed clinoamphiboles from the selbu tydal and forsback tarnaby areas of the scandinavian caledonides exhibit a well defined subgrain microstructure from a transmission electron microscopy study tem the subgrain boundaries are shown to consist of arrays of positive and negative screw dislocations with burgers vector b 001 locally expanded loops are present having long screw segments the subgrain boundaries are parallel to rational crystallographic planes of the type hk 0 the density of isolated dislocations within the subgrains is low in addition planar defect structures parallel to 010 and bounded by screw dislocations with b x 001 x are observed based on metamorphic criteria the p t values at the time of the amphibole growth have been estimated at 450 600 c and 4 6 kbar and these represent maximum conditions for the deformation the present results indicate that slip on hk 0 001 is an operative deformation mechanism in naturally deformed clinoamphiboles

crystallographic fabrics of omphacite rutile and quartz in vendee eclogites armorican massif france consequences for deformation mechanisms and regimes abstract this study aims at further understanding of the mechanisms how lattice preferred orientations lpo develop during deformation in the main eclogite minerals microstructures and textures of deformed eclogites from the les essarts complex western france were investigated using optical microscopy and electron backscatter diffraction ebsd in the scanning electron microscope microfabric analyses of eclogite facies minerals are used to identify their deformation mechanisms which define the rheology at high pressure metamorphic conditions mechanisms of intracrystalline deformation by dislocation movement dislocation creep result usually in a non linear flow law typically power law while diffusive processes diffusion creep correspond to linear flow laws general microstructural observations may suggest intracrystalline deformation dislocation creep of omphacite the omphacite lpo vary between s and l type and correlate with oblate or prolate grain shape fabrics respectively until now these lpo types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene an alternative interpretation is given in terms of anisotropic growth and dissolution with grain boundary diffusion as the rate controlling process there are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite in omphacite around a hollow garnet crystallographic and shape fabrics align with the c 001 axes parallel to the grain elongations defining the mineral lineation which rotates locally with the inferred flow direction in this part the grain sizes of omphacite and rutile are larger than in the surrounding matrix the geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime directions and ratios of the principal stresses the lpo of rutile duplicate the lpo of omphacite and a similar distinction between s and l type was used rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep quartz mainly occurs as an interstitial phase with weak lpo patterns interpreted as random no representative obliquity of the lpo in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria however the rutile lpo was slightly rotated relative to the omphacite lpo consistently in most samples the results suggest that diffusion processes are strongly involved in the deformation of eclogites a linear flow law should be taken into account in tectonic models where eclogites are incorporated

dislocation microstructures in naturally deformed silicate garnets abstract dislocation microstructures of naturally deformed silicate garnets and olivines in garnet peridotites and silicate garnets in eclogites from four localities have been observed with a transmission electron microscope tem to clarify the dislocation characteristics of silicate garnets we have obtained the following results 1 dislocation densities of garnets in all the garnet peridotites 105 107 cm 2 are always nearly an order of magnitude lower than those of co existing olivines 2 dislocation densities of garnets in eclogites 105 108 cm 2 which are embedded in garnet peridotites are almost an order of magnitude higher than those of garnets in the surrounding garnet peridotites 3 the dominant burgers vector b of mainly edge dislocations in garnet is 100 for specimens with dislocation density 105 106 cm 2 while b 1 2 111 for specimens with 107 108 cm 2 result 1 indicates that the observed dislocations in garnets were formed by plastic deformation under the same stresses as for co existing olivines and that there is a similar relationship between applied stress and dislocation density for garnets as for olivines result 2 suggests that the stress concentration occurred around eclogites embedded in garnet peridotites and the resulting differential stress in garnets in eclogites was further elevated by the interlocking of neighboring hard garnet grains finally result 3 indicates that the dominant burgers vector of mainly edge dislocations in garnet changes from 100 to 1 2 111 with increasing applied differential stress

the malpica lamego line a major crustal scale shear zone in the variscan belt of iberia abstract the malpica lamego line mll is a deformation zone in the variscan belt of nw iberia nw spain and n portugal that runs parallel to the chain for at least 275 km bearing i type granodiorite plutons along most of its length the mll affects previous structures by which high pressure and ophiolitic rocks were exhumed and emplaced on the iberian plate during earlier deformation phases correlation and reconstruction of the stratigraphy of these sheets or tectonic units at both sides of the shear zone allows a preliminary estimate of the accumulated vertical and horizontal offsets after the tectonic activity of the fault the value of the separations of crustal scale proportions reaches a maximum 15 km of vertical offset that decreases gradually to the south the structural record found in the rocks indicates a strike slip regime that in general does not fit the geometry of the offsets we suggest that the mll went through two different stages during the same orogenic cycle a first dip slip episode a reverse faulting event overprinted by a later strike slip reactivation

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

how is strain localized in a meta granitoid mid crustal basement section spatial distribution of deformation in the central aar massif switzerland abstract this study investigates strain distribution in granitoid rocks formerly in the middle crust in the central aar massif switzerland and places the deformation behavior in the tectonic framework of the alpine orogeny strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones sz 80 sz km with sz thicknesses the majority of the steep shear zones preferentially accommodated upward motion by the southern block leading to an increase in peak metamorphic conditions from 250 c in the north to 450 c in the south of the aar massif the shear zones initiated at about 18 20 km depths during a stage of crustal thickening handegg phase subsequent deformation reactivated some shear zones with a gradual transition from reverse dip slip over oblique slip to strike slip shear zones under local transpressional conditions oberaar phase

transposed high pressure granulite fabrics cabo ortegal nw spain implications on the scales of deformation localization abstract high pressure granulite facies rocks of the bacariza formation cabo ortegal nw spain were syn metamorphically deformed at the contacts with the bounding units peridotite and eclogite massifs this enabled the formation of meter thick spectacular shear zones with reworked and transposed foliations and lineations the texturally stable mineral assemblage of the new fabrics records an intense ductile deformation of the mineral aggregate at temperatures of 700 800 c associated with amalgamation of eclogite high pressure granulitic rocks and ultramafic sheets in deep portions of a subduction channel the lattice preferred orientation of the main constituent minerals garnet augite amphibole plagioclase quartz and biotite discloses the active deformation mechanisms at the scale of the mineral grains and the relationships with the deformation at larger scales overprinting relationships of the metamorphic assemblages demonstrates that partitioning and deformation localization occurred at different scales under similar high grade conditions complete macroscopic transposition in the shear zones was complementary to meso and microscopic partitioning of deformation intensity and mechanisms between different lithological layers and mineral species

petrofabrics of high pressure rocks exhumed at the slab mantle interface from the point of no return in a subduction zone sivrihisar turkey the highest pressure recorded by metamorphic rocks exhumed from oceanic subduction zones is 2 5 gpa corresponding to the maximum decoupling depth mdd 80 10 km identified in active subduction zones beyond the mdd the point of no return exhumation is unlikely the sivrihisar massif turkey is a coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record p t fluid deformation conditions during exhumation from 80 to 45 km crystallographic fabrics and other features of high pressure metasedimentary and metabasaltic rocks record transitions during exhumation in quartzite microstructures and crystallographic fabrics record deformation in the dislocation creep regime including dynamic recrystallization during decompression and a transition from prism slip to activation of rhomb and basal slip that may be related to a decrease in water fugacity during decompression 2 5 to 1 5 gpa phengite lawsonite and omphacite or glaucophane in quartzite and metabasalt remained stable during deformation and omphacite developed an l type crystallographic fabric in marble aragonite developed columnar textures with strong crystallographic fabrics that persisted during partial to complete dynamic recrystallization that was likely achieved in the stability field of aragonite p 1 2 gpa results of kinematic vorticity analysis based on lawsonite shape fabrics are consistent with shear criteria in quartzite and metabasalt and indicate a large component of coaxial deformation in the exhuming channel beneath a simple shear dominated interface this large coaxial component may have multiplied the exhuming power of the subduction channel and forced deeply subducted rocks to flow back from the point of no return

variation in u 238 and th 232 enrichment in u mineralized zone and geological controls on their spatial distribution singhbhum shear zone of india the determination of the natural radioactivity has been carried out by means of gamma ray spectrometry in exposed rock samples collected from various geological formations across the singhbhum shear zone of india the spatial variation in the concentration of primordial radionuclides reflects in general the geochemical variance among the analyzed rocks and affirms the earlier views on genesis of uranium mineralization this study further speculates new possibilities of thorium depletion during shearing and metamorphism all the obtained u values were more than the worldwide average u concentration which is about 40 bq kg interestingly u concentration in mica schists from singhbhum group of rocks and in slates and argillaceous quartzites from iron ore group of rocks was five to seven times higher than the global values in contrast the granites in study area showed comparatively lower u concentration the overall results suggest the control of complex metamorphism and shearing on u distribution in the terrane the contribution of 238u 232th and 40k to the total radiation dose in the study area was found to be significant ranging from 110 to 310 ngy h in comparison with the lower global as well as indian average values

deformation and recrystallization mechanisms in naturally deformed omphacites from the sesia lanzo zone geophysical consequences buatier m van r rmund h l m drury m r and lardeaux j m 1991 deformation and recrystallization mechanisms in naturally deformed omphacites from the sesia lanzo zone geophysical consequences teetonophysrc c 195 11 27 naturally deformed omphacites clinopyroxenes from the sesia lanzo zone western italian alps have been studied using light and transmission electron microscopy techniques deformation and recrystallization of omphacite took place at 5 50 600 c and minimum pressures of 14 16 kbar tem reveals isolated free dislocations loops nodal points tiltwalls complex dislocation networks minor twins stacking faults and antiphase domain boundaries the activated slip systems are l z llo llo l llo and al assisted by climb in nodal points and complex dislocation networks dislocations with burgers vector l 2 112 are present in low strain domains most of the subgrain transitional and new grain boundaries subparallel to wl develop from 110 tiltwalls which accommodate lattice bending produced by the l 2 110 110 slip system a component of the misorientation across such newly formed sub grain boundaries is however also formed by sub rain boundary migration grain rotations in the intermediate and high strain domains are more complex but can be explained by the activated slip systems the omphacite microstructures indicate that initial strain induct rec stallisation occurred by a rotation mechanism where single crystals become polycrystals by the development of numerous high angle grain boundaries at a subsequent more advanced stage migration recrystallization is also activated and a different kind of recrystallization mechanism occurs which involves the discontinuous development of new strain free grains these grains are derived from the extremum of the orientations produced by heterogeneous strain and slip system activity the evident ductility of omphacite during low to intermediate eclogite metamorphism is inferred to be facilitated by hydrolitic weakening effects as such the plastic behaviour of omphacite should be incorporated in geophysical flow models dealing with subducted oceanic and or lithosperic plate margins

tectonic overpressure and underpressure in lithospheric tectonics and metamorphism the lithostatic pressure concept is most commonly applied on a geological scale for lithospheric processes and related evolution of metamorphic rock complexes here various aspects of non lithostatic overpressure and underpressure phenomena in lithospheric tectonics and metamorphism are reviewed on the basis of recently published literature the main conclusion from this short review is that these phenomena certainly exist in nature on all time and space scales including geological ones they are in particular responsible for some geological processes which are otherwise difficult to explain such as downward water suction into the interior of subducting slabs magnitudes of overpressure and underpressure are strongly variable and may potentially reach up to 100 of the lithostatic pressure and up to a gpa level these magnitudes depend mainly on the rheology of deforming rocks and on the nature of related tectonic process rheological heterogeneity of deforming rock units which is common in nature has a tendency to enhance overpressure and underpressures large overpressure can typically be expected in rheologically strong dry bending rock units in particular in the mantle lithosphere however rheological weakness of rocks and small local deviatoric stresses do not guarantee the absence of large overpressures in these rocks therefore the influence of significant tectonic overpressure and or underpressure cannot be excluded for any metamorphic complex a priori but should be instead tested by exploring realistic thermomechanical models for envisaged tectono metamorphic scenarios many lithospheric rocks subjected to large overpressures and underpressures cannot be studied as they do not exhume to the surface some controversy exists concerning overpressure magnitudes for the ultrahigh pressure uhp rocks and several conflicting hypotheses are proposed which need to be thoroughly tested in the future in this respect the alpine region may offer a unique opportunity for the testing of geological scale overpressures in u hp rocks by combining structural geological and petrological data with realistic lithospheric scale numerical modelling

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

rheological contrast between garnet and clinopyroxene in the mantle wedge an example from higashi akaishi peridotite mass sw japan abstract garnet clinopyroxenites occur within foliated dunite in the higashi akaishi peridotite mass located within the subduction type high pressure low temperature sanbagawa metamorphic belt the garnet clinopyroxenites contain 3 80 garnet and garnet and clinopyroxene are homogeneously distributed garnet crystals contain extensive regular dislocation arrays and dislocation networks suggesting that dislocation creep was the dominant deformation mechanism analyses of crystallographic orientation maps indicate similar grain sizes and aspect ratios for garnet and clinopyroxene regardless of modal composition indicating that these minerals deformed with similar degree of plasticity moreover indexes of crystallographic fabric intensity i e j index and m index for both garnet and clinopyroxene tend to increase with increasing modal composition of garnet fourier transform infrared spectroscopy analysis revealed that water content in garnet is 60 ppm whereas that in clinopyroxene is 70 ppm olivine crystal preferred orientations in the higashi akaishi peridotite mass characterized by 0 0 1 0 1 0 are thought to have developed during deformation under wet conditions consequently we argue that the presence of water could act to enhance garnet plasticity during deformation the results reveal contrasting influences of water on the deformation of garnet and diopside under wet conditions compared with dry the strain rate increases by two orders of magnitude for garnet but by an order of magnitude for diopside given the influence of water on the creep strength of garnet garnet within the higashi akaishi mass may have become significantly as weak as clinopyroxene during deformation

the dislocation substructures of naturally deformed omphacites abstract a transmission electron microscopy study of naturally deformed omphacite has revealed twins antiphase boundaries and a variety of dislocation substructures consisting of curved free dislocations loops isolated nodal points and networks both free dislocations and dislocation boundaries contain nodes that involve dislocations with the following burgers vectors 001 1 2 110 and 1 2 112 this implies a reaction of the type 001 1 2 110 1 2 112 by analogy with quartz and olivine the dislocation substructures in the omphacites are considered to be indicative of dislocation creep mechanisms and responsible for most of the optical defect structures using microstructural and geochemical data the metamorphic conditions during the creep deformation are estimated to be minimum 8 kbar 550 c maximum 15 kbar 625 c

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

deformation inside a paleosubduction channel insights from microstructures and crystallographic preferred orientations of eclogites and metasediments from the tauern window austria the eclogite zone of the tauern window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high pressure metasediments the rocks were exposed to 600 c and 20 25 kbars and then retrogressed during their exhumation first under blueschist facies and later under amphibolite facies metamorphism to gain insights into the deformation within the subduction channel during subduction and exhumation both fresh and retrogressed eclogites as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations cpos pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite a misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep the omphacite cpos of the eclogites correspond to dominant sl fabrics characteristic of plane strain deformation though there are local variations towards flattening or constriction within the paleosubduction channel the glaucophane cpos in retrogressed eclogites match those of omphacite suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions plastic deformation of the host high pressure metasediments outlasted that of the eclogites as indicated by white mica fabrics and quartz cpo the latter is consistently asymmetric pointing to the operation of non coaxial deformation the microstructures and cpo data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

plastic deformation and recrystallization of garnet a mechanism to facilitate diffusion creep elongate and deformed garnets from glenelg nw scotland occur within a thin shear zone transecting an eclogite body that has undergone partial retrogression to amphibolite facies at circa 700 c optical microscopy back scattered electron imaging electron probe microanalysis and electron back scatter diffraction reveal garnet sub structures that are developed as a function of strain subgrains with low angle misorientation boundaries occur at low strain and garnet orientations are dispersed around rational crystallographic axes across these boundaries towards high strain areas boundary misorientations increase and there is a loss of crystallographic control on misorientations which tend towards random in high strain areas a polygonal garnet microstructure is developed the garnet orientations are randomly dispersed around the original single crystal orientation some garnet grains are elongate and ca rich garnet occurs on the faces of elongate grains oriented normal to the foliation commonly the garnet grains are admixed with matrix minerals and where in contact with other phases garnet is well faceted we suggest that individual garnet porphyroclasts record an evolution from low strain conditions where dislocation creep and recovery accommodated deformation through increasing strain where dynamic recrystallization occurred by subgrain rotation to highest strains where recrystallized grains were able to deform by diffusion creep assisted grain boundary sliding with associated rotations

development of lattice preferred orientation in clinoamphiboles deformed under low pressure metamorphic conditions a sem ebsd study of metabasites from the aracena metamorphic belt sw spain the mechanical properties of the lower continental crust are intimately related to the rheology of calcic clinoamphibole naturally deformed clinoamphibole typically shows brittle behaviour under a wide range of deformation conditions whereas crystal plasticity is rarely observed in this study the microfabric of clinoamphiboles from metabasites deformed under low pressure medium to high temperature metamorphic conditions is presented amphiboles from all samples have developed lpo that can be attributed to different deformation mechanisms depending on deformation temperature fluid content structure and phase strength contrasts rigid body rotation of amphibole prisms within a weaker plagioclase matrix corresponds to metabasites deformed under medium deformation temperatures that show a weak layer structure and a high phase strength contrast between plagioclase and clinoamphibole lower temperatures and the presence of fluids mean that one of the studied rocks was affected also by dissolution precipitation creep and cataclastic flow in the final sample dislocation creep was accommodated by recovery and subgrain rotation dynamic recrystallization in monominerallic hornblende layers at higher temperatures which could be representative of the rheology of amphiboles in a lower continental crust at high temperature conditions

extensional flow during gravitational collapse a tool for setting plate convergence padron migmatitic dome variscan belt nw iberia abstractplate convergence analysis in collisional orogens is usually based on the study of major contractional structures and strike slip shear zones here we show how the structural analysis of extensional structures may report the regional or far stress field during relatively local gravity driven extensional collapse of a thickened continental crust and how this information may be used to constrain the broad vectors of plate convergence at that time the padron migmatitic dome is a synconvergent extensional system developed in the axial zone of the variscan belt exposed in the nw part of the iberian massif of spain this system affected the allochthonous and autochthonous sequences involved in pangaea s assembly in southern europe it includes three major extensional shear zones which have been analyzed in detail to provide a wide ground data set for the discussion of the proposed model the tectonic flow in the padron migmatitic dome and in other coeval structures is characterized by vectors ranging

dynamic recrystallization processes in plagioclase porphyroclasts abstract recrystallization processes in plagioclase porpyroclasts were studied in samples from an anorthositic gabbro of the jotun complex south norwegian caledonides the deformation took place at a temperature of approximately 700 c and at pressures below 900 mpa two populations of plagioclase porphyroclasts can be distinguished based on microstructural observations the crystallographic orientation and grain shapes for the two populations are interpreted as soft and hard orientations of the dominant slip system 010 001 with respect to the kinematic reference system the orientations of porphyroclasts and recrystallized grains were measured with a u stage microscope analysis of misorientations between porphyroclasts and adjacent recrystallized grains yields different results for the two types of porphyroclasts in type 1 porphyroclasts soft orientation the misorientation is inferred to be caused by subgrain rotation due to the activation of the 010 001 slip system resulting in a host control orientation relationship whereas in type 2 porphyroclasts hard orientation no dominant misorientation relationship could be identified the slip systems of the dislocations inferred from transmission electron microscopy tem analysis correspond well with the results of the u stage misorientation analysis in type 2 porphyroclasts hard orientation micro fracturing was observed in the tem whereas microfractures are absent in type 1 porphyroclasts soft orientation micro fracturing is likely to be the mechanism for nucleation of new grains in type 2 porphyroclasts resulting in a no host control orientation relationship between porphyroclasts and recrystallized grains

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

impacts of uplift of northern tibetan plateau and formation of asian inland deserts on regional climate and environment abstract based on the geological evidence that the northern tibetan plateau ntp had an uplift of a finite magnitude since the miocene and the major asian inland deserts formed in the early pliocene a regional climate model regcm4 1 with a horizontal resolution of 50 km was used to explore the effects of the ntp uplift and the related aridification of inland asia on regional climate we designed three numerical experiments including the control experiment representing the present day condition the high mountain experiment representing the early pliocene condition with uplifted ntp but absence of the asian inland deserts and the low mountain experiment representing the mid miocene condition with reduced topography in the ntp by as much as 2400 m and also absence of the deserts our simulation results indicated that the ntp uplift caused significant reductions in annual precipitation in a broad region of inland asia north of the tibetan plateau tp mainly due to the enhanced rain shadow effect of the mountains and changes in the regional circulations however four mountainous regions located in the uplift showed significant increases in precipitation stretching from the pamir plateau in the west to the qilian mountains in the east these mountainous areas also experienced different changes in the rainfall seasonality with the greatest increases occurring during the respective rainy seasons predominantly resulted from the enhanced orographically forced upwind ascents the appearance of the major deserts in the inland asia further reduced precipitation in the region and led to increased dust emission and deposition fluxes while the spatial patterns of dust deposition were also changed not only in the regions of uplift impacted topography but also in the downwind regions one major contribution from this study is the comparison of the simulation results with 11 existing geological records representing the moisture conditions from miocene to pliocene the comparisons revealed good matches between the simulation results and the published geological records therefore we conclude that the ntp uplift and the related formation of the major deserts played a controlling role in the evolution of regional climatic conditions in a broad region in inland asia since the miocene

crystal plasticity of natural garnet new microstructural evidence scanning electron microscope sem orientation contrast images of mantle nodules show that garnets contain cellular domains of different crystallographic orientation electron backscatter diffraction shows small crystallographic mismatches

tectonic evolution of a continental subduction exhumation channel variscan structure of the basal allochthonous units in nw spain a regional study starting from detailed geological mapping has been carried out in the malpica tui complex of galicia in nw spain the complex is formed by two units representing pieces of the external edge of gondwana subducted and exhumed during the variscan collision the study shows that synsubduction and early synexhumation structures in continental subduction channels tends to be obscured and even erased once exhumation is complete detailed structural analysis matched with the knowledge of the history and available data for other galician basal units have elucidated the major structures developed during the subduction exhumation process the results include evidence of the plate convergence causing early variscan continental subduction of the gondwana margin subduction was followed by exhumation driven by ductile thrusting within the subduction channel which in turn provoked crustal duplication in the subducted slab and modified the initial tectonometamorphic architecture of the subduction wedge the next step was accretion to the adjacent continental domains placing the subduction wedge on top of unsubducted parts of the gondwana margin via ductile thrusting thrusting was preceded by progressive propagation of a train of recumbent folds toward the foreland that affected the previous structural stack subsequent transference of oceanic rheic and peri gondwanan terranes to the gondwana margin took place by out of sequence thrusting followed by crustal extensional collapse and strike slip tectonics

deformation enhanced metamorphic reactions and the rheology of high pressure shear zones western gneiss region norway microstructural and petrological analysis of samples with increasing strain in high pressure hp shear zones from the haram garnet corona gabbro give insights into the deformation mechanisms of minerals rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions scanning electron microscopy with electron backscattering diffraction sem ebsd compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization plagioclase with a shape preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the 0 0 1 1 0 0 slip system in omphacite crystallographic preferred orientations indicate slip on 1 0 0 0 0 1 and 1 1 0 0 0 1 during deformation fine grained garnet deformed by diffusion creep and grain boundary sliding ilmenite deformed by dislocation glide on the basal and at higher strains prism planes in the a direction relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500 650 c and 0 5 1 4 gpa and continued during prograde metamorphism to ultra high pressure uhp conditions both products and reactants show evidence of syn and post kinematic growth indicating that prograde reactions continued after strain was partitioned away the restriction of post kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away demonstrating that deformation played an important role in enhancing metamorphic reactions two important consequences of deformation observed in these shear zones are i the homogenization of chemical composition gradients occurred by mixing and grain boundary migration and ii composition changes in zoned metamorphic garnet by lengthening diffusion distances the application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100 150 mpa and lower limits for strain rates of 10 12 to 10 13 s 1 as deformation conditions for the shear zones in the haram gabbro that were produced during subduction of the baltica craton and resulted in the production of hp and uhp metamorphic rocks

late archaean early proterozoic source ages of zircons in rocks from the paleozoic orogen of western galicia nw spain abstract u pb data are reported for nine suites of zircons and three monazites from the paleozoic orogen in western galicia nw spain one paragneiss and six orthogneisses from the early paleozoic basement and two carboniferous ca 310 ma old intrusions of two mica granite new whole rock rb sr analyses along with earlier data indicate an age of ca 470 440 ma ordovician for the emplacement of the granitic precursors of the orthogneisses monazite from the paragneiss also yields an u pb age of ca 470 ma for all nine investigated suites of zircons the u pb systematics signal the presence of a minor proportion of precambrian zircon the zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3 0 ga and 2 0 ga and lower intercepts between ca 480 and 460 ma the former are interpreted as approaching the age of the old zircon component and the latter as reflecting either the time of crystallization of new zircons from the magma orthogneisses or the time of radiogenic lead loss from the old zircons paragneiss the suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470 440 ma ago in most cases leading to false discordias without geochronological significance similarly the zircons of the two investigated two mica granites do not produce meaningful discordias because of post paleozoic disturbance the monazite u pb systems of the latter granites indicate sub recent lead loss from the high initial 87 sr 86 sr ratios an involvement of precambrian continental crust material is evident in the generation of the early paleozoic suite of granites while the zircon u pb data give evidence of the presence of about 3 0 2 0 ga old late archaean early proterozoic components in the source material zircons from the oldest sedimentary rocks in the area now present as catazonal paragneisses and a likely source for the granites likewise reveal a provenance age of 3 0 2 0 ga the late archaean early proterozoic source rocks in western galicia probably formed part of an archaean continental crust underlying much of western europe but mostly modified beyond recognition by younger events

dissolution and precipitation processes in deformed amphibolites an example from the ductile shear zone of the ryoke metamorphic belt sw japan the granitic mylonite zone in the cretaceous ryoke metamorphic belt contains deformed amphibolites as thin layers the amphibolite layers do not exhibit pinch and swell or boudinage structures even when contained in a high strain granitic mylonite this mode of occurrence suggests that they were deformed as much as the surrounding granite mylonite in the highly deformed zone strongly foliated amphibolites contain ti rich brown amphibole porphyroclasts rimmed by ti poor green amphibole titanite and chlorite these porphyroclasts are elongated forming shear surfaces defined by preferential distribution of the chlorite and titanite porphyroclastic plagioclase in the strongly foliated amphibolites consists of two components an anorthite rich core and an anorthite poor rim based on these observations the mass balanced reaction occurring during deformation is defined as as the reaction products form a weak interconnected matrix the strain rate of the amphibolites may be controlled by the rate of dissolution precipitation through fluids weakly foliated amphibolites in the low strain zone exhibit cataclastic microstructures whereas the strongly foliated amphibolites do not exhibit such features these microstructural and chemical changes suggest that high strain amphibolites were initially deformed by cataclasis followed by deformation through metamorphic reactions during the metamorphism deformation old plagioclase grains with high xan were not stable and dissolved and new plagioclase grains with low xan crystallized at the old plagioclase rim dissolution of old plagioclase and precipitation of new plagioclase occurred normal to and parallel to the foliation respectively reflecting incongruent pressure solution due to differential stress and changes in p t h2o conditions the development of incongruent pressure solution is attributed to increased fluid flux in the strongly foliated amphibolites as evidenced by the greater abundance of hydration reaction products in the strongly foliated amphibolites than in the weakly foliated ones

dramatic effects of stress on metamorphic reactions thermodynamic calculations predict mineralogy from temperature and pressure and vice versa such calculations assume that stress is isotropic despite the fact that differential stresses prevail in earth resulting from large scale tectonics and or differences between fluid and rock pressures in porous rocks new calculations show that differential stress can have significant effects on thresholds for metamorphic reactions depending on the grain scale reaction pathways a differential stress may depending on the reaction pathway have an effect equivalent to a pressure difference on the order of assemblage volume reaction volume change differential stress the multiplying factor is typically 10 or more for example the onset of a garnet clinopyroxene breakdown reaction may be offset up or down by the equivalent of 500 mpa in pressure for a 50 mpa differential stress the effect is equivalent to a temperature difference on the order of assemblage volume reaction entropy change differential stress for example the onset of muscovite quartz breakdown may be offset up or down by the equivalent of 130 c for a 50 mpa differential stress much of earth is under differential stress so the new calculations invite a reappraisal of metamorphic mineralogy and microstructure indicating that new insights into stresses and fluid pressures on earth can be gained

the thermodynamics of deformed metamorphic rocks a review abstract the deformation of rocks is a disequilibrium and strongly non linear phenomenon with a number of interacting chemical thermal and microstructural processes operating simultaneously we review progress in this area over the past 30 years deforming chemically reacting systems are dissipative systems and hence are characterised by highly ordered structures that develop through cooperative processes once parameters such as critical strains strain rates fluid infiltration rates damage densities or temperatures are attained such criticality is the hallmark of deformed rocks at all length scales and is the basis for a diverse range of structures such as foliations and lineations produced by metamorphic differentiation rotation recrystallisation folding boudinage and micro to regional scale fracture systems criticality is identified with classical criticality and not self organised criticality the first and second laws of thermodynamics are used to show that such structural diversity arises from reaction diffusion deformation equations criticality of the system is associated with the stored energy becoming non convex and structures arise in order to minimise this non convex energy these structures are scale invariant and hence are characterised by fractal and minimal surface geometries thermodynamics is a powerful discipline to integrate seemingly unrelated processes in structural geology and produce an integrated approach to the subject that crosses all length scales

petrology structure and geotectonic implications of glaucophane bearing eclogites and related rocks from the malpica tuy mt unit galicia northwest spain abstract the malpica tuy mt unit is a narrow band of basement in galicia northwest spain composed of mica schists paragneisses and calc alkaline to peralkaline orthogneisses with minor amounts of metamorphosed basic rocks garnet omphacite glaucophane barroisite rutile phengite paragonite kyanite zoisite developed in eclogites s s and eclogitic impure sandstones garnet phengite rutile zoisite in the calc alkaline orthogeneisses and garnet phengite rutile chloritoid kyanite in the metasediments biotite breakdown in the orthogneisses and related hornfelses produced garnet with development of coronitic textures plagioclase porphyroblasts with aligned inclusions of rutile and phengite are a common feature of many paragneisses the minimum p t conditions for the equilibrium of garnet omphacite parageneses at the peak of the metamorphism were 600 c 12 2 kbar the mineral chemistry of the orthogneisses and metasediments is compatible with these conditions furthermore the structural data also suggest an in situ origin for the eclogites i e the same as the metamorphism of the enclosing rocks the crystalline area surrounding the mt unit is mainly composed of metasediments and anatectic granites the t at the peak of the metamorphism was higher than 700 c the p being of the order of 5 kbar the two domains represent different parts of the hercynian foldbelt tectonically juxtaposed the juxtaposition is the result of complex tangential tectonics three main phases of compressive deformation having been recognized a first stage of subduction of a thinned crust the materials of the mt unit and subsequent obduction development of hydrated minerals in eclogitic rocks with emplacement of these materials over an already metamorphosed crustal area is suggested the mt unit would represent a remnant of a hercynian thrust nappe

an application of eigenvalue methods to structural domain analysis the subdivision of a geologic map into structural domains involves the location of regions in which the rock fabric has certain geometrical characteristics typically foliation data should share a common axis the location of such domains involves working interactively with map and equal area projections of the data set a tedious and often subjective process eigenvector methods can quantify this type of analysis a set of four eigenvalue based indexes assists in discriminating among fabric distributions particularly between strong and weak cylindrical distributions these indexes form the basis of a triangular diagram for distinguishing among point girdle random and cylindrical fabrics a domain search proceeds by subdividing the data set and attempting to maximize the total cylindricity or other characteristic the method has been applied to a set of foliation data from a fold nappe in the western gneiss region of the norwegian caledonides the resulting domains define a late fold that refolds the recumbent nappe lineation and fold axis data from the area support the results of the domain search this method may be valuable in other areas of complex geometry

ductility of garnet as an indicator of extremely high temperature deformation discoidal shaped garnets alm63 prp33 grs3 sps1 occur in the foliation plane of lineated quartzofeldspathic mylonites of the morin shear zone located along the eastern boundary of the morin anorthosite complex grenville province quebec the flattened shape of garnets records a component of coaxial strain x y z different from the dominant rotational shear strain x y z in this mylonite tem observations indicate dislocation slip and recovery to be the main mechanism for the deformation of garnet extrapolation of available experimentally determined flow laws to natural strain rates shows an inversion of flow strength between quartz feldspar and garnet garnet much stronger than quartz and feldspar at temperatures below 700 c becomes weaker than quartz and feldspar above 900 c the coaxial ductile strain recorded by the garnets could be produced by a local uncharacterized deformation under extremely high temperature conditions at the roof of the rising anorthositic diapir 1155 ma subsequent lower temperature rotational deformation is related to strike slip movement along the morin shear zone 1020 ma

graphite and quartz petrofabrics examples from the ediacaran black quartzites of the ossa morena zone sw iberia abstract we study with the electron back scattered diffraction ebsd technique the fabric of metamorphosed and ductilely deformed phtanites or graphitic cherts a common lithotype in ediacaran supracrustals of the west european cadomian orogen currently they are black quartzites with a planolinear tectonite structure microscopically they present a sub mm scale alternation of coarse and fine grained dynamically recrystallized quartz bands we attribute intracrystalline plasticity partitioning to variations in graphite inclusion concentration constraining quartz grain boundary mobility during dynamic recrystallization under non coaxial strain regimes and moderate to elevated temperatures 400 650 c lattice preferred orientations of quartz c and axes are geometrically related to the external reference frame provided by foliations and lineations we also identify the involvement of 0001 r m and m c quartz intracrystalline slip systems deformation modes operated simultaneously and under identical temperatures in interleaved parallel domains mm to cm thick in adjacent coarse and fine grained bands medium to high t plasticity is congruent with the syntectonic temperature gradients associated with the amphibolite facies metamorphism of the country rocks we also present the first specific study published so far on natural deformation graphite lattice preferred orientation graphite inclusions as well as those of mica exhibit mineral shape fabrics that suggest operation of 0001 slip however ebsd measurements also record fabrics suggestive of m slip in spite of a rather small volumetric proportion graphite spatial organization at increased shear strains facilitated ductile deformation if a graphite network is established in the rock it can potentially increase rock electrical conductivity thus accounting for mid and lower crust anomalous electric conductivity

3d analysis of an ordovician igneous ensemble a complex magmatic structure hidden in a polydeformed allochthonous variscan unit the basal units of the allochthonous complexes of nw iberia represent a fragment of the external edge of gondwana subducted and subsequently exhumed during the variscan collision the structural analysis carried out in orthogneissic massifs of the southern part of one of these the malpica tui unit reveals the generation of recumbent folds and associated axial planar foliation during their exhumation these folds nucleated in irregular igneous bodies that were initially deformed during the subduction event and show east to southeast vergence down plunge projection of surface data and a series of regularly spaced cross sections have been used to build 3d models of the two main bodies of orthogneiss of calc alkaline and peralkaline compositions respectively the first is presently a lens shaped body folded in a recumbent syncline whereas the peralkaline gneiss also affected by a train of asymmetric recumbent folds in the south exhibits in the north a fold like structure which is interpreted as inherited from its primary geometry that of a ring dike pluton

role of the tectonic accretion channel in collisional orogeny high pressure relics studied in many collisional mountain belts are overprinted by subsequent barrovian metamorphism that may reach migmatite grade in the central parts of such orogens we propose that this evolution is linked to the development of a narrow tectonic accretion channel tac at the subducting plate boundary geologic evidence from the southern steep belt of the central alps and the tectono metamorphic record of this orogen guided us in constructing numerical models of a tac simulations indicate that the accretion of crust enriched in radioactive elements to mantle depth provides a mechanism to obtain p t t pressure temperature time trajectories in reasonable agreement with observations in the alps similarly the generation of granitoid melts predicted by the model during late orogenic exhumation of the tac is in line with the alpine record this case study suggests that accretion of upper crust fragments to mantle depth by underplating along a subduction fault and subsequent extrusion of parts of the tac along that same fault may be fundamental processes in the dynamic evolution of many collisional orogens

deformation mechanisms and reaction of hornblende examples from the bergell tonalite central alps abstract hornblendes in the bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization during the crystallization of the magma deformation commences by magmatic flow in this stage the hornblendes behave as rigid particles in a viscous matrix melt the rotation and alignment of hornblende as elongate rigid particles have produced a strong preferred orientation of both particle long axes and crystallographic directions with progressive solidification of the melt there is a gradual transition from magmatic flow to solid state deformation in the bergell tonalite the crystallographic and shape preferred orientation that have developed during solid state deformation are identical to those of magmatic flow this identical fabric development during solid state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods i e hornblendes have always acted as rigid particles in a viscous matrix tem observations show no evidence for intracrystalline plasticity in hornblende grains the modal abundance of hornblende decreases progressively with increasing solid state deformation to yield more biotite epidote and quartz small hornblende matrix grains form by cataclastic processes the observed changes in grain size shape and abundance of hornblende occur mainly by fracturing dissolution and reaction so that the solid state deformation of the tonalite is a complex process to which fracturing dissolution of hornblende and metamorphic reactions all contribute the relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction it is inferred that generally hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650 750 c in the presence of an aqueous fluid crystal plasticity could become dominant at higher temperatures and or lower aqueous fluid activities

experimental deformation of muscovite abstract the strength and mechanical anisotropy of muscovite have been investigated by shortening single crystals at 45 0 and 90 to the basal plane 001 at temperatures t from 20 to 400 c confining pressures pc from 10 to 400 mpa and strain rates e from 2 5 10 7 to 2 3 10 4 s 1 samples deformed as 45 to 001 exhibit smooth undulatory extinction in thin section associated with dislocation glide with few well developed low angle kink boundaries basal shear strengths depend on confining pressure at pc e a exp a exp q rt results from temperature and strain rate stepping experiments performed at pc 200 mpa yield an activation energy q of 47 19 kj mol 1 and an exponential constant a of 0 5 0 2 mpa 1 samples loaded parallel to the basal plane shorten by the development of sharply defined kink bands strengths associated with kink band formation are higher than those associated with glide in samples compressed at 45 to 001 and exhibit significant pressure dependence 0 8 up to confining pressures of 200 mpa kink bands are generated at the ends of samples where contact is made with the pistons samples loaded at 90 to 001 are much stronger than samples of all other orientations and deformation is accomplished by fracture fracture strengths exhibit a strong dependence on confining pressure 0 9 similar in magnitude to other silicates that are brittle at the same conditions by comparison with biotite muscovite is weaker in shear on 001 at the experimental conditions tested with an activation energy for glide q that is significantly smaller than that for glide in biotite

stretching lineations in high pressure belts the fingerprint of subduction and subsequent events malpica tui complex nw iberia the evolution of tectonic flow has been studied in an exposed continental subduction system of the variscan belt the malpica tui complex of nw iberia where structural data are used to establish the kinematics and flow geometry during the whole deformation history structural analysis reveals reorientation of successive lineations by subsequent deformation events and especially by late strike slip tectonics analysis of the deflection patterns in the lineation map permits us to establish the original trend of lineations the data expose the role of thrust and nappe tectonics and non coaxial deformation in the hinterland of a collisional belt showing a dominant orogen parallel lineation pattern following oblique subduction contractional ductile thrusts transported and emplaced a part of the subduction system onto the adjacent mainland following vectors normal to the orogenic trend however the finite stretching lineation in associated recumbent folds is oblique to the orogenic trend and suggests overprinting by dextral shearing probably owing to lateral components of the gondwana and laurussia convergence subsequently extensional structures dismembered the tectonic pile moving the pieces obliquely with respect to the trend of the collisional belt orogen parallel strike slip shear zones later produced major reorientation of the pre existing tectonic fabrics towards their shear planes

sense of shear in high temperature movement zones from the fabric asymmetry of plagioclase feldspars abstract a new method for determining the sense of shear in plagioclase bearing tectonites from the 010 orientation of plagioclase feldspar is presented the method is based on the asymmetry of the 010 plane with respect to the structural frame foliation and lineation and the dominant activity of the 010 slip plane in the high temperature plasticity of plagioclase feldspar using examples from the zabargad gneisses red sea the method is applied to plagioclases of an25 an45 and compared with other methods of shear sense determination quartz c axis fabrics and microstructural criteria

structural analysis of high pressure shear zones bacariza formation cabo ortegal nw spain abstract high pressure granulites of the bacariza formation cabo ortegal complex nw spain exhibit spectacular examples of ductile shear zones developed at different scales in rocks containing pre existing foliations a detailed structural analysis was carried out on these shear zones in order to unravel and compare the role of various parameters controlling the deformation process i e heterogeneous simple shear components of homogeneous deformation heterogeneous volume change and degree of non coaxiality although heterogeneous simple shear largely dominated negligible deviations from the ideal simple shear model were detected involving shortening along the structural directions perpendicular to the stretching axis within the foliation plane of the finite strain ellipsoid the relationship between displacement parallel to a half shear zone and the normal distance from its boundary provided the basis for the estimation of the stress exponent in the power law constitutive flow equation associated with each shear zone which is interpreted as a rheological indicator these geometric and rheological results and the thermobaric conditions of high pressure shear zone deformation indicate that these shear zones accommodated dominant plastic rock flow coeval with high pressure and high temperature deformations under moderate stress levels concomitant with elevated strain rates

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

preferred lattice orientations of plagioclase from amphibolite and greenschist facies rocks near the insubric line western alps abstract preferred lattice orientations of plagioclase measured by the universal stage from deformed amphibolite and greenschist facies rocks near the insubric line western alps reflect the type and orientation of the strain system either 001 or 010 are oriented parallel or subparallel to the schistosity and form more or less complete girdles around the stretching direction by the preferred orientation patterns dominant 010 001 slip is indicated and additionally slip on 001 parallel to the a direction possibly at somewhat higher temperatures and or at a higher an content of the plagioclase it is suggested that obliquities of the symmetry planes of the orientation patterns against the principle axes of strain are related to the sense of shearing

a new model for the hercynian orogen of gondwanan france and iberia abstract the hercynian orogen of gondwanan france and iberia is a collage of disparate terranes separated by faults plate reconstructions indicate that the orogen developed during a massive dextral transpression as laurentia slid along the boundary of gondwana and as laurentia baltica and east avalonia rotated clockwise to form pangea it is proposed therefore that the collage is the result of this transpression and that it is an amalgamation of displaced tectonostratigraphic terranes which represent the dismembering of the gondwanan shelf with its stable platform ordovician devonian sedimentary sequence and the neighbouring rheic ocean specifically it is proposed that 1 the slice of the gondwanan shelf that now forms north brittany was displaced dextrally from nw africa around iberia by more than 2000 km 2 the ophiolites oceanic material and volcanic arcs of south armorica and the massif central represent slices of the rheic ocean displaced dextrally along the gondwana margin and shuffled in amongst slices of gondwanan shelf 3 the convergent component of the transpression is represented by thrusts throughout the region the leon terrane of france and ophiolite bearing klippe in nw iberia the ibero armorican arc was formed by wrapping the mobile dextral shear belts about a rigid iberian basement block the dextral porto tomar shear zone represents the more important shearing in iberia and the iberian sinistral shears are interpreted as bookshelf type structures subsidiary to the dextral shearing

complete texture analysis of a deformed amphibolite comparison between neutron diffraction and u stage data abstract this paper describes a new technique of analysing complex neutron diffraction patterns allowing the quantitative texture analysis of each rock forming mineral up to trielinic lattice symmetry in polymineralic rocks the method is demonstrated using hornblende and plagioclase fabrics in alzenau amphibolite from the spessart mountains mid german crystalline rise hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements for control purposes crystallographic preferred orientations were also determined independently by individual grain measurements using a u stage from the neutron texture analyses we obtain only superposed pole figures hornblende and hornblende hornblende and plagioclase and plagioclase with plagioclase of low indexed lattice planes therefore the critical crystallographic directions were recalculated by the orientation distribution function odf which is given by texture components the optical and neutron derived pole figures are in a good agreement all pole figures display strong preferred orientation although the plagioclase texture is less pronounced the hornblende texture consists of a 001 maximum parallel to the stretching lineation with 010 showing a tendency to develop a girdle around the lineation 100 axes are distributed with a clearly split maximum representing the monoclinic angle between 100 and 001 the plagioclase texture is a rare one with 001 subparallel within the foliation and 100 oriented subparallel to the lineation 010 of plagioclase is concentrated normal to lineation within the foliation the technique outlined here is potentially the most useful one for texture analysis of polyphase rocks independent of crystal symmetry

sm nd isotope geochemistry and tectonic setting of the metasedimentary rocks from the basal allochthonous units of nw iberia variscan suture galicia the basal units of the allochthonous complexes of nw iberia are formed by thick metasedimentary rock sequences intruded by granitoids ranging in composition from calc alkaline c 493 ma to minor alkaline peralkaline massifs c 475 470 ma and mafic rocks the granitoids were transformed into variably deformed othogneisses and the associated mafic rocks were transformed into amphibolites blueschists and eclogites during eo variscan high p metamorphism dated at c 370 ma two different superimposed metasedimentary rock sequences can be distinguished the lower sequence maximum depositional age at c 560 ma is mainly composed of metagreywackes while the upper sequence maximum depositional age at c 500 ma consists of mica schists and other minor types major and trace element geochemistry of the metagreywackes of the lower sequence suggests that they were generated in relation to a peri gondwanan arc system built on the thinned continental margin although some chemical transition to passive margin greywackes is also observed this sedimentary sequence was probably deposited in an ediacaran early cambrian back arc setting or retro arc setting closer to the thinned platform of the continental margin the geochemical features of the sedimentary rocks of the upper sequence suggest some affinity with passive margin sediments they were probably deposited closer to the continental domain and to certain distance from the most active zones of the magmatic arc the nd model ages of 23 analysed samples are paleoproterozoic and range between 1782 ma and 2223 ma average value 1919 ma the nd model ages are slightly younger in the upper sequence than in the lower sequence but altogether they define a single population and therefore the two metasedimentary rock sequences can be clearly related sedimentation probably took place within the same basin located in the continental platform of gondwana the main source areas of these sedimentary rocks did not change during the late neoproterozoic and cambrian times the nd model ages are very old and they seem to be compatible with paleoproterozoic or archean source areas with only minor participation of younger sources probably represented by intrusive cadomian pan african granitoids

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

contrasting microstructures and deformation mechanisms in metagabbro mylonites contemporaneously deformed under different temperatures c 650 c and c 750 c

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

development of shape and lattice preferred orientations of amphibole grains during initial cataclastic deformation and subsequent deformation by dissolution precipitation creep in amphibolites from the ryoke metamorphic belt sw japan amphibolites from the ryoke metamorphic belt sw japan were deformed initially by cataclasis and subsequently by dissolution precipitation creep initial cataclastic deformation produced a rather weak shape preferred orientation spo of brown amphibole grains with small aspect ratios as well as a poorly developed amphibole lattice preferred orientation lpo with na a 100 axes scattered subnormal to the foliation and ng or c 001 axes scattered around the lineation during later deformation by dissolution precipitation creep preferential dissolution at grain boundaries subparallel to the foliation and simultaneous compaction normal to the foliation have likely produced a distinct spo of elongate brown amphibole grains subparallel to the foliation as well as their lpo such that their ng or c axes are scattered around the lineation while na a and nb b 010 are spread along a girdle normal to the lineation also during this deformation green amphibole precipitated as isolated grains or in pressure shadow regions around brown amphibole grains nucleation and anisotropic growth of isolated green amphibole grains according to the orientations of the principal stress directions produced an lpo of these grains such that their na a are oriented normal to foliation nb b within the foliation normal to the lineation and ng or c axes are parallel to the lineation in addition there is an associated spo growth of green amphibole in pressure shadow regions around brown amphibole grains occurs either syntaxially or anisotropically according to the orientations of the principal stress directions q 2003 elsevier ltd all rights reserved

on the relationship between kilometer scale sheath folds ductile thrusts and minor structures in the basal high pressure units of the cabo ortegal complex nw spain abstract large sheath folds in the basal high pressure nappes of the cabo ortegal complex are described and a kinematic interpretation provided the principal penetrative and map structures relate to regional d2 deformation which produced foliations s2 bearing mineral and stretching lineations l2 and several types of folds a type sheath like and folded folds the latter structures are subparallel to the trend of the orogen their attitude suggests that the units involved shared a common tectonic evolution during progressive ductile deformation of an anisotropic medium reconstruction of major geological structures was accomplished through projection of map scale features onto the ductile flow plane and the plane perpendicular to the ductile flow direction the structures reconstructed illustrate their development in the deeper structural levels of an orogenic channel subjected to high pressure metamorphism during the early phases of the hercynian orogeny in nw iberia we argue that orogen normal tectonic displacements of up to a few hundreds of kilometers represent the minor components of the transpression with possibly thousands of kilometers along strike dextral displacement between the intervening plates during subduction collision

high temperature microstructures and rheology of deformed granite erzgebirge bohemian massif abstract three stages of granite deformation were recognized in porphyric granitoids sheared under amphibolite facies conditions in the erzgebirge mts 1 weakly deformed granite in lozenge shaped pods 2 s c orthogneiss and 3 thin zones of banded mylonites weakly deformed granite forms a load bearing framework lbf structure in which quartz and feldspar aggregates exhibit similar strain intensities and shapes k feldspar deforms by fracturing and onset of dynamic recrystallization along clast margins plagioclase recrystallizes completely and quartz shows effects of grain boundary migration recrystallization and activity of basal a slip s c orthogneiss is interpreted as an interconnected weak layer iwl structure with high viscosity contrast between quartz and both feldspars concentration of deformation into thin quartz layers is indicated by intense strain of quartz grain size variations of recrystallized quartz and dominant prism a slip feldspars show well annealed structures and slip on 010 001 banded mylonite is marked by similar strain intensity of quartz and feldspar aggregates and low viscosity contrast between constituent phases quartz shows evidence for grain size increase and rhomb a c slip fully recrystallized feldspars show evidence for 010 100 slip the whole microstructural sequence and the transitions from lbf structure to iwl structures show the dependence of microstructural evolution of different materials on the bulk strain intensity

magmatism and early variscan continental subduction in the northern gondwana margin recorded in zircons from the basal units of galicia nw spain in situ uranium lead dating la sf icpms and sims and lu hf isotope analyses la mc icp ms of zircon from eclogite facies rocks from the basal units of the variscan belt in galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement the samples studied are two felsic gneisses two eclogites and one eclogitic gneiss of intermediate composition metatonalite oscillatory zoned zircon cores from the felsic samples gave a main clustering of u pb ages at 493 2 and 494 2 ma and some older ages that represent inherited cores zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 3 and 498 6 ma respectively interpreted as time of protolith crystallization variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 3 ma and very tiny zircons of one eclogite gave 350 2 ma both of which we interpret as metamorphic ages the new age data demonstrate that the calc alkaline magmatic suite described in the basal unit is ca 20 ma older than the alkaline to peralkaline plutonic suite of the same unit dated at 472 2 ma rodriguez et al 2007 and thus probably represents a distinct geologic event overgrowth rims are interpreted as metamorphic on the basis of their lu hf and th u ratios the 372 3 age is considered as dating the high pressure high p metamorphism and is essentially in agreement with previous ar ar and rb sr data this high p metamorphism marks the initial early variscan subduction of the gondwana margin the inherited zircon ages and hf isotopic composition of zircons point to a considerable input of crustal material with west african craton provenance to the felsic magma

microstructural evidence for the transition from dislocation creep to dislocation accommodated grain boundary sliding in naturally deformed plagioclase abstract we use quantitative microstructural analysis including misorientation analysis based on electron backscatter diffraction ebsd data to investigate deformation mechanisms of naturally deformed plagioclase in an amphibolite gabbro mylonite the sample is from lower oceanic crust exposed near the southwest indian ridge and it has a high ratio of recrystallized matrix grains to porphyroclasts microstructures preserved in porphyroclasts suggest that early deformation was achieved principally by dislocation creep with subgrain rotation recrystallization recrystallized grain average diameter 8 m microstructures indicate that subsequent grain boundary sliding gbs was active in the continued deformation of the recrystallized matrix the recrystallized matrix shows four grain junctions randomized misorientation axes and a shift towards higher angles for neighbor pair misorientations all indicative of gbs the matrix grains also exhibit a shape preferred orientation a weak lattice preferred orientation consistent with slip on multiple slip systems and intragrain microstructures indicative of dislocation movement the combination of these microstructures suggest deformation by dislocation accommodated gbs disgbs strain localization within the recrystallized matrix was promoted by a transition from grain size insensitive dislocation creep to grain size sensitive gbs and sustained by the maintenance of a small grain size during superplasticity

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

scales of deformation partitioning during exhumation in a continental subduction channel a petrofabric study of eclogites and gneisses from nw spain malpica tui allochthonous complex

does cation ordering in omphacite influence development of lattice preferred orientation three mantle eclogite xenoliths from the robert victor mine in south africa were studied the calculated pt conditions are ps4 5 gpa and t between 950 and 1145 8c the clinopyroxene has a jadeite component between 0 19 and 0 52 in situ at these compositions and temperatures clinopyroxene omphacite has the high temperature c2 c structure which has perfect cation disordering the lattice preferred orientation lpo of clinopyroxene in the samples of different microstructure and composition has been measured by ebsd the lpo for all three samples is characterized by a strong alignment of the 001 axes with the lineation typical of the l type or ls type patterns for omphacite these data disagree with a cation ordering model for dislocation slip which predicts a s type lpo pattern with the 001 axes forming a girdle in the foliation plane numerical simulation using the vpsc model has been undertaken to show the influence of the h110i 1 10 slip system on the lpo development in axially symmetric compression and extension and pure and simple shear the activity of the h110i 1 10 system should be very sensitive to changes in the cation ordering as the clinopyroxene goes towards the low temperature fully ordered p2 n structure increasing the critical resolved shear stress on this system from 1 to 10 resulted in a decrease of the slip activity of 80 but produced no significant changes to the lpo the most important slip systems for lpo development are 001 100 100 010 and 001 010 in all simulations the ls index is introduced to quantify the symmetry of omphacite lpo the index is based on the eigenvalues of the 010 and 001 pole figures and has a value of one for the end member l type zero for the end member s type and intermediate values for ls types the naturally deformed omphacite lpos have ls indexes of between 0 61 and 0 85 confirming they are ls types but closer to the l type end member in the vpsc models the s type end member develops in axially symmetric compression ls indexz0 whereas the l type end member develops in axially symmetric extension ls indexz1 in simple and pure shear the ls type fabric develops with ls index of between 0 55 and 0 60 the vpsc simulations show that the ls index is nearly constant with increasing strain for a constant macroscopic velocity gradient which prescribes the constant strain path we conclude that development of s and l type lpos in omphacite is controlled by the strain path and that the ls index is a useful quantitative indicator of fabric symmetry q 2005 elsevier ltd all rights reserved

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

deformation processes in eclogitic rocks evidence for the rheological delamination of the oceanic crust in deeper levels of subduction zones the deformation mechanisms and associated growth processes operating in omphacite during foliation development and veining have been studied in a km scale eclogitic ductile shear zone from the monviso meta ophiolitic complex italian western alps dislocation creep due to multiple slip and or climb is the dominant deformation mechanism whereas 100 and 001 twinning fracturing and intergranular fluid phase diffusional creep are subordinate active slip systems include 110 001 12 110 110 100 001 and 12 010 100 fracturing is interpreted in terms of brittle instabilities arising from strain and pore fluid pressure partitioning between adjacent omphacite domains the transition from plastic to brittle behaviour of rocks is cyclic in nature as the internally generated fluids can be released and trapped repetitively during progressive deformation the significance of the eclogitic shear zone is discussed in terms of a mechanically weak zone that developed during burial or at least prior to the peak of temperature at a minimum depth of 40 km in a subduction zone and which subsequently acted as a zone of detachment during a late stage of the alpine orogeny a rheological model for oceanic crust delamination in deeper levels of subduction zones is presented

dynamics of sediment subduction accretion at convergent margins short term modes long term deformation and tectonic implications a conceptual model is proposed for the short term operation of subduction accretion systems at convergent margins the operation is described in terms of the mass fluxes among the four system components namely the accretionary or prowedge wedge p an uplifted plug u a retrowedge r and the subduction zone the latter consists of a conduit c of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly a broad range of convergent margin types can be accommodated within the conceptual model framework with differing short term modes that depend on which of the p u r c components that are active finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible these experiments determine excitation of modes under controls that include model entry and exit mass fluxes flexural loading by accreted material and slab pull forces boundary velocities corresponding to subduction and subduction zone advance retreat distribution thickness and density of accreted sediment and internal frictional properties of the coulomb sediments that all modes with the exception of p in which only a prowedge is created are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized in addition the finite deformation predicted by the models is used to infer the long term tectonic styles associated with both single mode evolutions and those that exhibit mode switching a steady state mass balance can occur for modes 0 pure subduction p c i e where components p and c are active or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating the p c mode is favored by subduction zone retreat and or an increasing subduction load and creates an apparent landward dipping backstop p u c is predicted to be the most common mode and it creates an apparent seaward dipping backstop p u r c is an inefficient mode but is favored by easily detached retrocrust or sediment and or a decreasing subduction load

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

spider texture and amphibole preferred orientations abstract foliation in blueschist facies chert from california is defined by layers of oriented alkali amphibole which consistently curve towards and converge on pyrite and possibly pyrrhotite crystals these foliation nodes not previously described are called here spider texture the texture is interpreted in terms of perturbations of the stress field in a matrix undergoing strain about rigid pyrite or pyrrhotite crystals and it has important implications for understanding the mechanisms of amphibole preferred orientation development geometrical relationships between spider texture pressure shadows and quartz preferred orientations suggest that amphiboles grew with a strong preferred orientation along planes of maximum shearing stress the mechanism of foliation and preferred orientation development probably involved competitive anisotropic growth of amphibole prisms within the small gaps that open at steps on shear planes followed by additional micro porphyroblastic growth the first stage of the mechanism is similar to slickenfibre development

deformation high pressure metamorphism and exhumation of ultramafic rocks in a deep subduction collision setting cabo ortegal nw spain abstract new petrological and microstructural data from various metaperidotite boudins within large ductileshear zones in the cabo ortegal allochthonous complex in nw spain have important implications forthetectonicmodelsofthearea theperidotites myloniticgarnetharzburgite ti clinohumiteandmagnes ite olivine orthopyroxenite contain mineral assemblages that equilibrated at high to ultra high pressuremetamorphic conditions as well as microstructures of tectonic origin formed at temperatures well above800 c olivine and orthopyroxene fabrics resulted from ow at high temperature 1000 c and solid statenon coaxialplastic owatintermediatetemperature 800 900 c flowcauseddynamicrecrystalliz ationandformationofmoderatetostronglatticepreferredorientationsunderlowtomoderatediverentialstresses and strain rates characteristic of upper mantle and deep crustal deformation the microstructuresand textural relationships suggest that the mylonitic garnet harzburgite represents mantle fragments withlithospheric and asthenospheric imprints whereas the olivine orthopyroxenite resulted from serpentiniteburial to depths where it acquired a characteristic high ultra high pressure metamorphic signature bothtypes of ultrama tes converged to a common sitein a subduction zone that was later incorporated duringcontinental collision to the nw iberian massif as exotic allochthonous complexes that record structuraland metamorphic evidence of the earliest phases of the hercynian orogeny key words cabo ortegal metaperidotite petrofabric rheology ti clinohumite