provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

the earthquake prediction experiment at parkfield california since 1985 a focused earthquake prediction experiment has been in progress along the san andreas fault near the town of parkfield in central california parkfield has experienced six moderate earthquakes since 1857 at average intervals of 22 years the most recent a magnitude 6 event in 1966 the probability of another moderate earthquake soon appears high but studies assigning it a 95 chance of occurring before 1993 now appear to have been over simplified the identification of a parkfield fault segment was initially based on geometric features in the surface trace of the san andreas fault but more recent microearthquake studies have demonstrated that those features do not extend to seismogenic depths on the other hand geodetic measurements are consistent with the existence of a locked patch on the fault beneath parkfield that has presently accumulated a slip deficit equal to the slip in the 1966 earthquake a magnitude 4 7 earthquake in october 1992 brought the parkfield experiment to its highest level of alert with a 72 hour public warning that there was a 37 chance of a magnitude 6 event however this warning proved to be a false alarm most data collected at parkfield indicate that strain is accumulating at a constant rate on this part of the san andreas fault but some interesting departures from this behavior have been recorded here we outline the scientific arguments bearing on when the next parkfield earthquake is likely to occur and summarize geophysical observations to date

the evolution of perpendicular linear fabrics in synkinematically emplaced tourmaline granite central moravia bohemian massif abstract sheets of leucocratic peraluminous tourmaline granite were synkinematically emplaced into metasediments of the svratka crystalline unit during the variscan orogeny the granite was deformed under hightemperature conditions close to the solidus as indicated by quartz microstructures and the presence of prism c glide in quartz decrease in the temperature of deformation causing orthogneissification of the granite is documented by transition from prism c to prism a and basal a glide operating in quartz grains two mutually perpendicular stretching lineations originated during high temperature deformation of the granite detailed three dimensional analysis of tourmaline preferred orientations underlining both lineations has been performed and the distributions have been compared with theoretical predictions of jeffery and march for rotation of rigid linear markers the origin of mutually perpendicular linear fabrics is discussed on the basis of numerical models transpression considered as a combination of northwestward shearing accompanied by shortening across and lengthening along the shear plane is proposed to explain the perpendicular linear fabrics observed

deformation of partially molten granite a review and comparison of experimental and natural case studies experimental and natural investigations of partially molten granite are compared and reviewed experiments suggest that deformation of partially molten granite with low viscosity melt 104 pa s exhibits a rheological critical melt percentage rcmp in case of high viscosity melts 108 pa s however the relationship between melt fraction and log viscosity of the partially molten granite may be linear considerations about viscosity rheological thresholds and segregation of natural melts suggest that low viscosity melt experiments simulate natural conditions more realistically therefore an rcmp is to be expected under natural conditions both diffusion creep and dislocation creep may occur under natural conditions whereas cataclastic flow is only observed under experimental conditions a melt induced transition from dislocation creep to diffusion creep occurs under experimental and natural conditions melt topology is controlled by the magnitude of differential stress under experimental conditions if differential stress is higher than 100 150 mpa melt pockets are elongate and oriented at a low angle to the maximum compressive stress in contrast in nature melt pockets tend to be oriented subparallel to the foliation plane i e presumably at a high angle to the maximum compressive stress

the loma prieta earthquake 1989 and earth strain tidal amplitudes an unsuccessful search for associated changes the loma prieta earthquake of 1989 pro vided an opportunity for a sensitive test of suggestions that earthquakes may be preceded by variations in earth tidal strain amplitudes such variations have been pro posed as providing an advantageous technique for detect ing precursory chkuges in elastic parameters in a seismo genic zone we have analyzed data from two borehole strainmeters continuously operating within 40 km of the epicenter and within about 10 km of the southern end of the rupture we are unable to identify any precursory changes in m2 and o 1 tidal amplitudes and estimate that any large scale changes in young s modulus must have been less than about 2 if these results apply gener ally we would conclude that variations in elastic material properties prior to earthquakes do not occur throughout substantial volumes of the subsequent hypocentral region

is there a relationship between magmatic fabrics and brittle fractures in plutons a view based on structural analysis anisotropy of magnetic susceptibility and thermo mechanical modelling of the tanvald pluton bohemian massif abstract the present study focuses on the thermal and mechanical aspects of cooling of plutonic systems with respect to their internal structure analysis of the tanvald pluton and the nearby liberec granite the krkonose jizera plutonic complex bohemian massif using the anisotropy of magnetic susceptibility ams method indicates the decoupled magnetic fabrics formed by different processes in these two plutons two regional sets of vertical fractures cut across the plutons and their host rocks they are interpreted as having been formed as opening mode fractures joints after emplacement and cooling our thermo mechanical finite element modelling indicates that the thermal stresses within the tanvald pluton were compressive and that fracture criteria were not met during pluton cooling these results imply that the emplacement depth and confining pressure significantly affect the state of stress within a pluton during cooling and may play a crucial role in the formation of early fractures in plutons consequently the formation of early fractures during pluton cooling is driven by external conditions overriding the role of internal magmatic fabrics we argue that no genetic geometric or temporal correlation can exist between magmatic fabrics and fractures in granitoid plutons and thus that the original concept of hans cloos based on fabric joints relationships should be reconsidered instead an alternative classification of fractures in plutons should include cooling fractures syntectonic fractures uplift fractures and post uplift fractures

high pressure metamorphism in the early variscan subduction complex of the sw iberian massif abstract several units exposed in the boundary area of the ossa morena zone omz and the south portuguese zone spz preserve petrographic and thermobarometric evidence for an early metamorphic episode m1 developed under a high p low to intermediate t gradient related to early variscan subduction in the sw iberian massif in the omz these are the cubito moura unit p min 9 2 kbar and t 395 45 c blueschists from its basal melange p min 12 4 kbar and t 310 11 c and the underlying fuenteheridos group p 10 9 0 4 kbar and t 449 31 c the equivalent units in the spz are the la minilla formation p 8 7 0 4 kbar and t 388 16 c and the lawsonite pseudomorphs bearing pulo do lobo formation all these units formed part of an approximately ne verging orogenic wedge present coordinates developed by the accretion of subducted slabs of the outermost margin of gondwana and other elements of the rheic ocean realm from at least the middle devonian to the lowermost tournaisian high pressure rocks were subsequently emplaced on more internal zones of the omz that only experienced a younger high t low to intermediate p metamorphism m2 this high t event was coeval with magmatic activity from the uppermost devonian to the middle mississippian probably as a consequence of transtensional lithospheric thinning and or delamination of the lower crust and mantle lithosphere of the gondwana margin pre late devonian synorogenic sedimentation in forearc and back arc basins of the subduction complex evolved to a late devonian middle mississippian foreland basin system in early collisional stages finally a new middle late mississippian fold and thrust belt with opposite sw vergence and new foreland basins developed during late collisional stages

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

measurements of the strain field associated with episodic creep events on the san andreas fault at san juan bautista california over the past 6 years repeated strain steps recorded on a tensor strainmeter installed close to the san andreas fault trace at san juan bautista in california correlated closely with episodic creep events registered on a nearby creepmeter the strain events were remarkably similar in character were of about 1 hour duration and were followed within hours to days by creep events of 2 6 days duration the episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data we postulate that the additional stress transferred to this region by the loma prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region thus accounting for the increased frequency of the creep strain events following the loma prieta earthquake

magmatic fabric acquisition mechanisms in a syenite results of a combined anisotropy of magnetic susceptibility and image analysis study the structural significance of the anisotropy of magnetic susceptibility ams and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the archean lebel syenite intrusion ontario canada bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples intensities shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding ams ellipsoids differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre full crystallization textures and structural overprinting in samples deformed after crystallization subfabrics of clinopyroxene and magnetite in samples showing pre full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes the resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite which is similar to the overall petrofabric of the samples crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains analogous to a logjam in a river the final characteristics of the ams and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma rather than transient late features

baltic provenance of top famennian siliciclastic material of the northern rhenish massif rhenohercynian zone of the variscan orogen the provenance of top famennian sedimentary rocks linked to the hangenberg crisis from the northern rhenish massif germany was investigated by the means of detrital zircon u pb geochronology based on the obtained age spectra three main tectonothermal domains are recognized as possible sources paleo and mesoproterozoic 2000 1000 ma units of baltica and early paleozoic caledonian orogen 500 400 ma our interpretation of the detritus having been derived from northern source areas i e baltica and the scandinavian caledonides with a minor input of german polish rugen pomeranian caledonides contradicts the traditional view that during the upper devonian the northern rhenish massif was supplied by detritus from the south complementary mineralogical textural and geochemical analyses point to a derivation of the detritus of drewer and hangenberg sandstones mainly from felsic recycled continental crust the elevated concentrations of pb and zn in the studied sections are a feature attributed to hydrothermal alteration related to the terminal devonian synsedimentary volcanism or post depositional variscan deformation

birth of a volcanic passive margin in cambrian time rift paleogeography of the ossa morena zone sw spain abstract mapping at 1 10000 scale of both north n and south s flanks of the variscan monesterio antiform in the ossa morena zone southern spain in association with detailed petrographic and major element analyses of magmatic rocks has revealed the building of a passive but volcanically active oceanic margin in cambrian time this margin was located in northern gondwana and represents the beginning of an extensional tectonic regime that concluded with the opening of the rheic ocean at time of eruption middle cambrian the n flank was located on the northern coastline of a marine basin in the process of passive rifting the vents mainly located in its western part were subaerial continental and or shallow marine on the s flank eruptions and deposits were shallow marine occurring as lava flows and pillows interstratified within shallow marine sediments on both flanks intrusives were emplaced after the extrusives lateral thickness variations of the sedimentary deposits testify to the building during four distinct periods of a rifted passive margin with a shifting locus of volcanic activity over time the composition of magmas was bimodal the mafic magmas are silica saturated alkaline to sub alkaline and somewhat ti rich perhaps originating from partial melting of a subcontinental mantle that was less metasomatized with time the youngest magmatism with an affinity close to that of morb likely had an asthenospheric source the alkaline felsic products are diverse and may have originated from a very heterogeneous continental crust in each set partial melting relationships prevail suggesting that the melts rose along open fractures as soon as they formed in the context of lithospheric thinning it is possible that mantle and crustal melting was triggered by decoupling between rheologically different materials

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

multiple magmatic fabrics in the sazava pluton bohemian massif czech republic a result of superposition of wrench dominated regional transpression on final emplacement abstract the sazava pluton 354 ma of the central bohemian plutonic complex was emplaced syntectonically into the upper crustal neo proterozoic and lower paleozoic host rock along the se margin of the tepla barrandian zone bohemian massif czech republic the host rock is characterized by transpressional fabric dominated by steep se nw dipping foliations and subhorizontal ne sw stretching lineations associated with flattening to a plane strain ellipsoid the strain intensity degree of oblateness and temperature conditions of deformation increase in the host rock towards the pluton margin in the pluton quartz microfabric indicates progressive cooling of the syntectonically emplaced magma the ams method revealed multiple magmatic fabrics and lateral variations in the shapes of the ams ellipsoid and the magnetic fabric intensity in the pluton the central part of the pluton yields prolate shapes of the ams ellipsoid and a low degree of anisotropy with preserved steep magnetic lineations whereas along the pluton margins oblate ams ellipsoids are associated with a high degree of anisotropy we interpret the multiple fabrics and the partially decoupled fabric pattern between the host rock and the pluton as being a result of superposition of regional tectonic strain during highly oblique transpression on emplacement related intrusive strain where outer solidified and more rigid edges of the pluton enabled preservation of older steep fabrics in its interior

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

the ca 350 ma beja igneous complex a record of transcurrent slab break off in the southern iberia variscan belt abstract we report the results of an isotopic study of the large gabbro dioritic beja igneous complex bic in the boundary between the highly contrasting ossa morena om and south portuguese sp zones of the southern iberian variscan orogen this boundary is interpreted as a major suture zone materialized by discontinuous scattered strips of mafic ultramafic rocks the so called beja acebuches ophiolite complex baoc and by melange deposits of middle to late devonian age in the pulo do lobo accretionary prism plap the beja gabbro was interpreted either as part of the ophiolite like units or as a broadly arc related massif reflecting the northward subduction of oceanic lithosphere u pb zircon id tims dating of two diorites and a granodiorite points to igneous emplacement ages of 350 2 ma serpa 352 2 ma torrao and 353 4 ma sao pedro respectively whereas a felsic dyke yields a slightly younger age of 345 2 ma these results show that published ar ar dates do not represent igneous crystallization ages but merely reflect regional cooling below ca 500 c at least 10 ma after the major intrusive event probably as a result of uplift of the omz side of the suture zone relative to the subsiding spz 87sr 86sr350 and end350 display a large range of values from 0 7041 to 0 7093 and from 4 0 to 6 1 respectively which documents a rather complex petrogenetic history with an important role played by crustal contamination processes the more primitive sr and nd isotope signatures are measured in the mafic cumulates while radiogenic sr and unradiogenic nd isotope compositions occur in the more evolved rock types the broad trend of decreasing end350 with decreasing sm nd and increasing sio2 concentration is reminiscent of crustal assimilation combined with fractional assimilation afc end values of flasergabbros and associated cumulates ascribed to the ophiolite like unit in the guadiana valley are close to zero or even slightly negative irrespective of the age chosen for the correction for in situ radioactive decay of 147sm in the range 350 500 ma assumed to encompass the true geological age this observation conflicts with a truly oceanic derivation of these rocks which could rather represent early stage deformed and metamorphosed equivalents of the beja gabbro most metabasalts of the same area have positive albeit variable end consistent with mildly depleted mantle sources and or significant crustal assimilation only a few samples reach highly positive values end350 500 ca 8 similar to those typical for n morbs as do the acebuches amphibolites near aracena end350 500 ca 9 these data suggest that the baoc is not only disrupted but also composite since the gabbroic rocks are not cogenetic with the basalts based on the late devonian age of the collision between the omz and spz the emplacement of the beja gabbro cannot be related to an active subduction of oceanic lithosphere instead the generation in the mantle and emplacement in the overlying continental crust of copious volumes of mafic magmas with concomitant lp ht metamorphism is tentatively ascribed to a process of transcurrent slab break off that followed oblique subduction and collision

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

a multidisciplinary study on the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part i structural geology ams and paleomagnetism during the cretaceous the south china block scb experienced a widely distributed extensional event including numerous plutons emplacement and basin opening investigations on the tectonic regime coeval with pluton emplacement and emplacement mechanism of the pluton remain relatively rare in the scb in order to address these questions a multidisciplinary approach including field structural and petrographic observations anisotropy magnetic susceptibility ams and paleomagnetic analyses was carried out on the qingyang jiuhua granitic massif which intrudes into the lower yangtze fold belt in the northeastern part of scb the qingyang jiuhua massif is composed of the granodioritic qingyang and monzogranitic jiuhua plutons dated by zircon u pb method at ca 142 ma and ca 131 ma respectively our structural observations show that the intrusion of the qingyang jiuhua massif does not modify the fold strike a weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism in the contact aureole of the massif the foliation follows the pluton contour and the mineral lineation is rare when present it exhibits a down dip attitude field and microstructural observations indicate isotropic magmatic textures in most parts of the massif the ams analysis of 93 sites reveals weak values for the anisotropy degree pj 0 for most of the measured samples two principal foliation patterns are identified horizontal foliations in the center of the plutons and vertical foliations on the boundaries magnetic lineation strike is largely scattered and weakly inclined at the scale of the entire massif the paleomagnetic investigations indicate that a the younger jiuhua pluton did not produce a remagnetization in the older qingyang pluton b no relative movement can be observed between these two plutons c the entire massif did not experience any important relative movement with respect to south china considering the paleomagnetic uncertainties integrating the newly obtained results with previous observations our study favors a permissive emplacement mechanism for the two plutons i e vertical magma intrusion into an opening space controlled by the nw se brittle stretching of the upper crust which is in agreement with a weak extensional regional tectonic framework of the scb

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

multiple magmatic fabrics in plutons an overlooked tool for exploring interactions between magmatic processes and regional deformation this paper elaborates on the concept of multiple magmatic fabrics in plutons after a general overview of various types of multiple fabrics that may develop in magmatic rocks two case examples of porphyritic granite and melasyenite plutons in the bohemian massif are examined in the jizera granite complex variations in k feldspar phenocryst shape fabric revealed by image analysis of a 200 m long section of an underground tunnel are in contrast with homogeneously oriented magnetic ams fabric carried by coaxial contributions of biotite magnetite and maghemite in the kni eci stolec melasyenite pluton emplacement related margin parallel feldspar foliation was overprinted by flat lying foliation the latter is interpreted to record regional tectonic strain at the two stations examined in detail the crystallographic preferred orientation of biotite and amphibole in the inter phenocryst matrix measured using electron back scatter diffraction ebsd differed from both feldspar fabric and also from the ams principal directions multiple magmatic fabrics in these two plutons are interpreted in terms of fabric superposition where late weak strain is superposed onto a high strength phenocryst framework but is accommodated preferentially by small mineral grains biotite magnetite in the melt bearing matrix this mechanism may explain the discrepancy between mesoscopic feldspar fabric and ams we conclude that multiple magmatic fabrics in plutons may thus result from accumulated strain caused by different processes during final crystallization and as such may serve as a sensitive indicator of the evolving interactions between magmatic and tectonic processes in the earth s crust

sigmoidal structures featuring dextral shear during emplacement of the hercynian granite complex of cauterets panticosa pyrenees abstract the three plutons of the cauterets panticosa granite complex are studied by the anisotropy of magnetic susceptibility ams technique as expected from the dominantly paramagnetic nature of the rocks the magnetic susceptibility zonation accurately compares with the lithological zonation detailed examination of the linear and planar structural patterns of the plutons allows the difficult problem of their formation mechanisms to be explored the sigmoidal pattern of the linear structures only results from regional tectonics and points unequivocally to a dextral shear component for the responsible tectonic event the concentric planar structures while also affected by regional tectonics retain some imprint of magma emplacement the microstructures observed in the granitoids point to a continuous deformation from the magmatic state to the high temperature solid state in the country rocks structures and kinematic criteria underline the transpressive character of the d2 hercynian event of the pyrenees which resulted in a partitioning between zones where either compression or dextral strike slip were dominant all these data constrain a new model of d2 syn tectonic emplacement for the plutons of the cauterets panticosa complex and remarkably illustrate structural features resulting from a transpressive regime

continuous measurements of crustal deformation for the 1992 landers earthquake sequence abstract we describe and attempt to interpret continuous measurements of strains and tilts made at pinon flat observatory pfo before during and after the landers and joshua tree earthquake sequences these data show substantial transient deformation following the landers mainshock with a total amplitude of several percent of the co seismic deformation and a decay time of at least several days comparing data from the many types of instruments at pfo allows us to infer possible sources for this deformation the immediate postseismic transient was nearly the same size on three long base strainmeters suggesting either broadscale deformation or local motion near one part of the observatory the latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source possible mechanisms for broad scale deformation include postseismic fault slip time dependent creep in near surface rocks and elastic or thermal responses to water table changes the first two agree best with the observations from pfo but if postseismic fault slip is the source it must have been distributed differently than the co seismic slip and may have included faults other than those that ruptured seismically if one of the other mechanisms is the main source the pfo data imply that the postseismic slip must have been very much smaller than the seismic slip perhaps 2 or less no significant preseismic deformation was observed at a level of 2 10 3 of the co seismic deformation for the days to minutes before the earthquake

a medium term precursor to the loma prieta earthquake the loma prieta earthquake ms 7 1 17 october 1989 provided unique near field borehole tensor strain observations the medium term data at the san juan bautista site showed a clear anomalous change in the fault parallel shear strain rate beginning about a year before the event the anomaly ultimately reached 30 of the coseismic offset the signal shows similarities with changes in strain rates reported from the geodetic record suggesting a broad regional anomaly the limited spacial sampling available prevents determination of a causal link useful for prediction between these data and the earthquake

role of magma pressure tectonic stress and crystallization progress in the emplacement of syntectonic granites the a type estrela granite complex carajas mineral province brazil the archaean syntectonic a type estrela granite complex carajas mineral province brazil consists of three plutons emplaced in a greenstone sequence under low pressure conditions 180 p 310 mpa it is composed mainly of annite ferropargasite hedenbergite and ilmenite bearing monzogranites the contact aureole is affected by a subvertical penetrative schistosity conformable with the limits of the plutons meso to microstructures and mineral reactions in the granites indicate that deformation occurred in a continuum from above solidus to low t subsolidus conditions two distinct planar structures are observed i a concentrical primary foliation s0 corresponding to rhythmic isomodal phase layering associated with a faint grain shape fabric it is horizontal in the centre and vertical towards the edges of the plutons and ii a steep to subvertical foliation s1 associated with the deformation of s0 and accompanied with emplacement of synplutonic dykes and veins of leucocratic granites and pegmatites emplacement differentiation and consolidation of the estrela granite complex are considered to result from a continuous evolution under decreasing temperatures in a single stage strained crust transpression with two main periods 1 the first period is controlled by body forces and it corresponds to inflation with magma ponding as long as the rheology is melt dominated magma pressure is the critical parameter and almost no strain is recorded with decreasing t magmas crystallize and differentiate leading to a concentrical magmatic phase layering the growing magmatic bodies are mechanically decoupled from the country rocks and their evolution depends on internal magma chamber processes 2 for higher amount of crystallization residual melt fraction f 0 5 the role of magma pressure becomes insignificant establishment of a continuous crystal framework leads to the coupling of plutons with their surroundings and deformation in response to tectonic stress most of the strain is recorded during this period which starts from the rigid percolation threshold and extends to subsolidus low grade conditions this leads to deformation of the partially crystallized volume and redistribution of fluid enriched differentiated melts the amount of crystallization through the rheological thresholds appears as the critical parameter determining the transition from magma controlled processes inflation and differentiation of the magma chamber with development of a phase layering to tectonic controlled processes deformation of the phase layering and redistribution of residual melts this accounts for the fact that syntectonic plutons commonly display intermingled boudinaged layers with distinct modal compositions and in some cases well preserved rhythmic layering

direct observation of deformation processes in crystal mushes isothermal microstructural development in deforming melt bearing crystal mushes is studied using synkinematic microscopy and a polyphase thiocyanate system the aim of the work is to contribute to the basis for understanding the origin of microstructures in rocks which have undergone melt present deformation during high strain rate pure shearing experiments microstructures indicating crystal plasticity and dynamic recrystallization are observed during low strain rate pure shearing experiments a pressure solution like process called contact melting is active resulting in optically strain free crystals crystal boundary sliding is active in both regimes framework scale processes such as filter pressing grain flow and development of a micro shear zone are illustrated some of these processes are cryptic microstructurally and may be difficult or impossible to recognize in rocks the validity of the experimental system as an analog for magmatic systems is discussed we conclude that the experiments can serve as a source of ideas to test against the evidence from rocks but that no direct analogy to the behavior of natural crystal mushes has been or will readily be established

fabric evolution of rigid inclusions during mixed coaxial and simple shear flows abstract in a slowly moving viscous fluid a population of non interacting rigid inclusions evolves in an oscillatory fashion or creates a stable fabric depending on the flow geometry and the shape of the inclusions using jeffery s equations for the motion of a rigid ellipsoidal inclusion a mixed coaxial and simple shear flow was investigated both analytically and by numerical modeling exact results were obtained for spheroidal inclusions depending on the axial ratio of the inclusion and the amount of the coaxial component of flow individual inclusions can move cyclically or rotate to a stable orientation in one of two mutually orthogonal directions equations were found which allow us to distinguish between these types of motion and to determine the stable inclusion orientation the relative motions of individual inclusions determine the preferred orientation in a population of inclusions in a heterogeneous population with diverse axial ratios the fabric evolves as the superposition of subfabrics corresponding to subsets with different axial ratios the orientations of stable subfabrics depend on axial ratios and could thus be used as kinematic indicators in cases of a sufficiently large coaxial component of flow the fabric created by a heterogeneous population may comprise two stable mutually orthogonal subfabrics no exact results are available for general i e non spheroidal ellipsoidal inclusions and numerical modeling must be used if the coaxial component of flow is sufficiently strong the non spheroidal inclusions move much like the spheroidal ones orthogonal subfabrics may appear as transient features in the evolution of a homogeneous population of non spheroidal ellipsoidal inclusions

the cadomian unconformity in the saxo thuringian zone germany palaeogeographic affinities of ediacaran terminal neoproterozoic and cambrian strata abstract the most complete ediacaran terminal neoproterozoic to early cambrian record in the saxo thuringian zone is provided by the cadomian deformed ediacaran rothstein formation and the unconformably overlying early cambrian zwethau formation in the torgau doberlug syncline tds conglomerates and greywackes of the marine rothstein formation are of continental magmatic arc provenance and record detrital zircon shrimp ages that indicate a late cryogenian to early ediacaran 700 580 ma age for the arc source and its emplacement into palaeoproterozoic 2000 ma crust tuffaceous intercalations record extrabasinal eruptions of evolved calc alkaline lavas that point to ongoing cadomian continental arc magmatism shrimp zircon ages from a tuffaceous layer date this volcanic activity and the formation s deposition as late ediacaran 566 10 ma close correlation in provenance and age to other ediacaran units of the saxo thuringian zone points to a common palaeogeographic setting in the avalonian cadomian belt detrital and inherited zircon ages and nd isotopic ratios from these ediacaran siliciclastic rocks suggest a position on the active margin of gondwana near the west african craton the early cambrian zwethau formation records the erosion of the underlying ediacaran rocks followed by the successive evolution of i a carbonate dominated subtidal ramp with calcimicrobial archaeocyathan buildups ii a shallow subtidal to intertidal mixed ramp with peritidal sediments and oolite shoal complexes and iii a more siliciclastic depositional environment calcimicrobial archaeocyathan buildups oolite shoals and sulfate nodules in intertidal sediments indicate warm arid to semi arid climatic conditions and a palaeogeographic setting in equatorial to sub equatorial latitudes archaeocyaths the trilobite taxon dolerolichia and the sedimentary facies assemblages compare closely to those of the mediterranean early cambrian constraining the palaeogeographic setting to the european sub equatorial western gondwana shelf realm archaeocyaths record a mid early cambrian middle issendalenian ovetian age making these the oldest cambrian sediments in both the saxo thuringian zone and the bohemian massif the middle cambrian lithological and palaeontological records of the tds closely resemble those of the frankenwald area of the saxo thuringian zone as well as those of other fragments of the western gondwana shelf in spain morocco and bohemia archaeocyaths early cambrian trilobites and correlations with morocco suggest cambrian deposition commenced at ca 520 ma such that the cadomian unconformity represents a time gap of about 35 55 ma latest ediacaran to early lower cambrian this gap which is related to the cadomian orogeny and so common to all areas of the cadomian belt obscures some of palaeobiological palaeogeographic and climatic change that marks the precambrian cambrian transition

metamorphic and deformational imprint of cambrian lower ordovician rifting in the ossa morena zone iberian massif spain abstract the high temperature metamorphism recorded in the valuengo and monesterio areas constitutes a rare occurrence in the ossa morena zone of southwest iberia where low grade metamorphism dominates the metamorphism of the valuengo area has been previously considered either cadomian or variscan in age whereas that of monesterio has been interpreted as a cadomian imprint however these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution the metamorphism of the valuengo and monesterio areas affects late proterozoic and early cambrian rocks and is syn kinematic with a top to the north mylonitic foliation which was subsequently deformed by early variscan folds and thrusts the u pb zircon age 480 7 ma we have obtained for an undeformed granite of the valuengo area is consistent with our geological observations constraining the age of the metamorphism we propose that this high temperature metamorphic imprint along a nw se ductile extensional shear zone is related to the crustal extension that occurred in the ossa morena zone during the cambro ordovician rifting in the same way the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one

mechanisms and orientation of melt segregation paths during pure shearing of a partially molten rock analog norcamphor benzamide abstract in situ deformation experiments were performed on partially molten analog materials norcamphor in the presence of a benzamide norcamphor melt undergoing pure shearing at a constant melt fraction of 0 13 melt in the samples induces a strain dependent transition from purely dislocation creep to dislocation creep associated with minor intergranular fracturing and grain boundary sliding gbs intergranular fractures drain the melt from initially isotropic melt pockets to grain boundaries along such boundaries grain boundary migration recrystallization is inhibited while gbs occurs intergranular melt pockets occur along grain boundaries oriented subparallel to the shortening direction but melt must have migrated parallel to the elongation direction of the samples as indicated by melt accumulations at both extruding ends of the sample intergranular melt pockets parallel to the elongation direction were only rarely observed because melt was rapidly expelled from these sites nevertheless these grain boundaries are the pathways of melt segregation in the samples

measurements and interpretations of tilt strain gauges in seismically active areas abstract an overview of different aspects of deformation measurements is given an introduction to the general theory of point crustal deformations with an application to the stress strain relations at the earth surface is made the instrumentation used in crustal deformation measurements is briefly explained emphasizing different constructions of tilt and strainmeters the problems connected with the housing of the instruments i e the cavity and topography effects and with the ambient noise factors as the hydrological and pressure effects are discussed the last part of the paper is concerned with the relation between deformation measurements and seismic events both from a theoretical and observational standpoint a number of examples are shown that report on pre co and post seismic and the secular term deformation in different areas of the world

comparing tiltmeters for crustal deformation measurement a preliminary report a collection of high precision tiltmeters is being operated at pinon flat observatory southern california both to compare instruments and to measure tectonic deformation we report on 1 2 years of data from four of these two michelson gale long fluid tiltmeters one long center pressure tiltmeter and a shallow borehole tiltmeter the three long base instruments are all located on the same baseline with a precise leveling line running between their end monuments at nontidal frequencies only the two michelson gale instruments show some coherence 3 for periods of 2 to 4 days while the center pressure instrument is correlated with air temperature at periods from a few days to a few weeks the most stable tilt record shows a secular rate of 0 28 rad a which may be real over much longer times leveling to specially stabilized benchmarks should confirm this comparing instruments has identified more and less successful measurement techniques it appears that low noise data will most probably be produced only by relatively complex and expensive instruments though even for these the operating costs over any reasonable lifetime will exceed the capital cost even the best existing sensors must be improved to measure continuous tectonic motions

y ree nb ta ti oxide ab2o6 minerals from rel ree euxenite subtype pegmatites of the třebic pluton czech republic substitutions and fractionation trends this paper describes subtitutions and fractionation trends in the y ree nb ta ti oxides aeschynite and euxenite group minerals from nyf euxenite subtype pegmatites of t ebic pluton

magma emplacement during exhumation of the lower to mid crustal orogenic root the jihlava syenitoid pluton moldanubian unit bohemian massif in this study we present structural and ams data from the w335 ma ultrapotassic jihlava syenitoid pluton which intruded the lower to midcrustal orogenic root moldanubian unit in the bohemian massif central european variscides the emplacement of the pluton was accommodated by multiple processes such as ductile host rock shortening formation of sheeted zones by magma wedging magmatic stoping and possibly host rock displacement within a wide transtensional zone magmatic fabrics preserved in the pluton reflect both intrusive processes and regional strain margin parallel and weneewsw foliations which probably formed by strain during emplacement of inner magma pulses were overprinted by tectonic strain within a zone of distributed wrench dominated dextral transtension this zone probably accommodated exhumation of different segments in the eastern part of the moldanubian unit during pluton emplacement in contrast to existing models we emphasize that the jihlava pluton as well as other ultrapotassic plutons widespread in the moldanubian unit are structurally highly variable bodies emplaced by multiple intrusive processes our case study illustrates how careful documentation of structural relations around these ultrapotassic plutons may constrain the kinematic framework and local exhumation histories in different segments of the orogenic root during and shortly after the w340 ma mechanical event in the central european variscides 2006 elsevier ltd all rights reserved

significance of low symmetry fabrics in magmatic rocks abstract the significance of low symmetry fabrics monoclinic and triclinic in magmatic rocks is studied using theoretical models describing the motion of rigid markers embedded in a slowly deforming ductile matrix in agreement with curie s symmetry principle the development of these fabrics requires a non coaxial strain path simulations of monoclinic fabrics indicate that their characteristics depend on a the strain regime and the amount of strain and b the morphological composition of the population of rigid markers natural monoclinic fabrics contain precious information about the strain regime they result from type of the non coaxial strain path involved simple shear non coaxial flattening etc orientation of its axes and sense of shear the magnitude of the finite strain may eventually be estimated provided that the shape of the markers is precisely known

overprinting of magnetic fabrics in granites by small strains numerical modelling abstract the anisotropy of magnetic susceptibility ams is used to determine weakly defined mineral preferred orientation fabrics in granites however the effects of small strains superimposed on the low intensity ams fabrics typical of these rocks is not well understood numerical simulations are presented which investigate the effects of small superimposed strains on the shapes magnitudes and orientations of pre existing ams fabrics which are similar to those reported in the literature for undeformed granites a computer generated biotite petrofabric is subjected to progressive strain using march s strain response model for pure shear plane strain axially symmetric shortening and simple shear regimes results indicate that a pre existing ams fabric might be significantly modified or overprinted by small strains 10 15 strain in the pure shear and axially symmetric shortening regimes 0 5 1 0 for simple shear a new ams fabric may develop with an orientation similar to that expected for shearing of an initially isotropic petrofabric at low strains the evolution of the shape of the ams ellipsoid depends strongly on the orientation of the initial petrofabric with respect to the strain framework the ams ellipsoid may evolve towards either oblate or prolate shapes and the evolution in the ellipsoid shape may be reversed during progressive strain this modelling suggests that small strains affecting a pluton following its emplacement could mask ams fabrics developed earlier the results also suggest that in simple shear the orientation of the ams ellipsoid may serve as a shear sense indicator for small strains even if the earlier fabric is not isotropic

four magmatic fabrics in the tuolumne batholith central sierra nevada california usa implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust for the first time four distinct magmatic fabrics are documented in a composite plutonic body the tuolumne batholith central sierra nevada california usa one type of fabric was formed by strain caused by highly localized magma flow type 1 whereas the other three chamber wide fabrics recorded strain increments during boundary processes along batholith margins type 2 superimposed by increments of heterogeneous regional tectonic strain types 3 and 4 our present work indicates that in contrast to studies that consider magmatic fabrics to be simple structures formed by a single process magmatic lineations and foliations in plutons may reflect accumulated finite strain and form as composite structures recording multiple strain increments in relatively static actively deforming and rheologically complex crystal mushes at lower melt percentages we demonstrate that multiple magmatic fabrics in a single batholith may record remarkably different processes and thus preserve a temporal record of strain in the batholith from strain during internal chamber processes to postem placement regional tectonic strain however it may be commonly very problematic to infer the exact nature of flow or even fabric forming process from the preserved rock record our study also exemplifies how examination of magmatic fabric patterns in plutons complemented with geochronology may provide crucial constraints on the interplay among successive magma emplacement fabric preservation temporal evolution of strain fields in a crystallizing magma chamber and the development of crystal mush zones

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

fabric evidence for granodiorite emplacement with extensional shear zones in the variscan gredos massif spanish central system abstract three granitoid bodies in the central part of the gredos massif spanish central system batholith are tabular around 1 km in thickness and intruded into a migmatitic middle crust during the d3 deformation phase of the variscan orogeny petrologically they are composed of bt granodiorite and crd monzogranite and they show varying abundance of large cm scale feldspar megacrysts a detailed study of the shape preferred orientation spo magmatic fabric defined by these megacrysts together with a kinematic analysis of the structures due to interactions between them and the measurement of quartz c axis fabrics in migmatites and granitoids suggests that granitic magma and country rocks were mechanically coupled during deformation the emplacement took place along large scale extensional shear zones active during the first stages of the d3 phase the shape of the spo ellipsoids varies from constrictional at the centre of the granitic bodies to flattening or even oblate at their external contacts with the migmatitic host rocks the favoured interpretation of this spatial fabric variation is the overprinting of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments followed by shear zone development commonly at the top of the granitic bodies the entire structure was later folded during the last stages of the d3 phase

the amphibolites from the ossa morena central iberian variscan suture southwestern iberian massif geochemistry and tectonic interpretation abstract basic rocks included in the ossa morena central iberian variscan suture of sw iberia have been studied in order to decipher the pre orogenic evolution along this major boundary these rocks appear as garnet free amphibolites and garnet bearing amphibolites previous geochronological data on these basic rocks suggest the existence of two different protolith ages upper precambrian and lower palaeozoic ordovician upper precambrian amphibolites form small size up to several metres thick lense dike or elongated shaped bodies while lower palaeozoic ones form elongated bodies with thicknesses up to several hectometres the chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic magmas as for the originally basaltic rocks their geochemical characteristics indicate a shallow melting with different proportions of at least three end members namely i a morb like asthenospheric source ii a more enriched plume derived or subcontinental lithosphere mantle source and iii a continental crust component moreover upper precambrian and lower palaeozoic amphibolites have different geochemical affinities in the former group we envisage an advanced intra continental rifting situation or an intra continental back arc setting related to the cadomian orogeny while for the latter we propose an environment of oceanic crust formation predating variscan subduction and collision the recognition of these remains of oceanic affinity basic rocks along the omz ciz boundary allows us to propose that some sort of ocean may well have existed between the two zones during early palaeozoic times and that this ocean was presumably closed at a later stage probably during the devonian by oceanic subduction under the ciz crust

tonalite sill emplacement at an oblique plate boundary northeastern margin of the bohemian massif abstract a tonalitic sill has been examined at the variscan transpressive boundary of the lugian and silesian plates at the ne margin of the bohemian massif a structural petrological and geochronological study reveals that it was emplaced syn tectonically with major ductile shearing in lower crustal rocks magmatic and pre rheological critical melt percentage rcmp fabrics are concordant with the hanging wall structures but discordant with those of the footwall the ams study shows the predominance of flattening strain at the margins and plane strain fabrics in the core numerical modelling of ams fabrics is in good agreement with the hypothesis of magma flow and deformation in oblique transpression a tectonic model was developed explaining emplacement and syn tectonic deformation of progressively cooled tonalitic intrusion

early paleozoic rifting and bimodal volcanism in the ossa morena zone of south west spain lower paleozoic volcanic members have been investigated by geological petrographical and geochemical means in a traverse across the ossa morena zone omz in south west spain

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

lattice preferred orientations of late variscan granitoids derived from neutron diffraction data implications for magma emplacement mechanisms the lattice preferred orientation lpos of two late variscan granitoids the meissen monzonite and the podlesi dyke granite were determined from high resolution time of flight neutron diffraction patterns gained at the diffractometer skat in dubna russia the results demonstrate that the method is suitable for the lpo analysis of polyphase relatively coarse grained 0 1 6 mm rocks the meissen monzonite has a prominent shape preferred orientation spo of the non equidimensional minerals feldspar mica and amphibole whereas spo of the podlesi granite is unapparent at the hand specimen scale the neutron diffraction data revealed distinct lpos in both granitoids the lpo of the non equidimensional minerals feldspar mica and amphibole developed mainly during magmatic flow in the case of the meissen monzonite the magmatic flow was superimposed by regional shear tectonics which however had no significant effect on the lpos in both samples quartz shows a weak but distinct lpo which is atypical for plastic deformation and different in the syn kinematic meissen monzonite and the post kinematic podlesi granite we suggest that first of all the quartz lpo of the meissen monzonite is the result of oriented growth in an anisotropic stress field the quartz lpo of the podlesi granite which more or less resembles a deformational lpo in the flattening field of the local strain field developed during magmatic flow whereby the rhombohedral faces of the quartz crystals adhered to the 010 faces of aligned albite and to the 001 faces of zinnwaldite due to shape anisotropy of their attachments the quartz crystals were passively aligned by magmatic flow thus magmatic flow and oriented crystal growth are the major lpo forming processes in both granitoids for the meissen monzonite the solid state flow was too weak to cause significant crystallographic re orientation of the minerals aligned by magmatic flow finally the significance of our results for the evaluation of the regional tectonic environment during magma emplacement is discussed the discussion on the regional implications of the more methodologically oriented results provides the basis for future more regionally aimed studies in view of the fabric characteristics of such plutons and their developing mechanisms

a shear strain anomaly following the loma prieta earthquake borehole tensor strain instruments deployed along the san andreas fault for the past ten years have provided sufficient resolution and stability to sample regional tectonic processes potentially enhancing earthquake prediction capability data obtained from the instrument at san juan bautista in the near field region of the 17 october 1989 loma prieta earthquake ms 7 1 provide the first opportunity to observe shear strain processes associated with a large earthquake we previously reported1 a prominent shear strain anomaly in those data for more than a year before the earthquake this anomaly ceased immediately after the earthquake but as we report here a new and higher rate of fault parallel shear accumulation 2 microstrain per year was established about four months later and has continued to the present associated changes in creep rate are apparent at a number of sites on the surface trace of the fault within 30 km of the strain meter we propose that the observed strain accumulation results from increased slip around a nearby locked section of the fault this slip arising from loading by the failed loma prieta source region to the north this model is consistent with suggestions of an increased probability of a moderate earthquake near san juan bautista9 10 and with evidence12 that interactions between fault regions are important in earthquake processes

design parameters for borehole strain instrumentation the response of a borehole strain meter to hydrostatic and shear deformations in an isotropic medium is calculated to facilitate optimum instrument design and produce instrument response factors for parameters typically encountered in installed instruments results for an empty borehole are first compared with results for an instrument in intimate contact with the surrounding rock the effects of the grout used to install the instrument are then examined where possible analytic forms for the response factors are given results for typical installations are then presented in graphical form for optimizing instrument design in an environment of known elastic parameters alternatively the results may be applied in the measurement of unknown strain signals to correct for instrument response or to provide in situ estimates of the elastic properties of the environment by examination of observed strain response to known strain signals

the elusive nature of the rheic ocean suture in sw iberia the rheic ocean suture resulted from pre carboniferous oceanic subduction followed by late devonian carboniferous variscan collision in sw iberia this suture has been classically located along the boundary between the ossa morena and south portuguese zones based on the presence of three units i a conspicuous metamafic unit beja acebuches that crops out along this boundary and has been interpreted as a pre carboniferous rheic ocean ophiolite ii a low grade metasedimentary unit with minor mid ocean ridge basalt like lithologies pulo do lobo unit thought to represent a rheic ocean subduction related accretionary prism and iii the allochthonous cubito moura unit that contains high pressure and ophiolitic like rocks we report new structural and geochronological data that allow us to reinterpret the origin and internal structure of the beja acebuches and the pulo do lobo units thus both the beja acebuches protoliths and the pulo do lobo metabasalts would have been formed in the context of an intracollisional extensional stage that interrupted the variscan collision at early carboniferous time after the rheic ocean consumption and the first continental collision later on collision was resumed in an oblique left lateral regime that gave way to coeval frontal folds and thrusts and lateral shear zones and strike slip faults structures with variable pressure temperature conditions and space distribution along time as a consequence of the superposition of transtension and complex transpression the rheic suture in sw iberia has an obscure nearly cryptic appearance

emplacement related microstructures in the margin of a deformed pluton the san jose tonalite baja california mexico abstract in the san jose tonalite pluton baja california mexico interior magmatic fabrics give way progressively to subparallel solid state fabrics with s c structures near the margins the outer rim marginal northern unit approximately 1 km thick is inferred to have been deformed during the emplacement of the inner parts of the magma chamber the rocks show a transition from strongly deformed at the outer edge to progressively less deformed towards the inner edge of the marginal northern unit a small amount of melt appears to have been present during the deformation as indicated by 1 apparent contact melting between some grains of plagioclase and 2 coarse grained aggregates and single grains of quartz k feldspar and or more sodic plagioclase in fracture controlled openings in primary plagioclase crystals the more sodic plagioclase typically grew as rims on the walls of the openings in twin continuity with the primary plagioclase the deformed rocks have a prominent foliation marked mainly by aggregates of biotite with contributions by hornblende and titanite grains as well as apparently recrystallized aggregates of quartz and or feldspar at least some of which may have originated as zones of fragmentation many primary grains of plagioclase hornblende biotite and titanite show evidence of brittle deformation and fragmentation the shapes of aggregates in the folia relationships between incipient folia and residual primary grains and occurrences of many folia in narrow zones transecting strong minerals plagioclase and hornblende suggest that brittle processes may have controlled at least some of the foliation development however evidence of strain induced grain boundary migration and bulge nucleation of new grains of quartz and plagioclase indicates that dislocation creep also played a part in the deformation recrystallization of plagioclase together with intrusion of the pluton at greenschist facies ambient temperatures confirms high temperature deformation with heat derived from the cooling magma

borehole tensor strain measurements in california two continuous borehole plane strain monitoring sites have been operational in california since late 1983 using borehole tensor strain monitors implanted at a depth of 150 m shear strain data at subtidal sensitivies were available immediately after installation without contamination by bond curing or thermal recovery signals at pinon flat observatory data indicate a constant shear strain accumulation of 0 6 microstrain per annum with the axis of maximum compression oriented 50 5 west of north this result differs significantly from regional geodetic estimates the amplitude being dominated by continued viscoelastic response of the hole variations of measured strain rate exceeding 100 nstrain yr would be significant at this instrument site at san juan bautista measurements of the angle of maximum compression 10 3 west of north agree closely with previous geodetic and hydrofracture estimates after 2 years of operation the residual shear strain rate is 1 strain yr preliminary analysis of strain steps observed at san juan bautista during the morgan hill earthquake of april 24 1984 show good agreement with calculations from seismically determined source parameters for this event

near field high resolution strain measurements prior to the october 18 1989 loma prieta ms 7 1 earthquake high resolution strain recordings were made in deep boreholes throughout california prior to during and following the october 18 1989 ms 7 1 loma prieta earthquake the nearest dilational strainmeters sensitivity 1010 and 3 component tensor strainmeters sensitivity 10 9 were 37 km to 42 km respectively from the main shock high quality data including details of strain offsets were recorded on both instruments through the earthquake these data have been searched for indications of short intermediate and long term strain redistribution and or fault slip that might have indicated imminent rupture short and intermediate term changes in both tensor strain and dilational strain several nanostrain if any during the minutes to months before the earthquake are at least 1000 times smaller than that generated by the earthquake itself if short term preseismic slip did occur at the nucleation point of the earthquake during the previous week and if the type of slip is similar to that during the earthquake its moment could not be more than 1024 dyne cm stated another way slip equivalent to that expected for a m 5 3 earthquake could have occurred in the hypocentral region without the strainmeters detecting it at these distances and azimuthal positions long term changes appear to have occurred in mid 1988 and mid 1989 these changes were both followed by ml 5 earthquakes in the hypocentral region on june 27 1988 and august 8 1989 respectively and since they correspond approximately to changes in geodetic strain rate over the epicenter may indicate precursory strain redistribution in the epicentral region minor postseismic strain recovery 14 occurred in the month following the main shock

petrogenesis of early paleozoic peralkaline rhyolites from the macedo de cavaleiros region ne portugal the petrology and geochemistry of variscan peralkaline gneisses from ne portugal discovered in 1962 is presented major and trace element data for the oversaturated sequence of the macedo de cavaleiros area depict some specific geochemical patterns high fe2o3 t even higher than in the ethiopian main rift high ree concentrations and small eu anomalies field and petrographical observations also suggest that liquid immiscibility has played an important petrogenetic role in the crystallization trends the alternating pyroxenefeldsparhematite rich bands observed in outcrop are interpreted as resulting from fluctuating fo2 conditions petrological and geochemical data show that these pantelleritic and comenditic rocks are the end members of an alkaline transitional basaltic sequence by which they are overlain an uppermost sequence of oceanic tholeiitic basalts completes the picture of an evolving rift environment where mantle processes generate crustal thinning and eventually oceanic crust

near surface in situ stress 1 strain relaxation measurements along the san andreas fault in southern california strain relaxation stress measurements were made in the mojave desert southeast of palmdale california at two sites during the summers of 1979 and 1980 using the u s bureau of mines technique to depths of about 30 m the field data and finite element modeling studies demonstrate that thermally induced stress dominates the results obtained in the upper 6 m at depths greater than 6 m the average orientation for the horizontal maximum compressive stress at these sites is n22 w 9 at 2 km south of the san andreas fault and n13 w 2 at 20 km north of the fault these azimuths compare favorably with the average of n21 w determined with nearby hydrofracture stress measurements zoback et al 1980 savage et al 1981 also found a nnw orientation for the maximum shortening from a geodetic network with a 15 km aperature in the palmdale area the fact that essentially the same orientation is recovered by three different techniques which sample to different depths and over different areal extents argues strongly for a contemporary tectonic origin for the stress finite element models of the san andreas fault in southern california develop a stress field similar to that observed regionally 15 e for the maximum compressive stress away from the fault when displacements corresponding to relative motion between lithospheric plates are applied on the boundaries of the models near the fault however the model principal stresses are rotated counterclockwise similar to those measured near palmdale demonstrating the influence of the faults on the principal stress orientations

quantitative comparison of microfabric and magnetic fabric in black shales from the appalachian plateau western pennsylvania u s a abstract anisotropy of magnetic susceptibility ams has been shown to be a good proxy for crystallographic preferred orientation texture however it is not clear in detail how different factors e g modal composition and preferred orientation define the total ams in a rock black shale samples from a drill core on the appalachian plateau in western pennsylvania were analyzed with respect to their microfabric as determined by texture and microstructure and ams low and high field ams which was measured at room temperature reveals that the ams of the samples is dominated by the para and diamagnetic phases synchrotron diffraction was applied to determine the texture of all relevant mineral phases incorporated in the samples muscovite and chlorite pole figures show single maxima perpendicular to the foliation reflecting dominant flattening strain from these textures and the modal compositions ams models were calculated using the intrinsic magnetic anisotropy of the single crystals the modeled and measured ams are comparable in terms of their principal directions and shapes of the anisotropy ellipsoid for the dominantly paramagnetic samples with the maximum susceptibility axes oriented subhorizontal to the ne sw both textures and ams indicate that the samples have undergone largely bedding compaction with a weak tectonic overprint linked to the alleghenian orogeny our results suggest that the microfabric and the magnetic fabrics on the plateau 200 km away from the allegheny front detect the tectonic shortening the ams modeling demonstrates how the contribution of the intrinsic ams of each mineral controls the ams of the whole rock

strain rate profile across the elsinore san jacinto and san andreas faults near palm springs california 1973 81 a profile of the strain accumulation rate along a line trending n50 e across the subparallel elsinore san jacinto and san andreas faults near palm springs california has been constructed from trilateration surveys in the 1973 81 interval the strain accumulation is principally right lateral shear across a vertical plane parallel to fault strike n40 w the strain rate profile for that component exhibits two clearly resolved maxima one centered on the san jacinto fault max 0 35 0 02 rad a and the other on the san andreas fault max 0 40 0 02 rad a no maximum is associated with the elsinore fault this result clearly implies that slip at depth on both the san andreas and san jacinto faults is loading the shallower sections of these faults and that eventually rupture can be expected on both

grain scale processes in actively deforming magma mushes new insights from electron backscatter diffraction ebsd analysis of biotite schlieren in the jizera granite bohemian massif abstract in the porphyritic jizera granite bohemian massif three distinct types of lattice preferred orientations of biotite grains were revealed in schlieren delineated magmatic structures using the electron backscatter diffraction ebsd method 1 biotite basal planes 001 reorient from schlieren subparallel near the schlieren base to schlieren perpendicular in the upper part of the schlieren both orientations share subhorizontal n s to nne ssw trending a axes 2 in some domains the a axes are steep and at a high angle to the schlieren plane while the c axes plunge shallowly and rotate around an ill defined a axis 3 in other domains the ebsd coincides with background magnetic fabric of the host granite revealed using the anisotropy of magnetic susceptibility ams method that is the a axes plunge shallowly to the se or nw while the c axes are subhorizontal and cluster around the ne sw trend these multiple biotite orientations in the schlieren are interpreted to reflect 1 velocity gradient in laminar magma flow along channel like conduits localized within the high strength host phenocryst framework 2 grain scale gravity driven constrictional deformation of the magma mush and 3 overprinting background tectonic deformation transmitted across large parts of the magma chamber prior to its final crystallization the grain scale mechanisms of biotite fabric acquisition in the schlieren presumably involved rotation of biotite crystals during flow with the biotite alignment reflecting the flow geometry and kinematics replaced after flow cessation by melt aided grain boundary sliding of those biotite crystals still enclosed in melt pockets within otherwise static highly crystallized magma mush the latter process was sufficient to reorient biotite grains but not to cause destruction of the schlieren using the jizera granite as a case example we argue that the lattice preferred orientation of mineral grains in mafic schlieren is highly sensitive to reorient in response to processes both associated with the schlieren formation e g localized magma flow and those that occur later and are superimposed onto the effectively solid high strength magma mush

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

numerical modeling of the motion of rigid ellipsoidal objects in slow viscous flows a new approach abstract a simple algorithm for modeling the rotation of rigid ellipsoidal objects in viscous flows based on jeffery s 1922 proceedings of the royal society of london a102 161 179 theory is presented and is implemented in a fully graphic mathematics application mathcad http www mathsoft com the orientation of ellipsoidal objects is specified in terms of polar coordinate angles that can be easily converted to the trend and plunge angles of the three principal axes rather than the euler angles with the mathcad worksheets presented in the supplementary data associated with this paper modeling the rotation paths of individual rigid objects the development of inclusion trail geometry within syn kinematic porphyroblasts and the development of preferred orientation and shape fabrics for a population of rigid objects becomes as easy a task as using a spreadsheet the shape and preferred orientation fabrics for a population of rigid objects can be presented in both a three dimensional form and a two dimensional form allowing easy comparison between field data and model predictions the modeler can customize the type and format of the output to best fit the purpose of the investigation and to facilitate the comparison of model predictions with geological observations application examples are presented for various types of modeling involving rigid objects

earthquakes and sea level space and terrestrial metrology on a changing planet the complacency with which man inhabits continent ocean margins or plate boundaries is based on the assumption that sea level rise and periodic seismicity are acceptable risks the forecast of future sea level or future seismicity is essential for informed assessment of these risks the historical data from which forecasts may be derived are in each case of limited antiquity and uneven spatial distribution developments in space geodesy and crustal deformation metrology in the past two decades offer the means to acquire a homogeneous global data set for monitoring relative vertical and horizontal motions of the earth s surface to within several millimeters these new tools are reviewed in terms of their application to forecasting trends in sea level rise and forecasting damaging earthquakes

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

timing and nature of magmatic fabrics from structural relations around stoped blocks abstract structural relations around several large 100 m stoped blocks in the granodiorite of castle creek sierra nevada california suggest that magmatic fabrics formed as or after the space making phase of pluton emplacement had ended the stoped granite blocks are approximately 360 m below the exposed pluton roof magmatic foliations in granodiorite surrounding the blocks commonly follow km scale orientation trends to within 1 m of block contacts where they become deflected parallel to the block boundaries locally undeflected foliations occur within a few centimeters of block contacts mafic enclave fabric ellipsoid intensities 0 2 we draw the following conclusions concerning magmatic fabrics in the granodiorite of castle creek 1 magmatic fabrics formed during and or after the latest stages of pluton emplacement and thus contain little or no information about space making processes 2 magmatic fabrics had poor strain memory and 3 during magmatic fabric formation pluton strains were partially decoupled from host rock strains published data from other plutons indicate that these relationships may be common particularly in the brittle upper crust it follows from these conclusions that even in the best case magmatic fabrics contain only qualitative information about pluton emplacement mechanisms regional tectonics and internal magma chamber processes interpretations of magmatic fabrics must be corroborated by evaluating causes kinematics and magnitudes of syn emplacement host rock deformation in the worst case the causes of magmatic fabrics cannot be uniquely determined

granite fabrics and regional scale strain partitioning in the serido belt borborema province ne brazil 1 fabrics of the brasiliano age granitoid plutons of the serido belt northeastern brazil were organized following the partitioned regional strain field which shaped larger areas of the basement and its metapelitic cover nine plutons located at the central and western portions of the belt have been investigated by means of anisotropy of magnetic susceptibility these plutons emplaced between 580 and 575 ma are mainly potassic calk alcalic magnetite bearing monzogranites the presence of primary coarse grained magnetite as the main magnetic mineral is indicated by optical and electronic microscopy magnetic susceptibilities hysteresis ratios and thermomagnetic curves the plutons show a strong internal coherence of magnetic fabric with foliation poles distributed around a zone axis parallel to the gently plunging mean lineation the magnetic fabric was used to trace the strain partitioning along the belt in the central transpressive shear belt the magnetic lineations are parallel to north trending regional stretching on the other hand in the western basement block a voluminous granitic magmatism was emplaced as a consequence of a transtensional extensional deformation lineations in these granites trend in a ne direction which is parallel to the transport inferred from kinematic indicators along the shear zones a crustal scale east trending dextral shear zone connects both the transtensional and transpressional domains

a record of crustal scale stress igneous foliation and lineation in the mount waldo pluton waldo county maine abstract the late devonian mount waldo pluton is a porphyritic granite that has a magmatic foliation and lineation foliation is defined by 1 preferred orientation of potassium feldspar phenocrysts and biotite crystals and 2 the anisotropy of magnetic susceptibility ams ams alone defines the lineation the carrier of the ams signal is multidomain magnetite confirmed by petrographic observation high bulk susceptibility generally 3 e3 si units of samples and results of isothermal remanent magnetization experiments ams data from 26 sites in the pluton consistently show a subhorizontal north south trending lineation defined by the orientation of the maximum axis of the ams ellipsoid and a subvertical north south striking foliation whose pole is the minimum axis of the ams ellipsoid at individual sites the feldspar and ams foliations are parallel the lack of evidence for significant solid state deformation in the pluton suggests that both the lineation and foliation are entirely magmatic in origin the pluton s relatively small size 15 km diameter and shallow emplacement depth 3 kbar suggest that it was in its magmatic state for a short time probably less than one million years the orientation of the lineation and foliation is consistent with their formation due to a small increment of crustal scale strain absorbed by the pluton in its geologically brief magmatic state if so these fabrics record a late devonian crustal scale strike slip stress regime that may also have driven dextral movement on the neighboring norumbega fault zone

age and magnetic fabric of the tres corregos granite batholith evidence for ediacaran transtension in the ribeira belt se brazil anisotropy of magnetic susceptibility ams and u pb shrimp zircon ages in the eastern part of the tres corregos batholith ribeira belt se brazil indicate a well defined fabric pattern acquired between 600 and 595 ma the batholith consists mostly of porphyritic granites distributed in the ribeirao branco barra do chapeu and itaoca plutons late fluorite bearing alkaline granites some containing sn polymetallic greisen type deposits intruded the ribeirao branco pluton and the low grade metasedimentary host rocks the magnetic fabric of the ribeirao branco granite is dominantly oblate and oblique to the pluton elongation while that in barra do chapeu is mostly concentric on both plutons ams records the preferred orientation of coarse homogeneous ti poor magnetite grains such fabric patterns indicate a partitioned strain field dominated by strike slip left lateral shear deformation in the ribeirao branco and extension in the barra do chapeu pluton the zircon ages of these plutons are 600 6 ma and 595 4 ma likewise the ages of alkaline plutons were in the range of 597 and 595 ma registering a fast transition between the typical syntectonic batholithic magmatism to the late highly evolved and specialized magmas these results indicate that the ribeira belt was deformed by transtension in the middle ediacaran the geological setting is consistent with a continental arc with the tres corregos batholith emplacing at the middle upper crust

possible cryptic tectono magnetic fabrics in post tectonic granitoid plutons of the canadian shield two late archean granitic plutons forcefully intruded and thermally metamorphosed archean schists feldspar magacrysts aligned concentrically with the margins during inflation substantial tectonic deformation never affected the plutons nevertheless both plutons show the same consistently oriented cryptic magnetic fabric revealed by anisotropy of low field susceptibility ams from 134 samples they have an ne sw vertical magnetic foliation and a magnetic lineation trending 060 15 eigenvalues of the orientation distribution of principal susceptibilities show that the magnetic fabrics have almost identical strengths orientations and symmetry in both plutons we propose a post magmatic reactivation of the earlier regional tectonic shortening this imparted a cryptic tectonic magnetic fabric on the plutons that overprinted the magmatic ams fabric in most outcrops the plutons ams fabrics are subparallel to the much older schistosity of the country rocks because multidomain magnetite provides 99 of the low field susceptibility of the rocks it controls the ams however we see no alignment of the magnetite grains thus magnetic anisotropy may be due to stress alignment of intragranular domain walls and not controlled by grain shape as usually assumed for magnetite

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow