magnetic susceptibility anisotropy strain and progressive deformation in permian sediments from the maritime alps france abstract the relationships among magnetic susceptibility anisotropy finite strain and progressive deformation have been studied in permian red shales and slates of the maritime alps southeastern france these rocks contain deformed reduction spots which serve as finite strain indicators the magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements the magnetic fabric changes progressively from oblate to prolate and back to oblate as deformation increases the quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined they differ between the less and more deformed areas perhaps due to a change in deformation mechanism accompanying an increase in metamorphism the relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control

strain and displacement across the pinaleno mountains shear zone arizona u s a abstract the 27 km long 0 5 km thick mylonite zone of the pinaleno mountains metamorphic core complex is interpreted as a normal displacement simple shear zone on the basis of major minor and microscopic scale structures shear strains calculated as a function of the angle between the foliation and the lower boundary of the shear zone range from 2 to 19 and have a mean value of 3 5 corresponding to a mean angular shear of 74 these values are the same as those calculated independently from rotations of sheared lithologic contacts integration of the calculated shear strains yields a minimum translation estimate of 2 9 km

high angle normal faulting at the tangra yumco graben southern tibet since 15 ma abstractseveral active graben systems in tibet and the himalaya are the expression of ongoing east west extension but the significance and history of normal faulting in this large region are still

quantification of the late quaternary throw rates along the yadong rift southern tibet abstract southern tibet is characterized by seven main n s trending rifts bounded by normal faults which accommodate e w extension quantifying the throw and extension rates along these rifts is essential to understand their activity onset timing regional seismic hazard and kinematic role in the tectonic evolution of the tibetan plateau the yadong gulu rift ygr near lhasa is the longest and most prominent rift in southern tibet and one of the few that crosses the himalayas however it remains poorly studied and understood here we focus on the yadong rift i e the southern section of the ygr south of the yarlung zangbo suture we determine the late quaternary holocene throw rates at four sites where the active yadong normal fault vertically offsets glacial and fluvial geomorphic surfaces using terrestrial lidar uav and kinematic gps we precisely measured vertical offsets up to 17 m and constrained the age of abandoned surfaces using 32 10be cosmogenic surface exposure ages 10 20 ka one optically stimulated luminescence age and two travertine samples dated using u series dating the offset age reconstructions yield uniform throw rates of 0 9 0 3 mm yr along the entire yadong rift taking a fault dip of 50 as measured in the field we obtain late quaternary e w extension rates of 0 8 0 3 mm yr which is similar to but at the lower end of those along other rifts in southern tibet 1 2 mm yr

magnetic vs structural fabrics whipple detachment structure whipple wash california abstract orientations of ellipsoids of anisotropy of magnetic susceptibility ams are compared with those of mesoscopic structural features for the breccia and upper mylonite zones for the whipple detachment zone in whipple wash whipple mountains eastern california the anisotropy of the breccia which ranges up to 5 is much less than that of the mylonitic gneiss which can exceed 100 slight differences in trend are evident between the ams ellipsoid elongations and those of mineral lineations maximum axes 1 of the ams ellipsoids for both breccia and mylonite trend generally ssw nne with shallow plunge whereas mineral stretching lineations in the mylonite gneiss have trends near ne sw with shallow plunge measured fold axes are close to the 1 axes ssw nne 1 axes in the breccia show more variation e w and ssw nne at different structural levels mesoscopic mineral lineations in the breccia were not discernible for the planar elements ams ellipsoid flattening is generally more closely parallel to foliation surfaces in the mylonite than in the breccia this is evident from overlapping minimum susceptibility axes 3 and foliation pole clusters 3 axes are more scattered than are poles of foliation the scatter in 3 is possibly the superimposed susceptibility effect of several structural surfaces of different ages joints small faults transposed metamorphic foliations and mylonitic foliation especially interesting are the discovery of good 1 definitions in the breccia the apparent lack of correlation of 1 axes with the very prominent stretching lineations in the mylonite and the similarity of the 1 axial trends with those of the fold fracture pole patterns

evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the main central thrust zone garhwal himalaya india inferences from finite strain and ams analyses abstract the footwall of the main central thrust mct zone along the bhagirathi valley comprises a wide zone of mylonitic quartzite and deep level tectonites the systematic variation of finite strain parameters es k and v in the mylonites indicates heterogeneous deformation which is determined to vary between simple shear and non coaxial flattening type in such a strain regime the outer boundary of the quartz clasts are no longer preserved thus leading to an error in finite strain measurement in order to supplement the finite strain studies anisotropy of magnetic susceptibility ams analyses were carried out on the mylonitic quartzites a systematic variation in degree of anisotropy p with distance from the mct is documented and is interpreted to be tectonic in origin based on these results it is concluded that p can be used as a strain intensity gauge at least on an outcrop scale where a systematic variation in p values from one part of the outcrop to the other can be established however the quantitative relation between principal axes of finite strain ellipsoid and ams axes magnitude of principal susceptibility difference k1 and k3 and finite strain magnitude e1 ln 1 e1 and e3 ln 1 e3 were related by a logarithmic relationship with a correlation coefficient of 0 844

study of ductile shear zones galicia spain using texture goniometry and magnetic fabric methods abstract a joint magnetic and mica fabric study has been carried out on rocks from recognised shear zones in galicia spain the two main aims of the study are to determine strain gradients in a foliated material without any significant strain markers and to obtain a quantitative relationship between the magnetic susceptibility anisotropy ellipsoid and the mica fabric axes the intensity parameters of the fabric on profiles intersecting the studied areas indicate the dominant influence of two major shear zones on the fabric development the regional fabric shows less significant variations across minor less developed shear zones the empirical relationship between magnetic fabric and strain reported by rathore 1979 1980 and rathore and henry 1982 is found to hold in the magnetic and mica fabrics comparison the relationship found in this study is i j l i l j 0 240

magnetic susceptibility anisotropy of mylonites from the brevard zone north carolina u s a abstract this study presents the results of magnetic susceptibility anistropy measurements on mylonites from the brevard zone of the southern appalachians the brevard zone is a major crustal mylonite zone and the samples analyzed for this study represent the progressive increase of mylonitization decrease in grain size of the henderson gneiss with increasing proximity to the brevard zone our results suggest that magnetic fabric measurements in mylonites are at least qualitatively significant with respect to the strain history of mylonites the orientations of the principal susceptibilities are parallel to rock fabric elements indicating that the magnetic fabric is a deformationally induced phenomenon although the henderson gneiss appears to undergo a smooth progressive mylonitization with increasing proximity to the brevard fault zone our magnetic measurements suggest that strain states do not increase smoothly as the brevard zone is approached however the magnetic measurements do suggest that at some sites the strain is very high the highest site mean anisotropy max min mean 100 we have measured is 120 cgs which could based on strain to magnetic susceptibility anisotropy msa correlations by other workers indicate strain ratios 1 e 1 1 e 3 of between 186 and 1 4 1017 because we cannot correlate our magnetic fabric measurements with strain in these rocks we cannot prove that such high strain states in fact exist in the brevard mylonites however these appear to the highest magnetic anisotropies yet recorded our data also indicate that the susceptibility ellipsoid is severely flattened with k values commonly lower than than 0 1 we also observe an inverse relationship between k values and anistropy similar relationships have been observed in other shear zones and suggest that mylonites follow a strain path of increasing departure from plane strain k 1

degree of magnetic anisotropy as a strain intensity gauge in a saturated finite strain zone abstract anisotropy of magnetic susceptibility ams analyses were carried out on mylonitic quartzites from the footwall of the main central thrust zone garhwal higher himalaya india a systematic increase in degree of magnetic anisotropy p with distance from the main central thrust is documented even in a saturated finite strain zone on the basis of correlation between orientation of principal ams axes and multidomain magnetite and the mylonitic foliation and lineation direction strain related recrystallization was inferred to be the cause of development of the magnetic fabric based on these results it is concluded that if p is not controlled by k m then the former can be used as a strain intensity gauge supplementary material three dimensional finite strain parameters and rock magnetic and ams datasets are available at http www geolsoc org uk sup18328

remanence stability and magnetic fabric development in synthetic shear zones deformed at 500 c shear experiments were performed on magnetite bearing calcite aggregates to examine magnetic fabric development and remanence stability in a deforming system using elevated temperature and pressure to encourage deformation by crystal plastic processes samples composed of 1 wt pseudo single domain magnetite 1 2 m in a calcite matrix were created with either strong or weak initial fabrics and deformed in coaxial simple shear to strains up to 1 5 at constant strain rates between 6 10 5 and 1 10 4 s 1 at 500 c and confining pressure of 300 mpa samples were given weak field thermal remanent magnetizations prior to deformation demagnetization of postdeformation remanence reveals that a primary remanent magnetization can withstand deformation at pressures and temperatures approximately equivalent to greenschist facies metamorphic conditions on laboratory time scales but this stability is found to depend on the character of the predeformation fabric the origin of secondary remanence components acquired during deformation is uncertain but is likely to partially result from thermal viscous remagnetization complete postdeformation remagnetization in initially anisotropic samples appears to involve a stress softening or piezoremanent magnetization mechanism postdeformation anisotropy measurements show progressive changes in magnetic fabric strength with strain in the absence of a strong initial magnetic anisotropy magnetic fabric intensity increases linearly as a function of strain however deformation that overprinted an existing fabric results in an apparent decrease of the initial anisotropy at low strains followed by rapid increases in magnetic fabric strength with increasing strain our results underscore the important role that initial fabric can play in determining the character of deformation fabrics

the transition region mode water of the north pacific and its rapid modification abstractusing argo float data this study examined the formation region spatial distribution and modification of transition region mode water trmw which is a recently identified pycnostad in the subtropical subarctic transition region of the north pacific the basin scale boundary region between subtropical and subarctic water masses analyses of the formation fields of water masses within and around the transition region reveal that trmw forms in a wide area from the western to central transition region and is separated from the denser variety of central mode water d cmw to the south by a temperature and salinity front trmw has temperatures of 4 9 c and salinities of 33 3 34 0 making it colder and fresher than d cmw trmw has a density range of 26 3 26 6 and thick trmw is widely distributed in the transition region however the range of the t s properties at trmw cores is substantially reduced downstream within 10 20 longitude from the formation region by gradually losing its fresh and c

revisiting growth and decline of late quaternary mega lakes across the south central tibetan plateau abstract lake basins record critical interactions between the hydrosphere cryosphere and atmosphere their sediments and shoreline landforms archive timing duration and intensity of past climatic variability and environmental impacts over a variety of timescales thousands of lake systems spread across the internally drained tibetan plateau today of which many are only the salty remains of much more expansive paleolakes in the past this study presents new shoreline ages integrated with quantitative digital topographic analysis and regional geomorphic evidence to reconstruct the sizes and extents of late quaternary rift lake systems for the south central tibetan plateau this study presents optically stimulated luminescence osl and the first k feldspar post infrared infrared pirir stimulated luminescence ages for zabuye caka and dawa tso and tests if these lakes were once part of a singular pleistocene mega lake established across four n s rift systems our new results show that two large but separate paleolake systems developed in the latest pleistocene earliest holocene and no lake larger than 6460 km2 has existed since at least 41 ka in the lunggar region early holocene paleolakes expanded up to 7 modern 220 335 km3 with two to four fold asymmetric lake expansion in the west compared to the east findings corroborate earlier investigations implying that dramatic runoff increase and or evapotranspiration reduction prompted rapid earliest holocene lake expansion precipitation variability in combination with permafrost degradation temperature and wind strength governed paleolake moisture balance open system lake behavior and evolving drainage configurations across complex topography explain delayed onsets of lake regression and punctuated lake decline this suggests abrupt holocene climate shifts are not necessarily required for punctuated lake decline lastly unique geomorphological and paleoclimatic similarities between tibetan and east african rift lakes highlight the potential for some tibetan paleolakes to function as natural climate amplifiers over both short and geologic timescales

effect of progressive deformation on remanent magnetization of permian redbeds from the alpes maritimes france abstract the relationships among the anisotropy of magnetic susceptibility the finite strain and the characteristic remanent magnetization chrm have been investigated in deformed red sediments from the alpes maritimes in southeast france the magnetic properties of these rocks are due to hematite the finite strain is known from field studies of the shapes and orientations of green reduction spots and from optical microscopy and x ray texture goniometry studies of chlorite and illite the strain data permit a classification of the magnetic results according to four main stages of progressive deformation in the compaction stage magnetic anisotropy is flattened in the bedding due to compactional loading the chrm directions approximate to a fisherian distribution about a mean value close to the known permian direction for the region as deformation proceeds pencil structure first develops and the anisotropy is characterized by a prolate pattern with maximum axis parallel to the subhorizontal azimuth of the pencil structure and fold axes with increasing deformation the magnetic susceptibility ellipsoid becomes oblate and finally triaxial at these stages of high deformation the anisotropy patterns show that the hematite grain orientations are strongly influenced by the cleavage the remanence directions in deformed rocks do not correspond to a fisher distribution and eventually deviate strongly from permian directions these results show that paleomagnetic vectors are affected by the distortional strains which accomodate shape change in the rock simple tilt correction without compensation for the strain would yield incorrect paleopole positions an attempt has been made to compensate for the effects of deformation by regarding the remanent vector as a passive marker march 1932 equations for strain removal were applied to the remanence in two ways 1 a using strain data directly obtained in the field for each site and 2 b by converting the magnetic susceptibility anisotropy of each sample to a corresponding finite strain value correcting for the finite strain by both methods improves the within site grouping of paleomagnetic vectors in strongly deformed rocks strain removal assuming the march model results in overcorrection of the remanent magnetization data in low strain regions distortion may produce mechanical rotation of the hematite particles but recrystallization may alter the deformation mechanism in regions of high strain the limits of applicability of the passive marker treatment could not be tested adequately in the deformed red sediments of the alpes maritimes

detachment faulting in a bivergent core complex constrained by fault gouge dating and low temperature thermochronology abstract the central menderes massif in western turkey is a bivergent metamorphic core complex characterised by two detachment faults that operated with opposite sense of shear here we present k ar ages and hydrogen isotope data for fault gouge as well as fission track and u th he ages for apatite and zircon from bedrock samples to constrain the timing of detachment faulting comparison of the k ar ages and low temperature thermochronologic ages indicates brittle faulting at temperatures between 240 and 70 c at the western termination of the gediz detachment k ar ages between 12 and 5 ma suggest rather continuous faulting since the mid miocene a pliocene phase of rapid slip is mainly recorded in the eastern part of this detachment in contrast k ar ages from the buyuk menderes detachment cluster at 22 16 and 9 3 ma and provide evidence for two phases of faulting which is in accordance with new and published low temperature thermochronologic data hydrogen isotope ratios of 76 to 115 indicate infiltration of meteoric water during detachment faulting and gouge formation the youngest k ar ages and cooling histories for hanging and footwall rocks imply that the activity of both detachments ended 3 ma ago when the currently active grabens were established

displacement calculations across a metamorphic core complex mylonite zone pinaleno mountains southeastern arizona minimum offset of 7 km across the pinaleno mountains metamorphic core complex is calculated by integrating the shear strains across the exposed width of the mylonite zone the calculated displacement equals the offset on the associated detachment fault estimated from offset marker beds the method of determining displacement by strain integration may be directly applicable to many other metamorphic core complexes

effects of tectonic deformation on the remanent magnetization of rocks paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis however the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known anisotropy of magnetic susceptibility ams provides a quantitative basis for strain determination the principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid a linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types if the correlation method used is valid ams observations can be used to establish the magnitudes and directions of the principal strains strain modifies bedding planes and realigns the grains that carry magnetic remanence compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain the effects of strain on bedding can be corrected using passive marker theory attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive during deformation linear and planar elements deflect away from the axis of maximum shortening the passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions the passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix

crustal structure from three dimensional gravity modeling of a metamorphic core complex a model for uplift santa catalina rincon mountains arizona gravity modeling shows that the observed fluctuations in gravity over the catalina rincon metamorphic core complex of southeastern arizona are caused primarily by shallow crustal density contrasts the wilderness suite granites the probable source of a 20 mgal low over the central part of the core complex may extend to depths of 7 12 km below the surface seismic refraction information is consistent with the existence of a deep crustal root under the catalina rincon core complex we hypothesize that the voluminous wilderness suite granites are related to a crustal thickening event that built the root in early tertiary time modeling of flexural isostatic uplift resulting from an excess crustal root predicts two episodes of uplift for the catalina rincon metamorphic core complex 1 low relief flexural uplift in the middle tertiary and 2 complete uplift precipitated by high angle basin and range faulting at 10 15 ma the uplift and exposure of core complexes therefore can be an isostatically driven process that occurs in regions of locally thickened crust

strain and magnetic fabric in the santa catalina and pinaleno mountains metamorphic core complex mylonite zones arizona anisotropy of magnetic susceptibility ams is capable of recording finite strain in weakly magnetized rocks ams was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history ams measurements were compared with finite strains determined from dike rotations and from foliation orientations in one zone the santa catalina mountains the orientations of susceptibility and finite strain ellipsoids are in excellent agreement and there is a logarithmic relationship between susceptibility difference ki ki k k and finite strain magnitude in the second zone the pinaleno mountains minimum susceptibility is perpendicular to the finite flattening plane but the maximum susceptibility does not parallel the maximum extension direction and there is no systematic relationship between susceptibility magnitude and strain magnitude oriented polished thin sections indicate that magnetite in the protolith of the santa catalina mylonite occurs as randomly oriented elongate grains with subsequent deformation the long axes are rotated into the maximum extension direction in the pinaleno mylonites both equant and elongate magnetite grains are present with deformation the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane ams in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric the final orientation of the elongate grains is a function of their initial orientation as well as the finite strain therefore despite similar deformational histories the two zones display different ams patterns due to the differences in occurrence initial orientation and shape of ferromagnetic grains

kinematics and strain distribution in an orogen scale shear zone insights from structural analyses and magnetic fabrics in the gavarnie thrust pyrenees abstract this work aims to characterize the gavarnie thrust one of the large scale thrusts that define the alpine structure of the west central sector of the pyrenees a detailed comparison of structural analysis and magnetic fabrics is carried out for the paleozoic phyllonites of the gavarnie thrust in order to decipher strain distribution and transport direction the ams at room rt ams and low lt ams temperature and the airm can be correlated with the structural patterns kmin axes are mainly parallel to the pole of the s c or c planes and kmax axes are parallel to the transport direction and related to ductile s c structures furthermore the pj t changes across the shear zone characterize strain variations larger pj and t are found in the basal most deformed part of the shear zone and lower values are found where the interaction between alpine and variscan related petrofabrics is stronger we also interpret the reactivation of variscan inherited fabrics within the alpine shear zone in spite of the heterogeneous strain markers indicate a common top to the south n190e alpine transport direction which contrasts with the strong obliquity of the genetically related structures developed in the southern mesozoic sedimentary cover in this sense our data suggest a complete decoupling between basement and cover units during the alpine compression