models of magnetic anisotropy variations in sedimentary thrust sheets abstract magnetic anisotropy variation in sedimentary thrust sheets was modelled through the extension of strain models in thrust sheets coward and kim 1981 sanderson 1982 using the empirical relationship between the magnetic anisotropy and strain and respecting the pre deformational anisotropy of sedimentary rocks in thrust sheets deformed by simple shear and lateral lengthening the magnetic foliations remain near the bedding while in thrust sheets deformed by simple shear and lateral shortening the magnetic foliation may deviate strongly from the bedding even creating a girdle pattern in its poles

paleomagnetism magnetic fabric and 40ar 39ar dating of pliocene and quaternary ignimbrites in the arequipa area southern peru 40ar 39ar ages and paleomagnetic correlations using characteristic remanent magnetizations chrm show that two main ignimbrite sheets were deposited at 4 86 0 07 ma la joya ignimbrite lji and at 1 63 0 07 ma arequipa airport ignimbrite aai in the arequipa area southern peru the aai is a 20 100 m thick ignimbrite that fills in the arequipa depression to the west of the city of arequipa the aai is made up of two cooling units an underlying white unit and an overlying weakly consolidated pink unit radiometric data provide the same age for the two units as both units record exactly the same well defined paleomagnetic direction 16 sites in the white unit of aai dec 173 7 inc 31 2 95 0 7 k 2749 and 10 sites in the pink unit of aai dec 173 6 inc 30 3 95 1 2 k 1634 showing no evidence of secular variation the time gap between emplacement of the two units is unlikely to exceed a few years the 50 m thick well consolidated white underlying unit of the arequipa airport ignimbrite provides a very specific magnetic zonation with low magnetic susceptibilities high coercivities and unblocking temperatures of nrm above 580 c indicating a ti poor titanohematite signature the anisotropy of magnetic susceptibility ams is strongly enhanced in this layer with anisotropy values up to 1 25 the fabric delineated by ams was not recognized neither in the field nor in thin sections because most of the aai consists in a massive and isotrope deposit with no visible textural fabric pumices deformation due to welding is only observed at the base of the thickest sections ams within the aai ignimbrite show a very well defined pattern of apparent imbrications correlated to the paleotopography with planes of foliation and lineation dipping often at more than 20 toward the expected vent buried beneath the nevado chachani volcanic complex in contrast with the relatively small extent of the thick aai the la joya ignimbrite covers large areas from the altipano down the piedmont ti poor titanomagnetites are the dominant magnetic carriers and ams values are generally lower than 1 05 magnetic foliations are sub horizontal and lineations directions are scattered in the lji the ams fabrics are probably controlled by post depositional compaction and welding of the deposit rather than transport dynamics

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

tectonic rotations within the bolivian altiplano implications for the geodynamic evolution of the central andes during the late tertiary paleomagnetic results from 61 sites in tertiary red beds and volcanic rocks from the bolivian andes allow quantification of tectonic rotations within the altiplano domain a total of 16 sites were also obtained in lower miocene ignimbrites that cover the forearc of the north chilean andes near arica in the southern altiplano lipez region a local clockwise rotation of up to 38 is recorded in lower miocene volcanic rocks farther north near the salar de uyuni there is no evidence of significant rotations counterclockwise rotations are observed in the northern altiplano the largest counterclockwise rotation 28 is found in eocene oligocene red beds tiwanaku formation along the eastern border of the corque syncline middle miocene sediments located within the center of the corque basin record a counterclockwise rotation of only 10 about 200 km north of the corque basin eocene oligocene sedimentary rocks near the locality of viacha record only 10 counterclockwise rotation paleomagnetic results in lower miocene ignimbrites from the forearc near the arica bend do not show evidence for late cenozoic rotation of the forearc these new results suggest that the 20 to 25 counterclockwise rotation of the southern peruvian forearc occurred prior to the miocene anisotropy of magnetic susceptibility ams data show a magnetic fabric mostly controlled by bedding but slight ams lineations are found almost parallel to the major regional structural trends such as fold axes the apparent relationship between tectonic rotations ams lineations and structural trends suggests that rotations occured after or during the final stage of folding paleomagnetic data obtained in paleozoic rocks show evidence of clockwise rotation of the southern sub andean ranges tectonic rotations during the neogene are mostly localized on the eastern side of the central andes the curvature of the occidental margin near arica was likely acquired prior to the last stage of andean deformation although the magnitude of the rotations may vary from one locality to the other there is a consistent pattern showing counterclockwise rotations to the north and clockwise to the south a large scale regional mechanism is needed to explain this pattern of tectonic rotations within the central andes we propose a model in which along strike segmentation of the andean foreland and indentation of the altiplano by a curved forearc are the major factors controlling tectonic rotations within the altiplano eastern cordillera and sub andean ranges

anisotropy of magnetic susceptibility of heated rocks abstract heating produces changes which does not always correspond to simple enhancement of the magnetic fabric two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating the first diagonalizes the tensor from the difference between each tensor term before and after heating the second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment when the same magnetic fabric is obtained from several thermal steps it cannot be related to randomly oriented ferrimagnetic minerals instead the newly formed fabric must be related to characteristics of the pre existing rock by comparing this ferrimagnetic minerals fabric with the initial whole rock fabric we can distinguish cases where heating simply enhances pre existing fabric from those where thermal treatment induces a different fabric relative to the pre heating fabric this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction related to petrostructural elements other than those defining the initial fabric

oroclines in the tasmanides abstract oroclines are features of orogenic belts that exhibit curvature in plan on a map scale with the additional requirement that this curvature resulted from relative rotation of the limbs around a vertical or near vertical axis much debate has centred on contrasts between thin skinned and thick skinned systems and on the question of whether the structural fabric was imposed prior to or synchronous with the development of curvature leading to a variety of inconsistent terminology a simple hierarchy in which all map scale curved orogenic features which pass a test for rotation are termed oroclines as distinct from primary arcs and in which these are then subdivided by timing into progressive synchronous fabric and curvature and secondary pre existing fabric is then rotated and by mechanism into thin or thick skinned is applied to a suite of curved orogenic features in the tasmanides of eastern australia this analysis includes the first formal application of the orocline test palaeomagnetic or palaeocurrent directions plotted against structural strike to the tasmanides the neoproterozoic to early mesozoic tasmanides comprise four orogens each of which contains curved orogenic features which are highlighted by extensive aeromagnetic datasets the delamerian orogen developed on the gondwana margin exhibits two curved structures in the neoproterozoic to cambrian adelaide fold belt termed the fleurieu and nackara arcs previous structural interpretations have indicated that the nackara arc is non rotational and hence not oroclinal re examination of existing palaeomagnetic data including a positive palaeomagnetic orocline test shows the nackara arc to in fact be a hinge of the adelaide orocline curvature in the adelaide orocline was dominantly progressive and thin skinned but may have been influenced by secondary rotation of the pre existing partly underlying curnamona craton mega kinks in a buried delamerian arc volcanic chain revealed in aeromagnetic images follow the curvature of the nackara arc suggesting an accompanying thick skinned secondary oroclinal deformation further from the continental margin orogenic curvature in the palaeozoic lachlan orogen has only been widely recognised since the advent of tilt filtered aeromagnetic images and a model of oroclinal folding with two hinges riverina and tambo and strike slip offset during the silurian bindian orogeny has recently been proposed to explain perplexing repetitions of sequence and inversions of structural vergence and directions of sediment provenance palaeomagnetic data indicates rotation of elements within the proposed orocline and a preliminary study indicates a positive palaeomagnetic orocline test for the riverina hinge palaeocurrent data around the tambo hinge provide a positive if scattered orocline test an independently conceived numeric model of the roll back response to congestion of a subduction zone has generated an oroclinal structure which shares many features with the proposed lachlan orocline and deep crustal ambient noise seismic data confirms that the major curved and strike slip features persist throughout the crust the lachlan orocline appears to be secondary and thick skinned orogenic curvature in the largely covered thomson orogen occurs along its southern border with the lachlan orogen the putative olepoloko orocline and around its geometrically complex boundary with the mossman orogen in northern queensland the charters towers orocline rotation of the olepoloko structure cannot be confirmed and competing models for its formation leave open the question of whether this is a primary arc a thin skinned progressive orocline or a thick skinned secondary orocline palaeomagnetic data from the charters towers orocline are limited to two poles but a change in declination between the silurian pole and the devonian pole matches the apparent rotation of the orocline which appears to have been thick skinned and secondary in contrast to the other orogens consensus that at least part of the curvature in the palaeozoic to triassic new england orogen is oroclinal has been broad although not universal there remains substantial debate over the number of hinges from two to four and mechanism palaeomagnetic poles have previously been cited as evidence of rotation of blocks within the orocline but this paper presents the first formal palaeomagnetic orocline test which is positive for the manning and texas hinges a palaeocurrent orocline test of the manning hinge in younger rocks than the palaeomagnetic sample is negative constraining rotation in the southern manning hinge to the carboniferous before 322 ma while rotation in the northern texas hinge appears to be latest carboniferous or permian the existence of a hastings hinge is questionable but if real its rotation also appears to be younger than that of the manning hinge thick skinned secondary rotation of the hinges of the new england orocline appears to have been diachronous

quaternary sediment patterns in the weddell sea relations and environmental conditions sediment patterns such as texture composition and facies from three selected areas of the antarctic continental margin of the weddell sea are discussed in relation to environmental variations of the quaternary hydrosphere and cryosphere advance and retreat of ice shelves as well as oscillations in sea ice coverage are reflected by particular sediment facies the distribution of ice rafted detritus tracks the antarctic coastal current and the weddell sea bottom water contour current can be recognized by its distinctive winnowing and erosion pattern distribution and abundance of biogenic sediment components are mainly controlled by the duration of sea ice coverage reflecting the long term climatic evolution

surface rupture associated with a 5 3 mb earthquake the 5 april 1986 cuzco earthquake and kinematics of the chincheros quoricocha faults of the high andes peru abstract the shallow mb 5 3 1986 earthquake that affected the cuzco region reactivated the 18 km long chincheros qoricocha faults in the central part of the cuzco fault system although the event has a moderate magnitude clear surface ruptures were observed to total 3 km in length with maximum 10 cm normal displacements fault kinematics related to the earthquake analyzed where the qoricocha fault had a maximum surface fault displacement is in agreement with roughly n s extension the normal fault mechanism proposed for this earthquake is compatible with the surface kinematic analysis seismic activity prior to the 1986 earthquake is deduced from analysis of holocene scarps trench exposures give evidence for at least three fault displacements suggesting an average recurrence interval of the order of a few thousand years for surface rupturing seismic events these fault reactivations with vertical displacement ranging from 0 6 to 2 m were related to seismic events of mw magnitude probably ranging from 5 8 to 6 9 structural analyses of holocene faults indicate that the chincheros qoricocha fault activity was related to roughly n s trending extension characterized by a horizontal n s trending 3 axis and a vertical 1 axis the n s extensional tectonics that controlled the holocene and 1986 chincheros qoricocha fault activity are the typical deformational pattern of the peruvian high andes and seem to be in agreement with compensated high topography

seismic investigations in the weddell sea embayment abstract seismic multi channel data collected during norwegian antarctic research expeditions in 1976 1977 and 1978 1979 outline aspects of the cenozoic depositional environment in the weddell sea embayment acoustic basement probably representing the east antarctic craton is exposed in a 50 100 km wide swath along the ice barrier between 78 s 75 5 s on the eastern side of the crary trough the shelf prograded westward and northward from the craton into a subsiding basin colinear with the transantarctic mountain range measured sediment thicknesses exceed 5 km during middle and late tertiary times a submarine fan complex the crary fan developed on the southeastern margin of the weddell sea embayment the glacially eroded crary trough is located at the contact between the craton and a sedimentary basin to the west the entire sedimentary section is undisturbed by faulting or folding which indicates that any movements related to cenozoic uplift of the trans antarctic mountains and or relative motion of east antarctica had little effect in the area north of the filchner ice shelf east of 41 w

tectonic versus mineralogical contribution to the magnetic fabrics of epimetamorphic slaty rocks an example from the ardennes massif france belgium abstract the discrimination of the respective contribution of strain and mineralogy of the bulk anisotropy of low field magnetic susceptibility ams of rocks is addressed by a study of the ardennes variscan thrust belt unmetamorphosed to low medium grade rocks in the study area define a succession of magnetic parageneses hematite maghemite hematite maghemite magnetite magnetite pyrrhotite a direct control of the magnetic mineralogy on the magnitude of ams axes is shown in addition in all cases the interpretation of the directions of ams axes as kinematic indicators also applies to these rocks we show a progressive change in shearing direction from the south of the rocroi massif where the nappe transport is n directed in the north the vergence changes to northwestward

the curved magallanes fold and thrust belt tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study the magallanes fold and thrust belt ftb presents a large scale curvature from n s oriented structures north of 52 s to nearly e w in tierra del fuego island we present a paleomagnetic and anisotropy of magnetic susceptibility ams study from 85 sites sampled in cretaceous to miocene marine sediments magnetic susceptibility is lower than 0 0005 si for 76 sites and mainly controlled by paramagnetic minerals ams results indicate that the sedimentary fabric is preserved in the undeformed areas of tierra del fuego and the more external thrust sheets units where an incipient lineation due to layer parallel shortening is recorded prolate ams ellipsoids indicating a significant tectonic imprint in the ams fabric are observed in the internal units of the belt ams results show a good correlation between the orientation of the magnetic lineation and the fold axes however in peninsula brunswick the ams lineations are at 20 counterclockwise to the strike of the fold axes pretectonic stable characteristic remanent magnetizations chrm were determined in seven sites a counterclockwise rotation 21 2 9 2 is documented by chrm data from four sites near the hinge of the belt in peninsula brunswick and near canal whiteside while there is no evidence of rotation near the nearly e w oriented vicuna thrust within tierra del fuego the curved shape of the cenozoic magallanes ftb is not related to vertical axis rotation and thus the magallanes ftb can be considered as a primary arc

post middle oligocene origin of paleomagnetic rotations in upper permian to lower jurassic rocks from northern and southern peru abstract we report paleomagnetic data from 28 sites of upper permian to lower jurassic strata from northern and southern peru in northern peru 6 s a stable magnetic component from six permo triassic sites passes fold and reversal tests the overall mean pole agrees well with late permian to triassic poles from cratonal south america suggesting this part of peru has experienced neither significant rotation nor latitudinal transport since the permo triassic in southern peru 13 to 16 s thermal demagnetization isolates stable magnetic components in 16 of 22 upper permian to lower jurassic sites collected along the transition between the altiplano and the eastern cordillera these 16 sites are rotated 14 to 147 counterclockwise and pass an inclinations only fold test within the same structural zone three other permo triassic sites as well as 10 paleocene sites also show important counterclockwise rotations roperch and carlier j geophys res 97 1992 17233 17249 butler et al geology 23 1995 799 802 the large magnitude and exclusively counterclockwise sense of rotation suggest that the tectonic regime included an important sinistral shear component no correlation exists between rotation amount and rock age suggesting the rotations are post paleocene in age because the rotations occur along the fringe of the eastern cordillera they were likely produced during its structural formation hence from the late oligocene to present sinistral shear acting in the northern part of the bolivian orocline appears much more pronounced than that north of the abancay deflection which likely arises from differences in convergence obliquity

a convective model for the weddell polynya abstract mechanisms are considered which may induct the large over 105 km2 area of open water or polynya which frequently occurs within the weddell sea winter sea ice we propose that when surface cooling and ice formation decrease the temperature and increase the salinity of the surface water the latter by salt rejection during ice formation in a preconditioned area static instability with intense vertical mixing can occur the upwelled warm salty deep water can then supply enough heat to melt the ice or prohibit its formation even in the middle of winter a simple two level model is derived to test this theory and is found to agree well with observations the process is found to be irregular due to different times of ice onset from one year to the next and to a lesser extent from variations in surface heating and cooling further it is shown that unless the freshwater input exactly balances the increased salinity from the overturn each year the system will either gain or lose salt yearly an

an ams structural and paleomagnetic study of quaternary deformation in eastern sicily abstract an integrated structural anisotropy of magnetic susceptibility ams and paleomagnetic study was carried out on plio pleistocene sedimentary basins in eastern sicily these basins belong to three main tectonic domains the tyrrhenian hinterland domain the catania foredeep domain and the hyblean foreland domain we sampled 329 oriented samples from 25 sites in selected areas from the different tectonic domains the ams is typical for weakly deformed sediments with a magnetic foliation sub parallel to the bedding plane and a well defined magnetic lineation the orientation of the magnetic lineation is strongly controlled by the main tectonic deformation recorded in the basins structural and ams data define a transition from nw se extension in the tyrrhenian hinterland domain to e w compression in the catania foredeep domain to e w extension in the hyblean foreland domain respectively reliable paleomagnetic results have been obtained in 12 out of 25 sampled sites data show that no significant rotations occurred in any of the studied basins at least since the middle pleistocene these results allow us to define an upper limit to the large rotations about vertical axes that have been previously found in the calabria and sicily regions

tectonic rotations and transcurrent deformation south of the abancay deflection in the andes of southern peru we report new paleomagnetic results from 55 out of 76 sites sampled at different localities along a transect from nazca to cuzco where the general structures of the peruvian andes are strongly offset across the abancay deflection nine new 39ar 40ar ages better constrain the timing of volcanism along the western edge of the western cordillera at the latitude of nazca a mean paleomagnetic result from 22 sites in the lower miocene volcanics does not show significant rotation r 2 3 7 7 of the western margin of the central andean plateau since the early miocene within the western cordillera we sampled three structural blocks bounded to the north by the abancay fault system in the westernmost block a large counterclockwise rotation r 65 0 11 1 is found in mesozoic limestones and paleocene eocene red beds magnitude of rotation decreases toward the east from r 35 6 12 8 in the central block to r 4 5 8 4 south of the town of cuzco the anisotropy of magnetic susceptibility ams recorded by the red beds sediments is the consequence of compaction and tectonic strain during the early stages of deformation we show that the magnetic lineations were also rotated counterclockwise as the remanent magnetizations the present study confirms results from the peruvian fore arc showing that rotations are not older than circa 40 ma and likely not younger than circa 20 ma the spatial variation in the amount of counterclockwise rotation suggests a large component of shear along the abancay deflection concomitant with a broad late eocene oligocene oroclinal deformation in southern peru

magnetic anisotropy of plio pleistocene sediments from the adriatic margin of the northern apennines italy implications for the time space evolution of the stress field abstract the anisotropy of magnetic susceptibility ams of the marine fine grained plio pleistocene sediments that crop out at the eastern adriatic front of the central northern apennines italy indicates a prevalent sedimentary compactional magnetic fabric with variable overprint of the tectonic strain the degree of anisotropy and the geometry of the ams ellipsoids suggest a subdivision of the studied sediments in two distinct ages early middle pliocene and late pliocene early pleistocene the early middle pliocene sediments show a weak but well defined magnetic lineation parallel to the main fold and thrust axes throughout the region analogously to the messinian sediments located in a more internal western position of the apenninic chain since the late pliocene the ams data are not regionally coherent and indicate a reduced and locally variable tectonic influence on the magnetic fabrics the ams data have been integrated with the available geophysical information of the present day stress in the region with the aim to reconstruct the time spatial evolution of the stress field in the adriatic margin of the central northern apennines during the past 5 ma the present day stress field was evaluated using borehole breakout in deep wells about 50 wells focal mechanisms of crustal earthquakes 2 5 m d

re computing palaeopoles for the effects of tectonic finite strain abstract the pre messinian limestone cover 58 8 ma to the troodos ophiolite 88 ma of southern cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and in many sites stylolitic cleavage these define a gently n dipping foliation and an n plunging extension south vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south disturbed by faulting the magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de strained assuming continuum behaviour the optimum de straining 30 40 shortening in a flattening strain is compatible with the stylolitic cleavage development restores bedding to the near horizontal and restores the characteristic remanent magnetization vectors chrms to concentrated symmetrical fisherian distributions the strain corrected chrms yield more reasonable palaeopole locations for the lefkara and pakhna limestone and more uniform micro plate rotation rates corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the troodos plate since the creation of the late cretaceous 88 ma ocean lithosphere it did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at 1 5 ma 1 since 58 ma

using anisotropy of magnetic susceptibility to better constrain the tilt correction in paleomagnetism a case study from southern peru 1 we report a combined study of anisotropy of low field magnetic susceptibility ams and paleo magnetism from 16 sites in a sedimentary sequence of eocene early oligocene red beds in southern peru incipient tectonic strain is recorded during the early stages of deformation nonhorizontal magnetic linea tion in geographic coordinate suggests either non cylindrical folding and or interference of two phases of compressive deformation and tectonic rotation applying the classic tilt correction results in significant dispersion in paleomagnetic declinations and apparent clockwise and counterclockwise relative tectonic rota tions a dispersion in the orientation of the magnetic lineation also arises from a simple classic tilt correction inducing apparent local rotation in paleostress determi nation the magnetic lineation is a good proxy to detect a complex history of folding when the finite strain is not large enough to reset the magnetic fabric acquired during the early stages of deformation and when detailed geo logical field mapping is not available or not possible in the present study a double correction rotating first the lineation to the horizontal reduces significantly the dis persion of the paleomagnetic data with respect to con ventional tilt correction fisher parameter k increases from 14 to 35 the interest of this double correction must obviously be evaluated for each study according to the complexity of the folding and the intensity of the deformation assuming a mean age of 40 ma for the sedimentary sequence no significant rotation 4 5 8 4 is observed in this area of the peruvian andes citation roperch p v carlotto and a chauvin 2010 using anisotropy of magnetic susceptibility to better constrain the tilt correction in paleomagnetism a case study from southern peru tectonics 29 tc6005

the magnetic fabric of weakly deformed late jurassic shales from the southern subalpines chains french alps evidence for sw directed tectonic transport direction abstract extensive magnetic fabric investigations have been conducted on the late jurassic sedimentary black shales 37 sites 362 samples the so called terres noires formation from the southern subalpine chains western alps this study completes a previous study done in the northern subalpine chains which demonstrated that the terres noires formation preserves a subtle magnetic lineation related to tectonic transport direction in the southern subalpine chains diois devoluy embrunais digne nappe similar magnetic fabrics have been found the shape of the magnetic fabric is oblate and locally mirrors the gradient of deformation in the embrunais footwall the magnetic foliation is parallel to the bedding or to the slaty cleavage about 50 of the magnetic lineations have down dip magnetic directions while the other magnetic lineations are sub horizontal or parallel to the intersection bedding cleavage the tectonic magnetic fabrics recorded in the embrunais confirm the ability of the terres noires to record tectonic transport direction the magnetic fabric geometry suggests sw directed tectonic transport in the southern subalpine chains the diois appears as a tip zone where transport directions are abruptly clockwise rotated from nw directed in the northern subsalpine chains to sw directed the magnetic fabric from the digne nappe exhibits a fan pattern of transport directions in close agreement with the arcuate shape of the thrust

magnetic fabrics in the western central pyrenees an overview abstract in the western central pyrenees numerous investigations during the past years have yielded an exceptional high density of localities more than 700 sites where the ams and rock magnetic properties have been determined this unique ams dataset helps in understanding the orogenic evolution of the pyrenees and its foreland basins processes related to ams development in different structural units permit to identify i an early recording of strain related to the transtensional or transpressional stages during the early triassic and early cretaceous ii a moderate shortening related to the fold and thrust belt and foreland basin evolution during the cretaceous tertiary orogenic stage and iii a relatively strong deformation that produced cleavage related to the emplacement of basement thrusts in the axial zone during the middle late eocene the main achievement of this contribution is the integration in cross section and map view of magnetic fabrics orientation and parameters in the western central pyrenees which allow constraining the processes affecting magnetic fabrics at the orogen scale ams results indicate i a dominant bedding controlled foliation very sensitive to subtle layer parallel shortening lps processes especially in the foreland basins ii magnetic lineations parallel to fold axes or strike of thrusts outside the cleavage front that can be deflected by vertical axis rotations in particular areas and iii an overall decrease of the definition of ams fabrics magnetic lineation or the anisotropy of the ellipsoid associated with incipient cleavage and intersection lineation fabrics in the internal zones of the orogen therefore ams is a sensitive indicator to delineate the strain patterns at the orogen scale in particular in those areas undergoing a modest degree of deformation

reliability of magnetic fabric of weakly deformed mudrocks as a palaeostress indicator in compressive settings abstract in this work we compare the results of anisotropy of magnetic susceptibility ams with palaeostress analysis at local scale the ams data refer to 14 sites from weakly deformed mudrocks lower and middle miocene lacustrine sediments from two areas of the internal part of the ebro foreland basin the bardenas reales and monegros areas nine sites display a magnetic fabric related to a subtle tectonic overprint showing a roughly e w magnetic lineation whereas the rest of sites most of them located at the monegros area show a sedimentary fabric without a well defined magnetic lineation palaeostress data result from fault populations and joint sets exposed in the limestone beds interbedded with the mudrocks at the nearest possible outcrop from the ams site the origin of the magnetic lineation is related to a n s compression coeval with the earliest diagenesis of the studied sediments prior to consolidation the comparison between magnetic fabric and palaeostress data demonstrates the coaxiality between the ams and palaeostress ellipsoids both at local and regional scales thereby validating the reliability of ams as palaeostress indicator in compressive settings

magnetostratigraphy of the miocene corque basin bolivia implications for the geodynamic evolution of the altiplano during the late tertiary a magnetostratigraphic study of a thick miocene continental red bed sequence totora formation located in the north central altiplano the corque syncline yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the altiplano paleomagnetic results obtained in 653 samples from a composite 4 5 km thick red bed sequence across the corque syncline indicate that most of the totora sequence was deposited in the middle miocene from 14 to 9 ma with high sedimentation rates up to 970 m myr from 12 to 9 ma the high rate of infill of the corque basin demonstrates active subsidence of the northern altiplano during the middle miocene deformation ceased in the northern altiplano domain only at the end of the miocene two major tuff beds within the sequence have been previously dated by 39 ar 40 ar of sanidine minerals marshall et al 1992 the excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by cande and kent 1995 the numerous paleomagnetic results obtained in middle miocene sediments within the corque syncline a structure which can be traced for more than 100 km along its axis demonstrate that the corque syncline rotated counterclockwise by 10 8 2 9 since 9 ma as a consequence of the internal deformation of the altiplano inclination flattening of 17 is recorded in these red bed sediments this result provides additional evidence that detrital magnetizations especially in red beds should be taken with caution when estimating paleolatitudes or long term variations of the geomagnetic field

paleomagnetism of miocene sediments from the turfan basin northwest china no significant vertical axis rotation during neotectonic compression within the tian shan range central asia abstract paleomagnetic results are reported from 377 samples of miocene red sediments in the turfan basin of east tian shan northwest china thermal demagnetization for most specimens isolates a low temperature component that groups around the present geomagnetic field a high temperature characteristic component is isolated from four localities and interpreted as primary remanent magnetization from apparent pre folding origin and positive reversal test as for previous results obtained from tertiary rocks in central asia the magnetic inclination is significantly shallower than the reference field computed from the eurasian apw path at 10 and 20 ma magnetic susceptibility measured in 324 specimens exhibits low anisotropy of less than 1 05 for 80 of the sample the low anisotropy degrees ams shape and orientations suggest that the studied rocks are unlikely to have undergone significant strain during compaction or tectonism we argue that the shallow inclination is likely to result from an initially shallow detrital remanent magnetization comparison of the paleomagnetic declination from eurasia north china and tarim suggests that the turfan basin has been subjected to little or no vertical axis rotation since the miocene together with paleomagnetic declinations on both sides of the talas ferghana fault we propose that the tian shan to the east of the talas ferghana fault has remained relatively stationary with respect to adjacent blocks since the miocene

vertical axis rotations across the puna plateau northwestern argentina from paleomagnetic analysis of cretaceous and cenozoic rocks between 10 s and 30 s the central andes are marked by both a major topographic anomaly the altiplano puna plateau and a westward concave geometry whose origin remains controversial the arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes indeed in the central andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc throughout peru northernmost chile and northern bolivia and clockwise rotations on the southern limb throughout southern bolivia northwestern argentina and northern chile to fill a gap in data from northern argentina and to contribute to the ongoing debate on the origin of rotations in the central andes we have undertaken a paleomagnetic study of 373 cores taken at 29 sites grouped into seven localities the samples are from sediments and lava flows of cretaceous to tertiary age located in intermontane basins of the puna plateau in northwestern argentina vertical axis rotations calculated from paleomagnetic declinations are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central andes however significant variations in the amount of rotation occur from one locality to another suggesting that they are at least in part influenced by local tectonics as most faults in the puna plateau have reverse dip slip components we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults whether or not this mechanism has operated across the entire area of thickened crust in the central andes remains to be demontrated even if such faulting has locally influenced rotations cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central andes

strain magnetic fabric and paleomagnetism of the deformed red beds of the pont rean formation brittany france a joint study of strain anisotropy of magnetic susceptibility ams and paleomagnetism has been conducted in the deformed red beds of the ordovician pont rean formation brittany france analysis of results from 13 sampling sites shows that homogeneous strain estimated from the shape of elliptical reduction spots in each site controls the development of a magnetic fabric during the hercynian folding of this formation the good correlation between strain and ams tensors is interpreted as mainly arising from a strain induced progressive rotation of planar hematite that tends to align within cleavage plane this rotation of magnetic carriers in turn induces a rotation of net measurable pretectonic remanent magnetization toward cleavage because of the high angle between the initially subvertical ordovician remanent magnetization and the regionally subhorizontal shortening direction this deviation of the pretectonic magnetization is achieved without any scattering but with a probable clustering effect upon within site initial dispersions for the same reason the overall distribution of mean site pretectonic magnetization vectors is clustered at the in situ deformed state this results in an anomalous behavior of the classical fold test that could erroneously lead to assignment of a syntectonic to post tectonic age to the magnetizations finally the strain removal technique based upon the passive behavior hypothesis is applied to the deviated vectors it allows recovery of the initial direction of these pretectonic ordovician magnetizations d 235 i 75 95 6 5

strain removal applied to paleomagnetic directions in an orogenic belt the permian red slates of the alpes maritimes france abstract detailed investigations of relationships between strain and directions of characteristic remanent magnetization chrm have been conducted on the permian red series of the alpes maritimes france the analysis of the results obtained by structural and paleomagnetic studies on 14 sites leads to the following observations in most sites the mean direction of the strain shortening axis is close to the expected permian paleomagnetic direction in this case we observe a large scatter of the chrm directions within each site which is interpreted as an increase of an initial dispersion due to strain moreover this dispersion more or less increases with strain intensity in other cases where the angle between the shortening axis and the paleomagnetic direction is high the whole population is deflected towards the cleavage xy plane these observations lead us to assume a material line like behaviour of the chrm vectors and consequently an attempt has been made to recalculate these directions by strain removal these calculations give positive results at two levels 1 a statistically significant improvement in the clustering of the within site distributions and 2 after tilt correction using unstrained bedding planes a significant improvement in the clustering of the between site distribution the formation mean direction after strain removal is in good agreement with known permian results for stable europe

tectonic evolution of the pamir recorded in the western tarim basin china sedimentologic and magnetostratigraphic analyses of the aertashi section the northward indentation of the pamir salient into the tarim basin at the western syntaxis of the india asia collision zone is the focus of controversial models linking lithospheric to surface and atmospheric processes here we report on tectonic events recorded in the most complete and best dated sedimentary sequences from the western tarim basin flanking the eastern pamir the aertashi section based on sedimentologic provenance and magnetostratigraphic analyses increased tectonic subsidence and a shift from marine to continental fluvio deltaic deposition at 41 ma indicate that far field deformation from the south started to affect the tarim region a sediment accumulation hiatus from 24 3 to 21 6 ma followed by deposition of proximal conglomerates is linked to fault propagation into the tarim basin from 21 6 to 15 0 ma increasing accumulation rates of fining upward clastics is interpreted as the expression of a major dextral transtensional system linking the kunlun to the tian shan ahead of the northward pamir indentation at 15 0 ma the appearance of north pamir sourced conglomerates followed at 11 ma by central pamir sourced volcanics coincides with a shift to e w compression clockwise vertical axis rotations and the onset of growth strata associated with the activation of the local east vergent qimugen thrust wedge together this enables us to interpret that pamir indentation into tarim had started by 24 3 ma reached the study location by 15 0 ma and had passed it by 11 ma providing kinematic constraints on proposed tectonic models involving intracontinental subduction and delamination

paleomagnetic study along the southeastern edge of the altiplano puna plateau neogene tectonic rotations in northwestern argentina the transition zone between the puna and the sierras pampeanas was deformed in late miocene and pliocene time structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression this study contributes new paleomagnetic results from four sites in cretaceous rocks and 19 sites in neogene sedimentary sequences when compared with reference paleomagnetic data for stable continental south america the results show an average inclination shallowing error of 10 5 and a pattern of clockwise rotations up to 29 study of the anisotropy of magnetic susceptibility ams on 278 specimens provided for most sites ams tensors with an oblate shape controlled by sedimentation and compaction explaining the observed inclination shallowing a slight ams lineation was measured and we argue that this magnetic lineation reflects late miocene pliocene compression a correlation observed between magnetic lineations deduced from the ams analysis and rotation estimates from remanent magnetizations demonstrates that the inferred shortening directions have also been rotated clockwise

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

reconstructing the exhumation history of the lesser himalaya nw india from a multitechnique provenance study of the foreland basin siwalik group 1 this research presents the first multitechnique provenance study of the siwalik group in the himalayan foreland basin in india using the jawalamukhi section magnetostratigraphically dated at 13 5 ma combined with provenance data from a dharamsala formation sedimentary section 21 13 ma located close by it forms the longest temporally continuous record of himalayan erosion in the indian foreland basin sandstone petrography and heavy mineral analysis conglomerate clast composition ar ar dating of detrital white micas and sm nd analyses on siltstones conglomerate matrix and conglomerate clasts was undertaken to determine 1 shifts in source region through time and 2 changes in detrital lag times related to exhumation rates in the hinterland together interpreted in the light of thrusting events we interpret the data to show a slow down in exhumation rate of the higher himalaya by 16 17 ma after which time the locus of thrusting propagated south of the main central thrust and erosion of the low grade haimanta formation to the south became significant the nonmetamorphosed inner lesser himalaya breached its haimanta cover by 9 ma with the metamorphosed inner lesser himalaya lesser himalayan crystalline series exhuming to surface by 6 ma this event caused sufficient disruption to established drainage patterns that all higher himalayan material was diverted from this location at this time

tertiary geodynamics of sakhalin nw pacific from anisotropy of magnetic susceptibility fabrics and paleomagnetic data abstract sakhalin has been affected by several phases of cretaceous and tertiary deformation due to the complex interaction of plates in the northwest pacific region a detailed understanding of the strain is important because it will provide constraints on plate scale processes that control the formation and deformation of marginal sedimentary basins anisotropy of magnetic susceptibility ams data were obtained from fine grained mudstones and siltstones from 22 localities in sakhalin in order to provide information concerning tectonic strain ams data reliably record ancient strain tensor orientations before significant deformation of the sediments occurred paleomagnetically determined vertical axis rotations of crustal rocks allow rotation of the fabrics back to their original orientation results from southwest sakhalin indicate a n035 e directed net tectonic transport from the mid paleocene to the early miocene which is consistent with the present day relative motion between the okhotsk sea and eurasian plates reconstruction of early late miocene ams fabrics in east sakhalin indicates a tectonic transport direction of n040 e in west sakhalin the transport direction appears to have remained relatively consistent from the oligocene to the late miocene but it has a different attitude of n080 e this suggests local deflection of the stress and strain fields which was probably associated with opening of the northern tatar strait a northward directed tectonic transport is observed in miocene sediments in southeast sakhalin mid eocene sediments in east sakhalin and in late cretaceous rocks of west and northern sakhalin which may be associated with northwestward motion and subduction of the pacific plate in the tertiary period the boundaries of the separate regions defined by the ams data are consistent with present day plate models and therefore provide meaningful constraints on the tectonic evolution of sakhalin

reorientation of remanent magnetism during tectonic fabric development an example from the waynesboro formation pennsylvania u s a abstract a paleomagnetic study of the late early early middle cambrian waynesboro formation in south central pennsylvania reveals a pre folding magnetization which is close to the expected cambrian field at three sites while other site mean directions are rotated by as much as 82 from the expected cambrian field these results are based on detailed thermal demagnetization analysis of 84 red siltstone and sandstone samples from nine sites collected over a 163 km 2 area local faulting fault block rotation and remanence acquisition during rapid apparent polar wander do not appear to be responsible for the observed remanence deviations total strain at the sites as indicated semiquantitatively by the anisotropy of magnetic susceptibility ams of 28 samples has been studied to test the hypothesis that deformation has rotated the remanence directions ams data vary considerably both between sites and within sites and show a path of progressive deformation on a flinn diagram there is a strong positive correlation between the deviation of a sample s remanence from the expected cambrian field and its position along the deformation path suggesting that finite strain fabric development has rotated the magnetizations the study area is structurally contained in the south mountain anticlinorium whose southern margin forms the displacement transfer zone to the southern appalachian alleghenian thrust sheets elliott 1976 if thrusting had continued and incorporated the waynesboro rocks into these thrust sheets the rotated magnetizations could have been incorrectly interpreted to result from rigid thrust sheet rotation

layer parallelisation an unrecognised mechanism for palaeomagnetic rotations in fold belts abstract we report a new driving mechanism for vertical axis rotation in fold belts during directed folding there always are layers which are tilted oblique to the regional shortening direction these layers are geometrically incompatible with fold closure and must become accommodated towards parallelism with the other layers in the course of further shortening accommodation is achieved through vertical axis rotations of the layers towards the shortening direction a revision of palaeomagnetic data sets from fold belts shows that 1 incompatible layers occur in almost all data sets reflecting a certain degree of non cylindrical folding and 2 a parallelisation of the layers by true vertical axis rotations occurred when folding became intense layer parallelisation is a potential source of disturbance for palaeomagnetic and tectonic interpretation it can be the explanation for some of the frequent outliers in palaeomagnetic data sets and a basic model for the rotation pattern of palaeomagnetic directions

sedimentation processes on the antarctic continental margin at kapp norvegia during the late pleistocene sedimentological analyses concerning ice rafted debris grain size distribution biogenous components and clay mineral composition of four sediment cores from the antarctic continental margin off kapp norvegia reveal a cyclical pattern of three different sediment facies these are classified into warm and cold types representing warm and cold a climatic periods and short transition period from cold to warm events

paleogene building of the bolivian orocline tectonic restoration of the central andes in 2 d map view using available information on the magnitude and age of tectonic shortening as well as paleomagnetically determined tectonic rotations we have run a series of 2 d map view restorations of the central andes neogene shortening in the foreland belt induced only slight orogenic curvature of the central andes the constraints on the ages of the large observed fore arc rotations average of 37 counterclockwise in southern peru and 29 clockwise in northern chile indicate that the bolivian orocline formed during the eocene oligocene as a consequence of differential shortening focused in the eastern cordillera to minimize local block rotations along the fore arc the restoration that best fits the central andean rotation pattern requires about 400 km of total paleogene plus neogene shortening near the arica bend this value corresponds to the upper bound of estimates of maximum horizontal shortening for the central andes along strike variations in horizontal shortening in the back arc induced bending of the continental margin block rotations with fore arc along strike extension and or orogen parallel transport of upper crustal material toward the symmetry axis of the orocline