secondary fabrics revealed by remanence anisotropy methodological study and examples from plutonic rocks summary the fabric of fine grained magnetite is isolated by partial remanence anisotropy paar measurements that combine cycles of anhysteretic remanence acquisition and alternating frequency af tumbling demagnetization of the low coercivity fraction since remanence intensities of the most coercive fine grained magnetite are very weak rarely exceeding 10 per cent of the measured remanence in the studied samples a methodological study is presented that defines optimal measurement schemes and examines the role of gyroremanence and background remanences due to hard remanence carriers this procedure is applied to some reddened granites from tana corsica and wichita oklahoma and to the tellnes ilmenite rich norite norway the paar fabrics for fine grained magnetites are compared to the fabrics of the coarse grained primary magnetite grains as shown by both the anisotropy of magnetic susceptibility ams and the remanence anisotropy of the low coercivity fraction in tellnes fine grained magnetite is believed to be of secondary origin this is strongly suggested by the correlation between the paar fabrics and the orientations of sets of microfractures decorated by oxides in the reddened granites although no direct correlation between paar fabrics and microstructures could be evidenced the comparison between unaltered specimens and hydrothermally altered ones reveals changes in magnetic properties and fabrics that are ascribable to this secondary geological process

modelling granite migration by mesoscale pervasive flow abstract mesoscale pervasive magma migration leads to granite injection complexes common in hot crustal terranes pervasive migration is limited by magma freezing when intruding cold country rock here we explore numerically the feedback mechanism between magma intrusion and heating of the country rock which allows younger intrusive batches to reach increasingly shallower cooler levels this process relies on the higher solidus temperature of a rock compared to that of its melt once melt is segregated we define the free ride layer as the region above the melt source where magma may freely migrate because rock temperature is above melt solidus the top of the free ride layer which corresponds to the melt solidus t s isotherm is at the limiting depth z s after magma passes through the free ride layer the magma front is always at the limiting depth we modeled the thickening and heating of the crust above the source as melt at its liquidus t l intrudes it pervasively from below we found that a magma quickly warms crust below z s to about t l forming a step in temperature at z s b the front z s moves up through the crust as more magma is intruded c as magma is emplaced at the front a mingled layer of about half magma half crust forms below it so that the total rise of the front corresponds approximately to half of the thickness of magma added to the free ride layer d the rate of rise of the front depends on the temperature difference between crust and t l and slows down as the magma front rises e for most reasonable intrusion rates and volumes the crust above z s feels little influence of the intrusion because the diffusion time scale is much smaller than the rise rate of the front in summary pervasive migration is an efficient way of heating the lower to middle crust and can result in an injection complex several kilometers thick consisting of about half magma and half original crust

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

magnetic susceptibility anisotropy a new petrofabric tool in migmatites 1 the rheological behavior of the lower to middle continental crust during orogenic deformation is poorly understood migmatite common at this crustal level remains difficult to analyze from a structural viewpoint because of pervasive recrystallization we present new anisotropy of magnetic susceptibility ams data from 217 specimens of migmatites of the superior province in minnesota with the aim of investigating its potential as a petrofabric tool quarry exposures provide the ideal sampling playground for such a study the migmatites are coarse grained are granitic to tonalitic in composition and display a centimeter scale compositional layering the layering and the regional foliation are subhorizontal to shallowly dipping to the ne mineral lineations are difficult to observe due to recrystallization but trend around 70 n with gentle plunges magnetic properties are carried mainly by multidomain md primary magnetite grains ranging in size from 20 to 200 m the magnetic susceptibilities are generally high around 5 10 10 3 si the degree of magnetic anisotropy is large between 1 2 and 1 3 the principal axes of the ams ellipsoids coincide with those of the finite strain and the degree of anisotropy broadly correlates with the mineral fabric intensity this study is the first detailed investigation of magnetic fabrics in migmatites these preliminary results suggest that the ams technique is an easy and powerful tool for the petrofabric analysis of migmatites

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

timing of the ht lp transpression in the neoproterozoic serido belt borborema province brazil constraints from upb shrimp geochronology and implications for the connections between ne brazil and west africa abstract the serido schist belt is a classical domain of the borborema province ne brazil recording crustal melting at ht lp conditions in a tectonic setting dominated by dextral transpressive deformation a comparable tectonometamorphic setting has been described in the pan african igarra and ilesha schist belts of south nigeria africa the ht transpression of the serido has previously been imprecisely dated by using a range of isotopic methods on a suite of syntectonic granites we provide new u pb zircon ages of the acari granite and santa luzia migmatite that constrain the ht lp metamorphism of the serido at c 575 ma anisotropy of magnetic susceptibility of the santa luzia migmatite shows that the magnetic fabric records the ne trending dextral kinematics similar to those previously described in the acari pluton on the other hand the u pb ages of the totoro pluton provide evidence for an older mostly mafic to intermediate magmatism intruding the pelitic schist that precedes the ht lp metamorphism by approximately 15 m y in contrast with the acari pluton and santa luzia migmatite the magnetic fabrics of totoro show a concentric steep dipping structure apparently related to magma ascent thermobarometric studies in the serido igarra and ilesha schist belts indicate that a large domain in the northern borborema and south of the nigeria shields was deformed at a high temperature and at relatively shallow depths although the tectonic setting for such a post collision ht event remains an open issue it allows us nevertheless to refine the across atlantic geological correlation between the continents

viscoplastic flow in migmatites deduced from fabric anisotropy an example from the naxos dome greece 1 many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of orogenic crustal flow field and microstructural methods may be insufficient to characterize the planar and linear elements of rock fabric in migmatites due to obscured flow fabrics or protracted deformation in the naxos dome greece we test the anisotropy of magnetic susceptibility ams as a tool for recovering mineral fabric symmetry and the kinematic axes of flow in migmatites measurements of 155 migmatite samples yield dominantly low values 300 10 6 si of bulk magnetic susceptibility km consistent with biotite being the dominant carrier of the ams higher values of km thermomagnetic hysteresis and microstructural data however suggest a ferromagnetic contribution from magnetite in a subset of samples n 15 using electron backscatter diffraction ebsd analysis we establish the correspondence of the biotite subfabric with the ams and structural fabric of the naxos migmatites ebsd data from biotite suggests that magnetic lineation in these dominantly paramagnetic migmatites arises from a zone axis orientation of biotite crystals organized about the direction of viscoplastic flow over a range of spatial scales migmatitic foliation and magnetic foliation are well correlated the magnetic lineation recovered by ams displays a coherent organization despite the heterogeneous structure and composition of the naxos migmatites these data suggest that the apparent complexity of migmatites masks a simpler flow regime controlled by bulk viscoplastic flow furthermore our study demonstrates the utility of the ams method for studying the dynamics of partially molten orogenic crust

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

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

the neoproterozoic brasiliano orogeny in northeast brazil 40ar39ar and petrostructural data from ceara abstract the borborema province in ne brazil comprises paleoproterozoic basement and its proterozoic cover that experienced a complex tectonometamorphic history during the brasiliano pan african orogeny in ceara four major domains separated by continental scale steeply dipping dextral shear zones have been identified 40 ar 39 ar step heating of mineral separates and laser probe fusion of single grains have been used in conjunction with petro structural observations to delineate the cooling tectonic history of these domains to the north of the senador pompeu shear zone spsz amphibole and biotite pairs from low pressure granulite belts exposed along the granja shear zone and spsz record plateau ages in the range 575 560 ma corresponding to cooling rates close to 20 c m y 40 ar 39 ar laser probe muscovite dates from the chaval augen gneiss point to a similar age for shearing responsible for the exhumation and fast cooling of the granulite belts a muscovite biotite pair from the central ceara high pressure nappes lying between the shear zones documents a lower cooling rate in the interval 534 525 ma related to the decompression and uplift of previously deelly buried rocks to the south of the spsz only moderate crustal thickening occurred within the various schist belts a distinct cooling pattern is indicated for these belts and the western patos shear zone attesting to a different crustal development 40 ar 39 ar dates of amphibole phlogopite muscovite and biotite range from 550 to 522 ma recording a postmetamorphic cooling rate of 6 9 c m y slower cooling rates of 3 4 c m y between amphibole and biotite closure ages in the range 540 500 ma are reported for the eastern part of the palos shear zone system the diachronous cooling recorded along this shear zone is interpreted to reflect the variations in thickness of the high temperature mylonitic belts earlier more rapid cooling occurred where the belts are narrow in the west late slower cooling occurred in wider zones to the east these thermochronological data reveal an age and cooling rate distribution at the scale of the borborema province that provides new constraints on the development of the brasiliano orogeny between 580 and 500 ma

results of a sea floor electromagnetic survey over a sedimented hydrothermal area on the juan de fuca ridge summary the eastern part of middle valley on the northern juan de fuca ridge is characterized by locally high heat flow evidence of hydrothermal activity and near surface mineralization a magnetometric offshore electrical sounding moses survey was conducted in this area to determine the electrical structure of the sediment and upper crust two perpendicular lines of data were obtained across a mound a 500m diameter uplifted hill where sulphide mineralization and hydrothermal activity have been observed the best fitting model for the east west regional line consists of a sediment layer of resistivity 0 70 f 0 06 qm and thickness 500 f 100 m overlying basaltic basement of resistivity 2 3 f 0 6 qm basement porosities are estimated to be of the order of 10 14 per cent using measured vent fluid temperatures of 275 300 c consistent with previous estimates of the porosity in fractured oceanic crust results for the north south line along the mound indicate the presence of a 1 35 f 0 15 qm 100 f 20 m thick resistive cap overlying deeper sediments of much lower resistivity of the order of 0 1 qm the resistivity structure across a localized high heat flow anomaly is similar but the resistive cap is absent the phase lags calculated from the resistivity model deviate significantly from the measured lags and provide the clearest evidence for transverse isotropy the interbedding of very conductive with less conductive layers the conductive thin layers are two orders of magnitude less resistive than the resistive layers because of massive sulphide mineralization a result now confirmed by drilling additional data were gathered at a site centred about the southern end of a vent field 3 4km northwest of the hydrothermal mound the resistivities can be fit using either the regional model or a 1 1 qm halfspace there are some anomalous values which appear to be spatially correlated with basement topographic highs

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

proterozoic links between the borborema province ne brazil and the central african fold belt the congo cc and the sao francisco sfc cratons were joined at about 2 05 ga northern parts of palaeoproterozoic basement subsequently underwent extension at about 1 ga forming intracratonic basins neoproterozoic metasedimentary rocks in these basins yield detrital zircons as young as 630 ma the brasiliano and pan african c 620 580 ma assembly of west gondwana extensively altered this system the sergipano domain occurs north of the sfc and the comparable yaoundedomain occurs north of the cc crust north of the sergipano domain com prises the pernambuco alagoas peal domain the ne sw striking tchollire banyo fault in cameroon may extend southwestwards between the peal and sergipano domains defining northern limits of abundant sfc cc basement the adamawa yadedomain in africa does not appear to extend into brazil the transverse domain of brazil is a collage of palaeoproterozoic crustal blocks the 1 0 ga cariris velhos orogen cvo late neoproterozoic basins and brasi liano granites the cvo extends ene for more than 700 km in brazil but eastern continuation into africa has not been identified north of the transverse domain contiguous c 2 15 ga gneisses comprise basement of rio grande do norte and cearadomains which continue eastwards into western nigeria and western sahara

geology of the northern borborema province ne brazil and its correlation with nigeria nw africa the borborema and benin nigeria provinces of ne brazil and nw africa respectively are key areas in the amalgamation of west gondwana by continental collision during the brasiliano pan african orogenies both are underlain by complex basement nigeria has c 3 05 ga archaean crust but no known palaeoproterozoic rocks 2 0 ga in ne brazil 2 6 3 5 ga archaean rocks form small cores within palaeoproterozoic gneiss terrains affected by plutonism at c 2 17 ga both regions exhibit late palaeoproterozoic c 1 8 ga rift related magmatism and metasedimentary sequences overlying the basement the serido group of ne brazil 0 65 ga is similar to the igarra sequence in sw nigeria the ceara group which may date back to c 0 85 ga is a passive margin deposit on crust thinned during initiation of an oceanic domain in both provinces basement and sedimentary cover were involved in tangential tectonics that resulted in crust thickening by nappe stacking associated with closure of this ocean frontal collision between c 0 66 and 0 60 ga later evolved to an oblique collision generating north south continental strike slip shear zones at c 0 59 ga in ne brazil the main pan african suture is probably buried beneath the parnaiba basin the transbrasiliano lineament interpreted as the prolongation of the kandi 4 50 lineament in hoggar may represent a cryptic suture

migmatites formed by water fluxed partial melting of a leucogranodiorite protolith microstructures in the residual rocks and source of the fluid abstract the opatica subprovince in the canadian shield is a late archaean 2761 2702 ma plutonic arc formed above a north dipping subduction zone anatexis 2690 2677 ma of leucogranodiorite and leucotonalite orthogneisses in the opatica generated migmatites in an area of north vergent back thrusts visible at the surface and in l ithoprobe seismic profile 48 schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them the modal mineralogy microstructure and whole rock major trace and oxygen isotope compositions of the protolith and migmatites were investigated to 1 determine the melting reaction 2 find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome and 3 to determine the source of the fluid involved in anatexis partial melting of the protolith did not change the mineral assemblage but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks quartz plagioclase and microcline show evidence for dissolution and biotite does not thus water fluxed melting of quartz plagioclase microcline occurred a mass balance indicates 25 30 partial melting the melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase extraction of 80 90 of the melt left a thin film of melt on the grain boundaries and crystallization of these in the plagioclase domains created diagnostic microstructures microcline fills the last remaining pore space and forms high aspect ratio crystals between plagioclases or triangular crystals at grain junctions quartz shows a range of morphologies from high aspect ratio films through the string of beads to isolated rounded grains as the microstructure progressively equilibrated after crystallisation most accessory phases including zircon remained in the residuum however almost all the schollen migmatites have high contents of th u nb ta and ree relative to the protolith due to contamination by accessory phases derived from mafic rocks disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed average whole rock 18 o for the protolith and migmatites are similar ca 8 2 and the small difference between melt rich 8 6 and residuum rich rocks 8 0 is consistent with fractionation thus the fluid that caused melting was probably of metamorphic origin with 18 o similar to the protolith the seismic profile shows several reflectors extending to a present depth of 20 km ca 40 km in the late archaean under the migmatites these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface a new type of neosome reported in this study may have formed along fractures that the fluids migrated along however these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites

strain transfer at continental scale from a transcurrent shear zone to a transpressional fold belt the patos serido system northeastern brazil during the brasiliano pan african orogeny a complex continental scale pattern of east west transcurrent shear zones and northeast trending fold belts formed in the northern and central borborema province of northeastern brazil the east west shear zones have been usually regarded as slightly younger features but the study of the most spectacular case of intersection between these two structures the patos shear zone and the serido transpressional belt leads to a different tectonic model satellite imagery and structural petro logical and geophysical data support the interpretation that these structures 1 are in structural continuity and 2 formed simultaneously under amphibolite facies metamorphic conditions that led to partial melting this suggests a model of strain transfer at the scale of the orogen at the eastern end of the patos east west dextral shear zone the strain that accommodated the relative motion of the northern block was transferred to the northeast trending serido belt where it resulted in folding strike slip faulting and stretching parallel to the strike of the belt

strain distribution across a partially molten middle crust insights from the ams mapping of the carlos chagas anatexite aracuai belt east brazil abstract the easternmost part of the neoproterozoic aracuai belt comprises an anatectic domain that involves anatexites the carlos chagas unit leucogranites and migmatitic granulites that display a well developed fabric microstructural observations support that the deformation occurred in the magmatic to submagmatic state structural mapping integrating field and anisotropy of magnetic susceptibility ams revealed a complex 3d structure the northern domain displays gently dipping foliations bearing a nw trending lineation southward the lineation trend progressively rotates to ew then sw and the foliation is gently folded the eastern domain displays e w and ne sw trending foliations with moderate to steeply dips bearing a dominantly ns trending lineation magnetic mineralogy investigation suggests biotite as the main carrier of the magnetic susceptibility in the anatexites and ferromagnetic minerals in the granulites crystallographic preferred orientation cpo measurements using the electron backscatter diffraction ebsd technique suggest that the magnetic fabric comes from the crystalline anisotropy of biotite and feldspar grains especially the delineation of several structural domains with contrasted flow fabric suggests a 3d flow field involving westward thrusting orthogonal to the belt northwestward orogen oblique escape tectonics and ns orogen parallel flow this complex deformation pattern may be due to interplay of collision driven and gravity driven deformations

rapid extensional unroofing of a granite migmatite dome with relics of high pressure rocks the podolsko complex bohemian massif the podolsko complex bohemian massif is a high grade dome that is exposed along the suprastructure infrastructure boundary of the variscan orogen and records snapshots of its protracted evolution the dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high to ultrahigh pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards our new u pb zircon ages indicate that the leucocratic migmatites were derived from early ordovician c 480 ma felsic igneous crust the same age is inferred for melting the proto source of the metapelitic migmatites the relics of high to ultrahigh pressure rocks suggest that at least some portions of the complex witnessed an early variscan subduction to mantle depths followed by high temperature anatexis and syntectonic growth of the podolsko dome in the middle crust at c 340 339 ma subsequently the dome exhumation was accommodated by crustal scale extensional detachments similar c 340 ma ages have also been reported from other segments of the variscan belt yet the significance of this tectonothermal event remains uncertain here we conclude that the 340 ma age post dates the high pressure metamorphism the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and finally the extensional unroofing of the complex requires a minimum throw of about 8 10 km we use this as an argument for significant early carboniferous palaeotopography in the interior of the variscan orogen

fabrics of migmatites and the relationships between partial melting and deformation in high grade transpressional shear zones the espinho branco anatexite borborema province ne brazil abstract the espinho branco anatexite located within a transcurrent high temperature shear zone in ne brazil was the subject of a comprehensive petrostructural study anisotropy of magnetic susceptibility ams anisotropy of anhysteretic remanence aar electron backscatter diffraction ebsd to evaluate the compatibility of different fabrics with the kinematics of melt deformation magnetite dominates susceptibilities larger than 1 msi and biotite displays 001 lattice directions consistent with ams k 3 axes in contrast migmatites with a susceptibility lower than 0 5 msi and no visible mesoscopic foliation provide crystallographic fabrics distinct from ams and aar however aar remains consistent with the regional strain field these results suggest that the correlation of field ams and crystallographic fabrics is not always straightforward despite the relatively simple organisation of the magnetic fabric in the anatexite we conclude that ams recorded the final stages of the strain field in the migmatite irrespective of its complex mesoscale structures and contrasting crystallographic fabrics

thermal history of the pan african brasiliano borborema province of northeast brazil deduced from 40ar 39ar analysis abstract detailed step heating 40 ar 39 ar analyses were performed on different minerals from magmatic and metamorphic rocks of the borborema province this region located in northeast brazil belongs to the pan african brasiliano belt which is about 1000 km wide and 600 km long twenty six single grains of amphibole muscovite and biotite were extracted from eighteen samples selected in an area of about 17 500 km 2 250 km by 70 km in the patos region this region has been affected by a continental scale shear zone system well defined 40 ar 39 ar plateau ages and an u pb analysis on zircon leterrier j jardim de sa e f bertrand j m and pin c 1994 ages u pb sur zircon de granitoides brasilianos de la ceinture du serido province borborema ne bresil c r acad sci paris 318 1505 1511 allow the definition of a homogeneous and unusual slow cooling history 3 4 c ma and suggest a rather slow uplift rate between 580 and 500 ma followed by a fast cooling of the whole studied area around 500 ma rapid cooling is suggested by concordant plateau ages on muscovite and biotite the existence of such an event at the end of the belt history is in agreement with data obtained on other major belts such as the alpine himalayan system or the hercynian belt

the montagne noire migmatitic dome emplacement french massif central new insights from petrofabric and ams studies in the southern french massif central the montagne noire axial zone is a ne sw elongated granite migmatite dome emplaced within visean south verging recumbent folds and intruded by syn to late migmatization granitoids the tectonic setting of this dome is still disputed thus several models have been proposed in order to better understand the emplacement mechanism of this dome petrofabric and anisotropy of magnetic susceptibility ams studies have been carried out in the granites and migmatites that form the dome core magmatic texture and to a lesser extent weak solid state texture are dominant as a paramagnetic mineral biotite is the main carrier of the magnetic susceptibility on the basis of 135 ams sites the magnetic fabrics appear as independent of the lithology but related to the dome architecture coupling our results with previous structural and geochronological studies allows us to propose a new emplacement model between 340 325 ma the palaeozoic series underwent a compressional deformation represented by nappes and recumbent folds involving the thermal event leading to partial melting until 325 310 ma the dome emplacement was assisted by diapiric processes an extensional event took place at 300 ma after the emplacement of the late to post migmatitic granitic plutons in the northeast side of the dome a brittle normal dextral faulting controlled the opening of the graissessac coal basin

contribution of ams measurements in understanding the migmatitic terrains of pointe geologie terre adelie east antarctica a detailed magnetic mapping using anisotropy of magnetic susceptibility ams technique was carried out in pointe geologie archipelago terre adelie east antarctica that represents a hot crust having experienced a long lived anatectic event during paleoproterozoic times 1 69 ga ago ams measurements allowed to better analyse the tectonic structure of the crystalline basement that is built up by rocks affected by various degrees of partial melting and then devoid of clear strain markers ams sampling was performed from main rocks types of pointe geologie migmatites including leucosomes and melanosomes coarse grained pink granites anatexites and mylonitic gneisses for melanosomes the magnetic foliation is dominantly in agreement with the observed field foliation i e dominantly n s sub vertical in shear zones and gently inclined in dome structures ams technique reveals a sub horizontal magnetic lineation in migmatites from shear zones and a gently plunging one in dome structures magnetic properties of leucosomes and of coarse pink granitic dykes contrast with melanosomes the bulk susceptibility and anisotropy degree are significantly lower in granitic magmas that in melanosomes in addition in well defined leucosomes granitic dykes and anatexites the magnetic ellipsoid is characterized by a higher plunge of the magnetic lineation which tends to be vertical this is associated to a rheological contrast between the solid state deformation suffered by oxide grains in the melanosomes and their reorientation in a viscous flow during the transfer of felsic melt to the granitic dykes magnetic structure of leucosomes granitic dykes and anatexites highlights the role of the gravity induced upwelling of a crust undergoing high degree of partial melting in a transpressional regime

paleoseismology of the southern panamint valley fault implications for regional earthquake occurrence and seismic hazard in southern california 1 paleoseismologic data from the southern panamint valley fault pvf reveal evidence of at least four surface ruptures during late holocene time 0 33 0 48 ka 0 9 3 0 ka 3 3 3 6 ka and 4 1 ka these paleo earthquake ages indicate that the southern pvf has ruptured at least once and possibly twice during the ongoing 1 5 ka seismic cluster in the mojave section of the eastern california shear zone ecsz the most recent event mre on the pvf is also similar in age to the 1872 owens valley earthquake and the geomorphically youthful mre on the death valley fault the timing of the three oldest events at our site shows that the pvf ruptured at least once and possibly thrice during the well defined 2 5 ka seismic lull in the mojave section of the ecsz interestingly the 3 3 3 6 ka age of event 3 overlaps with the 3 3 3 8 ka age of the penultimate i e pre 1872 rupture on the central owens valley fault these new pvf data support the notion that earthquake occurrence in the ecsz may be spatially and temporally complex with earthquake clusters occurring in different regions at different times coulomb failure function modeling of the panamint valley and garlock faults reveals significant stress interactions between these two faults that may influence future earthquake occurrence specifically our models suggest a possible rupture sequence whereby an event on the southern panamint valley fault can lead to the potential triggering of an event on the garlock fault which in turn could trigger the mojave section of the san andreas fault

magma migration folding and disaggregation of migmatites in the karakoram shear zone ladakh nw india effi cient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt bearing channels during deformation we investigate rocks that have undergone deformation and melting within the karakoram shear zone of ladakh nw india in which leucosome distribution is inferred to record the permeable network for magma extraction delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids resulting from the interplay between folding and magma migration during folding magma migrated from rock pores into layer parallel and axial planar sheets forming a stromatic migmatite or metatexite with two communicating sets of sheets intersecting parallel to the fold axis once the network was developed folding and stretching was eased by magma migration and slip along axial planar magma sheets folding and magma migration led to layer disaggregation transposition and the formation of a diatexite where rock coherency and banding were destroyed a number of structures developed during this process such as cuspate fold hinges disharmonic folds truncated layering shear along axial planar leucosomes and fl ow drag and disruption of melanosomes in this system magma migration was an integral part of deformation and assisted the folding and stretching of metatexites while folding gave rise to a magma sheet network now preserved as leucosomes as well as the pressure gradients that drove magma migration and the breakup of the metatexite thus metatexite folding increased melt interconnectivity while magma mobility increased strain rate and released differential stresses

review of the low temperature magnetic properties of magnetite from a rock magnetic perspective the low temperature magnetic properties of magnetite are reviewed and implications for rock magnetism considered the behaviour of fundamental properties of magnetite at low temperatures near the verwey transition tv are documented and attention is given to various verwey transition theories the low temperature behaviour of the magnetic energies that control domain structure is reviewed in detail for the first time in rock magnetic literature the low temperature anomaly in spontaneous magnetization ms is documented and the differences between the saturation magnetization and ms near the verwey transition are discussed it is argued that the low temperature behaviour of the magnetocrystalline anisotropy and in particular the anomaly at tv is most likely to affect multidomain remanence during low temperature cycling for multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through tv is likely to reduce the stability of closure domains