structures and way up criteria in migmatites with application to the velay dome french massif central abstract a detailed structural analysis has been applied to the migmatite sequence of the southern limb of the velay dome france in an attempt to study the structural implications of the occurrence of metatexites and diatexites metatexites have a plastic behaviour presumably dominated by the solid skeleton which largely takes up the deformation in contrast diatexites have a predominantly viscous behaviour because the solid framework has been disrupted and they are dominated by the granitic melt rather than the solid component therefore the metatexite to diatexite transition has a major rheological and tectonic importance because it separates regions that record plastic stable deformation from regions that record viscous unstable deformation where gravitational instabilities may develop mesoscopic syn migmatitic way up criteria which assume that melt migration is partly driven by gravity give evidence for a southward overturning of the velay dome this overturning is supported by sedimentary and plutonic way up criteria seen in schists and granitic plutons respectively the n s asymmetry of the velay dome is related to late variscan asymmetric extension on a regional scale consistency of data provides further evidence that syn migmatization way up criteria are regionally reliable structural tools applicable to other migmatitic terrains

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

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

deformation partitioning during transpression in response to early devonian oblique convergence northern appalachian orogen usa abstract transpressive deformation was distributed heterogeneously within the central maine belt shear zone system which formed in response to early devonian oblique convergence during the acadian orogeny in the northern appalachians straight belts are characterized by tight folds s l fabrics and sub parallel form lines and asymmetric structures that together indicate dextral se side up kinematics in contrast intervening zones between straight belts are characterized by open folds and l s fabrics within both types of zone metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade including a penetrative moderately to steeply ne plunging mineral lineation thus we interpret accumulation of plastic deformation and regional metamorphic re crystallization to have been synchronous across the central maine belt shear zone system discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in straight belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the central maine belt shear zone system perturbations in the flow were caused by serially developed thrust ramp anticlines in the stratigraphic succession immediately above the avalon like basement at which decollement of the shear zone system was initially rooted general shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow in contrast deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow part of the thickening stratigraphic succession exceeded tsolidus reflected by the occurrence of migmatites and granites the latter were partly sourced from the underlying avalon like basement that was involved in the deformation and melting

the middle to upper paleolithic transition in la guelga cave asturias northern spain abstract la guelga cave is located at the bottom of a mountain valley in the eastern part of asturias northern spain 186 m above sea level and 15 km far away from the coast it currently comprises a group of caves that were occupied during the middle and upper paleolithic periods mp and up respectively in recent years we have studied the levels of la guelga cave d zone proposing a sequence for the mp up transition mousterian aurignacian ch telperronian which we review in this paper after a detailed analysis of the chronostratigraphy we also provide new radiocarbon dates for the mousterian aurignacian and ch telperronian levels finally the geoarchaeology taphonomy archaeological data and chronology suggest that the interstratification initially identified on the basis of stratigraphic observations during excavation cannot be maintained in any case the stratigraphy of the internal d zone of la guelga cave is one of the most interesting from the cantabrian region for analyzing the last phases of the mousterian and the rise of the aurignacian in the north of the peninsula

experimental study of lateral restraint effects on the potential heave of expansive soils this paper investigates the capability of swell parameters obtained under oedometer and triaxial loading conditions to accurately predict the potential heave of expansive soils the applicability of the oedometer and triaxial swell data was examined using the results of an instrumented field experimental station the field behaviour was predicted using pressure suction and moisture heave prediction methods when the results of the oedometer tests were used in the calculations all three techniques markedly overestimated the field heave measurements contrarily using triaxial swell parameters gave a satisfactory agreement between observed and predicted heave values comparison of oedometer and triaxial results corroborated by back calculations of the field data clearly indicates that the mismatch of observed heaves and predictions based on the oedometer data was primarily attributed to the fully lateral restraint conditions applied in the oedometer swell tests applying a correction factor to the heave estimated based on oedometer data a reasonable agreement was achieved between the estimated heave and field measurements the value of the correction factor was found to be dependent on the method of prediction however an overall average value in the order of one third is considered not unreasonable for the three heave prediction methods considered in this study this factor converts the volume change obtained from the oedometer data to a vertical heave and it also accounts for the discrepancy between field and oedometer soaking conditions

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

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

influence of melt induced mechanical anisotropy on the magnetic fabrics and rheology of deforming migmatites central vosges france in the central vosges france we describe and model the structural relationships existing between orthogneiss domains and their surrounding viscous migmatites these domains show a structural and anisotropy of magnetic susceptibility ams zonation associated with an increasing amount of melt from the core to their margins a pre extension oblate to plane strain fabric is preserved in the core of the orthogneiss domains where the granite veins were emplaced along a dilated mechanical anisotropy the internal margin of the orthogneiss bodies exhibits development of a prolate fabric and active buckling of an orthogneiss granite multilayer the external margin of the orthogneiss bodies is characterized by oblate fabrics resulting from a layer perpendicular shortening of a melt orthogneiss multilayer pinch and swell structures and extensional kink bands along which granite veins have been injected the surrounding metasedimentary migmatites show a similar structural history but the degree of fabric resetting is significantly higher this systematic fabric and structural succession is due to contrasting rheological evolution between orthogneiss and metasedimentary migmatites during progressive melting and continuously evolving degree of mechanical anisotropy in the stronger orthogneiss numerical modeling confirms the role of the relationship between initial mechanical anisotropy and superimposed deformation overprints on finite ams fabrics in migmatites

crustal scale rheological transitions during late orogenic collapse abstract orogeny involves crustal thickening followed by thermal relaxation and radiogenic heat production in the thickened crust culminating in crustal melting and magma intrusion which decrease the crustal viscosity by several orders of magnitude and cause late orogenic collapse collapse of the canadian cordillera is expressed in the early tertiary shuswap metamorphic core complex british columbia which displays a three layer crustal section separated by two fundamental rheological discontinuities 1 the brittle ductile transition across which high angle normal faults in the upper crust control basin formation merge into a low angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic to low temperature conditions and 2 the metatexite diatexite transition across which the rocks lose their solid framework and behave like a viscous magma this transition has the potential to mechanically decouple the upper crust from the rest of the lithosphere during a late orogenic gravitational collapse

the timing of partial melting barrovian metamorphism and granite intrusion in the naxos metamorphic core complex cyclades aegean sea greece abstract the relationship between partial melting and granite intrusion in a classic barrovian metamorphic terrane has been assessed thirteen samples were dated by shrimp u pb zircon geochronology from the island of naxos greece one of the aegean metamorphic core complexes the effect of partial melting during peak barrovian metamorphism on naxos is recorded by fine

magnetic anisotropy of some phyllosilicates magnetic susceptibility anisotropy of susceptibility and hysteresis of single microcrystals of chlorite biotite phlogopite muscovite zinnwaldite and fuchsite were measured in low and high magnetic fields with an alternating gradient force magnetometer micromag their properties are sufficient to account for the low field susceptibility ams of most micaceous rocks nearly all samples show some ferromagnetic contribution at low fields due to inclusions of pseudosingle domain and multidomain magnetite the paramagnetic contribution isolated at high fields usually exceeds the ferromagnetic contribution the paramagnetic susceptibility is intrinsic to the silicate lattice and agrees with values predicted from chemical composition within the limits of error the minimum susceptibility is nearly parallel to c another axis is parallel to b and the third susceptibility usually the maximum is close to a the paramagnetic susceptibility has a disk shaped magnitude ellipsoid with strong anisotropy p 2 the ferromagnetic contributions at low fields have more variably shaped ellipsoids with greater eccentricity p 5 the silicate lattice does not constrain their orientation our technique cannot determine the principal axes of the ferromagnetic component however its principal values usually correspond with the paramagnetic principal susceptibilities in order of magnitude thus the combined paramagnetic ferromagnetic anisotropy recognised in routine studies of ams should faithfully represent the petrofabric of most micaceous rocks nevertheless nearly 10 of our samples have incompatible anisotropy ellipsoids for the silicate host and magnetite inclusions these yield a net inverse ams that does not correctly represent the orientation of the silicate lattice therefore some caution is necessary in petrofabric ams studies of micaceous rocks

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments abstract the effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility ams of igneous rocks despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks on this work a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non interactive fractions in a rock results of the model show that a variable proportion of magnetically interactive grains the relative values of the bulk susceptibility degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock ams experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported and supports the model predictions both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains the contribution to the ams of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed

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

spatial distribution of melt bearing structures in anatectic rocks from southern brittany france implications for melt transfer at grain to orogen scale abstract in this study we present quantitative spatial information on the one and two dimensional distribution of inferred melt bearing structures in anatectic supracrustal rocks of the southern brittany migmatite belt south of the transcurrent south armorican shear zone sasz based on these data we infer the mechanism of melt extraction from partially molten crust former melt bearing structures include foliation parallel leucosomes and cross cutting granitic leucosomes that infill inter boudin partitions and extensional shear surfaces as well as discordant dykes of granite petrographic i e mineralogical and microstructural continuity of granite from structure to structure suggests that they once formed a continuous melt bearing network measurements along one dimensional line traverses perpendicular to layering of stromatic migmatite exposed in clean sub horizontal outcrop surfaces provide information about thickness and spacing distributions of foliation parallel leucosomes most leucosome thicknesses fall in the range of 1 10 mm with upper limits around 20 30 mm the number of thicker layers decreases abruptly with increasing thickness which is inconsistent with scale invariance this suggests that leucosome formation was controlled by short range melt movement along grain boundaries to form melt rich layers constrained by pre existing compositional layering spacing distributions also are not scale invariant however the large percentage of leucosomes 40 60 in these line traverses suggests that spacing distributions may be controlled in part by impingement of leucosomes making it difficult to derive genetic information from these data qualitative observation of inferred melt bearing structures in mutually perpendicular two dimensional exposures from the same outcrop reveals anisotropy of the leucosome network related to a well developed sub horizontal quartz feldspar lineation reflecting stretching associated with transcurrent movement along the sasz analysis of these two dimensional distributions using the box counting method corroborates the observed anisotropy but indicates that leucosome morphology and perhaps distribution is not scale invariant the applicability of the box counting method or of fractal analysis to understanding melt movement in migmatites is discussed in light of these results based on the anisotropy of melt bearing structures we infer that melt movement in structures now represented by layer parallel leucosomes was primarily sub horizontal these layers fed steeply dipping structures now represented by cross cutting leucosomes in particular those developed at inter boudin partitions and granite dykes the formation and orientation of these steeply dipping structures was in part controlled by far field stresses related to dextral displacement along the sasz melt extraction is inferred to have occurred along these steeply dipping structures extracted melt accumulated in plutons at higher crustal levels such as the quiberon sarzeau and guerande granites

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

mobility of trace elements and oxygen in zircon during metamorphism consequences for geochemical tracing this study addresses the mobility of the rare earth elements ree o u and pb during the formation of metamorphic zircon by overgrowth and coupled dissolution reprecipitation for this purpose the texture and ree 18o and u pb compositions of zircon crystals from a metabasite an anatectic granite and a metapelite sampled on naxos island greece have been characterised the behaviour of o and ree during the formation of the zircon metamorphic domains shows that two end members can be distinguished i closed or partially closed system behaviour for oxygen and trace elements in zircon formed by coupled dissolution reprecipitation metamorphic domains partially preserve the textural and geochemical characteristics of the precursor zircon chemical or isotopic equilibrium between the metamorphic zircon and the matrix is not completely reached for all the elements such zircons have to be used with caution for geochemical tracing when re equilibration is not complete the u pb system is also not completely reset and the metamorphic domains yield old scattered ages ii open system behaviour for oxygen and trace elements in zircon formed by overgrowth these metamorphic zircons do not show an inherited shape and are devoid of relic magmatic zoning their composition reflects growth in an open system at the scale of the zircon grain with the fluid phase and the surrounding minerals because equilibrium between the metamorphic zircon and the matrix was thus favoured zircon crystallisation can be more confidently related to the p t history and fluid circulation events by geochemical tracing

magnetic fabric interpretation complicated by inclusions in mafic silicates abstract the expected relationships between anisotropy of low field magnetic susceptibility ams and crystal symmetry of mafic silicates are disturbed by the presence of magnetite inclusions measurements of ams anisotropy of anhysteretic remanence aarm and theoretically predicted bulk susceptibilities from chemical composition all favour the exercise of great caution in the interpretation of preferred orientation distributions of silicates from a rock s ams these results pertain mainly to the mafic silicates of lower crustal rocks pyroxene orthopyroxene amphibole and some of their low grade metamorphic alterations serpentine epidote

migmatite structures in the central gneiss complex boca de quadra alaska migmatite structures in the coast plutonic metamorphic complex are well exposed in the inlet of boca de quadra southeast alaska two types of anatectic migmatites are present patch migmatites formed by in situ melting and subsequent crystallization of melt diktyonitic migmatites comprise a discontinuous veined network of leucocratic material in which leucosomes enclose boudins of host rock the margins of these boudins show the development of both melanosomes and shear band fabrics strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20 27 this volume decrease is attributed to melt extraction into the adjacent fracture filling leucosomes thus diktyonitic migmatites formed by shear induced segregation of partial melt whereas in patch migmatites the lack of shear stresses inhibited melt segregation the variable structural style of anatectic migmatites in boca de quadra is not related to host rock composition but may be due to differences in the amount of differential stress during migmatization these in turn may be controlled by host rock strength and or diachroneity of migmatization and deformation determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels