pull apart emplacement of the margeride granitic complex french massif central implications for the late evolution of the variscan orogen a microstructural magnetic fabric and gravity study is performed on the carboniferous margeride granitic complex that crops out in the central part of the variscan french massif central this complex consists of three facies namely a main porphyritic monzogranite a two mica granite and late leucogranite dykes and stocks in spite of local variations the magnetic lineation mainly trends nw se with a shallow plunge throughout the complex the magnetic foliation pattern is more complex with various strikes and generally a moderate dip this fabric pattern complies with the late orogenic nw se regional extensional deformation already recognised in this part of the variscan belt new gravity measurements complete available data and are used to build up a gravity map and a cross section with which four gravity minima are identified three gravity minima are interpreted as extensional fractures consistent with a nw se maximum stretching the last gravity low corresponds to an extensional jog between the two westernmost fractures the structural and gravity features of the complex are used to propose a feeding and emplacement model controlled by the regional late orogenic extensional tectonics the margeride complex is interpreted as a kilometre scale laccolith like pluton emplaced in a transtensional setting controlled by a nw se opening direction such a model strengthens the relationships between pluton emplacement and late orogenic collapse of the variscan belt

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

a 2 5d interpretation of the eastern galicia magnetic anomaly northwestern spain geodynamical implications abstract the eastern galicia magnetic anomaly egma shows intensities up to 190 nt and is one of the major features of the magnetic field in northwestern spain taking into account the seismic refraction and surface geological data a 2 5d model of the magnetic properties of the crust in the area of the egma has been carried out models show that the anomaly can be produced by a middle to lower crustal layer a few kilometers thick with magnetic susceptibility of approximately 0 045 si emplaced with a ramp flat like geometry in higher crustal levels and giving rise to horizontal displacements of about 50 km on the basal eastern galicia thrust this structure is in close agreement with surface geology since the culmination of the lugo dome and the extensional viveiro fault can be interpreted in relation with the displacement along the eastern galicia thrust the southern termination of the egma can be related to the termination of the anomalous layer on a lateral hanging wall ramp of this thrust rocks in the magnetic layer are interpreted as mainly lower crust mafic and or ultramafic rocks in the northern sector at least part of the anomaly must be attributed to ultramafic hercynian intrusives cropping out in the vicinity of the viveiro fault

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

structural pattern and ascent model in the central extremadura batholith hercynian belt spain abstract the central extremadura batholith constitutes an important alignment of plutonic rocks occupying part of the internal zones of the iberian hercynian belt it comprises 13 plutons and numerous minor intrusions and covers a wide compositional range from quartz diorites to alkali feldspar granites structural study of the individual plutons reveals that their internal structural patterns may be correlated correlation between the different structural patterns in the plutons and interpretation of the superimposed deformation in the metasedimentary host rocks indicate that the plutons were emplaced and deformed in an e w dextral intracontinental shear zone developed during the second deformation phase 302 320 ma a qualitative ascent model has been established as follows 1 development of extensional fractures at 45 to the e w dextral shear zone 2 intrusion of granitic magmas along these fractures and 3 rotation of the earlier intrusions in a dextral sense inducing antithetic shear zones n s sinistral and a transverse shortening perpendicular to the major axis of the earlier deformed plutons as well as the forced emplacement of the later plutons

intrusion level of granitic massifs along the hercynian belt balancing the eroded crust abstract hercynian granitoid intrusions form a long 3200 km belt comparable in size to other batholiths in the world six massifs have been selected which encompass cabeza de araya extremadura spain guitiriz galicia spain pontivy and mortagne brittany france la marche massif central france and fichtelgebirge bavaria germany detailed gravity surveys over these massifs and subsequent inversion provide their shape at depth correlation of the deeper zones with internal structures determine the place of the root zones the shape of the massifs is examined along the strike of the chain the emplacement of individual massifs is controlled by local tectonics most granites are not deeply rooted but one massif cabeza de araya spain shows a root zone presently as deep as 14 km most have about half of their volume in the first 3 km below the present surface estimates of the magma volume transferred result in 1500 km 3 issued from one specific feeder yielding a total of 70 000 km 3 of magma intruded all along the chain the depth of emplacement of the granitic massifs does not show any significant trend along the strike of the chain the shallower massifs in the french massif central correspond to more deeply eroded areas in the center of the chain their root zone as well as the change in the dip of the walls are presently observed at depths ranging between 4 and 6 km in hercynian granites both variations are interpreted as being related to the brittle ductile transition at the time of emplacement gross thermal considerations place the transition at its former place during magma emplacement indicating that the upper crust has not been eroded by more than 6 8 km this estimate severely contrasts with models involving a doubled crust

the yarra granite an intradeformational pluton associated with ductile thrusting lachlan fold belt southeastern australia the yarra granite is a deformed s type pluton on the eastern edge of the wyangala batholith intruded into deformed early paleozoic metasedimentary rocks detailed structural studies of the pluton include the character and geometry of the foliation and its history of development in both the pluton and wall rock the relationship of the foliation to the pluton wall rock boundary and microstructural relationships in the contact aureole evaluation of the above and regional data indicate that pluton emplacement is broadly synorogenic in detail however it is post tectonic relative to an early phase and pretectonic relative to later phases of deformation and it may be more accurately classified as intradeformational structures in the wall rock have a complex history which indicates that at least three cycles of transposition of planar elements have occurred giving rise to composite structures often exhibiting largely parallel orientation and comparable morphology these multiple composite structures show a gentle to moderate westerly dip a down dip orientation of stretching lineations and rotated fold axes and consistent west over east asymmetry of various kinematic markers field and laboratory data indicate that transposition occurred in a gentle to moderately dipping zone of ductile thrusting under upper greenschist facies conditions the yarra granite was deformed while it still contained considerable magmatic heat which promoted and facilitated ductile deformation of the wall rocks and pluton during the thrusting event we speculate that movement along the zone of ductile thrusting may have attenuated the pluton at depth and possibly detached it from its source if this is so it implies that the yarra granite and possibly the eastern part and or all of the wyangala batholith may have been brought up from some depth and displaced eastward along the zone of ductile thrusting

ore genesis at the camaqua copper mine a neoproterozoic sediment hosted deposit in southern brazil abstract the camaqua copper mine includes the uruguai and the sao luiz orebodies hosted by sandstone and conglomerate of the neoproterozoic to early paleozoic camaqua basin despite a great controversy regarding the evolution of the camaqua basin it is generally accepted that the basin was initially marine and progressively changed into a definite continental environment the ore consists of massive sulfides in veins pipes and stringers and disseminated sulfides the sulfide paragenesis in the primary ore consists of chalcopyrite bornite chalcocite and pyrite other sulfides such as wittichenite idaite molybdenite and carrollite are minor phases native gold is closely associated with hematite a number of supergene minerals occur in the weathering zone including covellite digenite antlerite chrysocolla brochantite cuprite malachite azurite tenorite and native copper traces of gold and silver have also been found in the oxidized zone close to surface primary ore formation could be the result of multiple mineralization events in a discrete mineralizing event quartz was the first mineral to form followed by pyrite chalcopyrite and bornite carbonate and barite appeared by the end of the process sulfur isotope analyses of pyrite chalcopyrite and bornite show little variation 34 s cdt clustering around 0 indicating a magmatic origin for the sulfur on the other hand 34 s values of 10 30 to 14 24 for barite indicate probable oxidation due to the mixing of magmatic and meteoric so 4 2 rich fluids analyses of pb isotopes in chalcopyrite pyrite and bornite from the camaqua deposit plot conformably with the reference isochron of 592 ma built for regional igneous rocks shrimp u pb zircon analyses specifically the lavras cacapava granite microthermometric measurements in fluid inclusions of quartz veins indicated temperatures varying from 236 to 60 c fluid inclusion in carbonate displayed trapping temperatures between 150 and 50 c whereas the trapping temperatures in barite are below 80 c the fluid salinity varies largely from one mineral to another 2 7 wt eq nacl to near pure water in quartz 14 wt eq nacl in carbonate 8 to 10 wt eq nacl in barite the structural control of mineralization the occurrence of superimposed mineralizing events the temperature of deposition of the early formed ore minerals 330 to 190 c and the sulfur isotope ratios indicate that the camaqua mine may be considered as a hydrothermal type deposit resulting of distal magmatic fluids focused into a relatively oxidized clastic succession

magnetic fabric and 3d geometry of the hombreiro sta eulaliapluton implications for the variscan structures of eastern galicia nw spain abstract in this paper the internal structure and 3d geometry of the hombreiro sta eulalia granitic pluton hercynian belt nwspain are described characterization of the internal structure was performed from surface data based on microstructural analysis and anisotropy of magnetic susceptibility studies the geometrical features of the pluton morphology at depth were derived from gravity data the pluton is made up of paramagnetic granites and leucogranites k values ranging between 1 1 and 13 3 10 5 si with well developed magnetic fabrics anisotropy degree p 1 05 for 78 of the sampling localities the results obtained enable us to depict a flat lying magnetic structure with associated n120 e trending magnetic lineations the gravity survey corroborates the subhorizontal laminar geometry of the pluton which has an estimated thickness of 2 km and an almost planar floor the 3d morphology is elongate at depth parallel to the linear internal fabric this would imply that magma emplacement took place along this preferential direction whereas the deepest zones of the pluton trend wnw ese elongate parallel to regional 1 it is almost rootless we present data showing that the hombreiro sta eulalia pluton emplacement was closely related spatially and chronologically along with other granitoid plutons from the lugo dome with two major structures of eastern galicia the basal shear zone of the mondonedo nappe and the extensional vivero fault

magmatic history and geophysical signature of a post collisional intrusive center emplaced near a crustal scale shear zone the plechý granite pluton moldanubian batholith bohemian massif the plech pluton southwestern bohemian massif represents a late variscan complexly zoned intrusive center emplaced near the crustal scale pfahl shear zone the pluton thus provides an opportunity to examine the interplay among successive emplacement of large magma batches magmatic fabric acquisition and the late variscan stress field associated with strike slip shearing the magmatic history of the pluton started with the emplacement of the porphyritic plech and haidmuhler granites based on gravity and structural data we interpret that the plech and haidmuhler granites were emplaced as a deeply rooted ne sw elongated body its gross shape and internal fabric steep ne sw magmatic foliation may have been controlled by the late variscan stress field the steep magmatic foliation changes into flat lying foliation particularly recorded by ams presumably as a result of divergent flow magnetic lineations correspond to a sub horizontal ne sw finite stretch associated with the divergent flow subsequently the t istolicnik granite characterized by steep margin parallel magmatic foliation was emplaced as a crescent shaped body in the central part of the pluton the otherwise inward younging intrusive sequence was completed by the emplacement of the outermost and the most evolved garnet bearing granite the marginal granite along the southeastern margin of the pluton

granite emplacement in an extensional setting an ams study of the magmatic structures of monte capanne elba italy abstract monte capanne elba is an excellent example of a granite pluton 42 km 2 emplaced 6 m y ago in a well known extensional environment that of the tyrrhenian sea its internal fabric has been studied using the magnitude of magnetic susceptibility and anisotropy measurements at 74 regularly distributed stations susceptibility magnitudes reveal a normal and asymmetrical petrographical zoning in the pluton the magnetic lineations are remarkably well organized mostly of nw se azimuth with slight plunges and always normal to the walls of the late aplitic dykes these lineations are also parallel to the stretching lineations of the immediate country rocks of the pluton in the western and probably deepest part of the pluton a ne sw trending magnetic magmatic lineation is also imprinted the following scenario is proposed for the emplacement of monte capanne during the opening of the tyrrhenian sea an approximately e w trending sinistral transform fault is taken as being responsible for the creation of a pull apart void that permitted upwelling of the magma orientations of the magmatic structures at depth are parallel to the elongation of the feeding zone of this void i e ne sw the very top part of the pluton was sheared parallel to the granite country rock interface i e nw se parallel to the local stretching of the tyrrhenian sea at the time of emplacement

emplacement setting of the granite sheeted pluton of esperanca brasiliano orogen northeastern brazil abstract anisotropy of magnetic susceptibility ams and isotopic u pb sm nd data were combined to study the emplacement setting of the granite sheets that constitute the esperanca pluton in the borborema province northeastern brazil the sheets dip moderately to the se along the contact zone between the paleoproterozoic basement rocks and early neoproterozoic orthogneisses and metasediments granite fabrics were determined mainly using ams in 136 sites distributed within the central and western part of the pluton the sheets normally have susceptibility lower than 0 35 msi but locally where a ti poor magnetite appears with titanite the susceptibility increases up to 5 msi comparison between the silicate fabric and ams showed inconsistencies between the shape of mineral and magnetic ellipsoids despite of their orientations that fit fairly well to each other ams indicated the deformation was partitioned between the lower tonalite syenogranite and upper leucogranite and coarse porphyritic granite sheets in the lower sheets the curvilinear lineation trajectory is attributed to a dominant heterogeneous pure shear event that flattened laterally the still molten tonalite and syenogranite into the regional foliation the associated microstructures are typically magmatic zircon u pb data of the syenogranite yielded a crystallization age of 592 5 ma in the upper sheets the fabric recorded a component of simple shear deformation that displaced the coarse porphyritic granite and the top gneissic host rocks to the southwest microstructures are mostly of post full crystallization type tdm model ages and nd t 0 values indicate that the magma contaminated by partial melting of the regional host rocks sheet propagation at the emplacement level would have exploited the contact zone between crustal blocks of different rheologies when the melt pressures would be able to tensionally fail the anisotropy of the host rocks

opposite p t t paths of hercynian metamorphism between the upper units of the cabo ortegal complex and their substratum northwest of the iberian massif abstract in the cabo ortegal region two metamorphic groups can be distinguished which have followed opposite p t paths in their hercynian metamorphism the upper group comprises the three uppermost units of the allochthonous complex with mafic and related lithologies of cabo ortegal the lower metamorphic group includes the two basal units of the complex and the lithological sequence outcropping in the ollo de sapo anticlinorium the lithologies of the upper group were affected by high p and medium high t eo hercynian metamorphism during a subduction episode related to the onset of the hercynian orogenic cycle as a result of such metamorphism the basic rocks were transformed into b type eclogites and also into high p mafic granulites the advance of the hercynian collision produced the progressive uplift of the subducted lithologies and finally their emplacement as large thrust sheets over the lower metamorphic group this final evolution is clearly reflected in the p t t paths of the high p lithologies which show pronounced decompressions with only minor cooling the evolution of the lower metamorphic group does not reflect high p events the characteristic regime is a barrovian type medium p metamorphism showing p t t paths with a progressive compression during heating at least partially the burial of the rock association was due to the overthrusting of a thick allochthonous pile this is represented by the upper metamorphic group

the guitiriz granite variscan belt of northern spain extension controlled emplacement of magma during tectonic escape abstract the ollo de sapo domain of the northern part of the variscan belt of spain contains precambrian and ordovician metamorphic rocks intruded by the guitiriz granite the domain is bounded by two n s transcurrent shear zones detailed structural and anisotropy of magnetic susceptibility studies of the guitiriz pluton reveal a syntectonic emplacement relative to movement on the two n s trending bounding shear zones characterized by 1 development of n s trending structural and magnetic fabrics 2 concordant structures in granites and country rocks and 3 development of shear zones along the east and west granite margins gravity data show two roots trending 110 e extending down to 4 5 km from an otherwise thin

precise time and conditions of peak taconian granulite facies metamorphism in the southern appalachian orogen u s a with implications for zircon behavior during crustal melting events abstract the time of peak thermal conditions in the granulite facies zone of the southern appalachian orogen corresponds to the time of formation of garnet bearing leucosomes generated by biotite dehydration melting in garnet sillimanite gneisses leucosomes contain unusually abundant zircon occurring as inclusions in euhedral plagioclase phenocrysts and garnet isotope dilution thermal ionization mass spectrometric analysis of 13 elongate euhedral zircon crystals from the leucosome yields a concordant u pb age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9

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

distal magmatic hydrothermal origin for the camaqua cu au ag and santa maria pb zn cu ag deposits southern brazil abstract the camaqua cu au ag and santa maria pb zn cu ag deposits are the largest base metal deposits discovered so far in the sedimentary clastic sequences of the neoproterozoic age camaqua basin the origin of the camaqua santa maria deposits has been the centre of dispute with three alternative genetic hypotheses proposed a syngenetic model a diagenetic model and a magmatic hydrothermal model in detail this mineralization has been suggested to be related to sedimentary clastic diagenetic processes volcanic related processes or deep granitic magmatism reevaluation of previous data and new studies in the area yield the following conclusions 1 mineralization is fracture controlled and magmatic hydrothermal in origin rather than stratiform syngenetic or diagenetic 2 the temperature of deposition of the main ore minerals was 210 to 300 c 3 the 34 s of sulphides of around 0 indicates an external magmatic hydrothermal source of sulphur 4 pb isotope ratios of sulphides indicate that metals were derived at the end of the brasiliano cycle from a large crustal source with very primitive pb and that 5 the age of mineralization is 594 ma as constrained by u pb shrimp sensitive high resolution ion microprobe determinations on zircons of the lavras granite thus the camaqua and santa maria deposits are interpreted to be of magmatic hydrothermal origin with the metals derived from an old crustal basement source during the end of dom feliciano collisional orogeny at 594 ma late in the brasiliano cycle the interpretation above is critical for base metal exploration in the sul riograndense shield previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits however the occurrence of mineralization along fractures within specific wall rocks requires consideration of alternative exploration parameters these include 1 ancient ew and nw trending regional fractures and their intersections which are potential structural sites for base metal mineralization and 2 preferential wall rock sites either rocks with high initial porosity or secondary fracture induced porosity or soluble rocks which are susceptible to replacement processes post collisional plutonism of the dom feliciano orogeny was the most likely heat source and also the source of sulphur and base metals gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry style cu au deposits

granites built by sheeting in a fault stepover the sanabria massifs variscan orogen nw spain the sanabria region central iberian zone variscan belt of spain shows an asymmetric thermal dome marked by migmatites accompanied by the ribadelago and the sotillo plutons these small plutons display pronounced mineralogical variations biotite granodiorites and tonalites prevail and granites and gabbros are common both plutons are confined in the releasing stepover of a transpressional shear zone that strikes 120 e and dips 70 sw most of the igneous rocks form sheets parallel to the shear zone magnetic foliations and lineations in the igneous rocks are parallel respectively to the shear bands and stretching lineations observed in the shear zone the formation of these igneous sheets at high angle to the main axis of the regional field stress is explained by a combination of the fault valve behaviour of the shear zone with the power of melt overpressure to open and ascend through previously formed planar structures like s or c planes

kinematic evolution of the mbakop pan african granitoids western cameroon domain an integrated ams and ebsd approach abstract this study integrates anisotropy of magnetic susceptibility microstructural and crystallographic preferred orientation cpo data from the mbakop granitic pluton mgp pan african age in order to decipher its kinematic evolution the mgp lies close to ne sw branch of central cameroon shear zone ccsz and is emplaced in gneissic basement high mean magnetic susceptibility and presence of multi domain magnetite are recorded quartz cpo measured using electron backscatter diffraction reveals dominance of rhomb prism and prism slip in different samples which is consistent with microstructures developed under upper greenschist amphibolite facies conditions quartz cpo along with other kinematic indicators feldspar porphyroclasts mineral fish indicate non coaxial deformation was important during tectonic evolution of the mgp contrasting sense of shear is recorded implying multi stage mylonitization in the western cameroon domain top towards south sense of shear is related to regional d 2 deformation 613 585 ma while top towards north is related to d 3 585 540 ma the magnetic fabric in mgp records d 3 the obliquity between mean orientation of magnetic foliation n12 e and the ne sw ccsz branch n48 e gives kinematic vorticity number of 0 95 this indicates dominantly simple shear with a minor pure shear component it is concluded that regional transpression was important during mgp emplacement

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

magnetic mineralogy of some granites from the french massif central origin of their low field susceptibility abstract with a view to a better understanding of the origin of the low field susceptibility in the variscan granitoids of the french massif central for which extensive structural mapping using the anisotropy of magnetic susceptibility ams technique is currently applied a detailed study of magnetic carriers has been undertaken origin of the susceptibility has been studied using high field and low temperature measurements in a cryogenic magnetometer together with standard rock magnetic techniques additional determinations of the opaque phases have been performed with the metallographic microscope x ray microdiffraction and electron microprobe techniques in almost all the granite types the magnetic susceptibility is essentially carried by the paramagnetic biotite crystals minor amounts of ubiquitous oxides ilmenite rutile titano haematite do not contribute significantly to the susceptibility this is also the case for rare occurrences of magnetite in some granitoids mainly the cordierite bearing anatexites sulphides are found to give a large contribution magnetic mineralogy identifies a mixture of ferromagnetic pyrrhotite and antiferromagnetic fe9s10 hexagonal pyrrhotite hpo the latter species is identified as forming the larger unaltered pyrrhotite grains the ferromagnetic pyrrhotite is identified to be mainly smythite fe9s11 a species formed by supergenic alteration of monoclinic pyrrhotite located along the basal cleavage planes the microfractures and the grain boundaries of the hpo this study which constitutes the first report of the magnetic properties of smythite in rocks indicates that magnetic carriers derived from low temperature alteration have to be taken into account for purposes of structural studies using ams

mesoscopic and magnetic fabrics in arcuate igneous bodies an example from the mandi karsog pluton himachal lesser himalaya field microstructural and anisotropy of magnetic susceptibility ams data from the palaeozoic mandi karsog pluton in the lesser himalayan region reveal a concordant relationship between fabric of the proterozoic host rock and the granite the pluton displays a prominent arcuate shape on the geological map the margin parallel mesoscopic and magnetic fabrics of the granite and warping of the host rock fabric around the pluton indicate that this regional curvature is either synchronous or pre dates the emplacement of the granite body mesoscopic fabric magnetic fabric and microstructures indicate that the northern part of the pluton preserves its pre himalayan magmatic fabric while the central and southern part shows tectonic fabric related to the tertiary himalayan orogeny the presence of nw se trending aplitic veins within the granite indicates a post emplacement stretching in the ne sw direction shear sense indicators in the mylonites along the margin of the pluton suggest top to the sw shearing related to the himalayan orogeny based on these observations it is envisaged that the extension that gave rise to this rift related magmatism had a ne sw trend that is normal to the trend of the aplite veins subsequently during the himalayan orogeny compression occurred along this same ne sw orientation these findings imply that the regional curvature present in the himachal lesser himalaya is in fact a pre himalayan feature and the pluton has formed by filling a major pre himalayan arcuate extension fracture

modelling the geometry of coulomb thrust wedges internal geometry and detachment of coulomb thrust wedges is examined in a combined gravity and pressure force field under various boundary conditions and coulomb parameters frontally buttressed sand wedges accreted with narrow tapers above soles with negligible friction in these models serial imbricates stepped up in conjugate sets which operated synchronously or serially with increases in sole friction model coulomb wedges accreted in piggyback style with enhanced taper but narrower spacing of imbricates in this mode flat topped box fold anticlines nucleated at the frontal tip of a sole thrust prior to the fore limb failing as a ramp the initial ramp thrusts subsequently rotated and steepened to acquire concave upwards listric geometry when cohesion was a significant component of strength and the sand grains interlocked thoroughly ramp climb during piggyback thrusting occurred through the generation of a stack of backthrusts the taper of cohesionless sand wedges which lack any length scale was consistent with the critical cohesionless coulomb wedge taper in contrast the taper of experimental cohesive sand wedges which are scale dependent was consistent with the theoretical cohesive coulomb narrow taper only when the cohesive length scale was lr sr rar 10 6 this was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

crustal structure of the external variscides in northwest spain from deep seismic reflection profiling abstract recent deep crustal seismic reflection profiling escicantabrica 1 provides a detailed image of the external parts of the variscan belt in northwest spain these seismic data together with surface geological information provide constraints on the crustal structure of the foreland thrust and fold belt cantabrian zone and the transition to the hinterland areas narcea antiform area the image in the cantabrian zone includes strong inclined reflections in the upper 5 s that correspond to mappable paleozoic rocks at the surface which have been deformed by thrusting and folding during the carboniferous some of the major thrust faults imaged show a ramp flat geometry at depth and merge into a highly reflective band between 5 and 6 s that corresponds to the cantabrian zone detachment the precambrian basement is almost transparent underneath the cantabrian zone detachment there are a few subhorizontal discontinuous reflections at 11 s that may correspond to the reflection moho at the transition to the hinterland the seismic profile includes short inclined reflections grouped in several reflective bands in the upper 6 s these correspond to the narcea antiform which comprises at the surface a stack of thrust sheets containing precambrian rocks the narcea antiform is placed above a crustal scale ramp imaged by strong w dipping reflections between 6 and 9 s these reflections merge upwards into the cantabrian zone detachment at 6 s and downwards into a reflective lower crustal zone located between 9 and 12 s these midcrustal reflectors are interpreted as a major variscan shear zone that marks the transition between the thin skinned tectonics of the external areas and the thick skinned tectonics of the hinterland areas the lower crust is highly reflective below the narcea antiform area and reflectivity fades out to the east beneath the foreland thrust and fold belt the different seismic responses at middle and lower crustal levels in the foreland and hinterland areas suggest that variscan tectonics strongly affected deep crustal levels in the hinterland areas whereas they remained undeformed beneath the foreland

granite emplacement during tectonic exhumation the adirondack example sensitive high resolution ion microprobe u pb zircon ages for late to posttectonic leucogranites fix the timing of extensional collapse of a portion of the mesoproterozoic grenville orogen of eastern north america plutons of lyon mountain granite lmg were emplaced within the carthage colton shear zone synchronously with formation of extensional mylonite at 1045 1037 ma leucogranite melts were generated in the hot granulite facies core of the adirondack highlands central granulite terrane that served as the lower plate for down to the northwest extension the lmg suite is associated with hightemperature hydrothermal magnetite deposits in the adirondack highlands and widespread cl 1 co2 hydrothermal alteration of upper plate rocks is localized along the carthage colton shear zone where lmg granites are present the relationships between melt generation granite intrusion high strain rates extensional collapse and hightemperature hydrothermal activity provide a framework for understanding midcrustal processes in modern and ancient orogenic belts

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

anatomy of the mondonedo nappe basal shear zone nw spain this paper analyses a major shear zone from the iberian hercynian belt which forms the basal thrust of the mondonedo nappe the shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle ductile deformation in greenschists facies folds in the shear zone are asymmetric very tight 1c or similar class and frequently developing sheath geometries the sheath folds originated by non coaxial flow superimposed on earlier irregularities the fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non mylonitic tectonites c axis fabrics vary across the shear zone but show a dominant monoclinic symmetry the blastomylonitic rocks include the fabrics representing the highest temperatures the main foliation of the schists results from flattening of an earlier foliation recording occasional microfolds the use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen

emplacement of the aya variscan granitic pluton basque pyrenees in a dextral transcurrent regime inferred from a combined magneto structural and gravimetric study magmatic structure three dimensional shape and emplacement mode of the aya pluton variscan pyrenees have been studied by combining magneto structural and gravimetric studies the aya pluton displays a fabric acquired using the anisotropy of magnetic susceptibility technique comparable to that of several other plutons in the pyrenees with shallow plunging lineations and steep foliations trending e w and ne sw forming an s pattern in map view gravity modelling was used to determine the probable root of the pluton directly below the most basic gabbro dioritic facies this has a funnel shape with an e w trending elliptical horizontal section and a maximum depth of 3 5 km these different data demonstrate that this pluton was synorogenically emplaced during the main d2 variscan tectonic phase in a dextral transcurrent regime

three dimensional shape and emplacement of the cardenchosa deformed pluton variscan orogen southwestern iberian massif the cardenchosa pluton is a lower carboniferous variscan granite located in the southwestern iberian massif it intruded along the contact between the sierra albarrana and azuaga tectonic units to the northwest the pluton connects with the left lateral azuaga fault the pluton appears in the footwall of the low angle normal casa del cafefault which crops out to the west of the granite gravimetric modelling shows the pluton to have a flat bottom at a depth of 2 km strain analysis of post emplacement deformation of the pluton indicates that a the deformation of the pluton accommodates the displacement of the azuaga fault and b the pluton prior to the solid state deformation was a lens shaped laccolith of approximately 10 km diameter and 2 km thickness the cardenchosa pluton was a single pulse of magma trapped in a rheological discontinuity of the upper crust the contact between the sierra albarrana and azuaga units the magma would ascend through dikes since no root has been detected the tectonic scenario during the intrusion was one of regional extension 2000 elsevier science ltd all rights reserved

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

gravimetric and magnetic fabric study of the sintra igneous complex laccolith plug emplacement in the western iberian passive margin the geometry and emplacement of the 96 km2 late cretaceous sintra igneous complex sic ca 80 ma into the west iberian passive margin is presented based on structural data gravimetric modeling and magnetic fabrics a granite laccolith 76 km2 1 km thick according to gravimetric modeling surrounds a suite of gabbro diorite syenite plugs 20 km2 4 km deep and is encircled by cone sheets and radial dykes anisotropy of magnetic susceptibility was interpreted from 54 sites showing fabrics of para and ferro magnetic origin most fabrics can be interpreted to have a magmatic origin according to the scarcity of solid state deformation in most part of the massif magnetic foliations are shallowly dipping in the granite laccolith and contain a sub horizontal ene wsw lineation the gabbro syenite body displays concentric magnetic foliations having variable dips and steeply plunging lineations the sic can be interpreted to be intruded along an nnw sse 200 km long fault perpendicular to the magnetic lineation within the laccolith and was preceded by the intrusion of basic sills and plugs the sic intruded the mesozoic series of the lusitanian basin during the post rift passive margin stage and its geometry was only slightly modified during the paleogene inversion that resulted in thrusting of the northern border of the intrusion over the country rocks

reassessment of copper mineral occurrence through electrical tomography and pseudo 3d modeling in camaqua sedimentary basin southern brazil the reassessment of mineral occurrences is a common and economically rational technical procedure because it involves studies directed to targets defined in previous studies in mineral exploration the targets definition can be based on indications of mineralization on the surface or in the contrast of measurable physical properties using geophysics and intrinsic to certain types of deposits this paper discusses the results by the combination of the use of geophysical methods dc resistivity and induced polarization on detailed study of a copper mineral occurrence from the adaptation of techniques of modeling of mineral deposits in mining the indications of mineralization in the area consist of malachite quartz and barite in veins recognized in volcanic tufts structurally embedded in andesites and metaconglomerates nine parallel lines of electrical tomography in schlumberger arrangement were performed which resulted in 2 d inversion models interpolated in models of blocks maps and pseudo 3 d isosurface models in a mineral deposits modeling program the results allowed the recognition of areas with high chargeability 2 5 mv v and high resistivity 800 m relating to areas with disseminated sulphide ores and silicification according to recognized evidence around the area distribution in regional recognition this is where the pseudo 3 d products enabled an estimate morphology of the deposit and clearly defined targets for sampling by drilling and quantification of results the results allow the recommendation of this procedure in mineral exploration activities supported by the quickness and possible global reduction of project costs with real increase in the chances of incorporation of new deposits this reduces the costs for research development and minimizes the environmental impacts associated with mining as well as the application of these methods for environmental diagnosis

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

supercontinent evolution and the proterozoic metallogeny of south america abstract the cratonic blocks of south america have been accreted from 2 2 to 1 9 ga and all of these blocks have been previously involved in the assembly and breakup of the paleoproterozoic atlantica the mesoproterozoic to neoproterozoic rodinia and the neoproterozoic to phanerozoic west gondwana continents several mineralization phases have sequentially taken place during atlantica evolution involving au u cr w and sn during rodinia assembly and breakup and gondwana formation the crust dominated metallogenic processes have been overriding responsible for several mineral deposits including au pd sn ni cu zn mn fe pb u p 2 o 5 ta w li be and precious stones during rodinia breakup epicontinental carbonate siliciclastic basins were deposited which host important non ferrous base metal deposits of cu co and pb zn ag in africa and south america isotope pb pb analyses of sulfides from the non ferrous deposits unambiguously indicate an upper crustal source for the metals a genetic model for these deposits involves extensional faults driving the circulation of hydrothermal mineralizing fluids from the archean paleoproterozoic basement to the neoproterozoic sedimentary cover these relations demonstrate the individuality of metal associations of every sediment hosted neoproterozoic base metal deposit of west gondwana has been highly influenced by the mineralogical and chemical composition of the underlying igneous and metaigneous rocks

kinematic analysis using ams data from a deformed granitoid abstract this study highlights the usefulness of anisotropy of magnetic susceptibility data from a deformed granitoid in deciphering its kinematic evolution vis a vis shear zone data are presented from the chakradharpur granitoid ckpg that lies to the north of the northerly dipping ene wsw striking singhbhum shear zone ssz eastern india whilst the foliation recorded in the field in some parts of the granitoid is parallel to the ssz the magnetic foliation is n54 e 90 mean orientation it is suggested that the magnetic fabric provides a window into an evolutionary stage prior to the final shearing thrusting event the evidence of which is preserved on the mesoscopic scale it is envisaged that during the initial stages of deformation there was simple shear along the evolving ssz that resulted in sinistral strike slip movement the vorticity axis at this stage was steeply plunging and sense of rotation was anticlockwise space was generated in a direction n25 e perpendicular to maximum instantaneous stretching axis into which ckpg emplaced synchronously with regional deformation and evolving ssz with continued deformation there was thrusting along the ssz the vorticity axis flipped to a sub horizontal orientation thus leading to the development of down dip stretching lineations and sheath folds within the ssz however at the same time the vorticity axis responsible for fabric evolution within the syntectonically crystallizing cooling ckpg was steeply plunging with clockwise rotation the magnetic foliation mean orientation n54 e 90 developed during the final stage of syntectonic crystallization however deformation in the region and thrusting along the ssz continued even after the ckpg had fully crystallized and solidified which led to the development of the ene wsw striking mesoscopic foliation that is parallel with the ssz we propose that the angle between the magnetic foliation and the ssz foliation recorded in the field enables to decipher the kinematic vorticity number of flow responsible for fabric evolution of the ckpg it is concluded that transpression was an important mechanism and during regional deformation whilst the ssz developed structures by dominantly simple shear the ckpg underwent dominantly pure shear

stratiform copper deposits hosted by low energy sediments ii nature of source rocks and composition of metal transporting water several factors suggest that hematite bearing sediments composed principally of alluvial fan sediments were the source of metal transporting water and of metals in stratiform copper deposits hosted by low energy sediments first such sediments are associated with all examples of larger stratiform copper deposits hosted by low energy sediments second a zonal sequence corresponding to an approximate sulfide solubility product increase is developed laterally into the basin from sandy sediments in contact with the hematite bearing sediments and upward from the contact of the ore hosting sediments with the hematite bearing sediments third the zonal sequence is developed basinward from pinchouts and wedge downs of the hematite bearing sediments and copper mineralization is absent from host horizons where hematite bearing sediments are not developed fourth authigenic mineral assemblages with the exception of hematite sulfides and organic carbon are similar in both the host sediments and in the hematite bearing sediments the authigenic and evaporite mineral assemblages and the nature of associated sediments indicate that most occurrences of the hematite bearing sediments were probably deposited in closed continental basins published sedimentological and paleogeographic analyses show that the rotliegende associated with the kupferschiefer hosted deposits was deposited in a closed continental basin no consistent composition of the hematite bearing sediments is evident although basalt flows or basalt derived clastic components occur in the alluvial fan sediments associated with three of the larger stratiform copper deposits authigenic and evaporite minerals and compositions of modern continental closed basin waters show that the waters originally within the closed basins were na cl so 4 types with variable mg ca and so 4 contents in equilibrium with hematite and with a ph constrained either by carbonate minerals atmospheric carbon dioxide or by quartz microcline muscovite chloride concentrations were likely to have been about six times seawater waters with higher mg and lower ca contents were likely to be associated with alluvial fan sediments containing basalt or dolomite derived clastic components water rock ratios in the alluvial fan sediments were probably 33 percent or less if the 50 cm hypothesis i e that chalcocite in stratiform copper deposits hosted by low energy sediments was precipitated as a result of bacterial sulfate reduction within 50 cm of the surface of the host sediment is true then metal transporting waters were being released from the hematite bearing sediments very early in their burial history if so the metal transporting water was the original closed basin water with a temperature of less than 50 degrees c

geochemistry of archean metasedimentary rocks from west greenland abstract archean metasedimentary rocks occur as components of the isua supracrustals akilia association and malene supracrustals of southern west greenland primary structures in these rocks have been destroyed by metamorphism and deformation their chemistry and mineralogy is consistent with a sedimentary origin but other possible parents e g acid volcanics altered pyroclastic rocks cannot be excluded for some of them there is little difference in the composition of metasedimentary rocks from the early archean isua supracrustals and probable correlative akilia association both have a wide range in rare earth element ree patterns with la n yb n ranging from 0 61 5 8 the ree pattern of one akilia sample with low la n yb n compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources other samples with very steep ree patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the isua supracrustals samples with intermediate ree patterns are best explained by mixing of basaltic and felsic end members metasedimentary rocks from the malene supracrustals can be divided into low silica 55 sio2 and high silica 77 sio2 varieties these rocks also show much variation in la n yb n 0 46 14 0 and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks the wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation accordingly these data do not rigorously test models of early archean crustal composition and evolution the systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early archean crust

modification of the regional stress field by magma intrusion and formation of tabular granitic plutons granitic intrusions typically have large aspect ratios with an horizontal major axis about 4 8 times the vertical one combined structural and geophysical gravity data allows distinction between two main types of plutons flat floored plutons the most represented are rather thin 3 4 km and extend in every horizontal direction with a gently dipping floor toward several root zones these contrast with the thick 10 km wedge shaped plutons more elongated in one direction with a few root zones we interpret the shape of flat floored plutons as the result of a switch in the stress pattern caused by the emplacement of magma magma is preferentially emplaced into the plane 1 2 perpendicular to the least principal stress component 3 this plane is initially vertical except for compressional conditions this dilation causes a local re organization of the stress field by increasing the minor and intermediate principal stress components when they overcome the lithostatic load a drastic change in the orientation of the opening plane results switching from vertical to horizontal this constitutes a change from vertically oriented dike shaped intrusions to sub horizontal laccoliths crustal anisotropies vertical faults or horizontal zones of different rheology also contribute to modify the shape of the intrusions the continual exchange of the intermediate stress components also explains the rounded shape of intrusions in strongly extended regions our analysis suggests that the concept of magmas rising to a level of neutral buoyancy is not applicable to many settings rather we suggest that the feedback between displacements resulting from magma intrusion and the local stress pattern controls the geometry of magma emplacement

a non cylindrical model for the northwest iberian allochthonous terranes and their equivalents in the hercynian belt of western europe abstract a case is made for a non cylindrical development of the european hercynian belt far travelled allochthonous terranes in the belt consist of a mixture of units including catazonal units ophiolites and units derived from the continental margin of gondwana they often depict a metamorphic history of intermediate to high pressure type and caledonian to eo hercynian age and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of gondwana considerations upon the disposition of the terranes their relationships with the paleogeography of the gondwana realm and the structures in them suggest that their emplacement took place in an oblique fashion at a small angle to the margin of the continent the proposed allochthon could be related to the development of an island arc or a wedge like subplate in the southern part of the baltica plate later convergence between gondwana and laurussia involving transcurrent mostly dextral movements combined with tectonic escape phenomena would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost europe

diapiric emplacement in the upper crust of a granitic body the la bazana granite sw spain abstract the ascent and emplacement of granites in the upper crust is a major geological phenomenon accomplished by a number of different processes the active processes determine the final geometry of the bodies and in some favourable cases the inverse problem of deducing mechanisms can be undertaken by relying on the geometry of plutons this is the case of the la bazana granitic pluton a small variscan igneous body that intruded cambrian rocks of the ossa morena zone sw iberian massif in the core of a large late upright antiform the granite shows no appreciable solid state deformation but has a late magmatic foliation whose orientation derived from field observations defines a gentle dome the regional attitude of the main foliation in the country rock parallel to the axial plane of recumbent folds is nw se but just around the granite it accommodates to the dome shape of the pluton flattening in the host rock on top of the granite is indicated by boudinaged and folded veins and appears to be caused by an upward pushing of the magma during its emplacement the dome shaped foliation of the granite geometrically and kinematically congruent with the flattening in the host rock can be related in the same way to the upward pushing of the magma the level of final emplacement was deduced from the mineral associations in the thermal aureole to be of 7 10 km in depth models of the gravity anomaly related to the granite body show that the granite has a teardrop pipe shape enlarged at its top diapiric ascent of the magma through the lower middle crust is inferred until reaching a high viscous level where final emplacement accompanied by lateral expansion and vertical flattening took place this natural example suggests that diapirism may be a viable mechanism for migration and emplacement of magmas at least up to 7 10 km in depth and it provides natural evidence for theoretical discussion on the ability of magmatic diapirs to pierce the crust

on the role of diapirism in the segregation ascent and final emplacement of granitoid magmas only bodies of magma with a high crystal content and partially molten crustal country rocks can ascend as diapirs once such an envelope is pierced the diapiric ascent of the pluton is arrested by the high viscosity of a solid aureole deformation by shortening of the carapace of these bodies may lead to the expulsion of a magma with a relatively low crystal content which may then continue ascent via fractures and dykes the details of the mechanisms of granitoid magma segregation are still unknown but it appears that many magmas hegin their ascent through the crust as mushes with at least 50 melt and that such magmas are rheologically able to ascend through a thickness of crust this ascent mechanism explains the dearth of structures attributable to the ascent of granitoids in contrast to the abundance of structures that developed during their final emplacement when a magma becomes too crystalline melt 25 to continue its ascent via dykes it is immobilised at approximately this stage a hydrous magma may become saturated with water and release fluids into the aureole making it particularly susceptible to deformation magma that continues to arrive at this level is also immobilised and the pluton grows as a ballooning diapir these characteristically deform themselves and their aureoles by bulk shortening magmas that are able to ascend to shallow depths largely by virtue of lower water contents and higher initial temperatures tend to become finally accommodated by such brittle processes as stoping and cauldron subsidence high level intrusions lend to be tabular are also fed by dykes or conduits and assemble in tabular batholiths

structural evidence for the relationship between thrusts extensional faults and granite intrusions in the variscan belt of galicia spain abstract new structural and petrofabric data are presented from the hombreiro pluton a synkinematically emplaced granite located in the northern part of the variscan belt of spain this pluton shows the imprint of two plastic deformations the first is related to the motion of the mondonedo nappe with a top to the east shear sense and the later one to the vivero fault an extensional shear zone with a top to the west motion these deformations initiated at high temperatures close to the granite solidus as shown by the preservation of c slip fabrics in quartz this fact indicates that extensional faulting was activated soon after the thrusting of the mondonedo nappe during which the hombreiro granite was emplaced we propose that the position of the vivero fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault

structural metamorphic and magmatic history of the mondonedo nappe hercynian belt nw spain abstract the mondonedo nappe is one of the main units in the internal zone of the hercynian belt in the iberian peninsula it consists of low to medium grade metasediments late precambrian and palaeozoic in age folded by large d 1 e verging recumbent structures and thrusted several tens of km towards the east during a d 2 deformation episode at the base of the thrust sheet a shear zone of up to 3 km thick was developed granitoid bodies which were intruded after d 1 and before or during d 2 were deformed in this basal zone the geometry of d 1 and d 2 structures and that of the isograds are described as well as the relationships between metamorphism and deformation a structural evolution is inferred which shows an episode of generalized ductile deformation d 1 followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone d 2 and culminated with the thrusting along an important fracture the shear zone originated in a thermally softened level which according to metamorphic data was at temperatures in the range 550 600 c the structural metamorphic and magmatic features of the mondonedo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs this could be a consequence of the collision against another continental mass to the west

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

anisotropy of magnetic susceptibility and magmatic structures in the guerande granite massif france a multidisciplinary approach has been used in order to map out the syntectonic magmatic structures of the guerande leucogranite in southern brittany western france on one hand extensive field work and microscopic observations and on the other a detailed magnetofabric investigation have been carried out the former showed a magmatic flow plane trending generally to the ene wsw with a moderate dip to the north and a flow direction slightly dipping toward nne these observations suggest a blade shaped body which is not deeply rooted the magnetofabric as defined by the anisotropy of magnetic susceptibility is mainly related to hematite ilmenite grains with a mean rock susceptibility of about 3 10 6 g oe there is an excellent agreement between the magnetic lineation kmax and the magmatic flow direction the agreement between the magnetic foliation kmax kint plane and the magmatic flow plane is not as good the quantitative analyses of the anisotropy allows one to group the data into two domains the one with planolinear ellipsoids corresponding to the central part of the granitic body where flow was mainly achieved on the viscous state and the other in the flattening domain related to the northern border area which exhibits incipient orthogneissification that is some deformation on the solid state such a favorable comparison with the classical structural method is encouraging for using the anisotropy of magnetic susceptibility technique more routinely for structural mapping of granitic bodies provided that the microstructural state of the rock is already well known

pluton emplacement along an active ductile thrust zone piute mountains southeastern california interaction between deformational and solidification processes plutons that are emplaced synchronously with regional deformation offer the opportunity to understand the partitioning of deformation and metamorphism in time and space during orogeny the 85 ma east piute pluton of the eastern mojave desert is an unusually clear example of a syntectonic pluton it was emplaced during thrusting in the southern cordillera combined petrologic and structutal studies lead to empirical models for the dynamic interaction of magmatic and deformational processes during granitoid emplacement and final crystallization trace and major element and isotopic data suggest that closed system differentiation of a single magma batch was responsible for compositional variation granodiorite to leucocratic granite correlation between degree of chemical evolution of compositional units and intrusive sequence suggests that discrete melt segregation events punctuated fractional crystallization modeling of trace element data suggests segregation of monzogranite melts after 50 70 crystallization of the magma and segregation of leucocratic granite after about 80 crystallization deformation promoted segregation of melt fractions at discrete times during crystallization during final ascent and after the magma had crystallized sufficiently 50 70 to attain shear strength monzogranite liquid was incompletely segregated along the active thrust zone at the southern margin of the pluton at essentially the present crustal level and after about 80 crystallization the remaining leucocratic granite melts were more effectively segregated and migrated into thrust related extensional openings including tension gashes undeformed pegmatite and aplite dike arrays indicate that final crystallization coincided with final increments of shear zone movement solid state deformational fabrics and brittle synthetic thrusts that offset the pluton margin record sharp temperature decline during and after crystallization because plutons crystallize rapidly they can be used to estimate strain rates during orogeny thermal modeling suggests that the transition from rheological liquid to solid in the east piute pluton occurred during an interval of about 10 4 yr as final increments of ductile deformation coincided with final crystallization of evolved compositional units total strain recorded by fabric in the pluton apparently accumulated during this interval shear strains estimated from the s c fabric in the granodiorite unit and rotation of pegmatite filled vein arrays are on the order of 1 thus strain rates in the shear zone were on the order of 10 12 sec during crystallization of the pluton this corresponds to displacements of tens of centimeters per year these high strain rates suggest punctuated melt enhanced movement episodes during contractional orogeny

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

coalescence of lateral spreading magma ascending through dykes a mechanism to form a granite canopy el hongo pluton sierras pampeanas argentina this study deals with the structure of the el hongo trondhjemite a ene wsw elongate pluton belonging to the famatinian magmatic arc that developed from late cambrian to silurian times in the sierras pampeanas argentina anisotropy of magnetic susceptibility ams measurements combined with structural and microstructural data permit the correlation of magmatic and magnetic fabrics within this ferromagnetic pluton magnetic foliations strike north south and dip steeply west but in the marginal facies the foliation becomes parallel to the ene wsw pluton contacts magnetic lineations mostly trend north south and plunge se gravity data reveal that the pluton is in the form of a very thin horizontal sheet resting on two north south trending root zones detected below the central portion of the pluton these root zones represent feeder dykes that exploited tension fractures the emplacement of the el hongo pluton took place during the action of a buried sinistral shear zone parallel to the elongation of the pluton corresponding to a transfer fault associated with the late extensional exhumation of the famatinian domain the emplacement of other famatinian granites of the sierras pampeanas could be related to similar late shear zones

a new mechanism of granite emplacement intrusion in active extensional shear zones one of the principal ways in which continental crust grows is by the incorporation from deeper within the earth of large volumes of granitic magma a space problem exists1 as to how these magmas are accommodated in the crust traditionally two main emplacement mechanisms have been emphasized forceful intrusion whereby buoyancy driven magmas physically push the crust aside creating granitic diapirs and balloons and passive emplacement characterized by replacive mechanisms such as cauldron subsidence and stoping although more recent work2 3 has demonstrated that space for granites may be created within bends and offsets of large transcurrent faults the simple view of either forceful or passive still fails to account for the intrusive mechanisms of many granites here we report the discovery of a new igneous intrusion mechanism in the superbly exposed proterozoic continental crust of south greenland we have observed that rapakivi granite was intruded as large scale sheets along ductile extensional shear zones that were active during emplacement in such a process the space problem seems to be simply resolved

geophysical and metalogenetic modelling of the copper occurrence in camaqua sedimentary basin brazilian southern the geophysics application is indicated procedure in mineral exploration activities by the quickness and possible global reduction of project cost this work presents the results of the dc resistivity and induced polarization methods joint application integrated to geological and structural data from a potentially mineralized area with copper sulfides located in the northern border of the camaqua sedimentary basin brazil southern the mineral occurrence is characterized by malachite and azurite impregnation on silicified neoproterozoic metarenite outcrop of passo da promessa formation marica group the geophysics acquisition consists in six electrical tomography lines in schlumberger arrangement with 440 m each the lines were separated on two groups disposed at orthogonal form where the center area is located the mineral occurrence in surface the results indicate a low resistivity and high chargeability zone 60 m deep related to a sulfided zone and surrounded by elevated resistivity which point silicification areas the structural analysis has revealed two main fracture systems in the crystalline basement ne sw and nw se one in the metarenite ne sw and fold with a n20 axis the 3d visualization model reveals that the mineralization is oriented in the n125 direction nw se the nw se fracture family is secondary but is present in the occurrence area and coincident with the relaxing direction of the folding event this context corroborates with the hypothesis of filling nw se fractures result which indicates the relevance of this preferential direction in future mineral research campaigns in the camaqua basin