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

the maladeta granite polydiapir spanish pyrenees a detailed magnetostructural study abstract the maladeta granitic complex spanish central pyrenees has been subjected to a detailed magnetostructural study through the systematic measurement of magnetic susceptibility and anisotropy of magnetic susceptibility at 253 regularly spaced sampling sites due to the dominant paramagnetic property of the granites a first order correlation can be derived between the susceptibility magnitude and the bulk rock iron content hence the petrographic nature of the rock the maladeta complex is shown to be composed of four principal juxtaposed plutons each one being normally zoned i e becoming more basic towards the periphery the largest pluton namely the colomers unit is itself made of a number of sub plutons due to distinct but more or less coeval batches of magmas the various magnetic data complemented by field and microstructural observations allow us to propose that the maladeta polydiapir probably exploited ne and se trending openings of the brittle crust for its emplacement that accompanied or just preceded a sw verging compressive event a later n s compression marked by inverse dextral and e w trending shears is preferably attributed to late variscan rather than to alpine tectonics

magnetic fabric of the barrington passage pluton meguma terrane nova scotia a two stage fabric history of syntectonic emplacement abstract structural analysis and the anisotropy of magnetic susceptibility ams are used to document the internal magmatic to high temperature subsolidus fabrics of the tonalitic late devonian barrington passage pluton the ams fabric orientations compare favorably with mesoscopic scale fabrics in general the k1 k2 principal plane of the ams is close to the observed mineral foliation defined by biotite and plagioclase and the k1 axis is aligned parallel to a mineral lineation defined by the same minerals and to a locally developed weak extension lineation defined by quartz measurement of the anisotropy of anhysteretic remanence aar and image analysis of biotite preferred orientations confirm that the orientation of the ams is principally controlled by biotite locally anomalous ams orientations are attributed to the influence of fine grained magnetite crystals the presence of which is revealed by measurements of hysteresis and coercivity properties a two stage fabric history is revealed first horizontal fabrics were formed foliation compositional banding during emplacement of a laccolith like pluton the second stage involved synmagmatic folding of the horizontal foliation and banding in response to regional contraction associated with the acadian orogeny with development of a lineation that records stretching parallel to the fold axis the results demonstrate that the barrington passage pluton was emplaced during the main phase of the acadian orogeny a model is proposed that depicts the barrington passage pluton as the folded base of a larger syntectonic pluton that is now largely eroded off

internal structure and emplacement of the papoose flat pluton an integrated structural petrographic and magnetic susceptibility study the papoose flat pluton of eastern california is commonly cited in the geologic literature as a classic example of a forcibly emplaced pluton although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial reexamination of this late cretaceous 83 ma pluton using a combination of new structural petrographic microstructural and anisotropy of magnetic susceptibility ams data on the pluton wall rock system has shown that the pluton is an inclined and internally zoned tabular structure assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12 15 km initial pluton formation involved magma ascent in a vertical west northwest striking feeder dike which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying cambrian metasedimentary rocks leading to formation of a southwest dipping sill subsequent sill inflation accompanied by horizontal infilling from the feeder dike at the base of the sill resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof facilitated by thermal weakening as the wall rock temperatures progressively rose during emplacement of successive magma pulses cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton we have been unable to document any regional scale structures e g of equivalence to similar age synplutonic strike slip shear zones in the sierra nevada batholith to the west that may have controlled emplacement of the papoose flat pluton however this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time indeed tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level simple thermal modeling using microstructural and thermobarometric data indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional scale deformation

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

the granite pluton of zahedan se iran a petrological and magnetic fabric study of a syntectonic sill emplaced in a transtensional setting abstract the 32 ma nw se elongate 75 km long granite pluton of zahedan is one of the most conspicuous geological features of southeast iran it is composed of metaluminous to weakly peraluminous rocks corresponding to magnetite and ilmenite bearing i type granitoids ranging from diorites to granites all rocks including the late andesitic to dacitic dykes belong to the high k calk alkaline series field and structural relationships as well as evidence for mixing mingling can be used to unravel the magmatic history of the different rock types microstructural examinations and magnetic fabric measurements were performed on a collection of oriented specimens from the northern part of the pluton ubiquitous flat lying foliations argue in favour of a sill like body except for the diorite the biotite bearing granitic facies at the pluton s periphery was the first batch of magma which initiated the sill this sill probably behaved as a n s directed decollement zone marked by conspicuous magmatic cataclastic microstructures and n s treading lineations the intrusion of the hornblende bearing granodiorite with typical magmatic structures inflated the sill deformation occurred at the same time with e w to nw se stretching marked by lineations the main dioritic stock forming the floor of the sill is considered to be a remnant of the feeder zone an abundant dyke swarm marks the end of the syntectonic emplacement of the zahedan sill a three dimentional model of emplacement is proposed in the context of the latest episodes in the closure of the neo tethys during which the lut block ended its northward migration with respect to the afghan blocks

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

c slip in quartz from subsolidus deformed granite abstract the slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions 700 800 c of deformation is studied by lattice preferred orientation lpo and microstructural techniques lpos with a point maximum of c axes parallel to the shear direction which is close to the stretching lineation are presented and interpreted with the aid of light and electron microscopy in terms of c as the dominant slip direction during plastic deformation microscopy of such samples reveals the presence of 0001 tilt boundaries and free 0001 burgers vector dislocations in edge orientations a change in lpo in a decreasing temperature environment from c axes parallel to the shear direction to c axes normal to the shear direction documents a fabric transition from the rare c slip to the more common a slip fabric it is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant c slip during deformation at geological strain rates

new geological geochronological and geochemical investigations on the khoy ophiolites and related formations nw iran abstract this paper gives a detailed geological description of the region of khoy nw iran and its ophiolites and presents a new geological map the main conclusion is that there are not one but two ophiolitic complexes in the khoy area 1 an old poly metamorphic ophiolite tectonically included within a metamorphic subduction complex whose oldest metamorphic amphiboles yield a lower jurassic apparent 40kn 40ar age and whose primary magmatic age should logically be pre jurassic 2 a younger non metamorphic ophiolite of upper cretaceous age overlain by a turbiditic flysch like volcanogenic series of upper cretaceous lower paleocene age this latter ophiolite was created at a slow spreading oceanic center according to the lherzolitic mantle sequence the small volume of gabbroic rocks the absence of a diabasic sheeted dike complex and the abundant phyric basalts in the extrusive sequence a scenario for the geodynamic evolution of the khoy oceanic basin is proposed in conclusion

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

relative rheological evolution of chemically contrasted coeval magmas example of the tichka plutonic complex morocco the tichka plutonic complex presents many unusual structures related to physical interaction between mafic and felsic coeval magmas in this paper a rheological approach is used to attempt to understand the various observed structures after a general presentation of rheological laws and a discussion about the present state of our understanding of the rheology of suspensions we analyse the main steps of rheological evolution of magmas during crystallization taking into account the effect of cooling increase of silica content and increase of crystallinity on viscosity during thermal evolution of magma a general rheological diagram is presented two main rheological thresholds at about 35 crystallinity th1 and about 65 70 th2 allow us to distinguish three major steps in the rheological behaviour of magmas during cooling comparative study of evolution curves for viscosity of coeval mafic m and felsic f magmas shows that an inversion temperature ti exists at which viscosities of both magmas are the same this point has major petrologic significance because the viscosity ratio m f inverses during cooling when the temperature of magma falls below the ti point a model for the evolution of the tichka magmas during cooling has been derived from the knowledge of the chemical and mineralogical composition of the different rocks representing the original m and f magmas as well as some hybrid and differentiated samples viscosities and crystal fractions of magmas as functions of temperature have been calculated from chemical and mineralogical data curves representing the evolution of apparent viscosity of the three initial magmas of the complex are presented from these vicosity temperature curves and from the calculted temperatures corresponding to rheological thresholds th1 and th2 for the different coeval magmas five main evolution stages are defined the different structures observed at the contracts between m and f magmas are developed throughout the entire cooling history of the pluton each structure being related to one of the main evolution stages

the pan african toro complex northern nigeria magmatic interactions and structures in a bimodal intrusion the toro complex is one of the pan african older granites of nigeria first described as a reversely zoned pluton made of a central dioritic mass surrounded by a broad granitic rim it has been thoroughly reinvestigated both from the petrographic and structural points of view with the help of systematic anisotropy of magnetic susceptibility ams measurements the granite main body is a hornblende biotite porphyritic monzogranite characterized by an early submagmatic fabric displaying a concentric pattern of foliations and west plunging lineations stage 1 this fabric is overprinted by a later one due to solid state strain along north south subvertical dextral shear zones stage 2 in the vicinity of the diorite an evengrained granite displays magmatic structures that are contemporaneous with this strike slip event the diorite granite contact is a complex zone where field petrographic and geochemical data enable recognition of the effects of mixing and mingling between a mafic and a felsic magma tonalites cropping out within this contact zone are interpreted as hybrid rocks the reverse zonation of the diorite itself is also the result of some hybridization process magmatic interactions mainly resulted from in situ infiltration of granitic liquid into the dioritic mass the detailed history of this bimodal intrusion began with the emplacement of the granitic magma acquiring a first stage fabric before full crystallization of the granitic core intrusion of the dioritic magma permitted reheating of the granitic magma that then crystallized with specific structural characters the second stage structures whether characterized by magmatic fabric near the diorite or by solid state strain features in north south shear zones elsewhere in the granite are related to late pan african dextral strike slip tectonics in the basement of northern nigeria the bimodal toro complex is therefore considered as a late pan african syntectonic pluton

magnetite grain shape fabric and distribution anisotropy vs rock magnetic fabric a three dimensional case study abstract shape fabric and distribution anisotropy of magnetite grains in a highly ferromagnetic s l syenite are determined for three mutually perpendicular planes on about 600 magnetite grains the grains are lenticular with an average aspect ratio of about 2 0 their preferred orientation determined by the inertia tensor method is consistent with the principal directions of the magnetic susceptibility ellipsoid and the axial ratios of the magnetic fabric ellipses are very close to those of the shape fabric ellipses e g 1 41 and 1 39 in the section plane perpendicular to the magnetic foliation and parallel to the magnetic lineation the anisotropic wavelet transform can detect and quantify the anisotropic distribution of the magnetite grains which are located interstitially along the feldspar grain boundaries this anisotropic distribution has no noticeable effect on the rock s anisotropy of magnetic susceptibility ams since only a few percent of grains are close enough to interact magnetically therefore it is realistic to consider that the ams of ferromagnetic granitic rocks originates mainly from the shape fabric of anisotropic magnetite grains and a close correlation between the magnetic fabric and the magnetite grain shape fabric is expected

plate tectonic evolution of the southern margin of eurasia in the mesozoic and cenozoic abstract thirteen time interval maps were constructed which depict the triassic to neogene plate tectonic configuration paleogeography and general lithofacies of the southern margin of eurasia the aim of this paper is to provide an outline of the geodynamic evolution and position of the major tectonic elements of the area within a global framework the hercynian orogeny was completed by the collision of gondwana and laurussia whereas the tethys ocean formed the embayment between the eurasian and gondwanian branches of pangea during late triassic early jurassic times several microplates were sutured to the eurasian margin closing the paleotethys ocean a jurassic cretaceous north dipping subduction boundary was developed along this new continental margin south of the pontides transcaucasus and iranian plates the subduction zone trench pulling effect caused rifting creating the back arc basin of the greater caucasus proto south caspian sea which achieved its maximum width during the late cretaceous in the western tethys separation of eurasia from gondwana resulted in the formation of the ligurian penninic pieniny magura ocean alpine tethys as an extension of middle atlantic system and a part of the pangean breakup tectonic system during late jurassic early cretaceous times the outer carpathian rift developed the opening of the western black sea occurred by rifting and drifting of the western central pontides away from the moesian and scythian platforms of eurasia during the early cretaceous cenomanian the latest cretaceous paleogene was the time of the closure of the ligurian pieniny ocean adria alcapa terranes continued their northward movement during eocene early miocene times their oblique collision with the north european plate led to the development of the accretionary wedge of the outer carpathians and its foreland basin the formation of the west carpathian thrusts was completed by the miocene the thrust front was still propagating eastwards in the eastern carpathians during the late cretaceous the lesser caucasus sanandaj sirjan and makran plates were sutured to the iranian afghanistan plates in the caucasus caspian sea area a north dipping subduction zone jumped during paleogene to the scythian turan platform the shatski terrane moved northward closing the greater caucasus basin and opening the eastern black sea the south caspian underwent reorganization during oligocene neogene times the southwestern part of the south caspian basin was reopened while the northwestern part was gradually reduced in size the collision of india and the lut plate with eurasia caused the deformation of central asia and created a system of nw se wrench faults the remnants of jurassic cretaceous back arc systems oceanic and attenuated crust as well as tertiary oceanic and attenuated crust were locked between adjacent continental plates and orogenic systems

melt topology and seismic anisotropy in mantle peridotites of the oman ophiolite this paper presents shape measurements of plagioclase and clinopyroxene inclusions assumed to reflect melt topology in peridotites of the uppermost mantle section of the oman ophiolite the plagioclase and clinopyroxene grains are devoid of any intracrystalline deformation in all samples in contrast the olivine in these rocks has recorded a high temperature plastic deformation with different strengths of the crystallographic preferred orientation cpo of the olivine grains individual melt pockets are first described by ellipses in two dimensions they are more elongated when they have a larger area and they are preferentially oriented parallel to the lineation the x structural axis of the sample with a better defined preferred orientation for samples that have a stronger cpo in a second step an average melt phase shape is defined in three dimensions for each sample using the image autocorrelation technique the average shape is nearly spherical for the samples with weak cpos and it is ellipsoidal with a long axis parallel to x and the short axis parallel to z normal to the foliation for samples with strong cpos the long axis of the ellipsoid is 3 times as long as the short axis for the sample with the strongest cpo we use an anisotropic differential effective medium method to estimate the seismic properties of partially molten upper mantle peridotites the melt pockets were modelled as basalt filled inclusions with the average shape and orientation given by the image analysis the cpo of the olivine crystals was used to calculate the elastic properties of the anisotropic background medium the calculated p wave seismic anisotropies ranged from 5 to 15 with the anisotropy increasing with the cpo strength and melt fraction the maximum p wave velocities are found along x with velocities above 8 km s at 0 melt and an average 0 5 km s reduction for 10 of melt the minimum p wave velocities are found along z with velocities generally below 7 5 km s at 0 melt and an average reduction of 0 8 km s for 10 of melt 1998 elsevier science b v all rights reserved

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

oblique magmatic structures of two epizonal granite plutons hoggar algeria late orogenic emplacement in a transcurrent orogen abstract two granite plutons in the pan african belt of hoggar algeria are studied using the anisotropy of magnetic susceptibility technique ams at low field together with a study of microstructures the tesnou complex is made of coalescent plutons at the margins of which dextral n s oriented shearing imprints solid state textures in the granite microstructures and low magnetic anisotropy magnitudes indicate a magmatic state weak deformation consistent nw se trending sub horizontal magnetic lineations making an overall oblique pattern with respect to bordering shear faults is attributed to low n s trending dextral shear strain magnitudes

syntectonic granite emplacement at different structural levels the closepet granite south india abstract the closepet granite in south india is a large 400 km long but only 30 km wide elongate late archaean granitic body structural levels from deep crust to upper levels crop out as evidenced by a shallowing of paleo depths from north to south all along the closepet granite this allows the study of the emplacement of the same granitic body at various crustal levels four zones have been identified a root zone where magmas are collected in active shear zones a transfer zone featuring large scale magma ascent and crystal liquid partitioning in the granitic mush a gap where the mush was filtered allowing only the liquids to rise shallow intrusions filled with this liquid the closepet granite was emplaced syntectonically field work and anisotropy of magnetic susceptibility allowed documentation of steep foliations with subhorizontal lineations both in the root and transfer zones and in the shallow intrusions remote sensing evidenced a network of shear zones bounding the closepet granite in the porphyritic root and transfer zones magmas cooled slowly thus developing strong fabrics during large scale dextral shearing ascent of residual liquids amidst the crystallizing solid framework was not recorded in the fabrics however these liquids were channelised through the gap and infilled the homogeneous shallow intrusions where rapid cooling only permitted the development of feint although wholly consistent fabrics

internal fabric and strike slip emplacement of the pan african granite of solli hills northern nigeria the solli hills pluton 250 km2 in northern nigeria is an example of a pan african granite emplaced circa 580 ma ago in the jos adamawa block its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the pan african orogeny in this block we used field observations and measurements petrographic examinations and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations most samples are ferromagnetic i e belong to the magnetite series granites and present predominantly magmatic microstructures particularly remarkable are the asymmetrical zoning of the petrographic types the susceptibility magnitudes and the planar linear magnetic fabrics the magnetic foliations and lineations are very consistent in the pluton around n 20 e 60 w and n 205 e 10 respectively field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country rock envelope during a dextral strike slip synplutonic episode it is suggested that the solli hills pluton was emplaced in a late pan african dextral tear fault system the striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model

the tourmaline bearing granite pluton of bodmin cornwall uk magnetic fabric study and regional inference the magnetic fabrics and microstructures of the variscan granite pluton of bodmin cornwall have been studied its low susceptibility magnitude consistent with its lack of magnetite comes from biotite and tourmaline as the magneto crystalline behaviour of tourmaline is inverse compared with that of the phyllosilicates the magnetic fabric was remeasured after heating the specimens to enhance the biotite magnetic signal by the growth of mimetic magnetite the roughly similar magnetic fabrics before and after heating demonstrate that the effect of tourmaline is not important as long as the tourmaline content does not exceed that of the phyllosilicates regionally this study reveals a well defined nnw trend of the magnetic lineations with opposite plunges on each side of a separation line crossing the pluton centre this pattern very similar to that documented in the nearby carnmenellis pluton is ascribed to unroofing during emplacement it agrees with the late to post magmatic nw stretch that characterizes the devonian to carboniferous formations located to the north of this region the thrust and fold structure of which is attributed to southward compression we conclude that syntectonic emplacement may have characterized the whole cornubian batholith at the very end of the variscan orogeny in cornwall

magnetic fabrics and microstructures of the jurassic shah kuh granite pluton lut block eastern iran and geodynamic inference abstract the jurassic shah kuh granite pluton was emplaced in the northeastern part of the lut block eastern iran while this block was a part of the active margin under which the tethys ocean that separated arabia from central iran was subducting since this time the lut block has rotated and migrated northward up to its present position this structural study of the shah kuh aims at strengthening the evidence of its original geodynamical location thanks to a systematic collection of oriented samples in the field well defined fabric and microstructural patterns were deduced from magnetic fabric measurements and optical microscopy observations the overall magnetic fabric of the pluton yields vertical north south striking foliations and shallow lineations plunging to the north the corresponding microstructures attest to their formation in the magmatic stage i e during emplacement subsequent deformation characterized by sub mylonitic microstructures has modified the original fabric into a northwest and locally a west northwest trend inside a two kilometre wide corridor this corridor reflects the trace of a sinistral shear zone that offsets the previous north south magmatic pattern of the pluton the latter magmatic pattern is proposed to result from the shear component of strain that was parallel to the ancient active margin during pluton emplacement as a result of strain partitioning a situation frequently documented at convergent margins sinistral shear along an e w directed south facing active margin assuming a northeastward slip vector for the tethys ocean during the jurassic and accepting a 150 counterclockwise rotation of the lut block after emplacement of the shah kuh pluton best explains both the magmatic and the solid state lineation patterns

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

the urumieh plutonic complex nw iran record of the geodynamic evolution of the sanandaj sirjan zone during cretaceous times part ii magnetic fabrics and plate tectonic reconstruction abstract the urumieh complex to the north of the sanandaj sirjan zone nw iran belongs to a plutonic arc that took place above the northeastward dipping subduction of arabia under iran during late cretaceous times seven granitoid bodies occupying an area of 300 km 2 can be sorted into three suites according to the isotope chronology study of ghalamghash et al ghalamghash j nedelec a bellon h vousoughi abedini m bouchez j l in press the urumieh plutonic complex a magmatic record of the geodynamic evolution of the sanandaj sirjan zone nw iran during cretaceous times part ii petrogenesis and 40k 40ar dating journal of asian earth sciences the two first suites were emplaced during the same event at 100 ma and the third one was emplaced 20 ma later 1 the diorites form the largest bodies and comprise the ghamishlu and dourbeh stocks 2 the biotite granites are composed by the sehkani nari and doustak bodies and 3 the younger bodies are represented by the bardkish syenite and the dourbeh granite these bodies were subjected to systematic microstructural observations and magnetic fabric measurements that yield information about their emplacement kinematics the magnetic lineations of the diorites and biotite granites the early suites call for a dominant nw trending stretching during their intrusion attributed to the transpressive deformation of the overriding sanandaj sirjan microplate during the north to northeastward motion of the subducting western branch of the neo tethys oblique plate motion with 20 of strain partitioning along a nnw trending plate boundary accounts for the observed magmatic structures intrusion of the younger bodies took place after consumption of this western oceanic domain at about 80 ma the nw trending lineations of the syenite suggest that the transpressive regime was continuing while the steep lineations and the peculiar microstructures of the dourbeh granite call for a forceful intrusion our study suggests that the motion of arabia with respect to central iran was more northerly directed than estimated before for the 100 80 ma time interval during which plate tectonic markers are not available

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

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

magnetic fabric and rheology of co mingled magmas in the nasavrky plutonic complex e bohemia implications for intrusive strain regime and emplacement mechanism abstract the fabrics of mafic microgranular enclaves mme and of the host granodiorite of the old granodiorite intrusion of the nasavrky plutonic complex e bohemia were investigated by means of magnetic anisotropy the magnetic fabric in mme is oriented coaxially with the magnetic fabric of the host granodiorite which is undoubtedly magmatic intrusive in origin consequently the magnetic fabric in mme originated during the same process as the magnetic fabric in granodiorite i e during the granodiorite emplacement in addition also the degree of anisotropy and the shapes of susceptibility ellipsoids are very similar in mme and in granodiorite which probably indicates that the mme had probably a similar viscosity to that of granodiorite and behaved as passive markers whose shapes resembled those of the intrusive strain ellipsoids petrological and geochemical approaches coupled with rheological modelling allow the prediction of the physical states of magmas and of the development of their internal fabrics observed fabrics were compared to the results of numerical modelling of multiparticle systems slowly moving in viscous fluid the steep magnetic foliations and almost vertical magnetic lineations suggest that today s erosion level represents a relatively deep intrusive level where magma flowed vertically and its intrusion was controlled by the shape of the feeder zone used for magma ascension

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

granite emplacement and its relation with regional deformation in the aravalli mountain belt india inferences from magnetic fabric abstract this contribution presents field microstructural and anisotropy of magnetic susceptibility ams data from the godhra granite a ca 1 ga pluton at the southern margin of the aravalli mountain belt western india that lies near the central indian tectonic zone citz the northern and southern margins of the granite are high strain zones with north dipping e w striking foliations and nw plunging stretching lineations the presence of preferentially oriented feldspars suggests that the granite was subjected to magmatic deformation during its emplacement high temperature solid state deformation fabrics are recorded from microstructural studies which imply that the granite is deformed ams data show magnetic foliations and magnetic lineations in the granite with orientations similar to the field data the magnetic foliation in the granite is parallel to the d 3 foliation in the gneissic host rock both the stretching lineations and the magnetic lineations at the granite margin high strain zones are parallel with the axes of d 3 reclined folds in the gneiss therefore the emplacement of the godhra granite is inferred to have taken place syntectonically while the gneiss was undergoing the d 3 phase of deformation the orientation and character of these lineations and the vergence of the d 3 folds imply oblique simple shear which is inferred to have controlled the emplacement of the granite the magnetic fabric data are compared with the regional structural trends of the metasedimentary rocks of the southern aravalli region and the available geochronological data from rocks of the southern aravalli region are summarised based on this information it is suggested that the emplacement of godhra granite deformation of the southern parts of aravalli mountain belt and tectonothermal events along the citz and rodinia supercontinent assembly were related events

the urumieh plutonic complex nw iran a record of the geodynamic evolution of the sanandaj sirjan zone during cretaceous times part i petrogenesis and k ar dating the urumieh plutonic complex is located in the northwestern part of the sanandaj sirjan zone iran the urumieh plutonic complex comprises three components i e a dioritic suite a granitic suite consisting of biotite bearing granites and younger stocks i e an alkaline suite alkali syenite and alkali granite and a muscovite bearing granite based on k ar chronology of separated minerals amphibole biotite muscovite and feldspar and field observations the urumieh complex was built during two distinct episodes the diorite and granite suites were emplaced between 100 and 92 ma whereas the younger suites were emplaced between 82 and 80 ma the dioritic suite has sio2 contents in between 56 59 wt and major and trace elements characteristic of mafic i type granitoids a ckn molar ratio 0 81 0 88 this suite is characterized by relatively high mg 49 low na2o content 3 5 wt and a relatively high abundance of lile incompatible elements their chondrite normalized ree patterns are relatively flat la yb n 7 in average and show small or no negative eu anomalies eu eu 0 9 in average some diorites show mingling and mixing textures that may support local crustal contamination of the dioritic melt geochemical features suggest that diorites formed in a subduction setting by partial melting of metasomatized mantle the coeval granites are peraluminous 1 04 molar a ckn 1 37 with sio2 contents between 66 69 wt their chondrite normalized ree patterns are slightly enriched in lree relative to hree la yb n 11 in average with negative eu anomalies eu eu 0 4 in average these granites have biotite as the main mafic mineral and locally contain restitic xenoliths they originated by partial melting of metasediments the younger alkaline syenite and granite are only slightly peraluminous molar a cnk 1 04 in average they show enrichment in hfse high contents of ree and relatively high feo mgo and ga al ratios typical of a type magmas the other younger granite pluton dourbeh granite has peculiar mineralogical and geochemical features none of these late syenite and granites were intruded by mafic dikes and they do not contain any comagmatic mafic enclaves they seem to have been derived only from crustal melting changing of magmatic sources during the 100 80 ma time interval is discussed with respect to the geodynamic context the first magmatic episode 100 92 ma is regarded as subduction related whereas the second one at ca 80 ma likely ocurred during the collision between the arabian margin and the sanandaj sirjan zone

the pombal granite pluton magnetic fabric emplacement and relationships with the brasiliano strike slip setting of ne brazil paraiba state abstract the pombal pluton 500 km 2 a suite of diorite syenite and porphyritic granite bodies is here used to constrain kinematics of brasiliano age tectonic episodes in northeast brazil the pluton intrudes high grade to migmatitic gneiss forming the western basement of the serido belt and is located at the intersection between two sets of continental scale dextral strike slip shear zones the northern set of shear zone strikes ne sw and branches southwards into the e w patos mega shear zone a detailed microstructural and low field magnetic susceptibility study was performed to unravel the relationships between solid state deformation in the country rocks and magma emplacement porphyritic granite and syenite have quite high magnetic susceptibilities 10 3 10 2 si units indicative of magnetite as the principal carrier of susceptibility the magnetic fabric is remarkably homogeneous in orientation throughout the pluton it is characterized by a shape preferred alignment of magnetite itself parallel to the shape fabric of mainly biotite amphibole i e to the magmatic fabric even close to the contact with the high temperature mylonites of the patos shear zone south of pombal no imprint of the e w trending structures is observed in the fabrics of either the granite or the host rocks granite emplacement and its internal fabric development is concluded to be independent of the movement of the patos shear zone in the southwestern border of the pluton a low dip foliation bearing a ne sw striking lineation is shared in both the magmatic fabric of the pluton and the solid state fabric farther to the north approaching the ne sw strike slip shear zone the magmatic fabric is characterized by a steeply dipping ne striking foliation carrying a subhorizontal lineation transition from low to steep dips of the planar fabrics is progressive two models are proposed for emplacement of the pombal pluton one considers magma injection during an early episode of tangential tectonics responsible for the gently dipping foliations evolving later to strike slip deformation the other model considers that the pluton was emplaced in a pull apart domain developed in the overlapping sector of a right hand en echelon system of a dextral shear zone compatibility of these models with the tectonic evolution of the serido belt is discussed

granite fabrics and regional scale strain partitioning in the serido belt borborema province ne brazil 1 fabrics of the brasiliano age granitoid plutons of the serido belt northeastern brazil were organized following the partitioned regional strain field which shaped larger areas of the basement and its metapelitic cover nine plutons located at the central and western portions of the belt have been investigated by means of anisotropy of magnetic susceptibility these plutons emplaced between 580 and 575 ma are mainly potassic calk alcalic magnetite bearing monzogranites the presence of primary coarse grained magnetite as the main magnetic mineral is indicated by optical and electronic microscopy magnetic susceptibilities hysteresis ratios and thermomagnetic curves the plutons show a strong internal coherence of magnetic fabric with foliation poles distributed around a zone axis parallel to the gently plunging mean lineation the magnetic fabric was used to trace the strain partitioning along the belt in the central transpressive shear belt the magnetic lineations are parallel to north trending regional stretching on the other hand in the western basement block a voluminous granitic magmatism was emplaced as a consequence of a transtensional extensional deformation lineations in these granites trend in a ne direction which is parallel to the transport inferred from kinematic indicators along the shear zones a crustal scale east trending dextral shear zone connects both the transtensional and transpressional domains

slip partitioning and inclined dextral transpression along the zagros thrust system iran abstract flexural duplex structures and shear zones are well developed in the heneshk area in southwestern iran the zagros thrust system in this area consists of eight sheets of nw striking ne dipping dextral strike slip duplexes that are linked with imbricate fans and oblique slip thrusts the zagros thrust system is a portion of the internal zone of the zagros orogenic belt this internal zone is characterized by penetrative plastic deformation and metamorphism based on the kinematic vorticity number w k 0 73 0 02 calculated from quartz porphyroclasts of the goshti shear zones along the zagros thrust system the estimated angle between the maximum instantaneous strain axis isa and the transpressional zone boundary is 33 the estimated angle of the plate convergence or the flow apophysis of the displacement field is 25 the presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence the mean estimated finite deformation w m value indicates relative contributions of 47 pure shear and 53 simple shear for the deformation which involved both strike slip and oblique slip displacements in this inclined transpression about 40 strike slip partitioning was required to accommodate the finite strain and re orientation of instantaneous strain

grain boundary migration microstructures in a naturally deformed quartzite abstract in this study several grain scale microstructures are presented that are thought to demonstrate the migration direction of once mobile grain boundaries in a naturally deformed quartzite an analysis is presented of the sense of migration of the boundaries and the characteristics of the patterns of relative grain growth and shrinkage grain boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz quartz grain boundaries and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane

strain partitioning and interplate friction in oblique subduction zones constraints provided by experimental modeling physical modeling of oblique subduction is performed to study the mechanism of strain partitioning the model is two layer and includes the elasto plastic lithosphere the overriding and subducting plates and the low viscosity liquid asthenosphere the subduction is driven by a push force from a piston and a pull force when the density contrast ap between the subducting plate and the asthenosphere is positive we vary both ap and the interplate friction frictional stresses slip partitioning is obtained only in the models with high interplate friction and only when the overrid ing plate contains a weak zone this zone in the models corresponds either to locally thinned litho sphere or to cut fault the horizontal trench normal component of the interplate friction force ffh can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force fph in the subduction zone ffh is always negative compression while fph can be either negative compressional subduction regime or positive extensional regime high friction which promotes partitioning can completely cancel the extensional suction force fph back arc tension and strike slip faulting appear thus as conflicting processes although they can coexist in the same subduction zone depending on the relative values of relevant forces it ap pears that high friction can exist only in compressional subduction zones where partitioning should develop more easily this conclusion is supported by the comparison of two oblique subduction zones having similar geometry the compressional southern kurile zone strong partitioning and extensional southern ryukyu zone no lithospheric scale partitioning other factors controlling the strain partitioning are the length of the oblique subductlon zone the boundary conditions at the transverse limits of the forearc sliver and of course the obliquity of subduction