brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

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

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

episodic metamorphism and deformation in low pressure high temperature terranes in many cases low pressure high temperature metamorphism must be the consequence of advective heat transfer e g the ascent of granitic magma whereas such heating mechanisms are necessarily short lived the lifetime of the underlying orogenic processes is likely to be much longer and it is conceivable that during the evolution of an orogen repeated transient heating at shallow crustal levels is caused by episodes of segregation and upward passage of melts generated at deep crustal levels the number and timing of these events would depend on such factors as critical melt segregation volume moho temperature and strain rate evolution we investigate some of the controls on this multiple event superposition using a simple thermomechanical model designed to predict simultaneously the strain rate and the isostatic and thermal evolution of convergent orogenesis subject to a constant driving force an appealing aspect of this formulation is that in contrast to models that have attempted to explain event cyclicity through episodic processes acting on the orogenic boundaries it accounts for the repeated occurrence of events through processes inherent to the orogenic system itself

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

an experimental deformation study of partially molten amphibolite application to low melt fraction segregation an amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low melt fraction segregation processes experiments were performed under fluid absent conditions at 1 8 gpa between temperatures of 650 c and 1000 c these conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere at 850 c melt is granitoid in composition and its viscosity is that of wet granite 103 105 pa s the results can be summarized as follows 1 under subsolidus conditions tests at 650 c and 750 c the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample 2 at near solidus conditions 800 c 0 5 vol melt fractures 1 10 m in width displace hornblende and plagioclase grains and melt formed in situ is found in some of these cracks the formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration hydration reactions that involve plagioclase the dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt embrittlement process trapping melt in pockets of lower strain with higher melt fractions 10 15 vol broad melt bearing shear zones form and grains within these zones are brittlely deformed 3 at 20 vol melt 1000 c additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow the results show that at low melt fractions 15 vol fluid absent melting reactions can induce fracture in previously ductile rocks this suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or cmf of 26 40 vol in general estimating the fraction of melt at the onset of segregation cannot be predicted by the cmf melt segregation models have to be adapted to variations in such major factors as pressure temperature type of melting reaction rate of melting and strain rate such models will greatly assist our understanding of continental growth and evolution during orogenesis

mega fold interference patterns in the beishan orogen nw china created by change in plate configuration during permo triassic termination of the altaids abstract kilometer size fold interference patterns in the beishan orogenic collage boc in the southernmost altaids formed by fold superimposition in fossiliferous permian sedimentary rocks first phase f 1 upright and almost north trending folds were refolded by e to ene trending f 2 folds whose axial planes and axes are vertical or subvertical from east to west there is a regional change in style of interference patterns from lobate cuspate to crescent to mushroom shape this variation is accompanied by a westward decrease in the f 2 interlimb angle and related to a higher percentage of coarse grained clastic rocks suggesting a dependence of the f 2 deformation on lithology axial planar slaty cleavages are well developed in f 1 and poorly developed in f 2 folds the superposed folds mainly underwent flexural slip and flexural flow folding to give rise to the lobate cuspate pattern and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids the two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter arc basin during the final amalgamation of the boc in the latest permian to early to mid triassic the two folding events bracketed between a maximum detrital zircon age of

mechanisms of continental growth in extensional arcs an example from the andean plate boundary zone during the late triassic jurassic and early cretaceous an extensional magmatic arc was formed in the andean margin of northern chile plutons emplaced at ramps within a hinterland propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle phases of volcanism separated phases of plutonism and our model demonstrates that plutonism was favored only when upper plate extensional fault systems were active when they were not dikes cut across inactive faults and magma rose directly to the surface during volcanic phases of arc growth

the transition from magmatic to high temperature solid state deformation implications from the mount stuart batholith washington criteria for syntectonic emplacement of plutons are commonly ambiguous the strongest single criterion is probably the preservation of a continuous transition from submagmatic to high temperature solidstate deformation this transition is not commonly documented an exception is the mount stuart batholith which displays a variety of syntectonic structures within its sheared northeastern margin including the following 1 s c fabrics that range from submagmatic to high temperature solid state 2 solid state foliation and lineation defined by amphibolite facies assemblages that overprint and are parallel to magmatic foliation and lineation 3 folds of magmatic and solid state foliation that have hinge lines parallel to magmatic and solid state lineation and to equivalent structures in wall rocks 4 dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite 5 pegmatite dikes and mineralized joints that are sub perpendicular to magmatic and solid state lineation and 6 submagmatic and high temperature solidstate ductile shear zones we suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation syntectonic features are most likely preserved in plutons cooled at slow to moderate rates or in plutons deformed at high strain rates during emplacement the optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid crust such as the mount stuart batholith and in intrusions at deeper levels that were rapidly exhumed and or intruded during the waning stages of regional deformation

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

the influence of large ductile shear zones on the emplacement and deformation of the wyangala batholith se australia abstract the wyangala batholith consists of many small s and i type plutons most of which are elongate in a n s direction our studies of this batholith indicate that n s striking magmatic foliations exist in some of the individual plutons but are not strongly developed a few individual plutons show evidence of forceful emplacement but in general most do not intense post emplacement deformation occurs in a series of e directed thrusts that border the eastern edge of individual plutons or the batholith as a whole this thrusting formed gneissic layering mylonites and secondary foliations in the batholith and complex polyphase folding and cleavage development in the wall rocks we suggest that the batholith was emplaced as sheet like bodies possibly along steep reverse faults data presently available indicate that the pre emplacement deformation of the wall rocks is middle silurian that the batholith was emplaced in the late silurian and that post emplacement thrusting is late silurian to early carboniferous finally the close association of batholiths and regions of complex deformation in the lachlan fold belt suggest to us that these elements may represent shallow level equivalents of hollister crawford s surge tectonics geology 14 558 561 1986

the competition between rates of deformation and solidification in syn kinematic granitic intrusions resolving the pegmatite paradox abstract while fully crystallized granites rich in feldspar generally serve to strengthen the continental crust their precursor melts are assumed to be important agents of crustal weakening many syn tectonic granitic pegmatites are deformed within shear zones but ubiquitously preserve undeformed primary magmatic textures implying that they were largely molten during shearing yet the shapes of pegmatite bodies indicate that they deformed with a greater competence than their surroundings this co located pair of material behaviours is paradoxical we interpret field relationships in a typical pegmatite shear zone association torrisdale nw scotland and propose a mechanism by which syn tectonic granitic melts may in effect act as competent bodies while not yet fully crystallized competence was rapidly increased by preferential crystallization on intrusion margins that served to encapsulate residual melt inside stiff rinds further crystallization may have been pulsed as the concentrations of crystallization inhibitors fluxes increased in residual fluids postulating the existence of initial stiff rinds also consistent with modern estimates for rates of feldspar crystallization cms yr from undercooled hydrous silicic magma to form pegmatites these greatly outpace strain rate estimates for shear zones thus fully liquid granitic melts may only be present fleetingly and have little opportunity to weaken deforming crust before crystallization begins

termination of a continental scale strike slip fault in partially melted crust the west pernambuco shear zone northeast brazil continental scale strike slip faults rooted in the upper mantle have been recently interpreted in deep seismic profiling these shear zones transect the entire crust and it may be assumed that the deformation pattern associated with their propagation at different depths was strongly influenced by the rheological properties of the rocks the continental scale west pernambuco shear zone provides information on the deformational behavior of a hot and partially melted crust that has undergone large scale shearing this late precambrian shear zone terminates eastward in a fanlike structure passing to a wide transpression zone at a high angle to the shear direction the main fault branches into numerous splay faults over a distance of about 50 km severe perturbation of the mylonitic flow pattern commonly occurs at branch points where subsidiary shear zones diverge from the main zone in these areas problems of strain compatibility are raised by the sharp bending and decrease in dip of the foliation surface we propose that this unusual deformation pattern is a result of the low strength of the continental crust at the considered level of observation

structure magnetic fabric and emplacement of the archean lebel stock sw abitibi greenstone belt abstract the anisotropy of magnetic susceptibility ams has been determined in a suite of samples from the ca 2680 ma old sub circular syenitic lebel stock which is bound to the north by a major late archean shear zone larder lake cadillac deformation zone lcdz the ams defines a magnetic foliation which is parallel to a strong planar preferred mineral orientation fabric considered to have been acquired during flow of a crystalladen magma during emplacement the geometry of the foliation indicates that the stock has an horizontal dish shape at depth the syenites also contain a well defined magnetic lineation which consistently points toward the lcdz apart from localities in the south where it trends e w parallel to the stock s margin ams ellipsoids are dominantly of oblate type except in samples showing solid state strain and alteration overprints which often have plane to prolate ellipsoids the intensity of the magnetic fabric shows a weak spatial variation being strongest at the southern margin the magnetic and magmatic fabric pattern ams parameter variation and the sense of displacement of the western wall rocks are used to propose an emplacement mechanism involving southward migration of a relatively thin sheet of syenitic magma from a linear source corresponding to the present location of the lcdz

role of melt in the formation of a deep crustal compressive shear zone the maclaren glacier metamorphic belt south central alaska the maclaren glacier metamorphic belt is an exhumed portion of a deep crustal shear zone where hot upper amphibolite facies rocks were emplaced over cooler lower grade rocks it is located in south central alaska within the collision zone between terranes previously accreted to north america and the wrangellia superterrane crosscutting relationships and orientations of thin granitoid sills within the hanging wall show that melt was repeatedly intruded into the shear zone during overall compression in addition a 1 km thick tonalite sill the valdez creek tonalite was emplaced into the shear zone while it was active this conclusion is based on the presence of a well developed foliation within the sill which is concordant to the fabric of the shear zone the alignment and tiling of plagioclase laths the presence of highly strained mafic enclaves within a matrix which shows no evidence of any subsolidus deformation and the preservation of a delicate magmatic texture which indicates that a magmatic fluid was still present after deformation in the sill had ceased a one dimensional thermal model for sill shaped plutons of varying thicknesses which are emplaced into country rocks at different initial temperatures indicates that melt can be present for long periods of time in the deep crust 1 m y an important factor controlling the length of time that a sill remains molten is the temperature of the surrounding rocks which in the lower crust can be as high as the solidus temperature of granitoid melts if the amount of time for melt to crystallize is sufficiently long potentially large amounts of strain will be accommodated by zones containing melt due to the low strength of melt compared to rock in the maclaren glacier metamorphic belt the amount of time needed for the valdez creek tonalite to fully crystallize is calculated to be about 90 000 years this is a minimum value since the original thickness of the sill is not known nevertheless using the 90 000 year value and assuming that the bulk of convergence between north america and wrangellia was concentrated within the valdez creek tonalite a displacement of at least 10 km could have been accommodated across the sill while melt was present

shear zone controlled magma emplacement or magma assisted nucleation of shear zones insights from northeast brazil abstract in the borborema province of northeast brazil neoproterozoic granitoids and large scale transcurrent shear zones are spatially associated suggesting a genetic link between magma bodies and shear zones in some cases magma emplacement was clearly favored by shear zone activity but for several plutons this model is not satisfactory in these plutons pre full crystallization strike slip deformation evidenced by parallelism of magmatic foliations and lineations with the solid state mylonitic fabric and by a transition from magmatic to solid state flow is restricted to the vicinity of the shear zones evidence of shear zone activity prior to magma emplacement is lacking and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older gently dipping regional gneissic foliation field and anisotropy of magnetic susceptibility mapping together with petrographic and geochemical studies performed in one of the magmatic complexes of the borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone controlled emplacement model away from the shear zones this complex has retained a stratification inherited from the mixing of crystal poor magmas of contrasting composition and a magmatic fabric characterized by low to moderate dip magmatic foliations bearing a nw trending lineation which contrast with vertical foliations bearing ne or e w trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development based on evidence that magma emplacement predated strike slip shearing and on information about the transition from magmatic to solid state deformation observed in the studied plutons we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation we propose that in the studied cases deformation first concentrated within the plutons where shear zones nucleated and grew subsequently as the solid phase component increased the crystal mush began to behave as a solid and the shear zones propagated into the country rocks

variations in deformation fields during development of a large volume magmatic arc central sierra nevada california mid to late cretaceous plutons in the central sierra nevada magmatic arc show widely preserved magmatic foliation whereas regionally developed solid state foliation is absent relatively slow cooling of these plutons and expected strain rates 10 14 suggest that the plutons were emplaced in a neutral or weakly extensional deformation regime domains of solid state ductile shear of only slightly younger age than the plutons on the other hand indicate a contractional regime timing of pluton emplacement and movement on the shear zones have been constrained using pb u zircon and 40 ar 39 ar hornblende and biotite geochronology both plutons and ductile shear zones become younger toward the east the four more westerly shear zones which were active between ca 100 and 90 ma show steeply plunging stretching lineations whereas the most easterly and or youngest zones active between ca 88 and 78 ma show mostly oblique and or subhorizontal stretching lineations indicating a change in kinematics at ca 90 ma the above events define a complex deformation pattern in which strain regimes fluctuated in time and space between neutral or weak extensional and contractional we propose a tectonic model in which thenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neutral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid and upper levels probably as dikes via fractures slab flattening caused the neutral or weak extensional regime to move eastward away from the trench increased coupling between upper and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons and domains of ductile shear formed in progressively younger plutons to the east the above events were accompanied by an oblique convergence vector between north america and farallon plates engebretson et al 1985 which imposed a relatively small component of right lateral shear onto the arc that increased with time we estimate that at ca 100 ma the convergence vector made an angle obl 20 to the arc normal and we suggest that around ca 90 ma obl passed through a critical value conceivably 20 oblcrit 30 at this juncture the component of right lateral shear became sufficiently large to induce significant arc parallel strike slip movement on the most easterly shear zones these kinematics continued as the dominant scheme possibly as late as ca 78 ma

magnetic fabric vs magnetite and biotite shape fabrics of the magnetite bearing granite pluton of gameleiras northeast brazil abstract a well defined orientation pattern has been evidenced by the magnetic fabric lineation and foliation of the granite pluton of gameleiras northeast brazil this magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves and by the planar arrangement of tabular k feldspar phenocrysts low field anisotropy of magnetic susceptibility properties as well as magnetic mineralogy studies and optical microscopy observations indicate that multidomian iron rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy to examine in more detail the relationships between the magnetic and magmatic fabrics measurements of two dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections angular departure has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen we found that the fabric of biotite correlates strongly with the magnetic fabric 7 8 the fabric of magnetite although not as well defined mainly because of its weak shape anisotropy still correlates with the magnetic fabric 12 18 origin of low field magnetic fabric in the pluton of gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains magnetic interactions between the grains of magnetite where they form clusters may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy concerning the orientation of the magnetic fabric as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals particularly biotite and titanite the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite however as no correlation exists between shapes or clustering of magnetites and strain undergone by the deforming magma the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

role of melt during deformation in the deep crust deformation in the deep crust is strongly influenced by the presence of melt injected melt or magma weakens the crust because strain will tend to localize where melt is present the amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate for plutons that intrude into rocks which are near the solidus temperature of the melt crystallization times can be quite long 1myr partial melting of deep crustal rocks can lead to melt enhanced embrittlement this occurs because the volume change for most melting reactions is positive therefore when the rate of melt production outpaces the rate at which melt can leave the system the melt pressure increases eventually the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the central gneiss belt canada this suggests that melt enhanced embrittlement occurred in these rocks an expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived assuming that migmatite has a small tensile strength differential stresses are 20 mpa in migmatitic rocks at the time melt enhanced embrittlement occurs the occurrence of melt enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting furthermore repeated episodes of melt enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains

major precambrian nappes of the brazilian belt ceara northeast brazil the precambrian of ceara northeast brazil includes the reactivated archean basement and proterozoic metasediments that have gone through one single stage of metamorphism and deformation during the brazilian orogeny that culminated at about 600 ma preliminary petrostructural studies show that crystalline nappes with associated himalayan type reverse metamorphism have been thrust westward to southwestward over more than 150 km geometrical relationships indicate that some steep shear zones could be lateral ramps to the nappes thus this precambrian segment of the brazilian belt has the petrostructural signature of a former overthickened continental crust similar to that generated by himalayan type continent continent collisions

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

melt enhanced deformation a major tectonic process convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks anatectic and or mantle derived melts combine to form a melt weakened zone in the thickened lower crust the accumulation of melt eventually leads to crustal failure along melt lubricated shear zones rapid 1 mm yr vertical component movements of large crustal blocks result we refer to these crustal displacements as tectonic surges the melt lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons rocks that formed at different crustal levels are juxtaposed across these shear zones one result of surges with large lateral component of movement is metamorphic inversion with high pressure and high temperature metamorphic rocks structurally over lower pressure and temperature assemblages large and rapid vertical surges may displace crust containing abundant melt and may result in high t p including granulite facies metamorphism and preservation of metamorphic textures caused by rapid decompression

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

the great tonalite sill emplacement into a contractional shear zone and implications for late cretaceous to early eocene tectonics in southeastern alaska and british columbia the late cretaceous to early tertiary great tonalite sill of southeast alaska and british columbia is a very long 1 000 km and thin 25 km orogen parallel composite batholith which lies between the insular superter rane including the alexander and wrangellia terranes and the intermontane superterrane including the stikine and cache creek terranes the batholith is delineated by many steep sheet like plutons which are dominated by northwest southeast striking concordant fabrics with steep lineations that formed during deformation in a country rock shear zone prior to the complete crystallization of the magmas deformation in this shear zone is dominated by northeast southwest directed contraction orthogonal to the orogenic strike associated with a component of northeast over southwest high angle shear the steep multiple dike like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike wedging mechanisms along the deep reaching shear zone the remarkable narrowness of the great tonalite sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length such a shear zone of late cretaceous to early tertiary age lying along 800 km of the possible boundary between the insular and intermontane superterranes implies that it may represent the actual boundary between them if this hypothesis is correct it implies that the large scale tectonic regime during emplacement of the great tonalite sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data

extensional pluton emplacement models do they work for large plutonic complexes shallow crustal to mid crustal magma emplacement models commonly rely on local or regional extension to solve the room problem we argue that existing extensional models explain the emplacement of sheeted dike complexes or internally layered plutons but are not as easily reconciled with the emplacement of elliptically shaped plutons or batholiths for the following reasons 1 rock strengths and crustal stresses prevent large voids from existing at depth and therefore require wall rock extension to be balanced by magma influx 2 rates of extension are controlled by fault slip rates of millimetres to centimetres per year and thus require numerous small injections of magma and 3 because of their sheetlike shapes and small thicknesses these small magma injections will cool rapidly and form sheeted plutons or dike swarms extension may however facilitate emplacement of elliptically shaped plutons in concert with other mechanisms if certain kinematic constraints are satisfied

shearing of magma along a high grade shear zone evolution of microstructures during the transition from magmatic to solid state flow abstract syntectonic plutons may record short lived geological events related to crustal melting and deformation of the continental crust therefore interpretation of microstructure and orientation of fabrics is critical in order to constrain space time temperature deformation relationships during pluton crystallization here we describe the transition from magmatic to solid state deformation in the late variscan diorite granite suite dgs emplaced along the santa lucia shear zone the systematic collection of meso microstructural and quartz axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics we identify three magmatic microfabric types thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization our data suggest that diffusion creep dislocation creep and grain scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate we suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions gt 0 40 and that most of the hypersolidus fabrics formed during horizontal flow reflecting the stress field imposed by the shear zone and preserving no memory of the ascent stage

igneous emplacement in a transpressive shear zone ox mountains igneous complex the central and southwestern ox mountains inlier in western ireland comprises amphibolite facies metasediments and metavolcanic rocks which have been deformed by a major sinistral shear zone granitoid emplacement occurred in two distinct styles in the ox mountains shear zone the older pluton the ox mountains granodiorite was emplaced syn kinematically with respect to the main sinistral transpressional events in its wall rocks it was emplaced as a series of compositionally distinct sheets in the central part of the shear zone system thus suggesting the formation of transient dilational sites occurred in response to the transpressive deformation the younger lough talt and easky lough adamellites were emplaced as individual intrusions in dilational cavities produced by reactivated movements on a discrete zone within the ox mountains inlier the different intrusive styles are considered to be a product of the tectonic regimes operating during emplacement intrusion into transient dilational cavities may be one method by which granitoid magmas are emplaced into fault zones undergoing active contractional deformation

the role of sub milankovitch climatic forcing in the initiation of the northern hemisphere glaciation mechanisms responsible for the initiation of major glaciation in the northern hemisphere at about 2 75 million years ago are poorly understood a laminated terrestrial sequence from pula maar hungary containing about 320 000 years in annual layers between 3 05 and 2 60 million years ago provides a detailed record of rates of climatic change across this dramatic transition an analysis of the record implies that climatic variations at sub milankovitch frequencies less than or equal to 15 000 years were an important driving force during this transitional interval and that as the threshold was approached these increased in frequency and amplitude possibly providing the final trigger for the amplification of northern hemisphere ice sheets

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

submagmatic microfractures in granites microstructures in hercynian granite plutons of the pyrenees indicate fracturing of plagioclase crystals in the presence of melt this is represented mainly by quartz and or feldspar that fills fractures electron microprobe analyses demonstrate that in compositional agreement with the rims of the zoned plagioclase the feldspar infillings are more sodic with decreasing melt fraction fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic i e when the melt was below the critical fraction for magmatic flow material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures because during cooling the volume fraction of melt remains low for a large temperature range a strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas

magnetic and magmatic structures of the emas granodioritic pluton cachoeirinha belt ne brazil relationships with pan african strike slip fault systems abstract the emas granodiorite was intruded during the brasiliano pan african orogeny into the metapelites of the cachoeirinha belt ne brazil this pluton was chosen for a petrofabric study of magmatic emplacement structures using magnetic anisotropy because of its conspicuous position in the junction area between the large e w trending patos shear zone and the ne sw trending cachoeira da mina strike slip fault the magnetic behavior of this calc alkaline granite is dominated by paramagnetic minerals which is optimal for the application of the anisotropy of magnetic susceptibility technique used for this study almost all microstructures correspond to the magmatic state so the measured magnetic lineations and foliations may be interpreted respectively as directions and planes of magmatic flow these magnetic magmatic lineations and foliations are dominantly n s and ne sw oriented and are gently to moderately dipping the anisotropy of magnetic susceptibility ratios range from 1 to 5 4 most of these values corresponding to magmatic alignment the lowest ratios are located chiefly in the centre of the pluton and the highest on the northern and western margins from these data we propose that the emplacement of the emas pluton was controlled by a ne sw trending left lateral strike slip zone parallel to the cachoeira da mina fault giving rise to openings in the metapelitic country rocks infilled by two succesive magmas the principal result of this study is the unexpected independence between the emplacement of the emas pluton and the functioning of the patos dextral shear 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

rates of processes in magmatic arcs implications for the timing and nature of pluton emplacement and wall rock deformation abstract the construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions it is critical therefore to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton wall rock systems our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following 1 crystal growth rates in magma of 10 4 cm year 1 2 growth rates of metamorphic porphyroblasts between 10 5 and 10 2 cm year 1 3 long term magma supply rates of 10 1 km 3 year 1 and short term rates of up to 350 km 3 year 1 4 diapiric ascent rates for mafic plutons of 1 3 m year 1 5 cooling of plutons to ambient wall rock temperatures in 10 5 10 6 years 6 final crystallization of plutons in a small fraction of the time needed for complete cooling 7 fault displacements of 3 cm year 1 8 development of cleavages in fault zones in less than 10 6 years at strain rates of 10 13 s 1 or higher and 9 the development of regional cleavages in 10 6 years at strain rates of 10 14 s 1 these rates indicate that processes operating in magmatic arcs are relatively rapid pluton emplacement cleavage development etc occur over time spans of tens of thousands to no more than a few million years however the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed at deeper levels or in zones undergoing faster strain rates the time spans of the various processes approach one another thus plutons with otherwise similar characteristics emplaced in regions undergoing fast strain rates or at deeper crustal levels may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration finally we argue that the structural and other characteristics of pluton wall rock systems will depend on rates of various processes involved and that these rates at the very least influence and sometimes invalidate the timing criteria previously published by us and others

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

the role of melt in the uplift and exhumation of orogenic belts abstract in some orogenic belts rapid pulses of uplift occur when melt is present detailed work on the relation of melt to structural and metamorphic features in several localities shows that hot metamorphic rocks and especially anatectic migmatite can be physically transported from the lower to middle crust on decollements the decollements form in the lower crust because even though the lower crust is structurally weak throughout due to thermally activated crystal plastic deformation mechanisms the crust is substantially weakened where melt forms or is intruded melt in the lower crust is present long enough to accomodate strain whereas in the upper crust it crystallizes too fast to be a factor in strain localization following crustal loading due to thrusting exhumation results in the formation of additional melt in the migmatites due to decompression melting support of this sequence of events includes 1 evidence for near isothermal decompression of high temperature metamorphic rocks with early high pressure minerals having formed pre or synchronous with penetrative deformation and with lower pressure minerals having formed after the deformation 2 the occurrence of magma sheets that occur along decollements between high temperature metamorphic rocks and lower temperature metamorphic rocks in either thrust or normal configurations 3 nearly concordant k ar cooling dates for biotite and hornblende suggesting rapid cooling rates and 4 post tectonic intrusions of leucogranite or leucotonalite that may represent the products of decompression melting the crystalline slabs of the high himalaya the compressional surges of the coast orogen of british columbia and alaska the cordillera of southern british columbia and the southern belt of the hercynian chain are examples where melt assisted rapid pulses of uplift appear to have occurred during the terminal phases of uplift and following uplift these orogenic belts were exhumed during extension while melt was still present

the case for simultaneous deformation metamorphism and plutonism an example from proterozoic rocks in central arizona abstract the syntectonic 1 70 ga crazy basin monzogranite provides an example of the complex spatial and temporal interactions between metamorphism deformation and plutonism synchronous plutonism and deformation is indicated by syn shortening dikes sills and veins parallel magmatic and solid state fabrics fabrics in xenoliths and a foliation triple point synchronous plutonism and metamorphism is indicated by a systematic increase from 400 c to 630 c towards the pluton at a constant pressure of 300 mpa 3 kb temperatures are consistent with a conductive cooling model in which a 700 c pluton was emplaced into country rocks undergoing greenschist facies regional metamorphism synchronous deformation and metamorphism is indicated by porphyroblast inclusion geometries that document the synmetamorphic development of the s2 cleavage the pluton was emplaced adjacent to the shylock shear zone during progressive shortening emplacement of granite as ne trending sheets was facilitated by temporal partitioning of transpressional convergence into strike slip and dip slip components at the scale of the pluton s aureole and on the relatively rapid time scale of 103 106 y regional deformation and metamorphism were punctuated by thermal softening and increased diffusion rates data suggests that accretion of proterozoic arcs in arizona involved diachronous pluton enhanced deformation and associated high temperature low pressure regional metamorphism

structural controls on metamorphism and syn tectonic magmatism the portuguese hercynian collision belt upper palaeozoic collisional movements along the ibero armorican arc were accommodated on oblique strike slip ductile shear zone systems in north central portugal these shear zones are coincident with long linear belts of high temperature low pressure metamorphism and s type granitic plutons a model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12 15 km this was achieved by a combination of i increased heat flow ii lowering of melting temperatures caused by fluxing of hydrous fluids and iii a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts shear zones are proposed as the most important influence in controlling the site of development of such belts anatexis of metasedimentary crust and emplacement of the resulting s type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source