flow and fabric development during the diapiric rise of magma scaled isothermal experiments of the internal circulation induced by viscous drag at a diapir country rock interface are compared with published theoretical studies of flow induced finite deformation inside a rising sphere and new numerical results are presented for the incremental strain pattern developed in spherical diapirs the results predict a time dependent deformation history within rising diapirs that produces finite strains characterized by strong oblate flattening strains at the margin and strong vertical prolate strains in the center with an intervening zone of low plane strain finite strains are discussed in terms of the different sub fabrics expected to form in granitoids after considering the history of rheology and temperature distribution in rising magma strong rheological and temperature gradients are predicted for the margins of internally circulating diapirs that can lead to the formation of strong penetrative foliations at the margin and magmatic or even isotropic fabrics in the i

unroofing of active metamorphic core complexes in the d entrecasteaux islands papua new guinea metamorphic core complexes in the d9entrecasteaux islands are forming as a result of active extension at the western end of the woodlark basin spreading center high grade metamorphic rocks exist in the cores of gneissic domes including eclogites formed at minimum depths of 45 75 km i e 13 21 kbar high temperatures were maintained during uplift of basement rocks from 13 21 kbar t 730 900 c through 7 11 kbar t 570 730 c to 5 6 kbar t 730 c subsequent rapid cooling 100 c m y of the basement occurred between 1 and 2 ma on the basis of 40 ar 39 ar geochronology the core zone rocks were transected by kilometre scale amphibolite facies ductile shear zones active at depths of 25 35 km extensive intrusion of granodiorite plutons occurred and the age of granodiorite intrusions constrains late stage shear zone activity to 2 ma and younger unroofing was accomplished by faults and shear zones bounding the gneiss domes juxtaposing them against ultramafic rocks and sedimentary deposits surface uplift continues today and has led to topographic elevations of up to 2 5 km

a hf isotope perspective on continent formation in the south peruvian andes convergent continental margins are the primary host of both growth and loss of continental crust continental growth largely occurs via subduction driven magmatism whereas continental loss largely occurs via subduction erosion and sediment subduction because the latter typically involves partial recycling into magmas both growth and loss of continental crust can be represented in the magmatic record the degree of crustal recycling can be estimated from the initial hf isotope signatures in both magmatic and detrital zircon grains recent insights into the geodynamic evolution of the peruvian margin in combination with a new dataset of hf isotopic data on zircon from the carboniferous to early cretaceous enable us to 1 compare the geodynamic history of the southern peruvian margin with its hf isotopic evolution and 2 quantify the crustal growth between 500 and 135 ma the data exhibit a correlation with trends in isotope composition v time and reflect the dominantly extensional regime that prevailed from the onset of subduction from 530 ma to c 135 ma this study demonstrates that the peruvian margin experienced continental growth with juvenile input to arc magmatism of 30 45 on average and illustrates the use of u pb and hf isotopes in zircon as a tool to trace episodes of crustal growth through time supplementary material hf istopic analyses on zircon a1 and a2 and new u pb zircon ages a3 are available at http www geolsoc org uk sup18661

two dimensional strain determination by the inverse surfor wheel abstract a simple method of two dimensional strain determination from the preferred orientation of deformed lines is presented it is based on the change of probability of intersecting lines on a given traverse as a function of traverse orientation the method can be applied to real lines such as grain boundary outlines or to virtual lines such as the tie lines between centre points of neighbouring grains in this manner estimates for the strain of surfaces i e of the particles enclosed by the surfaces and of bulk strain can be obtained

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

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

fracturing and diking in incompletely crystallized granitic plutons abstract dikes in granitic plutons commonly have mineralogic and textural characteristics indicating a genetic relation to their host the contact between the dike and the host is marked signifcantly by interlocking unfractured crystals rather than by truncations and overgrowths although megascopically the contacts between dike and host pluton may be exceedingly sharp and planar an explanation of these relationships by fracturing an incompletely crystallized host rather than by entirely post magmatic fracturing with subsequent diking is postulated to be consistent with experimental date and certain petrologic observations published experimental research indicates that fracturing of crystal melt systems may occur if there is at least 70 crystals in the system at which point the system is capable of transmitting compressive and shear stresses via a crystalline framework dilatancy pumping of melt into fracture zones and subsequent hydraulic fracturing is thought to be one possible way late stage fluids of granitic systems evolve into dikes in granitic plutons such as leucogranites aplites and pegmatites interlocking textures between dike and host may also develop if magma from some distant source is interjected along the plane of fracturing in crystal melt systems

deformation of incompletely crystallized magma systems granitic gneisses and their tectonic implications certain textures in two feldspar calc alkaline granitic gneisses have been identified as indicators of the presence of intercrystalline melt during deformation of crystal melt mushes fluid relocation or fracting is to be expected in an incompletely crystallized magmatic system as deviatoric stresses are applied to it relocation of late stage fluids capable of crystallizing k feldspar quartz microaplite and myrmekite to low pressure zones such as the pressure shadow region around earlier crystals occurs within some developing mylonitic gneiss fabrics and results in a distinctive assemblage of textures strain in these rocks is expressed by various brittle and ductile features and by fluid relocation textures the petrogenesis of these particular gneisses is therefore both magmatic and metamorphic contrasting with gneiss forming processes that had no participating magmatic component migmatitic complexes evolving by partial anatexis of non magmatic rocks in a tectonically active environment are not

the ascent of felsic magmas in dykes abstract the results of a thermal and fluid dynamical analysis are presented that allow geologists to estimate the critical dyke or fracture width w c necessary for the transport of felsic magma through the crust without control by solidification the principal geological variables required to estimate w c are the magmatic viscosity the density and temperature contrasts and t between magma and country rock and the height of ascent h the results show that for typical values of and and for transport distances of 20 to 30 km the critical dyke width is constrained to lie between about 2 and 20 m

how the development of magma chambers affects collapse calderas insights from an overview here we review the studies of pluton emplacement and caldera collapse proposing a model linking the two processes the shallow rise of magma occurs by dyking and its emplacement occurs along major anisotropies many plutons are emplaced at subhorizontal discontinuities forming sills these will eventually grow forming laccoliths the most common mechanism to store shallow magma calderas are a surface expression of these reservoirs and can be approximated by a piston cylinder which sinks due to 1 underpressure in the reservoir producing outward dipping reverse faults 2 overpressure within a silllike chamber undergoing doming and 3 overpressure within a laccolith generating apical tensile stresses in the latter two cases inward dipping normal faults form we suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions tabular intrusions generating ring faults mainly at their tips form calderas with a similar width to the intrusion laccoliths may generate ring faults along the intrusion roof forming calderas narrower than the intrusion the outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

the cordilleran ribbon continent of north america the north american cordilleran orogen is the result of a two stage process a triassic jurassic accretion within panthalassa forming saybia a composite ribbon continent and b late cretaceous collision of saybia with north america this model requires that a large portion of the continental foreland of the orogen is exotic the exotic continental component of saybia cassiar platform is distinguished from the autochthon on the basis of its a triassic eurasian fauna b involvement in a major late triassic early jurassic orogenic event and c young in part grenvillian basement and mantle a mid cretaceous magmatic arc records west dipping subduction beneath the east margin of saybia the related accretionary prism consists of imbricated shale chert and deep water limestones the medial basin and overlies an isotopically juvenile mantle domain carbonatite complexes delineate the cryptic suture separating saybia and the autochthon paleomagnetic and paleobotanical data place saybia 2000 km to the south relative to the autochthon at 80 ma late cretaceous thrust belt development records transpression between the north moving ribbon continent and the autochthon pinning against the okhotsk chukotka arc in siberia buckled saybia giving rise to the alaskan promontory

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

heterogeneous deformation and quartz crystallographic fabric transitions natural examples from the moine thrust zone at the stack of glencoul northern assynt quartz crystallographic fabric transitions in well exposed mylonites immediately beneath the moine thrust at the stack of glencoul nw scotland have been investigated by optical microscopy x ray texture goniometry and orientation distribution function analysis a progressive change is observed from asymmetrical kinked single girdle c axis fabrics at 0 5 cm beneath the moine thrust through asymmetrical type i cross girdle fabrics to symmetrical type i cross girdle fabrics at 30 cm beneath the thrust this c axis fabric transition is accompanied by a transition from asymmetrical single a axis maximum fabrics 0 5 cm beneath the thrust through asymmetrical two maxima fabrics to essentially symmetrical two maxima a axis fabrics odf analysis of these s l and l s tectonites indicates that c axis positions on the leading edge of the fabric skeleton are related by a common a direction oriented within the xz plane at a moderate angle to the lineation x in contrast c axis positions on the peripheral trailing edge are related by a positive r rhomb pole oriented close to z a directions lying within this common rhomb plane progressively change through 180 in orientation traced around the c axis fabric skeleton such contrasting single crystal rhomb a preferred orientations on the leading and trailing edges of the fabric skeleton are interpreted as indicating localized grain scale plane strain and flattening deformation respectively they result in tectonites with essentially symmetrical c and a axis fabrics which display strongly asymmetrical positive r and negative z rhomb pole figures the observed transition in quartz c and a axis fabrics is interpreted as indicating an increasing importance of non coaxial plane strain deformation as the moine thrust is approached even immediately 1 cm beneath the thrust however flow has still significantly departed from bulk simple shear and involved an important heterogeneous component of contemporaneous flattening deformation

structure of late variscan millevaches leucogranite massif in the french massif central ams and gravity modelling results in the limousin area variscan leucogranitic plutons are spatially associated with normal faults and major strike slip shear zones that are a continuation of the south armorican shear zone our study focuses on the large n s trending millevaches granitic massif massif central france and intends to highlight through gravity modelling structural and anisotropy of magnetic susceptibility ams the massif structure at depth and to discuss the mode of emplacement of granites within a strike slip tectonic context the mica subfabric suggests that the magnetic foliations display a general nw se sub horizontal pattern on both sides of the n s pradines dextral wrench fault zone that deforms the core of the massif on 5 km width the magnetic lineation trend exhibits a sigmoidal pattern n s in the pradines fault zone and nw se on both sides of it which are consistent with a dextral wrench component the horizontal magnetic foliations and lineations are consistent with the thin granite laccolith model there is no significant imprint of the extensional variscan belt collapse on the internal fabric of millevaches granites than the tectonic dextral transcurrent movement prevailing in this area

shaping mobile belts by small scale convection claudio faccenna and thorsten becker compute global mantle flow based on a recent high resolution seismic tomography model to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean they show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region and conclude that vigorous small scale convection in the uppermost mantle may also hold the key for understanding other complex mobile belts such as those in the north american cordillera and the himalayan tibetan collision zone here global mantle flow is computed on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation in the mediterranean region the findings show that mantle flow explains much of the observed dynamic topography and microplate motion in the region small scale convection in the uppermost mantle may also hold the key to understanding complex mobile belts elsewhere mobile belts are long lived deformation zones composed of an ensemble of crustal fragments distributed over hundreds of kilometres inside continental convergent margins1 2 the mediterranean represents a remarkable example of this tectonic setting3 the region hosts a diffuse boundary between the nubia and eurasia plates comprised of a mosaic of microplates that move and deform independently from the overall plate convergence4 surface expressions of mediterranean tectonics include deep subsiding backarc basins intraplate plateaux and uplifting orogenic belts although the kinematics of the area are now fairly well defined the dynamical origins of many of these active features are controversial and usually attributed to crustal and lithospheric interactions however the effects of mantle convection well established for continental interiors5 6 7 should be particularly relevant in a mobile belt and modelling may constrain important parameters such as slab coherence and lithospheric strength here we compute global mantle flow on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean we show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region more generally vigorous small scale convection in the uppermost mantle may also underpin other complex mobile belts such as the north american cordillera or the himalayan tibetan collision zone

granite magma migration and emplacement along thrusts this paper investigates the influence exerted by brittle tectonic structures in the emplacement of granite plutons in contractional settings we address both cases where contractional tectonics and magma intrusion are 1 coeval to study how active contractional tectonics controls the transport of magma and 2 diachronous to study the role of pre existing structures on the transport of magma in light of new experimental models we show that magma can rise along thrusts ramps and flats this phenomenon occurs for both low viscosity magma basalts to andesite and high viscosity magma dry granite the experimental results also allow the evaluation of the role played by magma viscosity in determining pluton geometries in addition a review of literature demonstrates a spatial and causal relationship between granites and thrusts and highlights the geometric control of magma pathways in the pluton final shape the abundance of subhorizontal and tabular granitic intrusions indicates that the location of inflating granitic sills along thrust flats can be common we argue that active and pre existing flats and ramps thrusts provide a preferential continuous planar anisotropy susceptible to become a granitic magma migration pathway

polyphase wrench tectonics in the southern french massif central kinematic inferences from pre and syntectonic granitoids in the variscan french massif central the south limousin area consists of low to medium grade metamorphic rocks intruded by two granitic bodies the structural and textural analyses of these plutons undertaken in parallel with the structural analysis of their host rocks allow us to characterize and to date different stages in the tectonic evolution of this area this study shows that the south limousin area experienced successivelly two strike slip events along two geographically distinct shear zones from north to south the left lateral estivaux and the right lateral south limousin strike slip faults respectively these ductile faults subdivide the south limousin into three structural units from north to south they are the upper gneiss unit thiviers payzac unit and genis unit the two granitic bodies intrude the thiviers payzac unit only the younger estivaux granite is a syntectonic pluton which emplaced during left lateral wrenching 40ar 39ar dates from biotites indicate an early carboniferous age 346 3 ma the older granite is a pretectonic body it is the ordovician saut du saumon augen orthogneiss in which detailed structural analyses show the polyphase nature of the solid state deformation our microtectonic data indicate that the right lateral motions overprint the left lateral ones and produce apparently symmetrical fabrics

70 m y history 139 69 ma for the caribbean large igneous province it is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots mantle plumes new 40ar 39ar ages 111 139 ma and geochemical results from the nicoya peninsula costa rica extend the age range of volcanism in the caribbean large igneous province to 70 m y 69 139 ma our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the galapagos instead we propose that multiple oceanic intraplate igneous structures such as plateaus and hotspot tracks were accumulated through the subduction process the igneous structures could be remnants of the earlier history of the galapagos hotspot making it one of the oldest active hotspots on earth alternatively they could have been derived from several spatially distinct mantle melting events that sampled similar source material e g oceanic lithosphere of similar age

shear zone hosted polymetallic sulfides in the south limousin area massif central france remobilized sulfide deposits related to variscan collisional tectonics and amphibolite facies metamorphism this paper investigates the relationship between deformation and mineralization developed in amphibolite facies shear zones in the south limousin area using structural microstructural and textural analyses the investigated shear zone is a ductile thrust fault that stacked low grade metamorphic rocks onto a highly metamorphosed basement with both being folded during a late orogenic tectonic phase the shear zone hosts sulfide bearing mylonitic quartzites composed of dynamically recrystallized quartz syntectonic pyrite pyrrhotite sphalerite chalcopyrite and late covellite mylonites display northwest southeast trending lineation and asymmetric microstructures including sigmoidal quartz aggregates and shear bands that indicate a northwestward movement along the shear zone sulfides occur mainly along shear bands and exhibit primary undeformed textures attesting to their syntectonic emplacement based on petrographic analyses of the metamorphic host rocks mineralization formed at p 5 0 to 5 5 kbars and t 300 to 600 c consistent with p t conditions for recrystallization of metapelites and metabasites involved in the shear zone all data indicate that ductile deformation and emplacement of mineralization were coeval the occurrence of similar pretectonic sulfide deposits in the hanging wall of the mineralized shear zone suggests that the sulfides in the thrust fault were remobilized from preexisting mineralization by fluid that migrated toward the thrust this is the first evidence for thrust zone hosted mineralization within the massif central and attests to the role of crustal scale thrust faults as occasional pathways for mineralized fluids during the upper paleozoic continental collision these results suggest that ductile thrust faults may be important sites of ore deposition in the variscan belt of western europe and in similar collisional belts elsewhere

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

questions about myrmekite in deformed rocks abstract several questions about the development of myrmekite in deformed felsic and metapelitic rocks remain unanswered although a general relationship between deformation and myrmekite appears to be well established the fact that myrmekite grows into relatively non deformed k feldspar while being simultaneously deformed and recrystallized at the rear of the growing lobes suggests that strain energy may not be a major contributor to the advance of the growth interface of the myrmekite colony furthermore because myrmekite typically nucleates on existing plagioclase rather than in k feldspar strain energy may not contribute directly to nucleation of myrmekite either at least in many instances however strain energy probably contributes indirectly by facilitating access of fluids to growth sites thereby altering the local chemical environment and so promoting development of myrmekite because myrmekite recrystallizes readily in deformed felsic rocks it is one of the main contributors to the development of folia

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

episodic exhumation of hp rocks inferred from structural data and p t paths from the southwestern massif central variscan belt france abstract in the southwestern massif central france hp rocks are found in lower allochthonous units i e hp units sandwiched between upper allochthonous units i e lp units and the parautochthonous unit both devoid of hp rocks hp rocks were formed at minimum conditions of 1 7 gpa 700 c in response to the northward subduction of gondwana beneath armorica during the silurian 430 390 ma their exhumation results from an 80 ma long sequence of tectonic processes effective at various scales and active specifically at each main stage of the evolution of the gondwana lithosphere hp rocks were first exhumed by upward southwestward extrusion during the early devonian 390 380 ma in relation to late stage of subduction in the northern area exhumation is achieved by rifting during the upper devonian 365 ma the most obvious event corresponds to the northwestward thrusting of lp units above hp units during early stages of continental collision 355 ma hp rocks were exhumed for a second time during orogen parallel transpression in response to late stage of continental collision 350 345 ma exhumation in internal zones is achieved by crustal scale normal faulting and erosion during the middle carboniferous 335 315 ma synorogenic extension while compression was still active in the southern external zones exhumation of the whole region is completed by regional uplift and erosional denudation during the upper carboniferous 310 290 ma postorogenic extension subduction and collision have played a key role for driving hp rocks to the surface but their efficiency and the resulting strain pattern depend on the angle of convergence upper devonian rifting and the subsequent early carboniferous crustal thickening prevents to consider exhumation of variscan hp rocks as a single and continuous process and open new questions on the related paleozoic lithosphere history

intracontinental subduction and hinged unroofing along the salmon river suture zone west central idaho the salmon river suture zone in west central idaho juxtaposes volcanic arc rocks of the wallowa terrane directly against cratonic north america detailed metamorphic studies along a 10 km traverse perpendicular to the suture indicate that the arc and two crystalline fragments thrust upon it each record different pressure temperature p t histories from lowest to highest structural level the wallowa terrane shows only subgreenschist metamorphism the rapid river plate rrp records unroofing and cooling from 8 kbar and 550 c to 6 kbar and 475 500 c and the pollock mountain plate pmp shows evidence for polymetamorphism and records burial and heating paths to final equilibration conditions of 9 11 kbar and 600 625 c ar ar hornblende ages combined with the p t data suggest that currently exposed levels of the rrp and pmp were juxtaposed against one another at 15 20 km depth at or prior to 118 ma indicating that 10 20 km of uplift and hence also the onset of collision related metamorphism occurred before 118 ma correlation of the metamorphic and age data with geometric constraints from the initial sr 0 706 line and the dimensions of the rrp and pmp permit construction of large scale retrodeformable sections of the west side of the suture from late jurassic through late cretaceous time the abrupt nature of the sr 0 706 line implies that the arc continent boundary extends vertically through most of the crust which requires sharp downwarping of the arc lithosphere in order to account for the pmp metamorphic data narrow zoned overgrowths on pmp garnets record this burial event and require initially rapid 3 km m y uplift rates in order to be preserved we suggest that the onset of rapid uplift resulted from the separation of the negatively buoyant lithospheric root from the downwarped arc allowing buoyant rise of fragments of thickened crust detachment of the root is suggested to change the environment of crustal shortening from one in which footwalls of thrusts or shear zones sink to one in which hanging walls rise this mechanism represents an alternative to cessation of shortening or onset of tectonic denudation as an explanation for the transition from burial to uplift of high pressure metamorphic terrains subsequent uplift appears to have been slow and to have occurred in a hinged fashion such that mineral and whole rock ages decrease systematically towards the suture zone the consumption of lithosphere during 40 km of shortening between two crustal blocks implies that the salmon river suture is the trace of an intracontinental subduction zone burial and collision apparently began before about 130 ma and thus any precollision strike slip faulting or tectonic escape of intervening terranes was likely accomplished in jurassic and earliest cretaceous time

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

characteristics and mode of emplacement of gneiss domes and plutonic domes in central eastern pyrenees abstract gneiss domes and plutonic granitoid domes make up almost 50 of the pre hercynian terrains in the central and eastern pyrenees from a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism the study also shows that the internal structure the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes this appears to be in good agreement with h ramberg s 1967 gravity deformation and the earth s crust academic press london 1970 model studies in relation to intrusion of plutonic bodies in mechanisms of igneous intrusion edited by newall g rast n geol j spec issue 2 261 286 model studies showing that dome or mushroom like structures similar to those observed develop when there is a small viscosity ratio between the rising body and its enclosing medium this implies a high crystal content for the granitoid magma this crystal content has been estimated by i calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension using the model and program of j p carron et al 1978 bull soc geol fr 20 739 744 ii using the recent results of experimental deformation of partially melted granites of i van der molen m s paterson 1979 contr miner petrol 70 299 318 and ii comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes the minimum crystal content for the development of a dome like structure has been thus estimated to about 70 i e a value very close to that estimated by van der molen paterson 1979 to be the critical value separating the granular framework flow from suspension like behaviour the effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments these small variations appear to exert a strong control on the shape and rate of rise of the domes they are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes finally more complex experiments with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data are reported their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed

thermochronology and tectonics of the central and western cordilleras of colombia early cretaceous tertiary evolution of the northern andes abstract new thermochronological data record a complex cooling history in the central and western cordilleras of colombia that is a function of early cretaceous to late miocene tectonic events alkali feldspar 40ar 39ar cooling ages of 138 130 ma immediately post date the cessation of jurassic arc magmatism and a major unconformity within the retro foreland region of the northern andes we interpret these ages as cooling driven by exhumation in response to either compression driven by subduction of a seamount or extension and oceanward migration of the slab during the earliest cretaceous giving rise to the lower cretaceous quebradagrande arc sequence biotite and alkali feldspar 40ar 39ar data from the palaeocontinental margin reveal the presence of a younger cooling event at 117 107 ma which was contemporaneous with hornblende 40ar 39ar cooling ages obtained from medium high p t metamorphic relicts of a late jurassic early cretaceous subduction channel this cooling event is attributed to exhumation driven by the collision and accretion of a fringing arc against the continental margin and obduction of the subduction channel onto the forearc inverse modelling of zircon and apatite fission track and u th he data from throughout the central and western cordilleras reveals three periods of rapid cooling since the late cretaceous the earliest phase is recorded by jurassic and cretaceous granitoids that cooled rapidly during 75 65 ma we attribute cooling to exhumation of the continental margin during 75 70 ma 1 6 km my which was forced by the collision and accretion of the caribbean large igneous province in the campanian the central cordillera exhumed at moderate rates of 0 3 km my during 45 30 ma which are also observed over widely dispersed regions along the andean chain and were probably caused by an increase in continent ocean plate convergence rates exhumation rates drastically increased in the middle late miocene with the greatest amount occurring in southern colombia as a consequence of the collision and subduction of the buoyant carnegie ridge at 15 ma

the basaltic volcanism of the dumisseau formation in the sierra de bahoruco sw dominican republic a record of the mantle plume related magmatism of the caribbean large igneous province abstract the basaltic volcanism of the dumisseau fm in the sierra de bahoruco sw dominican republic offers the opportunity to study on land the volcanism of the caribbean large igneous province clip it consists of an at least 1 5 km thick sequence of submarine basaltic flows and pyroclastic deposits intruded by doleritic dykes and sills three geochemical groups have been identified low ti tholeiites group i high ti transitional basalts group ii and high ti and lree enriched alkaline basalts group iii these geochemical signatures indicate a plume source for all groups of basalts which are compositionally similar to the volcanic rocks that make up various clip fragments in the northern region of the caribbean plate trace element modelling indicates that group i magmas are products of 8 20 melting of spinel lherzolite group ii magmas result 4 10 melting of a mixture of spinel and garnet lherzolite and group iii basalts are derived by low degrees 0 05 4 of melting of garnet lherzolite dynamic melting models suggest that basalts represent aggregate melts produced by progressive decompression melting in a mantle plume there is no compositional evidence for the involvement of a caribbean supra subduction zone mantle or crust in the generation of the basalts two 40 ar 39 ar whole rock ages reflect the crystallisation of group ii magmas at least in the late campanian 74 ma and the lower eocene 53 ma all data suggest that the dumisseau fm is an emerged fragment of the clip which continues southward through the beata ridge

a cretaceous back arc basin in the coast belt of the northern canadian cordillera evidence from geochemical and neodymium isotope characteristics of the kluane metamorphic assemblage southwest yukon the coast belt of the northern cordillera in canada is the locus of the boundary between accreted and ancient north american margin rocks the largest exposure of metasedimentary rocks in the coast belt is the kluane metamorphic assemblage kma a northwest striking belt 160 km long of graphitic mica quartz schist and gneiss with minor interfoliated olivine serpentinite the kma does not appear to correlate with other sedimentary or metamorphic rock assemblages in the canadian cordillera to determine its tectonic setting and protolith provenance we analyzed trace element rare earth elements and neodymium isotope compositions of the kma of the adjacent pericratonic aishihik metamorphic suite ams of the yukon tanana terrane and of adjacent slates of the dezadeash formation df filling a late jurassic early cretaceous flysch basin the end 0 values of analyzed kma samples range from 1 4 to 5 6 and depleted mantle model ages t dm range from 1 16 to 1 45 ga kma samples are intermediate between more evolved ams samples average end 0 25 t dm 2 6 ga and more juvenile df samples end 0 1 9 t dm 0 95 ga the intermediate characteristics of the kma samples cannot be linked to a known source region and are interpreted to reflect homogeneous mixing from predominantly juvenile and minor evolved sedimentary sources a compatible tectonic setting is a back arc basin within influence of a continental source eastward subduction of the kma beneath ancient north america collapsed the back arc basin by latest cretaceous time

extensional orogenic collapse captured by strike slip tectonics constraints from structural geology and upb geochronology of the pinhel shear zone variscan orogen iberian massif abstract the late paleozoic collision between gondwana and laurussia resulted in the polyphase deformation and magmatism that characterizes the iberian massif of the variscan orogen in the central iberian zone initial continental thickening d1 folding and thrusting was followed by extensional orogenic collapse d2 responsible for the exhumation of high grade rocks coeval to the emplacement of granitoids this study presents a tectonometamorphic analysis of the trancoso pinhel region central iberian zone to explain the processes in place during the transition from an extension dominated state d2 to a compression dominated one d3 we reveal the existence of low dipping d2 extensional structures later affected by several pulses of subhorizontal shortening each of them typified by upright folds and strike slip shearing d3 d4 and d5 as identified by superimposition of structures the d2 pinhel extensional shear zone separates a low grade domain from an underlying high grade domain and it contributed to the thermal reequilibration of the orogen by facilitating heat advection from lower parts of the crust crustal thinning decompression melting and magma intrusion progressive lessening of the gravitational disequilibrium carried out by this d2 shear zone led to a switch from subhorizontal extension to compression and the eventual cessation and capture of the pinhel shear zone by strike slip tectonics during renewed crustal shortening high grade domains of the pinhel shear zone were folded together with low grade domains to define the current upright folded structure of the trancoso pinhel region the d3 tamames marofa satao synform new dating of syn orogenic granitoids shrimp u pb zircon dating intruding the pinhel shear zone together with the already published ages of early extensional fabrics constrain the functioning of this shear zone to ca 331 311 ma with maximum tectonomagmatic activity at ca 321 317 ma the capture and apparent cessation of movement of the pinhel shear zone occurred at ca 317 311 ma

folding and granite emplacement inferred from structural strain tem and gravimetric analyses the case study of the tulle antiform sw french massif central abstract in the south limousin french massif central the tulle anticline is a kilometre scale upright open fold which re folds an already foliated variscan stack of nappes three main granodioritic plutons associated with numerous folded dykes and boudinaged sills crop out in the core of the tulle anticline microstructural study including quartz c axis petrofabrics and transmission electron microscope tem analysis allows us to recognize the timing of deformation during pluton emplacement and cooling the earliest fabric is a pre full crystallization deformation the second stage corresponds to a high temperature ca 650 c solid state deformation the last deformation is responsible for the c axis preferred orientation at low temperature several methods of strain analysis applied in various mineral phases show that pluton emplacement is coeval to a constrictional strain this strain is characterized by a nw se maximum stretching axis x parallel to the tulle anticline axis and an intermediate ne sw trending shortening axis y also responsible for large open folding of the host rocks gravimetric modelling allows us to determine a laccolithic shape for the plutons hot stokes diapirism is ruled out and the hypothesis of granitic magma channelling through dykes is preferred a laccolith emplacement model based on a syn folding decollement level is proposed

on granitoid emplacement and related structures a review granitoid magmas are emplaced at the upper crust under a complex interaction of gravitational processes and horizontal tectonics natural intrusive bodies show a wide variety of structural patterns that must be strongly related with their dynamic history of ascent and emplacement comparisons of natural structural patterns with experimental and numerical models are necessary to interpret in a coherent fashion the emplacement mechanism involved from natural experimental numerical and theoretical considerations one can reach important conclusions on the problem of ascent and final emplacement of granitoid magmas in the earth s crust the present paper reviews the principal emplacement mechanisms referred to in the literature that is doming diapirism ballooning stoping cauldron subsidence and dike propagation dike propagation is the most effektive process in magma transport from deep zones in the crust and upper mantle extensional fractures for dike propagation and magma ascent can be developed at depth in the crust according to the modern dike propagation theory in orogenic domains magmas reach the upper crust through narrow channels or dikes and are accumulated in a final reservoir in the form of a discordant supracrustal pluton or a ballooning pluton if during emplacement any regional deformation acts simultaneously

using pluton ages to date regional deformations problems with commonly used criteria studies of pretectonic plutons in the foothills terrane central sierra nevada california along with a review of studies on syntectonic and post tectonic plutons indicate that no single criterion can establish the relative timing of pluton emplacement and regional deformation criteria most often used are whether a pluton is deformed cuts regional structures or is associated with porphyroblasts that postdate regional structures however all three types of plutons may 1 appear deformed or nondeformed 2 have contacts that appear to cut structures in the wall rocks and 3 have porphyroblasts in their aureoles that display varied timing relations the ambiguity of these criteria emphasize the need for careful studies of the structures present throughout the pluton and surrounding wall rock with particular attention paid to regions where the pluton contact is at high angles to regional structures porphyroblast cleavage relations are an important tool in this regard but also must be used with caution

pluton accommodation at high strain rates in the upper continental crust the example of the central extremadura batholith spain abstract emplacement in the tensional bridge of a stepped dextral shear zone system is proposed for the central extremadura batholith spain the country rocks show a pervasive anisotropy that conditioned the style of the structures developed as a consequence of the transference of displacement from the stepped shear zones to the releasing area the kinematic evolution of the resulting megakink fold provided the volume increase necessary for the granite emplacement thermal and kinematic models suggest that the growth of individual plutons took place in periods of no more than several hundred to a few thousand years fast strain rates 10 10 10 11 s 1 must concentrate in local structures e g initiation of kink folds even in zones deforming as a whole under typical strain rates 10 14 1 s 1 granite plutons might be used as strain rate gauges for syn plutonic structures

deep mantle structure as a reference frame for movements in and on the earth earth s residual geoid is dominated by a degree 2 mode with elevated regions above large low shear wave velocity provinces on the core mantle boundary beneath africa and the pacific the edges of these deep mantle bodies when projected radially to the earth s surface correlate with the reconstructed positions of large igneous provinces and kimberlites since pangea formed about 320 million years ago using this surface to core mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander we have developed a model for absolute plate motion back to earliest paleozoic time 540 ma for the paleozoic we have identified six phases of slow oscillatory true polar wander during which the earth s axis of minimum moment of inertia was similar to that of mesozoic times the rates of paleozoic true polar wander 1 my are compatible with those in the mesozoic but absolute plate velocities are on average twice as high our reconstructions generate geologically plausible scenarios with large igneous provinces and kimberlites sourced from the margins of the large low shear wave velocity provinces as in mesozoic and cenozoic times this absolute kinematic model suggests that a degree 2 convection mode within the earth s mantle may have operated throughout the entire phanerozoic

a 3 d kinematic model of fabric development in polycrystalline aggregates comparisons with experimental and natural examples abstract a 3 d generalization of the approach previously developed by etchecopar for the simulation of fabrics in polycrystalline aggregates is presented the model is based on a geometric minimization of gaps overlaps and boundary displacements between deformed neighbouring cells each cell may deform using a small number

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

intra panthalassa ocean subduction zones revealed by fossil arcs and mantle structure the vast panthalassa ocean once surrounded the supercontinent pangaea subduction has since consumed most of the oceanic plates that formed the ocean floor so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean s history since the cretaceous period 1 2 and the triassic jurassic plate tectonic evolution of the panthalassa ocean remains largely unresolved 3 4 geological clues come from extinct intra oceanic volcanic arcs that formed above ancient subduction zones but have since been accreted to the north american and asian continental margins 4 here we compile data on the composition the timing of formation and accretion and the present day locations of these volcanic arcs and show that intra oceanic subduction zones must have once been situated in a central panthalassa location in our plate tectonic reconstructions 5 7 to constrain the palaeoposition of the extinct arcs we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic wave tomographic models 8 9 we suggest that a series of subduction zones together called telkhinia may have defined two separate palaeo oceanic plate systems the pontus and thalassa oceans our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the telkhinia subduction zones and panthalassa ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics the spatial uncertainty in reconstructions of oceanic basins increases the further we go back in time because oceanic plates subduct plate reconstructions for the triassicjurassic history of the panthalassa ocean once surrounding the pangaea supercontinent are based on the premise that the pacific plate formedinthemiddlejurassicasatriangle 2 originatingfromatriple junction between the farallon phoenix and izanagi plates fig 1 the preserved area of jurassic palaeoplates represents only a small portion 10 of the oceanic crust of the jurassic panthalassa ocean the remaining part along with pre jurassic oceanic plate configurations is entirely unconstrained however the geology of the circum panthalassa continental margins provides evidence that early mesozoic subduction related volcanism occurred in the panthalassa ocean 4 10 on subduction continental fragments volcanic arcs ophiolites and ocean floor sediments collided with or accreted to the continental margins of in particular western north america far east asia and japan marine microfossils in these sediments and palaeomagnetic data constrain their age and palaeolatitude but their palaeolongitudes remain undetermined palaeogeographic reconstructions therefore show uncorrelated intra panthalassa exotic terranes essentially

quartz fabric transition in a cooling syntectonic granite hermitage massif france abstract the hermitage granite situated in the northwest massif central france is a syntectonic hercynian leucogranite emplaced along an active transcurrent shear zone during emplacement and cooling the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands c s mylonites increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline a slip and rotation recrystallization migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction we discuss relevant deformation mechanisms and rheological implications for syntectonic plutons in particular we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid state flow

spreading of salt structures in the gulf of mexico abstract past and present overhangs on diapirs ductile intrusions of salt are potential hydrocarbon traps and increasing numbers of larger overhangs are being recognised as seismic acquisition and processing improves salt overhangs develop by three process drag by ductile surroundings sinking around salt diapirs thinning of diapiric stems or the topic considered here gravity spreading gravity spreads salt where it is easier for salt to flow sideways than it is to float or sink the potential level to which individual salt diapirs rise depends on the pressure applied to their source by the overburden load and any lateral forces the potential level to which salt diapirs rise can be independent of both the top free surface and the level of neutral buoyancy of the salt even the most vigorous diapirs cannot rise indefinitely they gravitationally spread at barriers they cannot penetrate below at or above their level of neutral buoyancy it has been suggested that salt diapirs spread below their level of neutral buoyancy in weak layers beneath stiff barriers however no case of deep subsurface salt spreading appears to have been documented instead shales rise with salt along the us gulf coast some salt diapirs in the gulf of mexico may simulate ductile ice flows and spread both upwards and downwards to a subsurface level of neutral buoyancy in surroundings of similar strain rate however most diapirs in the gulf are driven above their level of neutral buoyancy so that they spread downwards back towards it these spread over denser or stifter layers in less dense and weaker barriers of air water or unconsolidated sediments this work focuses on the geological implications of the shapes of the tops of small scale secondary salt bodies that spread superficially in weaker and less dense barriers under the northern gulf of mexico analytical material and natural models are used to show that the shape of the top free surface of spreading salt contains information about both the properties of the materials involved and their kinematics many salt diapirs in the gulf spread downslope from viscous salt fountains in muds over sand salt flowing downslope towards a lower level of neutral buoyancy exhibits dynamic extrusion the temporary dynamic nature of salt sheets is emphasised by restating standard arguments for overburden strength in terms of low strain rates contrary to recent views that salt spreading occurs in weak horizons during fast sedimentation it is argued that most salt spreads superficially during periods of slow sedimentation small salt laccoliths advance downslope at long term rates between 1 and 5 mm a after burial spreading and gliding of their overburdens can detach such laccoliths from their source and smear them to salt sills that advance downslope at 8 mm a as development wells are likely to be complicated by such active salt flow we should begin now direct measurements of the rates at which salt spreads at superficial levels beneath the northern gulf of mexico

architecture of late orogenic plutons in the aracuai ribeira fold belt southeast brazil abstract post collisional to late orogenic magmatism 580 to 480 ma in the aracuai ribeira fold belt se brazil is characterized by the predominance of high k metaluminous allanite titanite bearing granitoids small lenses of coronitic gabbro anorthosite pyroxenite and phlogopite peridotite are also common in deeper exposed areas of this fold belt in the region of southern espirito santo state the deep erosional level associated with a steep topography reveals the internal architecture of the intrusions a tendency to funnel shaped bodies with sub vertical hemi ellipsoidal conical roots changing upwards to shallow dipping tops associated stocks sills and dykes of basic and acid rocks generally intrude the enclosing gneisses along the foliation planes local ductile shear zones and parallel to fold hinges the contact between intrusions and the enclosing rocks is sharp in deeply eroded plutons e g santa angelica venda nova mimoso do sul and varzea alegre but at shallow levels e g castelo pedra azul and conceicao de muqui agmatic stoping zones occur along the borders a magmatic foliation within the granitoids is usually well marked but the schistosity in the surrounding gneisses wraps around the plutons the intrusions have a bimodal chemical distribution and generally are reversely zoned with mafic cores gabbro diorite to tonalite surrounded by a mingled marble cake zone where basic and acid rocks are interfingered and an external zone of syenomonzonite and granite widespread evidence of mingling and mixing between contrasting magmas of gabbroic and granitic and or syenomonzonitic compositions is characteristic for all intrusive complexes we suggest that replacement of lithospheric mantle by hot asthenospheric mantle induced partial melting of the crust the mantle exchange was due to lithospheric mantle delamination and slab breakoff following collisional orogenesis the bimodal plutons result from interaction among the contrasting magmas ascent and emplacement followed older regional structures such as regional fold hinges and ductile shear zones

temporal variation of oceanic spreading and crustal production rates during the last 180 my we present a re evaluation of seafloor spreading and generation rates mainly based on a direct measurement of the remaining surfaces of oceanic crust and isochron lengths defined in the most recent isochron maps j y royer r d muller l m gahagan l a lawyer c l mayes d nurnberg j g sclater a global isochron chart tech rep 117 austin univ of tex inst for geophys 1992 r d muller w r roest j y royer l m gahagan j g sclater digital isochrons of the world s ocean floor j geophys res 102 1997 3211 3214 our evaluation of the amount of oceanic crust per unit age da dt as a function of age which can be expressed as da dt co 1 t tm is in fairly good agreement with previous determinations j g sclater b parsons c jaupart oceans and continents similarities and differences in the mechanisms of heat loss j geophys res 86 1981 11 535 11 552 d b rowley rate of plate creation and destruction 180 ma to present geol soc amer bull 114 2002 927 933 with co 2 850 0 119 km2 year 1 and tm 180 2 9 7 ma dividing these da dt by the ridge lengths l defined as the isochron length at each epoch allowed us to compute the evolution of global half spreading rates these have been roughly constant at 25 9 3 3 mm year 1 for at least the last 150 ma we propose that the global seafloor surface generation rate is roughly constant as well with a mean half value of 1 298 0 284 km2 year 1 and varying 20 with time this study corroborates the recent conclusion of rowley d b rowley rate of plate creation and destruction 180 ma to present geol soc amer bull 114 2002 927 933 of a constant generation rate since 180 ma and completely contradicts the commonly accepted idea of high seafloor spreading and surface generation rates during a large part of the cretaceous combining the oceanic surface generation rates derived here with crustal thicknesses deduced from the chemical composition of old oceanic crusts and seismic measurements e humler c h langmuir v daux depth versus age new perspectives from the chemical compositions of ancient crust earth planet sci lett 173 1999 7 23 the magmatic flux at young 0 80 ma oceanic ridges appears to be about 18 1 3 4 km3 year 1 and was possibly 15 to 30 higher during the mesozoic we propose that mantle temperature variation provides an alternative mechanism to spreading rate for the cretaceous highstand in sea level and atmospheric co2 generation

role of ocean continent contrast and continental keels on plate motion net rotation of lithosphere and the geoid three dimensional spherical shell models of mantle convection have been formulated to investigate the effects of ocean continent contrast i e different viscosity for continental and oceanic lithospheres and continental keels on plate motion net rotation of lithosphere i e degree 1 toroidal plate motion and the geoid the models include relatively realistic plate rheology i e strong plate interiors and weak plate margins and continental keel structure based on seismic models the mantle flow in the models is driven by slab buoyancy the models demonstrate that the ocean continent contrast and continental keels have minor effects on the long wavelength geoid for degrees 2 5 however continental keels overriding subducted slabs for example in north america can produce large negative gravity anomalies at a regional scale this may have important implications to interpreting the gravity anomalies in north america our models show that when plates bounded by weak plate margins have the same thickness neither weak plate margins nor the ocean continent contrast efficiently excite net rotation of lithosphere although they excite toroidal motion at higher harmonic degrees however plate thickness variations like continental keels excite the net rotation in spite of their ability to excite the net rotation continental keels as thick as 300 km cannot provide the necessary coupling to the deep mantle to produce the observed net rotation of lithosphere and slow continental motion if there is a weak asthenosphere underlying them if the thermomechanic structure of north american continental upper mantle derived from seismic and heat flow studies is representative our models suggest that the temperature and pressure dependent mantle rheology may not produce sufficiently high viscosity below continental keels that is needed to explain the observed plate motion our study hints the necessity to include more realistic treatment of plate boundaries in future studies to assess possible effects of plate plate coupling on plate motion

paleogeographic record of eocene farallon slab rollback beneath western north america eocene terrestrial strata in western north america are now dated with sufficient precision to examine the hypothesis that rollback of the farallon flat slab created a trenchward migrating wave of dynamic and thermal topography we use patterns of accumulation and unconformity lake type stratigraphy and physical and isotopic provenance tools to track its migration across the laramide foreland between 53 ma and 47 ma hydrologic ponding and coincident unconformity development began at 53 ma culminating at 51 8 ma in the hydrologic closure of an 900 000 km2 area between the cordilleran and rocky mountain divides four subsequent stream diversions and a southwest expanding regional unconformity record trenchward migration of uplift from 50 to 47 ma volcaniclastic detritus from magmatic centers coincident with the uplifted region progressively filled the green river formation lakes from north to south these findings are consistent with recent numerical and conceptual models of slab rollback that predict initial dynamic subsidence above the slab hinge followed by uplift and volcanism triggered by influx of asthenosphere beneath the overriding plate based on the surface record rollback proceeded systematically at a rate of 6 cm yr across the wyoming craton from 53 to 47 ma resulting in removal of the entire shatsky conjugate oceanic plateau from the base of the north american lithosphere

the nonmarine lower cretaceous of the north american western interior foreland basin new biostratigraphic results from ostracod correlations and early mammals and their implications for paleontology and geology of the basin an overview abstract the timespan represented by the hiatus between nonmarine upper jurassic early berriasian and unconformably overlying lower cretaceous deposits throughout the north american western interior foreland basin has been under discussion for the entire 20th century and remains controversial to date ongoing research in revision of early cretaceous nonmarine ostracods of some respective north american formations leads to a breakthrough concerning the verification of their biostratigraphic utility as well as their subsequent application these ostracods are not as endemic as hitherto believed and can be used for supraregional and regional correlation as well as improvement of the age determination of north american units new results strongly suggest a maximum age of late berriasian to valanginian 142 138 ma for the lower part of the lakota black hills area south dakota and cedar mountain utah formations a pre aptian maximum age for the lakota formation is supported by early mammals these biostratigraphic results affect the correlatable formations as well and therefore have broad implications on basin related geologic and paleontologic topics that are overviewed and discussed herein the central issue hampering an integrated synthesis of the foreland basin is its yet imprecise chronostratigraphic framework and documentation temporal relationships between the gologic processes of the basin and their control factors are still insufficiently calibrated or controversial detailed ongoing revision of north american early cretaceous nonmarine ostracods demonstrates their applicability utility and further potential as tool for improvement of the chronostratigraphy of the western interior foreland basin at both small and large scales these ostracods also foster understanding of animal e g early mammals and dinosaurs and plant angiosperms evolution on the north american continent and show promise of providing age determinations for single sample horizons in the near future

influence of continental roots and asthenosphere on plate mantle coupling the shear tractions that mantle flow exerts on the base of earth s lithosphere contribute to plate driving forces and lithospheric stresses we investigate the sensitivity of these tractions to sub lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity lateral viscosity variations generally do not change the direction of shear tractions but deeply penetrating continental roots increase traction magnitudes by a factor of 2 5 compared to 100 km thick lithosphere a low viscosity asthenosphere decreases traction magnitudes by a smaller amount and is important only if 100 km thick increased plate mantle coupling beneath thick continental lithosphere may increase plate driving forces surface deformation and mantle derived lithospheric stresses in these regions by contrast a low viscosity asthenosphere does not decouple the lithosphere from mantle flow highlighting the geological importance of mantle tractions on the lithosphere

extensional tectonics within a subduction type orogen the case study of the wugongshan dome jiangxi province southeastern china abstract in southeastern china the wugongshan massif consists of metamorphic rocks and granites generally considered as basement rocks formed during an early paleozoic orogeny our structural and radiometric reappraisal of this massif shows that it results from a mesozoic doming in the metamorphic rocks the foliation defines an e w elliptical shape and bears a n s stretching lineation the axial zone of the dome is occupied by oriented plutons the outer part of the dome consists of devonian quartzite carboniferous permian limestone and triassic jurassic sandstone ductile deformation observed in the permian carbonates and older units develops contrasting kinematics the northern and southern flanks present top to the north and top to the south motions respectively the same divergent pattern of motion is recorded from brittle structures up to the jurassic sandstone the late cretaceous red beds are undeformed in the axial zone coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates along the n and s flanks of the dome post folial recumbent folds are symmetrically overturned to the n and s respectively but at the two extremities of the dome the fold axes are parallel to the stretching lineation 40 ar 39 ar ages of biotite and muscovite from mylonite and granite suggest a late triassic age for the ductile deformation and an early cretaceous age for the final doming the wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime a model which emphasizes plutonism is discussed from permian to early cretaceous southeastern china was a subduction type orogen the crust of which was thickened by accretion of mantle derived magmas the emplacement of a large amount of mesozoic granitoids in southeastern china promoted thermal softening and gravitational instability that allowed the crust to extend the formation of the wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology

amphibolitization of metagabbros in the scottish highlands compositions of actinolite hornblende and cummingtonite together with pyroxene and plagioclase are studied in basic intrusions in the dalradian of north east scotland and the glen scaddle complex in the west moine amphibolitization is due to influx of water from the country rocks pyroxene compositions are found to have adjusted to the regional metamorphic environment owing to the difficulty of diffusion of al and si calcic amphiboles are zoned and commonly contain quartz blebs discontinuities in zoning give rise to actinolite hornblende pairs compared with north east scotland disequilibrium is less strong in the glen scaddle area in the latter plagioclase compositions have been greatly changed na partition between hornblende and plagioclase is close to equilibrium the maximum al content of hornblende is lower and zoning patterns are more consistent the fe mg ratio in calcic amphiboles varies with al content while approaching equilibrium partition with other minerals both zoning patterns and fe mg partition with cummingtonite suggest that fe mg of the calcic amphiboles increases more strongly with increasing alvi fe3 than can be explained simply by substitution of al fe3 for mg on m2 model reactions for amphibole formation are constructed cummingtonite formed at lower chemical potential of cao than actinolite ca was exchanged for mg fe between orthopyroxene derived and clinopyroxene derived local systems both cummingtonite and actinolite were formed because of kinetic constraints as intermediate reaction products actinolite hornblende pairs represent disequilibrium this work suggests that many occurrences of actinolite with hornblende where the minerals are zoned may also be due to diffusion kinetics

strain patterns within the late variscan granitic dome of velay french massif central the velay granites are intruded as a late variscan dome in the french massif central strain patterns within the velay granites reflect the geometry and kinematics of late carboniferous deformation at the site of final emplacement the internal deformation developed during granitic magmatism is not homogeneously distributed from inner to outer zones increasing deformation is marked by changes from high temperature foliations to s c orthogneisses strain concentration at the margins reflects a strength contrast due to cooling and subsequent magma hardening according to the internal strain field two zones can be distinguished in the velay dome i a central and southern zone where concentric patterns indicate a southward expansion of granites and ii a northern zone where the lateral expansion is limited and combined with top to the north extensional shearing along the mont pilat detachment zone and high temperature wrenching these asymmetric strain patterns relate to the expansion and deformation of granites emplaced below a detachment fault during the uplift of a thickened area post thickening combination of extensional and wrench tectonics is inferred

stability of active mantle upwelling revealed by net characteristics of plate tectonics global scale mantle flow patterns can be deduced from the net behaviour convergence and divergence of surface plate motions persistent quadrupole divergence in central africa and the central pacific suggest sustained stationary upwelling beneath these locations in the mantle earth s long history of intense geological activity mountain building and plate tectonic motion included is correlated with viscous flow of the underlying mantle clinton conrad and co authors show here that despite their apparent complexity the net characteristics of plate tectonics reflect simple and steady large scale patterns of viscous flow within the underlying mantle they tracked the geographic locations of net convergence and divergence from tectonic reconstructions covering the past 250 million years and found that the positions of divergence have not moved significantly and are found above two large upwellings beneath africa and the pacific ocean the authors conclude that these lowermost mantle regions may have remained relatively stationary over geological time and thereby organized global mantle flow viscous convection within the mantle is linked to tectonic plate motions1 2 3 and deforms earth s surface across wide areas4 5 6 such close links between surface geology and deep mantle dynamics presumably operated throughout earth s history but are difficult to investigate for past times because the history of mantle flow is poorly known7 here we show that the time dependence of global scale mantle flow can be deduced from the net behaviour of surface plate motions in particular we tracked the geographic locations of net convergence and divergence for harmonic degrees 1 and 2 by computing the dipole and quadrupole moments of plate motions from tectonic reconstructions8 9 extended back to the early mesozoic era for present day plate motions we find dipole convergence in eastern asia and quadrupole divergence in both central africa and the central pacific these orientations are nearly identical to the dipole and quadrupole orientations of underlying mantle flow which indicates that these net characteristics of plate motions reveal deeper flow patterns the positions of quadrupole divergence have not moved significantly during the past 250 million years which suggests long term stability of mantle upwelling beneath africa and the pacific ocean these upwelling locations are positioned above two compositionally and seismologically distinct10 regions of the lowermost mantle which may organize global mantle flow11 as they remain stationary over geologic time12

thermal and barometric constraints on the intrusive and unroofing history of the black mountains implications for timing initial dip and kinematics of detachment faulting in the death valley region california unroofing of the black mountains death valley california has resulted in the exposure of 1 7 ga crystalline basement late precambrian amphibolite facies metasedimentary rocks and a tertiary magmatic complex the 40ar 39ar cooling ages obtained from samples collected across the entire length of the range 55 km combined with geobarometric results from synextensional intrusions provide time depth constraints on the miocene intrusive history and extensional unroofing of the black mountains data from the southeastern black mountains and adjacent greenwater range suggest unroofing from shallow depths between 9 and 10 ma to the northwest in the crystalline core of the range biotite plateau ages from 13 to 6 8 ma from rocks making up the death valley turtlebacks indicate a midcrustal residence with temperatures 300 c prior to extensional unroofing biotite 40ar 39ar ages from both precambrian basement and tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest subparallel to the regional extension direction diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest the cooling age pattern and geobarometric constraints in crystalline rocks of the black mountains suggest denudation of 10 15 km along a northwest directed detachment system consistent with regional reconstructions of tertiary extension and with unroofing of a northwest deepening crustal section mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment the well known amargosa chaos and perhaps the badwater turtleback are examples of this splaying process considering the current distance of the structurally deepest samples away from moderately to steeply east tilted tertiary strata in the southeastern black mountains these data indicate an average initial dip of the detachment system of the order of 20 similar to that determined for detachment faults in west central arizona and southeastern california beginning with an initially listric geometry a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward this pattern may be explained by a model where migration of footwall flexures occur below a scoop shaped hanging wall block one consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading this process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

the biards sb au bearing shear zone massif central france an indicator of crustal scale transcurrent tectonics guiding late variscan collapse structural textural mineralogic microthermometric and isotopic data obtained on the biards sb au district located in the south limousin area massif central france provide new evidence for the tectonic setting of stibnite and gold deposition during the late orogenic variscan collapse in western europe a variety of geologic and geophysical data define a major crustal scale left lateral strike slip shear zone that acted as a trap for sb au mineralization during these movements fault fill veins and then extensional veins of limited extent were created along rheologic heterogeneities that focused brittle deformation geometry and orientation of both vein sets indicate two main episodes of emplacement of ore related to a progressive wrench movement in response to an counterclockwise rotation of the regional stress axes both veins sets are filled by quartz and sulfides in three stages an early arsenopyrite pyrite stage with fine grained quartz a sphalerite pb sb sulfosalt native gold stage with comb quartz and a late stibnite berthierite stage with comb quartz combined fluid inclusion and stable isotope studies on quartz muscovite and sulfides reveal that the three stages result from the continuous evolution of a deep seated hydrothermal system in which the latest stages mixed with meteoric water these data argue for cooling and a pressure drop during ore deposition and indicate a shallow level of emplacement for sb au veins a comparison of the biards deposits with other upper paleozoic gold and antimony deposits in the massif central the western variscides and elsewhere in the world illustrates their common association with crustal scale strike slip shear zones the upper carboniferous age 305 300 ma of the sb au deposits indicates that they formed during the early stages of the postthickening extension of the variscan lithosphere sb au bearing strike slip shear zones resulted from the late variscan crustal collapse guided by the very large scale dextral shearing between central europe northern africa and the northern appalachians and associated with the opening of the paleo tethys and the closing of the theic ocean during the late paleozoic gondwana laurasia translation

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

extensional deformation assisted by mineralised fluids within the brittle ductile transition insights from the southwestern massif central france abstract extensional deformation assisted by mineralized fluids within the brittle ductile transition is inferred using structural microstructural and textural investigations applied to the sarlande shear zone of the southwestern massif central france the shear zone is a low angle normal fault that bounds the meuzac antiform to the south these structures were formed during the nw se trending synorogenic extension of middle carboniferous age pre existing lithologies involved in the shear zone led to partitioning of behaviour and hydrothermal alteration in response to crustal extension and channelling of mineralising fluids intense fluid rock interactions produce softening and weakening reactions induced by alteration of k feldspar remobilisation of graphite and rutile and growth of white micas chlorite and polymetallic sulphides extensional shear deformation and mineralization progressively and concomitantly developed within the brittle ductile transitional crust 9 13 km depth the derived crustal scale model emphasises the importance of collision which led to the production of large quantities of granite derived fluids and the creation of a crustal permeability that drove these fluids near to the brittle ductile transition late orogenic extension associated with fluid rock interactions led to growth of mineralizing low angle shear zones and footwall domes the role of mineralising fluids fluid pressure appears to be essential in promoting deformation within the brittle ductile transition of the extending crust

tabular intrusion and folding of the late archaean murehwa granite zimbabwe during regional shortening the murehwa granite belongs to the late archaean chilimanzi suite of granites that occupy more than 50 of the zimbabwe craton the granite has compositional layers on metre to tens of metre scales with generally shallow dips and dip directions towards the centre of the granite near its margins the layering is systematically folded about sub horizontal east west axes microcline phenocrysts are generally strongly aligned parallel to the layering but locally crosscut the layering because they are axial planar to the folds crystals are not significantly deformed fabrics are thus magmatic the granite was intruded in a tabular sheet that may have been only about 6 km thick and fed by dykes contrary to previous concepts of diapirism ballooning for some archaean granites of the zimbabwe craton the systematic orientation of magmatic folds and axial planar fabrics demonstrate that the granite was intruded during regional north south shortening this shortening is consistent with the orientation of cratonic scale fractures that were intruded by the great dyke within 25 ma of the intrusion of the murehwa granite the granite may have intruded in a relatively short interval of less than 1 26 ma within the longer duration of the entire chilimanzi granite event

folding of granite and cretaceous exhumation associated with regional scale flexural slip folding and ridge subduction kitakami zone northeast japan abstract the early cretaceous granitic plutons intrude the kitakami zone northeast japan whose southern and northern regions consist of forearc basin and accretionary complex rocks respectively all country rock of the kitakami zone exhibit prominent pressure solution cleavage and associated folds formed during shortening with a small component of sinistral shear whereas most plutons show only igneous textures the kesengawa granite and some other plutons however have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks we document a tectonic fold with axial planar foliation in part of the kesengawa granite the metamorphic minerals associated with the contact aureole and country rocks of the kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled the kesengawa pluton and country rocks of the southern kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern kitakami zone are overturned and verge to the east the kitakami basement is not bounded by normal or reverse faults so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions nor is the exhumation associated with extensional doming instead the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines the regional shortening of accretionary prism forearc basin and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation the adakitic magmatism and forearc shortening resulted from subduction of the buoyant izanagi kula ridge a regional event known as the oshima orogeny

calcite twinning strains from syn faulting calcite gouge small offset strike slip normal and thrust faults we have evaluated the stress strain behavior of calcite precipitated and mechanically twinned in small offset strike slip normal and thrust faults of a variety of ages and from a variety of tectonic settings n 3001 twin measurements 63 strain analyses from 18 field sites five strike slip faults with syn faulting horizontally striated calcite rake 0 were studied and we report the orientations of the contemporaneous stress strain field associated with each fault intrusion of the marathon large igneous province mafic dikes 2 1 ga in archean crust minnesota usa post keweenaw rift 1 1 ga faulting island lake fault central ontario canada subduction associated with metamorphic core complex formation cretaceous china subduction cretaceous to miocene italy and continental extension recently active furnace creek fault death valley california usa seven normal faults with synfaulting dip slip striated calcite were studied and are from the following tectonic settings a normal fault slip surface in an ordovician piedmont fold appalachian s paleo subduction associated with cretaceous metamorphic core complex formation china 3 sites the paleo extensional atlantic margin 55 ma ireland 1 site with a u pb calcite age continental extension 1 active site mojave desert a transcurrent margin jamaica 1 active fault site and subduction 2 active faults along the eur african margin in italy with calcite u th disequilibria ages and crete respectively six thrust fault examples are all from convergent orogenic settings the basal thrust of the penokean 1850 ma fold and thrust belt the penokean orogen foreland in mesabi range banded iron formation folds an offset breccia body in the permian gondwanide belt ellsworth mountains antarctica the frontal thrust of the gondwanide cape belt south africa the paleocene frontal prospect thrust sevier belt wyoming and an alpine foreland back thrust lulworth cove u k for each strike slip fault system the twinning shortening strain is horizontal and at an angle of 0 60 to the respective fault plane dextral or sinistral although in the majority of cases the shortening axis is parallel to fault strike 13 of 23 results in each normal fault example dip slip kinematic striations dominate the faulted surface yet the orientation of the maximum principal compressive stress 1 and shortening strain axis e1 are not 45 to the fault plane as predicted but are sub horizontal and either strike parallel 25 of 35 results or strike normal 10 of 35 results thrust faults preserve shortening strain axes parallel to the dip slip kinematic direction within the fault plane plane strain and not at 45 to the principal plane 5 of 5 results none of the fault stress strain field results reported here support the andersonian or mohr coulomb criteria for stress strain relations predicted along faults

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

late orogenic carboniferous extensions in the variscan french massif central the variscan french massif central experienced two successive stages of extension from middle carboniferous to early permian in the northern massif central the first stage began in the late visean immediately after nappe stacking and is well recorded by namurian westphalian synkinematic plutonism the middle carboniferous leucogranites widespread in the nw massif central limousin and sioule area were emplaced within a crust extending along a ne sw direction at the same time the hanging wall or gueret extensional allochton moved toward the se several examples of the synextensional plutonism are also recognized in central limousin saint mathieu dome la porcherie and cornil leucogranites these examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by nw se stretching and ne sw shortening in the central and southern massif central cevennes ch taigneraie and margeride areas plutonism is dominantly granodioritic and exhibits the same structural features nw se maximum stretching and overturning to the se middle carboniferous namurian westphalian extension was parallel to the variscan belt both in the massif central and southern armorican area this extensional regime was active from the late visean in the north while compression dominated in the southernmost domains montagne noire and pyrenees the second extensional stage occurred from late carboniferous to early permian this event was responsible for the opening of intramontane coal basins brittle deformation in the upper crust and ductile normal faulting localized on the margin of cordierite granite migmatite domes data from the coal basins show that the half graben is the dominant structural style except for basins located along submeridianal left lateral faults which have pull apart geometries late carboniferous extension occurred along the ne sw direction the ne sw maximum stretching direction can be found in the whole massif central but is more developed in the eastern part the extensional direction is transverse to the general trend of the belt and top to the ne shearing is dominant correlations of these two extension directions with neighboring variscan massifs are discussed

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

space accommodation by roof lifting during pluton emplacement at amiata italy the amiata area is characterized by an uplift attributed to pluton emplacement during the pliocene epoch the shape of the buried pluton is constrained by gravity data fieldwork was carried out to identify possible deformations within the overburden in order to evaluate the mechanism of emplacement the attitudes of lower pliocene marine sediments deposited immediately before the uplift were measured also the overall pattern of the deposits defines a dome approximately 2 5 km high and 35 50 km wide consistency between these field data and the existing geophysical data suggests that the dome results from the emplacement of a laccolith like intrusion which was responsible for lifting and flexuring the overburden the amiata case shows that roof lifting is a viable process by which space in the upper crust to a depth of 7 km can be accommodated during pluton emplacement

isotopic evidence for rapid continental growth in an extensional accretionary orogen the tasmanides eastern australia the trace element signature of earth s continental crust resembles that of arc lavas but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra subduction zone magmatism we explore the role of coupled arc back arc accretionary processes in crust generation by considering the tectonic context of whole rock nd isotope and zircon hf o isotope data from igneous rocks of the australian tasmanides 515 230 ma which is thought to have evolved by the repeated opening and closure of sediment filled back arc basins behind a long lived subduction zone the significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective granitic rocks in this area define striking secular end ehf 18o trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity these trends reveal that juvenile magmatic input was enhanced during extensional back arc rifting episodes that immediately followed crustal thickening suggesting a relationship between slab rollback and continental growth interaction between juvenile magma and sedimentary units deposited during a preceding back arc rifting cycle was integral to the formation of stable continental material this highlights the importance of back arc environments for both the generation and differentiation of continental crust the juvenile component within the tasmanide igneous rocks increased from the cambrian to the triassic consistent with a diminished input from cratonderived metasedimentary material as the subduction zone migrated outboard of the gondwana margin subduction zone retreat formed large tracts of new crust in eastern australia at comparable rates to crust generation at modern island arcs providing a mechanism for rapid continental growth at convergent margins using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation