crustal scale mass transfer geotherm and topography at convergent plate boundaries end member cases for the fate of the crust at convergent lithospheric plate boundaries accommodated by subduction are considered combining i plate kinematics slab retreat vs slab advance and ii mechanical coupling decoupling within the subducting slab slab advance is accommodated by thickening of the overriding lithospheric mantle whereas slab retreat is associated with thinning crust mantle mechanical coupling results in continental subduction whereas decoupling leads to continental accretion the thermal and topographic evolutions of the associated orogenic belts are functions of the crust mantle thickness ratio resulting from the combined effects of crust mantle coupling decoupling and slab advance retreat crustal accretion associated with slab advance results in crustal thickening and contributes to an increase in radioactive heat production and buoyancy which is impeded by concomitant thickening of the conductive and dense lithospheric mantle in contrast crustal accretion combined with slab retreat results in thickening simultaneous with thinning of the lithospheric mantle this peculiar situation is the most favourable for generating a high temperature and buoyant orogenic belt

strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

thermochronology of central ribeira fold belt se brazil petrological and geochronological evidence for long term high temperature maintenance during western gondwana amalgamation abstract the studied sector of the central ribeira fold belt se brazil comprises metatexites diatexites charnockites and blastomylonites this study integrates petrological and thermochronological data in order to constrain the thermotectonic and geodynamic evolution of this neoproterozoic ordovician mobile belt during western gondwana amalgamation new data indicate that after an earlier collision stage at 610 ma zircon u pb age peak metamorphism and lower crust partial melting coeval with the main regional high grade d1 thrust deformation occurred at 572 562 ma zircon u pb ages the overall average cooling rate was low 650 c during sub horizontal d2 shearing until 510 470 ma garnet wr sm nd ages 1 2 c ma being then rapidly exhumed cooled 8 30 c ma during post orogenic d3 deformation with late granite emplacement at 490 ma zircon u pb age cooling rates based on garnet biotite fe mg diffusion are broadly consistent with the geochronological cooling rates a metatexites were cooled faster at high temperatures 6 c ma and slowly at low temperatures 0 1 c ma decreasing cooling rates with time b charnockites show low cooling rates 2 c ma near metamorphic peak conditions and high cooling rates 120 c ma at lower temperatures increasing cooling rates during retrogression the charnockite thermal evolution and the extensive production of granitoid melts in the area imply that high geothermal gradients were sustained for a long period of time 50 90 ma this thermal anomaly most likely reflects upwelling of asthenospheric mantle and magma underplating coupled with long term generation of high hpe heat producing elements granitoids these factors must have sustained elevated crustal geotherms for 100 ma promoting widespread charnockite generation at middle to lower crustal levels

cordierite as a sensor of fluid conditions in high grade metamorphism and crustal anatexis cordierite h2o and co2 volatile saturation surfaces derived from recent experimental studies are presented for p t conditions relevant to high grade metamorphism and used to evaluate fluid conditions attending partial melting and granulite formation the volatile saturation surfaces and saturation isopleths for both h2o and co2 in cordierite are strongly pressure dependent in contrast the uptake of h2o by cordierite in equilibrium with melts formed through biotite dehydration melting controlled by the distribution of h2o between granitic melt and cordierite dw dw wt h2o melt wt h2o crd is mainly temperature dependent dw 2 5 6 0 for the h2o contents 0 4 1 6 wt percentage typical of cordierite formed through biotite dehydration melting at 3 7 kbar and 725 900 c this range in dw causes a 15 30 relative decrease in the total wt of melt produced from pelites cordierite in s type granites are h2o rich 1 3 1 9 wt and close to or saturated in total volatiles signifying equilibration with crystallizing melts that achieved saturation in h2o in contrast the lower h2o contents 0 6 1 2 wt percentage preserved in cordierite from many granulite and contact migmatite terranes are consistent with fluid absent conditions during anatexis in several cases including the cooma migmatites and broken hill granulites the cordierite volatile compositions yield ah2o values 0 15 0 4 and melt h2o contents 2 2 4 4 wt compatible with model dehydration melting reactions in contrast h2o leakage is indicated for cordierite from prydz bay antarctica that preserve h2o contents 0 5 0 3 wt which are significantly less than those required 1 0 0 8 wt for equilibrium with melt at conditions of 6 kbar and 860 c the co2 contents of cordierite in migmatites range from negligible 0 1 wt to high 0 5 1 0 wt and bear no simple relationship to preserved cordierite h2o contents and ah2o in most cases the cordierite volatile contents yield total calculated fluid activities ah2o aco2 that are significantly less than those required for fluid saturation at the p t conditions of their formation whether this reflects fluid absence dilution of h2o and co2 by other components or leakage of h2o from cordierite is an issue that must be evaluated on a case by case basis

metamorphic rates in collisional orogeny from in situ allanite and monazite dating the prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates in individual samples from the northern lepontine central alps allanite is partly replaced by monazite at 560 580 c relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions 430 450 c coexisting monazite and allanite were dated in situ to delimit in time successive stages of the barrovian metamorphism in situ sensitive high resolution ion microprobe shrimp u th pb dating of allanite 31 5 1 3 and 29 2 1 0 ma and monazite 18 0 0 3 and 19 1 0 3 ma constrains the time elapsed between 430 450 c and 560 580 c which implies an average heating rate of 8 15 c m y combined with new fission track ages zircon 10 9 ma apatite 7 5 6 5 ma metamorphic rates of the entire orogenic cycle from prograde to final cooling can be reconstructed

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

two models for the formation of magma reservoirs by small increments abstract it is now clear that magma reservoirs develop over long time intervals out of a large number of individual intrusions evidence from some large volcanic deposits including those of the bishop tuff california and fish canyon tuff colorado indicates that such reservoirs may experience pervasive and rapid heating before catastrophic eruptions injection of hot primitive melt in the reservoir is often invoked to explain these observations but this requires the rapid emplacement of a large magma volume at least 25 km 3 in less than 100 years for the bishop tuff and efficient heat transfer through a thick cumulate pile within a magma body that grows incrementally temperatures rise progressively and may eventually lead to a permanent volume of melt temperature and crystallization follow two completely different paths depending on the thickness of individual intrusive sheets in one limit corresponding to thick sheets crystallization proceeds at equilibrium initial magma batches crystallize completely and a permanent body of melt develops progressively once ambient temperatures in the complex rise above the solidus in the other limit corresponding to thin sheets crystallization is kinetically controlled such that initial intrusions do not crystallize completely and preserve a glassy residue once temperatures within the growing magma pile reach a certain threshold value nucleation and growth of crystals get reactivated in the residual glass crystallization then proceeds catastrophically in a positive feedback loop involving latent heat release and temperature rise thus an initial phase with no evolved melt present ends with the sudden formation of a large volume of evolved melt in a crystal mush after this initial phase new intrusions get emplaced in heated surroundings and follow a thermal path similar to that of the equilibrium case with melts that become increasingly more primitive with time evidence for kinetic controls on crystallization in the natural environment is provided by the significant amounts of glass up to 40 that are found in thin isolated basaltic sills and dykes owing to sluggish crystallization kinetics more glass should be generated in andesitic and dacitic magmas emplaced in similar conditions devitrification of such quantities of glass accounts for the rapid heating of a thick sill complex the two models of magma reservoir formation involve different requirements the intrusion thickness must be smaller than a critical value in the kinetic model and the average magma input rate must exceed a threshold value in the equilibrium model for a given injection rate a permanent magma reservoir forms in a shorter time interval and over a smaller cumulative magma thickness in the kinetic model than in the equilibrium one the kinetic model is such that the amount of evolved melt generated increases with decreasing injection rate in contrast to the equilibrium model for which the opposite relationship holds

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

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

causes and consequences of protracted melting of the mid crust exposed in the north himalayan antiform gneiss domes exposed by the north himalayan antiform in southern tibet provide evidence for protracted melting of the mid crust during convergent tectonics anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low viscosity crustal sheet sr nd isotope systematics establish that the cores of the domes dominated by cambrian gneiss form a window of the high himalayan crystalline series hhcs indian plate combined trace element and isotopic analyses from 4 two mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the high himalayan crystalline series by the incongruent breakdown of muscovite and to a lesser extent biotite at temperatures from 680 to 760 c u pb dating of accessory phases extends the known period of granite magmatism from the late oligocene to mid miocene a ca rich garnet bearing leucogranite of oligocene age 27 5 ma provides a rare insight into the pre miocene history of the underthrust indian plate reflecting prograde melting at greater depths during a period of crustal thickening the origin of the miocene granites 15 10 ma can be modeled from the exhumation of an extruding channel at a rate of ca 1 mm year constrained by ar isotope data melting over a protracted time scale along the leading edge of the indian plate less than 50 km from the suture accelerated horizontal shortening within the collision zone and provided a positive feedback mechanism between magmagenesis and deformation

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

the late neoproterozoic early palaeozoic evolution of the west congo belt of nw angola geochronological u pb and ar ar and petrostructural constraints u pb zircon and ar ar amphibole and biotite ages from rocks of the west congo belt of angola indicate that this area underwent two main deformation events of amphibolite facies grade at c 540 and 490 ma which were followed by tectonically assisted exhumation during eastward thrusting of the hinterland domain onto the foreland domain high grade conditions in the west congo belt are 20 40 ma younger than in the ribeira aracuai belt of brazil its south american counterpart or than in the westernmost kaoko belt of namibia its african correlative in the present state of knowledge a more appropriate counterpart to the southern part of the west congo belt may be restricted to the cabo frio terrane in the eastern ribeira belt which yields a broadly similar evolution

strain distribution across a partially molten middle crust insights from the ams mapping of the carlos chagas anatexite aracuai belt east brazil abstract the easternmost part of the neoproterozoic aracuai belt comprises an anatectic domain that involves anatexites the carlos chagas unit leucogranites and migmatitic granulites that display a well developed fabric microstructural observations support that the deformation occurred in the magmatic to submagmatic state structural mapping integrating field and anisotropy of magnetic susceptibility ams revealed a complex 3d structure the northern domain displays gently dipping foliations bearing a nw trending lineation southward the lineation trend progressively rotates to ew then sw and the foliation is gently folded the eastern domain displays e w and ne sw trending foliations with moderate to steeply dips bearing a dominantly ns trending lineation magnetic mineralogy investigation suggests biotite as the main carrier of the magnetic susceptibility in the anatexites and ferromagnetic minerals in the granulites crystallographic preferred orientation cpo measurements using the electron backscatter diffraction ebsd technique suggest that the magnetic fabric comes from the crystalline anisotropy of biotite and feldspar grains especially the delineation of several structural domains with contrasted flow fabric suggests a 3d flow field involving westward thrusting orthogonal to the belt northwestward orogen oblique escape tectonics and ns orogen parallel flow this complex deformation pattern may be due to interplay of collision driven and gravity driven deformations

δ44 42 ca in gas hydrates porewaters and authigenic carbonates from niger delta sediments we report ca isotope ratios for gas hydrates pore waters and authigenic carbonate concretions recovered by coring at cold seep locations from the niger delta deep province our aim in this study has been to understand the mechanisms controlling ca isotope fractionation during biogeochemical processes at cold seeps carbonate precipitation in cold seep environments is closely related to the anaerobic oxidation of methane aom which is mediated by a consortium of micro organisms and releases bicarbonate hco3 and sulfide hs into surrounding pore waters 1 we found that carbonate precipitation causes ca enrichment in the surrounding porewaters as expected from previous work demonstrating that calcite prefers isotopically light ca 2 our measured ca values for pore waters are consistent with values estimated previously by teichert et al 3 in our studied sediment cores the highest ca ratios in pore waters were measured at the aom sediment depth dashed line in fig 1 indicating on going carbonate formation at these horizons porewater and carbonate ca measurements may therefore offere potential to identify the location and intensity of aom processes now and in the past we will also discuss the mineralogical control of ca isotope fractionation at cold seeps

u pb and 40ar 39ar geochronology of the saint urbain and lac allard havre saint pierre anorthosites and their associated fe ti oxide ores quebec evidence for emplacement and slow cooling during the collisional ottawan orogeny in the grenville province abstract intrusive suites containing anorthosite mangerite charnockite granite amcg are a characteristic feature of mesoproterozoic magmatic activity in the grenville province of eastern north america in this study the u pb and 40ar 39ar geochronology of two amcg suites saint urbain and havre saint pierre and associated rutile bearing hemo ilmenite deposits within the allochthonous polycyclic belt of the grenville province in quebec are used to constrain their crystallization ages and cooling histories and to assess their tectonic setting both amcg suites separated by 700 km along the st lawrence river have similar crystallization ages ca 1050 1060 ma and durations of magmatism 10 15 million years the 450 km2 saint urbain anorthosite is a composite intrusion with a distinctly younger central leuconorite 1046 2 3 1 ma compared with anorthosite to the north and south 1055 0 2 4 ma and 1053 6 2 6 ma respectively the age of a deformed oxide apatite gabbronorite 1057 4 1 5 ma along the northwest margin of the intrusion is within analytical uncertainty of the anorthosites whereas the oldest intrusion in the saint urbain area is the regionally extensive saint anne du nord orthopyroxene granodiorite 1060 8 2 8 ma the large 11 000 km2 havre saint pierre anorthosite suite comprises a number of intrusive phases including the 2700 km2 lac allard anorthosite which contains the giant lac tio ilmenite deposit and big island ilmenitite dyke u pb zircon ages of three anorthosites from the lac allard intrusion including the host to the lac tio deposit overlap within error 1061 6 3 0 ma 1060 5 1 9 ma 1057 4 8 4 ma and the crosscutting big island dyke yields an age of 1052 9 6 5 ma these results indicate that the havre saint pierre anorthosite should be divided into an older geon 11 part ca 1129 ma and a younger geon 10 part ca 1060 ma average cooling rates of 3 4 c million years were estimated for the two amcg suites by combining crystallization ages u pb zircon and cooling ages u pb rutile 40ar 39ar biotite plagioclase and reflect emplacement of the intrusions into a hot long duration orogen emplacement of these two amcg suites at ca 1050 1060 ma is related to regional extensional tectonics near the end of peak metamorphism associated with the collisional ottawan orogenic phase of the grenville orogeny the results of this study provide strong support for proposals that link amcg magmatism associated with the grenville orogeny to convective thinning or delamination of the lithosphere and subsequent melting of upwelling asthenospheric mantle

tectonic implications of precambrian sm nd dates from the southern sao francisco craton and adjacent aracuai and ribeira belts brazil abstract the archean and paleoproterozoic sao francisco craton of eastern brazil is surrounded on all sides by brasiliano pan african orogens the n ne trending orogen that separates the eastern edge of the southern sao francisco craton from the atlantic coast can be divided into the largely greenschist and amphibolite facies aracuai belt on the west and the largely granulite facies ribeira belt on the east a pronounced linear gravity and magnetic anomaly the abre campo discontinuity defines the boundary between these two belts we obtained sm nd mineral ages and whole rock sm nd model ages for garnet bearing metamorphic rocks along an e w transect across the southern sao francisco craton the southern aracuai belt and the ribeira belt at about latitude 20 s a recrystallization age of 2 1 ga from metasediments recrystallized during the development of the classic dome and keel province of the southern sao francisco craton the quadrilatero ferrifero indicates that dome emplacement occurred during the waning stages extensional collapse of the transamazonian collisional orogeny seven mineral ages from the southern aracuai and ribeira belts date the thermal peak of metamorphism at between 538 and 589 ma confirming that these belts were pervasively remobilized during the brasiliano event samples from the aracuai belt yield either archean 2 6 ga or transamazonian 2 1 2 3 ga tdm model ages indicating that the protoliths were either fragments of the sao francisco craton crust or were sediments derived from that craton which presumably had been stretched to form a thinned continental margin during meso and neo proterozoic rifting events notably a mixed meta pelite and metabasite sequence in the southern aracuai belt the dom silverio group has transamazonian ancestry and thus may represent oceanic sediments deposited on or east of this stretched margin and then thrust back onto the continent to mark a collisional suture between the sao francisco block and an archean crustal sliver to the east the rocks of the ribeira belt i e the region east of the abre campo discontinuity have younger model ages tdm 1 6 2 0 ga indicating that this belt was not originally part of the sao francisco craton it may represent an accreted transamazonian terrane therefore the abre campo discontinuity marks an important crustal boundary possibly a suture brasiliano shear zones appear to steepen and root into the abre campo discontinuity suggesting that it formed in brasiliano time

complex 3d strain patterns in a synkinematic tonalite batholith from the aracuai neoproterozoic orogen eastern brazil evidence from combined magnetic and isotopic chronology studies abstract this work combines structural and geochronological data to improve our understanding of the mechanical behaviour of continental crust involving large amount of magma or partially melted material in an abnormally hot collisional belt we performed a magnetic and geochronological u pb study on a huge tonalitic batholith from the neoproterozoic aracuai belt of east brazil to determine the strain distribution through space and time anisotropy of magnetic susceptibility combined with rock magnetism investigations supports that the magnetic fabric is a good proxy of the structural fabric field measurements together with the magnetic fabrics highlight the presence in the batholith of four domains characterized by contrasted magmatic flow patterns the western part is characterized by a gently dipping orogen parallel ns magmatic foliation that bears down dip lineations in agreement with westward thrusting onto the sao francisco craton eastward the magmatic foliation progressively turns sub vertical with a lineation that flips from sub horizontal to sub vertical over short distances this latter domain involves an elongated corridor in which the magmatic foliation is sub horizontal and bears an orogen parallel lineation finally the fourth narrow domain displays sub horizontal lineations on a sub vertical magmatic foliation oblique n150 e to the trend of the belt u pb dating of zircons from the various domains revealed homogeneity in age for all samples this together with the lack of solid state deformation suggests that 1 the whole batholith emplaced during a magmatic event at 580 ma 2 the deformation occurred before complete solidification and 3 the various fabrics are roughly contemporaneous the complex structural pattern mapped in the studied tonalitic batholith suggests a 3d deformation of a slowly cooling large magmatic body and its country rock we suggest that the development of the observed 3d flow field was promoted by the low viscosity of the middle crust that turned gravitational force as an active tectonic force combining with the east west convergence between the sao francisco and congo cratons

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

reactive monazite and robust zircon growth in diatexites and leucogranites from a hot slowly cooled orogen implications for the palaeoproterozoic tectonic evolution of the central fennoscandian shield sweden monazite in melt producing poly metamorphic terranes can grow dissolve or reprecipitate at different stages during orogenic evolution particularly in hot slowly cooling orogens such as the svecofennian owing to the high heat flow in such orogens small variations in pressure temperature or deformation intensity may promote a mineral reaction monazite in diatexites and leucogranites from two svecofennian domains yields older coeval and younger u pb sims and emp ages than zircon from the same rock as zircon precipitated during the melt bearing stage its u pb ages reflect the timing of peak metamorphism which is associated with partial melting and leucogranite formation in one of the domains the granite and diatexite belt zircon ages range between 1 87 and 1 86 ga whereas monazite yields two distinct double peaks at 1 87 1 86 and 1 82 1 80 ga the younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late orogenic shear zones magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to th u ratios and preferential growth sites of zircon reflecting the transition from melt to melt crystallisation of the magma in the adjacent ljusdal domain peak metamorphism in amphibolite facies occurred at 1 83 1 82 ga as given by both zircon and monazite chronology pre partial melting 1 85 ga contact metamorphic monazite is preserved in spite of the high grade overprint by combining structural analysis petrography and monazite and zircon geochronology a metamorphic terrane boundary has been identified it is concluded that the boundary formed by crustal shortening accommodated by major thrusting

near and mid infrared microspectroscopy of the ronda peridotite 1 microspectroscopy is a potentially useful tool for planetary applications as individual mineral grains can be targeted and measured in situ a series of closely coupled experiments was designed to investigate and compare the laboratory measurement of optical properties of a peridotite sample in the macroscale and microscale the ronda peridotite consists largely of olivine with significant orthopyroxene and minor clinoproxene and serpentine microscopic transmission spectra of unmounted thin sections produce strong well defined absorption features in the near infrared and band positions can be directly compared to macroscopic and microscopic reflectance spectra for near infrared reflectance where photons are multiply scattered by different grains measurement of a microscopic target means most photons are scattered out of the field of view and few photons return to the detector from the target area this precludes most microspectroscopic reflectance measurements of small features in the near infrared in the mid infrared where reflectance measurements involve only surface interactions microspectroscopic measurements detect well defined absorption features for most samples although the intensity of absorption features varies between grains band positions are consistent with macroscopic diffuse reflectance and emittance measurements of bulk samples and can be used to extract general chemical information such as iron magnesium ratios of olivines

slow cooling and crystallization of the roots of the neoproterozoic aracuai hot orogen se brazil implications for rheology strain distribution and deformation analysis the aracuai ribeira belt formed during the amalgamation of west gondwana in the late neoproterozoic its evolution included a main tectonometamorphic peak at 600 580 ma and a minor peak associated with the final collision with the western congo at 540 530 ma this belt has the characteristics of a hot orogen including a high thermal gradient 30 c km pervasive partial melting of the middle crust emplacement of large volumes of magmas resulting from partial melting of the lower crust and underlying mantle and slow cooling after the peak temperature we report 21 new amphibole biotite and muscovite 40ar 39ar ages that complement previously published data these data suggest slow cooling 3 5 c myr over several tens of million years after the peak temperature 800 c at 600 ma followed by faster cooling 10 c myr after the final amalgamation of west gondwana we estimate that 30 myr was required to heat the middle crust to the peak temperature and that anatectic and plutonic bodies remained in the magmatic state for 40 myr this protracted thermal evolution likely had major effects on the rheology of the middle crust and on the tectonic evolution of this orogen for example the correlation of u pb zircon crystallization ages and 40ar 39ar biotite cooling ages in the anatectic core of the orogen denotes a diachronic thermal evolution likely related to 3d deformation involving successive upwelling of anatectic components within a thrust unit crosscutting the pre existing fabric channel flow like this study also highlights that classical structural analysis techniques relying on changes in pressure or temperature conditions to identify the succession of deformation phases are not efficient at deciphering the tectonic evolution of hot slowly cooling orogenic belts where the temperature varies slowly over tens of million years allowing diachronic episodes of deformation to occur under nearly similar pressure and temperature conditions

models of large hot orogens containing a collage of reworked and accreted terranes we describe a classification scheme for orogens using temperature magnitude t m diagrams and use this framework for modelling large hot orogens that evolve in continents comprising cratonic nuclei bordered by a series of juvenile accreted reworked and metamorphosed terranes modelling the complete evolution of an orogen is difficult particularly large orogens with multiple orogenic phases early phases during which a continent is assembled produce a tectonic and metamorphic fabric that needs to be taken into account when modelling the main collisional orogeny this inherited fabric is represented in a simple way in models described here by a series of lower crustal blocks that are arranged to be systematically stronger toward the cratonic continental interiors we investigate how this fabric influences the development of the model orogen during the main collisional phase using upper mantle scale ums and crustal scale cs finite element models the models exhibit a diachronous three phase evolutio