calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

late proterozoic granitoid magmatism in the state of sao paulo southeastern brazil abstract the precambrian basement in the state of sao paulo is made up of a series of metamorphic units exposed at various erosional levels and in part separated by large faults a strong overprint of the brasiliano cycle 700 500 ma is evident from isotopic data over most of the region nevertheless a large part of the metamorphic belts belong to older archaean to middle proterozoic reworked sequences about 30 of the total basement area consists of intrusive granitoids of late proterozoic age these can be grouped into six major groups on the basis of petrography geochemistry rock association associated manifestations and whenever possible age relationships gneissic mangerite charnockite hastingsite granite associations appear as folded tabular bodies within high grade regions the sao jose do rio pardo suite is chemically comparable to the aluminous a type granitoid series and its emplacement possibly signals a period of extensional tectonics prior to the onset of the brasiliano cycle 1000 ma a series of elongated batholiths 650 ma with predominant porphyritic hornblende biotite granitoids is present mainly in high grade areas the compositions are generally granitic to granodioritic but intermediate e g monzodioritic rocks are locally common these high k calc alkaline suites are in some aspects comparable to the cordilleran i type granitoids and their origin might be related to a subductional environment migmatitic granitoids constitute a peculiar association occurring as a series of small bodies within high grade allochthonous domains these polyphase heterogeneous often nebulitic granitoids are source related products of crustal anatexis syn to late orogenic brasiliano granitoids in the low to medium grade domains are dominantly porphyritic calc alkaline biotite granites and granodiorites reminiscent of caledonian i types in contrast a peraluminous association porphyritic biotite granitoid to equigranular two mica granite predominates in high grade domains which possibly reflects a different tectonic evolution for the two regions late to post orogenic brasiliano granitoids intruded after the major orogenic development of the region 610 590 ma i type calc alkaline and aluminous a type granitoids are geographically associated in the itu belt and appear to show some temporal and geochemical overlapping wiborgitic facies and some sn w mineralization are locally present in the a type suites a post orogenic extensional period is marked in the south of the area by the emplacement of shallow a type granitoids in the serra do mar belt

upb geochronology of the central ribeira belt brazil and implications for the evolution of the brazilian orogeny abstract the ribeira belt in southeastern brazil was generated during the brazilian orogeny the south american equivalent of the pan african orogeny 700 450 ma the central sector of the belt is characterised by a series of northwest vergent crustal scale thrust stacks emplaced under amphibolite grade conditions major shear zones separate four lithotectonic domains andrel ndia to the northwest juiz de fora paraiba and coastal domains analyses of zircon monazite and titanite from a variety of rock types from the four domains lead to the following conclusions all domains record an important tectono thermal event at 590 565 ma represented by partial melting granitoid intrusion and remobilisation of older basement gneisses and corresponding to thrusting and development of dextral shear zones also recorded are older metamorphic ages 611 604 ma of unknown significance only the paraiba and coastal domains in the south record a younger metamorphic event m2 at 535 520 ma most likely associated with another episode of thrusting and shear zone development the absence of m2 metamorphism in the andrel ndia and juiz de fora domains can be explained by post 535 520 ma emplacement of the paraiba domain late to post tectonic activity is represented by 503 492 ma metamorphism precursors of the basement gneisses are 2185 2134 ma old rocks generated during the transamazonian orogeny some of which contain a significant archean component with minimum ages of 2981 2846 ma the ages presented above define specific metamorphic events for the brazilian orogeny for the first time in southeastern brazil in addition together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central ribeira belt represent an extension of the units present in the southern sao francisco craton and indicate that the foreland was more extensive than the presently defined craton

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

elemental and sr nd isotope geochemistry of two neoproterozoic mangerite suites in se brazil implications for the origin of the mangerite charnockite granite series abstract neoproterozoic 625 ma mangerites are the dominant rock type in a 100 km long belt of deformed tabular bodies in the socorro guaxupe nappe sgn a high grade terrane southwest of the sao francisco craton southeastern brazil the mangerites form two suites sao pedro de caldas shows a wide compositional variation and abundant felsic rocks mangerite hornblende granite leucogranite initial 87 sr 86 sr of 0 707 and initial e nd of 4 to 5 divinol ndia is dominated by more mafic mangerites with minor charnockite granite and diorite and has initial 87 sr 86 sr of 0 710 and initial e nd of 10 to 12 the probable sources of the sao pedro de caldas mangerites were granulites depleted by previous melting events in radiogenic elements and the light rare earth element primary melts of sao pedro de caldas were generated at 950 c p 10 kbar from a biotite free and k feldspar bearing source with plagioclase as an important residual phase the amount of melting was low as required by low mg enrichment in nb and ta relative to potential sources and high nb ta and was probably controlled by exhaustion of k feldspar in the source contribution from infracrustal mafic melts cannot be ruled out and would help explain the weakly negative e nd and high nb but is not essential the divinol ndia rocks depart from typical within plate mangerites towards more mafic rocks with orogenic signatures e g higher mg sr ca lower zr radiogenic isotope data require sources with greater crustal residence time than those envisioned for sao pedro de caldas elemental data require that these sources were more depleted e g higher k rb granulites with these characteristics are exposed in the northern portion of the sgn these were probably melted at slightly higher temperatures than the sao pedro de caldas suite 970 c before a sufficient amount of melt was generated and the magmas could leave their sources in this case k feldspar was exhausted and plagioclase in amounts exceeding those of the granite minimum was dissolved

continental subductions beneath tibet and the himalayan orogeny a review argand 1924 and later holmes 1965 proposed that the himalayan orogeny and tibet uplift were the result of large underthrusting of the indian continental crust beneath tibet since then plate motion studies have yielded new constraints on the relative displacements of india and asia and a large number of geological and geophysical investigations have helped to determine the structure of the himalayas and tibet the best natural laboratory for understanding mountain belts the aim of this paper is to review briefly the various models of himalayan mountain building in light of recent geological and geophysical data and to estimate the role of continental subductions

tectonometamorphic evolution of the himalayan metamorphic core between the annapurna and dhaulagiri central nepal the metamorphic core of the himalaya in the kali gandaki valley of central nepal corresponds to a 5 km thick sequence of upper amphibolite facies metasedimentary rocks this greater himalayan sequence ghs thrusts over the greenschist to lower amphibolite facies lesser himalayan sequence lhs along the lower miocene main central thrust mct and it is separated from the overlying low grade tethyan zone tz by the annapurna detachment structural petrographic geothermobaro metric and thermochronological data demonstrate that two major tectonometamorphic events charac terize the evolution of the ghs the first eohimalayan episode included prograde kyanite grade metamorphism during which the ghs was buried at depths greater than c 35 km a nappe structure in the lowermost tz suggests that the eohimalayan phase was associated with underthrusting of the ghs below the tz a c 37 ma 40ar 39ar hornblende date indicates a late eocene age for this phase the second neohimalayan event corresponded to a retrograde phase of kyanite grade recrystallization related to thrust emplacement of the ghs on the lhs prograde mineral assemblages in the mct zone equilibrated at average t 880 k 610 c and p 940 mpa 35 km probably close to peak of meta morphic conditions slightly higher in the ghs final equilibration of retrograde assemblages occurred at average t 810 k 540 c and p 650 mpa 24 km indicating re equilibration during exhumation controlled by thrusting along the mct and extension along the annapurna detachment these results suggest an earlier equilibration in the mct zone compared with higher levels as a consequence of a higher cooling rate in the basal part of the ghs during its thrusting on the colder lhs the annapurna detachment is considered to be a neohimalayan synmetamorphic structure representing extensional reactivation of the eohimalayan thrust along which the ghs initially underthrust the tz within the upper ghs a metamorphic discontinuity across a mylonitic shear zone testifies to significant late to post metamorphic out of sequence thrusting the entire ghs cooled homogeneously below 600 700 k 330 430 c between 15 and 13 ma middle miocene suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels

potential geologic hazards on the eastern gulf of cadiz slope sw spain abstract geologic hazards resulting from sedimentary oceanographic and tectonic processes affect more than one third of the offshore gulf of cadiz and are identified by interpreting high resolution seismic profiles and sonographs hazards of sedimentary origin include the occurrence of slope instability processes in the form of single or multiple slumps occupying up to 147 km 2 mainly concentrated in the steeper upper slope area besides the presence of steep slopes the triggering of submarine landslides is probably due to seismic activity and favoured by the presence of biogenic gas within the sediment gassy sediments and associated seafloor pockmarks cover more than 240 km 2 in the upper slope hazards from oceanographic processes result from the complex system of bottom currents created by the interaction of the strong mediterranean undercurrent and the rough seafloor physiography the local intensification of bottom currents is responsible for erosive processes along more than 1900 km 2 in the upper slope and in the canyons eroded in the central area of the slope undermining slopes and causing instability the strong bottom currents also create a mobile seafloor containing bedforms in an area of the gulf that extends more than 2500 km 2 mostly in the continental slope terraces hazards of tectonic origin are important because the gulf of cadiz straddles two major tectonic regions the azores gibraltar fracture zone and the betic range which results in diapir uplift over an area of more than 1000 km 2 and in active seismicity with earthquakes of moderate magnitude also tsunamis produced by strong earthquakes occur in the gulf of cadiz and are related to the tectonic activity along the azores gibraltar fracture zone

shear wave splitting in se brazil an effect of active or fossil upper mantle flow or both we investigated the structure of the upper mantle beneath southeastern brazil using teleseismic shear wave splitting measurements measurements were performed on seismic data recorded in the ribeira and brasilia neoproterozoic belts which wrap around the southern termination of the sa o francisco craton and disappear westward under the paranabasin in the northern ribeira belt dominated by thrust tectonics the fast shear wave polarization planes trend on average n080 e whereas in the central domain dominated by strike slip tectonics fast shear waves are polarized parallel to the structural trend n065 e stations located above the main transcurrent fault display large delay times s 2 5 s such values among the largest in the world require either an unusually large intrinsic anisotropy frozen within the lithosphere or a contribution from both the lithospheric and asthenospheric mantle within the southern brasilia belt fast split shear waves are polarized parallel to the structural trend of the belt at a high angle from the apm although part of our data set strongly favors an origin of anisotropy related to a fabric frozen in the lithospheric mantle since the neoproterozoic a contribution of the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters 2003 elsevier science b v all rights reserved

active crustal deformation from the azores triple junction to the middle east the co seismic crustal deformation along the africa eurasia eurasia arabia plate boundary from the azores triple junction 30 w up to the karliova triple junction 41 e is examined based on the moment tensor summation of mainly post 1964 earthquakes with ms 5 0 strain rates range from 10 8 yr to 10 6 yr while values exceeding 10 7 yr were calculated for the back arc aegean area and for the north anatolian fault zone a remarkable stability in the azimuth of the component of compression and of extension is observed in the area that extends west of italy 17 e up to the azores triple junction along this region the deformation is taken up by compression along a mean direction of n140 with rates from 1 1 to 2 8 mm yr and by extension along a mean direction of n50 with rates from 2 4 to 12 0 mm yr at the gloria fault and in the surrounding area dextral strike slip motion occurs at a rate of 11 mm yr from sicily up to the eastern end of the hellenic arc the crust is compressed at azimuths in the range n20 n31 with an average value of 26 5 this compression is occurring at a rate of 1 21 mm yr with an average value of 9 8 mm yr in the back arc aegean area extension is occurring along a mean azimuth of n156 with a rate of 5 2 3 8 mm yr the average shortening rate along the convex side of the hellenic arc is 12 mm yr from the eastern end of the north anatolian fault 41 e up to the northern aegean trough extension along n68 n179 and compression along n158 n89 occurs which results in dextral strike slip motion the rate of extension reduces from east to west from 30 mm yr to 5 mm yr while the rate of compression reduces from 17 mm yr to 5 mm yr the northwards motion of arabia results in a n31 trending compression at a rate of 7 mm yr along the east anatolian fault zone

mud volcanoes revealed and sampled on the western moroccan continental margin we surveyed and sampled two active and 3 inactive mud volcanoes on the atlantic continental margin of morocco the active mud volcanoes named yuma and ginsburg are each about 4 km in diameter rise between 150 250 meters above the seafloor and are the first active methane related mud volcanoes to be identified in this region the inactive volcanoes range in diameter from 1 3 km with relief between 50 and 80 meters gravity cores from the crest of the active mud volcanoes yielded methane hydrates and mud breccia deposits chemosynthetic communities of pogonophora worms and the bivalve solemya were found on the surface of the active mud volcanoes sediment cores from the inactive volcanoes contained hemipelagic sediments rich in foraminifera and ahermatypic coral debris overlying mud breccia

calculated mineral equilibria in the pelite system kfmash k 2 o feo mgo al 2 o 3 sio 2 h 2 o

the mara rosa arch in the tocantins province further evidence for neoproterozoic crustal accretion in central brazil the mara rosa volcano sedimentary sequence consists of several nne trending belts of metavolcanic metabasalts to metarhyolites and metasedimentary rocks micaschists quartzites cherts exposed over large areas of the tocantins province a neoproterozoic brasiliano orogenic region in the central part of brazil the supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post orogenic granitic and dioritic bodies these volcano sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite greenstone terrain of the archaean basement in this paper we present geochronological and isotopic data for rocks of the metavolcanic metaplutonic rock association samples of a felsic metavolcanic rock from the posse gold mine and a metatonalite yielded u pb zircon ages of 862 8 ma and 856 7 13 ma respectively these are interpreted as crystallization ages of the igneous protoliths titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632 4 ma sm nd isotopes for these rocks indicate primitive compositions with nd t of 4 6 and 3 7 and tdm model ages of 1 0 ga a syn tectonic dioritic intrusion has an u pb crystallization age of 630 6 ma and nd t of 1 9 and tdm of 1 0 ga additional sm nd analyses on metasedimentary rocks and post orogenic granites of the mara rosa region yield tdm ages between 1 2 and 1 0 ga the isotopic data combined with preliminary trace element results suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system off the coast of the sao francisco congo continent at around 860 ma recrystallization and deformational ages of 630 ma may represent the timing of final ocean closure and continental collision the rock associations structural pattern geochronological and isotopic characteristics of the rocks from the mara rosa region are very similar to those of neoproterozoic arc terrains exposed 300 km to the southwest in the arenspolis area proving the regional importance of the neoproterozoic crustal accretion event in the central part of brazil

iberian plate kinematics a jumping plate boundary between eurasia and africa the rotation of iberia and its relation to the formation of the pyrenees has been difficult to decipher because of the lack of detailed sea floor spreading data although several models have been proposed1 7 here we use detailed aeromagnetic measurements from the sea floor offshore of the grand banks of newfoundland to show that iberia moved as part of the african plate from late cretaceous to mid eocene time with a plate boundary extending westward from the bay of biscay when motion along this boundary ceased a boundary linking extension in the king s trough to compression along the pyrenees came into existence finally since the late oligocene iberia has been part of the eurasian plate with the boundary between eurasia and africa situated along the azores gibraltar fracture zone

geoid anomalies over gorringe ridge north atlantic ocean abstract gorringe ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the azores gibraltar plate boundary the geoid over this structure derived from seasat altimeter data exhibits a 9 m height anomaly with a north south lateral extension smaller than 200 km an attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity which has been compiled as a function of depth it is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded as it assumes a continuous unfractured elastic plate models involving local heterogeneities are then tested the comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation uncompensated models previously proposed to explain the gravity anomalies are tested using the geoid one model purdy and bonnin in bonnin 11 which involves an uplift of upper mantle material at depth generates too strong geoid anomalies and must be discarded another model which represents a nascent subduction zone le pichon et al 25 fits both the gravity and geoid anomalies but leads to difficulties in explaining the deep seismicity north of gorringe ridge a model in isostatic equilibrium is also able to fit both gravity and geoid anomalies this model involves a deep root of density 3 0 g cm 3 as has been previously proposed for many oceanic ridges and plateaus this model is compatible with the deep seismicity but the origin of this low density material at great depth is up to now an unresolved question more likely dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered

structural and metamorphic control on the exhumation of high p granulites the carvalhos klippe example from the oriental andrelândia nappe system southern portion of the brasilia orogen brazil abstract high pressure granulites are formed within the lowermost continental crust if exhumation is driven only by erosion these granulites will remain at their site of formation indefinitely their exposure then requires a complex process in this study the andrel ndia nappe system is used to investigate the formation and exhumation of high pressure granulites related to subduction collision extrusion processes this nappe system is part of the southern portion of the neoproterozoic brasilia orogen in brazil which comprises a pile of syn metamorphic rootless nappes in which high pressure granulites are an important constituent the andrel ndia nappe system is thrusted by a magmatic arc allochthon and it overrides the allochthonous passive margin that is represented by rocks of the carrancas nappe system metamorphism within the andrel ndia nappe system presents an inverted pattern that varies from top to bottom and from granulite to amphibolite facies conditions under a high pressure regime the carvalhos klippe is part of the uppermost nappe and is located 60 km ahead of its original allochthon this klippe is composed of rutile kyanite garnet k feldspar granulites and it displays ternary feldspars and associated mafic granulites that in turn contain garnet clinopyroxene plagioclase quartz as a peak mineral assemblage both granulite types crystallized at a higher pressure than the rocks within the orthopyroxene stability field the kyanite bearing granulites show a clockwise p t path which is inferred from reaction textures and thermobarometry peak conditions were attained at 825 c and 12 kbar at 617 7 1 3 ma which is determined by an id tims analysis of monazite the convergent kinematic flow of the klippe was directed eastward then northeastward and finally changed northward to northeastward which is contemporaneous with extension and thinning under high temperature conditions melt filled dilation shear zones were observed and the melt worked as a lubricant within the shear zones facilitating granulite exhumation extrusion

blueschist facies metamorphism shear heating and p t t paths in subduction shear zones the low temperatures recorded by blueschist facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15 50 km in this paper steady state and transient pressure temperature time p t t paths followed by subducted material are calculated using analytical expressions presented by molnar and england 1990 supplemented by two dimensional numerical calculations in the absence of shear heating calculated steady state p t t paths are very cold intersecting 1 gpa 10 kbar at t 200 c for a wide range of subduction parameters for the case of constant shear stress in the subduction shear zone and typical thermal parameters steady state p t t paths intersect the blueschist facies for 10 60 mpa and 0 100 mpa for convergence rates of 10 and 3 cm yr 1 respectively blueschist facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 mpa for the case of shear stress increasing linearly with depth steady state p t t paths intersect the blueschist facies when 1 9 p and 0 14 p for convergence rates of 10 and 3 cm yr 1 respectively subduction zone shear stresses may be considerably lower and possibly zero if blueschist terrains represent material metamorphosed during the early stages of subduction first 5 10 m y while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation subduction zone shear stresses of several tens of mpa at 35 km depth or a few percent of lithostatic pressure are consistent with a steady state model of franciscan metamorphism a progressive underplating model for the formation of the pelona schist inverted metamorphic gradient and surface heat flow measurements in the ne japan subduction zone

correlation of neoproterozoic terranes between the ribeira belt se brazil and its african counterpart comparative tectonic evolution and open questions

terrane accretion and upward extrusion of high pressure granulites in the neoproterozoic nappes of southeast brazil petrologic and structural constraints the high grade crystalline nappes exposed southeast of the sao francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of neoproterozoic deep continental crust the socorro guaxupe nappe above derived from an arc terrane and the tres pontas varginha nappe below metamorphism in the tres pontas varginha nappe is characterized by the exceptional preservation of kyanite granulites 700 750 c 15 kbar and followed by limited retrogression maximum temperatures around 900 950 c were reached toward the base of the overlying socorro guaxupe nappe during the intrusion of charnockitic mangeritic magmas lower pressure metamorphism accompanied by anatexis prevailed at shallower crustal levels our petrological results document an inverted thermal structure with isobaric heating of the top of the high pressure granulite nappe both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units both metamorphosed to high pressure but lower temperature conditions significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes the proposed geodynamic scenario considers that continental subduction took place westward underneath neoproterozoic oceanic lithosphere the two granulite units crystallised at 45 km depths under distinct paleogeotherms within this subduction zone around 630 ma the kyanite granulites were rapidely exhumed through the mechanism of low angle forced extrusion whereas syncollisional collapse affected the soft anatectic middle crust of the overlying arc terrane the final emplacement of the thinned nappe pile onto the cold sao francisco craton and its platform cover with at most anchizonal to greenschist facies metamorphism occurred around 600 ma

the cadiz margin study off spain an introduction abstract the cadiz continental margin of the northeastern gulf of cadiz off spain was selected for a multidisciplinary project because of the interplay of complex tectonic history between the iberian and african plates sediment supply from multiple sources and unique mediterranean gateway inflow and outflow currents the nature of this complex margin particularly during the last 5 million years was investigated with emphasis on tectonic history stratigraphic sequences marine circulation contourite depositional facies geotechnical properties geologic hazards and human influences such as dispersal of river contaminants this study provides an integrated view of the tectonic sediment supply and oceanographic factors that control depositional processes and growth patterns of the cadiz and similar modern and ancient continental margins

late quaternary seismic facies of the gulf of cadiz spanish margin depositional processes influenced by sea level change and tectonic controls abstract seismic facies and quaternary depositional systems of the gulf of cadiz have been characterized by means of high resolution seismic profiles airguns geopulse and ore 3 5 khz a total of 11 pleistocene and two holocene sequences separated by type 1 or type 2 discontinuities were identified the age of the depositional sequences was determined based on foraminifera ages from boreholes and by correlation with seismic profiles of the slope areas with uniform deposition and without significant discontinuities these seismic units have been mapped throughout the margin allowing to establish the correlation with eustatic sea level curves and global climatic trends the distribution and geometry of the units reveals that most of the sedimentary record was developed during episodes of forced regression ten regressive periods corresponding to 5th order cycles arranged into 4th order were distinguished absolute water depths for the development of coastal deposits in each unit were calculated using depth from present day sea level corrected for tectonic subsidence in the area the model for the evolution of the depositional system of the gulf of cadiz continental margin highlights the strong influence of tectonism in the development of depositional bodies controlling the distribution of deposits generally produced during episodes of sea level fall

crustal architecture and tectonic evolution of the gulf of cadiz sw iberian margin at the convergence of the eurasian and african plates 1 the gulf of cadiz located at the southwestern iberian margin is characterized by widespread seismicity compressional and strike slip fault plane solutions and by a large elongated positive free air gravity anomaly the gulf of cadiz gravity high gcgh multichannel seismic profiles across and along gcgh together with bathymetric and gravity data allow us to study in detail the tectonic architecture and crustal structure of the gulf of cadiz the upper shelf and slope of the gulf of cadiz includes the main structural domains of the betic fold and thrust belt in the middle part of the gulf the paleozoic basement crops out on the shallow guadalquivir bank and is associated with the largest signature of the gcgh whereas toward the outer part of the gulf the basement deepens progressively a large nw se normal fault and conjugate ne sw faults define a prominent basement high associated with the gcgh modeling of the gcgh suggests localized crustal thinning of 10 km along the central part of the gulf of cadiz probably generated during the mesozoic rifting episode between the iberian and african plates concentric wedges of fold and thrust belts and large allochthonous masses were emplaced in the gulf of cadiz during the neogene compressional phase the final emplacement of these units becomes progressively young from the se pre early langhian toward the foreland in the nw late tortonian seafloor surface ruptures pockmarks and submarine landslides provide evidence of active faulting in the gulf of cadiz to accommodate the present day convergence between the african and eurasian plates previously extensional faults have probably been reactivated and inverted at depth as suggested by the intermediate depth seismicity

40ar 39ar thermochronology of high pressure granulite nappes in the southern brasilia belt brazil implications for nappe exhumation neoproterozoic orogenesis in the southern sector of the southern brasilia belt was the result of subduction and terminal collision between the socorro guaxupe arc and paranapanema block to the west present coordinates and the passive margin along the southern portion of the western side of the sao francisco craton to the east we report the results of a thermochronologic study of the uppermost passive margin derived high pressure granulite facies nappes of the andrel ndia nappe complex to better constrain the timing of exhumation of these nappes based on 40 ar 39 ar biotite plateau ages the tres pontas varginha nappe at the top of the andrelandia nappe complex was the first nappe to be exhumed through 300 c during the interval 591 to 567 ma this was followed by exhumation of the underlying carmo da cachoeira nappe through 300 c at circa 540 ma initially the tres pontas varginha nappe cooled slowly at a rate of 20 c ma 1 this increase is attributed to tectonically driven exhumation during orogenic collapse triggered by regional extension as a far field effect of eastward subduction of the passive margin on the eastern side of the sao francisco craton during accretion of a magmatic arc terrane associated with the formation of the ribeira belt to the east the average rate of cooling in the underlying carmo da cachoeira nappe was 4 to 5 c ma 1 this slower rate reflects collapse of the nappe stack which is likely to have led to faster exhumation of nappes higher in the stack exhumation of the andrelandia nappe through 300 c circa 50 ma after exhumation in the correlative carmo da cachoeira nappe demonstrates that the two nappes were exhumed separately the delayed exhumation of the andrelandia nappe may be due to reworking during orogenesis associated with formation of the ribeira belt the andrelandia nappe records cooling from the bottom up this was likely a consequence of final emplacement on the colder sao francisco craton

u pb geochronology of the southern brasilia belt se brazil sedimentary provenance neoproterozoic orogeny and assembly of west gondwana abstract the brasilia belt borders the western margin of the sao francisco craton and records the history of ocean opening and closing related to the formation of west gondwana this study reports new u pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the sao francisco craton and date the orogenic events leading to the amalgamation of west gondwana ages of detrital zircons by id tims and la mc icpms were obtained from metasedimentary units of the passive margin of the sao francisco craton from the main tectonic domains of the belt the internal allochthons araxa group in the araxa and passos nappes the external allochthons canastra group serra da boa esperanca metasedimentary sequence and andrel ndia group and the autochthonous or cratonic domain andrel ndia group the patterns of provenance ages for these units are uniform and are characterised as follows archean paleoproterozoic ages 3 4 3 3 3 1 2 7 and 2 5 2 4 ga paleoproterozoic ages attributed to the transamazonian event 2 3 1 9 ga with a peak at ca 2 15 ga and to the ca 1 75 ga espinhaco rifting of the sao francisco craton ages between 1 6 and 1 2 ga with a peak at 1 3 ga revealing an unexpected variety of mesoproterozoic sources still undetected in the sao francisco craton and ages between 0 9 and 1 0 ga related to the rifting event that led to the individualisation of the sao francisco paleo continent and formation of its passive margins an amphibolite intercalation in the araxa group yields a rutile age of ca 0 9 ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin detrital zircons from the autochthonous and parautochthonous andrel ndia group deposited on the southern margin of the sao francisco craton yielded a provenance pattern similar to that of the allochthonous units this result implies that 1 6 1 2 ga source rocks must be present in the sao francisco craton they could be located either in the cratonic area which is mostly covered by the neoproterozoic epicontinental deposits of the bambui group or in the outer paleo continental margin buried under the allochthonous units of the brasilia belt crustal melting and generation of syntectonic crustal granites and migmatisation at ca 630 ma mark the orogenic event that started with westward subduction of the sao francisco plate and ended with continental collision against the parana block and goias terrane continuing collision led to the exhumation and cooling of the araxa and passos metamorphic nappes as indicated by monazite ages of ca 605 ma and mark the final stages of tectonometamorphic activity in the southern brasilia belt whilst continent continent collision was proceeding on the western margin of the sao francisco craton along the southern brasilia belt eastward subduction in the east was generating the 634 599 ma rio negro magmatic arc which collided with the eastern sao francisco margin at 595 560 ma much later than in the brasilia belt thus the tectonic effects of the ribeira belt reached the southernmost sector of the brasilia belt creating a zone of superposition the thermal front of this event affected the proximal andrel ndia group at ca 588 ma as indicated by monazite age the participation of the amazonian craton in the assembly of western gondwana occurred at 545 500 ma in the paraguay belt and ca 500 ma in the araguaia belt this together with the results presented in this work lead to the conclusion that the collision between the parana block and goias terrane with the sao francisco craton along the brasilia belt preceded the accretion of the amazonian craton by 50 100 million years

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

syn collisional lower continental crust anatexis in the neoproterozoic socorro guaxupe nappe system southern brasilia orogen brazil constraints from zircon u pb dating sr nd hf signatures and whole rock geochemistry abstract the southern brasilia orogen occurs along the southernmost border of the sao francisco craton this orogen is structured as a pile of ediacaran syn metamorphic thicked skinned nappes that diachronically migrate toward the cratonic margin a magmatic arc environment is represented by the socorro guaxupe nappe sgn which is a thick segment of partially molten lower to middle continental crust the sgn comprises three main units a lower granulitic an intermediate diatexitic and an upper metatexitic high temperature metamorphic peak was reached at 625 ma when large volumes of high k diatexitic rocks were formed charnockitic and mangeritic magmatic rocks sao jose do rio pardo suite and in situ leucosomes of mafic composition are inferred to have formed under hornblende dehydration melting conditions i e t 850 c granitic diatexites pinhal type were also formed and zircon thermometry indicates t 850 c elemental and isotopic geochemistry indicate that these diatexites and in situ leucosomes derived from melting of lower to middle continental crust tims zircon u pb ages are presented for the lower and middle units u pb dating coupled to lu hf analysis la mc icp ms was performed on charno enderbitic in situ leucosomes from metatexitic rocks that also resulted from hornblende dehydration melting two different zircon typologies were recognized the first comprises bipyramidal prismatic grains with oscillatory zoning and high luminescence sometimes preserving low luminescence 670 ma cores the high luminescence grains show 19 concordant ages at 621 16 ma the second typology comprises isometric or soccer ball type grains these grains show sector zoning and a concordia age of 608 4 ma th u ratios from soccer ball grains vary from 1 253 to 2 107 whereas prismatic grains vary from 0 118 to 1 774 initial hf signatures between 13 and 21 in both typologies provide evidence of a reworked crustal reservoir the older prismatic grains may be related to the onset of metamorphic peak 625 ma whereas the isometric ones provide evidence of long lived high t conditions until 610 ma

pseudo and real inverted metamorphism caused by the superposition and extrusion of a stack of nappes a case study of the southern brasilia orogen brazil the southern brasilia orogen is a neoproterozoic belt that occurs along the southernmost border of the sao francisco craton where the andrel ndia nappe system represents the subducted sedimentary domain and is divided into three allochthonous groups of which the ages and p t conditions of metamorphism are studied here the basal unit the andrel ndia nappe exhibits an inverted metamorphic pattern the base of the structure composed of staurolite garnet biotite kyanite quartz and muscovite marks the metamorphic peak whereas at the top the association of the metamorphic peak does not contain staurolite the liberdade nappe the middle unit presents a normal metamorphic pattern its base close to the andrel ndia nappe shows paragneiss with evidence of in situ partial melting and towards the top coarse grained staurolite schist is found the staurolite out and melt in isograds are coincident and parallel to the main foliation thus the shear zone that limits the nappes is syn metamorphic reheating the underlying andrel ndia nappe and influencing the establishment of metamorphic inversion this suggestion is supported by the monazite chemical ages which indicates that the andrel ndia nappe metamorphic peak 586 15 ma is younger than that of the liberdade nappe 622 3 7 6 ma the upper unit the serra da natureza klippe bears a typical high pressure granulite mineral assemblage that is composed of kyanite garnet k feldspar rutile and leucosome as well as a metamorphic peak at 604 5 6 1 ma this tectonic assembly with inverted and non inverted metamorphic patterns and generation of klippen structures is consistent with exhumation models and a strong indentor located in the lower continental crust

high pressure granulite facies metamorphism in the pan african dahomeyide orogen west africa high pressure hp granulite facies rocks are associated with eclogites in the pan african dahomeyide orogen west africa the hp granulite and eclogite facies mineral assemblages occur in mafic gneisses within the suture zone exposed in southeastern ghana and the adjoining part of togo thermobarometric calculations and phase equilibria considerations indicate that the hp metamorphic rocks preserve a record of i prograde garnet crystallization during eclogite facies metamorphism as temperature increased to a maximum of 600 c and pressure reached 18 kbar followed by ii hp granulite facies metamorphism when temperatures reached 800 900 c at p 14 kbar resulting in partial melting to produce leucocratic veins decompression along a retrograde path is inferred from granulites that yield temperatures down to 700 c at 10 kbar the inferred p t path of the metamorphic record defines a clockwise loop in the kyanite stability field compatible with orogeny involving subduction during the pan african as

geochemical studies in the proterozoic metamorphic terrane of the guaxupe massif minas gerais brazil a discussion on large ion lithophile element fractionation during high grade metamorphism abstract the trace element composition of high grade metamorphic rocks in the guaxupe massif is investigated the large ion lithophile element composition is compared in both the granulite facies rocks and the associated amphibolite facies gneisses the k rb th contents in the granulite facies rocks from other parts of brazil show that the depletion of these elements is variable the covariance of k rb ratios with k suggest the importance of mineralogical control protolith composition nature of fluid etc the distribution of u and th appears to indicate the protolith composition of the rock types

post brasiliano pan african high k granitic magmatism in central brazil the role of late precambrian early palaeozoic extension abstract in western goias brazil the emplacement of large high k postorogenic granites and associated small gabbro dioritic intrusions followed immediately after the last deformational events of the brasiliano pan african orogeny at 600 ma well fitted whole rock rb sr isochrons indicate ages which suggest two discrete intrusive events the older between 588 and 560 ma and the younger between 508 and 485 ma the older granites display general petrographic and geochemical characteristics of highly differentiated calc alkaline i type granitoids whereas the younger intrusions are more alkaline similar to a type granites initial 87 sr 86 sr ratios vary from 0 703 to 0 710 and initial nd isotope ratios yield nd t values in the range between 4 0 and 3 0 there are no major differences in initial isotopic compositions between the two granite groups suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the 940 640 ma basement arc type metatonalites metagranodiorites and associated arc metavolcanics the major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust with younger melts being produced by the refusion of anhydrous depleted crustal sources left after the extraction of previous batches of more hydrated calc alkaline magmas the heat input required to promote extensive remelting of the continental crust was most likely provided by mantle derived mafic magmas that invaded and probably underplated the crust during uplift and extension the two intrusive events are bimodal in nature and are interpreted as shallow level extension related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the proterozoic and early palaeozoic the older at 600 ma and the younger between 550 and 510 ma the onset of the granite magmatism at 590 ma shortly post dating the brasiliano orogeny 600 ma is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic paraguay belt in brazil and its correlative in bolivia the tucavaca belt which probably correspond to rift deposits related to the break up of laurentia from gondwana at the end of the proterozoic and beginning of the palaeozoic granites of the younger group cut the deformational structures of the paraguay belt metasediments and pre date by between 40 and 20 ma the initial stages of subsidence and sedimentation of the parana basin

implications structurales et paleogeographiques de la presence de messinien a l ouest de gibraltar structural and paleogeographical implications of the existence of messinian layers west of gibraltar abstract a core sampled west of gibraltar shows at its base lower messinian sediments typical of the mediterranean area and at its top pleistocene sediments of atlantic type these observations lead us to consider the existence of a basin west of gibraltar which would have been isolated from the atlantic during the upper miocene though still communicating with the mediterranean through a south rifan strait the isolation of this basin during the messinian would be the result of the creation of a south iberian spur made up of the series folded during miocene events between the moroccan and iberian margins at about 9 w

u pb age of detrital zircon from the embu sequence ribeira belt se brazil abstract the embu complex crops out in the southeastern part of sao paulo state in brazil and consists mainly of neoproterozoic metasedimentary rocks belonging to the interference zone between the brasilia and ribeira fold belts we present u pb ages la ipcms supported by cathodoluminescence images for detrital zircon from these rocks in order to constrain the formation and evolution of the source area of this complex in this paper we choose to individualize within the embu complex a paleoproterozoic basement a metasedimentary sequence the embu sequence and neoproterozoic intrusive bodies now largely orthogneisses six distinct rock types were recognized in the embu sequence biotite gneiss biotite schist quartzite amphibolite calc silicate rock and talc schist the sequence is strongly deformed by several deformation phases and metamorphosed into amphibolite facies an important shear zone the buquira shear zone separates the embu complex from the socorro guaxupe nappe to the north considered to be part of the upper plate of the southern brasilia belt collision however our mapping showed that the shear zone cuts obliquely into the embu complex diminishing considerably its regional importance three samples were separated for dating a schist north of the shear zone and two quartzites south of the shear zone the results show that the ages of detrital zircon of these samples are mainly paleoproterozoic with a minor archean component the youngest igneous zircon grain from the schist produced an age of about 1000 ma indicating the maximum sedimentation age for the sequence no significant difference appears between the samples north and south of the shear zone reforcing the interpretation that this zone is only of local importance important metamorphic overgrowth of zircon in the age range 670 640 is present in all three samples it is interpreted as related to the installment of a continental magmatic arc in the socorro guaxupe nappe and in the embu complex a second metamorphic pulse between 621 and 599 ma is only apparent in one sample according to the literature this was the period of continental collision between the paranapanema and sao francisco paleocontinents leading to the formation of the southern brasilia belt possibly the fact that the analyzed samples are localized some 30 km away from the suture in the upper plate explains that the increase in temperature during the collision in this area was insufficient to grow significant new zircon a third metamorphic pulse with ages of 576 574 ma possibly related to collision in the ribeira belt is only detected in three analyses of metamorphic rims in the schist sample earlier reported ages of about 790 ma both for igneous crystallization and metamorphism in a more westerly part of the embu complex are only recognized in one analysis of metamorphic overgrowth 794 24 ma from the schist sample and two probably hybrid analyses of 830 19 ma and 767 14 ma from the northern quartzite sample it is concluded that the results show that the detritic zircon is probably derived from the paranapanema paleocontinent

petrology and geochemistry of the neoproterozoic guaxupe granulite facies terrain southeastern brazil abstract geochemical and oxygen isotopic data on the neoproterozoic granulite facies rocks enderbites charnockite and charnokitic augen gneisses of guaxupe brazil are assembled geochemical discrimination function calculations show an igneous origin for majority of the rock types in the area major and trace element data especially ree are similar to tonalite and granodiorite oxygen isotope composition for charnockitic gneisses and mafic granulites yield 18o values in the range of 8 8 to 11 and fall in the range of quartzofeldspathic rocks of medium to high grade terrains elsewhere a small scale variation of 18o values compatible with the chemical data imply the preservation of pre metamorphic oxygen isotope compositions minimum peak pressure temperature conditions are 8 5 kbar and 850 c the high grade metamorphism did not appear to have altered significantly the geochemical and oxygen isotope compositions of the protoliths approximately 2 5 kbar of pressure decrease accompanied by 200 c cooling has been recorded for a portion of the cooling path the p t path is consistent with a single event namely the formation and uplift of the guaxupenappe in the neoproterozoic brasiliano cycle

geoelectric crustal structures off the sw border of the sao francisco craton central brazil as inferred from a magnetotelluric survey summary a magnetotelluric mt survey of the crust beneath sedimentary basins and thrust sheets along the southwestern edge of the sao francisco craton central brazil reveals intricate electrical characteristics that are interpreted in the context of proterozoic collision tectonics and horizontal transport of allochthonous rock units emplacement of cretaceous hypabyssals lavas and diatremes of ultrapotassic mafic composition and occurrence of induced seismicity the data exhibit strong distortions represented by 3 d induction effects and galvanic disturbances resulting from shallow structures frequency and site dependence of electrical strike and inhomogeneous anisotropic layers with smoothly varying phase split conductance and azimuth of the highly conductive direction geoelectrical and orthogonal phase difference directions 2 d inversion and forward modelling characterize three distinct subhorizontal sections showing two anisotropic conductors within a highly resistive crust laterally segmented into unique blocks the model for the uppermost crust section has e w geoelectric directions and a 15 30 s anisotropic zone at a depth of approximately 1 2 km along the entire profile this conducting layer is interpreted to represent a brine filled fracture layer possibly controlled by the present day state of crustal stresses as disclosed from reservoir triggered quakes the mid crust section presents a deeper conducting zone located at depths below 10 km it is defined by stronger mt responses having phase split directions oscillating from wnw ese to e w beneath sites in the central western area parana basin and allochthonous cover units and nne ssw in the northeastern region autochthonous platform units and sanfranciscana basin anisotropy is greater than an order of magnitude in the highly conductive direction with conductance in the range of 250 400 s conjecturally the source of this anomalous feature would come from interconnected grain boundary phases and hypersaline fluids exsolved and precipitated from upwelling cretaceous magma in the central western area favourable trapping of conductors was constrained along a nearly e w direction feasibly associated with relic structures inherited from brasiliano pan african continental collisions along the northeast however the coincidence with superficial nne ssw structural directions suggests a localized direct causal relationship with the trend of extension related to magma emplacement the lower crust has a highly resistive quasi 1 d section along the entire profile that prevails also at uppermost mantle depths thus whereas the brittle crust would have reconciled subhorizontal strain with fluid percolation related to uplift and magma emplacement a mechanically coupled and stronger lower crust upper mantle would have controlled the deep magma generation during cretaceous distention pulses

constraining timing and p t conditions of continental collision and late overprinting in the southern brasilia orogen se brazil u pb zircon ages and geothermobarometry of the andrelândia nappe system abstract the andrel ndia nappe system ans is an important component of the neoproterozoic southern brasilia orogen sbo in se brazil and its evolution is related to the convergence and subsequent collision between the passive margin of the sao francisco paleocontinent and paranapanema block during the brasiliano orogeny the ans is mainly composed of a pile of deformed meta sedimentary rocks that record an inverted metamorphic pattern ranging respectively from amphibolite facies at the bottom to high p granulite conditions at the top the ans is here simplified into the lower middle and upper nappes four samples from different tectonic settings within the ans were collected for zircon isotopic investigation in addition geothermobarometry modelling were performed on seven samples from the middle nappe retro eclogite slices associated to basement gneissic rocks occur along the contact between the upper and middle nappes and have zircon grains with metamorphic rims grown in high p conditions of 12 16 kbar with temperatures of 700 800 c during the continental subduction dated here by u pb shrimp in zircon at 625 6 ma a period of nappe emplacement accompanied by melting in the middle nappe is evidenced by a garnet muscovite staurolite kyanite schist with leucosome parallel to the main foliation dated by u pb shrimp in zircon at 618 5 ma re equilibrium at pressures of 4 7 kbar with temperatures of 500 700 c is ascribed to a second heat pulse under lower pressure conditions the period of melting associated to this event in the middle nappe occurred at 586 9 ma and is marked by an anatectic tourmaline garnet muscovite bearing s type granite this granite body is part of a ne sw belt of granites with ages between 605 and 563 ma and covering both the central ribeira orogen cro and the sbo the age and the alignment of this granite belt justify the interpretation that it is genetically related to the cro thus the new data provide evidence that tectonothermal events related to the collisional front of the cro extended up to 200 km overlapping the consolidated sbo at ca 580 ma

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

self indentation of a heterogeneous continental lithosphere the neoproterozoic ribeira aracuai belt stretches along the southeastern edge of the sao francisco craton of southeastern brazil and extends south of the cratonic domain for 1000 km the termination of the craton is spatially correlated with a significant modification of the deformation pattern in the belt 1 the structural trend bends from due north to northeast 2 the dominant tectonic flow shifts from orogen transverse to orogen normal and 3 the metamorphic conditions of deformation decrease southwestward from high to medium grade finite element modeling suggests that the presence of a craton within a continent favors strain localization initiation of continental scale shear zones and differential vertical deformations the southward termination of the sao francisco craton may have triggered the development of the complex deformation pattern that characterizes the ribeira aracuai belt

rotated garnets a mechanism to explain the high frequency of inclusion trail curvature angles around 90 and 180 abstract this paper presents numerical data from garnets with inclusion trail curvature angles of up to 260 three hundred and twenty five garnets were studied from an outcrop of greenschist facies phyllite in southern minas gerais state brazil apart from the inclusion trail curvature angle also the aspect ratio r and the angle between the long axis of the garnets and the foliation were measured the results show a remarkable concentration of at 180 and a minor one at 90 r varies between 1 and 2 showing that the garnets deviate from sphericity and shows that all garnets have their long axis in the forward rotated quadrant supporting the rotational interpretation a model is proposed to explain the concentrations of based on preferential growth of the garnets into the mica rich strain caps orthogonal to the foliation causing elongated crystals that because of their shape and position would experience accelerated rotation until relatively stable positions with their long axes parallel to the foliation would be attained renewed growth again into the mica rich strain caps orthogonal to the foliation would first restore the spherical shape and then produce an elongated shape again perpendicular to the foliation forcing a repetition of the process it is concluded that this model is capable of explaining the concentration of in multiples of 90 in a rotational model where this concentration was considered earlier as an argument in favor of the non rotational model

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

orogen parallel westward oblique uplift of the qinling basement complex in the core of the qinling orogen china an example of oblique extrusion of deep seated metamorphic rocks in a collisional orogen abstract both vertical uplift and lateral extrusion of rocks in convergent orogens have been widely documented this article using the basement core of the qinling orogen as an example demonstrates orogen parallel westward uplift with both vertical and horizontal displacement components the qinling complex of the qinling orogen is a tectonic slice of a precambrian crystalline basement it is an elongate lens between younger terranes bounded by the shangdan suture to the south and by the zhuxia zone to the north during the middle paleozoic the complex underwent large scale orogen parallel westward oblique ductile uplift supporting evidence for this includes the following 1 opposite shear senses of the two bounding shear zones i e the thrust sinistral shear on the south and thrust dextral shear on the north 2 well developed eastward plunging orogen parallel a type structures in the complex such as stretching lineation and sheath folds 3 strong decompression metamorphism of the complex f

tectonometamorphic evolution of ghana togo and benin in the light of the pan african brasiliano orogeny abstract the south pan african belt of west africa ghana togo and benin province can be divided into three domains 1 1 the external nappes thrust over the west african craton derived from passive margin sedimentary deposits middle to late proterozoic and displaying middle to high pressure metamorphism such as the kante and atacora nappes 2 2 the intermediate nappes including the suture zone rocks with eclogitic assemblages protoliths ca 800 ma and mainly monocyclic metasedimentary rock and orthogneiss that are found in central togo and northwest benin 3 3 the internal nappes composed of high grade commonly anatectic gneiss anatexis ca 600 ma including orthogneiss eburnian plutons elongated granulite belts reworked archean basement and an intrusive granite and charnockite association ca 600 ma which are exposed in benin and nigeria the p t t d paths calculated from rocks in these three domains show a prograde retrograde evolution 1 1 an initial clockwise evolution related to an early burial stage hp ht in the intermediate nappes followed by oblique thrusting in a sw direction that caused nappe stacking and subsequent uplift 2 2 a second anti clockwise evolution with late temperature and pressure increases only recorded in the more internal units this temperature increase was a consequence of the previous crustal thickening with subsequent doming extension and anatexis the pressure increase was related to movement along the kandi fault working as a dextral strike slip fault with a transpressional component under late e w shortening the retrograde trend corresponds to the final uplift the evolution of the belt can thus be divided into three successive stages 1 1 a first stage was characterized by an oblique collision inducing sw oblique thrusting with associated syn foliation reverse metamorphism 2 2 during a second stage anatectic doming took place followed by 3 3 a third stage at still high temperature with dextral wrench faulting and associated n s post foliation folding under late e w shortening comparison between the west african north brasiliano and central african late proterozoic belts shows very strong similarities between their lithology kinematics and metamorphism and indicates that the pan african brasiliano chain resulted from collision between three major continental domains the west african and sao francisco congo cratons and a late proterozoic mobile domain

seismic crustal structure in the southwest of the iberian peninsula and the gulf of cadiz abstract the crust under the southwestern iberian peninsula and the gulf of cadiz has been sampled by 1200 km of deep seismic refraction wide angle reflection profiles together with many seismic reflection lines and bore holes wide angle seismic data were collected during the last three decades commercial multichannel data provide a detailed image of the uppermost crust improving the confidence about the models of the deeper structures p wave velocities within the thick column of sediments in the gulf of cadiz range from 2 0 to 3 8 km s while the algarve and the sines areas have higher velocities of 4 3 to 4 8 km s the top of the palaeozoic basement rises to the northwest outcropping in the south portuguese zone of the iberian massif and is characterized by p wave velocities of 5 7 5 9 km s high velocities of 6 4 km s have been found at shallow depths of 7 to 10 km in the south portuguese zone that could be related to the mafic and ultramafic rocks in the beja acebuches zone lower crustal velocities are in the range of 6 7 6 9 km s the crustal thickness shows important lateral changes from 29 km beneath the guadalquivir basin iberian massif contact to 35 km in the southeastern part of the south portuguese zone from the interpretation of these seismic data a geodynamic model of the evolution of the crust in southwestern iberia and the gulf of cadiz is proposed the guadalquivir basin and its continuation at sea the gulf of cadiz is a flexure area of the crust that could be related to the overloading due to the overthrusting of the alboran domain over the iberian plate

thermobarometric constraints on the thermal history of the main central thrust zone and tibetan slab eastern nepal himalaya the main central thrust mct south of mt everest in eastern nepal is a 3 to 5km thick shear zone separating chlorite bearing schist in the lower plate from sillimanite bearing migmatitic gneiss in the overlying tibetan slab the metamorphic grade increases through the mct zone toward structurally higher levels previous workers have suggested that either post or synmetamorphic thrust movement has caused this inversion of metamorphic isograds in an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault four well calibrated thermobarometers were applied to pelitic samples collected along two cross strike transects through the mct zone and tibetan slab results show an increase in apparent temperature up section in the mct zone from 778 k to 990 k and a decrease in temperature to 850 k in the lower tibetan slab which is consistent with synmetamorphic thrust movement a trend in calculated pressures across this section is less well defined but on average decreases up section with a gradient of 28mpa km resembling a lithostatic gradient pressure temperature paths for zoned garnets from samples within the mct zone modelled using the gibbs method show a significant decrease in temperature and a slight decrease in pressure from core to rim which might be expected for upper plate rocks during synmetamorphic thrust movement samples from the uppermost tibetan slab yield higher temperatures and pressures than those from the lower tibetan slab which may be evidence for later resetting of thermobarometers by intrusion of the large amounts of leucogranite at that structural level

neoproterozoic granulite facies metamorphism and coeval granitic magmatism in the brasilia belt central brazil regional implications of new shrimp u pb and sm nd data abstract new shrimp u pb zircon ages combined with sm nd isotopic characteristics of granulites and associated granitic rocks of the anapolis itaucu complex in the central southern part of the brasilia belt are presented and discussed in this study igneous crystallization ages obtained in zircon grains of orthogranulite and granites vary between 760 and 650 ma growth of new zircon at ca 650 640 ma dates the high grade metamorphism zircon cores from paragranulites and granites give u pb ages between 2 0 and 0 8 ga tdm model ages of the granulitic rocks fall into two age intervals 2 3 1 9 and 1 7 1 4 ga end t values are negative varying between 9 29 and 1 42 shrimp u pb ages of zircon cores indicate that the granulite sedimentary protoliths were deposited after 800 ma ago contradicting previous models which assigned paleoproterozoic or archean ages for these rocks their nd isotopic signature indicate that paleoproterozoic sources within the sao francisco craton as well as younger sources such as the goias magmatic arc contributed to the sediment infilling of the former basin the intrusive granites display nd model age pattern similar to that of the granulitic rocks with most tdm model ages ranging between 1 45 and 1 2 ga end t values are between 2 61 and 7 96 indicative of assimilation of older material by the original magma shrimp u pb data for the granite intrusions indicate that granitoids metamorphosed under amphibolite facies show strong inheritance between 2 1 and 0 8 ga and magmatic metamorphic age at ca 660 650 ma the syn granulite facies granitoid ana 1 gave magmatic and metamorphic ages of ca 650 ma without any discernible inheritance the growth of new zircon occurred at the same time of the high grade metamorphism probably related with the collision event between the goias magmatic arc to the west southwest and the sao francisco craton to the east the data suggest that the anapolis itaucu complex instead of representing the ancient sialic basement of the supracrustal rock units of the brasilia belt constitutes a neoproterozoic high grade metamorphic complex related with the evolution of the brasiliano orogen

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

closure of the africa eurasia north america plate motion circuit and tectonics of the gloria fault we examine the closure of the current plate motion circuit between the african north american and eurasian plates to test whether these plates are rigid and whether the gloria fault is an active transform fault we also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries and compare the predicted direction of motion along the african eurasian plate boundary in the mediterranean with the direction of slip observed in earthquakes from marine geophysical data we obtain 13 transform fault azimuths and 40 3 m y average spreading rates 34 of which are determined from comparison of synthetic magnetic anomaly profiles to 140 observed profiles slip vectors from 32 earthquake focal mechanisms further describe plate motion detailed magnetic surveys north of iceland provide 11 rates in a region where prior plate motion models had few data magnetic profiles north of the azores triple junction record a rate of 24 mm yr 4 mm yr slower than used by prior models gloria and sea beam surveys accurately measure the azimuths of seven transform faults our plate motion model fits six of the seven within 2 two transform faults surveyed by gloria side scan sonar lie near famous area transform faults a and b and give azimuths 13 clockwise of them because recent studies show that short offset transforms such as transforms a and b are in many places oblique to the direction of plate motion we exclude azimuths from transforms with less than 35 km offset the best fitting and closure enforced vectors fit the data well except for a small systematic misfit to the slip vectors on right lateral slipping transforms slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths whereas on left lateral slipping transforms slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths we search the long eurasia north america boundary for evidence of an additional plate but find no systematic misfits to the data in particular if a spitsbergen plate exists and moves relative to eurasia its motion is less than 3 mm yr an africa eurasia euler vector determined by adding the eurasia north america and africa north america euler vectors is consistent with the gloria fault trend and with slip vectors from eastern azores gibraltar ridge focal mechanisms a small circle centered at the africa eurasia closure enforced pole fits the trace of the gloria fault the model in which closure was enforced predicts 4 mm yr slip across the azores gibraltar ridge and west northwest convergence near gibraltar 45 more oblique than suggested by a recent model based on compressive axes of focal mechanisms moreover our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between gibraltar and sicily because closure enforced vectors fit the data nearly as well as the best fitting vectors we conclude that the data are consistent with a rigid plate model and with the gloria fault being a transform fault

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

207pb 206pb ages of zircons from syn collisional i type porphyritic granites of the central ribeira belt se brazil abstract in the central segment of the ribeira belt southeast brazil several foliated porphyritic granitic bodies intrude high grade migmatitic gneisses of the andrel ndia and juiz de fora domains and embu complex results of geological geochemical and geochronological investigations of the maromba pedra selada serra do lagarto and funil porphyritic i type granites provide profound similarities except for the distinct geochemical behavior of the funil granite perhaps related to a different crustal source these granitoids show similar structural textural and mineralogical features pb evaporation of single zircons provided ages of 586 6 579 6 6 3 586 3 4 8 and 584 5 ma for the granites respectively coincident with the syn collision i episode of the central ribeira belt the intrusion of i type porphyritic granitoids coeval with the main collisional event has not often been reported in the geological literature the most common syn collisional granitic magmatism has normally an s type signature or even a slightly peraluminous i type character however the occurrence of coeval i and s type syn tectonic granites along the central ribeira belt as observed in the investigated area and discussed in this paper is noteworthy

seismotectonics of the ibero maghrebian region abstract a study of the seismotectonics of the ibero maghrebian region is presented as deduced from main geological features seismicity and focal mechanism data the hypocentral distribution of shallow intermediate depth and deep earthquakes is analyzed the direction of the stress distribution is inferred from the focal mechanism of 24 selected events a seismotectonic framework summarizing these results together with the main geological features shows that a nw se general direction of horizontal compressive stresses is present in the region with a localized zone of horizontal tensions in the betics and the alboran sea the presence of intermediate depth and deep earthquakes is tentatively explained as caused by two consecutive processes of subduction

velocity analysis for wide aperture seismic data common midpoint data are now being collected with ever increasing source receiver offsets for wide aperture seismic data classical methods of interpretation fail since velocity analyses and signal to noise enhancement methods based on hyperbolic traveltime curves are no longer appropriate therefore the goals of increased velocity resolution and signal enhancement which motivate the increase in offset are not achieved approximate methods involving higher order traveltime curves or extrapolations have been developed for velocity analysis but these are ineffective in the presence of refracted arrivals and lack a physical basis these problems can be minimized by transforming the observational data to the domain of intercept or vertical delay time and horizontal ray parameter p in this domain headwave refractions are collapsed into points and both near vertical and wide angle reflections can be analyzed simultaneously to derive velocity depth information even in the presence of velocity gradients or low velocity zones

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

extension driven brittle exhumation of the lower middle crustal rocks a paleostress reconstruction of the neoproterozoic ambaji granulite nw india abstract the exhumation of the deep crustal rocks through brittle crust by extensional tectonics is recognized in orogens of all ages paleostress analysis are generally used to understand the brittle exhumation process in this study we reconstructed the paleostress of faults of the neoproterozoic ambaji granulite south delhi terrane of the aravalli delhi mobile belt nw india by analyzing the fault slip data in win tensor program several ne sw and wnw ese faults have been mapped in the area and found to be normal faults with a few strike slip faults the strike slip faults are pre kinematic to normal faults tensor solutions for 237 fault slip data points estimate wnw ese extension for strike slip faults and nw se direction extension for normal faults from these results we interpret that the ne sw striking orogen parallel normal faults were produced from a nw se directed extensional stress and are primarily responsible for brittle exhumation of the granulite through crustal extension and thinning at 764 650 ma this is comparable to earlier studies on brittle exhumation along the southern tibet detachment in the higher himalayas on a more regional scale our results are in agreement with the extensional tectonics that affected the entire aravalli delhi mobile belt and adjoining continents of the neoproterozoic cambrian gondwanaland supercontinent

magnetostratigraphy and paleontology of ait kandoula basin high atlas morocco and the african european late miocene terrestrial fauna exchanges abstract a magnetostratigraphic study has been carried out on a middle miocene to upper pliocene lacustrine sedimentary deposit in the central part of the ait kandoula basin which contains micromammal faunas and is situated in the southern high atlas morocco in total 113 samples were subjected to paleomagnetic analysis 60 out of the 113 studied samples representing 52 different stratigraphic levels yielded a paleomagnetic direction and at least the polarity could be recognized in 42 specimens eleven specimens were submitted to af demagnetization the mean direction for normal polarity samples was d 349 4 i 50 7 n 36 95 4 5 k 27 and for reversed polarity samples d 191 2 i 53 4 n 16 95 12 32 k 9 these results yielded a polarity sequence which we interpret as spanning from chron c5n 2n to the beginning of chron c3n 4n this interpretation relies on biostratigraphic data previously proposed for a part of the continental fauna found in the basin this result is in agreement with the 40ar 39ar dating previously carried out on a volcanic ash layer which provided an age of 5 9 0 5 ma 1 and which is shown here to be reversely magnetized this layer is correlated here with the reverse polarity zone corresponding to chron c3r biostratigraphic studies on the same section have shown that the micromammal levels extend here only from middle vallesian to upper turolian upper miocene four localities have yielded western european species of micromammals indicating trans mediterranean terrestrial faunal exchanges between these two continents during the late miocene the european murid rodent occitanomys is recorded for the first time in north africa in level 8 of the afoud section an age younger than 5 32 ma being assigned to this level by the present study level 1 of the same section yields the lagomorph prolagus cf michauxi with an age of 6 1 ma the magnetostratigraphic data suggest therefore that the beginning of terrestrial faunal exchanges between north africa and western europe took place in subchron c3an 1n at about 6 1 ma some 0 4 myr before the beginning of the messinian salinity crisis

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

structure and metamorphism of a tectonically thickened continental crust the yalu tsangpo suture zone tibet three tectonic units the active palaeomargin the palaeo oceanic domain and the passive palaeomargin are now juxtaposed and constitute the yalu tsangpo suture zone between india and eurasia in southern tibet we examine the deformation metamorphic and uplift histories of each of these units our observations are accounted for in terms of the processes whereby the crust was thickened to about 70 km we finally discuss the implications for ancient continental collision orogens

neoproterozoic high pressure metamorphism and tectonic constraint from the nappe system south of the sao francisco craton southeast brazil abstract the neoproterozoic giant nappe system exposed south of the sao francisco craton underwent a minimum of 150 km of near horizontal northeastward displacement this nappe system comprises an uppermost unit derived from a plutonic magmatic arc terrane with high pressure high temperature intermediate to mafic granulites at its base socorro guaxupe nappe underlain in sequence by a high pressure kyanite granulite nappe an amphibolite facies metapelite nappe and finally by a quartzite nappe which forms the lowest part of the pile and rests on a parautochthonous unit related to the passive margin of the craton the kyanite granulites crystallized ca 13 kbar 750 c and have only at their top sillimanite related to t increase overprinting t 890 c a consequence of downward heat advection from the overlying socorro guaxupe granulites equilibrated at 900 c pressures of 13 14 kbar are documented in the underlying metapelite nappe with temperatures of 640 670 c in which lenses of eclogite indicate p max of ca 17 5 kbar phyllites of the parautochthonous unit are directly overlain by hp granulites in the north and record p 7 kbar and t 500 c the distribution of metamorphic facies is reminiscent of inverted metamorphism with eclogites from subduction related metamorphism and kyanite granulites from early extrusion of subducted continental crust the metamorphic conditions of the parautochthon may represent a stage achieved during late metamorphic thinning of the nappe pile the overall picture suggests that high pressure units recrystallized in a subduction zone located to the wsw the frontal thrusts of the nappe system show a late north northeastward transported thin skinned pattern

high p t polymetamorphism dehydration melting and generation of migmatites and granites in the higher himalayan crystalline complex sikkim india the higher himalayan crystalline complex hhc in sikkim india consists of pelitic migmatites interlayered with calc silicate rocks and minor metabasites microstructural relationships between the mineral phases and deformational fabric elements and zoning characteristics of garnet indicate a prolonged and complex polymetamorphic history for the hhc the pelitic rocks in the upper part of the hhc contain the assemblage plagioclase quartz garnet k feldspar biotite sillimanite and are devoid of muscovite most of the mineral phases grew syn to post tectonically mineral growth coincided with development of a pervasive fabric s 2 during prograde metamorphism m 2 in the early stages of the collisional event some garnet grains display texturally distinct cores and rims which are separated by calcic plagioclase this texture suggests an earlier metamorphic episode m l m may represent pre himalayan metamorphism and decompression of the hhc later the collisional event led to renewed burial of the hhc and m 2 reactions m 2 is reflected by dehydration melting of muscovite and biotite to form granitic melts which either crystallized in situ to form leucosomes or migrated from their source regions to form larger granitic bodies geothermobarometric estimates for peak m 2 conditions indicate p 10 12 kbar t 800 850 c a subsequent metamorphic event m 3 occurred because of 5 kbar of decompression m 3 is recorded by the breakdown of porphyroblastic garnet in all hhc lithologies higher temperature and pressure estimates come from progressively higher structural levels of the complex the thermal gradient of 5 5 c km is anomalous and may be a consequence of thermal buffering during melting however the pressure gradient of 0 25 kbar km resembles a normal lithostatic gradient which suggests that the hhc in sikkim represents an inverted barrovian sequence this inverted zonation of the hhc is probably the result of large scale structural inversion and or tectonic juxtaposition because of ductile shearing

deformation conditions and quartz c axis fabric development along nappe boundaries the andrelândia nappe system southern brasilia orogen brazil abstract the liberdade and andrel ndia nappes se brazil record the western gondwana amalgamation in the southern brasilia orogen during the ediacaran the liberdade shear zone acted as the boundary between these two overlapping nappes quartz c axis fabrics and strain data from quartzite samples and petrological data from hosting metapelite rocks were analyzed to investigate the tectonic processes involved in nappe development further constraints were achieved by conducting petrological modeling and geothermobarometry using p t isochemical phase diagram approaches quartz axis plots show consistency with different kinematic criteria verified in the field in meso and macroscale the fabrics are associated to both coaxial deformation and non coaxial deformation quartz microstructures metamorphic assemblages and c axis concentrations primarily around y and x structural directions suggest that deformation was accommodated by grain boundary migration recrystallization under temperature above 500 c disregarding pressure effects the opening angles of quartz c axis fabrics suggest temperatures between 480 and 640 c muscovite quartzite and 650 720 c sillimanite quartzite considering pressure data the opening angles indicate conditions of 550 650 c and 7 10 kbar muscovite quartzite and 600 730 c and 4 7 kbar sillimanite quartzite reflecting primarily distinct baric regimes sillimanite bearing samples estimatives are supported by ubiquitous presence of chessboard subgrain patterns in quartz and by modeled isochemical phase diagrams that constrain conditions of 660 730 c and 6 7 kbar for garnet sillimanite biotite schist samples altogether these results indicate progressive tectonic evolution linked to oblique convergence between tectonic units represented by the andrel ndia and liberdade nappes and the paleoproterozoic basement besides recording distinct thermal and baric regimes the nappes also present different structural styles suggesting that pressure and temperature are important boundary conditions for the nappe structural style

the betic orogen and the iberian african boundary in the gulf of cadiz geological evolution central north atlantic abstract the study of the gulf of cadiz on the basis of multichannel seismic profiles and wells illustrates the stratigraphy and tectonics the evolution of the southern iberian margins was more complex than in most north atlantic margins since it entailed several phases of rifting convergence and strike slip motions three main tectonic provinces surround the internal zones of the gibraltar arc orogenic belt these include in the iberian margin of the gulf of cadiz the flysch units of the campo de gibraltar complex the betic external zones and the neogene basins of the guadalquivir valley fault bounded blocks of flysch and subbetic units crops out over large areas of the southeastern iberian shelf the basement of the northwestern area in contrast is represented by the paleozoic rocks of the hercynian massif of iberia half graben structures determined the main structural trends of the margin during the mesozoic which were affected by inversion structures during the neogene compressional stages the mesozoic and lower cenozoic units are best observed in wells and seismic profiles from the northern area these units are either obscure below a thick olistostrome deposit or are absent in most of the rest of the gulf of cadiz seven lithoseismic units from triassic to upper oligocene and another seven neogene and quaternary units are identified based on the relationship to the depositional sequence and the emplacement of the olistostrome the first tectonic phase was characterized by a passive margin which was controlled by the development of half graben extensional structures and carbonate platforms this evolution comprises the mesozoic and early cenozoic ocean spreading in the north atlantic induced extensional tectonics which deformed the cretaceous syn tectonic post aptian deposits increased amounts of terrigenous materials were supplied to the margin from aptian to albian times controlling depositional patterns while terrigenous siliciclastic facies replaced the jurassic carbonate platforms from middle eocene to early miocene the margin was influenced by the relative motions of iberia and africa and the development of the alpine orogeny the iberian african boundary in the gulf of cadiz experienced transpression and the mesozoic basins probably underwent subduction the emplacement of an olistostrome took place in the gulf of cadiz towards the central atlantic basin plains during the tortonian the end of the olistostrome emplacement during the late miocene coincides with accelerated tectonic subsidence while thick progradational and aggradational depositional sequences were developed the occurrence of closely juxtaposed regions of compression and extension during the miocene may reflect the westwards progression of the gibraltar arc mountain front over a subducting thinned tethys crust when the motion between iberian and africa was n s to nnw sse oriented the migration of the arcuate mountain front into the eastern gulf of cadiz is attributed to a mechanism of collision induced delamination of the mantle lithosphere in contrast to the betic rif belts however there is no geophysical evidence to postulate that continental collision took place in the gulf of cadiz during the cenozoic

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

determining the composition of late quaternary marine sediments from nuv vis and nir diffuse reflectance spectra abstract as the length of marine cores increases and sampling intervals decrease the need for rapid and inexpensive means of determining sediment composition has become apparent in this study we examine one potentially useful technique for assessing compositional changes in marine cores diffuse reflectance spectrophotometry we examined nearultraviolet visible and near infrared reflectance spectra from five data sets each data set consists of calibration samples and test samples the calibration samples spectra were related to a sediment component using multiple linear regression the resulting regression or calibration equations were then evaluated using the test samples calibration equations were written relating spectra to several sediment components including carbonate atlantic and east pacific rise odp site 847 organic carbon content atlantic and east pacific rise and opal content east pacific rise the correlation coefficients for the regression equations ranged from a high of 0 99 for carbonate and opal at odp site 847 to a low of 0 97 for atlantic carbonate indicating that spectral variations are highly correlated to sediment composition all of the equations include a substantial number of variables from shorter visible and longer near ultraviolet wavelengths suggesting that these wavelengths are especially important for devices designed specifically to scan marine cores although equations for estimating organic and carbonate content appear independent of other sediment components the opal equation is strongly dependent on carbonate content indicating that opal concentration is correlated to carbonate content tests of the calibration equations indicated that all our equations reasonably estimate the pattern of changes either down core or in surface sediments where our spectral estimates have difficulty is with absolute values frequently over or underestimating observed values by a substantial amount within these limitations diffuse reflectance spectrophotometry can be a useful tool for characterizing marine cores and as our understanding of the relationship between spectra and mineralogy improves so will estimates of absolute values

tectonic setting and u pb zircon dating of the plutonic socorro complex in the transpressive rio paraiba do sul shear belt se brazil the precambrian rio paraiba do sul shear belt comprises a 200 km wide anastomosing network of ne sw trending ductile shear zones extending over 1000 km of the southeastern coast of brazil granulitic gneissic migmatitic and granitoid terrains as well as low to medium grade metavolcano sedimentary sequences are included within it these rocks were affected by strong contractional tangential tectonics due to west northwestward oblique convergence of continental blocks subsequent transpressional tectonics accomodated large dextral orogen parallel movements and shortening the plutonic socorro complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic magmatic and metamorphic structures these relationships point to a continuous magmatic evolution related to regional thrusts and strike slip ductile shear zones the tectonic and magmatic structural features of the serra do lopo granite provide a model of emplacement by sheeting along shear zones during coeval strike slip and cross shortening of country rocks geochronological data indicate that the main igneous activity of socorro complex spanned at least 55 million years from the late stage of the northwestward ductile thrusting 650 ma through right lateral strike slip 595 ma deformation the country rocks yield discordant age data which reflect a strong imprint of the transamazonian tectono metamorphic event 1 9 to 2 0 ma we propose a model for the origin of calc alkaline granites of the ribeira belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins which were bounded by deep shear zones the transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones

titanite petrochronology of the southern brasilia orogen basement effects of retrograde net transfer reactions on titanite trace element compositions abstract combining u pb geochronology and trace element characteristics of accessory minerals more reactive during metamorphism and deformation than zircon such as titanite catisio5 is a powerful tool for elucidating pressure temperature time p t t evolution of orogenic basement complexes this study reports in situ titanite u pb and trace element data from upper amphibolite facies metatexitic orthogneisses of the pouso alegre complex in the basement of the neoproterozoic southern brasilia orogen se brazil the titanite data is combined with thermodynamic modeling major phase compositions and u pb zircon dating and used to constrain the p t t path of the complex during the neoproterozoic orogenesis the investigated garnet amphibole gneisses record peak metamorphic conditions of 750 775 c and 10 5 12 5 kbar associated with partial melting and dated at ca 630 620 ma based on u pb analyses of hree y depleted titanite cores inferred to have crystallized in the presence of garnet near the peak p t conditions extensive melt crystallization at ca 616 ma is constrained by u pb dating of leucosome zircons the exhumation of the complex and re equilibration to ca 550 625 c at 6 9 kbar led to widespread generation of biotite epidote clinozoisite partially replacing amphibole garnet along the main foliation trace element signatures of these retrograde net transfer reactions are recorded in the titanite rims which are hree y enriched strontium depleted and were dated at 605 7 6 2 ma the average zirconium contents of the titanite rims are higher than cores suggesting disequilibrium partitioning of zirconium into titanite during the retrograde path resulting in overestimated zr in titanite temperatures for the rims the exhumation of the complex was concomitant with the intrusion of folded and recrystallized leucogranite dykes dated at 607 0 5 7 ma attesting the persistence of deformation after their intrusion assuming that the pouso alegre complex is part of the sao francisco paleocontinent reworked during the brasilia orogeny the data constrain the minimum age for the collision between the sao francisco paleocontinent and the paranapanema block at ca 620 630 ma and agree with the idea of long lived metamorphism and deformation in the southern brasilia orogen