forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance

2 6 ga high si rhyolites and granites in the kursk domain eastern sarmatia petrology and application for the archaean palaeocontinental correlations abstract a 2 61 ga intraplate magmatism took place locally within the kursk domain of the eastern sarmatia forming a volcanic plutonic association of rhyolites and granites the zircon lu hf and sm nd whole rocks isotope data indicate the crustal source heterogeneity of these neoarchaean intraplate granitoids their protoliths were dominated by palaeoarchaean metapelites and mesoarchaean ttg associations the neoarchaean rhyolites of the kursk domain are characterized by the high crystallization temperatures of c 900 c according to the data of mineral geothermobarometry and geochemistry of zircons tiq and tizr the magmatic chambers were located at a depth of no more than 7 km the silicic melt parental for these 2 61 ga old rhyolites and granites could be formed as a result of 20 dehydration melting of the metapelite ttg mixture in equilibrium with the ilm bt opx pl restite at a pressure of no more than 4 kbar the atamansky complex granites crystallized at deeper levels and contain an admixture of opx pl restite or cumulus component this caused their geochemical differences from rhyolites when these phases were removed during the melts migration to higher levels of the crust the atamansky granites are considered as intermediate chambers where differentiation of the initial silicic magmas took place with the formation of the residual rhyolitic melts the heat source could be a mafic plume magmatism known in the eastern sarmatia there in intraplate settings hot deep plume generated mafic magmas intruded into the crust forming shallow interim chambers the latter initiated melting of the felsic host rocks predominantly metapelites and ttg in the upper parts of the crust producing the silicic volcanic plutonic association the simultaneous 2 61 ga old granitoid intraplate magmatism in the eastern sarmatia and dharwar craton caused by the mafic heat source adds one more argument in favor of an assumption that the sarmatia pilbara kaapvaal and possibly dharwar cratons composed a single supercontinent vaalbara in the neoarchaean the dharwar craton perhaps was at the periphery of the vaalbara supercontinent next to the eastern sarmatia

numerical modeling of liquefaction and comparison with centrifuge tests the prediction of liquefaction and resulting displacements is a major concern for earth structures located in regions of moderate to high seismicity conventional procedures used to assess liquefac

epidote in calcalkaline magmas an experimental study of stability phase relationships and the role of epidote in magmatic evolution experiments on tonalite and granodiorite were performed at conditions ranging from 2 1 to 18 kbar and 550 to 850 c to establish the magmatic stability field of epidote as a function of p t and f02 at water saturated conditions and f02 buffered by nno epidote has a wide magmatic stability field in tonalite at 13 kbar this field extends from the wet solidus at 630 to 790 0c the low pressure intersection of the magmatic epidote crystallization reaction and the h20 saturated tonalite solidus and hence the minimum pressure for magmatic epidote occurs at about 5 kbar the clapeyron slopes of epidote melting reactions are moderately positive in p t space at pressures below the intersection of the epidote melting and the gamet in reactions at 13 kbar 790 c at this intersection epidote reaches its maximal thermal stability 790 c in tonalite h20 in the presence of garnet that is above 14 kbar epidote melting reactions have steep negative clapeyron slopes in p t space at p t conditions near the low pressure intersection of the epidote melting reaction with the water saturated solidus experiments were also performed with f02 buffered by hm at these oxidizing conditions the magmatic stability field of epidote is enlarged down to a pressure of about 3 kbar in granodiorite the stability of epidote has been investigated at pressures from 7 5 to 18 kbar with respect to the tonalite the epidote out reaction is located almost at the same p t conditions at 10 kbar but the maximum thermal stability is about 50 c lower toward higher pressures although few experiments indicate that epidote stability does not increase dramatically with decreasing water activity at present state it is not possible to predict melting and crystallization behavior of epidote at waterundersaturated conditions modal abundances of epidote are low 05 vol in the presence of major 40 vol melt fractions thus epidote in calc alkaline magmatic rocks is not likely to affect major elements strongly by fractional crystallization

the development of subgrain misorientations with strain in dry synthetic nacl measured using ebsd abstract the development of subgrain boundary misorientations with strain in dry synthetic nacl polycrystals deformed at elevated temperature has been investigated using electron backscattered diffraction ebsd at low natural strains up to 0 5 average misorientations of subgrain boundaries increase with strain and a power law relationship exists between strain and average misorientations the average misorientations are strongly influenced by grain orientation suggesting that the misorientation strain relationship may also be texture dependent in materials with high plastic anisotropy like nacl a slight grain size dependency of the average misorientations was observed the results indicate that with suitable calibration average subgrain boundary misorientations may offer a method for estimating the strain accommodated by dislocation creep in nacl and thus perhaps in other geological materials although current theories for polycrystalline plasticity imply that misorientations may also depend on stress in some situations

from hot to cold the temperature dependence on rock deformation processes an introduction abstract understanding rock deformation processes in solid earth materials from the crystal to the tectonic plate scale is essential for characterising the evolution of the lithosphere and for predicting how rocks behave in the subsurface temperature is a key parameter that determines what rock deformation processes are active and therefore how tectonic structures form moreover it controls the migration of fluids and melt in the earth s crust that lead to phase transformations and changes in rock rheology this special issue gathers a collection of research papers following the 21st international conference on deformation mechanisms rheology and tectonics drt 2017 which was held in inverness scotland in april may 2017 organised by the university of aberdeen these contributions provide a significant advance in the study of rock deformation and fluid melt migration at multiple crustal levels from deformation bands near the earth s surface to shear zones in partially molten rocks in the lower crust in this introductory article we first provide an overview of how temperature controls deformation mechanisms and processes and then introduce the collection of research papers ordered from those analysing deformation processes occurring at high temperatures to contributions reporting deformation at shallow crustal conditions

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

importance of pressure solution and coble creep in the deformation of polymineralic rocks the theoretical framework for modeling pressure solution in polyphase rocks is presented it is often assumed that pressure solution is a deformation mechanism of secondary importance deep in the lithosphere it is shown here that the rheology of rock during pressure solution may be drastically reduced in a bimineralic and polymineralic rock relative to its monomineralic counterparts and therefore the importance of this deformation mechanism importance is perhaps underestimated this behavior contrasts with that of a polymineralic rock undergoing dislocation creep where existing models show that the rheology will be some average of the rheologies of individual phases for instance the rheology of a dunite in pressure solution is governed by the diffusion of both mgo and sio2 whichever is the slowest will control the viscosity of the rock conversely in a harzburgite with a specific texture flow will be governed by the fastest diffusing species thus its strength may be as low as that of a quartz rock of comparable grain size given the comparative strength of many phases undergoing dislocation creep relative to quartz this prediction implies a larger range of conditions under which pressure solution forms an important part of bulk rock deformation than inferred previously predictions on textural evolution of polymineralic aggregates are also made and it is shown that some minerals grains may become elongate parallel to the maximum principal stress during deformation even though the rock as a a whole shortens in this direction

basal slip and mechanical anisotropy of biotite the basal slip systems of biotite and their mechanical expressions have been investigated by shortening single crystals oriented to maximize and minimize shear stresses on 001 samples loaded at 45 to 001 exhibit gentle external rotations associated with dislocation glide high angle kink bands in these samples unlike those developed in micas loaded parallel to 001 are limited to sample corners samples shortened perpendicular to 001 show no evidence of nonbasal slip and fail by fracture over all conditions tested the mechanical response of biotite shortened at 45 to 001 is nearly perfectly elastic plastic stress strain curves are characterized by a steep elastic slope a sharply defined yield point and continued deformation at low mostly 1 stresses measured beyond the yield point are insensitive to confining pressure over the range 200 to 500 mpa and exhibit weak dependencies upon strain rate and temperature assuming an exponential relationship between differential stress d and strain rate e of the form e cexp d exp q rt the data collected over strain rates and temperatures of 10 7 to 10 4 s 1 and 20 to 400 c respectively are best fit by an exponential constant of 0 41 0 08 mpa 1 and an activation energy q of 82 13 kj mol a power law fits the data equally well with n 18 4 and q 51 9 kj mol samples oriented favorably for slip in directions 100 and 110 are measurably weaker than those shortened at 45 to 010 and 310 consistent with the reported burgers vectors 100 1 2 110 and 1 2 110 the anisotropy of biotite is further revealed by contrasting these plastic strengths with results of samples deformed parallel and perpendicular to 001 previous studies have shown that biotite loaded in the 001 plane is strong prior to the nucleation of kink bands the strength of biotite shortened perpendicular to 001 exceeds that measured parallel to 001 and is pressure dependent application of the results to deformation within the continental crust suggests that biotite oriented favorably for slip is much weaker than most other silicates over a wide range of geologic conditions its presence within foliated rocks and shear zones may limit locally the stresses that can be supported

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

paleoproterozoic granitoids of the don terrane east sarmatian orogen age magma source and tectonic implications abstract the pavlovsk complex granitoid plutons and associated gabbro diorites are confined to the east sarmatian orogen eso that compose the western part of the junction between the sarmatian and volgo uralia segments of the east european craton the plutons form the largest batholiths with an area of up to 2000 km2 and a total length of more than 330 km the conditions for their emplacement and crystallization sources of melts and tectonic position remain debatable here we present new results on geochronological geochemical and isotope data on the pavlovsk complex granitoids and associated gabbro diorites and discuss these in conjunction with previously published data the geochemical characteristics such as elevated concentrations of mg k sr and ba usually 500 ppm ratios la yb n 25 73 and sr y 23 223 in the absence of eu anomalies of the pavlovsk type plutons are similar to high mg archean granites zircon u pb data indicate the formation of the granitoids and associated gabbro diorites during a short span at 2060 2075 ma the magma emplacement occurred in a transitional environment from late collisional to post collisional after the collision of two crustal segments sarmatia and volgo uralia we identify a significant ancient archean continental crust in the magma source for the northern plutons and leucogranites including the presence of archean zircon cores and archean model tnd dm ages all the plutons are dominated by a mixed magma source generated through the interaction of juvenile lower crustal and enriched mantle components the nd isotopic composition of the studied plutons is significantly more radiogenic as compared with the host archean rocks end t from 0 3 to 4 2 our geochemical data suggest that that the mantle source of the gabbro diorites and granitoids were modified metasomatized by subduction related processes the contrasting geochemical and isotopic features of the granitoids in the eso as compared to the pavlovsk granitoids suggest different sources for the parent melts

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

peraluminous i type granites abstract s type granites always contain more al than the amounts of na ca and k in the rock required to form feldspars primarily owing to their derivation from source components that had previously been weathered those rocks are therefore always saturated in al or peraluminous many i type granites are also peraluminous despite i type source rocks typically not being saturated in al it has previously been suggested that this may result from the fractional crystallisation of amphibole however data from compositionally zoned high temperature plutons in the lachlan fold belt show that it is difficult to generate large quantities of peraluminous melt by removal of amphibole most of the i type granites 95 in the lachlan fold belt formed at lower temperatures and almost half of those rocks for which bulk chemical compositions are available are peraluminous among these granites there are 98 separate suites for which there are chemical data for two or more samples with 47 suites that include both metaluminous more mafic and peraluminous more felsic compositions the origin of those peraluminous compositions is fundamental to any understanding of i type granite petrogenesis in this region compositional variations caused by the assimilation or by partial melting of supracrustal rocks are very small as the isotopic variations within these rocks are dominantly between different suites not within suites the partial melting of more mafic source rocks rather than the fractional crystallisation of more mafic magmas is favoured for the origin of these rocks partial melting is the most likely process involved in the petrogenesis of felsic granites where broadly granodioritic monzogranitic batholiths are associated with lesser amounts of tonalite and very minor amounts of mafic rock experimental studies have shown that the melts generated by the partial melting of basaltic to andesitic rocks under crustal conditions are mostly peraluminous during the dehydrational melting of i type granite source rocks at pressures below the garnet stability field biotite and amphibole melt incongruently to yield pyroxenes the excess al is incorporated into the felsic liquid resulting in the generation of peraluminous melts in this instance the excess al in felsic i type granites is a function of the melting process and unrelated to the bulk composition of the source the observed gradation from peraluminous felsic granites to metaluminous compositions in less felsic rocks in largely isotopically closed systems could happen in two ways at higher temperatures of partial melting ca and other components of clinopyroxene could dissolve in the melt with the melt eventually becoming metaluminous alternatively minerals residual from partial melting dominantly pyroxenes and plagioclase could be incorporated in suspension in the melt so that the resulting bulk magma is metaluminous examples of these two extremes and of intermediate cases are well developed among the granites of southeastern australia

rigid slab foundation subjected to strike slip faulting mechanisms and insights in this paper the interaction of rigid slab foundations with a strike slip fault is studied a three dimensional finite element model of the soil foundation system is developed employing a thorou

time dependent cracking and brittle creep in crustal rocks a review abstract rock fracture under upper crustal conditions is driven not only by applied stresses but also by time dependent chemically activated subcritical cracking processes these subcritical processes are of great importance for the understanding of the mechanical behaviour of rocks over geological timescales a macroscopic manifestation of time dependency in the brittle field is the observation that rocks can deform and fail at constant applied stresses a phenomenon known as brittle creep here we review the available experimental evidence for brittle creep in crustal rocks and the various models developed to explain the observations laboratory experiments have shown that brittle creep occurs in all major rock types and that creep strain rates are extremely sensitive to the environmental conditions differential stress confining pressure temperature and pore fluid composition even small changes in any of these parameters produce order of magnitude changes in creep strain rates and times to failure three main classes of brittle creep model have been proposed to explain these observations phenomenological statistical and micromechanical statistical and micromechanical models explain qualitatively how the increasing influence of microcrack interactions and or the increasing accumulated damage produces the observed evolution of macroscopic deformation during brittle creep however no current model can predict quantitatively all of the observed features of brittle creep experimental data are limited by the timescale over which experiments are realistically feasible clearly an extension of the range of available laboratory data to lower strain rates and the development of new modelling approaches are needed to further improve our current understanding of time dependent brittle deformation in rocks

a new stylolite classification scheme to estimate compaction and local permeability variations we modeled the geometrical roughening of bedding parallel mainly layer dominated stylolites in order to understand their structural evolution to present an advanced classification of stylolite shapes and to relate this classification to chemical compaction and permeability variations at stylolites stylolites are rough dissolution seams that develop in sedimentary basins during chemical compaction in the zechstein 2 carbonate units an important lean gas reservoir in the southern permian zechstein basin in germany stylolites influence local fluid flow mineral replacement reactions and hence the permeability of the reservoir our simulations demonstrate that layer dominated stylolites can grow in three distinct stages an initial slow nucleation phase a fast layer pinning phase and a final freezing phase if the layer is completely dissolved during growth dissolution of the pinning layer and thus destruction of the stylolite s compaction tracking capabilities is a function of the background noise in the rock and the dissolution rate of the layer itself low background noise needs a slower dissolving layer for pinning to be successful but produces flatter teeth than higher background noise we present an advanced classification based on our simulations and separate stylolites into four classes 1 rectangular layer type 2 seismogram pinning type 3 suture sharp peak type and 4 simple wave like type rectangular layer type stylolites are the most appropriate for chemical compaction estimates because they grow linearly and record most of the actual compaction up to 40 mm in the zechstein example seismogram pinning type stylolites also provide good tracking capabilities with the largest teeth tracking most of the compaction suture sharp peak type stylolites grow in a non linear fashion and thus do not record most of the actual compaction however when a non linear growth law is used the compaction estimates are similar to those making use of the rectangular layer type stylolites simple wave like stylolites are not useful for compaction estimates since their growth is highly non linear with a very low growth exponent in the case where sealing material is collected at the tooth during dissolution stylolites can act as barriers for local fluid flow as they intensify sealing capabilities of pinning layers however the development of teeth and spikes offsets and thus destroys continuous stylolite seams so that the permeability across the stylolite becomes very heterogeneous and they are no continuous barriers this behavior is best shown in rectangular layer and seismogram pinning type stylolites that develop efficient fluid barriers at teeth tips but destroy sealing capabilities of layers by offsetting them at the flank leading to a permeability anisotropy along 2 d stylolite planes suture sharp peak stylolites can create fluid barriers if they collect enough sealing material however if the collecting material does not seal or if spikes offset the sealing material the stylolite leaks we propose that our classification can be used to realistically estimate chemical compaction in reservoirs and gives an indication on how heterogeneous the permeability of stylolites can be

dissolution and replacement creep a significant deformation mechanism in mid crustal rocks abstract zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and s c fabrics in these rocks were produced by dissolution precipitation and replacement processes even at epidote amphibolite facies metamorphic conditions the metamorphic fabric is defined by alternating layers and folia dominated by quartz feldspars and biotite epidote zoning patterns in most metamorphic plagioclase orthoclase epidote and sphene are truncated at boundaries normal to the shortening direction suggesting dissolution interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite epidote and myrmekitic intergrowths of plagioclase and quartz metamorphic plagioclase grains are also replaced by epidote we interpret these microstructures to reflect strain enhanced dissolution the cores of many grains show asymmetric overgrowths with at least two generations of beards all oriented on the ends of grains that face the extension direction we interpret these textures to reflect precipitation of components dissolved by deformation enhanced dissolution while biotite and quartz probably deformed by dislocation creep the overall deformation was accommodated by dissolution perpendicular to the shortening direction and precipitation parallel to it these chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks thus wet crust is likely to be weaker than calculated from these experimental studies

modelling the thermal evolution of a collisional precambrian orogen high heat production migmatitic granites of southern finland abstract we present results of thermal and geochemical modelling of paleoproterozoic southern finland which is characterized by formation of high heat production migmatitic granites post orogenic granitoids and high temperature low pressure metamorphism at 1 85 1 78 ga during late svecofennian time metamorphic grade is mostly amphibolite facies 4 5 kbar 650 700 c but also granulite facies 6 kbar up to 800 c occurs our results suggest that the thermal evolution of the area can be modelled as a process of rapid crustal thickening up to 70 km in a plate collision at ca 1870 1860 ma ago followed by conductive heating of the crust simultaneous exhumation and partial melting of middle crust and emplacement of the melts in the upper crust as late orogenic high heat production granites at about 1825 ma ago the upper crust collapsed gravitationally at about 1820 ma but the crust was still thick 57 km when the main orogenic era terminated about 1800 ma ago in modelling the thermal evolution we applied crustal heat production values constrained by geochemical data and a conservative estimate i e relatively low value for mantle heat flow density the modeled temperatures at 30 50 km depths in the thickened crustal stack exceed the dehydration melting temperatures of micas at 40 km 750 c for muscovite and 800 c for biotite respectively 25 35 ma after the collision and thickening of crust the results explain the high temperature low pressure metamorphism in southern finland as an inevitable consequence of crustal thicknening leading to an increase of the total heat production of the crust crustal thicknening is the main heat source for melting and metamorphism of the orogeny advective and latent heat brought by the melts further increased the temperatures at the emplacement depth and contributed to the granulite facies metamorphism that is spatially related to the migmatitic granites geochemical models suggest that the high concentrations of u and th in the migmatitic granites can be attributed to about 20 vol partial melting of deeply buried sediments and volcanic rocks about 36 71 of the original u 18 54 of th and 34 46 of k respectively in the source rock types entered the melt phase no anomalous concentrations of these elements in the source rock are required in addition to explaining the migmatitic late orogenic granite and post orogenic granitoid genesis the collision model implies that the middle lower crust and the upper mantle continued warming until the mesoproterozoic times which suggests a causal link between the svecofennian thermal evolution and the extensive bimodal rapakivi granite and anorthosite magmatism in the fennoscandian shield about 200 250 ma later

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

fingerprinting stress stylolite and calcite twinning paleopiezometry revealing the complexity of progressive stress patterns during folding the case of the monte nero anticline in the apennines italy in this study we show for the first time how quantitative stress estimates can be derived by combining calcite twinning and stylolite roughness stress fingerprinting techniques in a fold and thrust belt first we present a new method that gives access to stress inversion using tectonic stylolites without access to the stylolite surface and compare results with calcite twin inversion second we use our new approach to present a high resolution deformation and stress history that affected meso cenozoic limestone strata in the monte nero anticline during its late miocene pliocene growth in the umbria marche arcuate ridge northern apennines italy in this area an extensive stylolite joint vein network developed during layer parallel shortening lps as well as during and after folding stress fingerprinting illustrates how stress in the sedimentary strata did build up prior to folding during lps the stress regime oscillated between strike slip and compressional during lps before ultimately becoming strike slip again during late stage fold tightening our case study shows that high resolution stress fingerprinting is possible and that this novel method can be used to unravel temporal relationships that relate to local variations of regional orogenic stresses beyond regional implications this study validates our approach as a new powerful toolbox to high resolution stress fingerprinting in basins and orogens combining joint and vein analysis with sedimentary and tectonic stylolite and calcite twin inversion techniques

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

post deformational annealing at the subgrain scale temperature dependent behaviour revealed by in situ heating experiments on deformed single crystal halite the dynamics of substructures which encompass all structures present at the subgrain scale were investigated by static in situ annealing experiments deformed single crystal halite was annealed

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

fault rupture propagation through previously ruptured soil abstractsurface fault rupture during recent earthquakes has significantly damaged structures although several researchers have studied surface fault rupture the effects of fault rupture propagating through soil that has been ruptured previously have not been investigated yet faults rupture multiple times so that native soil deposits will have likely undergone previous ruptures that would have developed shear bands within the soil and the stress state of the soil will have evolved because of these events in addition the predominant modes of soil shearing during the fault rupture process have not been characterized fully numerical simulations are performed to analyze the mechanics of dip slip surface fault rupture and to explore the effects of previously ruptured soil the numerical results demonstrate that the soil rupture process occurs in two distinct stages first broad deformation occurs before strain localization which is followed by more localized deformation after shear band formation str

the preservation potential of microstructures during static grain growth grain growth simulations using the microstructure simulation system elle have been performed in materials with a pre existing grain shape foliation as might be expected the foliation is destroyed by the end of the experiment and grain areas have increased by a factor of seven the area of material swept by the migrating grain boundaries was monitored and it was found that at every stage virtually all of the grains which survived the grain growth process contain one and only one core of unswept material remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation this work suggests that even for samples where no equivalent protolith can be found it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock and perhaps even reconstruct the grain boundary kinematics in addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism as an example of a natural system showing these microstructures we describe a peridotite from almklovdalen norway this peridotite was infiltrated by aqueous fluids at several stages during late caledonian exhumation and retrogressive metamorphism grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite peridotite at the grain scale microstructural mapping of the fluid inclusion rich areas shows that as with the numerical simulations many of the grains retain exactly one core of unswept material examples of other natural systems discussed include dislocation density distributions and trace element zoning

fabric anisotropy controls faulting in the continental crust the influence of pre existing structural anisotropy on faulting in the continents is best tested in recently exhumed crust e g nanga parbat massif nw himalayas where earlier brittle structures have been annealed the kinematics of young faults formed in a single continuing tectonic regime nnw compression are distinctly different depending upon the orientation of the early ductile foliations around them faulting is subparallel and statistically simple where foliation is moderately dipping but highly complex where foliation is steeply dipping thus structural anisotropy does control faulting in the continental crust a result with important implications for seismogenesis fluid flow and basin evolution

shear fractures in anisotropic ductile materials an experimental approach abstract analogue models have been used to investigate the influence of planar mechanical anisotropy on the orientation of shear fractures in an elastoviscous brittle material multilayered models consisting of a mixture of plasticine vaseline and preferentially oriented paper flakes have been coaxially deformed for this purpose the evolution of the orientation length and connectivity of fracture sets has been systematically analysed with progressive coaxial deformation the experimental results show that the orientation of fractures with respect to the deformation axes depends on the orientation of transverse anisotropy two symmetrical sets of shear fractures are formed in models with layering parallel to the extension axis x while the fracture network is asymmetrical with respect to the deformation axes for cases with oblique anisotropy the average dihedral angles between fracture sets are higher than 100 with the obtuse bisector between the two sets oriented perpendicular to layers the stress fields calculated from fracture data differ from the boundary conditions applied by the deformation apparatus this misorientation is related to the degree of anisotropy in models with high oblique anisotropy both fracture arrays rotate in a dextral sense together with layers towards the x axis indicating that the presence of a strong anisotropy controls their evolution with progressive deformation

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

reassessment of proterozoic granitoid ages in ghana on the basis of u pb zircon and monazite dating abstract the west african craton in ghana is of early proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the birimian supergroup as well as younger conglomerates and quartzites of the tarkwaian group and extensive granitoid plutons no consensus exists on absolute and relative ages of plutons within belts and basins u pb geochronology of four different granitoid complexes indicates that contrary to long held views belt granitoid plutons ashanti belt 2172 2 ma sefwi belt 2179 2 ma are 60 to 90 ma older than basin plutons kumasi basin 2116 1 ma belt granitoid are approximately coeval with volcanic rocks of the birimian supergroup while basin plutons are late kinematic and postdate tarkwaian sedimentation the first high precision geochronological data for rocks from ghana i e the eastern part of the birimian domain on the west african craton presented here show that magmatism within this region occurred over a time span of at least 90 ma and provide further evidence that the period between 2 1 and 2 2 ga was one of important magmatism in west africa

tectonic controls on metamorphism partial melting and intrusion timing and duration of regional metamorphism and magmatism in the nigde massif turkey protracted heating and melting of orogenic middle crust followed by emplacement of a syn extensional pluton during decompression and cooling are demonstrated for a metamorphic core complex in central turkey new geochronologic data for the nigde massif of the central anatolian crystalline complex document an interval of melting crystallization and pb loss at elevated ambient temperature 85 92 ma determined by u pb shrimp within a single sample of peraluminous granite crustal melting was syn to post peak metamorphism 725 c dated at 91 0 2 0 ma by shrimp analysis of zircon rims in sillimanite schist that is part of the country rock to the pluton metamorphic monazite in sillimanite schist yields a younger age of 84 7 0 7 ma for matrix grains and inclusions in garnet hornblende 40ar 39ar ages from amphibolites interlayered with the sillimanite schists coincide with the timing of emplacement of magma into metamorphic country rock that had experienced cooling and decompression to 12 km and 550 c the similarity of 40ar 39ar ages for biotite in the pluton 79 ma and hornblende 79 81 ma biotite 74 76 ma and muscovite 78 83 ma in the metamorphic country rock document rapid cooling 100 c my following intrusion the coincidence of the youngest u pb magmatic zircon ages and the oldest 40ar 39ar ages combined with structural evidence for normal sense shearing and tectonic unroofing in the late cretaceous suggests that intrusion was driven by extension we propose that crustal melting occurred in the middle crust during prograde metamorphism and or initial decompression and that ascent and emplacement of magma as a pluton were only triggered once extension occurred and removed upper crustal rocks

continental margin magmatism and migmatisation in the west central fennoscandian shield the ljusdal batholith ljb is a major component of the central svecofennian domain in sweden it is separated from the bothnian basin to the north by the 1 82 1 80 cia crustal scale hassela shear zone hsz the ljb has emplacement ages of 1 86 1 84 cia is mainly alkali calcic metaluminous has epsilon nd values between 0 3 and 1 2 and was formed in a magmatic arc setting during the svecokarelian orogeny the ljb was affected by at least three fold episodes large scale folded screens of migmatised metasedimentary rocks occur in the eastern part of the batholith and to the north of the hsz there is a 50 km wide diatexite belt the transition belt trb consisting of 1 88 1 85 ga granitoids is located at the northwestern extension of this belt a calc alkaline and peraluminous composition combined with negative epsilon nd values 1 7 to 0 8 indicates a large proportion of metasediments in the source for these granitoids u pb sims data on zircon rims from migmatites and leucogranites to the north and east of ljb yield ages of 1 87 1 86 ga i e coeval with the granitoids of the ljb and the trb there is thus a close relationship between the ljb the trb and the migmatites in both space and time syn migmatitic shearing along the hsz indicates that a proto hsz was initiated already at c 1 86 ga and the location of the proto hsz is inferred to be controlled by two older nuclei present in the lower parts of the crust as crustal scale shear zone systems are known to act as ascent pathways for sheet like flow in active orogenies the trb may represents accumulations of melts that were attracted and extracted by the proto hsz and intruded in a block that was not pervasively affected by subsequent shear along the hsz an active continental margin setting for the ljb implies subduction at c 1 86 ga and provides a heat source for both the migmatites and the trb a later migmatisation at 1 82 ga has been recorded to the south of the hsz within the ljb the 1 82 ga stromatic migmatites are folded by f 2 folds and the fabric is truncated by 1 80 cia pegmatites

the geochemistry and petrogenesis of the archean gamitagama lake igneous complex southern superior province the field relationships petrography major element and trace element chemistry of the gamitagama lake complex are described it is emplaced in the rocks of the gamitagama greenstone belt in the superior province the complex comprises five rock groups which range from gabbros through diorites to tonalites and granites and quartz monzonites the granitoid rocks quartz monzonites and the three other groups are each shown to be derived from different sources the three early groups of rocks in the complex are predominantly mafic and were emplaced during three separate intrusive episodes of progressively more evolved hydrous high alumina mafic melts these melts resulted from the fractional crystallization at more than 9 kb of a more primitive basaltic olivine tholeiite melt each evolved by fractional crystallization at crustal levels between 6 km and 20 km 6 kb pt to produce a range of rock types the closest analogues to the gamitagama lake complex occur at convergent plate margins this may imply that some form of plate tectonics perhaps in a permobile regime was taking place during archean times

dynamic recrystallisation of ice aggregates during co axial viscoplastic deformation a numerical approach results of numerical simulations of co axial deformation of pure ice up to high strain combining full field modelling with recrystallisation are presented grain size and lattice preferred orientation analysis and comparisons between simulations at different strain rates show how recrystallisation has a major effect on the microstructure developing larger and equi dimensional grains but a relatively minor effect on the development of a preferred orientation of c axes although c axis distributions do not vary much recrystallisation appears to have a distinct effect on the relative activities of slip systems activating the pyramidal slip system and affecting the distribution of a axes the simulations reveal that the survival probability of individual grains is strongly related to the initial grain size but only weakly dependent on hard or soft orientations with respect to the flow field dynamic recrystallisation reduces initial hardening which is followed by a steady state characteristic of pure shear deformation

on estimating the strength of calcite rocks under natural conditions abstract field studies of calcite mylonites often document microstructures produced by dislocation creep in contrast flow laws derived from experiments predict that calcite rocks should deform mostly by diffusion creep during tectonic processes to investigate this apparent discrepancy we compare stresses estimated by microstructural piezometers to those obtained by extrapolation of experimentally derived flow laws considering shear zones from different geological settings a clear trend is observed of increasing recrystallized grain size with increasing temperature however there is a large spread in grain size and associated stress because separate flow laws have been defined for various different marbles and limestones the strengths predicted for a given set of conditions differ significantly the stress estimates based on the piezometers and strength extrapolated from the various experimentally derived dislocation creep flow laws agree qualitatively but no single flow law predicts all the palaeostress estimates even if experimental data are disregarded the field observations are not consistent with a hypothetical law for coble creep they are consistent with a power law for dislocation creep but only if the material constants are different from those currently determined in laboratory experiments

intraplate lithosphere deformation and the strength of the lithosphere summary the intraplate deformation of continental lithosphere in response to applied stress has been investigated using a mathematical model which incorporates the elastic ductile and brittle response of lithosphere material ductile deformation is assumed to be controlled in the crust by dislocation creep in quartz and in the mantle by dislocation creep and plasticity in olivine brittle failure is predicted using modified griffith theory a fundamental feature of the model is the redistribution of stress within the lithosphere following stress release by both ductile and brittle deformation this redistribution produces high levels of stress in the middle or lower crust immediately above the elastic ductile or brittle ductile transition lithosphere deformation is shown to be critically dependent on the temperature structure of which surface heat flow is a convenient indicator for higher geothermal gradients the release of stress in the lower lithosphere by ductile deformation is more rapid and complete and results in large stress levels in the upper lithosphere for sufficiently large applied stresses or steep geothermal gradients the stress levels in the upper and middle crust will cause complete fracture of the upper lithosphere whole lithosphere failure wlf then results by continued brittle and ductile deformation causing geologically significant strains the critical value of applied stress required to give wlf has been calculated for both tensional and compressional deformation as a function of surface heat flow the predicted lithosphere bulk strength is then compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads which are thought to give net stress levels in the continental lithosphere in the range 0 25 to 0 25 kb using these expected maximum stress levels the model predicts significant extensional deformation in regions of moderate heat flow with q c 60mwm e g central europe as well as for areas of high heat flow like the basinand range province significant compressional deformation is predicted for areas of high heat flow with q c 75 mwm 2 but only for restricted conditions of stress combination these results are in good agreement with

melt organisation and strain partitioning in the lower crust abstract partial melts can form as a result of crustal thickening due to orogenesis even small melt fractions weaken the crust so that partially molten volumes should accumulate significant amounts of strain however relatively little is known of how strain partitions in partial melts and how effective the melt expulsion processes from the partially molten crust are using examples from the western gneiss region wgr norway we consider a case of co existing migmatites and shear zones field image analysis and microanalytical methods allow semi quantification of melt volume rock mineralogy and mineral chemistry and microstructures integration of these analyses implies effective syn melt strain partitioning and subsequent freezing of both the shear zone and migmatite texture we propose a mechanism that allows i syn melt strain localisation at an outcrop scale through stress driven melt organisation resulting in significant relative competence differences in a partially molten rock volume and ii formation of fine grained rocks at outcrop that is entirely or mostly syn melt without subsequent mylonitic shearing in the solid state syn melt shear zones that have not acted as effective melt transport channels and or that have not accumulated post melt deformation may be more common than conventionally assumed

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

the significance of grain scale stresses in the kinetics of metamorphism during metamorphism phases grow and dissolve dominantly along grain boundaries bulk diffusion models for the development of metamorphic textures often assume that diffusion is homogeneous in regions of each phase rather than being localised along phase boundaries under isotropic stress chemical potentials of components involved in reactions are likely to be buffered along grain boundaries between phases thus inhibiting diffusion and growth motive forces must exist which overcome this and it is proposed here that local heterogeneous stresses will be important a mathematical theory for stress controlled growth is derived this theory predicts that the growth rates of two phases along their mutual interface will be linked in a quantifiable way it forms a basis for considering the important effects of stresses induced by local volumetric changes during metamorphism and it provides a link between these effects and those related to pressure solution in deforming rocks

evolution of shear zones through the brittle ductile transition the calamita schists elba island italy abstract a network of shear zones that evolved through the brittle ductile transition is exposed in the calamita schists elba island italy the shear zones formed during late miocene contractional deformation coeval with high grade contact metamorphism 650 c related to the emplacement of plutonic rocks at shallow crustal levels 7 10 km an early stage high metamorphic grade foliation was overprinted by mylonitic deformation that progressively localized on low metamorphic grade shear bands producing s c mylonites during cooling of contact aureole localization of deformation on shear bands was driven by temperature decrease that triggered strain partitioning between hard high grade relics and soft shear bands softening of shear bands occurred likely due to fluid influx and retrograde growth of fine grained phyllosilicates the interconnection of anastomosing shear bands and passive rotation of the relic high grade foliation caused widening of the shear bands producing mylonites with a composite mylonitic foliation and c shear bands an estimate of the vorticity number wk of the flow of 0 3 0 5 was obtained from the orientation of c shear bands measured at the meso and thin section scale close to the brittle ductile transition the growth of soft phyllosilicates allowed c shear bands to act as precursory structures to brittle deformation localized into an array of low angle faults and shear fractures

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

the microstructure of polar ice part ii state of the art an important feature of natural ice in addition to the obvious relevance of glaciers and ice sheets for climate related issues is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point without losing its polycrystalline character this fact together with its strong mechanical anisotropy and other notable properties makes natural ice an interesting model material for studying the high temperature creep and recrystallization of rocks in earth s interior after having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in part i of this work faria et al 2014 here in part ii we present an up todate view of the modern understanding of natural ice microstructures and the deformation processes that may produce them in particular we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure also known as the three stage model cf part i these results prove that grain growth in ice sheets is dynamic in the sense that it occurs during deformation and is markedly affected by the stored strain energy as well as by air inclusions and other impurities the strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal which demand large amounts of strain accommodation from the microstructural analyses of ice cores we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice additionally in fine grained high impurity ice layers e g cloudy bands strain may sometimes be accommodated by diffusional flow at low temperatures and stresses or microscopic grain boundary sliding via microshear in anisotropic ice sheared at high temperatures grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets evidence of nucleation of new grains is also observed at various depths provided that the local concentration of strain energy is high enough which is not seldom the case as a substitute for the tripartite paradigm we propose a novel dynamic recrystallization diagram in the three dimensional state space of strain rate temperature and mean grain size which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure dedicated to sepp kipfstuhl on occasion of his 60th birthday

centrifuge modeling of geotechnical mitigation measures for shallow foundations subjected to reverse faulting surface fault ruptures are particularly damaging to buildings lifelines and bridges located across or adjacent to active faults these structures should be designed in consideration of surface fa

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

relationships between zircon u pb shrimp ages and leucosome type in migmatites of the halls creek orogen western australia field relations and zircon zoning and shrimp u pb patterns introduction from an outcrop of deformed pelitic migmatites reveal relationships both the amount and isotopic variability of accessory between leucosome type and the amount of zircon growth during minerals incorporated into granitic magmas and their migmatization zircons from the oldest stromatic migmatites show constituent trace elements are a complex function of several pb pb intragrain analyses with 1850 1840 ma source and crystallization processes bea et al 1994 anatectic ages and distinctive cathodoluminescence cl patterns brown 1994 ayres et al 1995 barbey et al 1995 bea although the sample is dominated by inferred pre 1855 ma detrital 1996 it is recognized that initial bulk compositions x zircons subsequent thick contact sheath leucosomes and backand pressure p temperature t conditions of anatexis veins developed at 1845 1830 ma in response to intrusion of are major controls on the chemistry and subsequent mafic dykes into the stromatic migmatites zircons show cl evidence behaviour of granitic magmas e g clemens 1990 for substantial anatectic growth and overgrowths with post 1850 brown 1994 also there is increasing awareness that ma ages and a single population age 20 my younger than the deformation in the source region can also control the early other migmatites zircons from the structurally youngest shearand physical pathways and chemical evolution of magmas e g vein hosted leucosomes show few overgrowths and retain a large sawyer 1991 hand dirks 1992 brown et al 1995 detrital population correlation between shear zones and minimal rushmer 1995 watt et al 1996 oliver barr 1997 anatectic zircon growth is inferred to reflect rapid deformationwhich in turn may have a profound impact on subsequent induced melt extraction from the protolith and or rapid quenching granitic magma evolution for a given bulk source comthe earlier contact sheath and stromatic leucosomes probably had position magma extracted at low melt fractions from a combination of slow melt segregation larger interacting melt an actively deforming partially molten zone may differ volume and longer residence times at high temperatures results substantially in composition from one extracted at high suggest that rapid melt extraction by deformation will increase the melt volumes from rocks undergoing little deformation tendency for preservation of source rock isotopic patterns in some rushmer 1995 the link between frozen migmatites granites and complete granite bodies may not be direct clemens

fracpaq a matlab toolbox for the quantification of fracture patterns abstract the patterns of fractures in deformed rocks are rarely uniform or random fracture orientations sizes and spatial distributions often exhibit some kind of order in detail relationships may exist among the different fracture attributes e g small fractures dominated by one orientation larger fractures by another these relationships are important because the mechanical e g strength anisotropy and transport e g fluids heat properties of rock depend on these fracture attributes and patterns this paper describes fracpaq a new open source cross platform toolbox to quantify fracture patterns including distributions in fracture attributes and their spatial variation software has been developed to quantify fracture patterns from 2 d digital images such as thin section micrographs geological maps outcrop or aerial photographs or satellite images the toolbox comprises a suite of matlab scripts based on previously published quantitative methods for the analysis of fracture attributes orientations lengths intensity density and connectivity an estimate of permeability in 2 d is made using a parallel plate model the software provides an objective and consistent methodology for quantifying fracture patterns and their variations in 2 d across a wide range of length scales rock types and tectonic settings the implemented methods presented are inherently scale independent and a key task where applicable is analysing and integrating quantitative fracture pattern data from micro to macro scales the toolbox was developed in matlab and the source code is publicly available on github and the mathworks fileexchange the code runs on any computer with matlab installed including pcs with microsoft windows apple macs with mac os x and machines running different flavours of linux the application source code and sample input files are available in open repositories in the hope that other developers and researchers will optimise and extend the functionality for the benefit of the wider community

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

stylolites a review abstract stylolites are ubiquitous geo patterns observed in rocks in the upper crust from geological reservoirs in sedimentary rocks to deformation zones in folds faults and shear zones these rough surfaces play a major role in the dissolution of rocks around stressed contacts the transport of dissolved material and the precipitation in surrounding pores consequently they play an active role in the evolution of rock microstructures and rheological properties in the earth s crust they are observed individually or in networks in proximity to fractures and joints and in numerous geological settings this review article deals with their geometrical and compositional characteristics and the factors leading to their genesis the main questions this review focuses on are the following how do they form how can they be used to measure strain and formation stress how do they control fluid flow in the upper crust geometrically stylolites have fractal roughness with fractal geometrical properties exhibiting typically three scaling regimes a self affine scaling with hurst exponent 1 1 0 1 at small scale up to tens or hundreds of microns another one with hurst exponent around 0 5 to 0 6 at intermediate scale up to millimeters or centimeters and in the case of sedimentary stylolites a flat scaling at large scale more complicated anisotropic scaling scaling laws depending of the direction of the profile considered is found in the case of tectonic stylolites we report models based on first principles from physical chemistry and statistical physics including a mechanical component for the free energy associated with stress concentrations and a precise tracking of the influence of grain scale heterogeneities and disorder on the resulting micro structures experimental efforts to reproduce stylolites in the laboratory are also reviewed we show that although micrometer size stylolite teeth are obtained in laboratory experiments teeth deforming numerous grains have not yet been obtained experimentally which is understandable given the very long formation time of such geometries finally the applications of stylolites as strain and stress markers to determine paleostress magnitude are reviewed we show that the scalings in stylolite heights and the crossover scale between these scalings can be used to determine the stress magnitude its scalar value perpendicular to the stylolite surface during the stylolite formation and that the stress anisotropy in the stylolite plane can be determined for the case of tectonic stylolites we also show that the crossover between medium millimetric scales and large pluricentimetric scales in the case of sedimentary stylolites provides a good marker for the total amount of dissolution which is still valid even when the largest teeth start to dissolve which leads to the loss of information since the total deformation is not anymore recorded in a single marker structure we discuss the impact of the stylolites on the evolution of the transport properties of the hosting rock and show that they promote a permeability increase parallel to the stylolites whereas their effect on the permeability transverse to the stylolite can be negligible or may reduce the permeability depending on the development of the stylolite

paleoproterozoic granitoids of the losevo terrane east european craton age magma source and tectonic implications abstract the losevo terrane lies between the sarmatia and volgo uralia segments of the east european craton and is considered to be part of the east sarmatian orogen eso here we report the results from field petrologic geochemical sm nd isotopic and zircon u pb geochronological studies on a suite of granitoids from the losevo terrane lt the lt granitoids are divided into four types 1 migmatite leucosome from quartz diorite to granodiorite 2 tonalite trondhjemite and granodiorite ttg with high si na sr contents high sr y la yb ratios and low mg k y yb contents 3 trondhjemite granodiorite granite tgg spatially related to the ttg suite but showing higher k negative eu anomalies and flat ree patterns and 4 high k calc alkaline monzogranite and granodiorite of i type the early 2115 ma migmatites occur as lensoid and stromatic layers the ttg and tgg 2100 2075 ma constitute about 20 of the lt and form batholiths 150 540 km 2 and stocks small massifs of the i type granites 2081 12 ma intrude the migmatites the migmatites ttg and tgg contain inherited zircon cores that yield ages between 2130 and 2172 ma the granitic suite shows positive end values ranging from 2 1 to 5 7 for the ttg and tgg and 2 1 and 2 2 for the migmatites suggesting magma derivation from juvenile paleoproterozoic sources model ages 2255 2450 ma in contrast the i type granite shows slightly negative end value of 0 5 suggesting that the magma sources involved a mixture of archean and paleoproterozoic components all the granitoids from lt record varying degrees of fractional crystallization with or without crustal contamination the temperature of parental melts of the granites are estimated to be in the range of 793 33 c to 878 16 c the migmatites were derived mainly from metaigneous sedimentary protoliths under p 5 6 kbar whereas the ttgs were derived from lower crustal metabasites under p 15 kbar the tgg were derived from mid crustal metagreywackes under p

post collisional two stage magmatism in the east sarmatian orogen east european craton evidence from the olkhovsky ring complex the olkhovsky ring complex orc is composed of a two phase magmatic rock association with quartz diorite quartz monzodiorite granodiorite suite in the outer ring or and trondhjemite as the inner stock is zircon u pb dating reveals that the rocks of the or and is were emplaced at 2070 9 ma and 2044 13 ma respectively the or rocks are intermediate and felsic metaluminous low to moderate alkali both rocks are enriched relative to primitive mantle in rb ba sr k and depleted in nb and ti the or quartz diorite and is trondhjemite are characterized by positive values end t 2 2 and 1 6 respectively the orc belongs to intermediate i type granite with nil or weak crustal contamination whereas the is trondhjemite shows adakite like features that were contaminated with components from earlier magmatic pulses the low degree of contamination of the hypothetical melt its basaltic andesite composition the elevated potassium and magnesium contents and trace element patterns of whole rock and zircon suggest garnet free lower crust or metasomatized mantle wedge as the magma source for the or in a post collisional setting the mechanism for the inner stock trondhjemite was probably similar although the magma source was characterized by the presence of garnet with a moderate degree of melting supplementary material analytical methods and tabulated analytical data are available at https doi org 10 6084 m9 figshare c 3851104

the link between migmatites and s type granites in the turku area southern finland abstract metamorphism and compositions of metasedimentary migmatites and related small scale granites are investigated in the turku area sw finland phase assemblages and mineral compositions suggest peak p t conditions of 800 c and 6 kbar the portion of melt produced in the migmatites by dehydration melting is estimated to be 20 wt the partial melt is k 2 o rich and water undersaturated 4 wt h 2 o the leucosomes and small scale granites have almost the same proportions of felsic minerals but the mafic minerals grt crd bt are enriched in the granites along with the contents of tio 2 feo tot mgo and hfse the leucosomes have low total ree values and strongly positive eu anomalies the ree patterns of the granites are characterized by relatively high lree low hree and distinct negative eu anomalies the distribution of major elements and trace element signatures suggests that leucosomes and granites are related as cumulates and evolved residual melts respectively the decimeter to meter scale leucosomes are interpreted to be the product of crystal fractionation which occurred during near isobaric cooling and the granites are the crystallized residual melts which could move further on and form larger bodies within and outside of the migmatite complex

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance comment

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

zircon u pb ages and hf isotope compositions of the mayuan migmatite complex nw fujian province southeast china constraints on the timing and nature of a regional tectonothermal event associated with the caledonian orogeny abstract u pb ages trace elements and hf isotope compositions of zircons from the mayuan migmatite complex in nw fujian province have been determined to provide constraints on the source and genesis of anatexis and tectonothermal evolution related to the caledonian orogeny in south china the migmatites investigated consist of various amounts of mesosome leucosome and melansome zircons extracted from mesosome leucosome and granite samples are characterized by oscillatory overgrowths enclosing inherited cores or occur as newly grown grains the ages of the inherited zircons from the leucosome and granite samples are consistent with those of adjacent basement paragneiss in the study area suggesting that both leucosome and granite were generated by partial melting of the latter a comparison of hf isotopes between the newly formed zircons and inherited cores indicates that the former resulted from the breakdown of preexisting inherited zircons and or less hf rich minerals other than zircons at the source one mesosome sample contains typical metamorphic zircons that yielded a weighted mean 206 pb 238 u age of 453 3 ma they show enrichments in heavy rees lu n la n up to 22 709 indicating their growth prior to garnet crystallization the other mesosome sample in contrast contains both newly formed metamorphic rims and grains that gave a weighted mean 206 pb 238 u age of 442 8 ma they are characterized by relatively low th u ratios depletions in heavy rees lu n la n 117 396 and low 176 lu 177 hf ratios suggesting their growth synchronous with garnet crystallization the u pb ages of the mesosome samples are interpreted as recording the time of early ca 453 ma to peak 442 ma stages of a regional metamorphic event two leucosome and two granite samples yield consistent u pb ages of 438 5 ma to 442 4 ma which provide constraints on the timing of subsequent anatexis and magmatism the geochronological data reported here reveal a consecutive sequence of regional metamorphism anatexis and magmatism in nw fujian province lasting for at least 15 myr which was driven by the caledonian orogeny that have affected a major part of the scb

geotechnical mitigation strategies for earthquake surface fault rupture abstractsurface fault rupture can be damaging to structures built on or near active faults if the hazard is not addressed properly fault induced angular distortion and lateral ground strain can cause beams to yield and eventually lead to structural collapse when avoidance is not possible geotechnical mitigation strategies can be used these strategies include spreading fault displacement over a large area causing the structure to respond with rigid body movement and diverting the fault rupture around the structure the effectiveness of these strategies can vary from protecting life safety to preventing significant damage and can be effective for a range of dip slip fault displacements earth fills should be sufficiently thick and ductile to prevent the underlying fault dislocation from developing at the ground surface thick rc mat foundations proved to be especially effective in shielding the superstructure from the damaging effects of the underlying ground movements although more challenging to im

evolution source and tectonic significance of early mesozoic granitoid magmatism in the central asian orogenic belt central segment abstract numerous early mesozoic granitoids have been recognized from the central segment of the central asian orogenic belt caob they can be broadly classified into two groups according to zircon u pb ages an early stage group covering the time span from early to middle triassic 250 230 ma and a late stage group emplaced during late triassic to early jurassic ca 230 190 ma early stage 250 230 ma granitoids are mainly distributed in the western central mongolia erguna belt cmeb the western altai belt ab the south mongolia xing an belt smxb and the beishan inner mongolia jilin belt bijb they consist mainly of quartz diorites granodiorites and monzogranites mostly of i type with minor mafic intrusions with some of them showing adakite like signatures and some with s type features late stage 230 190 ma granitoids mainly occur in the north mongolia transbaikalia belt nmtb the eastern cmeb erguna massif and the eastern altai belt ab they are predominately syenogranites monzogranites and syenites associated with many alkaline granites and mafic intrusions and are a type and transitional i a type or highly fractionated i type granites whole rock sr nd and zircon hf isotopic data have been compiled for regional isotopic mapping the end t values show large variations from 7 0 to 7 4 and nd model ages tdm from 0 46 ga to 1 43 ga as well as the initial sr isotopic ratios sri from 0 7023 to 0 7174 the zircon ehf t values vary from 4 6 to 15 3 and give two stage hf model ages tdm2 from 0 30 ga to 2 09 ga the extremely large variations of whole rock sr nd and zircon hf isotopes demonstrate strong isotopic heterogeneity of the source regions that are mainly dominated by juvenile components with significant old crustal participation furthermore the late stage granitoids in the nmtb the cmeb and the ab generally have more negative end t values and more variable zircon ehf t values than those of the early stage granitoids in the same belt implying an increasing crustal signature from early to late stage in the assumed heterogeneous source regions within the same belt which probably results from melting of shallower crust in parallel with a shift to more alkaline chemistry of the late granitoid magmas by contrast most late stage granitoids in the smxb and the bijb have more positive end t values and homogeneous zircon ehf t values than those of the early stage granitoids in the same belt indicating more juvenile contribution to the source of the these granitoids the generation of the early mesozoic granitoid magmas in the nmtb and the cmeb was dominated by the ongoing closure of the mongol okhotsk ocean and some was probably related to a mantle plume process they were possibly derived from subducted materials melting or juvenile components with some probable contributions from ancient continental crust early mesozoic granitoid magmas in the smxb the ab and the bijb were generated in a post non orogenic setting after the closure of the paleo asian ocean and were the results of partial melting of crustal components in response to underplating of mantle derived magmas most likely linked to lithospheric thickening and delamination and asthenospheric upwelling early mesozoic granitoid magmatism provides critical information on mesozoic post accretionary tectonic evolution of the paleo asian ocean and transitional tectonic regimes from early mesozoic subduction to late mesozoic closure of the mongol okhotsk ocean as well as post accretionary continental growth

systematic use of trace element distribution patterns in log log diagrams for plutonic suites trace element modelling has been widely used for petrogenetic interpretation of basaltic systems this paper indicates how to select trace element pairs having very different bulk distribution coefficients d which when plotted on simple log log diagrams permit the identification of the main magmatic process magma mixing partial melting fractional crystallization involved in the genesis of plutonic rocks fractional crystallization gives a straight line on such a diagram with a strong decrease of the compatible element whereas the concentrations of the element with d 1 increase slowly a similar evolution of the solids in equilibrium is observed and when data of at least one of the cumulates are directly available it is possible to calculate the d and f parameters of the sequence of fractional crystallization an example of this procedure is shown for a french hercynian plutonic suite the basic suite of variscan corsica

what happens to deformed rocks after deformation a refined model for recovery based on numerical simulations deformation in large parts of the middle crust results in strained rocks consisting of grains with variable dislocation densities and microstructures which are characterized by gradual distortion

effects of layering and anisotropy on fault geometry the geometry and orientation of faults are examined using several field examples of small scale 1 with dihedral angles usually of about 50 as predicted by the coulomb theory of failure in layered rocks the geometry of faults varies with the orientation of layering with respect to the stress field where 1 is approximately normal to layering or anisotropy conjugate faults also develop symmetrically about 1 where rocks have interbedded layers with different mechanical behaviours however faults tend to initiate orthogonal to the more brittle layers i e originate as extension fractures sub parallel to 1 but oblique to the less brittle layers as the fault steps through the layering pull aparts are developed which may reduce the dihedral angle where 1 is oblique c 25 75 to anisotropy one set of faults is developed at a high angle to layering with another at a low angle usually showing a ramp flat geometry large dihedral angles up to 90 may result and 1 does not bisect this angle where 1 is approximately parallel to layering two cases can be recognized where 3 is approximately normal to layering faults with layer parallel flats and contractional ramps develop where 2 is approximately normal to layering conjugate faults develop which are symmetrical about 1 the geometry resembling that in isotropic rocks these observations are in agreement with rock deformation experiments which show the strong effect of anisotropy on fault orientation but the observations incorporate the effects of layering of materials with different deformation characteristics

geochemistry and zircon geochronology of the i type high k calc alkaline and s type granitoid rocks from southeastern roraima brazil orosirian collisional magmatism evidence 1 97 1 96 ga in central portion of guyana shield abstract the understanding of the geological evolution of the uatuma anaua domain in southeastern roraima central region of guyana shield is of major significance in the study of the amazonian craton this region lies between some major paleoproterozoic geological geochronological provinces tapajos parima or ventuari tapajos dominant maroni itacaiunas or transamazon northwest and central amazonian or central amazon southeast and east geological mapping of the northern area of uatuma anaua domain integrated with whole rock geochemistry data and previous and new pb evaporation and u pb zircon geochronology point out for a plutonic collisional magmatic event 1975 1968 ma represented by i type high k calc alkaline martins pereira and s type serra dourada granitoid rocks this magmatism was probably generated from crustal sources by partial melting during amalgamation of the ttg like anaua magmatic arc 2028 ma with transamazonian 2 2 2 0 ga and central amazonian older than 2 3 ga terranes local cumulatic leucogranites fills planar structures of the martins pereira granites these leucogranites show younger ages 1909 ma and several inherited zircons 2354 2134 1997 and 1959 ma suggesting origin from crustal sources

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

paragenesis and thermobarometry of ca amphiboles in the barcroft granodioritic pluton central white mountains eastern california mid mesozoic to tertiary igneous activity reflects arc evolution along the sierra white inyo sector of the continental margin the ca amphibole biotite augite bearing middle jurassic barcroft pluton consists of intergradational quartzose gabbro diorite metadiorite voluminous mafic to felsic granodiorite and rare alaskite aplite it was emplaced along the barcroft structural break a major ene striking suture juxtaposing mid mesozoic bimodal metavolcanics on the north against metamorphosed lower cambrian platform strata on the south the cretaceous two mica mcafee creek granite intruded both wall rocks and the barcroft granodioritic pluton on the east geologic mapping and petrochemical studies have documented the structure and metaluminous geochemistry of the barcroft body analyses of ca clinoamphiboles from 16 barcroft rocks allow the assignment of p t conditions of crystallization recrystallization based on experimentally determined and calculated clinoamphibole thermobarometers combined with earlier studies of country rock metamorphism and oxygen isotopic investigations of mineral assemblages in both plutons and wall rocks four stages in the p t evolution of the barcroft pluton have been identified stage 1 partial melting of a mafic protolith at 25 km or greater depth then onset of crystallization of refractory hornblende and other ferromagnesian phases at 915 25 c attending magma ascent stages 2 3 crystallization of progressively more actinolitic hornblende during decompression cooling and stalling of the pluton at a depth of 10 12 km as temperature declined over the interval 760 650 c stage 4 subsolidus recrystallization exsolution of nearly stoichiometric actinolite grunerite attending deuteric alteration over the range 395 545 c at a depth of 4 5 km mineral assemblages of stages 1 4 are disposed progressively inward indicating that the barcroft pluton lost heat along intrusive contacts with the wall rocks cooling annealing and re equilibrating in the process the cretaceous mcafee creek granite transected this ene trending thermal zonation but exerted little effect on ti al contents and 18o values of barcroft amphiboles

recent and future trends in paleopiezometry in the diagenetic domain insights into the tectonic paleostress and burial depth history of fold and thrust belts and sedimentary basins paleopiezometry provides an access to the past stress magnitude key to better understand the behaviour of the earth s crust over long period of time this contribution presents a review of some paleopiezometric techniques that can be used in the diagenetic domain in fold and thrust belts and sedimentary basins calcite twinning and stylolite roughness techniques have been selected and are presented through a critical description of their methodologies along with approaches to further reconstruct the complete effective stress tensor major geological lessons learned over the past decades from published studies are summarized and discussed along with a way forward to potential breakthroughs