low to medium pressure variscan metamorphism in galicia nw spain evolution of a kyanite bearing synform and associated bounding antiformal domains

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

metamorphism of the witwatersrand gold fields conditions during peak metamorphism pelitic assemblages from all major witwatersrand gold fields record metamorphic conditions of the greenschist facies with minimal regional grade changes over at least 200 km strike length diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites the actively exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons bulk rock composition has been a major control on assemblage development key metapelitic assemblages include pyrophyllite chloritoid chlorite and muscovite in each gold field with less common metamorphic biotite accessory minerals are pyrite tourmaline rutile and zircon the abundance of chloritoid and pyrophyllite in thin shaly units together with their minor but widespread distribution in quartzites and conglomerates indicate that metamorphic temperatures reached 350 c 50 c in all the gold fields pressures are less well constrained 1 2 kbar being inferred outside the gold fields higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions the temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to re mobilize gold and may explain its secondary textural features

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

shear wave splitting lithospheric anisotropy and mantle deformation beneath the arabia eurasia collision zone in iran article i nfo records of core refracted phases from permanent and temporary seismological stations are analysed to provide a map of seismic anisotropy covering most tectonic provinces of iran anisotropy is coherent at the scale of a single province but it differs from one province to another no significant shear wave splitting is observed in the mountain ranges of zagros alborz and kopeh dagh anisotropy with average time lags of 1 1 0 3 s between fast and slow shear waves with axes of the faster polarization oriented mostly nw se is observed in central iran but the faster orientation is ne sw in ne iran the orientations of fast axes do not coincide with the directions of absolute plate motion in the no net rotation frame or in the hot spot frame suggesting that the observed anisotropy cannot be explained by a simple model of asthenospheric flow the heterogeneous orientation of fast axes with differences in length scales of a few hundreds km suggests that the anisotropy originates in the lithospheric mantle in the area where strike slip shear dominates the fast axes lie approximately parallel to the orientations of maximum shear estimated from gps measurements either with right lateral shear associated with the ns trending right lateral faults in central iran or left lateral shear associated with ew trending left lateral faults in northeast iran in the mountain belts the null measurements may be related to horizontal shortening and thickening by pure shear as proposed from synthetic models of olivine fabric which both imply a large amount of vertical strain not seen by sks splitting our data therefore do not agree with previous explanations of shear wave splitting in the neighbouring regions of eastern anatolia and saudi arabia in terms of simple asthenospheric flow they rather support the view that iran deforms over a broad area with vertically coherent deformation at a lithospheric scale that includes both shear between blocks in central iran and shortening at the mountain belts

geochemistry and origin of late archean volcanics from the ventersdorp supergroup south africa abstract the ventersdorp supergroup 2700 2750 ma is comprised chiefly of mafic to intermediate subaerially erupted volcanics with smaller amounts of clastic sediments the lower unit the klipriviersberg group consists of basal basaltic komatiites overlain by basalts which show a progressive increase in mg number and ni content and a decrease in incompatible element contents with increasing stratigraphic height volcanics of the overlying platberg and pniel groups consist of basaltic andesites and andesites and some felsic volcanics all ventersdorp volcanics exhibit a subduction zone geochemical component and are similar in incompatible element distributions to volcanics from continental margin arc systems both tholeiitic and calc alkaline trends occur in the ventersdorp succession with the former dominating crustal contamination appears to have played a minor role in the evolution of ventersdorp magmas with the exception of basaltic komatiite the klipriviersberg lavas can be related by progressive melting of a garnet lherzolite source containing a subduction zone component followed by up to 30 of shallow fractional crystallization the basaltic komatiite can be produced from the same source at higher temperatures and the pniel lavas by shallow fractional crystallization of basaltic komatiite the platberg volcanics must come from a within plate enriched mantle source and mafic and felsic members of this group can be related by shallow fractional crystallization geochemical and geologic data are consistent with a model for ventersdorp magmas involving a subduction zone in which klipriviersberg and pniel magmas are produced from the mantle wedge and platberg magmas come from an enriched mantle lithosphere with a superimposed subduction zone component secular trends in klipriviersberg lavas can be explained by progressive adiabatic melting of an ascending plume rooted in the mantle wedge

estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data summary we developed an integrated technique for estimating crustal anisotropy with a horizontal axis using radial r and transverse t receiver functions the technique includes computing three individual and one joint objective function jof and a reliability analysis of the estimated anisotropy the individual objective functions iofs are designed to 1 maximize the peak energy of the stacked r receiver function after a cosine moveout correction in the ps arrival time 2 to maximize the correlation of the radial receiver functions after a full correction of anisotropy or 3 to minimize the total energy of transverse receiver functions stacked after a removal of crustal anisotropy the jof was computed by a weighted average of the three iofs and the reliability analysis uses the principle that stacking coherent signals can lead to an increase of signal to noise ratio we applied the technique to synthetic receiver functions generated with 30 60 per cent white noise from a variety of anisotropic and heterogeneous models the synthetic tests indicate that the proposed technique has good capability to recover the input models despite the presence of random and other coherent noises such as those caused by inhomogeneous structures in the data the technique can always provide accurate estimates of crustal anisotropy we applied the technique to two permanent seismic stations in western china and found significant crustal anisotropy beneath one station located at the northern edge of the tibetan plateau the observed fast polarization direction at this station follows the direction of the maximum horizontal tensile stress suggesting that the observed seismic anisotropy is likely caused by mineral alignment in the lower crust the station situated in the sichuan basin on the other hand shows little to no seismic anisotropy which may suggest that the crust beneath the basin is nearly rigid with very little deformation the developed technique can be applied to any broadband seismic stations that have a good backazimuthal coverage of teleseismic events

low thermal gradient hercynian metamorphism in the eastern pyrenees fluids with compositions in the system co2 h2o nacl were trapped in quartz veins enclosed in low grade metamorphic rocks chlorite zone on the southern flank of the canigo massif eastern pyrenees the veins which also contain arsenopyrite crystals were formed contemporaneously with the main hercynian foliation and metamorphism volumetric properties of the fluid and the results of arsenopyrite geothermometry suggest p t trapping conditions of 4 6 6 kbar and 450 530 c this implies that an episode of metamorphism with an average geothermal gradient of 25 c km 1 occurred during the main deformation event this episode preceded the low p high t metamorphism described around domes and to date considered as characteristic of the hercynian orogeny in the pyrenees

the archean proterozoic boundary in the kaapvaal province of southern africa abstract the kaapvaal province has long been considered unusual in that archean cover sequences developed over granite greenstone gneiss terranes 3060 25 ga whereas in other provinces the oldest cover sequences are generally 2500 ga however two unconformity bounded sequences 2 5 ga and of greater inter regional extent than the unconformably underlying archean sequences can now be recognized in the kaapvaal province the two basal unconformities of these sequences are candidates for the temporally defined 2 5 ga archean proterozoic boundary the older but geologically more discordant unconformity occurs at the base of the uppermost group of the ventersdorp supergroup the pniel group and in at least five other erosional remnants in central transvaal and adjacent botswana these other remnants include parts of the buffelsfontein and wolkberg groups which heretofore have been considered to be conformable protobasinal phases of the younger transvaal succession the pniel and the other remnants are truncated by the locally well known unconformity at the base of the transvaal succession on a world wide basis cratonization and the evolution of mineralogically mature cover sequences were and continue to be diachronous the kaapvaal province is unusual only in that it also contains archean cover sequences however with major conformities at about 2 5 ga and the presence of proterozoic unconformity bounded sequences the kaapvaal province seems little different than most other provinces that contain the archean proterozoic boundary

low p metamorphism following a barrovian type evolution complex tectonic controls for a common transition as deduced in the mondonedo thrust sheet nw iberian massif the mondonedo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low and medium p metamorphic domains this unit underwent an initial medium p event during the initial stages of variscan convergence related to crustal thickening subsequently the thrust sheet evolved to a low p baric type of metamorphism related to syn convergence thinning and exhumation its footwall cropping out in two tectonic windows registered a different evolution with a low p history that evolved from low to high t under a high geothermal gradient several different p t paths of the mondon edo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area following the initial crustal thickening two main syn convergence extensional shear zones developed one of them occurs in the hangingwall whereas the other affects the footwall unit both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet and their activity is viewed as a consequence of the need for gravitational re equilibration within the orogenic wedge the most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low p metamorphism is essentially a late phenomenon and is linked to late orogenic tectonic activity in the mondon edo thrust sheet our conclusions indicate that low p metamorphism may also develop during convergence and that this may occur in at least two cases one is tectonic denudation of an allochthonous unit during its emplacement and the other thinning and extension at the footwall unit of an advancing thrust sheet as a consequence the low p evolution may show different characteristics in different units of an orogenic nappe pile

basinal and shelf sedimentation in relation to the archaean proterozoic boundary abstract in most countries the archaean proterozoic boundary has been designated as an unconformity between the oldest shelf type quartz arenites and a basement consisting of greenstone belts intruded by granitic rocks or high grade gneisses the quartz arenites have been characterized as shallow water shelf deposits and sediments in the greenstone belts as resedimented basinal wackes of volcanogenic origin radiometric ages for rocks of the canadian shield led to initial designation of the archaean proterozoic boundary as thus defined at about 2 5 ga subsequent age determinations have clearly established that a number of quartz arenites originally assigned to the proterozoic are 2 7 2 94 ga old e g witwatersrand and pongola supergroups of south africa to accommodate these old quartz arenites some have suggested that the arch aean proterozoic boundary should be regarded as diachronous embracing a span of time about equivalent to that of the phanerozoic however quartz arenites also are abundant in 3 2 3 5 ga old greenstone belts and high grade metamorphic terranes of south africa western australia and the u s a likewise large regions of basinal volcanogenic sediments occur in post 2 5 ga terranes this greatly expands the diachroneity of geological units that were initially regarded as either uniquely archaean or proterozoic we therefore recommend that chronometric and chronostratigraphic sense be retained for the archaean proterozoic boundary divested of the requirement for all quartz arenites to be proterozoic according to this analysis of the precambrian record shelf sediments have increased in abundance relative to basinal sediments through time but at different rates in different areas this change is attributable to progressive cratonization of shield areas a process that was strongly diachronous on a worldwide scale

sr z sbnd nd isotopic record of multi stage interactions between mantle derived magmas and crustal components in a collision context the ultramafic granitoid association from vivero hercynian belt nw spain late stage epizonal calc alkaline granitoids of the hercynian belt are often associated with coeval more basic rocks such granitoids have been the focus of special attention when studying the role of mantle derived magmas in granitoid genesis these studies conclude that basic magmas are involved in granitoid production but their contribution to the chemical composition of granitoids is often controversial the vivero massif nw spain includes calc alkaline granitoids and coeval ultramafic and mafic rocks but these granitoids are syntectonic and intruded into middle crustal levels during an early stage scattered major and trace element chemistry together with heterogeneous sr nd isotopic results preclude an origin of this ultramafic to granitoid association by any closed system fractionation process ultramafic rocks include peridotites pyroxenites and homblendites with endi from 3 5 to 1 3 and 87sr86sr i ratio from 0 7059 to 0 7080 they are interpreted as cumulates formed by in situ fractional crystallization from contaminated basaltic magmas particularly by interaction with 87sr enriched fluids mafic rocks comprise biotite amphibole diorites quartz diorites and tonalites with ndi from 3 8 to 0 0 and 87sr86sr i ratio from 0 7058 to 0 7073 they represent either cumulates or more evolved magmas of calc alkaline affinity these rocks display an inverse correlation between the degree of differentiation and the degree of contamination inferred from isotope data this points to wall rock assimilation by thermal erosion during ascent of basic magmas up to the middle crust and supports the participation of mantle sources with positive end and 87sr86sr i 0 7058 granitoids range from amphibole biotite tonalites through biotite granodiorites to monzonitic granites they show an isotopic trend which diverges from that of ultramafic and mafic rocks endi 6 0 to 1 9 decreases while 87sr86sr i ratio 0 7061 to 0 7082 increases from tonalites to monzonitic granites reflecting an increasing contribution of crustal components in more differentiated rocks assimilation and fractional crystallization mixing included account for such an evolution contamination of basic magmas by this process occurring at deeper crustal level produced hybrid tonalites and related mafic magmatic enclaves only monzonitic granites are interpreted as pure crustal melts their hree fractionated patterns gd yb 4 5 suggest equilibrium with a garnet rich residuum partial melting of moderate time integrated rb sr protolith such as greywackes and or meta igneous rocks of intermediate acid composition could account for the isotopic composition of these monozogranites this example shows that at least some calc alkaline granitoids of collisional setting are not produced by pure intra crustal melting they are formed through a complex multi stage hybridization process involving mantle derived magmas crustal components and several concomitant magmatic processes

tracing crustal evolution in the nw iberian peninsula through the rbsr and upb systematics of palaeozoic granitoids a review abstract the abundant granitoids in the palaeozoic basement of the northwestern iberian peninsula include 470 440 ma old suites early middle ordovician now present as orthogneisses and several variscan suites the age data presently available suggest that the latter have been generated in three separate time intervals 330 320 ma ago late visean early namurian 310 300 ma ago westphalian and 290 280 ma ago late stephanian autunian rb sr whole rock and u pb zircon systematics along with field and chemical evidence testify to the generation of the early and late palaeozoic granitoids from lower palaeozoic and precambrian maximum age 1 7 1 6 ga sedimentary sequences respectively the source rocks contained zircons derived from a late archaean early proterozoic continental crust that probably underlies the whole of western europe although for the greater part modified beyond recognition by younger crustal reworking

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

complex seismic anisotropy beneath western tibet and its geodynamic implications a b s t r a c t shear wave splitting parameters obtained along a linear array of 23 stations deployed in western tibet during a two year period from 2011 to 2013 demonstrate strong and systematic azimuthal variations with a 90 degree periodicity suggesting the existence of double layer anisotropy with a horizontal axis of symmetry the predominant fast orientations are mostly e w for events from the east and southeast and ne sw or n s for events from the northwestern quadrant under the assumption of a two layer anisotropic structure we grid search for the two pairs of parameters that characterize the complex anisotropy the resulting fast orientation for the lower layer is mostly n s which is consistent with the direction of mantle flow caused by the subduction of the indian continent beneath tibet for the upper layer the fast orientation is ne sw which is significantly different from the strike of dominant surface structures and can be explained by lower crustal flow in western tibet this interpretation is supported by the ne sw orientation and significant strength with splitting times as large as 1 3 s of crustal anisotropy revealed using the sinusoidal moveout of the p to s converted phases from the moho to our knowledge this is the first time when upper layer anisotropy revealed from sws analysis is reliably constrained in the crust using independently determined crustal anisotropy measured by the p to s converted phase from the moho the observations provide additional evidence for the existence of a subducted indian slab that is undeformed and is decoupled from the significantly shortened crust extending to the northern margin of the qiangtang block 2015 elsevier b v all rights reserved

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

magnitude and symmetry of seismic anisotropy in mica and amphibole bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast tibetan plateau we calibrated the magnitude and symmetry of seismic anisotropy for 132 mica or amphibole bearing metamorphic rocks to constrain their departures from transverse isotropy ti which is usually assumed in the interpretation of seismic data the average bulk vp anisotropy at 600 mpa for the chlorite schists mica schists phyllites sillimanite mica schists and amphibole schists examined is 12 0 12 8 12 8 17 0 and 12 9 respectively most of the schists show vp anisotropy in the foliation plane which averages 2 4 for phyllites 3 3 for mica schists 4 1 for chlorite schists 6 8 for sillimanite mica schists and 5 2 for amphibole schists this departure from ti is due to the presence of amphibole sillimanite and quartz amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation forming orthorhombic anisotropy with vp x vp y vp z effects of quartz are complicated depending on its volume fraction and prevailing slip system most of the mica or amphibole bearing schists and mylonites are approximately transversely isotropic in terms of s wave velocities and splitting although their p wave properties may display orthorhombic symmetry the results provide insight for the interpretation of seismic data from the southeast tibetan plateau the n s to nw se polarized crustal anisotropy in the sibumasu and indochina blocks is caused by subvertically foliated mica and amphibole bearing rocks deformed by predominantly compressional folding and subordinate strike slip shear these blocks have been rotated clockwise 70 90 around the east himalayan syntaxis without finite eastward or southeastward extrusion in responding to progressive indentation of india into asia

probable anticlockwise p t evolution in extending crust hlinsko region bohemian massif in the hlinsko region variscan bohemian massif czech republic a major extensional shear zone separates low grade metasedimentary series hlinsko schists and high grade rocks of the moldanubian terrane svratka crystalline unit during late variscan extension a tonalite intruded syntectonically into the normal ductile shear zone and caused contact metamorphism of the overlying schists concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists in order to decipher the detailed petrological evolution of the hlinsko unit situated in the hangingwall of this tectonic contact a phase diagram approach and pctrogenetic grids calculated with the thermocalc computer program were used the crystallization deformation relationships and the paragenetic analysis of the hlinsko schists define a p t path with an initial minor increase in pressure followed by cooling calculated pseudosections constrain this anticlockwise p t evolution to the upper part of the andalusite field between 0 36 and 0 40 gpa for temperatures ranging from 570 to 530 c a low uhz0 is required to explain the presence of andalusite biotite bearing assemblages and could be related to the presence of abundant graphite in contrast the footwall rocks of the svratka crystalline unit record decompression from around 0 8 gpa at a relatively constant temperature followed by cooling thus the footwall and the hangingwall units display opposite but convergent p thistories decompression in the footwall rocks is related to a rapid exhumation we propose that the inverse anticlockwise p t path recorded in the hangingwall pelites is related to the rapid extension controlled sedimentation of the overlying flysch series

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

seismic anisotropy in eastern tibet from shear wave splitting reveals changes in lithospheric deformation abstract knowledge about seismic anisotropy can provide important insight into the deformation of the crust and upper mantle beneath tectonically active regions here we focus on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china we measured shear wave splitting of core refracted phases sks and skks at a temporary array of 25 iris passcal stations we calculated splitting parameters using a multi channel and a single record cross correlation method multiple layers of anisotropy cannot be ruled out but are not required by the data a fresnel zone analysis suggests that the shallow mantle between 60 and 160 km depth is the most likely source of anisotropy the polarization directions reveal a pronounced transition from primarily north south in the north sichuan to mostly east west orientations in the south yunnan in the southern part of the study region that is south of 26 n the fast polarization directions do not correlate well with known surface features and geodetic estimates of the crustal displacement fields whereas gps campaigns provide evidence suggesting north south crustal flow across the red river fault the pattern of anisotropy argues against such flow in the upper mantle these observations support models that allow differential movement of upper crust relative to lithospheric mantle in the northern part of the study region the relationships are more ambiguous and coherent deformation of the crust and mantle lithosphere cannot be excluded the interpretation of the shear wave splitting results is non unique but we suggest that the observed n s transition reflects a fundamental change in deformation regime across our study region it may be related to lateral variations in lithospheric rheology or may mark a transition from the direct impact of the continental collision to dominance of the far field strain field associated with regional subduction processes understanding the nature of the lateral change in deformation regime may prove critical for our understanding the geotectonic evolution of eastern tibet

a cavitation seal mechanism for ultramylonite formation in quartzofeldspathic rocks within the semi brittle field vivero fault nw spain abstract bands of multi phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions indeed they are one of the most typical manifestations of strain localization in high strained polymineralic rocks in the lithosphere especially in the semi brittle field yet many questions remain on their origin and existing models either do not explain all the observations or require the introduction of physical processes that are difficult to prove particularly for the inter grain mixing of phases here we present a case study of ultramylonite bands developed in a coarse grained granite deformed at conditions near the base of the seismogenic zone i e showing coseismic deformation microstructures overprinted by thermally activated ones we found that in the mylonite to ultramylonite transition two deformation mechanisms coexist promoting inter grain phase mixing through kfs precipitation 1 dislocation accommodated grain boundary sliding in quartz and 2 diffusion accommodated grain boundary sliding in feldspar the model proposed to explain grain mixing is based on a cavitation seal mechanism which is strongly dependent on grain boundary sliding for the opening of transient micro cavities and on the diffusivity of potassium feldspar for their progressive sealing

subdivision of precambrian time recommendations and suggestions by the subcommission on precambrian stratigraphy abstract the subcommission on precambrian stratigraphy consisting at the time of its last formal meeting in 1982 of 12 titular members and 30 correspondents has been actively working toward an acceptable subdivision of precambrian time for more than a decade comprehensive time rock charts have been prepared for major precambrian shields and this composite record has been analysed for patterns and events common to several or to all a geochronologic subdivision scheme has evolved that reflects these patterns of major events in earth history while still recognizing that those events are typically varied in intensity and timing from one region to another time unit boundaries have been selected so as to enclose or delimit principal cycles of sedimentation orogeny and magmatism but the boundaries are defined in years without specific reference to any bodies of rock several recommendations developed sequentially at formal meetings held at intervals of 2 4 years and announced in geologic journals with calls for comment and criticism are proposed 1 1 the recommendation to establish two major time units of eon rank proterozoic and archaean archean separated by a time boundary at 2500 ma is reaffirmed 2 2 the provisional recommendation to subdivide the proterozoic eon into three units of era rank provisionally labelled proterozoic i ii and iii with time boundaries at 2500 1600 900 and about 570 ma respectively is confirmed and 3 3 further subdivision of the proterozoic into eight time units of period rank as yet unnamed with time boundaries at 2500 2100 1800 1600 1400 1200 900 700 and about 570 ma respectively is provisionally recommended the nature of the subcommission s deliberations and the guidelines developed are described herein comment and criticism both on the procedure followed and on the recommendations themselves are invited

continental rifting and the archean proterozoic transition in the western australian shield the norseman wiluna belt dominated by lowermost sequences of deformed and metamorphosed basalts and komatiites and the fortescue group with successions of gently dipping little metamorphosed basalts and rare komatiites are typical examples of archean style greenstones and proterozoic style platform sequences respectively isotopic evidence however indicates that they are broadly coeval and both archean ca 2 7 0 05 ga and geologic evidence suggests that both relate to continental rifting their contrasting volcanism and subsequent deformation are inferred to result from their positions relative to rift axes and the state of the basement the norseman wiluna belt representing a failed rift in stretched relatively hot crust and the fortescue group representing a continental basalt succession on thick relatively cold crust the fortescue group immediately follows a major period of crustal reworking and stabilization whereas eruption of the greenstones of the norseman wiluna belt was closely followed by such an event thus the diachronous change in tectonic style from greenstones to platforms the archean proterozoic transition in this case appears to relate to a finite period of stabilization of continental crust rather than to a fundamental change in tectonic style or process with time

a comprehensive and densely sampled map of shear wave azimuthal anisotropy in the aegean anatolia region article history a better understanding of what drives surface motion in the rapidly deforming aegean anatolia region requires the comparison of mantle circulation models with reliable and densely spaced seismic anisotropy data we present a new set of 4279 high quality splitting data of core refracted shear waves measured at 216 permanent and temporary broadband seismic stations in turkey and greece and their neighboring countries when combined with previously published observations our dataset provides unprecedented dense spatial coverage of the area the delay time between the fast and slow shear waves is highest in the northern aegean sea and northwestern anatolia average 1 5 0 4 s and lowest in the southern aegean sea average 0 6 0 4 s the fast wave polarization axes are oriented ne sw over most of anatolia and the northern aegean sea these show steady counterclockwise rotation of 1 per degree of longitude from eastern anatolia to the northern aegean the only exceptions to this uniform pattern are nnw sse to nw se orientations in mainland greece and nw se orientations in the southwestern corner of anatolia the overall anisotropy pattern can be explained by instantaneous density driven mantle flow with additional local effects such as slab rollback in the aegean sea and a slab window beneath southwestern anatolia

evidence for the transition to an oxygen rich atmosphere in the rooiberg group south africa a note abstract the rooiberg felsite group is a rare example of a conformable succession that apparently spans the period of evolution of an oxygen rich atmosphere this conclusion is reached indirectly from a review of the proterozoic stratigraphy of south africa and directly from a consideration of the rooiberg itself the atmospheric transition coincides with the stratigraphic interval between the dark silicic lavas and associated grey volcaniclastics of the damwal formation and the overlying red rhyodacites and intercalated red bed sedimentary rocks of the selonsrivier formation radiometric age measurements indicate that this transition occurred after 2224 21 ma but before 2090 40 ma

growth and concentration of fibrous sillimanite related to heterogeneous deformation in k feldspar sillimanite metapelites in foliated k feldspar sillimanite metapelites fibrous sillimanite is commonly concentrated in folia that anastomose between lenticular pods of coarser grained aggregates rich in quartz feldspar and biotite with or without garnet cordierite and residual andalusite many of the folia appear to be limbs of crenulation microfolds the sillimanite concentrations may be due largely to the ability of fibrous sillimanite aggregates to undergo strong non coaxial deformation by grain boundary sliding fibre sliding without appreciable build up of dislocations whereas other minerals are unstable in these zones and so concentrate in lower strain interfolial zones initiation and especially concentration of the sillimanite in folia may be assisted by fluid flow and local base cation leaching whereby minerals unstable in zones of strong non coaxial strain are dissolved and removed from these zones

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

the variscan geology of the pyrenees abstract the main outlines of the geology of the variscan part of the pyrenees are discussed rocks involved in this cycle are high grade basement gneisses palaeozoic sediments and their metamorphic equivalents late intrusive granodiorites and early pre variscan granites the main features of the stratigraphy of the palaeozoic are given structures fall into two domains a low grade suprastructure essentially with steep folds and cleavages and a high grade infrastructure with dominantly low dipping foliations an important phase of early pre cleavage folding occurs in low grade rocks mainly along the southern border of the axial zone in high grade rocks most structures and the metamorphism postdate the main cleavage phase in low grade rocks the influence of the alpine orogeny on the variscan structures consists mainly of faults steep reverse faults in the northern and south directed thrusts in the southern part of the pyrenees metamorphism took place under high geothermal gradients and low pressures as indicated by the abundant occurrence of andalusite and cordierite

investigation of seismic anisotropy beneath the dead sea fault using dense networks of broadband stations 1 shear waveforms from core refracted sks phases recorded at 105 portable stations belonging to the desert and desire campaigns and nine permanent broadband stations of the israel seismological network are analyzed to study polarization seismic anisotropy beneath the region of the dead sea transform fault in the middle east shear wave splitting parameters exhibit variations with back azimuth initial polarization of the incoming sks waves the pattern of this variation is nearly constant along the strike of the fault suggesting a laterally uniform anisotropic structure beneath the dead sea region the modeling of the azimuthal variations of the shear wave splitting parameters and split waveforms yields two layered anisotropic models consisting of an upper layer with nearly n s symmetry axis and a deeper layer with around n25 e symmetry axis the split time is almost equally partitioned between the upper and lower layers allotting a value of 0 6 0 8 s to each layer 2 d finite difference modeling across the southern segment of the dead sea transform fault demonstrates that anisotropic structure in the strike normal direction is relatively uniform the dead sea transform fault appears not to have a significant role in the development of the regional anisotropic fabric the upper anisotropic layer is possibly related to a fossil fabric in the lithosphere inherited from the precambrian pan african orogeny the lower layer may be related to the mantle deformation due to the relative motion between the lithospheric plates and the asthenosphere and possibly affected by the local flow field due to mantle plumes as inferred by other studies

outline of upper precambrian and lower paleozoic evolution of the iberian peninsula according to u pb dating of zircons in the northwestern part of the iberian peninsula recent geological studies have defined the following stratigraphic sequence from bottom to top basal orthogneiss of miranda do douro is overlain by a complex of schist and greywacke which have been assigned to the cambrian or late precambrian these are followed by the ollo de sapo gneisses of volcanic sedimentary origin which in turn are overlain by the fossiliferous layers of the lower ordovician the schist and greywacke complex and the lower part of the ollo de sapo formation are transected by granitic dikes viana del bollo orthogneiss all these rocks were folded faulted metamorphosed and intruded by granite plutons during variscan times the emplacement of the miranda do douro and viana del bollo orthogneisses have been respectively dated at 618 9 ma and 465 10 ma by the u pb method on zircons these data yield lower and upper limits for the deposition of the ollo de sapo formation and demonstrate the occurrence of a precambrian basement pan african or cadomian orogeny in the variscan belt in the northwestern part of the iberian peninsula the emplacement of viana del bollo orthogneiss could be genetically related to the main phase of crustal thinning 470 520 ma characteristic of the prevariscan evolution of western europe typology and u pb data of the ollo de sapo zircons indicate the occurrence of inherited old precambrian zircons and yield an age of 325 3 ma for the variscan main thermal event in the area considered four possible evolutionary models for the u pb system of the ollo de sapo zircons are discussed

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

depth localized azimuthal anisotropy from sks and p receiver functions the tien shan summary shear wave splitting in the seismic sks phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations fast s wave polarization direction in collisional belts is often parallel to the trend of the belt although deformations of the mantle lithosphere in low angle thrusts would lead to the fast polarization direction normal to the trend of the belt these considerations suggested that the upper mantle in collisional belts is decoupled from the crust however sks technique is notable by a poor depth resolution and usually it assumes that the fast polarization direction is the same at any depth which is hard to justify here to investigate depth dependent azimuthal anisotropy in the mantle we invert jointly p receiver functions and sks particle motions at a number of seismograph stations the technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium a search for the optimum models is conducted with a technique similar to simulated annealing testing with synthetics demonstrates that this approach is robust the results for 10 seismograph stations in the tien shan the world s most active intracontinental collisional belt in central asia reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km in the mantle lithosphere at depths less than 100 km anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range this layer is not necessarily decoupled from the crust its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past in the lower layer asthenosphere the average azimuth of fast wave polarization is close to the trend of the belt whereas magnitude of s wave anisotropy is stable and large between 5 and 6 per cent this anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt at some stations the data require anisotropy in the crust there is no evidence for anisotropy at depths exceeding 150 250 km

a dynamic model of the structural evolution of the hercynian pyrenees abstract the structural history of the hercynian pyrenees is considered with respect to the relationships between the metamorphism and the successive deformations the characteristics and mode of emplacement of the gneissic and anatectic cores of the metamorphic domes the migmatization and the intrusions of plutonic and basic and ultrabasic rocks the metamorphism can be shown to have progressed upwards with time attaining the presently exposed levels just before during and a short time after the major penetrative regional deformation the intrusion of major plutonic anatectic and gneissic massifs which has determined the metamorphic domes in country rocks was roughly synchronous with the progression of the metamorphism but seems to have continued later in the uppermost levels the basic and ultrabasic rocks uplifted by the intrusion of the anatectic cores of the metamorphic domes have been deformed and foliated by the major regional penetrative deformation thus showing that their intrusion predated this deformation in the lowermost levels these results together with the presently available sedimentological data lead us to propose a dynamic model of the tectonic evolution of the hercynian pyrenees the first step was a crustal extension and thinning attaining its maximum during frasnian time marked in the upper levels by a horst and graben controlled sedimentation and in the lower crust by intrusions of basic magmas and the onset of the very low pressure metamorphism and anatexis characteristic of the hercynian pyrenees the second step was a crustal shortening marked by large scale crustal thrusting followed by widespread regional penetrative deformation the flysch troughs formed in front of the progressing and emerging thrusts in lower levels the thrusts are believed to have controlled the progressive ascent of the plutonic and anatectic magmas which continued their uplift and emplacement during major penetrative regional deformation in the actual mesozone and probably after it in the uppermost levels

structural and metamorphic evidence of local extension along the vivero fault coeval with bulk crustal shortening in the variscan chain nw spain abstract the vivero fault is a w dipping n s striking ductile shear zone separating the ollo de sapo antiform in its western hangingwall and the lugo dome in its eastern footwall two stages of deformation f 1 and f 2 produced nearly coaxial folds with sub horizontal axes a crenulation cleavage s 2 transposes an older s 1 three sets of shear bands in the hangingwall define a pervasive fabric consistent with an e w bulk shortening perpendicular to a composite s 1 2 foliation and nne stretching parallel to l 2 the vivero fault zone is marked by a mylonitic foliation with a steeply nw plunging stretching lineation and extensional crenulation cleavage ecc indicating normal slip in the vicinity of the fault sub horizontal nne trending f 3 folds with a crenulation cleavage s 3 deform earlier formed fabrics including a mylonitic foliation pressure temperature conditions obtained from mineral assemblages on both sides of the vivero fault yield a minimum throw of 5 5 km andalusite bearing pelite in the hangingwall was infolded by an f 2 synform into the kyanite field at 450 500 c the eastern edge of these rocks was later accreted to the footwall and heated to andalusite staurolite conditions at 600 c slip on the vivero and valdovino faults is kinematically related east west shortening during f 2 involved folding and sinistral strike slip on the valdovino fault which induced local extension along the newly generated vivero fault synkinematic emplacement of granitoids along the vivero fault is favoured by extension coeval slip on both faults took place during the later stages of f 2 folding geometrical constraints caused northwards escape of the crustal block bounded by the valdovino and vivero faults recorded by nne stretching defined by l 2

a calculated petrogenetic grid for the system k2o feo mgo al2o3 sio2 h2o with particular reference to contact metamorphosed pelites a petrogenetic grid is presented for the system kfmash k o feo mgo a1 0 sio h o including biotite muscovite k feldspar chlorite chloritoid staurolite cordierite garnet ortho amphibole orthopyroxene spinel andalusite sillimanite kyanite quartz and corundum with h o in excess which was calculated using the computer program thermocalc and the powell and holland internally consistent thermodynamic dataset by removing the normal constraint of having quartz in excess both quartz bearing and quartz absent equilibria are shown quartz absent equilibria are particularly relevant at high t and low p conditions because of their common occurrence at these conditions the calculated mineral assemblage and mineral compositional variations in terms of femg and fe mg siall exchange vectors are broadly compatible with observations on natural rocks particularly when non kfmash components are taken into account

setting new constrains on the age of crustal scale extensional shear zone vivero fault implications for the evolution of variscan orogeny in the iberian massif the vivero fault is crustal scale extensional shear zone parallel to the variscan orogen in the iberian massif belt with an associated dip slip movement toward the hinterland to constrain the timing of the extension accommodated by this structure we performed zircon u pb la icp ms geochronology in several deformed plutons some of them emplaced syntectonically the different crystallization ages obtained indicate that the fault was active at least between 303 2 and 287 3 ma implying a minimum tectonic activity of 16 5 ma along the fault the onset of the faulting is established to have occurred later than 314 2 ma the geochronological data confirm that the vivero fault postdates the main variscan deformation events in the nw of the iberian massif and that the extension direction of the late carboniferous early permian crustal scale extensional shear zones along the ibero armorican arc was consistently perpendicular to the general arcuate trend of the belt in sw europe

development of high thermal gradients by coeval transpression and magmatism during the variscan orogeny insights from the cap de creus eastern pyrenees abstract the cap de creus massif in the eastern pyrenees is a fragment of the variscan mid crust geological mapping and meso and micro scale analysis of the relationships between fold structures metamorphic mineral assemblages and fabrics affecting igneous bodies suggest the interaction of three processes deformation metamorphism and magmatism low pressure prograde metamorphism grades laterally from greenschist to amphibolite facies over a distance of 5 km zones of intense metamorphism coincide with transpressional deformation and with the intrusion of small sheets of diorite and granitoid sillimanite schists and small migmatite pods are distributed around the small intrusions of basic intermediate composition the inferred maximum thermal gradient is 80 c km this thermal pattern is similar to patterns observed in other variscan massifs in the pyrenees where high metamorphic gradients are interpreted in different ways in the ne cap de creus the type and distribution of prograde metamorphism was caused or at least enhanced by the intrusion of diorites and granitoids alternatively deformation developed at prograde conditions and synchronously with magmatism displays a highly heterogeneous distribution producing a sub vertical zone of high strain that coincides with domains of highest grade metamorphism and igneous activity this zone of high strain developed in a broad transpressive regime could have favoured the injection of magmatic sheets from deep to mid crustal levels progressive folding from prograde to retrograde metamorphic conditions might account for the narrowing of metamorphic zones

crustal anisotropy and ductile flow beneath the eastern tibetan plateau and adjacent areas abstract crustal anisotropy beneath 71 broadband seismic stations situated at the eastern tibetan plateau and the sichuan basin is investigated based on the sinusoidal moveout of p to s conversions from the moho and an intra crustal discontinuity significant crustal anisotropy is pervasively detected beneath the study area with an average splitting time of 0 39 0 18 s the resulting fast orientations are mostly parallel to the major shear zones in the songpan ganzi terrane and can be explained by fluid filled fractures favoring the model of rigid block motion with deformations concentrated on the block boundaries in the vicinity of the xianshuihe xiaojiang fault zone in the southern songpan ganzi terrane our results when combined with previously revealed high crustal poisson s ratio in the area support the existence of mid lower crustal flow the longmenshan fault zone and adjacent areas are dominated by strike orthogonal fast orientations which are consistent with alignments of cracks associated with compressional stress between the plateau and the sichuan basin the observations suggest that crustal thickening is the main cause of the high topographic relief across the longmenshan fault zone

thermobarometric constraints on the conditions of metamorphism in the canigou massif pyrenees implications for hercynian geothermal gradients quantitative thermobarometry of garnet biotite sillimanite f cordierite pelitic assemblages from the canigou massif and constraints based on consideration of high grade metapelitic reaction equilibria indicate that peak temperatures during hercynian metamorphism reached 725 25 c at depths of 16 2 km 4 5 0 5 kbar this corresponds to an average peak geothermal gradient of 45 50 c km 1 which is in broad agreement with results from studies of granulite facies assemblages in the pyrenees but is significantly lower than gradients proposed for some other amphibolite facies areas in the terrane it is suggested that the claims by numerous authors for peak hereynian geothermal gradients in the pyrenees in excess of 70 100 c km 1 fail to consider the effects of post metamorphic attenuation of metamorphic sequences and the unreliable nature of p estimates made in the absence of suitable assemblages for quantitative thermobarometry

structural metamorphic and magmatic history of the mondonedo nappe hercynian belt nw spain abstract the mondonedo nappe is one of the main units in the internal zone of the hercynian belt in the iberian peninsula it consists of low to medium grade metasediments late precambrian and palaeozoic in age folded by large d 1 e verging recumbent structures and thrusted several tens of km towards the east during a d 2 deformation episode at the base of the thrust sheet a shear zone of up to 3 km thick was developed granitoid bodies which were intruded after d 1 and before or during d 2 were deformed in this basal zone the geometry of d 1 and d 2 structures and that of the isograds are described as well as the relationships between metamorphism and deformation a structural evolution is inferred which shows an episode of generalized ductile deformation d 1 followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone d 2 and culminated with the thrusting along an important fracture the shear zone originated in a thermally softened level which according to metamorphic data was at temperatures in the range 550 600 c the structural metamorphic and magmatic features of the mondonedo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs this could be a consequence of the collision against another continental mass to the west

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

c o h s fluid composition and oxygen fugacity in graphitic metapelites c 0 h fluid produced by the equilibration of hzo and excess graphite must maintain the atomic h o ratio of water 2 1 this constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric isothermal conditions the o h20 and co fugacities j fhto and fc of such fluids have been estimated from equations of state and fit as a function of pressure and temperature these fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration e g pelitic lithologies because there are no compositional degrees of freedom for graphite saturated fluids produced entirely by dehydration the variance of the dehydration process is not increased in comparison with that in non graphitic systems thus compositional buffering of c 0 h fluids by dehydration equilibria a common petrological model requires that redox reactions decarbonation reactions or external h o 2 fluid sources perturb the evolution of the metamorphic system such perturbations are not likely to be significant in metapelitic environments but their tendency will be to increase the fo of the fluid phase at high metamorphic grades pyrite desulphidation reactions may cause a substantial reduction of fhz0 and slight increases info and fcs relative to sulphur free fluid at low metamorphic grade sulphur solubility in h o s 2 fluids is so low that pyrite decomposition must occur by sulphurconserving reactions that cause iron depletion in silicates a common feature of sulphidic pelites with increasing temperature and sulphur solubility pyrite desulphidation may be driven by dehydration reactions or infiltration of h o rich fluids the absence of magnetite and the assemblages carbonate aluminosilicate or pyrite pyrrhotite ilmenite from most graphitic metapelites is consistent with an h o 2 model for gcoh s fluid for graphitic rocks in which such a model is inapplicable a phase diagram variable that defines the h o ratio of gcoh s fluid is more useful than the conventional

characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions rocks in the continental crust are long lived and have the potential to record a wide span of tectonic history in rock fabric mapping rock fabric in situ at depth requires the application of seismic methods below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations cpos influenced by deformation processes interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor we compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements including electron backscatter diffraction ebsd and ultrasound on crustal rocks the majority of samples developed fabric at pressures corresponding to mid lower crustal depths tensor symmetry is a function of mineral modal composition with mica rich samples trending towards hexagonal symmetry amphibole rich samples trending towards an increased orthorhombic symmetry component and quartz feldspar rich samples showing a larger component of lower symmetries 77 of samples have a best fit hexagonal tensor with slow axis symmetry as opposed to mantle deformation fabric that usually has fast axis symmetry the best fit hexagonal approximation for crustal tensors is not elliptical but deviates systematically from elliptical symmetry with increasing anisotropy an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations we present empirical linear relationships between anisotropy and ellipticity for crustal rocks the maximum out of plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for non elliptical symmetry the elliptical assumption results in a bias of up to 1 4 times true anisotropy

inversion of teleseismic s particle motion for azimuthal anisotropy in the upper mantle a feasibility study summary we have developed a technique for the inversion of teleseismic s waveforms in terms of azimuthal anisotropy in the upper mantle we test different models of the earth upper mantle by transforming the observed horizontal components into a synthetic vertical component and comparing this with the observed vertical component the optimum model is found by minimizing the difference between the synthetic vertical component and the observed one using this method we explore the possibility of constraining the distribution of azimuthal anisotropy with depth we present examples of seismic observations where the data are clearly in favour of an anisotropic model these observations can be interpreted in terms of two anisotropic layers with different directions of fast velocity axes

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

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